A Novel Concept of a Phased-Array HIFU Transducer Optimized for MR-Guided Hepatic Ablation: Embodiment and First In-Vivo Studies

Abstract

PurposeHigh-intensity focused ultrasound (HIFU) is challenging in the liver due to the respiratory motion and risks of near-/far-field burns, particularly on the ribs. We implemented a novel design of a HIFU phased-array transducer, dedicated to transcostal hepatic thermo-ablation. Due to its large acoustic window and strong focusing, the transducer should perform safely for this application.Material and MethodsThe new HIFU transducer is composed of 256 elements distributed on 5 concentric segments of a specific radius (either 100, 111, or 125 mm). It has been optimally shaped to fit the abdominal wall. The shape and size of the acoustic elements were optimized for the largest emitting surface and the lowest symmetry. Calibration tests have been conducted on tissue-mimicking gels under 3-T magnetic resonance (MR) guidance. In-vivo MR-guided HIFU treatment was conducted in two pigs, aiming to create thermal ablation deep in the liver without significant side effects. Imaging follow-up was performed at D0 and D7. Sacrifice and post-mortem macroscopic examination occurred at D7, with the ablated tissue being fixed for pathology.ResultsThe device showed −3-dB focusing capacities in a volume of 27 × 46 × 50 mm3 as compared with the numerical simulation volume of 18 × 48 × 60 mm3. The shape of the focal area was in millimeter-range agreement with the numerical simulations. No interference was detected between the HIFU sonication and the MR acquisition. In vivo, the temperature elevation in perivascular liver parenchyma reached 28°C above physiological temperature, within one breath-hold. The lesion was visible on Gd contrast-enhanced MRI sequences and post-mortem examination. The non-perfused volume was found in pig #1 and pig #2 of 8/11, 6/8, and 7/7 mm along the LR, AP, and HF directions, respectively. No rib burns or other near-field side effects were visually observed on post-mortem gross examination. High-resolution contrast-enhanced 3D MRI indicated a minor lesion on the sternum.ConclusionThe performance of this new HIFU transducer has been demonstrated in vitro and in vivo. The transducer meets the requirement to perform thermal lesions in deep tissues, without the need for rib-sparing means.

Perfluorocarbon emulsion enhances MR-ARFI displacement and temperature in vitro: Evaluating the response with MRI, NMR, and hydrophone

“…Though, more adverse events have been reported, including biliary obstruction, fistula formation, osteonecrosis, and diaphragmatic rupture (30). A variety of techniques were developed to overcome some limitations of the procedure including rib shielding devices (32), a large acoustic window super-focusing transducer dedicated to intra-abdominal treatments (33), and techniques to account for respiratory motion (34)(35)(36)(37).…”

Threatment Strategies for Recurrent Hepatocellular Carcinoma Patients: Ablation and its Combination Patterns

“…Lorton et al designed a HIFU phased-array transducer dedicated to transcostal hepatic thermoablation. This transducer meets the requirement to perform thermal lesions in deep tissues without the need for rib-sparing methods 53 .…”

A new sonoablation using acoustic droplet vaporization and focused ultrasound: A feasibility study

“…[7][8][9] This may create problems in clinical practice because these treatments require a wide acoustic window for ultrasound (US) penetration, which rarely exist. 10,11 Recently, small transrectal transducers for histotripsy have also been designed, fabricated, and evaluated, however, they are only for prostate histotripsy. 12,13 The human body fundamentally lacks a good acoustic window, except for superficial organs such as the breast.…”

“…However, both of these approaches require a large‐array focusing transducer, sometimes as large as 20 cm in diameter, and a high‐amplitude peak negative pressure (PNP) from approximately 15 to 30 MPa 7–9 . This may create problems in clinical practice because these treatments require a wide acoustic window for ultrasound (US) penetration, which rarely exist 10,11 . Recently, small transrectal transducers for histotripsy have also been designed, fabricated, and evaluated, however, they are only for prostate histotripsy 12,13 .…”