A prototype RF dosimeter for independent measurement of the average specific absorption rate (SAR) during MRI

Abstract

To develop a scanner-independent dosimeter for measuring the average radio frequency (RF) power deposition and specific absorption rates (SAR) for human MRI exposure. A prototype dosimeter has a transducer with orthogonal conducting loops surrounding a small signal-generating MRI sample. The loops contain resistors whose values are adjusted to load the scanner's MRI coils equivalent to an average head or body during MRI. The scanner adjusts its power output to normal levels during setup, using the MRI sample. Following calibration, the total power and average SAR deposited in the transducer are measured from the root-mean-square (rms) power induced in the transducer during MRI. A 1.5 Tesla head transducer was adjusted to elicit the same load as the average of nine adult volunteers. Once adjusted, the transducer loads other head coils the same as the head does. The dosimeter is calibrated at up to 20 W total deposited power and 4.5 W/kg SAR in the average head, with about 5% accuracy. This dosimeter provides a simple portable means of measuring the power deposited in a body-equivalent sample load, independent of the scanner. Further work will develop SAR dosimetry for the torso and for higher fields.