(Tuesday, 7/16/2019) 1:45 PM - 3:45 PM
Room: 303
Purpose: Tissue mechanical properties can change following thermal ablation. In this work, we test the feasibility of measuring changes in tissue stiffness following MRgFUS ablation using the MR acoustic radiation force imaging (MR-ARFI).
Methods: In this Institutional Animal Care and Use Committee (IACUC) approved study, shear wave propagation speed in pig skeletal muscle was measured using MR-ARFI before and after thermal ablation in vivo. A thermal lesion (9.3 cc lesion volume with thermal dose > 240 EM at 43°C) was created using MRgFUS system (Sonalleve™, Philips Healthcare) in the thigh muscle of a pig.MR-ARFI: A custom written phase contrast sequence with motion encoding gradient (28.6mT/m, 8ms duration) captured tissue displacement caused by propagating mechanical wave emanating from the HIFU focus due to ARF (pulsed duration of 4ms) at progressively increasing time delays (3 ms intervals). Shear wave speed was calculated using a time-of-arrival algorithm.
Results: 1. Tissue displacements caused by shear wave induced at the HIFU focus were captured with adequate displacement sensitivity before and after ablation, and the thermal lesion maps measured corresponded closely with pathology.2. Before ablation, shear wave speed was anisotropic in pig thigh muscle with wave speed roughly twice as fast parallel to the direction of muscle fiber orientation (long axis) compared to perpendicular to the muscle fiber orientation (short axis).3. After ablation, the anisotropy of the shear wave speed with respect to muscle fiber orientation was diminished, and the wave speed was higher.
Conclusion: Preliminary findings from this in vivo study are as follows: (a) Tissue displacement caused by shear wave propagation within skeletal muscle can be measured in vivo. (b) Muscle fiber orientation has a significant influence in shear wave speed. (c) Following ablation, shear wave speed in ablated regions is faster, and shear wave anisotropy is lower.
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