EMF exposure variation among MRI sequences from pediatric examination protocols

Abstract

The magnetic resonance imaging (MRI) exposure environment is unique due to the mixture and intensity of magnetic fields involved. Current safety regulations are based on well‐known acute effects of heating and neuroexcitation while the scientific grounds for possible long‐term effects from MRI exposure are lacking. Epidemiological research requires careful exposure characterization, and as a first step toward improved exposure assessment we set out to characterize the MRI‐patient exposure environment. Seven MRI sequences were run on a 3‐Tesla scanner while the radiofrequency and gradient magnetic fields were measured inside the scanner bore. The sequences were compared in terms of 14 different exposure parameters. To study within–sequence variability, we varied sequence settings such as flip angle and slice thickness one at a time, to determine if they had any impact on exposure endpoints. There were significant differences between two or more sequences for all fourteen exposure parameters. Within–sequence differences were up to 60% of the corresponding between‐sequence differences, and a 5–8 fold exposure increase was caused by variations in flip angle, slice spacing, and field of view. MRI exposure is therefore not only sequence‐specific but also patient‐ and examination occurrence‐specific, a complexity that requires careful consideration for an MRI exposure assessment in epidemiological studies to be meaningful. Bioelectromagnetics. 40:3–15, 2019. © 2018 The Authors. Bioelectromagnetics Published by Wiley Periodicals, Inc.

Magnetic Resonance Examinations of Patients With Implanted Active Devices. A Low‐Cost Approach in Slew Rate Evaluation

“…In particular, the “whisper” gradient mode improves patient safety, reducing SR p to 75.3 T m ‐1 s ‐1 . More generally, all experimental findings highlight, in accordance with previous studies [Wilén et al, ; Trevisan et al, ; Frankel et al, ], that not only different types of sequences, but also scan settings can affect SR p values. In particular, regardless of the type of sequence, the “fast” gradient mode is always associated with increased slew rate.…”

“…This second group of patients was examined with optimized MR sequences. Table denotes optimized sequences with “O.” Even though slew rate reduction is achievable by changing several scan parameters [Frankel et al, ], the present study focused only on gradient modalities for the sake of simplicity. Moreover, as pointed out by Frankel [], erroneous setting of field of view, repetition time, flip angle, etc., can severely affect the image quality.…”

Valid Exposure Protocols Needed in Magnetic Resonance Imaging Genotoxic Research

“…Usually during clinical scanning, as many as 10 different sequences can be used to get the appropriate information. Based on our measurements, the peak values for the RF B 1 can reach several µT [Frankel et al, 2018;Frankel et al, 2019]. As the duty cycle in some sequences can be as low as a few percent, the short-term SAR value within each pulse can be rather high, in comparison with the time-averaged values of entire sequences.…”

“…The static field is the easiest to test as the dosimetry is not so complicated. The RF field used in MRI is always pulsed, and the duty cycle can vary between sequences with an order of magnitude [Frankel et al, 2019]. With this follows a large variation in the SAR values calculated by the scanner system; here we can have values from a few hundredths to somewhat higher than 1 W/kg.…”

“…If the SGF is included in the hypothesis, the isocenter is not a recommended position for the test sample as the time derivative is the lowest in that position. If clinical protocols are used, one must be aware that they consist of many sequences, which often involve different exposures in terms of dB/dt, time, and number of repetitions [Frankel et al, 2019]. How to combine these very different exposures into an appropriate exposure metric is not straightforward.…”