Youngseob Seo PhD, Presenter: Nothing to Disclose

Michael Craig Morriss MD, Abstract Co-Author: Nothing to Disclose

Alexander Ivanishev, Abstract Co-Author: Nothing to Disclose

Nancy Katherine Rollins MD, Abstract Co-Author: Nothing to Disclose

Zhiyue Jerry Wang PhD, Abstract Co-Author: Nothing to Disclose

Heating of patients by radiofrequency (RF) power in MRI is an important safety concern. An estimation of specific absorption rate (SAR) independent from the level indicated on the MRI scanner is desirable. SAR dosimeter has been described previously which employed digital oscilloscope to record the amplitude and waveform of the RF pulses. We developed a low-cost RF dosimeter measuring the change of resistance of a thermistor during scan.

A prototype 1.5T RF dosimeter was constructed. The B1 field was converted to electric voltages by a 3 × 3.8 cm2 copper-striped coil loop which was connected to a thermistor outside the magnet. The RF power dissipated in the thermistor caused its temperature to increase, leading to a change in the resistance. Using RF filtering circuitry, the DC voltage across the thermistor was measured under an applied DC current free of interference from the RF. The time constant of the reading in response to RF power change was adjusted through the RC constant of the circuit. The dosimeter was calibrated using continuous RF at 65 MHz with input voltages up to 1 volt. Heat deposition for 4 or 5 low-to-high SAR MRI sequences were measured on 2 scanners and the correlation between RF dosimeter reading and SAR levels calculated by the scanners was investigated. The baseline voltage prior to scan and the stabilized voltage during scan were measured and the difference between the 2 voltages was calculated for each SAR levels. The coil probe was placed on a water phantom bottle (15 cm diameter × 30 cm long) and the measurements at two orthogonal probe orientations were averaged for each sequence.

The meter reading changed linearly with the input power levels in the calibration range. The response time constant was 5 sec. The measured relative heat deposition increased linearly with the SAR values calculated by the 2 MRI scanners. Our findings indicate that one 1.5T MRI scanner had a 13% higher slope than the other, suggesting the former would generate more heat deposition than the latter if we would use the same sequence.

A novel scanner-independent prototype to measure RF power deposition was developed and tested. Relative RF power deposition can be readily measured with this dosimeter.

This prototype RF dosimeter can prevent patients from heating or buring.

Seo, Y, Morriss, M, Ivanishev, A, Rollins, N, Wang, Z, RF Dosimeter for the Measurement of Specific Absorption Rate (SAR) in MRI.  Radiological Society of North America 2012 Scientific Assembly and Annual Meeting, November 25 - November 30, 2012 ,Chicago IL. http://archive.rsna.org/2012/12030452.html