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

Abstract

Purpose: To develop a scanner-independent dosimeter for measuring the average radio frequency (RF) power deposition and specific absorption rates (SAR) for human MRI exposure.

Materials and methods: 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.

Results: 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.

Conclusion: 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.

Figure 1

Photograph (a) and circuit diagram for each loop (b) of the 1.5T head SAR dosimeter transducer. R_L is the total value of the load resistance (250 Ω) divided into a fixed reference resistor, R_R and a variable resistor used to match the Q_L of the transducer to that of the head.

Figure 2

Induced RF voltage measured across the loop load resistor, as a function of RF voltage applied to the MRI head coil.

Figure 3

a: Block diagram of the experimental setup used for calibration. The frequency synthesizer (a PTS 160) connects to an ENI MRI-2000 RF power amplifier, whose output is monitored by a Bird 4410A RF Wattmeter at the input to the quadrature hybrid on the MRI head coil. Voltage across the transducer reference resistors are monitored by a 200 MHz oscilloscope, which is also used to cross-check the input power. b: Block diagram of the proposed SAR dosimeter, with readout in W/kg.

Figure 4

Unloaded (crosses) and loaded 1.5T head coil Qs loaded with the human head (diamonds), and with the dosimeter (triangles) adjusted to provide a loaded Q equal to one of the authors for comparing the loaded Qs with the head and dosimeter in different head coils.

Figure 5

Power and SAR measurements from the prototype SAR dosimeter. The measured output power from the RF amplifier (blue diamonds and line of best fit), the measured input power to the birdcage coil (incident minus reflected power; green squares and line), and the total power deposited in the transducer as deduced from Eq. [1] (cyan crosses and line), all as a function of the power measured on the dosimeter load R_L on the horizontal axis. The cyan curve relating actual-to-measured power in the dosimeter, is used for calibration. The red line (red triangles) plot the SAR (vertical axis at right) for the average head in W/kg, as a function of the power measured in R_L on the horizontal axis. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]