Specific Absorption Rate and Specific Energy Dose: Comparison of 1.5-T versus 3.0-T Fetal MRI

Background: MRI performed at 3.0 T offers greater signal-to-noise ratio and better spatial resolution than does MRI performed at 1.5 T; however, for fetal MRI, there are concerns about the potential for greater radio frequency energy administered to the fetus at 3.0-T MRI. Purpose: To compare the specific absorption rate (SAR) and specific energy dose (SED) of fetal MRI at 1.5 and 3.0 T. Materials and Methods: In this retrospective study, all fetal MRI examinations performed with 1.5- and 3.0-T scanners at one institution between July 2012 and October 2016 were evaluated. Two-dimensional (2D) and three-dimensional (3D) steady-state free precession (SSFP), single-shot fast spin-echo, 2D and 3D T1-weighted spoiled gradient-echo (SPGR), and echo-planar imaging sequences were performed. SAR, SED, accumulated SED, and acquisition time were retrieved from the Digital Imaging and Communications in Medicine header. Data are presented as mean +/- standard deviation. Two one-sided tests with equivalence bounds of 0.5 (Cohen d effect size) were performed, with statistical equivalence considered at P< .05. Results: A total of 2952 pregnant women were evaluated. Mean maternal age was 30 years +/- 6 (age range, 12-49 years), mean gestational age was 24 weeks 6 6 (range, 17-40 weeks). A total of 3247 fetal MRI scans were included, with 2784 (86%) obtained at 1.5 T and 463 (14%) obtained at 3.0 T. In total, 93 764 sequences were performed, with 81 535 (87%) performed at 1.5 T and 12 229 (13%) performed at 3.0 T. When comparing 1.5- with 3.0-T MRI sequences, mean SAR (1.09 W/kg +/- 0.69 vs 1.14 W/kg +/- 0.61), mean SED (33 J/kg +/- 27 vs 38 J/kg +/- 26), and mean accumulated SED (965 J/kg +/- 408 vs 996 J/kg +/- 366, P < .001)were equivalent. Conclusion: Fetal 1.5- and 3.0-T MRI examinations were found to have equivalent energy metrics in most cases. The 3.0-T sequences,such as two-dimensional T1-weighted spoiled gradient-echo and three-dimensional steady-state free precession, may require modification to keep the energy delivered to the patient as low as possible. (C) RSNA, 2020