In Vivo Thermal Ablation of Deep Intrahepatic Targets Using a Super-Convergent MRgHIFU Applicator and a Pseudo-Tumor Model

Abstract

Background: HIFU ablation of liver malignancies is particularly challenging due to respiratory motion, high tissue perfusion and the presence of the rib cage. Based on our previous development of a super-convergent phased-array transducer, we aimed to further investigate, in vivo, its applicability to deep intrahepatic targets. Methods: In a series of six pigs, a pseudo-tumor model was used as target, visible both on intra-operatory MRI and post-mortem gross pathology. The transcostal MRgHIFU ablation was prescribed coplanar with the pseudo-tumor, either axial or sagittal, but deliberately shifted 7 to 18 mm to the side. No specific means of protection of the ribs were implemented. Post-treatment MRI follow-up was performed at D7, followed by animal necropsy and gross pathology of the liver. Results: The pseudo-tumor was clearly identified on T1w MR imaging and subsequently allowed the MRgHIFU planning. The peak temperature at the focal point ranged from 58–87 °C. Gross pathology confirmed the presence of the pseudo-tumor and the well-delineated MRgHIFU ablation at the expected locations. Conclusions: The specific design of the transducer enabled a reliable workflow. It demonstrated a good safety profile for in vivo transcostal MRgHIFU ablation of deep-liver targets, graded as challenging for standard surgery.

Computer-Aided Intra-Operatory Positioning of an MRgHIFU Applicator Dedicated to Abdominal Thermal Therapy Using Particle Swarm Optimization

“…The design of our super-convergent transducer [11] , [28] , spreading the incident energy over a large aperture and minimizing the secondary acoustic lobes, is the first key of its safe performance. As an additional layer, we describe the current optimization software.…”

“…The device is a phased-array transducer dedicated to transcostal liver ablation [11] , [28] (Imasonic, Voray-sur-l'Ognon, France) working at 650 kHz. The 256-element transducer is populated with elements widely distributed on 5 concentric segments of spheres of different radii (100, 111 and 124 mm) to increase the contact surface with the skin while guaranteeing a natural focal point at 10 cm-depth (Fig.…”

“…During post mortem examination, no skin lesions were detected on the thoracic and abdominal regions. A few millimeter-sized rib thermal lesions of grade 1 (6/6) were assigned, without any evidence found during post-mortem examination [11] . Here, the target of the EP was the center of mass of the effective thermal ablation.…”

“…Ultrasound propagation through tissues with significant impedance change is subject to undesirable effects at interfaces, including absorption, reflections, standing waves, and focal point shift. The higher energy absorption rate of the bone, approximately a factor of 12 compared to the soft tissues [11] may induce substantial heating in the near field (e.g rib cage) and may even propagate into skin burns or infiltrating thermal lesions [12] , [13] , [14] . Breathing motion has been addressed by numerous reports, for instance using two-dimensional ultrasound imaging [15] , [16] , [17] or a one-dimensional MR navigator [18] .…”

“…Very recently, Lorton et al [11] , [28] presented a new concept of transcostal HIFU transducer able to thermally ablate in vivo deep-seated targets of pig livers. The feasibility and targeting accuracy of tumor ablation located in regions considered as challenging to resect was demonstrated in 6 pig livers.…”