Olin Neuropsychiatry Research Center Magnetic Safety Information

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OVERVIEW OF THE EFFECTS OF THE 3.O TESLA ALLEGRA MRI

STATIC MAGNETIC FIELDS—
3. O TESLA is approximately 60,000 times the strength of earth’s magnetic pull. The magnetic effect will cause ferromagnetic metals to become a projectile in this environment. In addition, implants within the subject may have a translational torque or attraction. Discomfort or injury to the subject may occur. *In the 3 Tesla environment, the shape of the object will have an effect on the translational attraction for external and internal ferromagnetic objects. Consideration for the type of implant, its in vivo longevity and its shape must be made before bringing a subject into the magnetic field.

GRADIENT MAGNETIC FIELDS—occurs when there is rapid alteration in the gradient coils to provide spatial localization (tells the scanner where to scan).
Interference with peripheral nervous system or muscle stimulation may occur using the EPI (echo planar imaging) sequences due to greater gradient amplitudes. To prevent this effect, either, a) ensure a closed circuit is not made by the subject. By crossing the arms or legs or b) place a blanket/sheet between any types of skin-to-skin areas.
Acoustic Noise also occurs from the magnetic gradients. During EPI or 3D MPRAGE the highest decibels of up to 113 dB (peak) is produced. It is recommended by Shellock et al. that ear plugs and head phones be worn simultaneously in the 3T environment. In addition, foam pillows and cushions will be provided to further dampen the intense noise made by the EPI sequences of the scanner.

RADIOFREQUENCY (RF) ELECTROMAGNETIC FIELDS—this is the energy used to excite the tissue protons to be imaged.
Tissue heating may occur from the absorption of the RF energy. The term that is used for this phenomenon is Specific Absorption Rate (SAR), which is expressed in Watts/kg. The average SAR the given patient may absorb, total body is 1.5 Watts/kg with the maximum allowed being 3.0 Watts/kg. The scanner itself will help the investigator/scanner determine if a factor needs to be changed to keep the SAR within limits. ALWAYS PUT IN THE SUBJECT’S CORRECT WEIGHT, OTHERWISE THE SAR LIMITS WILL BE INCORRECT.
Electrical burns may also occur due to RF fields by producing electrical currents in conductive loops. Any type of wire or cable must not be looped with itself or with the subject or a severe burn may occur.

OTHER CONCERNS

*SHORT BORE SCANNERS—Because of the different position and magnitude of gradient amplitudes, short bore magnets will produce a higher deflection angle measurement for ferromagnetic and non-ferromagnetic materials and implants. Thus, a greater “pull” may be exerted on the object in question. Shape of the implant is also important in a 3T short bore scanner. A greater force is exerted on an object that is oblong or elongated in configuration.

Fringe Fields—the magnet itself is 3.0 Tesla at the center of the bore. Because it is actively shielded (there is an opposite magnetic gradient, and steel circumferencing the main magnetic field to keep it somewhat “contained”) the effect of the magnet actually “fades” exponentially as the distance grows from the center. Effectively, we measure the 5 gauss line (.005T) as the line in which persons that have not been screened or are not MR compatible may not cross over. These people include those with aneurysm clips, cardiac pacemakers, defibrillators, etc. Also, magnetic strip cards will be erased pass this “magical” line. At the Olin Center, this line lies in the surrounding walls of the scanner and room.

Magnetophosphenes—in magnets with higher static fields than 2.0 T, physiological effects can be felt such as vertigo, headache, visual sensations, metallic taste and nausea. These symptoms seem to be transient and do not have a harmful biological effect. The theory behind these effects is that rapid movement through the strong static magnet field may induce low local voltages and currents within the brain and/or cranial nerve areas. Once the person stops moving, or they have been sensitized to the effect, the symptom(s) stops.

Pregnancy—there are no known adverse effects of MRI on developing fetuses. However, there are discrepancies within the scientific community as to the validity of these findings. According to Dr. Shellock, *there may be discrepancies from different studies as to the effects on the fetus in an MRI scanner explained by the variations, intensities and timing of exposures within the MRI studies. For more information, please see “Reference Manual for Magnetic Resonance Safety” the 2003 edition, pages 57-60.
At the Olin Center, pregnant subjects will be excluded, and pregnant patients will follow Hartford Hospital guidelines for pregnant patients, including a signed consent and Radiologist follow-through.

Additional information—for up-to-date safety information, please see MRIsafety.com.

* Shellock, FG Reference Manual for Magnetic Resonance Safety, 2003 Edition, Amirsys Inc., Salt Lake City, Utah, pp. 4; 57-60; 98-103. Shellock, FG & Kanal, E, Magnetic Resonance: Bioeffects, Safety, and Patient Management, 2nd Edition, Lippincott-Raven, Philadelphia, 1996, chapters 1-3; 5.

Director—Godfrey Pearlson, M.D.
Contact—Safety Officer: Julie A. Raulukaitis, B.S. RT (M)