MRI safety

10
MRI safety




After reading this chapter, you will be able to:



  • Recognize the main hazards associated with MRI scanning.
  • Understand the mechanisms behind MRI-related injuries.
  • Identify and ameliorate potential risks to the patient undergoing an MRI procedure.

INTRODUCTION (AND DISCLAIMER)


MRI safety is a convoluted subject. Over the last 20 years, the field of MRI has increased greatly in complexity. Scanners now operate at a wide range of field strengths and manufacturers offer different machine configurations, some of which operate with powerful and rapidly switching gradient magnetic fields. Coil design, magnet bore width, and other factors also vary across different platforms. In addition to variations in hardware, patients present with a diverse variation in body habitus and an increasing variety of implanted devices and cosmetic body jewelry. Implanted devices may be conditionally safe below a certain field strength, spatial gradient magnetic field, or specific absorption rate (SAR). As a result, there are many combinations of factors that could potentially contribute to a safety hazard. This chapter is therefore not intended to be a definitive safety guide for the use of MRI but rather a general introduction to the topic of MRI safety. By the same token, pharmacological safety issues with gadolinium-based contrast agents are not within the brief of this book. Instead, this chapter aims to identify the generic safety factors relating to the MRI equipment and associated magnetic fields and explains the mechanisms behind the common types of injury that may occur during an MRI procedure.


The overarching philosophy behind MRI safety recognizes that every case is different and should be considered on its own merits. For patients with implanted devices, this is likely to require a careful risk/benefit analysis. On the one hand, patients should not be needlessly excluded from an MRI procedure that is necessary for their ongoing care. However, any potential risks need to be carefully considered. The primary responsibility for this task lies with the referring physician and the radiology lead. Crucially, the MRI practitioner should also be permitted to decide whether it is safe to proceed with an examination, or whether it should be postponed until sufficient evidence has been gathered related to the safety of the request. This is important because additional safety concerns (such as body piercings) may come to light when the patient attends for the examination. Such factors may not have been appreciated at the time of referral. Advice may also be sought from medical physicists and other professionals to gain a more informed and balanced view of the inherent risks in certain cases. As new devices are introduced to the market on a regular basis, it is vital that we keep up to date with the latest developments in the field of MRI safety and the latest safety research. There are recommended resources listed at the end of this chapter.


DEFINITIONS USED IN MRI SAFETY


The following definitions relate to the layout of the MRI department, the personnel who work in this environment, and the devices that are permitted to be taken into the MRI magnet room or subjected to MRI scanning.


Safety zones


The American College of Radiology (ACR) has published a guidance document on MRI safety [1] that makes recommendations related to policy and practice in the field. One of the key recommendations is that the MRI facility should be zoned according to risk. The zones are represented as a floor plan in Figure 10.1. The aim of using zones is to prevent unauthorized access to areas where the high magnetic field may cause injury or death. The ACR defines the zones as follows:



  • Zone I. “…all areas that are freely accessible to the general public. This area is typically outside the MR environment itself and is the area through which patients, healthcare personnel, and other employees of the MR site access the MR environment.”
  • Zone II. “…the interface between the publicly accessible, uncontrolled Zone I and the strictly controlled Zones III and IV. Typically, patients are greeted in Zone II and are not free to move throughout Zone II at will, but are rather under the supervision of MR personnel. It is in Zone II that the answers to MR screening questions, patient histories, medical insurance questions, etc. are typically obtained.”
  • Zone III. “…the region in which free access by unscreened non-MR personnel or ferromagnetic objects or equipment can result in serious injury or death as a result of interactions between the individuals or equipment and the MR scanner’s particular environment. These interactions include, but are not limited to, those involving the MR scanner’s static and time-varying magnetic fields. Zone III regions should be physically restricted from general public access by, for example, key locks, passkey locking systems, or any other reliable, physically restricting method that can differentiate between MR personnel and non-MR personnel.”
  • Zone IV. “…the physical confines of the room within which the MR scanner is located. Zone IV should also be demarcated and clearly marked as being potentially hazardous due to the presence of very strong magnetic fields. Zone IV should be clearly marked with a red light and lighted sign stating: The Magnet is On. Except for resistive systems, this light and sign should be illuminated at all times and should be provided with a backup energy source to continue to remain illuminated for at least 24 h in the event of a loss of power to the site.”
Diagram shows MRI safety zones having scanner and fringe field with lock, with unscreened MRI patients, screened MRI patients under direct supervision, et cetera.

Figure 10.1 MRI safety zones as recommended by the ACS Guidance Document on MR-Safe Practices 2013.


Full definitions are found in the ACR Guidance Document on MR Safe Practices: 2013 (see References).


Personnel


The ACR guidance document on MRI safety identifies three levels of personnel:



  • Non-MRI personnel include patients, visitors, or facility staff who have not undergone formal safety training (within the last 12 months) as designated by the MRI safety director.
  • Level 1 personnel include office staff and patient aides who have passed minimal safety education to ensure their own safety as they work within Zone III.
  • Level 2 personnel include MRI technologists, radiologists, and nursing staff who have been extensively trained in MRI safety, including issues relating to thermal loading, burns, and neuromuscular excitation from rapidly changing gradients.

Device safety


In 2005, the American Society for Testing and Materials International reviewed the terminology used to describe the MRI safety of implants and other medical devices that may pose risks in the MRI environment. These may include electromagnetic field interactions leading to heating or malfunction. Devices are now divided into three main categories: MRI safe, MRI conditional, and MRI unsafe (Figure 10.2). These are defined as follows:



  • MR safe. “An item that poses no known hazards in all MR imaging environments. With this terminology, MR safe items are nonconducting, nonmetallic, and 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.”
  • MR conditional. “An item that has been demonstrated to pose no known hazards in a specified MR environment with specified conditions of use. Field conditions that define the MR environment include static magnetic field strength, spatial gradient, time rate of change of the magnetic field (dB/dt), RF fields, and SAR. Additional conditions, including specific configurations of the item (e.g. the routing of leads used for a neurostimulation system), may be required. For MR conditional items, the item labeling includes results of testing sufficient to characterize the behavior of the item in the MR environment. In particular, testing for items that may be placed in the MR environment should address magnetically induced displacement force and torque, and RF heating. Other possible safety issues include but are not limited to: thermal injury, induced currents/voltages, electromagnetic compatibility, neurostimulation, acoustic noise, interaction among devices, the safe functioning of the item, and the safe operation of the MR system. Any parameter that affects the safety of the item should be listed and any condition that is known to produce an unsafe condition must be described.”
  • MR unsafe. “An item that is known to pose hazards in all MR environments. MR unsafe items include magnetic items such as a pair of ferromagnetic scissors.”
Diagram shows device-labeling icons like MR safe, MR conditional, and MR unsafe.

Figure 10.2 Device-labeling icons developed by the American Society for Testing and Materials International and recognized by the FDA.


Assessing the safety of MRI conditional devices can be a complex procedure. Importantly, devices that are conditionally safe at 1 T may not be safe at higher field strengths (see “Additional resources” at the end of this chapter for further information on this topic).


An MRI examination might subject the patient to adverse psychological and biological effects. Some of these effects are transient and have no long-term safety implications, and others may cause serious injury or death. The five main factors that are considered to have an impact on patient safety are as follows:



  • Psychological effects
  • The spatially varying static magnetic field
  • Electromagnetic (radiofrequency) fields
  • Time-varying gradient magnetic fields
  • Cryogens.

This chapter analyzes each of these areas in turn and provides a broad overview to assist you in making an informed decision on whether it is safe to scan.


PSYCHOLOGICAL EFFECTS


The design of MRI equipment, particularly closed-bore scanners, may increase levels of anxiety and emotional distress in patients. Early closed-bore scanners required a very long magnet bore to ensure a large imaging volume of appropriate homogeneity. One undesirable trade-off with this design was that patients were required to be almost entirely confined within the narrow bore. A study performed in 1998 revealed that 14.3% of patients required sedation or anesthesia to overcome feelings of anxiety or claustrophobia during the scan procedure [2]. Notably, a majority of these patients were scheduled for a brain examination. This requires the patient’s head to be positioned at the isocenter of the bore with the roof of the bore in proximity to the patient’s face. The use of a head coil also adds to the feeling of enclosure. More recent research from 2008 seemed to indicate that the confined space of an MRI scanner was still an issue. In addition to claustrophobia, patients expressed fears of suffocation and fear of fainting, and exhibited symptoms of panic attack. The factors that contribute to patient anxiety are complex. The narrow bore plays a significant role. However, patients also report feelings of isolation and other factors such as scan duration, acoustic noise, and fear of a negative diagnosis are also thought to contribute to emotional distress.


On first thoughts, patient noncompliance may appear to be nothing more than a minor inconvenience; however, there may be negative consequences to diagnostic outcomes. Firstly, a nervous patient may find it difficult to keep still or comply with instructions related to breath-holding. This is likely to degrade image quality due to phase mismapping artifacts (see Chapter 8). Furthermore, a patient suffering a panic attack may terminate the procedure before any diagnostic images are acquired. This may require the patient to be referred for alternative tests such as CT. A recent comparison revealed that CT was the preferred modality for 42% of patients undergoing cardiac imaging compared with just 12% who preferred MRI. The use of other modalities such as CT or isotope studies to yield a satisfactory diagnosis results in an unnecessary dose of ionizing radiation. Examinations prolonged due to patient intolerance may place a burden on staff, decreasing patient throughput. Failed examinations also waste scanner time and other valuable resources, incurring financial penalties. Finally, rescheduling may delay diagnosis, adversely affect waiting list times, and increase the burden on administrative staff. For these reasons, it is important to have a strategy for anxious or claustrophobic patients.


There are various approaches employed to reduce emotional distress in MRI patients. Aromatherapy has been used with varying degrees of success, but it is difficult to assess whether this offers a statistically significant benefit. Scanner design has improved in recent years; magnet bores are now considerably shorter and wider than earlier models, offering a brighter and more spacious environment for the patient. This has reduced the incidence of claustrophobia threefold [3]. The bore is often equipped with a fan to reduce the feeling of enclosure. Open scanners also appear to be better tolerated and allow relatives to sit close to the patient for comfort during the examination. Electrophysiology tests indicate that patients are most anxious at the very beginning of the procedure when they are moved into the bore and that the level of anxiety typically decreases throughout the procedure. This suggests that any strategies for combating anxiety should focus on this stage of the process. Equipment manufacturers have recently started to place more emphasis on the patient experience as a critical factor in the success of an MRI examination. Ambient lighting, wall and ceiling panels displaying relaxing scenery, and immersive in-bore virtual reality style video imagery are employed to shift the patient’s focus away from the mechanics of the procedure itself. Patients may even be invited to select a theme for their scan before entering the magnet room distracting them from any anxiety at this critical time. The in-bore video presentation may also include a count-down timer that keeps the patient informed about how long they are required to keep still and the remaining examination time. This may provide an additional motivation to complete the procedure successfully. For very young children, or patients who are very claustrophobic, sedation or anesthesia may be required. These patients require additional care and monitoring throughout the scan. Special care should be taken to ensure that monitor leads and other conductors are not left in contact with the skin of anesthetized or sedated patients.


THE SPATIALLY VARYING STATIC FIELD


The static field of the MRI scanner presents four main implications for patient safety:



  • Transient biological effects
  • Projectile hazards
  • Torque on implanted devices
  • Foreign bodies in the static field.

Transient biological effects


The main magnetic field B0 is responsible for certain transient biological effects experienced by patients, staff, and research volunteers, particularly when ultra-high-field systems are used. The sensitivity of living organisms to external magnetic fields is well known. Around 50 different animal species appear to be sensitive to the relatively insignificant magnetic field of the planet Earth (0.5 G). Migrating birds such as pigeons are thought to have a neural substrate that acts as a magnetoreceptor for navigational purposes. There are also species of bacteria, Gram-negative prokaryotes, that are known to be magnetotactic, passively aligning themselves and actively moving along the flux lines of the magnetic north pole.


As the external magnetic field increases to a magnitude that is 160 000 times more powerful than that of the earth (8 T), biological effects become more pronounced. Patients and staff who are required to enter magnetic fields of 3–7 T report transient symptoms including a metallic taste in the mouth and vertigo. Research indicates that these effects tend to occur when workers are required to move through the flux lines of the stray static magnetic field, and when patients are moved to the isocenter of the magnet. Vertigo, causing dizziness, nausea, nystagmus (involuntary eye movement), and postural instability, is predominantly elicited at fields of 7 T and above. Patients report a sensation of moving along a curved trajectory, despite the fact that the table movement occurs in a straight path along the z-direction of the magnet bore. The effect appears to be due to a temporary overcompensation by the vestibular system of the inner ear and can persist for a short time post examination. The mechanism of this effect has been theorized to be related to Lorentz force on the ions contained in the fluid within the semicircular canals. Any flow can cause vertigo to persist even when the patient is immobilized and stationary. The reason that this can be considered a safety hazard is that some patients appear to be more sensitive to the ultra-high field than others and, in some cases, may be advised to avoid driving or operating machinery until any symptoms subside. Normal function usually returns within 15 min from being removed from the magnetic field. Some studies have shown that the severity of induced vertigo increases in relation to the velocity at which the patient is introduced into the magnet bore. Using a slow table movement is therefore desirable when positioning the patient in an ultra-high-field system. The effects of vertigo do not seem to be influenced by RF or rapidly switching gradients.


In addition to the Lorentz force, there is Faraday’s law of electromagnetic induction to consider. This phenomenon causes the induction of an electrical current through a conductor that is moved through an external magnetic field (see Chapter 1). This is understood to be the mechanism behind phosphenes, whereby the optic nerve acts as a conductor and stimulation of the nerve causes the appearance of flashes and optical disturbances in the patient’s visual field. Metallic taste sensations are also reported by patients as they move along the static field gradient. Although the exact mechanism is still under investigation, it is likely that these sensations are caused by either nerve stimulation of the taste-buds or possibly electrolysis in the saliva. In terms of safety, the threshold for these effects is very high. Most patients, even those scanned on ultra-high-field equipment, do not report any perception of taste changes.


In summary, transient biological effects are not of great concern from a safety perspective. The International Committee on Non-Ionizing Radiation states that there is no evidence of serious adverse health effects from whole-body exposure up to 8 T. In some cases, some of the effects described by patients may be psychological in origin. Research participants assigned to a control group occasionally report symptoms when positioned inside a realistic dummy scanner having no active magnetic field.


Projectile hazards


Unlike the benign transient biological effects associated with the static field, ferromagnetic projectiles have the potential to be very dangerous. When we consider gradient magnetic fields in MRI, we often think about the time-varying gradients created by the gradient coils for spatial encoding. However, the static magnetic field also has a gradient. The static field is homogenous over the imaging volume at the isocenter of the magnet bore; however, the flux density changes dramatically over increasing distance from the end of the magnet bore. This creates a spatially varying gradient, rather than a time-varying gradient. Active shielding by the bucking coils considerably increases this effect because it is desirable to maintain the 5 G threshold as close to the magnet as possible (see Chapter 9). As a result, the magnetic field may vary from 3 T to 0.5 mT (5 G), for example, over just a few meters with the steepest part of the gradient at the end of the bore. The force required to accelerate a ferromagnetic item, such as a steel oxygen cylinder, toward the isocenter of the magnet is proportional to the product of the flux density of the magnet (field strength) and the spatial static-field gradient. In older, poorly shielded systems and some modern ultra-high-field systems, the static field gradient extends to such a distance that large ferromagnetic items are attracted from up to several meters away. This results in a greater capacity for acceleration along the gradient and a high terminal velocity.


However, modern actively shielded systems contain the fringe field very efficiently. The 0.5 mT (5 G) threshold may extend little further than the end of the patient transport assembly, which is 3 m from the end of the bore or less (Figure 10.3). From a projectile-safety viewpoint, this could be considered a double-edged sword. There is less risk of items being dragged from a long distance and accelerated along a powerful magnetic field. However, there is a much smaller margin of error if a ferromagnetic wheelchair or other such item is accidentally taken into the magnet room. On unshielded systems, we might notice a moderate attractive force exerted on a ferromagnetic item (for example, coins in pockets) while still some distance from the scanner and can therefore take corrective action. On actively shielded systems, by the time it becomes apparent that the item is being attracted to the magnet, there is nothing that can be done to prevent it becoming a projectile. Figure 10.4 shows an example of this, where a patient was taken almost to the end of the magnet bore in a ferromagnetic wheelchair. The wheelchair only became attracted to the magnet when the patient stood upright, pushing the chair into an intensely higher magnetic field. The same effect applies to smaller items such as pens, scissors, coins, hairgrips, pagers, phones, and other such items that may be inadvertently introduced into the magnet room if screening is not undertaken thoroughly. To date, there have been at least two cases of very serious injury caused by oxygen cylinder projectile incidents and two known fatalities. In addition to the human cost, a high-velocity projectile can cause severe damage to the scanner hardware resulting in days or weeks of lost imaging time.

Diagram shows magnetic field gradient with gamma camera, ultra scanner, CT scanner, cardiac pacemaker, credit card hearing and aid, et cetera.

Figure 10.3 Static magnetic field gradient of an actively shielded closed-bore MRI scanner. The values shown will vary with scanner model.

Photograph shows ferromagnetic wheelchair which is moving into actively shielded magnetic room.

Figure 10.4 Ferromagnetic wheelchair taken into an actively shielded magnet room. No attraction was appreciated until the chair was moved close to the end of the bore.


In the event of a large projectile incident, such as those involving oxygen cylinders or wheelchairs, it is possible that the superconducting magnet requires quenching to remove the projectile. This is an absolute necessity if there is an immediate danger to life or limb. There have been at least two reported cases where patients or staff have been trapped by oxygen cylinders and very seriously injured. On one occasion, the patient sustained facial fractures. In the other incident, a ward assistant sustained a fractured forearm, and a radiographer a fractured pelvis. In both these cases, the patients were physically trapped by the oxygen cylinder and were unable to be released because the quench circuit failed. The patient with facial fractures had to be extracted from the rear end of the magnet bore. The ward assistant and radiographer were trapped for approximately 4 h until an engineer managed to bypass the quench mechanism and free them. In this case, the radiographer had lost consciousness due to internal bleeding. Such cases make it very clear that all new staff members should be made aware of the location of the quench button. Quench circuitry should also be regularly checked by the service engineer.


In addition to the projectile effect, there is another potential safety concern when small ferromagnetic items are trapped inside the scanner. Such items may have a deleterious effect on image quality or image geometry/distortion. For example, some types of patient immobilization devices (“sand” bags) may be filled with steel-shot. In addition to a potential projectile hazard, they are counter-effective as immobilization devices, as they are attracted to the magnet. There have been at least two incidences where such bags have ruptured and shed their contents into the warm bore of the cryostat, permanently affecting image quality. Any small ferromagnetic items such as coins, hair grips, and safety pins that accumulate inside the MRI scanner may also adversely affect the homogeneity of the imaging volume. This causes image-quality concerns but may also become a safety issue when the examination is required to make accurate measurements of anatomy, for example, when planning image-guided surgery, RF ablation, radiotherapy, or pelvimetry. In all these examples, inaccurate measurement may have seriously unfavorable consequences for the patients, if their treatment plan is incorrect.


In summary, projectile hazards can be avoided by vigilance, the use of physical barriers, unambiguous warning signs, and a robust staff training program. If metallic items (such as halo-vests or external fixators) have to be taken into the magnet room, a powerful hand-held magnet helps to determine safety. Be aware that large devices or monitoring equipment may have a plastic cover, but a ferromagnetic chassis. For any patient monitoring, it is strongly recommended that only purpose-manufactured MRI certified devices are used.

Mar 9, 2019 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on MRI safety
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