Session 11

The session on high power, pulsed radio-frequency fields consisted of six papers that addressed issues of dosimetry, cellular effects, and thermophysiological effects that are associated with the radio-frequency (RF) signals used in nuclear magnetic resonance imaging (MRI) or spectroscopy (MRS). These papers provided a stimulating, insightful, up-to-date overview of this particular area of bioelectromagnetics. Four of the papers were concerned with theoretical dosimetry and employed numerical methods to predict the specific absorption rate (SAR) in the body during MRI. The other two papers were concerned with the biological responses to these fields in cells or in animals and humans where their effect on thermal physiology is of primary importance. The dosimetry papers included two presentations [(i) Gandhi and Chen and (ii) Grandolfo et a/ .] of research using numerical analysis of a complex, anatomically realistic model of the human body composed of several thousand individual cells or compartments (typically 1-1.5 cm in length). This modeling technique is known as the “impedance method” because each cell is characterized by its electrical impedance. Two other theoretical dosimetry papers [(i) Bottomley and Roemer and (ii) Boesiger ef al.] were based on numerical analyses of simpler geometric models of spheres and cylinders. Initial work in the field of dosimetry of MRI used the simpler geometric models, whereas the application of the impedance method to MRI is a new contribution. The individual papers discussed the assumptions, strengths, and weaknesses of each method. These dosimetry methods are directed toward the analysis of the primary known effect of radio-frequency exposure, namely, heating of tissues. The techniques must take into account or make assumptions for the many complex factors affecting the absorption of RF energy by the body, including frequency, variations in time and space of the intensity of the magnetic field, coupling efficiency between the R F coil and the body, duty cycle and waveform of the specific pulse sequence used in imaging, electrical properties of different tissues, geometry, and orientation of the body with respect to the polarity of the field. All of the authors deal with both average and local (or peak) S A R s for various MRI conditions. Considerations of local SARs in different regions of the body (e.g., skin) are the most difficult to determine and thus are the ones that received the most discussion. The impedance method has the capability to provide much more detailed information about internal current distribution and local S A R than the simpler geometric models. However, one of the points that stimulated the most discussion, in this session, was the difference of opinion over the significance of eddy currents in determining the SAR. Bottomley and Roemer presented challenging arguments