Foo, Thomas

PurposeTo 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 MethodsSimulation 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 magnetic resonance imaging (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.ResultsAs 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.ConclusionOur 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. J. Magn. Reson. Imaging 2016;44:653–664.

PurposeTo characterize peripheral nerve stimulation (PNS) of an asymmetric head‐only gradient coil that is compatible with a commercial high–channel‐count receive‐only array.MethodsTwo prototypes of an asymmetric head‐only gradient coil set with a 42‐cm inner diameter were constructed for brain imaging at 3T with maximum performance specifications of up to 85 mT/m and 708 T/m/s. Tests were performed in 24 volunteers to measure PNS thresholds with the transverse (x = left–right; y = anterior–posterior [A/P]) gradient coils of both prototypes. Fourteen of these 24 volunteers were also tested for the z‐gradient PNS in the second prototype and were scanned with high–slew‐rate echo planar imaging (EPI) immediately after the PNS tests.ResultsFor both prototypes, the y‐gradient PNS threshold was markedly higher than the x‐gradient threshold. The z‐gradient threshold was intermediate between those for the x‐ and y‐coils. Of the 24 volunteers, only two experienced y‐gradient PNS at 80 mT/m and 500 T/m/s. All volunteers underwent the EPI scan without PNS when the readout direction was set to A/P.ConclusionMeasured PNS characteristics of asymmetric head‐only gradient coil prototypes indicate that such coils, especially in the A/P direction, can be used for fast EPI readout in high‐performance neuroimaging scans with substantially reduced PNS concerns compared with conventional whole body gradient coils. Magn Reson Med 76:1939–1950, 2016. © 2015 International Society for Magnetic Resonance in Medicine