ISUOG Guidelines for fetal MRI: a response to 3‐T fetal imaging and limited fetal exams

We are writing in reference to the recently published ISUOG guidelines for the performance of fetal magnetic resonance imaging (MRI)1, which we applaud as an important tool for healthcare practitioners. In the section entitled ‘How should fetal MRI be performed?’, the merits of 1.5 Tesla (T) vs 3 T are discussed and it is stated that ‘higher field strength (i.e. 3 T) is currently not recommended for in-vivo fetal imaging’. We would like to express our disagreement with this statement. As its only reference of support, the guideline cites a paper from the Society of Obstetricians and Gynecologists in Canada, which in fact endorses the use of 3-T MRI, as follows: ‘fetal MRI is safe at 3.0 Tesla or less during the 2nd and 3rd trimesters’2. This Canadian guideline is recognized by the Agency for Health Research and Quality (AHRQ) as an evidence-based guideline. The AHRQ is a branch of the United States Department of Health and Human Sciences, charged with improving the safety and quality of America's healthcare system3. The theoretical advantages of 3-T fetal imaging are recognized in the ISUOG guidelines, which state that ‘3 T has the potential to provide imaging with higher resolution and better signal-to-noise ratio than does 1.5 T, while maintaining a comparable or lower energy deposition’, as illustrated in Figure 1. In fact, the higher resolution of 3 T compared with 1.5 T has been demonstrated objectively, in two recent publications from acknowledged centers of excellence in fetal MRI4, 5, to improve evaluation of fetal anatomical structures. Safety concerns for fetal MRI relate largely to the specific absorption rate (SAR), a measure of the energy deposition into tissue. Regulations in both Europe and the United States have been put in place, defining the maximum acceptable SAR values. As a result, all MR scanners used for clinical examinations have fail-safe mechanisms to halt the scanner when SAR regulatory limits have been reached, a safety mechanism that is independent of the magnetic field strength used and applies to both 1.5-T and 3-T magnets. It is critical to acknowledge that, to date, no safety concerns have been scientifically proven with respect to 3 T compared with 1.5 T for fetal imaging, as long as the MRI scanner is operated in ‘normal mode’. We are concerned that the recommendations put forward by the ISUOG guideline regarding 3-T fetal imaging will cause unwarranted alarm in the minds of pregnant patients and referring clinicians, resulting in reluctance to utilize 3-T MRI. We request that the esteemed ISUOG panel considers retracting the guideline statement that higher field-strength MRI is not recommended for fetal imaging and issue an updated guideline discussing the known safety data and the proven advantages of 3-T MRI in addition to 1.5-T MRI for fetuses in the second and third trimesters of pregnancy. The ISUOG guideline also notes that fetal MRI may be performed as a limited examination. We would like to stress that, in the United States, in order to bill for fetal MRI via the current procedural terminology (CPT) code, the entire fetus and pregnancy should be imaged.

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