Peripheral nerve stimulation properties of head and body gradient coils of various sizes

Abstract

Peripheral nerve stimulation (PNS) caused by time-varying magnetic fields has been studied both theoretically and experimentally. A human volunteer study performed on three different body-size gradient coils and one head-size gradient coil is presented in this work. The experimental results were used to generate average PNS threshold parameters for the tested gradient systems. It was found that the average stimulation threshold increases while gradient-region-of-uniformity size decreases. In addition, linear relationships between PNS parameters and diameter of homogeneous gradient spherical volume (DSV) were discovered: SR min and ⌬G min both vary inverse linearly with DSV. More importantly, the chronaxie value was found to vary inversely linearly with the DSV. This finding indicates that, contrary to the general understanding, the parameter "chronaxie" in the commonly accepted simple stimulation models cannot be considered to be a single-value, nerve-specific constant. A modified linear model for gradient-induced PNS based on these results was developed, which may permit, for the first time, Key words: gradient coils; peripheral nerve stimulation (PNS); diameter of homogeneous spherical volume (DSV); chronaxieHigh-strength, high-slew-rate gradient coils are desired in modern MRI for better image quality and faster imaging speed. However, rapidly time-varying magnetic fields produced by gradient coils induce electric fields in tissue, which can cause peripheral nerve stimulation (PNS). This significantly limits our ability to take full advantage of these high-performance gradient systems. Despite considerable research in this area in recent years, the relationship between PNS thresholds and gradient coil linear region dimensions or gradient performance parameters is still not well understood. In this study, we investigated this relationship.The fundamental law of electrostimulation is written as (1)where E stim is the threshold electric field to cause stimulation, is the interval over which this electric field is applied, E r is the electric field rheobase, and c is the chronaxie. The rheobase E r is defined as the minimum electric field to produce stimulation, and chronaxie c is defined as the stimulus duration for which the threshold E stim is twice the value of the rheobase E r . Currently accepted gradient-induced PNS models utilize these two parameters (rheobase E r and chronaxie c ) to define PNS threshold curves. It has been assumed that these two parameters are functions only of underlying nerve properties. Recently, Chronik and Rutt (2) reported different rheobase and chronaxie values for a head-size coil and a body coil, and attributed these differences to different nerve sensitivities in the head and body. On the other hand, various studies have reported different values of rheobase and chronaxie even within one class of gradient coils, such as whole-body coils (1-8). We hypothesized, therefore, that these "nerve-specific" PNS model parameters are actually dependent in a predictable way on gradient c...

Local bi‐planar gradient array design using conformal mapping and simulated annealing

“…functions, and j u (k) is the components of the Fourier transform of the current density on the cylinder as described in the Eq. [1]. Note that the inductance is calculated in the cylindrical geometry rather than bi-planar geometry for simulation speed since inductance in the cylindrical geometry and inductance in the bi-planar geometry has a monotonic relationship as shown in Fig.…”

“…These gradient performance factors are in turn limited by the potential for peripheral nerve stimulation (PNS) (1). The potential for PNS can be reduced by using gradients with reduced imaging volume, which helps to minimize the extent of associated magnetic field excursions and the resulting electric fields and currents induced in the patient.…”

Gradient coil design and intra-coil eddy currents in MRI systems

“…The electric field induced in the human body can result in undesirable PNS [2,3], which limits the gradient performance such as its strength and slew rate [4]. However, some techniques such as echo planar imaging (EPI) and diffusion imaging require enhanced gradient coils with rapid switching speed and high gradient strength to improve the imaging speed and diffusion contrast [5].…”

“…As smaller gradient coils produce less magnetic flux than the whole-body coils, therefore, local coils can be switched faster than whole-body coils without causing PNS [6][7][8]. Although local gradient coils have been demonstrated to be faster and more effective to achieve high performance and high gradient strength compared to the whole-body gradient coils [4,9,10], the applications of head coils have been restricted by the coil design and implementation [5]. Because the local gradient coils such as head coil inserts are designed to be asymmetric due to constraints from the dimensions of human shoulders and head, this leads to technical difficulties in maintaining a high gradient performance for the head coil inserts with very limited space.…”

“…One approach to improve performance and gradient strength as well as to solve PNS issues is to use local gradient coils, as in smaller gradient coils, coil wires are placed closer to the ROI can increase coil performance [4,66]. Local gradient coils do not need high-voltage amplifiers as do whole-body gradient systems at high gradient strength and fast switching speed.…”