Sunday, May 9, 2010

Ventilation


What is mechanical ventilationIn medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing.

This may involve a machine called a ventilator or the breathing may be assisted by a physician or other suitable person compressing a bag or set of bellows. Traditionally divided into negative-pressure ventilation, where air is essentially sucked into the lungs, or positive pressure ventilation, where air (or another gas mix) is pushed into the trachea.

It can be used as a short term measure, for example during an operation or critical illness (often in the setting of an intensive care unit). It may be used at home or in a nursing or rehabilitation institution if patients have chronic illnesses that require long-term ventilatory assistance.

Owing to the anatomy of the human pharynx, larynx, and esophagus and the circumstances for which ventilation is required then additional measures are often required to "secure" the airway during positive pressure ventilation to allow unimpeded passage of air into the trachea and avoid air passing into the esophagus and stomach. Commonly this is by insertion of a tube into the trachea which provides a clear route for the air. This can be either an endotracheal tube, inserted through the natural openings of mouth or nose or a tracheostomy inserted through an artificial opening in the neck. In other circumstances simple airway maneuvres, an oropharyngeal airway or laryngeal mask airway may be employed. If the patient is able to protect their own airway such as in non-invasive ventilation or negative-pressure ventilation then no airway adjunct may be needed.

Mechanical ventilation is often a life-saving intervention, but carries many potential complications including pneumothorax, airway injury, alveolar damage, and ventilator-associated pneumonia.[citation needed].

In many healthcare systems prolonged ventilation as part of intensive care is a limited resource (in that there are only so many patients that can receive care at any given moment). It is used to support a single failing organ system (the lungs) and cannot reverse any underlying disease process (such as terminal cancer). For this reason there can be (occasionally difficult) decisions to be made about whether it is suitable to commence someone on mechanical ventilation. Equally many ethical issues surround the decision to discontinue mechanical ventilation.

Contents
1 History
1.1 Negative pressure machines
1.2 Positive pressure machines
2 Indications for use
3 Types of ventilators
4 Modes of ventilation
4.1 Conventional ventilation
4.1.1 Breath termination
4.1.2 Breath initiation
4.2 APRV (Airway Pressure Release Ventilation)
4.3 High Frequency Ventilation (HFV)
4.4 Non-invasive ventilation (Non-invasive Positive Pressure Ventilation or NIPPV)
4.5 Proportional Assist Ventilation (PAV)
4.6 Adaptive Support Ventilation (ASV)
4.7 Neurally Adjusted Ventilatory Assist (NAVA)
5 Choosing amongst ventilator modes
6 Initial ventilator settings
6.1 Tidal volume, rate, and pressures
6.2 Sighs
6.3 Initial FiO2
6.4 Positive end-expiratory pressure (PEEP)
6.5 Positioning
6.6 Sedation and Paralysis
6.7 Prophylaxis
7 Modification of settings
7.1 When to withdraw mechanical ventilation
8 Connection to ventilators
9 Terminology
10 See also
11 References
12 Further reading
13 External links


 History
The Roman physician Galen may have been the first to describe mechanical ventilation: "If you take a dead animal and blow air through its larynx [through a reed], you will fill its bronchi and watch its lungs attain the greatest distention."[1] Vesalius too describes ventilation by inserting a reed or cane into the trachea of animals[2]. In 1908 George Poe demonstrated his mechanical respirator by asphyxiating dogs and seemingly bringing them back to life.[3]

Negative pressure machines
An Iron LungMain article: Iron Lung
The iron lung, also known as the Drinker and Shaw tank, was developed in 1929 and was one of the first negative-pressure machines used for long-term ventilation. It was refined and used in the 20th century largely as a result of the polio epidemic that struck the world in the 1940s. The machine is effectively a large elongated tank, which encases the patient up to the neck. The neck is sealed with a rubber gasket so that the patient's face (and airway) are exposed to the room air.

While the exchange of oxygen and carbon dioxide between the bloodstream and the pulmonary airspace works by diffusion and requires no external work, air must be moved into and out of the lungs to make it available to the gas exchange process. In spontaneous breathing, a negative pressure is created in the pleural cavity by the muscles of respiration, and the resulting gradient between the atmospheric pressure and the pressure inside the thorax generates a flow of air.

In the iron lung by means of a pump, the air is withdrawn mechanically to produce a vacuum inside the tank, thus creating negative pressure. This negative pressure leads to expansion of the chest, which causes a decrease in intrapulmonary pressure, and increases flow of ambient air into the lungs. As the vacuum is released, the pressure inside the tank equalizes to that of the ambient pressure, and the elastic coil of the chest and lungs leads to passive exhalation. However, when the vacuum is created, the abdomen also expands along with the lung, cutting off venous flow back to the heart, leading to pooling of venous blood in the lower extremities. There are large portholes for nurse or home assistant access. The patients can talk and eat normally, and can see the world through a well-placed series of mirrors. Some could remain in these iron lungs for years at a time quite successfully.

Today, negative pressure mechanical ventilators are still in use, notably with the Polio Wing Hospitals in England such as St. Thomas' (by Westminster in London) and the John Radcliffe in Oxford. The prominent device used is a smaller device known as the cuirass. The cuirass is a shell-like unit, creating negative pressure only to the chest using a combination of a fitting shell and a soft bladder. Its main use is in patients with neuromuscular disorders who have some residual muscular function. However, it was prone to falling off and caused severe chafing and skin damage and was not used as a long term device. In recent years this device has re-surfaced as a modern polycarbonate shell with multiple seals and a high pressure oscillation pump in order to carry out biphasic cuirass ventilation.

Positive pressure machines
Mechanical ventilatorThe design of the modern positive-pressure ventilators were mainly based on technical developments by the military during World War II to supply oxygen to fighter pilots in high altitude. Such ventilators replaced the iron lungs as safe endotracheal tubes with high volume/low pressure cuffs were developed. The popularity of positive-pressure ventilators rose during the polio epidemic in the 1950s in Scandinavia and the United States and was the beginning of modern ventilation therapy. Positive pressure through manual supply of 50% oxygen through a tracheostomy tube led to a reduced mortality rate among patients with polio and respiratory paralysis. However, because of the sheer amount of man-power required for such manual intervention, mechanical positive-pressure ventilators became increasingly popular.

Positive-pressure ventilators work by increasing the patient's airway pressure through an endotracheal or tracheostomy tube. The positive pressure allows air to flow into the airway until the ventilator breath is terminated. Subsequently, the airway pressure drops to zero, and the elastic recoil of the chest wall and lungs push the tidal volume -- the breath—out through passive exhalation.

This is an example of a neonatal (infant) ventilator.

 Indications for use
Mechanical ventilation is indicated when the patient's spontaneous ventilation is inadequate to maintain life. It is also indicated as prophylaxis for imminent collapse of other physiologic functions, or ineffective gas exchange in the lungs. Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease, the patient's underlying condition should be correctable and should resolve over time. In addition, other factors must be taken into consideration because mechanical ventilation is not without its complications (see below)

Common medical indications for use include:

Acute lung injury (including ARDS, trauma)
Apnea with respiratory arrest, including cases from intoxication
Chronic obstructive pulmonary disease (COPD)
Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) > 50 mmHg and pH < 7.25, which may be due to paralysis of the diaphragm due to Guillain-Barré syndrome, Myasthenia Gravis, spinal cord injury, or the effect of anaesthetic and muscle relaxant drugs Increased work of breathing as evidenced by significant tachypnea, retractions, and other physical signs of respiratory distress Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) < 55 mm Hg Hypotension including sepsis, shock, congestive heart failure Neurological diseases such as Muscular Dystrophy Amyotrophic Lateral Sclerosis

 Types of ventilators
 Ventilation can be delivered via: Hand-controlled ventilation such as: SMART BAG MO Bag-Valve-Mask ResuscitatorBag valve mask Continuous-flow or Anaesthesia (or T-piece) bag A mechanical ventilator. Types of mechanical ventilators include: Transport ventilators. These ventilators are small, more rugged, and can be powered pneumatically or via AC or DC power sources. ICU ventilators. These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure). This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time). Many ICU ventilators also incorporate graphics to provide visual feedback of each breath. NICU ventilators. Designed with the preterm neonate in mind, these are a specialized subset of ICU ventilators which are designed to deliver the smaller, more precise volumes and pressures required to ventilate these patients. PAP ventilators. these ventilators are specifically designed for non-invasive ventilation. this includes ventilators for use at home, in order to treat sleep apnea.

Modes of ventilation
  Conventional ventilation The modes of ventilation can be thought of as classifications based on how to control the ventilator breath. Traditionally ventilators were classified based on how they determined when to stop giving a breath. The three traditional categories of ventilators are listed below. As microprocessor technology is incorporated into ventilator design, the distinction among these types has become less clear as ventilators may use combinations of all of these modes as well as flow-sensing, which controls the ventilator breath based on the flow-rate of gas versus a specific volume, pressure, or time.
 Breath termination Modes of ventilation are classified by the means that they determine the inspired breath is complete. This is sensed by either pressure or volume. Volume ventilation - A predetermined tidal volume (Vt) is set for the patient and is delivered with each inspiration. The amount of pressure necessary to deliver this volume will fluctuate from breath to breath based on the resistance and compliance of the patient and ventilator circuit. If the tidal volume is set at 500ml, the ventilator will continue to inspire gas until it reaches its goal. Upon completion of the inspired volume, the ventilator will open a valve allowing the patient to passively exhale. Pressure ventilation - A predetermined peak inspiratory pressure (PIP) is determined based on the patient's condition and pathophysiology. The ventilator will flow gas into the patient until this set pressure is reached. Upon reaching the preset PIP, the ventilator allows for passive exhalation. Caution and close observation must be given in this mode due to potential for either hypoventilation or hyperventilation because the tidal volume is variable. Several manufactures have incorporated features from both of theses modes in an attempt to accommodate patients needs. These modes are flow-variable, volume-targeted, pressure-regulated, time-limited modes (for example, pressure regulated volume control - PRVC). This means that instead of providing an exact tidal volume each breath, a target volume is set and the ventilator will vary the inspiratory flow at each breath to achieve the target volume at the lowest possible peak pressure. The inspiratory time (Ti) limits the length of the inspiratory cycle and therefore the I:E ratio. Pressure regulated modes such as PRVC or Auto-flow (Draeger) can most easily be thought of as turning a volume mode into a pressure mode with the added benefit of maintaining more control over tidal volume than with strictly pressure-control. [edit] Breath initiation The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath. Similar to the termination classification noted above, microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications. Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above. Assist Control (AC). In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath. Traditional assist-control used only a pre-set tidal volume—when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV. However, the initiation timing is the same—both provide a ventilator breath with every patient effort. In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic. Although a maximum rate is not usually set, an alarm can be set if the ventilator cycles too frequently. This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt). Synchronized Intermittent Mandatory Ventilation (SIMV). In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 seconds - thus a respiratory rate of 12 results in a 5 second cycle time). Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor. When the ventilator senses the first patient breathing attempt within the cycle, it delivers the preset ventilator breath. If the patient fails to initiate a breath, the ventilator delivers a mechanical breath at the end of the breath cycle. Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath. However, SIMV may be combined with pressure support (see below). SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate, which requires the patient to take additional breaths beyond the SIMV triggered breath. Controlled Mechanical Ventilation (CMV). In this mode the ventilator provides a mechanical breath on a preset timing. Patient respiratory efforts are ignored. This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient. It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing. Pressure Support Ventilation (PSV). When a patient attempts to breathe spontaneously through an endotracheal tube, the narrowed diameter of the airway results in higher resistance to airflow, and thus a higher work of breathing. PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that "supports" ventilation during inspiration. Thus, for example, SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported. However, while the SIMV mandated breaths have a preset volume or peak pressure, the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (e.g. 10-25%). New generation of ventilators provides user-adjustable inspiration cycling off threshold, and some even are equipped with automatic inspiration cycling off threshold function. This helps the patient ventilator synchrony[4]. The peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath. PSV can be also be used as an independent mode. Continuous Positive Airway Pressure (CPAP). A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation, decrease the work of breathing, and decrease the work of the heart (such as in left-sided heart failure — CHF). Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths. In addition, no additional pressure above the CPAP pressure is provided during those breaths. CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs. Positive end-expiratory pressure (PEEP) is functionally the same as CPAP, but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle. After delivery of the set amount of breath by the ventilator, the patient then exhales passively. The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC). The FRC is primarily determined by the elastic qualities of the lung and the chest wall. In many lung diseases, the FRC is reduced due to collapse of the unstable alveoli, leading to a decreased surface area for gas exchange and intrapulmonary shunting (see above), with wasted oxygen inspired. Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC. [edit] APRV (Airway Pressure Release Ventilation) APRV begins from an elevated baseline (called Phigh or measured high pressure) and achieves tidal ventilation with a brief release of the Phigh. This brief release allows CO2 removal through passive exhalation secondary to elastic recoil. The exhalation time (Tlow) is shortened to usually less than one second to prevent alveolar derecruitment and collapse - it is essentially CPAP with a brief release. Ever increasing empirical evidence and clinical experience is showing that APRV is the primary mode of choice when ventilating a patient with ARDS or ALI (Acute Lung Injury). Advantages to APRV ventilation include: decreased airway pressures, decreased minute ventilation, decreased dead-space ventilation, promotion of spontaneous breathing, almost 24 hour a day alveolar recruitment, decreased use of sedation, near elimination of neuromuscular blockade, optimized arterial blood gas results, mechanical restoration of FRC (functional residual capacity), a positive effect on cardiac output (due to the negative inflection from the elevated baseline with each spontaneous breath), increased organ and tissue perfusion, potential for increased urine output due to increased renal perfusion. A patient with ARDS on average spends 8 to 11 days on a mechanical ventilator; APRV may reduce this time significantly and therefore reduce the incidence of VAP (ventilator acquired pneumonia), a risk that increases with each hour an intubated patient spends on the ventilator (VAP rate is 100% at 100 days on the vent) and carries with it a near 50% mortality rate. So, hospitals that are reporting a 0% incidence of VAP, may be improperly coding or improperly reporting. * A controlled clinical trial testing APRV against the current ARDSNet protocol must be initiated.

High Frequency Ventilation (HFV) Sensormedics 3100 High Frequency ventilator
 High frequency ventilation High-Frequency Ventilation refers to ventilation that occurs at rates significantly above that found in natural breathing (as high as 240-900 "breaths" per minute). Within the category of high-frequency ventilation, the three principal types are high-frequency jet ventilation (HFJV), high-frequency flow interruption (HFFI), and high-frequency oscillatory ventilation (HFOV). High Frequency Jet Ventilation employs a endotracheal tube adaptor in place for the normal 15 mm ET tube adaptor. A high pressure ‘’jet’’ of gas flows out of the adaptor and into the airway. This jet of gas occurs for a very brief duration, about 0.02 seconds, and at high frequency: 4-11 hertz. Tidal volumes ≤ 1 ml/Kg are used during HFJV. This combination of small tidal volumes delivered for very short periods of time create the lowest possible distal airway and alveolar pressures produced by a mechanical ventilator. Exhalation is passive. Jet ventilators utilize various I:E ratios--between 1:1.1 and 1:12-- to help achieve optimal exhalation. Conventional mechanical breaths are sometimes used to aid in reinflating the lung. Optimal PEEP is used to maintain alveolar inflation and promote ventilation-to-perfusion matching. Jet ventilation has been shown to reduce ventilator induced lung injury by as much as 20%. "HFFI" operates similarly to a conventional ventilator, providing increased circuit pressure during the inspiratory phase and dropping back to PEEP during the expiratory phase. In "HFOV" the pressure wave is driven by an electromagnetically controlled diaphragm similar to a loudspeaker. Because this can rapidly change the volume in the circuit, HFOV can produce a pressure that is lower than ambient pressure during the expiratory phase. This is sometimes called "active" expiration. In both types of high-frequency ventilation the pressure wave that is generated at the ventilator is markedly attenuated by passage down the endotracheal tube and the major conducting airways. This helps protect the alveoli from volutrauma that occurs with traditional positive pressure ventilation. Although the alveoli are kept at a relatively constant volume, similar to CPAP, other mechanisms of gas exchange allow ventilation (the removal of CO2) to occur without tidal volume exchange. Ventilation in HFOV is a function of frequency, amplitude, and I:E ratio and is best described graphically as the area under the curve of an oscillatory cycle. Amplitude is analogous to tidal volume in conventional ventilation; larger amplitudes remove more CO2. Seemingly paradoxical, lower frequencies remove more CO2 in HFOV whereas in conventional ventilation the opposite is true. As frequency decreases, there is less attenuation of the pressure wave transmitted to the alveoli. This results in increased mixing of gas and thus ventilation. I-time is set as a percentage of total time (usually 33%). Amplitude is a function of power and is subject to variability due to changes in compliance or resistance. Therefore, power requirements may vary significantly during treatment and from patient to patient. Patient characteristics and ventilator settings determine whether PaCO2 changes may be more sensitive to amplitude or frequency manipulation. In HFOV, mean airway pressure (MAP) is delivered via a continuous flow through the patient circuit which passes through a variable restriction valve (mushroom valve) on the expiratory limb. Increasing the flow through the circuit and/or increasing the pressure in the mushroom valve increases MAP. The MAP in HFOV functions similarly to PEEP in conventional ventilation in that it provides the pressure for alveolar recruitment. 

 Non-invasive ventilation (Non-invasive Positive Pressure Ventilation or NIPPV)
This refers to all modalities that assist ventilation without the use of an endotracheal tube. Non-invasive ventilation is primarily aimed at minimizing patient discomfort and the complications associated with invasive ventilation. It is often used in cardiac disease, exacerbations of chronic pulmonary disease, sleep apnea, and neuromuscular diseases. Non-invasive ventilation refers only to the patient interface and not the mode of ventilation used; modes may include spontaneous or control modes and may be either pressure or volume modes. Some commonly used modes of NIPPV include: Continuous positive airway pressure (CPAP). Bi-level Positive Airway Pressure (BIPAP). Pressures alternate between Inspiratory Positive Airway Pressure (IPAP) and a lower Expiratory Positive Airway Pressure (EPAP), triggered by patient effort. On many such devices, backup rates may be set, which deliver IPAP pressures even if patients fail to initiate a breath.(Wheatley 2000 et all) Intermittent positive pressure ventilation (IPPV) via mouthpiece or mask

Proportional Assist Ventilation (PAV)
Proportional Assist Ventilation (PAV) is a form of synchronised ventilator support based upon the Equation of Motion in which the ventilator generates pressure in proportion to the instantaneous patient effort. Unlike other modes of partial support, there is no target flow, tidal volume or pressure. PAV’s objective is to allow the patient to attain ventilation and breathing pattern his ventilatory control system desires. The main operational advantages of PAV are automatic synchrony with inspiratory efforts, exhalation and adaptability to change in ventilatory demand. Proportional Assist Ventilation Plus — PAV+ (Puritan Bennett – 840 ventilator range, Proportional Pressure Support — PPS (Drager Evita series)and Respironics BiPAP Vision PAV , are commercially available implementations of PAV which automatically amplify the patient's own spontaneous effort to breathe by increasing airway pressure during inspiration proportionally to a set amplification factor. In PAV+, the level of amplification, thus the level of work of breathing, is set through a single setting (%support) and the pressure applied is continuously and automatically adjusted based on measures (including automatic assessment of Elastance and Resistance) taken throughout the inspiratory cycle to maintain an appropriate level of support.

Adaptive Support Ventilation (ASV)
 Adaptive Support Ventilation (ASV) is a positive pressure mode of mechanical ventilation that is closed-loop controlled. In this mode, the frequency and tidal volume of breaths of a patient on the ventilator are automatically adjusted based on the patient’s requirements. The lung mechanics data are used to adjust the depth and rate of breaths to minimize the work rate of breathing. In the ASV mode, every breath is synchronized with patient effort if such an effort exists, and otherwise, full mechanical ventilation is provided to the patient. ASV technology was originally described as one of the embodiments of US Patent No. 4986268.[5] In this invention, a modified version of an equation derived in physiology in 1950 [6] to minimize the work rate of breathing in man, was used for the first time to find the optimum frequency of mechanical ventilation. The rationale was to make the patient's breathing pattern comfortable and natural within safe limits, and thereby stimulate spontaneous breathing and reduce the weaning time. A prototype of the system was built by the inventor in late 1980s. The inventor is Dr. Fleur T. Tehrani who is a university professor in the US. Shortly after the Patent was issued in 1991, Hamilton Medical, a ventilator manufacturing company, contacted the inventor and discussed marketing the technology with her. Some years later, Hamilton Medical marketed this closed-loop technique under license of this Patent as ASV. Since the issuance of the Patent, a number of articles have been published by the inventor and her colleagues that are related to the invention, and some of them describe further advancements of the closed-loop techniques presented in the Patent.

Neurally Adjusted Ventilatory Assist (NAVA)
Neurally Adjusted Ventilatory Assist (NAVA) is a unique positive pressure mode to mechanical ventilation based on neural respiratory output, in connectins with invasive and non-invasive NAVA. The act of taking a breath is controlled by the respiratory center of the brain, which decides the characteristics of each breath, timing and sice. The respiratory center sends a signal along the phrenic nerve, excites the diaphragm muscle cells, leading to muscle contraction and descent of the diaphragm dome. As a result, the pressure in the airway drops, causing an inflow of air into the lungs. With NAVA, the elctrical activity of the diaphragm (Edi) is captured, fed to the ventilatorand used to assist the patient's breathing in synchrony with and in proportion to the patients own efforts, regardless of patient category or size. As the work of the ventilator and the diaphragm is controlled by the same signal, coupling between the diaphragm and the SERVO-i ventilator is synchronized simultaneously. Reference: New method permits neural control of mechanical ventilation [edit] Choosing amongst ventilator modes Assist-control mode minimizes patient effort by providing full mechanical support with every breath. This is often the initial mode chosen for adults because it provides the greatest degree of support. In patients with less severe respiratory failure, other modes such as SIMV may be appropriate. Assist-control mode should not be used in those patients with a potential for respiratory alkalosis, in which the patient has an increased respiratory drive. Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease, hyperventilatory sepsis, and head trauma. Respiratory alkalosis will be evident from the initial arterial blood gas obtained, and the mode of ventilation can then be changed if so desired. Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients. It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis. High frequency oscillation is used most frequently in neonates, but is also used as an always alternative mode in adults with severe ARDS. Pressure Regulated Volume Control is another option. 

Initial ventilator settings
The following are general guidelines that may need to be modified for the individual patient. [edit] Tidal volume, rate, and pressures For adult patients and older children tidal volume (Tv) is calculated in milliliters per kilogram. Traditionally 10 ml/kg was used but has been shown to cause barotrauma, or injury to the lung by overextension, so 6 to 8 ml/kg is now common practice in ICU. Hence a patient weighing 70 kg would get a TV of 420–480 ml. In adults a rate of 12 strokes per minute is generally used. with acute respiratory distress syndrome (ARDS) a tidal volume of 6–8 ml/kg is used with a rate of 10–12 per minute. This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) but this elevation does not need to be corrected (termed permissive hypercapnia) For infants and younger children without existing lung disease—a tidal volume of 4–8 ml/kg to be delivered at a rate of 30–35 breaths per minute with RDS—decrease tidal volume and increase respiratory rate sufficient to maintain pCO2 between 45 and 55[vague]. Allowing higher pCO2 (sometimes called permissive hypercapnia) may help prevent ventilator induced lung injury As the amount of tidal volume increases, the pressure required to administer that volume is increased. This pressure is known as the peak airway pressure. If the peak airway pressure is persistently above 45 cmH2O (4.4 kPa) for adults, the risk of barotrauma is increased (see below) and efforts should be made to try to reduce the peak airway pressure. In infants and children it is unclear what level of peak pressure may cause damage. In general, keeping peak pressures below 30 cmH2O (2.9 kPa) is desirable. Monitoring for barotrauma can also involve measuring the plateau pressure, which is the pressure after the delivery of the tidal volume but before the patient is allowed to exhale. Normal breathing pattern involves inspiration, then expiration. The ventilator is programmed so that after delivery of the tidal volume (inspiration), the patient is not allowed to exhale for a half a second. Therefore, pressure must be maintained in order to prevent exhalation, and this pressure is the plateau pressure. Barotrauma is minimized when the plateau pressure is maintained < 30–35 cmH2O.  Sighs An adult patient breathing spontaneously will usually sigh about 6–8 times per hour to prevent microatelectasis, and this has led some to propose that ventilators should deliver 1½–2 times the amount of the preset tidal volume 6–8 times per hour to account for the sighs. However, such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma. Currently, accounting for sighs is not recommended if the patient is receiving 10-12 mL/kg or is on PEEP. If the tidal volume used is lower, the sigh adjustment can be used, as long as the peak and plateau pressures are acceptable. Sighs are not generally used with ventilation of infants and young children. [edit] Initial FiO2 Because the mechanical ventilator is responsible for assisting in a patient's breathing, it must then also be able to deliver an adequate amount of oxygen in each breath. The FiO2 stands for fraction of inspired oxygen, which means the percent of oxygen in each breath that is inspired. (Note that normal room air has ~21% oxygen content). In adult patients who can tolerate higher levels of oxygen for a period of time, the initial FiO2 may be set at 100% until arterial blood gases can document adequate oxygenation. An FiO2 of 100% for an extended period of time can be dangerous, but it can protect against hypoxemia from unexpected intubation problems. For infants, and especially in premature infants, avoiding high levels of FiO2 (>60%) is important.

 Positive end-expiratory pressure (PEEP)
Main article: Positive end-expiratory pressure
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC). At the end of expiration, the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure. The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange, thus reducing the size of the shunt. For example, if a large shunt is found to exist based on the estimation from 100% FiO2 (see above), then PEEP can be considered and the FiO2 can be lowered (< 60%) in order to maintain an adequate PaO2, thus reducing the risk of oxygen toxicity. In addition to treating a shunt, PEEP may also be useful to decrease the work of breathing. In pulmonary physiology, compliance is a measure of the "stiffness" of the lung and chest wall. The mathematical formula for compliance (C) equals change in volume divided by change in pressure. The higher the compliance, the more easily the lungs will inflate in response to positive pressure. An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC, since the partially inflated lung takes less energy to inflate further. Excessive PEEP can however produce overinflation, which will again decrease compliance. Therefore it is important to maintain an adequate, but not excessive FRC. Indications. PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function. As such, it should be judiciously used and is indicated for adults in two circumstances. If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60% If the initial shunt estimation is greater than 25% If used, PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2. As PEEP increases intrathoracic pressure, there can be a resulting decrease in venous return and decrease in cardiac output. A PEEP of less than 10 cmH2O (1 kPa) is usually safe in adults if intravascular volume depletion is absent. Lower levels are used for pediatric patients. Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring. More recent literature has failed to find outcome benefits with routine PA catheterisation when compared to simple central venous pressure monitoring.[8] If cardiac output measurement is required, minimally invasive techniques, such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives.[9][10] PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 < 40%. When withdrawing, it is decreased through 1–2 cmH2O decrements while monitoring haemoglobin-oxygen saturations. Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation. [edit] Positioning Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia. It improves FRC, drainage of secretions, and ventilation-perfusion matching (efficiency of gas exchange). It may improve oxygenation in > 50% of patients, but no survival benefit has been documented.

 Sedation and Paralysis
Most intubated patients receive intravenous sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress. Sedation also helps the patient tolerate the constant irritation of the endotracheal tube in their mouth, pharynx and trachea. Without some form of sedation and analgesia, it is common for patients to "fight" the ventilator. This fighting increases work of breathing and may cause further lung injury. Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning. These interruptions are frequently described as "sedation vacations" and have been shown to reduce the time patients stay on mechanical ventilation.

It is not uncommon for patients on a mechanical ventilator to be given a muscle relaxant or paralytic to aid in ventilation. These "neuromuscular blockades" prevent skeletal muscle from contracting and thereby stop all patient movement including respiratory efforts. These types of pharmaceutical agents must always be given in conjunction with sedation as the effects of the paralytics is not only uncomfortable but would cause significant psychological stress and anxiety.

Prophylaxis
To protect against ventilator-associated pneumonia, patients' beds are often elevated to about 30°.
Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults.
A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding, which has been associated with mechanical ventilation

 Modification of settings
In adults when 100% FiO2 is used initially, it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction. The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation. In normal physiology, gas exchange (oxygen/carbon dioxide) occurs at the level of the alveoli in the lungs. The existence of a shunt refers to any process that hinders this gas exchange, leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood).

When using 100% FiO2, the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg. For each difference of 100 mmHg, the shunt is 5%. A shunt of more than 25% should prompt a search for the cause of this hypoxemia, such as mainstem intubation or pneumothorax, and should be treated accordingly. If such complications are not present, other causes must be sought after, and PEEP should be used to treat this intrapulmonary shunt. Other such causes of a shunt include:

Alveolar collapse from major atelectasis
Alveolar collection of material other than gas, such as pus from pneumonia, water and protein from acute respiratory distress syndrome, water from congestive heart failure, or blood from haemorrhage

 When to withdraw mechanical ventilation
Withdrawal from mechanical ventilation—also known as weaning—should not be delayed unnecessarily, nor should it be done prematurely. Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation, and this should be assessed continuously. There are several objective parameters to look for when considering withdrawal, but there is no specific criteria that generalizes to all patients.

Trials of spontaneous breathing have been shown to accurately predict the success of spontaneous breathing. (Yang K, Tobin MJ. A prospective study of indexes predicting the outcome of weaning from mechanical ventilation. N Engl J Med 1991;324:1445–1450).

Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse, air leakage, and aspiration:

Face mask - In resuscitation and for minor procedures under anaesthesia, a face mask is often sufficient to achieve a seal against air leakage. Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway. These are designed to provide a passage of air to the pharynx through the nose or mouth, respectively. Poorly fitted masks often cause nasal bridge ulcers, a problem for some patients. Face masks are also used for non-invasive ventilation in conscious patients. A full face mask does not, however, provide protection against aspiration.
Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube. However, unlike tracheal tubes it does not seal against aspiration, making careful individualised evaluation and patient selection mandatory.
Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration. A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea. In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration. Intubation with a cuffed tube is thought to provide the best protection against aspiration. Tracheal tubes inevitably cause pain and coughing. Therefore, unless a patient is unconscious or anaesthetized for other reasons, sedative drugs are usually given to provide tolerance of the tube. Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis.
Esophageal obturator airway - sometimes used by emergency medical technicians and basic EMS providers not trained to intubate. It is a tube which is inserted into the esophagus, past the epiglottis. Once it is inserted, a bladder at the tip of the airway is inflated, to block ("obturate") the esophagus, and oxygen is delivered through a series of holes in the side of the tube which is then forced into the lungs.
Cricothyrotomy - Patients who require emergency airway management, in whom tracheal intubation has been unsuccessful, may require an airway inserted through a surgical opening in the cricothyroid membrane. This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access.[1]
Tracheostomy - When patients require mechanical ventilation for several weeks, a tracheostomy may provide the most suitable access to the trachea. A tracheostomy is a surgically created passage into the trachea. Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs. Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease, or in any patient who is expected to be difficult to wean from mechanical ventilation, i.e., patients who have little muscular reserve.
Mouthpiece - Less common interface, does not provide protection against aspiration. There are lipseal mouthpieces with flanges to help hold them in place if patient is unable.

MRI

What is Magnetic resonance imaging?
Magnetic resonance imaging (MRI), or nuclear magnetic resonance imaging (NMRI), is primarily a medical imaging technique most commonly used in radiology to visualize detailed internal structure and limited function of the body. MRI provides much greater contrast between the different soft tissues of the body than computed tomography (CT) does, making it especially useful in neurological (brain), musculoskeletal,cardiovascular, and oncological (cancer) imaging. Unlike CT, it uses no ionizing radiation, but uses a powerful magnetic field to align the nuclear magnetization of (usually) hydrogen atoms in water in the body. Radio frequency (RF) fields are used to systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. This signal can be manipulated by additional magnetic fields to build up enough information to construct an image of the body.

Magnetic resonance imaging is a relatively new technology. The first MR image was published in 1973 and the first cross-sectional image of a living mouse was published in January 1974.[4] The first studies performed on humans were published in 1977. By comparison, the first human X-ray image was taken in 1895.

Magnetic resonance imaging was developed from knowledge gained in the study of nuclear magnetic resonance. In its early years the technique was referred to as nuclear magnetic resonance imaging (NMRI). However, because the word nuclear was associated in the public mind with ionizing radiation exposure it is generally now referred to simply as MRI. Scientists still use the term NMRI when discussing non-medical devices operating on the same principles. The term magnetic resonance tomography (MRT) is also sometimes used.

Contents
1 How MRI works
2 Applications
2.1 Basic MRI scans
2.1.1 T1-weighted MRI
2.1.2 T2-weighted MRI
2.1.3 T*2-weighted MRI
2.1.4 Spin density weighted MRI
2.2 Specialized MRI scans
2.2.1 Diffusion MRI
2.2.2 Magnetization Transfer MRI
2.2.3 Fluid attenuated inversion recovery (FLAIR)
2.2.4 Magnetic resonance angiography
2.2.5 Magnetic resonance gated intracranial CSF dynamics (MR-GILD)
2.2.6 Magnetic resonance spectroscopy
2.2.7 Functional MRI
2.3 Interventional MRI
2.4 Radiation therapy simulation
2.4.1 Current density imaging
2.4.2 Magnetic resonance guided focused ultrasound
2.4.3 Multinuclear imaging
2.4.4 Susceptibility weighted imaging (SWI)
2.4.5 Other specialized MRI techniques
2.5 Portable instruments
2.6 MRI versus CT
2.7 Economics of MRI
3 Installation of the MRI unit
4 Safety
4.1 Implants and foreign bodies
4.2 Projectile or missile effect
4.3 Radio frequency energy
4.4 Peripheral nerve stimulation (PNS)
4.5 Acoustic noise
4.6 Cryogens
4.7 Contrast agents
4.8 Pregnancy
4.9 Claustrophobia and discomfort
4.10 Guidance
4.11 The European Physical Agents Directive
5 Three-dimensional (3D) image reconstruction
5.1 The principle
5.2 3D rendering techniques
5.3 Image segmentation
6 2003 Nobel Prize
7 See also
8 Footnotes
9 References
10 External links

How MRI works
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Main article: Physics of Magnetic Resonance Imaging
The body is largely composed of water molecules which each contain two hydrogen nuclei or protons. When a person goes inside the powerful magnetic field of the scanner, the magnetic moments of some of these protons align with the direction of the field.

A radio frequency transmitter is then briefly turned on, producing an electromagnetic field. In simple terms, the photons of this field have just the right energy, known as the resonance frequency, to flip the spin of the aligned protons. As the intensity and duration of the field increases, more aligned spins are affected. After the field is turned off, the protons decay to the original spin-down state and the difference in energy between the two states is released as a photon. It is these photons that produce the signal which can be detected by the scanner. The frequency at which the protons resonate depends on the strength of the magnetic field. As a result of conservation of energy, this also dictates the frequency of the released photons.

It is this relationship between field-strength and frequency that allows the use of nuclear magnetic resonance for imaging. Additional magnetic fields are applied during the scan in order to make the magnetic field strength depend on the position within the patient, providing a straightforward method to control where the protons are excited by the radio photons. These fields are created by passing electric currents through solenoids, known as gradient coils. Since these coils are within the bore of the scanner, there will be large forces between them and the main field coils, producing most of the noise that is heard during operation. Without efforts to dampen this noise, it can approach 130 decibels (the human pain threshold) with strong fields .

An image can be constructed because the protons in different tissues return to their equilibrium state at different rates. By changing the parameters on the scanner this effect is used to create contrast between different types of body tissue or between other properties, as in fMRI and diffusion MRI.

Contrast agents may be injected intravenously to enhance the appearance of blood vessels, tumors or inflammation. Contrast agents may also be directly injected into a joint in the case of arthrograms, MRI images of joints. Unlike CT, MRI uses no ionizing radiation and is generally a very safe procedure. Nonetheless the strong magnetic fields and radio pulses can affect metal implants, including cochlear implants and cardiac pacemakers. In the case of cardiac pacemakers, the results can sometimes be lethal[8], so patients with such implants are generally not eligible for MRI.

MRI is used to image every part of the body, and is particularly useful for tissues with many hydrogen nuclei and little density contrast, such as the brain, muscle, connective tissue and most tumors.

Applications
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In clinical practice, MRI is used to distinguish pathologic tissue (such as a brain tumor) from normal tissue. One advantage of an MRI scan is that it is believed to be harmless to the patient. It uses strong magnetic fields and non-ionizing radiation in the radio frequency range, unlike CT scans and traditional X-rays which both use of ionizing radiation.

While CT provides good spatial resolution (the ability to distinguish two structures an arbitrarily small distance from each other as separate), MRI provides comparable resolution with far better contrast resolution (the ability to distinguish the differences between two arbitrarily similar but not identical tissues). The basis of this ability is the complex library of pulse sequences that the modern medical MRI scanner includes, each of which is optimized to provide image contrast based on the chemical sensitivity of MRI.

For example, with particular values of the echo time (TE) and the repetition time (TR), which are basic parameters of image acquisition, a sequence will take on the property of T2-weighting. On a T2-weighted scan, water- and fluid-containing tissues are bright (most modern T2 sequences are actually fast T2 sequences) and fat-containing tissues are dark. The reverse is true for T1-weighted images. Damaged tissue tends to develop edema, which makes a T2-weighted sequence sensitive for pathology, and generally able to distinguish pathologic tissue from normal tissue. With the addition of an additional radio frequency pulse and additional manipulation of the magnetic gradients, a T2-weighted sequence can be converted to a FLAIR sequence, in which free water is now dark, but edematous tissues remain bright. This sequence in particular is currently the most sensitive way to evaluate the brain for demyelinating diseases, such as multiple sclerosis.

The typical MRI examination consists of 5–20 sequences, each of which are chosen to provide a particular type of information about the subject tissues. This information is then synthesized by the interpreting physician.




Basic MRI scans
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T1-weighted MRI
Main article: Spin-lattice relaxation time
T1-weighted scans use a gradient echo (GRE) sequence, with short TE and short TR. This is one of the basic types of MR contrast and is a commonly run clinical scan. The T1 weighting can be increased (improving contrast) with the use of an inversion pulse as in an MP-RAGE sequence. Due to the short repetition time (TR) this scan can be run very fast allowing the collection of high resolution 3D datasets. A T1 reducing gadolinium contrast agent is also commonly used, with a T1 scan being collected before and after administration of contrast agent to compare the difference. In the brain T1-weighted scans provide good gray matter/white matter contrast.

T2-weighted MRI
Main article: Spin-spin relaxation time
T2-weighted scans use a spin echo (SE) sequence, with long TE and long TR. They have long been the clinical workhorse as the spin echo sequence is less susceptible to inhomogeneities in the magnetic field. They are particularly well suited to edema as they are sensitive to water content (edema is characterized by increased water content).

T*2-weighted MRI
T*2 (pronounced "T 2 star") weighted scans use a gradient echo (GRE) sequence, with long TE and long TR. The gradient echo sequence used does not have the extra refocusing pulse used in spin echo so it is subject to additional losses above the normal T2 decay (referred to as T2′), these taken together are called T*2. This also makes it more prone to susceptibility losses at air/tissue boundaries, but can increase contrast for certain types of tissue, such as venous blood.

Spin density weighted MRI
Spin density, also called proton density, weighted scans try to have no contrast from either T2 or T1 decay, the only signal change coming from differences in the amount of available spins (hydrogen nuclei in water). It uses a spin echo or sometimes a gradient echo sequence, with short TE and long TR.

Specialized MRI scans
Diffusion MRI
Main article: Diffusion MRI
DTI imageDiffusion MRI measures the diffusion of water molecules in biological tissues.[9] In an isotropic medium (inside a glass of water for example) water molecules naturally move randomly according to turbulence and Brownian motion. In biological tissues however, where the Reynold's number is low enough for flows to be laminar, the diffusion may be anisotropic. For example a molecule inside the axon of a neuron has a low probability of crossing the myelin membrane. Therefore the molecule will move principally along the axis of the neural fiber. If we know that molecules in a particular voxel diffuse principally in one direction we can make the assumption that the majority of the fibers in this area are going parallel to that direction.

The recent development of diffusion tensor imaging (DTI)[3] enables diffusion to be measured in multiple directions and the fractional anisotropy in each direction to be calculated for each voxel. This enables researchers to make brain maps of fiber directions to examine the connectivity of different regions in the brain (using tractography) or to examine areas of neural degeneration and demyelination in diseases like Multiple Sclerosis.

Another application of diffusion MRI is diffusion-weighted imaging (DWI). Following an ischemic stroke, DWI is highly sensitive to the changes occurring in the lesion.[10] It is speculated that increases in restriction (barriers) to water diffusion, as a result of cytotoxic edema (cellular swelling), is responsible for the increase in signal on a DWI scan. The DWI enhancement appears within 5–10 minutes of the onset of stroke symptoms (as compared with computed tomography, which often does not detect changes of acute infarct for up to 4–6 hours) and remains for up to two weeks. Coupled with imaging of cerebral perfusion, researchers can highlight regions of "perfusion/diffusion mismatch" that may indicate regions capable of salvage by reperfusion therapy.

Like many other specialized applications, this technique is usually coupled with a fast image acquisition sequence, such as echo planar imaging sequence.

Magnetization Transfer MRI
Main article: Magnetization transfer
Magnetization transfer (MT) refers to the transfer of longitudinal magnetization from free water protons to hydration water protons in NMR and MRI.

In magnetic resonance imaging of molecular solutions, such as protein solutions, two types of water molecules, free (bulk) and hydration, are found. Free water protons have faster average rotational frequency and hence less fixed water molecules that may cause local field inhomogeneity. Because of this uniformity, most free water protons have resonance frequency lying narrowly around the normal proton resonance frequency of 63 MHz (at 1.5 teslas). This also results in slower transverse magnetization dephasing and hence longer T2. Conversely, hydration water molecules are slowed down by interaction with solute molecules and hence create field inhomogeneities that lead to wider resonance frequency spectrum.




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Fluid attenuated inversion recovery (FLAIR)
Main article: Fluid attenuated inversion recovery
Fluid Attenuated Inversion Recovery (FLAIR)[11] is an inversion-recovery pulse sequence used to null signal from fluids. For example, it can be used in brain imaging to suppress cerebrospinal fluid (CSF) so as to bring out the periventricular hyperintense lesions, such as multiple sclerosis (MS) plaques. By carefully choosing the inversion time TI (the time between the inversion and excitation pulses), the signal from any particular tissue can be suppressed.

Magnetic resonance angiography
Magnetic Resonance AngiographyMain article: Magnetic resonance angiography
Magnetic resonance angiography (MRA) is used to generate pictures of the arteries in order to evaluate them for stenosis (abnormal narrowing) or aneurysms (vessel wall dilatations, at risk of rupture). MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs (called a "run-off"). A variety of techniques can be used to generate the pictures, such as administration of a paramagnetic contrast agent (gadolinium) or using a technique known as "flow-related enhancement" (e.g. 2D and 3D time-of-flight sequences), where most of the signal on an image is due to blood which has recently moved into that plane, see also FLASH MRI. Techniques involving phase accumulation (known as phase contrast angiography) can also be used to generate flow velocity maps easily and accurately. Magnetic resonance venography (MRV) is a similar procedure that is used to image veins. In this method the tissue is now excited inferiorly while signal is gathered in the plane immediately superior to the excitation plane, and thus imaging the venous blood which has recently moved from the excited plane.

Magnetic resonance gated intracranial CSF dynamics (MR-GILD)
Magnetic resonance gated intracranial cerebrospinal fluid (CSF)or liquor dynamics (MR-GILD) technique is an MR sequence based on bipolar gradient pulse used to demonstrate CSF pulsatile flow in ventricles, cisterns, aqueduct of Sylvius and entire intracranial CSF pathway. It is a method for analyzing CSF circulatory system dynamics in patients with CSF obstructive lesions such as normal pressure hydrocephalus. It also allows visualization of both arterial and venous pulsatile blood flow in vessels without use of contrast agents.

Diastolic time data acquisition (DTDA). Systolic time data acquisition (STDA).
Magnetic resonance spectroscopy
 In vivo magnetic resonance spectroscopy
Main article: Nuclear magnetic resonance spectroscopy
Magnetic resonance spectroscopy (MRS) is used to measure the levels of different metabolites in body tissues. The MR signal produces a spectrum of resonances that correspond to different molecular arrangements of the isotope being "excited". This signature is used to diagnose certain metabolic disorders, especially those affecting the brain,[15] and to provide information on tumor metabolism.
Magnetic resonance spectroscopic imaging (MRSI) combines both spectroscopic and imaging methods to produce spatially localized spectra from within the sample or patient. The spatial resolution is much lower (limited by the available SNR), but the spectra in each voxel contains information about many metabolites. Because the available signal is used to encode spatial and spectral information, MRSI requires high SNR achievable only at higher field strengths (3 T and above).

Functional MRI
 Functional magnetic resonance imaging
A fMRI scan showing regions of activation in orange, including the primary visual cortex (V1, BA17).Functional MRI (fMRI) measures signal changes in the brain that are due to changing neural activity. The brain is scanned at low resolution but at a rapid rate (typically once every 2–3 seconds). Increases in neural activity cause changes in the MR signal via T*2 changes;[17] this mechanism is referred to as the BOLD (blood-oxygen-level dependent) effect. Increased neural activity causes an increased demand for oxygen, and the vascular system actually overcompensates for this, increasing the amount of oxygenated hemoglobin relative to deoxygenated hemoglobin. Because deoxygenated hemoglobin attenuates the MR signal, the vascular response leads to a signal increase that is related to the neural activity. The precise nature of the relationship between neural activity and the BOLD signal is a subject of current research. The BOLD effect also allows for the generation of high resolution 3D maps of the venous vasculature within neural tissue.

While BOLD signal is the most common method employed for neuroscience studies in human subjects, the flexible nature of MR imaging provides means to sensitize the signal to other aspects of the blood supply. Alternative techniques employ arterial spin labeling (ASL) or weight the MRI signal by cerebral blood flow (CBF) and cerebral blood volume (CBV). The CBV method requires injection of a class of MRI contrast agents that are now in human clinical trials. Because this method has been shown to be far more sensitive than the BOLD technique in preclinical studies, it may potentially expand the role of fMRI in clinical applications. The CBF method provides more quantitative information than the BOLD signal, albeit at a significant loss of detection sensitivity.

Interventional MRI
The lack of harmful effects on the patient and the operator make MRI well-suited for "interventional radiology", where the images produced by a MRI scanner are used to guide minimally-invasive procedures. Of course, such procedures must be done without any ferromagnetic instruments.
A specialized growing subset of interventional MRI is that of intraoperative MRI in which the MRI is used in the surgical process. Some specialized MRI systems have been developed that allow imaging concurrent with the surgical procedure. More typical, however, is that the surgical procedure is temporarily interrupted so that MR images can be acquired to verify the success of the procedure or guide subsequent surgical work.

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Radiation therapy simulation
Because of MRI's superior imaging of soft tissues, it is now being utilized to specifically locate tumors within the body in preparation for radiation therapy treatments. For therapy simulation, a patient is placed in specific, reproducible, body position and scanned. The MRI system then computes the precise location, shape and orientation of the tumor mass, correcting for any spatial distortion inherent in the system. The patient is then marked or tattooed with points which, when combined with the specific body position, will permit precise triangulation for radiation therapy.
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Current density imaging
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Current density imaging (CDI) endeavors to use the phase information from images to reconstruct current densities within a subject. Current density imaging works because electrical currents generate magnetic fields, which in turn affect the phase of the magnetic dipoles during an imaging sequence. To date no successful CDI has been performed using biological currents, but several studies have been published which involve currents applied through a pair of electrodes.

Magnetic resonance guided focused ultrasound
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In MRgFUS therapy, ultrasound beams are focused on a tissue—guided and controlled using MR thermal imaging—and due to the significant energy deposition at the focus, temperature within the tissue rises to more than 65 °C (150 °F), completely destroying it. This technology can achieve precise "ablation" of diseased tissue. MR imaging provides a three-dimensional view of the target tissue, allowing for precise focusing of ultrasound energy. The MR imaging provides quantitative, real-time, thermal images of the treated area. This allows the physician to ensure that the temperature generated during each cycle of ultrasound energy is sufficient to cause thermal ablation within the desired tissue and if not, to adapt the parameters to ensure effective treatment.

Multinuclear imaging
Hydrogen is the most frequently imaged nucleus in MRI because it is present in biological tissues in great abundance. However, any nucleus which has a net nuclear spin could potentially be imaged with MRI. Such nuclei include helium-3, carbon-13, fluorine-19, oxygen-17, sodium-23, phosphorus-31 and xenon-129. 23Na, 31P and 17O are naturally abundant in the body, so can be imaged directly. Gaseous isotopes such as 3He or 129Xe must be hyperpolarized and then inhaled as their nuclear density is too low to yield a useful signal under normal conditions. 17O, 13C and 19F can be administered in sufficient quantities in liquid form (e.g. 17O-water, 13C-glucose solutions or perfluorocarbons) that hyperpolarization is not a necessity.

Multinuclear imaging is primarily a research technique at present. However, potential applications include functional imaging and imaging of organs poorly seen on 1H MRI (e.g. lungs and bones) or as alternative contrast agents. Inhaled hyperpolarized 3He can be used to image the distribution of air spaces within the lungs. Injectable solutions containing 13C or stabilized bubbles of hyperpolarized 129Xe have been studied as contrast agents for angiography and perfusion imaging. 31P can potentially provide information on bone density and structure, as well as functional imaging of the brain.

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Susceptibility weighted imaging (SWI)
Main article: Susceptibility weighted imaging
Susceptibility weighted imaging (SWI), is a new type of contrast in MRI different from spin density, T1, or T2 imaging. This method exploits the susceptibility differences between tissues and uses a fully velocity compensated, three dimensional, RF spoiled, high-resolution, 3D gradient echo scan. This special data acquisition and image processing produces an enhanced contrast magnitude image very sensitive to venous blood, hemorrhage and iron storage. It is used to enhance the detection and diagnosis of tumors, vascular and neurovascular diseases (stroke and hemorrhage, multiple sclerosis, Alzheimer's), and also detects traumatic brain injuries that may not be diagnosed using other methods.[18][19]

Other specialized MRI techniques
MRI is a new and active field of research and new methods and variants are often published when they are able to get better results in specific fields. Examples of these recent improvements are T*2-weighted turbo spin-echo (T2 TSE MRI), double inversion recovery MRI (DIR-MRI) or phase-sensitive inversion recovery MRI (PSIR-MRI), all of them able to improve imaging of the brain lesions[20][21]. Another example is MP-RAGE (magnetization-prepared rapid acquisition with gradient echo)[22], which improves images of multiple sclerosis cortical lesions.

Portable instruments
Portable magnetic resonance instruments are available for use in education and field research. Using the principles of Earth's field NMR, they have no powerful polarizing magnet, so that such instruments can be small and inexpensive. Some can be used for both EFNMR spectroscopy and MRI imaging[24]. The low strength of the Earth's field results in poor signal to noise ratios, requiring long scan times to capture spectroscopic data or build up MRI images.

Research with atomic magnetometers have discussed the possibility for cheap and portable MRI instruments without the large magnet.

MRI versus CT
A computed tomography (CT) scanner uses X-rays, a type of ionizing radiation, to acquire its images, making it a good tool for examining tissue composed of elements of a higher atomic number than the tissue surrounding them, such as bone and calcifications (calcium based) within the body (carbon based flesh), or of structures (vessels, bowel). MRI, on the other hand, uses non-ionizing radio frequency (RF) signals to acquire its images and is best suited for non-calcified tissue, though MR images can also be acquired from bones and teeth as well as fossils.

CT may be enhanced by use of contrast agents containing elements of a higher atomic number than the surrounding flesh such as iodine or barium. Contrast agents for MRI are those which have paramagnetic properties, e.g. gadolinium and manganese.

Both CT and MRI scanners can generate multiple two-dimensional cross-sections (slices) of tissue and three-dimensional reconstructions. Unlike CT, which uses only X-ray attenuation to generate image contrast, MRI has a long list of properties that may be used to generate image contrast. By variation of scanning parameters, tissue contrast can be altered and enhanced in various ways to detect different features. (See Applications above.)

MRI can generate cross-sectional images in any plane (including oblique planes). In the past, CT was limited to acquiring images in the axial (or near axial) plane. The scans used to be called Computed Axial Tomography scans (CAT scans). However, the development of multi-detector CT scanners with near-isotropic resolution, allows the CT scanner to produce data that can be retrospectively reconstructed in any plane with minimal loss of image quality.

For purposes of tumor detection and identification in the brain, MRI is generally superior.[29][30][31] However, in the case of solid tumors of the abdomen and chest, CT is often preferred due to less motion artifact. Furthermore, CT usually is more widely available, faster, less expensive, and may be less likely to require the person to be sedated or anesthetized.

MRI is also best suited for cases when a patient is to undergo the exam several times successively in the short term, because, unlike CT, it does not expose the patient to the hazards of ionizing radiation.

Economics of MRI
The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page.
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MRI equipment is expensive. 1.5 tesla scanners often cost between $1 million and $1.5 million USD. 3.0 tesla scanners often cost between $2 million and $2.3 million USD. Construction of MRI suites can cost up to $500,000 USD, or more, depending on project scope.

MRI scanners have been significant sources of revenue for healthcare providers in the US. This is because of favorable reimbursement rates from insurers and federal government programs. Insurance reimbursement is provided in two components, an equipment charge for the actual performance of the MRI scan and professional charge for the radiologist's review of the images and/or data. In the US Northeast, an equipment charge might be $3,500 and a professional charge might be $350 [32] although the actual fees received by the equipment owner and interpreting physician are often quite less and depend on the rates negotiated with insurance companies or determined by governmental action as in the Medicare Fee Schedule. For example, an orthopedic surgery group in Illinois billed a charge of $1,116 for a knee MRI in 2007 but the Medicare reimbursement in 2007 was only $470.91 [33]. Many insurance companies require preapproval of an MRI procedure as a condition for coverage.

In the US, the 2007 Deficit Reduction Act (DRA) significantly reduced reimbursement rates paid by federal insurance programs for the equipment component of many scans, shifting the economic landscape. Many private insurers have followed suit.[citation needed]

Installation of the MRI unit
Heavy lifting equipment is used to install the MRI unit.An MRI unit is a rather large item, typically requiring heavy equipment (such as cranes) to move the unit to its final location. Once the MRI unit is in place, the room that houses it is usually "built up" around the unit itself. See this page for an example of the complexity involved in installing an MRI unit in a clinical setting.

Safety
Death and injuries have occurred from projectiles created by the magnetic field, although few compared to the millions of examinations administered.[35][36] MRI makes use of powerful magnetic fields which, though they have not been demonstrated to cause direct biological damage, can interfere with metallic and electromechanical devices. Additional (small) risks are presented by the radio frequency systems, components or elements of the MRI system's operation, elements of the scanning procedure and medications that may be administered to facilitate MRI imaging.

Of great concern is the dramatic increase in the number of reported MRI accidents to the U.S. Food and Drug Administration (FDA). Since 2004, the last year in which a decline in the number of MRI accidents was reported, the full spectrum of MRI accidents has increased significantly in the following years. The 2008 FDA accident report data culminates in a 277% increase over the 2004 rate.

There are many steps that the MRI patient and referring physician can take to help reduce the remaining risks, including providing a full, accurate and thorough medical history to the MRI provider.

Several of the specific MRI safety considerations are identified below:

Implants and foreign bodies
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Pacemakers are generally considered an absolute contraindication towards MRI scanning, though highly specialized protocols have been developed to permit scanning of select pacing devices. Several cases of arrhythmia or death have been reported in patients with pacemakers who have undergone MRI scanning without appropriate precautions. Other electronic implants have varying contraindications, depending upon scanner technology, and implant properties, scanning protocols and anatomy being imaged.

Many other forms of medical or biostimulation implants may be contraindicated for MRI scans. These may include vagus nerve stimulators, implantable cardioverter-defibrillators, loop recorders, insulin pumps, cochlear implants, deep brain stimulators, and many others. Medical device patients should always present complete information (manufacturer, model, serial number and date of implantation) about all implants to both the referring physician and to the radiologist or technologist before entering the room for the MRI scan.

While these implants pose a current problem, scientists and manufacturers are working on improved designs which will further minimize the risks that MRI scans pose to medical device operations. One such development in the works is a nano-coating for implants intended to screen them from the radio frequency waves, helping to make MRI exams available to patients currently prohibited from receiving them. The current article for this is from New Scientist.

Ferromagnetic foreign bodies (e.g. shell fragments), or metallic implants (e.g. surgical prostheses, aneurysm clips) are also potential risks, and safety aspects need to be considered on an individual basis. Interaction of the magnetic and radio frequency fields with such objects can lead to trauma due to movement of the object in the magnetic field, thermal injury from radio-frequency induction heating of the object, or failure of an implanted device. These issues are especially problematic when dealing with the eye. Most MRI centers require an orbital x-ray to be performed on anyone suspected of having metal fragments in their eyes, something not uncommon in metalworking.

Because of its non-ferromagnetic nature and poor electrical conductivity, titanium and its alloys are useful for long term implants and surgical instruments intended for use in image-guided surgery. In particular, not only is titanium safe from movement from the magnetic field, but artifacts around the implant are less frequent and less severe than with more ferromagnetic materials e.g. stainless steel. Artifacts from metal frequently appear as regions of empty space around the implant—frequently called 'black-hole artifact'. E.g. a 3 mm titanium alloy coronary stent may appear as a 5 mm diameter region of empty space on MRI, whereas around a stainless steel stent, the artifact may extend for 10–20 mm or more.

In 2006, a new classification system for implants and ancillary clinical devices has been developed by ASTM International and is now the standard supported by the US Food and Drug Administration:


MR Safe signMR-Safe — The device or implant is completely non-magnetic, non-electrically conductive, and non-RF reactive, eliminating all of the primary potential threats during an MRI procedure.

MR Conditional signMR-Conditional — A device or implant that may contain magnetic, electrically conductive or RF-reactive components that is safe for operations in proximity to the MRI, provided the conditions for safe operation are defined and observed (such as 'tested safe to 1.5 teslas' or 'safe in magnetic fields below 500 gauss in strength').

MR Unsafe signMR-Unsafe — Nearly self-explanatory, this category is reserved for objects that are significantly ferromagnetic and pose a clear and direct threat to persons and equipment within the magnet room.
Though the current classification system was originally developed for regulatory-approved medical devices, it is being applied to all manner of items, appliances and equipment intended for use in the MR environment.

In the case of pacemakers, the risk is thought to be primarily RF induction in the pacing electrodes/wires causing inappropriate pacing of the heart, rather than the magnetic field affecting the pacemaker itself. Much research and development is being undertaken, and many tools are being developed in order to predict the effects of the RF fields inside the body.

Patients who have been prescribed MRI exams who are concerned about safety may be interested in the 10 Questions To Ask Your MRI Provider.

MRI providers who wish to measure the degree to which they have effectively addressed the safety issues for patients and staff may be interested in the MRI Suite Safety Calculator provided through a radiology website.

Projectile or missile effect
As a result of the very high strength of the magnetic field needed to produce scans (frequently up to 60,000 times the Earth's own magnetic field effects), there are several incidental safety issues addressed in MRI facilities. Missile-effect accidents, where ferromagnetic objects are attracted to the center of the magnet, have resulted in injury and death. A video simulation of a fatal projectile effect accident illustrates the extreme power that contemporary MRI equipment can exert on ferromagnetic objects.

In order to help reduce the risks of projectile accidents, ferromagnetic objects and devices are typically prohibited in proximity to the MRI scanner, with non-ferromagnetic versions of many tools and devices typically retained by the scanning facility. Patients undergoing MRI examinations are required to remove all metallic objects, often by changing into a gown or scrubs.

New ferromagnetic-only detection devices are proving highly effective in supplementing conventional screening techniques in many leading hospitals and imaging centers and are now recommended by the American College of Radiology's Guidance Document for Safe MR Practices: 2007, the United States' Veterans Administration's MRI Design Guide and the Joint Commission's Sentinel Event Alert #38.

The magnetic field and the associated risk of missile-effect accidents remains a permanent hazard — as superconductive MRI magnets retain their magnetic field, even in the event of a power outage.

Radio frequency energy
A powerful radio transmitter is needed for excitation of proton spins. This can heat the body to the point of risk of hyperthermia in patients, particularly in obese patients or those with thermoregulation disorders. Several countries have issued restrictions on the maximum specific absorption rate that a scanner may produce.

Peripheral nerve stimulation (PNS)
The rapid switching on and off of the magnetic field gradients is capable of causing nerve stimulation. Volunteers report a twitching sensation when exposed to rapidly switched fields, particularly in their extremities. The reason the peripheral nerves are stimulated is that the changing field increases with distance from the center of the gradient coils (which more or less coincides with the center of the magnet).[citation needed] Note however that when imaging the head, the heart is far off-center and induction of even a tiny current into the heart must be avoided at all costs.[citation needed] Although PNS was not a problem for the slow, weak gradients used in the early days of MRI, the strong, rapidly switched gradients used in techniques such as EPI, fMRI, diffusion MRI, etc. are indeed capable of inducing PNS. American and European regulatory agencies insist that manufacturers stay below specified dB/dt limits (dB/dt is the change in field per unit time) or else prove that no PNS is induced for any imaging sequence. As a result of dB/dt limitation, commercial MRI systems cannot use the full rated power of their gradient amplifiers.

Acoustic noise
Switching of field gradients causes a change in the Lorentz force experienced by the gradient coils, producing minute expansions and contractions of the coil itself. As the switching is typically in the audible frequency range, the resulting vibration produces loud noises (clicking or beeping). This is most marked with high-field machines and rapid-imaging techniques in which sound intensity can reach 120 dB(A) (equivalent to a jet engine at take-off) .

Appropriate use of ear protection is essential for anyone inside the MRI scanner room during the examination.

Cryogens
As described above in #Scanner construction and operation, many MRI scanners rely on cryogenic liquids to enable superconducting capabilities of the electromagnetic coils within. Though the cryogenic liquids most frequently used are non-toxic, their physical properties present specific hazards.

An emergency shut-down of a superconducting electromagnet, an operation known as "quenching", involves the rapid boiling of liquid helium from the device. If the rapidly expanding helium cannot be dissipated through an external vent, sometimes referred to as 'quench pipe', it may be released into the scanner room where it may cause displacement of the oxygen and present a risk of asphyxiation.[39]

Liquid helium, the most commonly used cryogen in MRI, undergoes near explosive expansion as it changes from liquid to a gaseous state. Rooms built in support of superconducting MRI equipment should be equipped with pressure relief mechanisms[40] and an exhaust fan, in addition to the required quench pipe.

Since a quench results in rapid loss of all cryogens in the magnet, recommissioning the magnet is extremely expensive and time-consuming. Spontaneous quenches are uncommon, but may also be triggered by equipment malfunction, improper cryogen fill technique, contaminants inside the cryostat, or extreme magnetic or vibrational disturbances.

Contrast agents
Main article: MRI contrast agent
The most commonly used intravenous contrast agents are based on chelates of gadolinium. In general, these agents have proved safer than the iodinated contrast agents used in X-ray radiography or CT. Anaphylactoid reactions are rare, occurring in approx. 0.03–0.1%.[41] Of particular interest is the lower incidence of nephrotoxicity, compared with iodinated agents, when given at usual doses—this has made contrast-enhanced MRI scanning an option for patients with renal impairment, who would otherwise not be able to undergo contrast-enhanced CT.

Although gadolinium agents have proved useful for patients with renal impairment, in patients with severe renal failure requiring dialysis there is a risk of a rare but serious illness, nephrogenic systemic fibrosis, that may be linked to the use of certain gadolinium-containing agents. The most frequently linked is gadodiamide, but other agents have been linked too.[43] Although a causal link has not been definitively established, current guidelines in the United States are that dialysis patients should only receive gadolinium agents where essential, and that dialysis should be performed as soon as possible after the scan is complete, in order to remove the agent from the body promptly.[44] In Europe, where more gadolinium-containing agents are available, a classification of agents according to potential risks has been released.[45][46] Recently a new contrast agent named gadoxetate, brand name Eovist (US) or Primovist (EU), was approved for diagnostic use: this has the theoretical benefit of a dual excretion path.

Pregnancy
No effects of MRI on the fetus have been demonstrated.[48] In particular, MRI avoids the use of ionizing radiation, to which the fetus is particularly sensitive. However, as a precaution, current guidelines recommend that pregnant women undergo MRI only when essential. This is particularly the case during the first trimester of pregnancy, as organogenesis takes place during this period. The concerns in pregnancy are the same as for MRI in general, but the fetus may be more sensitive to the effects—particularly to heating and to noise. However, one additional concern is the use of contrast agents; gadolinium compounds are known to cross the placenta and enter the fetal bloodstream, and it is recommended that their use be avoided.

Despite these concerns, MRI is rapidly growing in importance as a way of diagnosing and monitoring congenital defects of the fetus because it can provide more diagnostic information than ultrasound and it lacks the ionizing radiation of CT. MRI without contrast agents is the imaging mode of choice for pre-surgical, in-utero diagnosis and evaluation of fetal tumors, primarily teratomas, facilitating open fetal surgery, other fetal interventions, and planning for procedures (such as the EXIT procedure) to safely deliver and treat babies whose defects would otherwise be fatal.

Claustrophobia and discomfort
Due to the construction of some MRI scanners, they can be potentially unpleasant to lie in. Older models of closed bore MRI systems feature a fairly long tube or tunnel. The part of the body being imaged needs to lie at the center of the magnet which is at the absolute center of the tunnel. Because scan times on these older scanners may be long (occasionally up to 40 minutes for the entire procedure), people with even mild claustrophobia are sometimes unable to tolerate an MRI scan without management. Modern scanners may have larger bores (up to 70 cm) and scan times are shorter. This means that claustrophobia is less of an issue, and many patients now find MRI an innocuous and easily tolerated procedure.

Nervous patients may still find the following strategies helpful:

Advance preparation
visiting the scanner to see the room and practice lying on the table
visualization techniques
chemical sedation
general anesthesia
Coping while inside the scanner
holding a "panic button"
closing eyes as well as covering them (e.g. washcloth, eye mask)
listening to music on headphones or watching a movie with a Head-mounted display while in the machine
Scan Rooms with lighting, sound and images on the wall. Some rooms come with images on the walls or ceiling.
Alternative scanner designs, such as open or upright systems, can also be helpful where these are available. Though open scanners have increased in popularity, they produce inferior scan quality because they operate at lower magnetic fields than closed scanners. However, commercial 1.5 tesla open systems have recently become available, providing much better image quality than previous lower field strength open models[49].


For babies and young children chemical sedation or general anesthesia are the norm, as these subjects cannot be instructed to hold still during the scanning session. Obese patients and pregnant women may find the MRI machine to be a tight fit. Pregnant women may also have difficulty lying on their backs for an hour or more without moving.
Acoustic noise associated with the operation of an MRI scanner can also exacerbate the discomfort associated with the procedure.

Guidance
Safety issues, including the potential for biostimulation device interference, movement of ferromagnetic bodies, and incidental localized heating, have been addressed in the American College of Radiology's White Paper on MR Safety which was originally published in 2002 and expanded in 2004. The ACR White Paper on MR Safety has been rewritten and was released early in 2007 under the new title ACR Guidance Document for Safe MR Practices.
In December 2007, the Medicines in Healthcare product Regulation Agency (MHRA), a UK healthcare regulatory body, issued their Safety Guidelines for Magnetic Resonance Imaging Equipment in Clinical Use.
In February 2008, the Joint Commission, a US healthcare accrediting organization, issued a Sentinel Event Alert #38, their highest patient safety advisory, on MRI safety issues.
In July 2008, the United States Veterans Administration, a federal governmental agency serving the healthcare needs of former military personnel, issued a substantial revision to their MRI Design Guide which includes physical or facility safety considerations.

The European Physical Agents Directive
The European Physical Agents (Electromagnetic Fields) Directive is legislation adopted in European legislature. Originally scheduled to be required by the end of 2008, each individual state within the European Union must include this directive in its own law by the end of 2012. Some member nations passed complying legislation and are now attempting to repeal their state laws in expectation that the final version of the EU Physical Agents Directive will be substantially revised prior to the revised adoption date.

The directive applies to occupational exposure to electromagnetic fields (not medical exposure) and was intended to limit workers’ acute exposure to strong electromagnetic fields, as may be found near electricity substations, radio or television transmitters or industrial equipment. However, the regulations impact significantly on MRI, with separate sections of the regulations limiting exposure to static magnetic fields, changing magnetic fields and radio frequency energy. Field strength limits are given which may not be exceeded for any period of time. An employer may commit a criminal offense by allowing a worker to exceed an exposure limit if that is how the Directive is implemented in a particular Member State.

The Directive is based on the international consensus of established effects of exposure to electromagnetic fields, and in particular the advice of the European Commissions's advisor, the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The aims of the Directive, and the ICNIRP guidelines upon which it is based, are to prevent exposure to potentially harmful fields. The actual limits in the Directive are very similar to the limits advised by the Institute of Electrical and Electronics Engineers, with the exception of the frequencies produced by the gradient coils, where the IEEE limits are significantly higher.

Many Member States of the EU already have either specific EMF regulations or (as in the UK) a general requirement under workplace health and safety legislation to protect workers against electromagnetic fields. In almost all cases the existing regulations are aligned with the ICNIRP limits so that the Directive should, in theory, have little impact on any employer already meeting their legal responsibilities.

The introduction of the Directive has brought to light an existing potential issue with occupational exposures to MRI fields. There are at present very few data on the number or types of MRI practice that might lead to exposures in excess of the levels of the Directive.[50][51] There is a justifiable concern amongst MRI practitioners that if the Directive were to be enforced more vigorously than existing legislation, the use of MRI might be restricted, or working practices of MRI personnel might have to change.

In the initial draft a limit of static field strength to 2 T was given. This has since been removed from the regulations, and whilst it is unlikely to be restored as it was without a strong justification, some restriction on static fields may be reintroduced after the matter has been considered more fully by ICNIRP. The effect of such a limit might be to restrict the installation, operation and maintenance of MRI scanners with magnets of 2 T and stronger. As the increase in field strength has been instrumental in developing higher resolution and higher performance scanners, this would be a significant step back. This is why it is unlikely to happen without strong justification.

Individual government agencies and the European Commission have now formed a working group to examine the implications on MRI and to try to address the issue of occupational exposures to electromagnetic fields from MRI.

Three-dimensional (3D) image reconstruction
The principle
Because contemporary MRI scanners offer isotropic, or near isotropic, resolution, display of images does not need to be restricted to the conventional axial images. Instead, it is possible for a software program to build a volume by 'stacking' the individual slices one on top of the other. The program may then display the volume in an alternative manner.

3D rendering techniques
Surface rendering
A threshold value of greyscale density is chosen by the operator (e.g. a level that corresponds to fat). A threshold level is set, using edge detection image processing algorithms. From this, a 3-dimensional model can be constructed and displayed on screen. Multiple models can be constructed from various different thresholds, allowing different colors to represent each anatomical component such as bone, muscle, and cartilage. However, the interior structure of each element is not visible in this mode of operation.

Volume rendering
Surface rendering is limited in that it will only display surfaces which meet a threshold density, and will only display the surface that is closest to the imaginary viewer. In volume rendering, transparency and colors are used to allow a better representation of the volume to be shown in a single image - e.g. the bones of the pelvis could be displayed as semi-transparent, so that even at an oblique angle, one part of the image does not conceal another.

Image segmentation
Where different structures have similar threshold density, it can become impossible to separate them simply by adjusting volume rendering parameters. The solution is called segmentation, a manual or automatic procedure that can remove the unwanted structures from the image.

2003 Nobel Prize
Reflecting the fundamental importance and applicability of MRI in the medical field, Paul Lauterbur of the University of Illinois at Urbana-Champaign and Sir Peter Mansfield of the University of Nottingham were awarded the 2003 Nobel Prize in Physiology or Medicine for their "discoveries concerning magnetic resonance imaging". The Nobel Prize committee acknowledged Lauterbur's insight of using magnetic field gradients to introduce spatial localization, a discovery that allowed rapid acquisition of 2D images. Mansfield was credited with introducing the mathematical formalism and developing techniques for efficient gradient utilization and fast imaging. The actual research for which the prize was awarded was done almost 30 years ago, while Paul Lauterbur was at Stony Brook University in New York.

The award was vigorously protested by Raymond Vahan Damadian, founder of FONAR Corporation, who claimed that he was the inventor of MRI,[3] and that Lauterbur and Mansfield had merely refined the technology.[52] An ad hoc group, called "The Friends of Raymond Damadian", took out full-page advertisements in the New York Times and The Washington Post entitled "The Shameful Wrong That Must Be Righted", demanding that he be awarded at least a share of the Nobel Prize.[53] Also, even earlier, in the Soviet Union, Vladislav Ivanov filed (in 1960) a document with the USSR State Committee for Inventions and Discovery at Leningrad for a Magnetic Resonance Imaging device[54], although this was not approved until the 1970s.[55] In a letter to Physics Today, Herman Carr pointed out his own even earlier use of field gradients for one-dimensional MR imaging.[56]

See also
Earth's field NMR (EFNMR)
Electron-spin resonance (spin physics)
History of brain imaging
Medical imaging
Magnetic immunoassay
Jemris (open source MRI simulator)
Magnetic Resonance Imaging (journal)
Magnetic resonance microscopy
Magnetic Particle Imaging (MPI)
Magnetic resonance elastography
Neuroimaging software
Nephrogenic fibrosing dermopathy
Nobel Prize controversies
Wikimedia Commons has media related to: Magnetic resonance imaging


Nuclear magnetic resonance (NMR)
2D-FT NMRI and Spectroscopy
Relaxation
Robinson oscillator
Rabi cycle
Virtopsy