The magnetic resonance imaging (MRI) physician’s mantra has long been that MRI is without “side effects” and in particular without the terrible side scourge of computed tomographic scanning: radiation. The lack of ionizing radiation with MRI is usually compelling compared to the known deoxyribonucleic acid damage that occurs following high doses of x-ray irradiation. New magnetic resonance scanners routinely operate at 3 T, and much more powerful scanners could potentially be approved by the US Food and Drug Administration, up to 8 T . Human MRI scanners operating up to 12 T have been manufactured, yet the impact of these powerful magnetic fields on the human body remains relatively unknown.
In the study by Schlamann et al , the technique of transcranial magnetic stimulation (TMS) was used to excite the motor cortex of normal research subjects prior to and after MRI of the brain performed in the same subjects both at 1.5 and 7 T. In TMS, a local varying magnetic field is applied to the brain that induces a current. If applied to the motor cortex, this current activates neuronal pathways and will affect peripheral nerves related to the stimulated portion of the motor cortex. The TMS technique used by Schlamann et al used two outcome parameters to detect brain motor cortex function: the motor threshold and the silent period. The motor threshold is a measure of the level of energy necessary to evoke a response in the associated peripheral nerve. After activation of the peripheral nerve, voluntary muscle activity is temporarily lost during a period of postexcitatory inhibition, termed the silent period. The investigators asked subjects to press their thumb and forefinger together. The silent period was measured as the time between TMS stimulation and the recovery of voluntary muscle contraction, as measured by electromyography, which involves the insertion of small needles into the muscle to record electrical muscle activity.
After TMS and measurement of the motor threshold and silent period duration, the subjects underwent MRI of the brain at 1.5 T, followed by repeat TMS testing. The entire procedure was repeated at 7 T using an MRI protocol designed to have similar overall energy deposition at both field strengths.
Schlamann et al indicate that after MRI at 1.5 T, the silent period was prolonged by an average of 36%. The motor threshold also increased, between 1% and 23% in 10 of 12 subjects. Values returned to normal within 15 minutes of MRI. After MRI at 7 T, the silent period was prolonged by an average of 33%. The motor threshold increased between 5% and 28% in 8 of 12 subjects. The authors conclude that the change in the TMS parameters was due to the intervening MRI scan. They further conclude that the TMS-detected changes were independent of field strength. Volunteers who were simply lying in the MRI scanner without undergoing scanning showed no changes in their TMS parameters. Together, these results suggest alterations of motor cortex function possibly due to either the gradient fields of the MRI scanner or radiofrequency energy deposition, as opposed to the powerful static magnetic field.
These results of Schlamann et al are unusual compared to those of other studies in the literature. Other studies assessing the effects of MRI scanning suggest that higher field strengths (≥3 T) are associated with an increased rate of transient side effects than lower field strengths such as 1.5 T . Side effects may include vertigo, light flashes, and perhaps muscle twitching. On the other hand, cognitive function was reportedly unchanged after exposure to a 0.05-T versus an 8-T static magnetic field .
What are the differences between an MRI scanner at 1.5 T and on at 7 T? The powerful static magnetic field interacts with the human body at both the molecular and cellular levels. Electric currents are produced in the body as a patient moves though the static magnetic field walking from the edge of the scan room to the gantry table. Rapid patient motion can result in symptoms of vertigo or nausea. When a patient is lying on the MRI table in a strong magnetic field, flowing blood results in the production of small currents, the so-called hydrodynamic effect. This effect is well known to cardiovascular radiologists because it alters the shape of the electrocardiogram measured during the MRI scan.
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References
1. Center for Devices and Radiological Health. Guidance for industry and FDA staff: criteria for significant risk investigations of magnetic resonance diagnostic devices. Rockville, MD: US Food and Drug Administration.
2. Schlamann M., Yoon M.-S., Maderwald S., et. al.: Short term effects of magnetic resonance imaging on excitability of the motor cortex at 1.5T and 7T. Acad Radiol 2010; 17: pp. 277-281.
3. Theysohn J.M., Maderwald S., Kraff O., et. al.: Subjective acceptance of 7 Tesla MRI for human imaging. MAGMA 2008; 21: pp. 63-72.
4. Chakeres D.W., Bornstein R., Kangarlu A.: Randomized comparison of cognitive function in humans at 0 and 8 Tesla. J Magn Reson Imaging 2003; 18: pp. 342-345.