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Abstract:
We have designed and built low-cost compact ultrasound imaging-guided robotic HIFU (High-Intensity Focused Ultrasound) ablation device for thermal damage of solid tumors in small animals. Before this device is used to treat animals, experimental studies on ex vivo tissues were necessary to assess the accuracy of its targeting, ensuring the safety of therapy. The objective of this study was to assess the targeting accuracy of our device in the focal and axial plane of the HIFU beam using ex vivo tissue embedded in a reference cylindrical chamber inside which a coaxial cylindrical volume with a smaller diameter was ablated. HIFU beams with selected acoustic parameters, generated by a singe-element bowl-shaped 64-mm HIFU transducer operating at 1.08 MHz or 3.21 MHz frequency, were propagated in two-layer media: water-tissue (50 mm-40 mm) and focused at 12.6-mm depth below the tissue surface. Cylindrical necrotic lesions of various size were created by moving the chamber using a computercontrolled precise positioning unit. Lesions formed were compared with those intended for treatment using various visualization methods and displacement between their centers were determined. The targeting accuracy in the focal and axial planes was found to be respectively about 98% and 86% when determined from photos and about 88% and 76% when determined from MR images. The displacement between the centers of the necrotic lesion formed and planned for treatment was about 1 mm in the focal plane and about 2 mm in the axial plane. Our ablation device can be used as an effective and safe tool to plan, monitor and treat solid tumors in small animals and to test new anti-cancer drugs in preclinical studies.

Abstract:
We have designed and built ultrasound imaging-guided HIFU ablative device for preclinical studies on small animals. Before this device is used to treat animals, ex vivo tissue studies were necessary to determine the location and extent of necrotic lesions created inside tissue samples by HIFU beams depending on their acoustic properties. This will allow to plan the beam movement trajectory and the distance and time intervals between exposures leading to necrosis covering the entire treated volume without damaging the surrounding tissues. This is crucial for therapy safety. The objective of this study was to assess the impact of sonication parameters on the size of necrotic lesions formed by HIFU beams generated by 64-mm bowl-shaped transducer used, operating at 1.08 MHz or 3.21 MHz. Multiple necrotic lesions were created in pork loin samples at 12.6-mm depth below tissue surface during 3-s exposure to HIFU beams with fixed duty-cycle and varied pulse-duration or fixed pulse-duration and varied duty-cycle, propagated in two-layer media: water-tissue. After exposures, the necrotic lesions were visualized using magnetic resonance imaging and optical imaging (photos) after sectioning the samples. Quantitative analysis of the obtained results allowed to select the optimal sonication and beam movement parameters to suport planning of effective therapy.

Keywords:
automated ultrasound imaging-guided HIFU ablation system, ex vivo tissue, ultrasonic exposure parameters, extent of necrotic lesions