Multiple‐point magnetic resonance acoustic radiation force imaging

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.