Room: Exhibit Hall | Forum 1
Purpose: We propose and test three non-contrast MR imaging metrics as biomarkers of thermal tissue damage in MRgHIFU in an ex vivo tissue model: (a) changes in tissue stiffness following ablation using the MR acoustic radiation force imaging (MR-ARFI); (b) tissue transverse relaxation rates (Râ‚‚), and (c) local tissue diffusion (D).
Methods: A 256 channel spherical shell transducer system (Sonalleve, Philips Healthcare) was used to ablate a freshly excised skeletal muscle tissue (cow) (12 cm diameter x 11.5 cm height). Thermal dose ( >240 EM at 43Â°C) was applied via 12 volumetric sonications (100-160 W) to ablate 55 cc.MR-ARFI: A custom written phase contrast sequence with motion encoding gradient (28.6 mT/m, 8 ms 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 (1 ms intervals). MR-DWI: Single shot spin-echo (SE) diffusion weighted images (12 b values spanning 0-1500 s/mm2) with TR/TE/flip angle = 3.0s/75ms/90Â° were acquired.MR-Râ‚‚ mapping: Râ‚‚ was computed from a 9 echo SE acquisition: TR/TE/Î”TE = 3000 ms/12.7 ms/12.7 ms.Data Analysis: Shear wave speed was calculated using a time-of-arrival algorithm from which tissue stiffness (Î¼) was calculated. D and Râ‚‚ were fitting respective signals to a mono-exponential decay curve.
Results: Compared to non-ablated regions or pre-ablation, stiffness in treated regions was (8.8Â±0.8 kPa versus 4.2Â±0.4 kPa) higher, Râ‚‚ in treated region was lower (14.5Â±2.1 Hz Vs 17.9Â±1.6 Hz), and D was higher (1.62Â±0.13 vs 1.06Â±0.11 mmÂ²/s ).
Conclusion: The changes in tissue stiffness, Râ‚‚ and D, following ablation suggest that MRgHIFU ablation induced changes can be observed in ex vivo tissue samples using these imaging biomarkers without the need for contrast administration on the same MRgHIFU platform. These findings need to be confirmed in vivo.
Funding Support, Disclosures, and Conflict of Interest: This work received final support from Research seed grant of American Assocaition of physicist in Medicine