On Peripheral Nerve Stimulation of a Compact, Asymmetric Head-Only Gradient Coil: Head Orientation Dependence
Seung-Kyun Lee1, Kishore V. Mogatadakala2, Dominic Graziani1, Jean-Baptiste Mathieu2, Thomas K.-F. Foo1, and Matt A. Bernstein3

1GE Global Research, Niskayuna, NY, United States, 2GE Healthcare, Florence, SC, United States, 3Mayo Clinic, Rochester, MN, United States


Head orientation dependence of the peripheral nerve stimulation (PNS) thresholds and the induced electric fields of a high-performance, asymmetric head-only gradient coil were studied experimentally and by numerical simulation. In the experiment, the gradient field direction was fixed and the subject head was rotated in the transverse plane. The subject-reported PNS thresholds nearly doubled when the head's anterior-posterior direction was parallel to the gradient compared to when the head was approximately perpendicular to the gradient. Human-body-model simulation suggested that the orientation dependence may be primarily due to locally concentrated electric fields in the corrugated regions of the face.


A compact, asymmetric head-only gradient coil with improved cooling and electromagnetic design has been shown to enable fast, high-quality imaging at slew rates up to 700 T/m/s [1-5] with reduced PNS compared to a whole-body gradient coil. Whereas the PNS advantage of a compact neuroimaging gradient coil is well-documented [6,7], the dependence of the PNS thresholds on the gradient direction in the head has been only sporadically reported in the literature [7,8] with limited quantitative explanation. In particular, the stronger PNS from a left-right (L/R) gradient coil as compared to an anterior-posterior (A/P) coil has been attributed to the difference in the cross sectional areas in the sagittal vs coronal mid-plane in the head [7]. However, this does not seem to fully explain a marked suppression of A/P-gradient PNS observed consistently on many subjects [1,4,5] with different head aspect ratios. Here we present an experiment in which the PNS thresholds of an asymmetric head-only gradient coil were measured as a subject rotated his head while the gradient direction was fixed in space. A numerical simulation probed electric field (E-field) hot spots at different head orientations.


The PNS measurement was conducted on a healthy male volunteer under an IRB-approved protocol using a compact head-only gradient coil described in [1-3] inside a whole-body 3T scanner (GE MR750W, Fig 1a). A train of bipolar trapezoidal gradient pulses (Fig. 2), with 1 ms flat-top time, a variable zero-to-peak amplitude ΔG and a variable zero-to-peak rise-time Δt, were applied to both the X (horizontal) and the Y (vertical) coils simultaneously, creating a 45°-oblique gradient field. The subject reported the onset of sensation as ΔG was incremented at one of three rise-times. The maximum ΔG was limited to 85 mT/m. The experiment was repeated as the subject rotated his head about the Z-axis by three different angles. After each measurement at a given angle, the subject was imaged with a 3-plane localizer (FSE, TR/TE/FOV/slice = 706 ms/79 ms/28 cm/8 mm) to record the head position in the gradient coil (Fig. 1b). Numerical simulation was performed on a human body model (electrical conductivity = 0.2 S/m) in Comsol Multiphysics (Burlington, MA, USA) (Fig. 3a,b). The gradient coil (X) was modeled after the electromagnetic design of the head-only gradient coil, and was driven with a 1 kHz sine-wave with 660A peak current. The induced E-fields at different regions of the head were recorded as the human model was rotated around the Z axis with respect to the gradient coil in 5° steps.


Figure (1c) shows the PNS thresholds at different angles and rise-times. The thresholds when the head was approximately parallel to the gradient field (θ = 10°) was nearly twice the thresholds at θ = 75° for the two shortest rise-times. The difference in thresholds between θ = 44° and θ = 75° was not as dramatic as the difference between θ = 44° and θ = 10°, indicating relatively sharp drop in PNS as the head aligns with the gradient direction. Figure (3c) shows the orientation dependence of the simulated E-field magnitudes. The E-field hot-spot occurs in the nose-bridge area for the PNS-sensitive orientation (i.e., when the head and the gradient are orthogonal, θ = ±90°), which is consistent with the PNS locations reported by a majority of subjects in earlier studies [1,4]. The concentrated E-field in this area exhibits a significant drop when the head aligns with the gradient field (Fig. 3c).

Discussion and Conclusion

The PNS thresholds of a compact asymmetric head gradient coil depend strongly on the relative orientation of the head with respect to the gradient field in the transverse plane [7,8]. Unlike in previous studies we obtained the experimental data by rotating the head while the gradient direction was fixed in space, thereby eliminating any possibility that different X, Y gradient coil designs contribute to the dependence. A human-body-model simulation suggested that the strong orientation dependence of the E-fields concentrated in certain corrugated areas of the face, e.g., the bridge of the nose, could be the main contributor to the observed PNS characteristics of a compact head gradient coil. Extension of the present work to include other degrees of freedom in head position/orientation and more sophisticated head models [9] is currently planned. The high PNS thresholds of the head gradient coil in the A/P direction can be exploited for high-speed Cartesian readout such as EPI.


This work was supported in part by the NIH grant 5R01EB010065. The views herein do not necessarily represent those of NIH.


[1] Mathieu et al, ISMRM 2015, #1019. [2] Huston et al, ISMRM 2015, #971. [3] Huston et al, ASNR 2015, #O-232. [4] Lee et al, ISMRM 2014, #310. [5] Lee et al, Magn Reson Med (accepted). [6] Chronik and Rutt, Magn Reson Med 46:386 (2001). [7] Schmitt et al, Echo-planar imaging: theory, technique, and application. Springer (1998), p212. [8] Goodrich et al, Concepts Magn Reson 44B:66 (2015). [9] Feldman et al, Magn Reson Med 68:1973 (2012).


Figure 1. (a) Setup for a volunteer PNS test and head imaging inside a head-only gradient coil. (b) Axial localizer images showing the three head orientations used. (c) PNS thresholds at three different rise-times at each head orientation.

Figure 2. PNS test pulse sequence. The waveform corresponds to ΔG(0-pk) = 85 mT/m, Δt(0-pk) = 120 µsec, tflattop = 1 msec. A given gradient amplitude was repeated 4 times before the next amplitude was applied.

Figure 3. (a) Gradient-induced E-field simulation setup in Comsol Multiphysics. (b) E-field magnitude in the face when the head A/P direction is along the Y axis, and the gradient field in X (θ = 90°). (c) Head orientation dependence of the maximum E-field magnitudes in different regions of the head.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)