High slew‐rate head‐only gradient for improving distortion in echo planar imaging: Preliminary experience

Abstract

Purpose To investigate the effects on echo planar imaging (EPI) distortion of using high gradient slew rates (SR) of up to 700 T/m/s for in-vivo human brain imaging, with a dedicated, head-only gradient coil. Materials and Methods Simulation studies were first performed to determine the expected echo spacing and distortion reduction in EPI. A head gradient of 42-cm inner diameter and with asymmetric transverse coils was then installed in a whole-body, conventional 3T MRI system. Human subject imaging was performed on five subjects to determine the effects of EPI on echo spacing and signal dropout at various gradient slew rates. The feasibility of whole-brain imaging at 1.5 mm-isotropic spatial resolution was demonstrated with gradient-echo and spin-echo diffusion-weighted EPI. Results As compared to a whole-body gradient coil, the EPI echo spacing in the head-only gradient coil was reduced by 48%. Simulation and in vivo results, respectively, showed up to 25-26% and 19% improvement in signal dropout. Whole-brain imaging with EPI at 1.5 mm spatial resolution provided good whole-brain coverage, spatial linearity, and low spatial distortion effects. Conclusion Our results of human brain imaging with EPI using the compact head gradient coil at slew rates higher than in conventional whole-body MR systems demonstrate substantially improved image distortion, and point to a potential for benefits to non-EPI pulse sequences.

Gradient and shim technologies for ultra high field MRI

“…For example, when imaging the head, body GCs still produce a high amplitude of time-varying magnetic fields over the entire torso cross-section, which causes PNS without improving imaging quality. The advantage of smaller-sized GCs with a reduced linearity region diameter is that fields are limited primarily to the head, decreasing the amplitudes of time-varying electric fields and induced currents significantly, thereby increasing PNS thresholds substantially (Wade et al, 2016; Tan et al, 2016; Weavers et al, 2016). This permits even higher useable gradient performance than expected from hardware specifications alone, compared to body-sized gradient coils.…”

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

“…However, both high SR and high G max have not been simultaneously achieved, which benefit the fast‐switching EPI readouts of functional MRI and that of diffusion‐weighted EPI used in measuring brain connectivity and microstructure imaging. In essence, faster SR translates to shorter echo spacings in EPI, which reduce image distortion and provide shorter TEs; shorter TEs lead to higher signal in short T 2 species. Higher G max not only provides shorter diffusion waveforms that lead to reduced TE, but also improves the image quality and sensitivity to shorter axonal length scales in microstructure imaging …”

Lightweight, compact, and high‐performance 3T MR system for imaging the brain and extremities

“…This is highly beneficial for minimizing geometric distortion, obtaining lower TE, and consequently results in fewer signal dropouts that plague typical EPI acquisitions . In previous work, improvements by 19% in signal intensity had been observed in regions susceptible to signal dropout, in which reductions in echo spacing (and image distortion) of up to 48% were also reported. Supporting Information Figure S2 shows the comparison between a gradient‐echo EPI acquisition with the C3T system and a whole‐body 3T scanner at the same TE time, in which the reduced distortion and signal dropout resulting from the high gradient slew rate is clearly seen.…”