Multiple‐point magnetic resonance acoustic radiation force imaging

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.

Efficient shear wave elastography using transient acoustic radiation force excitations and MR displacement encoding

“…41,42 A variety of MR ARF imaging techniques have also been used to evaluate changes in ex vivo tissue stiffness following an ablation. 20,[43][44][45] In this study, we demonstrated that MR-SWE can volumetrically image changes in ex vivo tissue shear wave speed following a FUS ablation. The same FUS hardware used to perform the ablation was used to produce the mechanical excitation.…”

“…US elastography has also been used to measure temperature induced changes in ex vivo tissue stiffness . A variety of MR ARF imaging techniques have also been used to evaluate changes in ex vivo tissue stiffness following an ablation …”

“…To improve acquisition efficiency, an MEG train following a single ARF impulse is used to encode the location of the propagating shear wave packet at multiple time‐points in each phase image. Leveraging the electronic steering capabilities of phased array FUS transducers and using an interleaving scheme described previously, phase images are acquired such that the ARF impulse spatial position for each image is different. The benefit of this multipoint approach is 3‐fold: (1) ARF induced shear wavefronts attenuate rapidly, which limits the spatial extent of the region over which the shear wave speed can be measured.…”

Advances and trends in ultrasound imaging and therapeutic technology

“…磁共振成像是超声治疗最理想的引导与监控手 段. 磁共振成像的多对比度机制不仅能够提供准确的 病灶定位、病灶区域组织在治疗前后的变化 [80] , 更重 要的是, 磁共振成像还具备温度与热剂量监控 [77] 、超 声作用点定位 [81] 等能力. 近年来, 高精度磁共振温度 监控朝着三维、实时、高分辨的方面 [82] , 后陆续被 批准在骨转移肿瘤 [84] 、前列腺癌 [85] 的治疗中应用.…”

Development and characterization of a tissue mimicking psyllium husk gelatin phantom for ultrasound and magnetic resonance imaging

“…Thermal dose [10] calculated from temperature change maps can be used independently or in conjunction with post-treatment contrast-enhanced imaging to evaluate the volume of the treated tissue. MR acoustic radiation force imaging (MR-ARFI) techniques [11][12][13] and MR-based elastography measurements [14][15][16] could also prove useful for HIFU treatment monitoring by providing additional information about the mechanical state of the tissue before, during, and after treatment.…”