On the potential for RF heating in MRI to affect metabolic rates and ¹⁸FDG signal in PET/MR: simulations of long‐duration, maximum normal mode heating

Abstract

Purpose To examine the possibility that MR-induced RF power deposition (SAR) and the resulting effects on temperature-dependent metabolic rates or perfusion rates might affect observed 18FDG signal in PET/MR. Materials and Methods Using numerical simulations of the SAR, consequent temperature increase, effect on rates of metabolism or perfusion, and [18FDG] throughout the body, we simulated the potential effect of maximum-allowable whole-body SAR for the entire duration of an hour-long PET/MR scan on observed PET signal for two different 18FDG injection times: one hour before onset of imaging and concurrent with the beginning of imaging. This was all repeated three times with the head, the heart, and the abdomen (kidneys) at the center of the RF coil. Results Qualitatively, little effect of MR-induced heating is observed on simulated PET images. Maximum relative increases in PET signal (26% and 31% increase respectively for the uptake models based on metabolism and the perfusion) occur in regions of low baseline metabolic rate (also associated with low perfusion and, thus, greater potential temperature increase due to high local SAR), such that PET signal in these areas remains comparatively low. Maximum relative increases in regions of high metabolic rate (and also high perfusion: heart, thyroid, brain, etc.) are affected mostly by the relatively small increase in core body temperature and thus are not affected greatly (10% maximum increase). Conclusions Even for worst-case heating, little effect of MR-induced heating is expected on 18FDG PET images during PET/MR for many clinically-relevant applications. For quantitative, dynamic MR/PET studies requiring high SAR for extended periods, it is hoped that methods like those introduced here can help account for such potential effects in design of a given study, including selection of reference locations that should not experience notable increase in temperature.

Integrated Positron Emission Tomography/Magnetic Resonance Imaging for Resting-State Functional and Metabolic Imaging in Human Brain: What Is Correlated and What Is Impacted

“…“Quieter” fMRI acquisition methods, such as sparse temporal sampling or interleaved silent steady state, could be applied to a less noisy background environment for BOLD-fMRI scan ( Andoh et al, 2017 ). In addition, MRI-induced RF power deposition and the resulting effects on temperature-dependent metabolic rates could also influence FDG uptake, with maximum relative increases of 26% for uptake models based on metabolism ( Carluccio et al, 2017 ). We speculate that these abovementioned factors synergistically influenced brain metabolism during the static phase of FDG uptake in our study.…”

Individualized and accurate SAR characterization method based on equivalent circuit model for MRI system

“…However, the RF field, time‐varying gradient field, and static magnetic field during the MRI scan might result in the potential harmful bio‐effects such as RF heating, peripheral nerve stimulation, force, and torque on ferromagnetic objects. Among them, excessive RF heating accumulated in the body could make the core temperature exceed the safety threshold, which associates with the patient discomfort, health deterioration, or potential tissue damage 1–6 . Given the potential hazards, the whole‐body specific absorption rate (SAR) is limited by international standards published by the International Electrotechnical Commission.…”

“…Among them, excessive RF heating accumulated in the body could make the core temperature exceed the safety threshold, which associates with the patient discomfort, health deterioration, or potential tissue damage. [1][2][3][4][5][6] Given the potential hazards, the whole-body specific absorption rate (SAR) is limited by international standards published by the International Electrotechnical Commission. For the medically diverse patients, such as pregnant women and patients with implants, there are more specific SAR limitations in the MR environment.…”