Non-invasive fast, large and selective in vivo HIFU ablation of the liver with a toroidal transducer

Treatments using high intensity focused ultrasound (HIFU) are routinely used to treat many kinds of diseases. Common HIFU systems use spherical transducers to create deep but small lesions. As an alternative, we have been developing intra-operative toroidal HIFU transducers allowing ablation rates 30 times faster than spherical transducers. This technology is currently under clinical validation for treating liver metastases. In the present study, we are extending our previous work to provide a completely non-invasive treatment of the liver with toroidal HIFU transducers. After numerical simulations and in vitro studies, in vivo experiments were conducted in pigs. The HIFU transducer has a toroidal shape with a diameter of 73 mm, a radius of curvature of 70 mm focusing on a circle of 30 mm in diameter. The transducer was divided into 32 concentric rings of equal surface. The working frequency was 2.5 MHz. An ultrasound imaging probe operating at 7.5 MHz was placed in the center of the HIFU transducer. Thirty-two power amplifiers were used to drive the 32 individual HIFU transducers for dynamic focalization. Inside each amplifier, a directional coupler allowed to measure the direct and reflected electrical power. A peristaltic pump drove coupling water at 8°C around a closed circuit to cool the transducer during treatment. In vivo trials were conducted in 4 pigs under general anesthesia. Animals were positioned as to have an anatomical access to approximately 50% of the liver whilst avoiding the rib cage. The exposure parameters were defined based on numerical simulations and preliminary in vitro experiments. In particular, three sets of exposure parameters were defined according to the: (i) the angle between the probe and the skin, (ii) the thickness of intermediate tissues (less than 10 mm, between 10 and 15 mm or between 15 and 20 mm) and (iii) the acoustic attenuation of intermediate tissues. The probe was held by hand and placed on the skin under ultrasound imaging guidance to target the liver. HIFU ablations were created using an acoustic energy between 3870 J and 6450 J corresponding to an exposure time between 30 and 50 seconds. Four lesions were created in the liver under in vivo conditions without skin burns and without damaging the intermediate tissues. The toroidal probe allowed for fast, large and precise ablations. Lesions were reproducible with an average diameter of 11.6±1.7 mm, an average length of 16.8±4.5 mm and a safety margin of 4.2±3.3 mm. Treatment parameters are sensitive to thickness variations of intermediate tissues within a 5 millimeter precision. This in vivo work provides a first demonstration of the feasibility and the safety of treating the liver non-invasively with a toroidal transducer. Ablations were large and obtained in less than one minute of treatment without resorting to mechanical scanning.