Introduction to MR bioeffects and safety

I. Bioeffects

A. Effects of static magnetic fields

Innumerable biologic effects have been associated with static magnetic fields (3), however none have been proven to be clinically significant. However, further research is needed in this area.

B. Effects of time varying magnetic fields (gradient fields)

Whenever there is time varying magnetic flux through a conducting loop an electric current is induced (Faraday's law of electromagnetic induction). In clinical imaging the conductor is the human body and the changing magnetic fields are the gradient fields. Small currents can therefore be induced in the body. The current induced during a typical clinical MR exam is estimated to be up to 5 µA / cm2(3).

The thermal (heating) effects of these small currents are negligible (3). However, there is a theoretical risk of direct effects including visual flashes (due to stimulation of the optic nerve), stimulation of nerve cells and muscle cells, and ventricular fibrillation. The current thresholds needed for all of these effects are much higher than the current induced in a clinical MR exam except for visual flashes which are estimated to have a threshold current of 17 µA /cm2. Visual flashes have been observed in systems with field strengths greater than 1.95 T (3).

One interesting effect of the current induced by the gradient fields is stimulation of bone healing much like that seen with implanted bone stimulators (3). However, repeated applications over a period of days or weeks would be needed to produce a significant effect.

C. Effects of radiofrequency radiation

The radiofrequency (RF) pulses used in MR can also induce currents in the body. The most significant effect of these currents is tissue heating. The amount of tissue heating depends on multiple variables including the absorbed power from the RF pulses, pulse sequence type, type of coil used, volume of tissue imaged, and multiple physiologic and environmental variables (3). The amount of body heating is greatest at the surface of the body and falls nearly to zero at the body core. Due to efficient heat loss mechanisms in the skin, the greatest heating actually occurs in the subcutaneous tissue (3).

The testis and eye are unusually susceptible to heat related damage, however no damage has been observed at the RF power levels used in clinical MR imaging (3).

One article (Shuman et al. Radiology1988; 167: 551-554) reported significant increases in temperature of internal organs in anesthetized dogs during an MR exam. These findings probably don't relate to conscious adults due to size differences but could have implications for imaging studies involving sedated children (3). Again more study is indicated.

D. Psychological effects and noise

2-10% of patients undergoing MR examination experience disturbing psychological effects such as panic, anxiety, claustrophobia, or depression (3). Two cases have been reported describing patients developing permanent debilitating claustrophobia following an MR exam. These reactions can be prevented to some extent by the following techniques:

1) Patient education regarding what to expect inside the magnet (physical dimensions, noise, duration)

2) Allowing a family member to remain in the room

3) Maintaining verbal / physical contact

4) Placing the patient in the prone position

5) Mirrored glasses /blindfold

Several instances of temporary and permanent hearing loss have been reported after MR examination. Acoustic noise within the magnet bore can reach 95 dB (3). This is equivalent to very heavy traffic noise or light road construction noise. Earplugs are the simplest mitigation device.

E. Pregnancy

According to the FDA the safety of MR in imaging the fetus has not been established (5). For this reason, as well as legal risk, MR imaging in the first trimester is relatively contraindicated. As with any imaging study however consideration should be given to the risks and benefits to the patient. Some authors feel that MR imaging during pregnancy may be indicated if other imaging modalities have not provided a diagnosis and the alternative test would involve ionizing radiation.

II. Foreign material or surgically implanted objects

A. Devices

Any mechanical, electrical, or magnetic implanted device is a contraindication to MR examination (2,5). This is due to the risk of device dysfunction, induction of electric currents, and possible device movement. A list of contraindicated objects includes:

1. Cardiac pacemakers

2. Implanted cardiac defibrillators

3. Cochlear implants

4. Neurostimulators

5. Bone stimulators

6. Drug infusion pumps

7. Thermodilution Swan-Ganz catheters

In regard to cardiac pacemakers, reprogramming can occur or the device can switch into anasynchronous operation mode due to reed switch closure. In addition, the wire leads may act like antennae and an electrical current can be induced which could lead to pacing of the heart at an abnormally high rate, ventricular fibrillation, or thermal injury. Even if a patient with an implanted device has safely undergone an exam in the past it doesn't preclude possible patient injury when imaged on a different system. Epicardial pacer leads left in after coronary bypass are also considered a contraindication by some authors due to the risk of induced currents (5).

B. Ferromagnetic objects

Ferromagnetic objects are those in which a strong magnetic field may be induced when they are exposed to an external magnetic field. The greater the ferromagnetism of an object, the greater the attractive force generated between the object and the magnet (3). The attractive force upon a ferromagnetic object is dependent on a variety of factors including the strength of the magnetic field, distance to the object, and the geometry of the object (3). The strength of the magnetic field falls off with distance. The field strength change is non-linear and depends on the shielding (if any) in the magnet housing (1,5). Obviously, if the forces are sufficient a ferromagnetic object may gain enough kinetic energy to become a dangerous projectile and can cause injury to patients or staff.

Surgical clips are generally ferromagnetic and thus theoretically could be displaced by the magnetic field. In most locations in the body, surgical clips are encased by fibrotic tissue after a period of time. After this occurs, MR imaging is considered safe. The time it takes for this fibrotic tissue to develop is probably variable from patient to patient and there is no clear-cut time after which it is clearly safe to image. The decision as to whether or not to image is therefore decided on a case by case basis.

The exception is in the central nervous system where fibrosis around clips doesn't occur. Therefore it is imperative that all aneurysm clips are certified as MR compatible or preferably are tested prior to insertion to prevent movement in the scanner and possible intracranial hemorrhage. Clips can vary in composition and therefore degree of ferromagnetism from lot to lot, therefore actually testing clips prior to insertion is the safest approach (3).

Possible occult ferromagnetic objects (intraocular metal fragments, shrapnel, bullets, etc.) become dangerous if sufficient attractive force is generated to displace them and they are in a biologically sensitive location (eye, spine, heart, etc.) (5).

One case has been reported in the medical literature describing the development of blindness (secondary to vitreous hemorrhage) in a patient with an unsuspected2.0 X 3.5 mm metal fragment, which moved within the eye during an0.35T MR exam. Fragments as small as 0.1 x 0.1 x 0.1 mm can be detected within the eye on plain radiographs (5). As the risk of movement of intraocular foreign bodies appears to correlate with size, it is felt that if the patient is asymptomatic and plain radiographs show no foreign body it is safe to image with MR (5). If a metallic foreign body is discovered by plain film, MR imaging is contraindicated. Metal objects in other locations have to be assessed based on a risk-benefit analysis.

C. Other surgically implanted devices

The bottom line on any other metal implant is that it has to be looked up in a reference book if there is a question regarding MR compatibility. Reference books are readily available in the Seimens control room. Some general guidelines follow:

The majority of orthopedic implants are non-ferromagnetic and can be safely imaged by MR (6). An exception is the Perfix interference screw, which is ferromagnetic and generates severe artifacts, which may render the scan uninterpretable.

Prosthetic heart valves are almost all ferromagnetic, however the deflection forces generated are usually small. These implants are therefore generally considered safe for MR imaging (6), however caution is advised in patients with suspected valve dehiscence (3).

Intravascular stents, coils, and filters vary in their ferromagnetic properties. These objects are thought to become firmly embedded in the vessel wall after about6 weeks. Even those objects that are ferromagnetic are safe to mage by MR 6 weeks after placement (6).

D. Tattooed makeup

Tattooed eyeliner is a relative contraindication to MR examination. A small number of patients have reported temporary skin irritation or swelling related to interaction of iron containing pigments in the eyeliner with the magnetic field (4). I could find reference to no instances of actual eye damage in such circumstances.

References

1) Gangarosa RE, Minnis JE, Nobbe J, Praschan D, GenbergRW. Operational Safety Issues in MRI. Magnetic Resonance Imaging1987; 5: 287-292.

2) Kanal E, Shellock FG, SMRI Safety Committee. Policies, Guidelines, and Recommendations for MR Imaging Safety and Patient Management. JMRI1992; 2: 247-248.

3) Kanal E, Shellock FG, Talagala L. Safety Considerations in MR Imaging. Radiology1990; 176: 593-606.

4) Shellock FG, Kanal E, SMRI Safety Committee. Guidelines and Recommendations for MR Imaging Safety and Patient Management III. Questionnaire for Screening Patients before MR Procedures. JMRI 1994; 4: 749-751.

5) Shellock FG, Kanal E, SMRI Safety Committee. Policies, Guidelines, and Recommendations for MR Imaging Safety and Patient Management. JMRI1991; 1: 97-101.

6) Shellock FG, Morisoli S, Kanal E. MR Procedures and Biomedical Implants, Materials, and Devices:1993 Update. Radiology1993; 189: 587-599.

M.P. Evitts D. O. 1/5/99