Fetal MRI: Radiofrequency Safety Assessment at 3 Tesla.

Abstract

Background: 3-T MRI can improve image quality of fetal imaging compared to 1.5-T MRI. However, concerns exist regarding increased local tissue heating at 3-T.

Purpose: To assess fetal MRI radiofrequency (RF) safety at 3-T by comparing simulated tissue heating to 1.5-T (using constant RF exposure) and by simulating tissue heating at 3-T using RF exposures from clinical fetal examinations.

Study type: Retrospective.

Population: Seven voxelized anatomical pregnant body models (gestational age [GA] 30 ± 3 weeks [mean ± standard deviation], maternal body mass index [BMI] 27.8 ± 8.5 kg/m²) were used. Maternal whole-body average specific absorption rate (wbSAR) logs were collected from 85 clinical examinations at 3-T (GA 25 ± 6 weeks, BMI 30.3 ± 6.8 kg/m²).

Field strength/sequence: 3-T, 1.5-T, HASTE, VIBE, TRUFISP, EPI, DTI.

Assessment: Simulated maternal and fetal peak and average SAR, temperature, and peak thermal dose were compared at 3-T and 1.5-T for 60 min 2 W/kg wbSAR using 7 body models and a 16-rung band-pass RF coil. Temperature and thermal dose were simulated in one body model using clinical wbSAR exposures at 3-T.

Statistical tests: Factorial analysis of variance was performed using 28 maternal and fetal temperature measurements from 7 body models to detect a difference between 3-T and 1.5-T. p < 0.05 was considered statistically significant.

Results: For constant RF exposure, we found no difference between 3-T and 1.5-T in peak maternal (1.5-T:40.38 ± 0.21°C; 3-T:40.40 ± 0.20°C; p = 0.85), peak fetal (1.5-T:39.21 ± 0.17°C; 3-T:39.09 ± 0.16°C; p = 0.19), and average maternal (1.5-T:37.32 ± 0.05°C; 3-T:37.33 ± 0.04°C; p = 0.68) temperature. We observed significantly higher average fetal temperatures at 1.5-T (1.5-T:37.75 ± 0.06°C; 3-T:37.70 ± 0.05°C). For 3-T clinical RF exposures, simulated peak temperatures exceeded the recommended limits. However, the thermal dose was below the recommended limit.

Data conclusion: For the same RF coil geometry, local heating was similar at 3-T and 1.5-T for constant RF exposure. Although realistic 3-T RF exposures could cause peak temperatures above the recommended limits, thermal dose was below the recommended limit.

Evidence level: 1.

Technical efficacy: Stage 1.