Physiological effects of human body imaging with 300 mT/m gradients.

Ralph Kimmlingen,Chantal M W Tax,Fabrizio Fasano,Malwina Molendowska,Derek K Jones,Slawomir Kusmia,Umesh Rudrapatna,C John Evans

doi:10.1002/mrm.29118

Abstract

The use of high-performance gradient systems (i.e., high gradient strength and/or high slew rate) for human MRI is limited by physiological effects (including the elicitation of magnetophosphenes and peripheral nerve stimulation (PNS)). These effects, in turn, depend on the interaction between time-varying magnetic fields and the body, and thus on the participant's position with respect to the scanner's isocenter. This study investigated the occurrence of magnetophosphenes and PNS when scanning participants on a high-gradient (300 mT/m) system, for different gradient amplitudes, ramp times, and participant positions. Using a whole-body 300 mT/m gradient MRI system, a cohort of participants was scanned with the head, heart, and prostate at magnet isocenter and a train of trapezoidal bipolar gradient pulses, with ramp times from 0.88 to 4.20 ms and gradient amplitudes from 60 to 300 mT/m. Reports of magnetophosphenes and incidental reports of PNS were obtained. A questionnaire was used to record any additional subjective effects. Magnetophosphenes were strongly dependent on participant position in the scanner. 87% of participants reported the effect with the heart at isocenter, 33% with the head at isocenter, and only 7% with the prostate at isocenter. PNS was most widely reported by participants for the vertical gradient axis (67% of participants), and was the dominant physiological effect for ramp times below 2 ms. This study evaluates the probability of eliciting magnetophosphenes during whole-body imaging using an ultra-strong gradient MRI system. It provides empirical guidance on the use of high-performance gradient systems for whole-body human MRI.

Highlights

Technical advances in gradient performance have led to tremendous improvements in MR imaging
These visual effects were more widely reported when the gradient amplitude was above 150 mT/m, where the gradient ramp times are longer
For the highest gradient amplitudes, when the corresponding gradient stimulus ramp times were above 3 ms, magnetophosphenes were reported by up to 86.7% of the participants; for lower gradient amplitudes, when ramp times were below 3 ms, fewer than 30% of participants reported magnetophosphenes

Summary

Introduction

Technical advances in gradient performance (including the availability of higher amplitude gradients) have led to tremendous improvements in MR imaging. Various high-performance head-only1–6 and whole body[7,8] gradient systems have been developed, which confer performance benefits for microstructural imaging9–11 in research and clinical studies.12–16 rapid switching of ultra-strong gradient systems produces rapidly time-varying, strong, magnetic fields which have physiological effects on the human body. These phenomena effectively limit the extent to which ultra-strong gradients technologies can be used safely for in vivo imaging. Magnetophosphenes are a biologically reversible effect and are considered non-harmful

Objectives

Methods

Results