Fetal MRI: maintaining safe temperature

Background Fetal MRI is increasingly used in clinical settings. Ongoing vigilance regarding safety is essential. Medicines and Healthcare Products Regulatory Agency (MHRA) 2007 limits whole body specific absorption rate (SAR) to 2W/Kg (normal mode) during MRI. Estimated fetal temperature must not exceed 38°C, assuming fetal-maternal temperature gradient ∼0.5°C. Temperature rise depends on the specific sequences used and length of image acquisition. Aim To audit maternal temperature during fetal MR scans in accordance with recommended safety guidance. Methods A prospective study of 100 women undergoing clinical and research fetal MR scan. Our typical protocol includes standard T2-weighted single-shot images (SAR 2 W/kg). Tympanic temperatures were measured within 5 min of the start and end of scanning and length of time in the scanner was documented. Results Mean pre and post-scan temperatures were 36.6°C (35.4–37.3°C) and 37°C (36–37.9°C) respectively. The mean temperature increase was 0.4°C (0–1.3°C). While an increase exceeding 1°C occurred in 3/100, none resulted in post-scan temperatures over 37.5°C. Six participants had a post-scan temperature above 37.5°C; of these 4 temperatures were above 37°C at the start of the scan. Mean length of time in the scanner was 59 min (25–110 min) of which approximately 75% involves image acquisition, according to our typical protocol. Conclusions Mean temperature rises during long MR examinations are acceptable. Screening maternal temperature pre-scan identifies women in whom fetal temperature is most likely to exceed 38°C. Women with pre-scan temperatures over 37°C should be imaged cautiously, with intervals between acquisitions to avoid heat absorption.