Synopsis

The feasibility of evaluating RF heating-induced temperature elevation in a human thigh in vivo during MRI was demonstrated . RF heating-induced temperature elevation in human thighs can be assessed in vivo during MRI. Reliable knowledge of RF heating allows safe and optimal utilization of MRI techniques. RF heating during MRI could change the observed mean diffusivity value and affects clinical interpretation of the results.

Abstract body

Purpose: Heating of patients or burning of biological tissues by RF power during MRI scan is a significant patient safety concern. We measured not only SAR values but also RF-induced temperature change at 1.5 and 3T MRI systems using human body-mimicking phantoms [1, 2]. However, the results in vivo could be completely different. The purpose of this study is to measure RF energy-induced temperature elevation in a human thigh in vivo using diffusion tensor imaging (DTI).

Method and Materials: Twenty-four normal males (35.4±8.6 years, 73.0±8.5 kg) participated in this IRB approved MRI study on a Magnetom Verio 3T scanner. The heating of thighs due to MRI scanning was assessed by DTI before and after low and high SAR image sequences as follows: 1 dummy DTI, 3×DTI, low SAR sequence, 3×DTI, high SAR sequence and 3×DTI, consecutively (SAR is the nominal value from the scanner). Image acquisition parameters are summarized in Table 1. A region-of-interest (ROI)-based diffusion coefficient calculation was performed (Fig.1). In data analysis, the average of mean diffusivity (MD) over 3 slices was obtained for each 3×DTI block. A 2.5% increase in MD approximately corresponds to 1°C temperature increase in skeletal muscles. In order to remove the effects of temperature differences among subjects, an offset adjustment of the MD was made to each subject so the average MD of all DTI acquisitions equals the group average. A paired two-tailed t-test for means was performed on MD and temperature changes (p<0.05 is significant).

Results: Mean ± SEM for diffusion coefficient and temperature change were (1.790±0.017)×10-3 mm²/s and 0.00±0.00 °C for the baseline, (1.789±0.018)×10^-3 mm²/s and -0.02±0.07 °C after the low SAR sequence (Time Point 2) and (1.820±0.003)×10^-3 mm²/s and 0.69±0.06 °C after the high SAR sequence (Time Point 3). The paired t-test showed significant difference in MD and temperature for baseline vs. Time Point 3 (p<0.0001) and for Time Point 2 vs. Time Point 3 (p<0.0002). Eight subjects(=33%) felt heating during the high SAR sequence.

Conclusion: We have demonstrated the feasibility of evaluating RF heating-induced temperature elevation in a human thigh in vivo during MRI. Bertelsmann et al [3] provided thermal discrimination thresholds in feet (0.15-0.30 ℃) and hand (0.15-0.50 ℃). RF heating-induced temperature elevation in human thighs can be assessed in vivo during MRI. Some heat appears to be carried away by blood circulation.

Clinical Relevance: Reliable knowledge of RF heating allows safe and optimal utilization of MRI techniques. RF heating during MRI could change the observed mean diffusivity value and affects clinical interpretation of the results.

Acknowledgements

References

1.Seo et al. “RF dosimeter for the measurement of specific absorption rate (SAR) in MRI,” RSNA 98th scientific assembly and annual meeting, Chicago, Illinois, 2012.

2.Seo Y and Wang ZJ, “MRI scanner-independent specific absorption rate measurements using diffusion coefficients,” J Appl Clin Med Phys 2017; 18(4):224-229.

3. Bertelsmann et al. “Thermal discrimination thresholds in normal subjects and in patients with diabetic neuropathy,” J Neurol NeuroSurg Psych 1985; 48:686-690.