[Tissue ablation by focused ultrasound].

Tissue lesions can be induced at the focal point of highly focused transducers with a frequency of 1 and 2.25 MHz for exposure times of less than 1 second. The energy generated by a high power amplifier (7.5 kilowatts effective at 1 MHz) is delivered in the form of series of impulses lasting between 10 and 1,000 milliseconds. The experimentation was conducted in the rat kidney (the left kidney, normally supplied by its vascular pedicle, was exteriorised during ultrasound treatment and then returned to the abdomen). The animal was sacrificed 3 days later and the lesions were studied by serial histological sections. 248 ultrasound shots were performed between January and September 1990. They allowed the definition of the time and intensity constants necessary to induce total destruction of the renal tissue at the focal point. Depending on the energy delivered, an elliptical cavity with a mean height of 1.2 to 4.6 mm and a mean diameter of 0.6 to 3 mm is observed at the focal point after a single shot. No cell structures were visible in the cavities and, in general, the cavity was prolonged by a cone-shaped region of coagulated necrosis with an inferior base. The mechanism responsible for this focused ultrasonic tissue destruction (FUTD) involves a variable combination of thermal and mechanical effects which depends on the ultrasound intensity delivered at the focal point of the transducer.