Ho, Vincent B.

PurposeAsymmetric gradient coils introduce zeroth‐ and first‐order concomitant field terms, in addition to higher‐order terms common to both asymmetric and symmetric gradients. Salient to compensation strategies is the accurate calibration of the concomitant field spatial offset parameters for asymmetric coils. A method that allows for one‐time calibration of the offset parameters is described.Theory and MethodsA modified phase contrast pulse sequence with single‐sided bipolar flow encoding is proposed to calibrate the offsets for asymmetric, transverse gradient coils. By fitting the measured phase offsets to different gradient amplitudes, the spatial offsets were calculated by fitting the phase variation. This was used for calibrating real‐time pre‐emphasis compensation of the zeroth‐ and first‐order concomitant fields.ResultsImage quality improvement with the proposed corrections was demonstrated in phantom and healthy volunteers with non‐Cartesian and Cartesian trajectory acquisitions. Concomitant field compensation using the calibrated offsets resulted in a residual phase error <3% at the highest gradient amplitude and demonstrated substantial reduction of image blur and slice position/selection artifacts.ConclusionsThe proposed implementation provides an accurate method for calibrating spatial offsets that can be used for real‐time concomitant field compensation of zeroth and first‐order terms, substantially reducing artifacts without retrospective correction or sequence specific waveform modifications.

PurposeWe investigate the importance of high gradient‐amplitude and high slew‐rate on oscillating gradient spin echo (OGSE) diffusion imaging for human brain imaging and evaluate human brain imaging with OGSE on the MAGNUS head‐gradient insert (200 mT/m amplitude and 500 T/m/s slew rate).MethodsSimulations with cosine‐modulated and trapezoidal‐cosine OGSE at various gradient amplitudes and slew rates were performed. Six healthy subjects were imaged with the MAGNUS gradient at 3T with OGSE at frequencies up to 100 Hz and b = 450 s/mm2. Comparisons were made against standard pulsed gradient spin echo (PGSE) diffusion in vivo and in an isotropic diffusion phantom.ResultsSimulations show that to achieve high frequency and b‐value simultaneously for OGSE, high gradient amplitude, high slew rates, and high peripheral nerve stimulation limits are required. A strong linear trend for increased diffusivity (mean: 8–19%, radial: 9–27%, parallel: 8–15%) was observed in normal white matter with OGSE (20 Hz to 100 Hz) as compared to PGSE. Linear fitting to frequency provided excellent correlation, and using a short‐range disorder model provided radial long‐term diffusivities of D∞,MD = 911 ± 72 µm2/s, D∞,PD = 1519 ± 164 µm2/s, and D∞,RD = 640 ± 111 µm2/s and correlation lengths of lc,MD = 0.802 ± 0.156 µm, lc,PD = 0.837 ± 0.172 µm, and lc,RD = 0.780 ± 0.174 µm. Diffusivity changes with OGSE frequency were negligible in the phantom, as expected.ConclusionThe high gradient amplitude, high slew rate, and high peripheral nerve stimulation thresholds of the MAGNUS head‐gradient enables OGSE acquisition for in vivo human brain imaging.