Brief applications of radio frequency electromagnetic fields are used during imaging to excite the spins and tilt them from their alignment with the static magnetic field. The absorbed energy from these radio frequency fields are later emitted by the spins at the same frequency, and these emitted radio frequency waves are the signals that we sample to create MR images. However, the coil transmits more radio frequency energy during excitation than is later emitted by the spins. This excess energy can penetrate the superficial surfaces of the body and become absorbed by the body's tissues. It is dissipated in the form of heat - either through convection, conduction, radiation, or evaporation. Thus, a potential concern in MRI is the heating of the body during imaging. Shellock(2000) has provided an excellent recent review of RF heating in MRI.
The Specific Absorption Rate (SAR) is the measure of the absorption of electromagnetic energy in the body (typically in watts per kilogram, or W/kg). The rate of absorption of RF energy depends upon the RF frequency and the size, geometry and conductivity of the absorbing object. The frequency of the RF used in imaging is dependent upon the static field strength (for H1, Radio Frequency = 42.58 MHz * Field Strength in Tesla). A frequency of 63.9 MHz is used to excite H1spins at 1.5 Tesla, and a frequency of 170.3 MHz is used at 4 Tesla. These frequencies are in the VHF or television range. Longer RF pulses (180�) deposit more RF energy than shorter pulses (90�), and SAR is greater for pulse sequences that employ many RF pulses per unit time (such as fast spin echo) than those that employ fewer (such as gradient echo EPI).
The SAR of imaging is limited to cause less than a one-degree (�C) temperature rise in core temperature. Higher frequencies are more energetic than lower frequencies, so there is a greater potential for heating at higher static field strengths. The original FDA MRDD (Magnetic Resonance Diagnostic Device) Guidelines stated a level of concern related to whole body RF heating of 0.4 W/kg. It was later found that operation at up to 4 W/kg was possible without incurring a core temperature rise of one-degree C. As a result, MRDDs have been cleared for market operating at up to 4 W/kg since reclassification (U.S. Department Of Health and Human Services, 1998). Operation above this level requires an approved human studies protocol under the IEC standard. Experimental studies by Shellock and colleagues (1994) exposed healthy subjects to a SAR of 6 W/kg while measuring a variety of physiological indices and temperatures. All measures stayed within physiological safety limits, indicating that the current standards have a considerable margin of safety for healthy individuals.
To avoid overheating any local area, the product of time and local SAR should not exceed (in Watts per minute per kilogram):
- 60 Wmin/kg averaged over the head
- 120 Wmin/kg averaged over the trunk
- 180 Wmin/kg averaged over the extremities,
provided that the instantaneous SAR does not exceed:
- 4 W/kg averaged over the head
- 8 W/kg averaged over the trunk
- 12 W/kg averaged over the extremities.
Actually determining SAR is difficult, and depends upon various models of heat conduction and body geometry. The calculation depends on the subject's weight and thus it is important to enter the subjects weight as correctly as possible. Subjects regulate heat dissipation through perspiration and blood flow changes, so it is important that the subject be made comfortable (bore fan, blanket if requested) and the subject be asked frequently how they are doing. For exposures to infants or pregnant women, a reduction of those values by a factor two is recommended by the FDA. Thermoregulation is impaired in patients with cardiocirculatory impairments, cerebral vascular impairment, and diabetes, and thus SAR should also be lowered. The FDA recommends 1.5 W/Kg for all such compromised patients.
SAR limits are enforced by software routines within the pulse sequence program (PSD) and by power monitors on the scanners that limit the duty cycle of the RF waveform delivered to the coils by the PSD.
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