Electric field calculation and peripheral nerve stimulation prediction for head and body gradient coils

Abstract

Purpose: To demonstrate and validate electric field (E-field) calculation and peripheral nerve stimulation (PNS) prediction methods that are accurate, computationally efficient, and that could be used to inform regulatory standards. Methods: We describe a simplified method for calculating the spatial distribution of induced E-field over the volume of a body model given a gradient coil vector potential field. The method is easily programmed without finite element or finite difference software, allowing for straightforward and computationally efficient E-field evaluation. Using these E-field calculations and a range of body models, populationweighted PNS thresholds are determined using established methods and compared against published experimental PNS data for two head gradient coils and one body gradient coil. Results: A head-gradient-appropriate chronaxie value of 669 µs was determined by meta-analysis. Prediction errors between our calculated PNS parameters and the corresponding experimentally measured values were ~5% for the body gradient and ~20% for the symmetric head gradient. Our calculated PNS parameters matched experimental measurements to within experimental uncertainty for 73% of ∆G min estimates and 80% of SR min estimates. Computation time is seconds for initial E-field maps and milliseconds for E-field updates for different gradient designs, allowing for highly efficient iterative optimization of gradient designs and enabling new dimensions in PNS-optimal gradient design. Conclusions: We have developed accurate and computationally efficient methods for prospectively determining PNS limits, with specific application to head gradient coils. K E Y W O R D Sasymmetric and symmetric gradient, E-field, electric field, gradient coil, head gradient, peripheral nerve stimulation, PNSDisplacements of brain center and eye center from top of head are included (bottom 2 rows), showing that brain center is located slightly superior to eye center as expected. The term "ellipse radii" refers to semi-major and semi-minor axis dimensions.

A Huygens’ surface approach to rapid characterization of peripheral nerve stimulation

“…This allows PNS constraints to be applied at multiple rise times, such as to avoid creating a coil that is better for some rise times but worse for others (eg, if the PNS threshold curves shown in Figure 4 were to cross due to differing chronaxie times). Note that the chronaxie has been observed to vary dramatically between 285 μs and 879 μs 1,23,45,46,48,50‐54 and is affected by the type of nerve 55 and the shape of the E‐field stimulus 56 . A change in nerve chronaxie as a result of winding modifications would represent a more complicated PNS relationship between unoptimized and optimized coils (although still fully characterized).…”

“…Linking E‐field metrics to PNS thresholds was achieved using a priori estimates of the nerve chronaxie (an IEC mandated value of 360 μs for body coils, or an adjusted value of 669 μs for head coils). However, gradient coil chronaxie times have been reported ranging between 351 μs and 770 μs for whole‐body gradients 1,23,45,46,48,50‐52 and between 285 μs and 879 μs for head‐only gradients 23,48,53,54 . Our PNS model requires a different sort of a priori parameters: those related to the nerve cable model, which are independent of the coil geometry.…”

“…Alternatively, the Huygens’ approach to PNS analysis could focus solely on the E‐fields rather than the PNS oracle. For example, recent works by Roemer et al 44,53 used fast surface E‐field metrics computed in homogeneous body models as a surrogate for PNS thresholds. These E‐field metrics can then be incorporated in the winding optimization as an additional constraint, similarly to our PNS‐constrained winding optimization 6 .…”

Occupational Exposure Assessment of the Static Magnetic Field Generated by Nuclear Magnetic Resonance Spectroscopy: A Case Study

“…Various studies assessing the health risk for MRI workers have been published [19][20][21][22][23]. Workers' exposure to static magnetic fields and their movements in fringe fields have been evaluated and discussed in sites that used MRI scanners ranging from 0.25 T up to 3.0 T [24][25][26].…”

Evaluation of Peripheral Electrostimulation Thresholds in Human Model for Uniform Magnetic Field Exposure

“…In addition, ICNIRP recommended that excitation modeling and threshold assessment be conducted to improve the accuracy of restrictions [ 11 ]. These recommendations and computational advances in incorporating neurons into realistic human models [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ] motivate revisiting the current protection limits of the standard and guidelines and provide rationality of the applied reduction factors as safety margins. A few studies have evaluated the threshold for central nervous system stimulation considering axonal stimulation indicating conservativeness with respect to the current guidelines/standard presented at low frequency [ 20 , 21 ].…”