Slice-selective RF pulses for in vivo B1+ inhomogeneity mitigation at 7 tesla using parallel RF excitation with a 16-element coil

Abstract

Slice-selective RF waveforms that mitigate severe B1+ inhomogeneity at 7 Tesla using parallel excitation were designed and validated in a water phantom and human studies on six subjects using a 16-element degenerate stripline array coil driven with a butler matrix to utilize the eight most favorable birdcage modes. The parallel RF waveform design applied magnitude least-squares (MLS) criteria with an optimized k-space excitation trajectory to significantly improve profile uniformity compared to conventional least-squares (LS) designs. Parallel excitation RF pulses designed to excite a uniform in-plane flip angle (FA) with slice selection in the z-direction were demonstrated and compared with conventional sinc-pulse excitation and RF shimming. In all cases, the parallel RF excitation significantly mitigated the effects of inhomogeneous B1+ on the excitation FA. The optimized parallel RF pulses for human B1+ mitigation were only 67% longer than a conventional sinc-based excitation, but significantly outperformed RF shimming. For example the standard deviations (SDs) of the in-plane FA (averaged over six human studies) were 16.7% for conventional sinc excitation, 13.3% for RF shimming, and 7.6% for parallel excitation. This work demonstrates that excitations with parallel RF systems can provide slice selection with spatially uniform FAs at high field strengths with only a small pulse-duration penalty.

Figure 1

a) The 16-channel transmit/receive (Tx/Rx) stripline coil and the butler matrix used in this work. b) Profiles for combined Tx-Rx, along with estimates of the separate Tx (B₁⁺) and Rx (B₁⁻) of the circularly-polarized mode-1 birdcage of the coil for a head shaped phantom, and c) axial section in human brain (subject 1). The axes of orientation are denoted by R/L (right/left), and A/P (anterior/posterior) on the right-most image.

Figure 2

Flowchart outlining the proposed quantitative B₁⁺ mapping technique, where, first a receive profile of the reception coil is estimated in steps 1-5, after which B₁⁺ maps of the transmit modes/coils can then be obtained via steps 6-7.

Figure 3

In vivo quantitative B₁⁺ mapping of the 1^st gradient transmit mode using a single low-flip-angle acquisition (subject 1). The B₁⁺ map is obtained by dividing the low-flip-angle image with the density-weighted receive profile estimate along with applying a sine inverse operation.

Figure 4

Magnitude (top) and phase (bottom) B₁⁺ maps of the 8 optimal modes for a) the head-shaped phantom, and b) an axial section in human brain (subject 1).

Figure 5

The optimized three- (a) and two-spoke (b) k-space excitation trajectories for the pulse design in the head-shaped phantom and human excitation experiment respectively. The optimized two-spoke placements in (k_x,k_y) for the in the in vivo experiments varied from subject to subject, but in all cases were two-spoke designs.

Figure 6

Head-shaped water phantom B₁⁺ mitigation. Flip-angle maps and line profiles for: a) birdcage mode with conventional 1-ms long sinc slice-selective excitation, demonstrating a 1:6.8 magnitude variation within the field-of-excitation (FOX); b) RF shimming, 1-ms long pulse, demonstrating a substantial residual flip-angle inhomogeneity as measured by the standard deviation and threshold metrics; and, c) three-spoke MLS, slice-selective 2.4-ms long pulse, demonstrating excellent mitigation of the B₁⁺ inhomogeneity.

Figure 7

Comparison of B₁⁺ mitigation by a) a least-squares, and b) a magnitude-least-squares three-spoke RF design with the same k-space trajectory (2.4 ms) and pulse shape (sinc, time-bandwidth product=4) as demonstrated on a head-shaped water phantom with substantial transmit inhomogeneity. On the left is a Bloch simulation of the magnitude and phase profiles, on the right are experimental results with line profiles through the magnitude image.

Figure 8

Comparison between the image phase due to the combined excitation and reception phase (left) and the acquisition phase measured at TE=5ms (right), which also includes the phase accrual due to B₀ inhomogeneity. Also shown on the far right is the estimated B₀ fieldmap. Clearly, the combined excitation and reception phase variation resulting from the MLS design is very small compared to the accrued phase due to B₀ inhomogeneity at TE=5ms. The phase resulting from the MLS design is slowly varying over the FOX, and thus does not introduce any intra-voxel dephasing.

Figure 9

B₁⁺ mitigation comparison for subject 1. The comparison includes slice selection based on mode-1 birdcage (top row), RF shimming (center row), and two-spoke (bottom row) excitation pulses. On the left of each row is the in-plane image of the excited slice after the removal of the receive profile. On the right is the flip-angle map estimate, along with the line profile plots.

Figure 10

B₁⁺ mitigation comparison for subject 5, who has the most severe B₁⁺ variation (in the mode-1 birdcage excitation) out of all the six subjects.

Figure 11

Two-spoke excitation for subject 4 with a 3D readout. a) Slice profile plot, where the solid line represents the predicted profile and the circles represent the experimental data. Each data point along the slice profile represents the average in-plane intensity at that particular z-location. b) In-plane images (a)-(j), at 1-mm separation along z, over a 1-cm range around the 0.5-cm excited slice.