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. 2019 Feb;81(2):1104-1117.
doi: 10.1002/mrm.27477. Epub 2018 Sep 26.

Multiple-point magnetic resonance acoustic radiation force imaging

Affiliations

Multiple-point magnetic resonance acoustic radiation force imaging

Henrik Odéen et al. Magn Reson Med. 2019 Feb.

Abstract

Purpose: To implement and evaluate an efficient multiple-point MR acoustic radiation force imaging pulse sequence that can volumetrically measure tissue displacement and evaluate tissue stiffness using focused ultrasound (FUS) radiation force.

Methods: Bipolar motion-encoding gradients were added to a gradient-recalled echo segmented EPI pulse sequence with both 2D and 3D acquisition modes. Multiple FUS-ON images (FUS power > 0 W) were interleaved with a single FUS-OFF image (FUS power = 0 W) on the TR level, enabling simultaneous measurements of volumetric tissue displacement (by complex subtraction of the FUS-OFF image from the FUS-ON images) and proton resonance frequency shift MR thermometry (from the OFF image). Efficiency improvements included partial Fourier acquisition, parallel imaging, and encoding up to 4 different displacement positions into a single image. Experiments were performed in homogenous and dual-stiffness phantoms, and in ex vivo porcine brain.

Results: In phantoms, 16-point multiple-point magnetic resonance acoustic radiation force imaging maps could be acquired in 5 s to 10 s for a 2D slice, and 60 s for a 3D volume, using parallel imaging and encoding 2 displacement positions/image. In ex vivo porcine brain, 16-point multiple-point magnetic resonance acoustic radiation force imaging maps could be acquired in 20 s for a 3D volume, using partial Fourier and parallel imaging and encoding 4 displacement positions/image. In 1 experiment it was observed that tissue displacement in ex vivo brain decreased by approximately 22% following FUS ablation.

Conclusion: With the described efficiency improvements it is possible to acquire volumetric multiple-point magnetic resonance acoustic radiation force imaging maps, with simultaneous proton resonance frequency shift MR thermometry maps, in clinically acceptable times.

Keywords: ARFI; FUS; HIFU; acoustic radiation force imaging.

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Figures

FIGURE 1
FIGURE 1
Pulse sequence diagram. Shown are 2 TRs with FUS ON (FUS power > 0 W), interleaved on the TR level with 1 TR with FUS OFF (FUS power = 0 W), in this case encoding 2 displacement positions per TR (one on each MEG lobe). The FUS is synchronized with the motion-encoding gradients (MEGs) using optical triggers. By electronically steering the FUS to different spatial positions for the positive and negative MEG lobe in a single TR, 2 different acoustic radiation force impulses (ARFI) displacement positions (labeled “Pos 1” and “Pos 2”) can be encoded into a single image. By electronically steering to 2 different positions during each of the 2 MEG lobes (not shown), a total of 4 different positions can be encoded into a single image
FIGURE 2
FIGURE 2
Experimental setup and k‐space sampling scheme. A, Experimental setup with FUS transducer coupled to phantom and ex vivo brain with bath of degassed and deionized water. A 5‐channel RF receive‐only coil was positioned around the sample. The approximate extent of the porcine brain within the skull with craniectomy is outlined in the dashed red line. B, The CAIPIRINHA sampling pattern used for 3D parallel imaging. For R = 2, the phase‐encoding slice‐encoding plane is subsampled with a “checker board” pattern, created by the shift of Δ = 1 from 1 k‐space slice encoding (kSE) to the next. This effectively shifts any remaining aliasing artifacts after reconstruction toward the corners of the 3D image volume
FIGURE 3
FIGURE 3
Two sonications per TR case of multiple‐point ARFI (mpARFI). A‐D, Individual points encoded using 2 sonications per TR for 3D acquisition. E, The corresponding maximum intensity projection (MIP) of all 16 points. The negative point is encoded first, and the created shear wave can be seen as a “ring” of positive displacement (red arrow in [A]) as encoded by the second MEG lobe. F, Proton resonance frequency shift (PRFS) temperature map from the FUS‐OFF image
FIGURE 4
FIGURE 4
Effect of spacing between MEGs. A‐F, Effect of increasing the spacing between the MEGs from 2 ms to 7 ms for the case with 2 sonications per TR. The top row shows 1 of the individual displacement maps, and the bottom row shows the corresponding MIP of 8 individual displacement maps. In the top row the first encoded position shows up as negative, and the second encoded position shows up as positive, as the MEG polarity is switched. The shear wave from the first position gets encoded during the second MEG lobe and is therefore also positive. For small pause times the second position coincides spatially with the shear wave, resulting in overestimated displacements. As the pause time is increased, the shear wave moves past the position of the second sonication, and accurate measurements are achieved
FIGURE 5
FIGURE 5
Homogenous phantom with 2D imaging. A,B, 1 and 2 sonications per TR, respectively. C,D, Parallel imaging with R = 2 for 1 and 2 sonications per TR, respectively
FIGURE 6
FIGURE 6
Homogenous phantom with 3D imaging. A,B, 1 and 2 sonications per TR, respectively. C,D, Parallel imaging with R = 2, for 1 and 2 sonications per TR, respectively
FIGURE 7
FIGURE 7
Three-dimensional mpARFI in dual-stiffness phantoms. A,B, Coronal and sagittal views of circular and 4 × 4 mpARFI trajectories in a dual-stiffness 125/175-bloom phantom, respectively. C,D Corresponding trajectories in a dual-stiffness 125/250-bloom phantom. To achieve the best contrast between the 2 different gelatins (which had different CuSO4 doping), the magnitude images shown are acquired with a 3D gradient-recalled-echo (GRE) sequence, whereas the overlaid displacement maps are acquired with the segmented EPI sequence
FIGURE 8
FIGURE 8
Effects of multiple sonications per TR, partial-Fourier, and parallel imaging on ex vivo mpARFI. Ex vivo porcine brain data using 4 sonications per TR for no additional speedup A, 33% speedup by partial Fourier (sampling 8 of 12 k-z slices) B, speedup of 50% by parallel imaging with R = 2 C, speedup of 67% by partial Fourier and parallel imaging D. E-G, Errors among (B), (C), (D) and (A), scaled to ±20% or the maximum measured displacement of 42.4 µm
FIGURE 9
FIGURE 9
Multiple-point ARFI before and after tissue ablation. A, Two orthogonal views of MIP displacement map of 13 FUS-ON acquisitions before ablation, overlaid on magnitude image. B, Two orthogonal views of PRFS temperature rise after 200 W sonication for 40 seconds at geometric focus (data acquired with 3D GRE-segmented EPI sequence without MEG), overlaid on magnitude image. C, Two orthogonal views of MIP displacement map after ablation. D, Two orthogonal views of displacement difference between before and after ablation. Red boxes magnify difference around geometric focus where the ablation took place

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