The static magnetic field of a magnetic resonance imaging (MRI) scanner exerts forces on ferromagnetic and other magnetic materials near the field. These forces can draw unrestrained objects, making them airborne, into the scanner’s magnet bore. This phenomenon is known as the projectile effect and can result in catastrophic consequences for individuals near the scanner and significant damage to equipment. To avoid serious or fatal injury from projectiles, magnetic resonance (MR) personnel must understand the principles of the projectile effect and properly screen individuals before entering the scanner room for ferromagnetic objects. In some cases, medical equipment needed in the scanner room for patient assessment or treatment (e.g., physiologic monitor, infusion pump) could also pose projectile risks. Additionally, the static magnetic field may cause medical equipment to malfunction, which may result in serious or fatal patient injury. MR personnel must know what equipment is safe to allow into the scanner room and what equipment is unsafe and should be left out of the room. Between June 2004 and December 2008, the Pennsylvania Patient Safety Authority received 27 reports of objects becoming projectiles in the MR environment, 16 ferromagnetic items that were brought into the MRI scanner room without becoming projectiles, and 5 ferromagnetic items almost allowed into the MRI scanner room. Proper MR screening practices for ferromagnetic items and establishing protocols for identifying and labeling equipment that can and cannot be brought into the scanner room will help reduce the risk of objects becoming projectiles within the MR environment.
A March 2009 Pennsylvania Patient Safety Advisory article discussed magnetic resonance (MR) safety screening practices. This article continues the discussion of MR safety, focusing on the safety concerns of ferromagnetic objects and ferromagnetic medical equipment entering the magnetic resonance imaging scanner room.
Ferromagnetic materials can be influenced by translational (linear) and torque (rotational) forces exerted by the static magnetic field of the magnetic resonance imaging (MRI) scanner. These forces, primarily translational forces, can draw unrestrained objects, making them airborne, into the magnet’s bore. This hazardous phenomenon is known as the projectile, or missile, effect, which can potentially result in serious or fatal injuries to individuals in the scanner room. Any object, regardless of size, can become a dangerous projectile. For example, paper clips and hairpins have been shown to travel at speeds up to 40 mph into a 1.5 T magnet.1
Some incidents of the projectile effect have been reported in the mainstream news media. For example, one fatal event involved a six-year-old child undergoing an MRI scan. During the scan, while the child was lying on the MRI table, an oxygen canister consisting of ferromagnetic materials was brought into the room. The force of the magnet pulled the oxygen canister into the magnetic bore, causing the canister to strike the child’s head. The child eventually died from the injuries.2
In another well-known event, an off-duty police officer was scheduled to undergo a MRI examination. A misunderstanding occurred between the officer and the magnetic resonance (MR) technologist when the technologist asked the officer to proceed to the MRI patient waiting area. The officer mentioned his holstered handgun to the technologist before entering the MRI dressing room. The technologist informed the officer to proceed to the patient waiting area with the gun; the technologist planned to secure the weapon in that room. However, the officer misunderstood and brought the gun into the MRI scanner room. While the technologist was entering the officer’s patient information into a computer, the officer entered the scanner room and attempted to place the gun on top of a cabinet, which was approximately 3 feet away from magnet bore. The force of the magnet pulled the gun from the officer’s hand and drew the gun to the magnet bore. When the gun struck the magnet, it spontaneously discharged a bullet (despite that fact that the firearm’s safety mechanism was engaged), which struck a wall in the scanner room. There were no injuries as a result of this incident.3
Between June 2004 and December 2008, the Pennsylvania Patient Safety Authority received 44 reports describing 27 ferromagnetic items that became projectiles, 16 ferromagnetic items that were brought into the MRI scanner room without becoming projectiles, and 5 ferromagnetic items that were almost brought into the MRI scanner room. (Forty-eight items were cited in 44 reports: in 2 projectile reports, an oxygen tank and a ventilator were reported; and in 2 other reports, an oxygen tank and a stretcher were reported.) Of the 44 reports, only 3 minor injuries were reported. See Tables 1 through 3 for information about the ferromagnetic items cited in the reports.
Table 1. Projectile Ferromagnetic Items in MRI Scanner Rooms
Reported to the Pennsylvania Patient Safety Authority,
June 2004 through December 2008
Table 2. Nonprojectile Ferromagnetic Items in MRI Scanner Rooms
Reported to the Pennsylvania Patient Safety Authority,
June 2004 through December 2008
Table 3. Ferromagnetic Items That Were Almost Brought into
the MRI Scanner Rooms Reported to the Pennsylvania Patient
Safety Authority, June 2004 through December 2008
For illustration, the following are examples of the narratives of Serious Events and Incidents involving projectiles reported to the Authority.
After completion of the MRI study, the technician proceeded to place the patient’s wheelchair at the door threshold of the MRI room. When she leaned over the wheelchair to lock the wheels, the wheelchair was attracted to the MRI and rolled into the equipment. There was no injury to the patient or staff. The MRI equipment suffered slight damage to the cover.
A patient was brought to the MRI room. [The patient’s] IV [intravenous line] became kinked. A technician entered the room to fix the IV tubing. The technician forgot to take scissors out of pocket. The scissors became attached to the machine. There was no apparent injury.
A post cardiac catheterization patient with a sandbag placed on right groin went to radiology for MRI. Patient was placed on the table. When the technician began advancing the table, the magnet pulled the sandbag from the patient’s groin onto the outer housing of the MRI unit.*
* For more information about sandbags in the magnetic resonance environment, refer to the article “Sandbags May Not Be What You Think” in the September 2006 issue of the Pennsylvania Patient Safety Advisory.
The critical care patient was taken to MRI with a nurse and respiratory technician. . . . The patient was hooked up to the MRI monitoring system and the respiratory technician acknowledged to the two MRI technicians that the oxygen tank could not go into the MRI room. The respiratory technician asked for the MRI-compatible ventilator stand for the ventilator (the patient was bagged at the time). The respiratory technician took the ventilator that had two oxygen tanks into the MRI room. [The technician] realized that the oxygen tanks were on the ventilator and went to the leave the room when the magnet took the tanks and the ventilator into the MRI. The patient was outside the MRI while this occurred.
A [brand omitted] [syringe] pump used in the nursery accompanied a patient to MRI. A label on the pump stated that it was MR-compatible and upon initial review it appeared to be safe. The pump was taken into the MRI suite with the patient, and the patient was positioned on the table. The pump was placed at the end of the table. As the table advanced in the bore of the magnet, the pump flew to the front of the magnet, sticking itself to the magnet cover. The baby was not injured and the pump was removed from the magnet cover. The pump was placed at a safe distance and the examination was performed as ordered.
The patient was placed on the MRI table and the [MR] safe monitor was connected to the patient. While positioning the patient, the monitor became attracted to the magnet, grazing the patient’s head.
The patient came to the [MRI] department and required anesthesia. The patient was [positioned] in the [MR] scanner when the anesthesiologist decided to use the laryngoscope. [The laryngoscope] was MR compatible without batteries. The handle went into the magnet and was retrieved without trauma to the patient or staff.
The reports above illustrate the dangers of ferromagnetic items making their way into the MRI scanner room. The risk of an object becoming a projectile increases the closer the object gets to the magnet bore. Individuals entering the MRI scanner room must understand that, for the vast majority of MRI systems, the magnet is always on and that the magnetic field is always present even when no scan is being performed. Precautions are necessary when bringing any item into the MRI scanner room.
Additionally, visual inspection alone may be inadequate to distinguish ferromagnetic projectile threats. Many objects that appear to be nonferromagnetic (e.g., wooden furniture) have been brought into MRI scanner rooms only to be discovered to have concealed ferromagnetic components. For this reason, effective screening of items, equipment, or belongings that have not been specifically tested and labeled for use in the MRI scanner room should be conducted. Many objects can be effectively tested with high-strength permanent magnets or, as discussed later in this article, ferromagnetic-only detection systems.
MRI Magnetic Field Strength Relative to Distance from Magnet
The magnetic field extends as a 3-D volume outside the scanner and is known as the fringe field. For convenience, the magnetic field is often mapped using gauss lines, which are lines of equal magnetic field strength; gauss is abbreviated “G.” For the translational (attractive) force, the field strength increases, usually very rapidly, approaching the entrance of the magnet bore. The gradient of the gauss lines are roughly analogous to the steepness of a mountain slope. If the grade (gradient) is gentle, the slope is reasonably safe, regardless of how high on the mountain you are. If it is very steep, such as at a cliff, the risk of falling becomes more immediate and dangerous, again regardless of how high you are on the mountain. It should be noted that the MR gradient is higher when the field strength lines are closer together.
The distance of these gauss lines from the magnet bore largely depends on the magnetic shielding used. When shielding is used, the magnetic field drops off very steeply, which means that an object needs only to move a small distance (e.g., about 1 inch) from experiencing no force effect to becoming uncontrollable. Modern systems use active shielding, which helps confine the fringe field close to the magnet’s bore. Each MRI system will have a fringe field that is unique to the specific supplier’s model of MRI system and the scanner room in which it is located. For illustration purposes only (these dimensions are not associated with specific suppliers of MRI systems), for an unshielded 1 tesla (T) (1 T = 10,000 G) MRI system, the 5 G line might be approximately 15 feet from the magnet bore; the 100 G line, stronger magnetic field, might be approximately 8 feet from the magnet bore; and the 10,000 G, the strongest field for a 1 T magnet, will be at the entrance to the bore. The gauss line distances are available from the specific MR system suppliers. The 5 G line is cited as the boundary at which the magnetic field strength has diminished sufficiently to pose no physical threat to the general public or, more specifically, to individuals with implanted cardiac devices. Ferromagnetic objects placed within the 5 G line could be drawn into the magnet, and many devices may fail to operate properly.1 Some medical devices are designed for use in the MR environment; however, these devices have conditional uses relative to specific MR environments (i.e., medical devices designed for use in one MR environment cannot be used in all MR environments).
Terminology for MRI-Specific Medical Devices
In 1997, in a draft guidance document,* the Center for Devices and Radiological Health (CDRH) of the U.S. Food and Drug Administration (FDA) adopted terminology from the American Society of Testing and Materials (ASTM) International—MR Safe and MR Compatible—to help characterize the safety of medical devices in the MR environment. The guidance called for devices designed for use in a MR environment to be marked with these terms to distinguish them from devices contraindicated for use in MR environments. The terms in the 1997 guidance document are defined as follows:4
* “A Primer on Medical Device Interactions with Magnetic Resonance Imaging Systems”4
MR Safe: “The device, when used in the MR environment, has been demonstrated to present no additional risk to the patient, but may affect the quality of the diagnostic information.”
MR Compatible: The device, when used in the MR environment, is MR Safe and has been demonstrated to neither significantly affect the quality of the diagnostic information nor have its operation affected by the MR device.”
The guidance document also states that the use of these terms “without specification of the MR environment to which the device was tested should be avoided since interpretation of these claims may vary and are difficult to substantiate rigorously.”4 This statement refers to the fact that any device designed for use in a MR environment has not been tested and validated for use in all MR environments. For example, a device designed and tested for use with MRI systems with magnetic fields up to 1.5 T may be contraindicated for use with systems above 1.5 T (e.g., 3 T MRI systems). Additionally, devices designed for use in MR environments typically have restrictions on the placement distance from the magnet bore. For example, an infusion pump designed for use in a 1 T MRI system may be restricted to no closer to the magnet than the 150 G line. In this example, the pump brought closer to the magnet than the 150 G line could be pulled to the magnet bore, potentially resulting in serious or fatal injuries to individuals near the magnet.
While, in theory, this terminology was developed to help users better understand the MR safety characteristics of medical devices, in practice, it has also created confusion. Distinguishing between the two terms may often be misunderstood if users are not aware that devices have been validated for use under specific MR conditions such as a maximum magnetic field strength. The terms are also often mistakenly used interchangeably, adding to the confusion. Both terms indicate no additional risks to individuals in specific MR environments. However, a device designated as MR Safe may affect the diagnostic information of the scan, while a device designated as MR Compatible will not affect the diagnostic information, which means that not all MR Safe devices are also MR Compatible. Because of this, users may not be sure when it is, or is not, appropriate to use equipment as MR Safe or MR Compatible.6
In 2005, in an effort to reduce or eliminate the confusion with the old terminology, ASTM International introduced a new standard* for marking medical devices for safety in the MR environment. The ASTM standard defines three terms to be used for permanently marking medical devices that may be brought into the MR environment. Unlike the old terminology, the new terminology does not include image quality considerations because, as stated in the standard, image artifact is not considered a safety issue for the purposes of the standard. The three new ASTM terms are MR SAFE, MR CONDITIONAL, and MR UNSAFE. (For the purposes of this article, to distinguish between the old terminology and the new terminology, italicized font represents the old terminology, and the new terminology is capitalized). The new terminology is defined as follows:5
* “Standard Practice for Marking Medical Devices and Other Items for Safety in the Magnetic Resonance Environment”5
MR SAFE: “An item that poses no known hazards in all MR environments.”
A note within the definition states “MR SAFE items include nonconducting, nonmagnetic items such as a plastic Petri dish. An item may be determined to be MR SAFE by providing a scientifically based rationale rather than test data.” [Emphasis added.]
MR CONDITIONAL: “An item that has been demonstrated to pose no known hazards in a specified MR environment with specified conditions for use. Field conditions that define the specified MR environment include field strength, spatial gradient, dB/dt (time rate of change of the magnetic field), radio-frequency (RF) field, and specific absorption rate (SAR). Additional conditions, including specific configurations of the item, may be required.”
MR UNSAFE: “An item that is known to pose hazards in all MR environments.”
A note within the definition states that “MR UNSAFE items include magnetic items such as a pair of ferromagnetic scissors.” [Emphasis added.]
Although the term “safe” is used in both nomenclature systems, the main difference between the old MR Safe term and the new MR SAFE term is that devices labeled safe under the old terminology are understood to be safe under specific MR conditions (e.g., magnetic field strength), whereas devices labeled safe under the new terminology are understood to be safe for all MR conditions without exception. The new MR SAFE term has no limitations or restrictions, unlike the old MR Safe term. Therefore, the new term should reduce confusion regarding devices brought into the MRI scanner room. The icon representing MR SAFE consists of two versions of the letters “MR” surrounded by a green square as shown in Figure 1.
Figure 1. ASTM Recommended Icon Associated with
the MR SAFE ASTM Term
The new MR CONDITIONAL term should also reduce confusion regarding devices brought into the MRI scanner room, since the word “conditional” will specifically alert clinicians that the device has limitations or restrictions when brought into the MRI scanner room. The icon for MR CONDITIONAL consists of the letters “MR” within a yellow equilateral triangle with a black band around the edge of the symbol, as shown in Figure 2. The new MR UNSAFE term should greatly reduce, if not eliminate, projectile risks due to the explicitness of the term (i.e., if a device is marked MR UNSAFE, it does not belong in the MRI scanner room). The MR UNSAFE icon consists of the letters “MR” surrounded by a red circle with a diagonal red line across the diameter of the circle over top of the letters, as shown in Figure 3.
Figure 2. ASTM Recommended Icon Associated with
the MR CONDITIONAL ASTM Term
Figure 3. ASTM Recommended Icon Associated with the
MR UNSAFE ASTM Term
The new MR safety marking standard will only be effective if it is well understood and properly implemented by device manufacturers and MR departments. Confusion may still exist, and there still may be an adjustment period as manufacturers and users migrate from the old nomenclature to the new nomenclature. Users may still see a mix of devices with the old MR safety markings and the new safety markings. During this period, users must know how to interpret the various device markings and how to ascertain which devices can and cannot be used in a particular MR environment.6 One way for facilities to reduce confusion between the old and new terminologies is to consult with equipment suppliers to obtain the information needed to relabel equipment with the new markings as soon as possible.
MR Safety Screening for Potential Projectiles
The March 2009 Pennsylvania Patient Safety Advisory article on MR safety screening processes focused on clinical screening of patients for exposure to metal that could compromise the safety of the patient or the quality of the image diagnostic information of the scan. However, MR safety screening also encompasses screening for ferromagnetic objects, external to the patient, which if allowed into the MRI scanner room, could potentially become dangerous projectiles. In addition to MR personnel checking to ensure that ferromagnetic objects do not make their way into the MRI scanner room, any facility personnel with access to the scanner room must be aware of which objects are permitted and restricted from entering the scanner room. This includes knowing and understanding the ASTM MR safety device markings and, as applicable, the safety conditions of each piece of equipment.
The Use of Ferromagnetic Detection Systems
A method that may help in screening for ferromagnetic objects that are external to the patient is the use of ferromagnetic-only detectors. The March 2009 Advisory article discussed the use of ferromagnetic-only detectors as an adjunct to the MR safety screening process that may significantly reduce the likelihood of objects becoming projectiles in the MR environment. Ferromagnetic-only detectors are designed to distinguish between ferromagnetic and nonferromagnetic materials. The detectors warn MR personnel of the presence of ferromagnetic items external to the body (the detectors are not currently approved for use in detecting ferromagnetic implants) before the item is brought into the MRI scanner room. The use of these detectors may significantly reduce the likelihood of objects becoming projectiles in the MR environment. A University of Pittsburgh Medical Center study demonstrated that ferromagnetic detectors were successful in detecting ferromagnetic items, even items as small as a safety pin, on patients before MRI scans.6 The University of Pittsburgh study also found that 44% of patients who indicated that they had complied with the MR screening instructions to remove all loose metallic objects before the MRI scan set off the ferromagnetic detector. This may suggest that patient compliance with MR screening instructions for metallic objects may not be comprehensive. Ferromagnetic-only detectors may not only be prudent for detecting larger ferromagnetic objects before entering the MRI scanner room, but may also prove useful as an adjunct practice in the MR patient screening process.
The use of ferromagnetic-only detectors has been recommended by the American College of Radiology (ACR) in its 2007 “ACR Guidance Document for Safe MR Practices” and the U.S. Department of Veterans Affairs (VA) in its 2008 “VA Magnetic Resonance Imaging Design Guide.” Use of ferromagnetic-only detectors has been cited as an approved means of screening verification in the Joint Commission’s Sentinel Event Alert Issue No. 38 “Preventing Accidents and Injuries in the MRI Suite.”7-9
Medical Device Malfunction in the MR Environment
In addition to the projectile effect, another safety concern is malfunction of medical devices in the MR environment. The proper operation of medical devices can be affected by the static, RF, and gradient magnetic fields of MRI systems. (For a discussion of the static, RF, and gradient magnetic fields, see the sidebar “Basic Operating Principles of Magnetic Resonance Imaging.”)
The static magnetic field can affect devices that incorporate components such as analog gauges or electric motors, which contain magnets and coils (e.g., ventilators, infusion pumps), or electronic components such as transformer or relays. The effects can cause devices to malfunction or completely stop operating, potentially resulting in serious patient harm. ECRI Institute cites one example of a patient-controlled analgesic infusion pump that malfunctioned in the presence of a static magnetic field.1 The field caused the pump’s motor to reverse direction without indication to the user. Because of the reversed motor action, the pump could have drawn blood from the patient into the IV line. However, the IV tubing incorporated a one-way valve that prevented backflow of blood.1
Devices that rely on magnetization to attach to a patient (e.g., otologic implants) can become demagnetized in the static magnetic field of the scanner.1 Implanted devices that are magnetically, electrically, or mechanically activated, which may be affected by the static magnetic field, are typically contraindicated for MRI scans. Other devices, such as those that measure physiologic electric signals (e.g., electrocardiogram [ECG] monitors) may be affected by high field strength MRI systems (greater than or equal to 1 T).1 For example, high field strengths can distort the ECG of patients within the field.1 The distortion may appear as an increase in the amplitude of the T wave or ST segment of the signal, causing clinicians to believe the patient’s physiologic condition changed.1
The RF magnetic field can affect devices, such as physiologic monitors, that incorporate lead wires (e.g., ECG monitors, pulse oximeters). The lead wires act as antennas in the presence of the RF energy. The RF energy of the MRI system can be electrically coupled to the lead wires if they are in close proximity to the MRI scanner. The coupling can result in temporary loss of the measured parameter or damage to the devices.1 The gradient magnetic field can mimic physiologic signals (e.g., ECG signals), and interfere with the ECG signal, which could cause misinterpretation of the ECG signal.1 However, through design of physiologic monitors, this interference can be eliminated using filtering and digital signal-processing techniques.1
The Price of Projectiles
Even in the absence of patient harm, the financial consequences of interrupted patient throughput or equipment damage, including emergency shutdown of the MRI scanner, resulting from ferromagnetic projectiles can be significant. The VA National Center for Patient Safety recently published information on the average cost of a ferromagnetic projectile event in a VA facility to be $43,172 (excluding lost revenue and legal expenses).10 The VA also indicated that the cost of an emergency shutdown of the MRI scanner, known as “quenching the magnet,” can range from $20,000 to $500,000.10
Ferromagnetic objects in the MRI scanner room can pose a serious, even catastrophic, projectile risk to individuals in the room. Additionally, the magnetic field can affect the operation of devices brought into the field. A good understanding of equipment and objects that can and cannot be brought safely into the MR environment will help reduce or eliminate the likelihood of projectiles or device malfunctions, thereby reducing the potential for serious harm to individuals within the MR environment.
As part of a risk reduction strategies to reduce or eliminate the possibility that objects will become projectiles or that medical devices will malfunction, consider the following:
- Identify the four MR safety boundaries as defined by ACR in its “ACR Guidance Document for Safe MR Practices.” Mark the boundaries with appropriate signage. Zone 4, the area containing the MRI scanner that is associated with the strongest magnetic field, should be clearly marked. Access should be restricted to this area without supervision by appropriate MR personnel.1 (For more information on the MR safety boundaries, see the article “Safety in the MR Environment: MR Safety Screening Practices” in the March 2009 issue of the Advisory.)
- Provide training on MR safety considerations to all MR staff and other personnel who may need access to the MRI scanner room. Include education on the new MR device terminology, the old MR device terminology, and the difference between the two nomenclatures.1
- Prohibit equipment and devices within Zone 4 without first verifying that (1) they have been tested by the device manufacturer or properly trained expert and (2) they have been labeled according to the ASTM device terminology—MR SAFE or MR CONDITIONAL—for the specific MR environment.1
- Maintain a list of MR SAFE, MR CONDITIONAL (including conditions for safe use), and MR UNSAFE equipment in every MR department or facility. When possible, identify the safety conditions directly on an object or device. If MRI systems are upgraded or newly purchased, the MR safety officer determines whether the equipment is still MR SAFE, MR CONDITIONAL, or MR UNSAFE with the upgraded or new system.1
- Do not alter MR SAFE or MR CONDITIONAL equipment. Altering equipment may negate the MR safety characteristics of the equipment.1
- Care should be taken when equipment containing ferromagnetic components is brought into the MR environment. For example, devices may be validated for use in some areas of the MR environment (e.g., field strengths not exceeding 150 G), but not other areas.1
All equipment brought into the MR environment should be properly labeled (e.g., MR SAFE, MR CONDITIONAL) and should be physically secured at a safe distance (defined by the equipment supplier) from the MRI system using nonmagnetic means (e.g., nonmagnetic bolts), as appropriate. The method of restraint should be adequately tested before implementation, and care should be taken to protect the integrity of the RF shielding for any attempt at providing anchorage within the MRI scanner room.
- Consider the use of ferromagnetic-only detection systems as an adjunct to your facility’s MR safety practices (based on the findings of the University of Pittsburgh Medical Center study,1 and recommendations from ACR7, VA8, and the Joint Commission9).
- ECRI Institute. The safe use of equipment in the magnetic resonance environment [guidance article]. Health Devices 2001 Dec;30(12):421-44.
- Chen DW. Boy, 6, dies of skull injury during MRI. NY Times [online]. 2001 Jul 31 [cited 2009 Mar 23]. Available from Internet: http://www.nytimes.com/2001/07/31/nyregion/
- Beitia A, Meyers S, Kanal E, et al. Case report: spontaneous discharge of a firearm in an MR imaging environment. AJR Am J Roentgenol 2002 May;178(5):1092-4.
- Center for Devices and Radiologic Health (CDRH) Magnetic Resonance Working Group. A primer on medical device interactions with magnetic resonance imaging systems [draft online]. 1997 Feb [cited 2009 Mar 23]. Available from Internet: http://www.fda.gov/cdrh/ode/primerf6.html.
- American Society of Testing and Materials (ASTM). Standard practice for marking medical devices and other items for safety in the magnetic resonance environment. ASTM F2503-05. 2005 Aug:1486-92.
- ECRI Institute. What’s new in MR safety; the latest on the safe use of equipment in the magnetic resonance environment [guidance article]. Health Devices 2005 Oct;34(10):333-49.
- Kanal E, Barkovich A, Bell C, et al. ACR guidance document for safe MR practices: 2007. AJR Am J Roentgenol 2007 Jun;188(6):1447-74.
- U.S. Department of Veterans Affairs (VA). VA MRI design guide [online]. 2008 Apr [cited 2009 May 20]. Available from Internet: http://www.cfm.va.gov/til/dGuide/dgmri02.pdf.
- Joint Commission. Preventing accidents and injuries in the MRI suite. Sentinel Event Alert Issue No. 38 [online]. 2008 Feb 14 [cited 2009 May 20]. Available from Internet: http://www.jointcommission.org/SentinelEvents/SentinelEventAlert/sea_38.htm.
- U.S. Department of Veterans Affairs (VA) National Center for Patient Safety. MR hazard summary [online]. 2008 Sept 29 [cited 2009 May 21]. Available from Internet: http://www.va.gov/ncps/SafetyTopics/mrihazardsummary.html.
Basic Operating Principles of Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) creates cross-sectional images of anatomic structures placed in the magnet bore using (nonionizing radiation) electromagnetic fields. The static magnetic field of the MRI scanner causes the protons in the body tissue to align in the direction of the static magnetic field. Electromagnetic pulses from a radio-frequency (RF) transmitter create an RF magnetic field that alters the static magnetic field. When this occurs, the direction of the magnetized protons in the static magnetic field changes alignment with that static field. When the pulses stop, the magnetized protons revert to their original position. While reverting to their original position, the protons emit RF energy that is detected by RF receivers of the MRI system. Computer analysis then converts the signals into images of the scanned anatomy. The gradient magnetic field is produced by coils inside the MRI system. The coils are rapidly pulsed on and off during the time the RF magnetic field is pulsed. The gradient magnetic field determines the location of the scanned anatomic section, the thickness, and the field of view of the section.
Source: ECRI Institute. The safe use of equipment in the magnetic resonance environment [guidance article]. Health Devices 2001 Dec;30(12):42-44.