Treatment by high-intensity ultrasound is well-established but requires extended treatment. A device composed of 32 elements arranged on a toroidal transducer was developed to increase the coagulated volume. The first clinical objective for this device will be to treat liver metastases with a maximal diameter of 2 cm. It has been previously demonstrated that HIFU toroidal-shaped transducers can create a single conical lesion of 7 cm3 in 40 s (diameter: 20 mm, depth: 25 mm). Presented here is the use of a toroidal HIFU device to enlarge the ablation volume by using electronic beam steering. The device can be used intra-operatively (during surgery) or extracorporally.
A spindle torus is generated by the rotation of a circle around an axis of revolution with a distance between the axis and the center of the circle lower than the radius of the circle. After rotation, the volume obtained is composed of two envelopes that can be used to create a toroidal transducer. To date, our previous work on toroidal transducers used the outer envelope as a reference surface. Here, the transducer geometry was based on the interior part of a torus. This produces a focus that is ring-shaped. In addition, due to this toroidal geometry, the ultrasound beams intersect between the principal focal ring and
the transducer surface to form a secondary focal zone, which contributes to reinforce the homogeneity of the lesion. The operating frequency was 2.5 MHz. The radius of curvature was 70 mm with a diameter of 67 mm. An ultrasound imaging probe was placed in a central circular opening of 26 mm in diameter. The transducer was also divided into 32 rings of 78mm2 each. Using a 32 channels amplifier with a phase resolution of 1.4 degrees, it was possible to change the diameter (0 to 15 mm) and depth (40 to 70 mm) of the focus to maximize dimensions of the lesion. Ablations were produced in vitro by placing the device directly in contact with the liver or in contact with a sample of intervening tissues (skin-fat-muscle).
Extracorporeal ablations were placed at an average depth of 32mm from the surface of the skin. Ten ablations were produced, each ablation was created in 55 seconds. The average depth of intervening tissues was 11±1 mm and the average depth of liver tissues that have been spared was 21±4 mm. No significant temperature rise in intervening tissues was measured (maximal temperature rise was 41°C). The dimensions of these ablations were an average diameter of 10±1 mm and a depth of 27±3 mm. In addition, 10 lesions were produced intra-operatively by placing the device directly on the liver surface. The dimensions of the HIFU lesions were a diameter of 34±4 mm and a depth of 45±2 mm were produced in vitro, in 370 seconds. This lesion is roughly spherical in shape. Therefore, if correctly positioned it would be possible to ablate a tumor of 3 cm in diameter with safety margins. This eliminates the need of juxtaposing mechanically single lesions.