Fast low-specific absorption rate B₀ -mapping along projections at high field using two-dimensional radiofrequency pulses

Abstract

Purpose: At 7 Tesla (T), conventional static field (B 0 ) projection mapping techniques, e.g., FASTMAP, FASTESTMAP, lead to elevated specific absorption rates (SAR), requiring longer total acquisition times (TA). In this work, the series of adiabatic pulses needed for slab selection in FASTMAP is replaced by a single two-dimensional radiofrequency (2D-RF) pulse to minimize TA while ensuring equal shimming performance. Methods: Spiral gradients and 2D-RF pulses were designed to excite thin slabs in the small tip angle regime. The corresponding selection profile was characterized in phantoms and in vivo. After optimization of the shimming protocol, the spectral linewidths obtained after 2D localized shimming were compared with conventional techniques and published values from

Prospective motion and B0 shim correction for MR spectroscopy in human brain at 7T

“…Spectra acquired from 1 subject with a more typical subject motion (slow continuous head rotation) are shown in Figure 3. The subject was instructed to slowly tilt the head to the right side (scans 5-10), then stay in this position until the end of the scan (scans [11][12][13][14][15][16]. This movement resulted in a maximum rotation between 5° and 10° along the Z-axis with associated translations between 12 and 22 mm in the X-direction.…”

“…B, Translation (in mm) and rotation (in degrees) parameters, showing comparable motion for each condition. C, Linewidth of NAA singlet (2.01 ppm), spectral SNR and total cumulative translation along Z measured at baseline (sum of scans 1-4) and after motion (sum of scans [13][14][15][16] under NoCo, MoCo and ShMoCo conditions. SNR was calculated in the frequency domain as the peak height of NAA divided by root mean square noise (measured between -1 and -4 ppm region), without any apodization alone is not sufficient to preserve the spectral quality during Z translation and rotation motions in the prefrontal cortex, and that concomitant B 0 shim correction is critical.…”

“…To minimize disturbance of magnetization, we used 2D radiofrequency (RF) pulses in the small tip angle regime to excite a cylindrical column passing through the VOI followed by an asymmetric echo‐planar readout train (3 echoes recorded per excitation, 5.5 ms delay between echoes, 256 complex points with 66.67 kHz readout bandwidth). The 2D‐RF pulses were generated based on a slew‐rate limited spiral trajectory as previously described . Briefly, using the maximal gradient coil slew rate of 200 mT/m/ms, spiral‐in gradient waveforms and the corresponding RF pulses (6.51 ms duration, Gaussian weighted, sampling density compensated) were computed on the fly inside the sequence to excite a cylinder of 7‐mm radius measured at full‐width half maximum.…”

“…The 2D-RF pulses were generated based on a slew-rate limited spiral trajectory as previously described. 16 Briefly, using the maximal gradient coil slew rate of 200 mT/m/ms, spiral-in gradient waveforms and the corresponding RF pulses (6.51 ms duration, Gaussian weighted, sampling density compensated) were computed on the fly inside the sequence to excite a cylinder of 7-mm radius measured at full-width half maximum. Each B 0 profile in each direction was acquired in 29.4 ms and were then computed as follows: for each echo, signals from each receive channel were phase corrected before summing.…”

B0 magnetic field homogeneity and shimming for in vivo magnetic resonance spectroscopy

“…An optimized FASTMAP implementation at 7 Tesla has been recently shown to allow localized B 0 shimming at low specific absorption rate (SAR), i.e. RF power deposition, in less than 1 minute (41). …”