Predicting Magnetostimulation Thresholds in the Peripheral Nervous System using Realistic Body Models

Abstract

Rapid switching of applied magnetic fields in the kilohertz frequency range in the human body induces electric fields powerful enough to cause Peripheral Nerve Stimulation (PNS). PNS has become one of the main constraints on the use of high gradient fields for fast imaging with the latest MRI gradient technology. In recent MRI gradients, the applied fields are powerful enough that PNS limits their application in fast imaging sequences like echo-planar imaging. Application of Magnetic Particle Imaging (MPI) to humans is similarly PNS constrained. Despite its role as a major constraint, PNS considerations are only indirectly incorporated in the coil design process, mainly through using the size of the linear region as a proxy for PNS thresholds or by conducting human experiments after constructing coil prototypes. We present for the first time, a framework to simulate PNS thresholds for realistic coil geometries to directly address PNS in the design process. Our PNS model consists of an accurate body model for electromagnetic field simulations, an atlas of peripheral nerves, and a neurodynamic model to predict the nerve responses to imposed electric fields. With this model, we were able to reproduce measured PNS thresholds of two leg/arm solenoid coils with good agreement.

Peripheral nerve stimulation limits of a high amplitude and slew rate magnetic field gradient coil for neuroimaging

“…More recently, attempts to include human body and nerve models have been used by electromagnetic field simulations to estimate PNS thresholds. [17][18][19] When compared with PNS experiments, these simulations provided good agreement with the location, magnitude, and orientation in whole body and head gradients, 19 and can be a powerful way to validate PNS studies and to improve gradient designs. They also confirm higher electric field (E-field) amplitudes with whole-body gradients (primarily in the torso regions) than with head gradients (in head and upper torso regions).…”

“…Although these regulatory safety guidelines apply in general to all MRI gradient systems, the need for PNS studies is especially important for MRI systems capable of higher gradient amplitude and slew rates. More recently, attempts to include human body and nerve models have been used by electromagnetic field simulations to estimate PNS thresholds . When compared with PNS experiments, these simulations provided good agreement with the location, magnitude, and orientation in whole body and head gradients, and can be a powerful way to validate PNS studies and to improve gradient designs.…”

Anatomical measurements correlate with individual magnetostimulation thresholds for kHz‐range homogeneous magnetic fields

“…Therefore, a lower fat percentage (i.e., a thinner fat layer) may position the peripheral nerves closer to the muscle tissue, causing them to experience a higher electric field, 18 which in turn would cause a reduction in B th . A recent electromagnetic simulation study on predicting PNS limits has also shown that electric field hotspots corresponded to regions of low-conductivity fat tissue positioned near a high-conductivity tissue such as muscle, 22 providing support for the abovementioned hypothesis and for the updated model in Eq. (4).…”

Parallel transmission to reduce absorbed power around deep brain stimulation devices in MRI: Impact of number and arrangement of transmit channels

“…The CT volumes were segmented into 3 classes (internal air, soft tissue, and bone) using the open‐source software 3D Slicer followed by manual cleanup . Segmented tissue voxel volumes were then meshed into 2‐manifold, watertight surface meshes using a previously described procedure . The final mesh and coil model were imported in Ansys Electronics (Canonsburg, PA).…”

“…BC, birdcage coil; DBS, deep brain stimulation devices; IPG, implanted pulse generator; pTx, parallel transmission described procedure. 44 The final mesh and coil model were imported in Ansys Electronics (Canonsburg, PA).…”