Motion-Compensated Slice-by-Slice ΔB₀ Shimming with an AC/DC Shim Coil and Dual-Echo vNavs

Synopsis

Subject motion results in intra-scan ΔB0 changes which are uncompensated in conventional ΔB0 shimming methods. Rapidly switchable shim currents and ΔB0 vNavigators together enable motion-compensated shimming. We have demonstrated successful measurement, calculation, and application of motion-compensated slice-by-slice shims using an AC/DC coil and vNav ΔB0 maps. Motion-compensated slice-by-slice homogeneity shimming improves shim robustness to subject motion and enables compatibility with changing slice prescriptions of prospective motion correction.

Introduction

Subject induced ΔB0 inhomogeneity caused by magnetic susceptibility discontinuities in the subject deteriorates image quality in key regions in brain imaging. Shim systems, ranging from scanner-integrated second order spherical harmonic arrays (2-SH) to local shim arrays, improve image quality by canceling subject-induced field perturbations. Rapidly-switchable shim systems, such as the AC/DC shim coil, enable further performance improvements in slice-based imaging by tailoring shim fields on a slice-by-slice basis.

Conventionally, shimming procedures are performed at the beginning of an exam, and optionally before each scan. However, over the course of a scan, subject motion invalidates the shim volume. Furthermore, rotation about either axis perpendicular to B0 will change the subject induced ΔB0 field.

In recent work, we demonstrated motion-compensated whole-brain AC/DC shimming [1] via low resolution 2-TE 3D EPI vNavs [2, 3]. This work expands previous capability by performing motion-compensated slice-by-slice (SBS) AC/DC shimming of 2D EPI, based on estimates of the changing ΔB0 field provided by vNavs acquired in each TR.

Methods

Proposed motion-compensated SBS shimming is performed as shown in Figure 1. The parent EPI sequence is also prospectively motion corrected based on the vNavs. Each TR, a 2-TE fieldmap vNav is used for prospective motion correction. The navigator and pre-measured coil calibration fields are interpolated into the next acquisition’s coordinate system, masked by a pre-acquired, BET computed mask [4]. Slice-by-Slice shim current calculation is posed as a linearly constrained quadratic optimization problem, solved with OSQP [5]. Shim current calculation for all parent EPI slices takes 200 ms each TR.

Figure 1:

Block diagram of motion-compensated shimming procedure. Subject-specific brain mask is acquired with a pre-acquisition EPI processed in fsl BET. During each TR of motion-compensated shimming, a low-res. EPI fieldmap vNav is acquired. Pre-measured subject-independent shim coil calibration data and the low-res. fieldmap are interpolated into next acquisitions coordinate system prospectively computed by the scanner. Slice-by-Slice shim currents are computed using interpolated coil fields and the interpolated subject ΔB0 map.

The motion-compensation shimmed EPI sequence is shown in Figure 2. A 2-TE 3D EPI vNav (8 mm iso, 32×32×32 matrix, TE1=6.2ms, TE2 =8.6ms, TR =16ms.) and a calculation delay executed prior to each TR of the motion-corrected parent 2D EPI (2.5 mm iso, 78×78 matrix, 60 slices, TE=30ms, TR=6.2s). Triggers from the scanner signal the change between slice-specific shim currents before slice excitation. Currents are zeroed after the EPI readout and during the EPI vNav.

Figure 2:

Motion-compensated shimmed EPI sequence diagram. Shim current calculation from Fig.1 executes between 2-TE 3D EPI and parent 2D EPI modules. Slice-specific shim currents are set before slice excitation and after prior-slice encoding.

In vivo motion shimming experiments were conducted in one human subject on a 3T Prisma scanner. Experiments were performed using a 32 channel AC/DC integrated RfRx ΔB0 shim array [6] with a 32 channel shim amplifier [7]. During motion-compensation shimmed EPI, the subject was instructed to move to and hold a series of poses for 9 TRs. Shim settings alternated between static whole-brain (WB) second order spherical harmonic (2-SH), static SBS AC/DC, and motion-compensated (MC) SBS AC/DC states. In each pose, successive EPI volumes were obtained with all shim options. Static WB AC/DC shim currents were computed for reference.

Static WB 2-SH shim settings were set via scanner adjustments. Static SBS AC/DC currents were set via offline shim optimization from a 2-echo 2D-GRE field map (110×110 matrix 75 slice, 2.5 mm iso) acquired preceding motion experiments.

Results

Predicted ΔB0 fields under each shim condition are computed by adding predicted shim fields to the interpolated vNav fieldmaps. Predicted fields from the static WB 2-SH, static WB AC/DC, static SBS AC/DC, and motion-compensated SBS AC/DC shims are shown for each pose in Figure 3. Effective-whole-brain ΔB0 standard deviation is shown as a function of the TR index. Partial-voxel and large-voxel effects from the vNav bias the computed predicted ΔB0. Thus, ΔB0 summary statistics may not be compared to ΔB0 statistics from high-res acquisitions, but provide useful relative reference. Static SBS AC/DC shim outperforms the motion-compensated shim in pose-1; the static shim currents were computed in pose-1 from high resolution field maps. In all other poses, including the return to the neutral pose, the proposed motion-compensated SBS AC/DC shim outperforms other shim conditions.

Figure 3:

Predicted ΔB0 fields from motion study. Subject moved through five poses: neutral, nod up, nod down, rotate left, neutral. Predicted motion-corrected ΔB0 fields from an axial slice and brain ΔB0 standard deviations are shown for static whole-brain (WB) 2-SH, static WB AC/DC, static slice-by-slice (SBS) AC/DC, and motion-compensated SBS AC/DC shims. Note that the low σΔB0 values arise from bias of large 8 mm vNav voxels. While σΔB0 values are representative of the change in ΔB0 field due, they may not be compared to summary statistics from high-resolution field mapping.

2D EPI motion study magnitude images are shown in Figure 4 (available online). For each shim condition, the animated Figure alternates between pose-1 and subsequent poses to demonstrate change in distortion. Motion-compensated SBS AC/DC shimming shows the lowest change in image distortion demonstrating the success of the method. However, motion-compensated shimming shows an increase in dephasing due to poor estimates of z-directed ΔB0 gradients as a result of the large vNav voxels.

Figure 4:

Animated figure showing magnitude images from EPI motion experiments. Axial and sagital slices registered via prospective motion correction are shown for three poses (nod up, nod down, rotate left) and static WB 2-SH, static SBS AC/DC, and motion-compensated SBS AC/DC shim conditions. The figure is animated to alternate between images acquired in the neutral pose and the listed pose to facilitate interpretation of change-in image distortion. The proposed motion-compensated SBS AC/DC images show the lowest distortion.

NIHMS2150156-video-Figure_4__animated_GIF_.gif ^{(1.2MB, gif)}

We validate predicted shimmed ΔB0 maps by examining phase data from the parent EPI. Though single echo EPI phase does not provide absolute ΔB0, we observe ΔB0 changes from TR-to-TR via phase differences. Figure 5 shows predicted and observed change in ΔB0 between each pose and pose-1. We find close agreement between predicted and observed fields showing the effectiveness of the motion-compensated shim method.

Figure 5:

Predicted and observed change in ΔB0 from pose-1 in EPI motion experiment under tested poses and shim configurations. Predicted fields are computed from interpolated vNav field estimates, pre-measured coil field patterns, and optimized shim currents. Observed fields are computed from changes in phase-unwrapped 2D-EPI phase. Predicted and observed fields show excellent correspondence modulo observed coil-combination singularities indicating in-vivo motion-compensated AC/DC shim performance is as predicted.

Discussion

We have demonstrated successful measurement, calculation, and application of motion-compensated slice-by-slice shims using and AC/DC coil and vNav ΔB0 maps. Motion-compensated slice-by-slice homogeneity shimming improves shim robustness to subject motion and enables compatibility with changing slice prescriptions of prospective motion correction. Anatomical consistency of EPI was improved with motion-compensated shimming over static comparisons in nodding and rotating poses. However, motion-compensated SBS shimming caused slight increase in signal dephasing, counter to what is observed in static SBS shimming studies. vNav resolution may limit estimation of large ΔB0 gradients and thus limit shimming’s mitigation of dephasing. Accelerated 2TE vNavs for reduced distortion and increased resolution may further improve the effectiveness of this motion-compensated SBS Homogeneity shimming method.