Medical MRI scanner

Magnetic resonance imaging (MRI) is in general a safe technique, although injuries may occur as a result of failed safety procedures or human error.[1] During the last 150 years, thousands of papers focusing on the effects or side effects of magnetic or radiofrequency fields have been published. They can be categorized as incidental and physiological.[2] Contraindications to MRI include most cochlear implants and cardiac pacemakers, shrapnel and metallic foreign bodies in the eyes. The safety of MRI during the first trimester of pregnancy is uncertain, but it may be preferable to other options.[3] Since MRI does not use any ionizing radiation, its use generally is favored in preference to CT when either modality could yield the same information.[4] (In certain cases, MRI is not preferred as it may be more expensive, time-consuming and claustrophobia-exacerbating.)

Structure and certification

In an effort to standardize the roles and responsibilities of MRI professionals, an international consensus document, written and endorsed by major MRI and medical physics professional societies from around the globe, has been published formally.[5] The document outlines specific responsibilities for the following positions:

  • MR Medical Director / Research Director (MRMD) – This individual is the supervising physician who has oversight responsibility for the safe use of MRI services.
  • MR Safety Officer (MRSO) – Roughly analogous to a radiation safety officer, the MRSO acts on behalf of, and on the instruction of, the MRMD to execute safety procedures and practices at the point of care.
  • MR Safety Expert (MRSE) – This individual serves in a consulting role to both the MRMD and MRSO, assisting in the investigation of safety questions that may include the need for extrapolation, interpolation, or quantification to approximate the risk of a specific study.

The American Board of Magnetic Resonance Safety (ABMRS) provides testing and board certification for each of the three positions, MRMD, MRSO, and MRSE. As most MRI accidents and injuries are directly attributable to decisions at the point of care, testing and certification of MRI professionals seeks to reduce the rates of MRI accidents and improve patient safety through the establishment of safety competency levels for MRI professionals.

Implants

MR-Safe sign
MR-Conditional sign
MR-Unsafe sign

All patients are reviewed for contraindications prior to MRI scanning. Medical devices and implants are categorized as MR Safe, MR Conditional or MR Unsafe:[6]

  • MR-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 – 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 – Objects that are significantly ferromagnetic and pose a clear and direct threat to persons and equipment within the magnet room.

The MRI environment may cause harm in patients with MR-Unsafe devices such as cochlear implants, aneurysm clips, and many permanent pacemakers. In November 1992, a patient with an undisclosed cerebral aneurysm clip was reported to have died shortly after an MRI exam.[7] Several deaths have been reported in patients with pacemakers who have undergone MRI scanning without appropriate precautions.[8] Increasingly, MR-conditional pacemakers are available for selected patients.[9]

Ferromagnetic foreign bodies such as shell fragments, or metallic implants such as surgical prostheses and ferromagnetic aneurysm clips also are potential risks. Interaction of the magnetic and radio frequency fields with such objects may lead to heating or torque of the object during an MRI.[10] MRI is contraindicated in those suspected with metallic foreign body in the eye. MRI maybe considered if there is strong suspicion of non-metallic foreign body.[11]

Titanium and its alloys are safe from attraction and torque forces produced by the magnetic field, although there may be some risks associated with Lenz effect forces acting on titanium implants in sensitive areas within the subject, such as stapes implants in the inner ear.[12]

Intrauterine devices with copper are generally safe in MRI, but may become dislodged or even expelled, and it is therefore recommended to check the location of the IUD both before and after MRI.[13]

Other implants that are contraindicated in MRI includes: magnetic dental implants, tissue expander, artificial limb, hearing aid, catheters with metallic components such as Swan-Ganz catheter and piercing.[14] However, tooth amalgam is not contraindicated in MRI.[15]

Risk of implant heating under MRI

Titanium and its alloys can heat from the radiofrequency field, as well as the switched gradient field (due to Faraday's law of magnetic induction).

Implants with metallic or electrically conductive parts may interact with the switched gradient and/or RF fields used in MRI, causing trauma or burns.[16]

Following Faraday’s law, the change of the magnetic flux through such a device induces eddy currents in the device and the metal subsequently converts electric energy into thermal energy... Furthermore, under specific conditions… gradient switching induced heating of other conductive material such as titanium, nitinol, or 316 stainless steel must be expected.

— Hansjörg Graf PhD Günter Steidle MS Fritz Schick PhD, MD, Heating of metallic implants and instruments induced by gradient switching in a 1.5‐Tesla whole‐body unit[17]

The amount of heating that takes place has a number of contributing factors:

A primary safety concern related to MRI is metallic medical implant heating by absorbing radiofrequency (RF) energy. This risk depends on the metal type, shape and orientation, the static magnetic field strength and the pulse sequence type and parameters.

— M Hasegawa, K Miyata, Y Abe, and T Ishigami, Radiofrequency heating of metallic dental devices during 3.0 T MRI[18]

Injuries have been reported by this heating of metallic implants:

Event Description: It was reported to Siemens that a patient suffered a second degree burn on her right forearm after examination on the magnetom trio system... The patient has a titanium rod and screws placed in the right humerus. Patient has no feeling in right arm. Approximately five hours after the examination, the patient reported redness and a second degree blister, approximately 8 cm in length and 1,5 cm in width on the upper right forearm... According to the investigation from our experts, the RF burn was likely caused due to the presence of a titanium rod and screws placed in the right humerus.

— FDA, MAUDE Adverse Event Report[19]

Projectile risk

The very high strength of the magnetic field may cause projectile effect (or "missile-effect") accidents, where ferromagnetic objects are attracted to the center of the magnet. Pennsylvania reported 27 cases of objects becoming projectiles in the MRI environment between 2004 and 2008.[20] There have been incidents of injury and death.[21][22] In one case, a six-year-old boy died in July 2001, during an MRI exam at the Westchester Medical Center, New York, after a metal oxygen tank was pulled across the room and crushed the child's head.[23][24] To reduce the risk of projectile accidents, ferromagnetic objects and devices are typically prohibited near the MRI scanner, and patients undergoing MRI examinations must remove all metallic objects, often by changing into a gown or scrubs. Some radiology departments use ferromagnetic detection devices to ensure that no ferromagnetic objects enter the scanner room.[25][26]

MRI-EEG

In research settings, structural MRI or functional MRI (fMRI) may be combined with EEG (electroencephalography) under the condition that the EEG equipment is MR-compatible. Although EEG equipment (electrodes, amplifiers, and peripherals) are either approved for research or clinical use, the same MR Safe, MR Conditional and MR Unsafe terminology applies. With the growth of the use of MR technology, the U.S. Food & Drug Administration [FDA] recognized the need for a consensus on standards of practice, and the FDA sought out ASTM International [ASTM] to achieve them. Committee F04 [27] of ASTM developed F2503, Standard Practice for Marking Medical Devices and Other Items for Safety in the Magnetic Resonance Environment.[28]

Genotoxic effects

There is no proven risk of biological harm from any aspect of an MRI scan, including very powerful static magnetic fields, gradient magnetic fields, or radio frequency waves.[29][23] Some studies have suggested possible genotoxic (i.e., potentially carcinogenic) effects of MRI scanning through micronuclei induction and DNA double strand breaks in vivo and in vitro,[30][31][32] however, in most, if not all cases, others have been unable to repeat or validate the results of these studies,[29][23] and the majority of research shows no genotoxic, or otherwise harmful, effects caused by any part of MRI.[29] A recent study confirmed that MRI using some of the most potentially-risky parameters tested to date (7-tesla static magnetic field, 70 mT/m gradient magnetic field, and maximum strength radio frequency waves) did not cause any DNA damage in vitro.[33]

Peripheral nerve stimulation

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.[34][35] 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).[36] 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 capable of inducing PNS. American and European regulatory agencies insist that manufacturers stay below specified dB/dt limits (dB/dt is the change in magnetic field strength 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.

Heating caused by absorption of radio waves

Every MRI scanner has a powerful radio transmitter that generates the electromagnetic field that excites the spins. If the body absorbs the energy, heating occurs. For this reason, the transmitter rate at which energy is absorbed by the body must be limited (see Specific absorption rate). It has been claimed that tattoos made with iron-containing dyes may lead to burns on the subject's body.[37][38] Cosmetics are very unlikely to undergo heating, as well as body lotions, since the outcome of the reactions between those with the radio waves is unknown. The best option for clothing is 100% cotton.

There are several positions strictly forbidden during measurement such as crossing arms and legs, and the patient's body may not create loops of any kind for the RF during the measurement.

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. As the switching typically is in the audible frequency range, the resulting vibration produces loud noises (clicking, banging or beeping). This behaviour, of sound being generated by the vibration of the conducting components, is described as a coupled acousto-magneto-mechanical system, solutions to which provide useful insight to the behaviour of the scanners.[39] This is most marked with high-field machines,[40] and rapid-imaging techniques in which sound pressure levels may reach 120 dB(A) (equivalent to a jet engine at take-off),[41] and therefore, appropriate ear protection is essential for anyone inside the MRI scanner room during the examination.[42]

Radio frequency in itself does not cause audible noises (at least for human beings), since modern systems are using frequencies of 8.5 MHz (0.2 T system) or higher.[43]

Cryogens

As described in the Physics of magnetic resonance imaging article, many MRI scanners rely on cryogenic liquids to enable the superconducting capabilities of the electromagnetic coils within. Although the cryogenic liquids used are non-toxic, their physical properties present specific hazards.[44]

An unintentional shut-down of a superconducting electromagnet, an event known as "quench", 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 a 'quench pipe', it may be released into the scanner room where it may cause displacement of the oxygen and present a risk of asphyxiation.[45]

Oxygen deficiency monitors usually are used as a safety precaution. Liquid helium, the most commonly used cryogen in MRI, undergoes near explosive expansion as it changes from a liquid to gaseous state. The use of an oxygen monitor is important to ensure that oxygen levels are safe for patients and physicians. Rooms built for superconducting MRI equipment should be equipped with pressure relief mechanisms [46] and an exhaust fan, in addition to the required quench pipe.

Because a quench results in rapid loss of cryogens from the magnet, recommissioning the magnet is expensive and time-consuming. Spontaneous quenches are uncommon, but a quench also may be triggered by an equipment malfunction, an improper cryogen fill technique, contaminants inside the cryostat, or extreme magnetic or vibrational disturbances.[47][48]

Pregnancy

No effects of MRI on the fetus have been demonstrated.[49] As opposed to many other forms of medical imaging in pregnancy, MRI avoids the use of ionizing radiation, to which the fetus is particularly sensitive. As a precaution, however, many guidelines recommend pregnant women only undergo MRI when essential, especially during the first trimester.[50]

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. The use of gadolinium-based contrast media in pregnancy is an off-label indication and may be administered only in the lowest dose required to provide essential diagnostic information.[51]

Despite these concerns, MRI is rapidly growing in importance as a way of diagnosing and monitoring congenital defects of the fetus because it is able to 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.[52][53]

Claustrophobia and discomfort

Although painless, MRI scans may be unpleasant for those who are claustrophobic or otherwise uncomfortable with the imaging device surrounding them. Older closed bore MRI systems have a fairly long tube or tunnel. The part of the body being imaged must 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. Some modern scanners have larger bores (up to 70 cm) and scan times are shorter. A 1.5 T wide short bore scanner increases the examination success rate in patients with claustrophobia and substantially reduces the need for anesthesia-assisted MRI examinations even when claustrophobia is severe.[54]

Alternative scanner designs, such as open or upright systems, may be helpful where these are available. Although open scanners have increased in popularity, they produce inferior scan quality because they operate at lower magnetic fields than closed scanners. Commercial 1.5-tesla open systems have become available recently, however, providing much better image quality than previous lower field strength open models.[55]

Mirror glasses may be used to help create the illusion of openness. The mirrors are angled at 45 degrees, allowing the patient to look down their body and out the end of the imaging area. The appearance is of an open tube pointing upward (as seen when lying in the imaging area). Even though one is able to see around the glasses and the proximity of the device is very evident, this illusion is quite persuasive and relieves the claustrophobic feeling.

For young children who cannot hold still or would be frightened during the examination, chemical sedation or general anesthesia are the norm. Some hospitals encourage children to pretend the MRI machine is a spaceship or other adventure.[56] Certain hospitals with Children's wards have decorated scanners for this purpose, such as that at the Boston Children's Hospital, which operates a scanner with a special casing designed to resemble a sandcastle.[57]

Obese patients and pregnant women may find the MRI machine a tight fit. Pregnant women in the third trimester also may have difficulty lying on their backs for an hour or more without moving.

MRI versus CT

MRI and computed tomography (CT) are complementary imaging technologies and each has advantages and limitations for particular applications. CT is more widely used than MRI in OECD countries with a mean of 132 vs. 46 exams per 1000 population performed respectively.[58] A concern is the potential for CT to contribute to radiation-induced cancer and in 2007 it was estimated that 0.4% of current cancers in the United States were due to CTs performed in the past, and that in the future this figure may rise to 1.5–2% based on historical rates of CT usage.[59] An Australian study found that one in every 1800 CT scans was associated with an excess cancer.[60] An advantage of MRI is that no ionizing radiation is used and so it is recommended over CT when either approach could yield the same diagnostic information.[4] Although the cost of MRI has fallen, making it more competitive with CT, there are not many common imaging scenarios in which MRI can simply replace CT, however, this substitution has been suggested for the imaging of liver disease.[61] The effect of low doses of radiation on carcinogenesis also are disputed.[62] Although MRI is associated with biological effects, these have not been proven to cause measurable harm.[63]

Iodinated contrast medium is routinely used in CT and the main adverse events are anaphylactoid reactions and nephrotoxicity.[64] Commonly used MRI contrast agents have a good safety profile, but linear non-ionic agents in particular have been implicated in nephrogenic systemic fibrosis in patients with severely impaired renal function.[65]

MRI is contraindicated in the presence of MR-unsafe implants, and although these patients may be imaged with CT, beam hardening artefact from metallic devices, such as pacemakers and implantable cardioverter-defibrillators, also may affect image quality.[66] MRI is a longer investigation than CT and an exam may take between 20 and 40 minutes depending on complexity.[67]

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 originally was 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 and Healthcare products Regulatory 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 U.S. 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,[68] that includes physical and facility safety considerations.

The European Directive on electromagnetic fields

This Directive (2013/35/EU – electromagnetic fields) [69] covers all known direct biophysical effects and indirect effects caused by electromagnetic fields within the EU and repealed the 2004/40/EC directive. The deadline for implementation of the new directive was 1 July 2016. Article 10 of the directive sets out the scope of the derogation for MRI, stating that the exposure limits may be exceeded during "the installation, testing, use, development, maintenance of or research related to magnetic resonance imaging (MRI) equipment for patients in the health sector, provided that certain conditions are met." Uncertainties remain regarding the scope and conditions of this derogation.[70]

References

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