Cylindrical thermal coagulation necrosis using an interstitial applicator with a plane ultrasonic transducer: in vitro and in vivo experiments versus computer simulations.

Extracorporeal HIFU techniques still cannot be used to treat tumours of the digestive tract, therefore an interstitial applicator has been developed to fill this gap. The object of the study was to validate the use of a plane ultrasonic transducer in an interstitial applicator to obtain large sector based or cylindrical coagulation necrosis. Two very different shot sequences were performed in vitro and in vivo and compared with numerical calculations. Each sequence consisted of 20 shots. After each shot the applicator was rotated through an angle of 18 degrees. Each shot in sequence 1 lasted 20 s, with a 2-min interval between shots which can be considered as independent. The second sequence involved coupling the shots so that each benefits from the heat deposited by the preceding ones. The first shot lasted 20 s to establish the lesion, then the duration of the subsequent 19 shots was 10 s to take into account the temperature rise due to preceding shots. In both cases, it was shown that cylindrical necrosis resulted in vivo and in vitro: 20 mm diameter and 8mm in height. The dimensions and the shapes of the necrosed volumes agreed with numerical predictions: the necrosed area induced by sequence 1 had a serrated border, whereas that of the second sequence was much more uniform. It was also shown that, for the two sequences, less than 20 s were necessary to coagulate the tissues in each direction. The results with sequence 2 showed that coupling the shots could be used to reduce treatment time without modifying the necrosed volume.