Ablation of tissue volumes using high intensity focused ultrasound.

Successful application of high intensity focused ultrasound to cancer treatment requires complete ablation of tissue volumes. In order to destroy an entire tumour it is necessary to place a contiguous array of touching lesions throughout it. In a study of how best to achieve this, exposures were selected to give single lesions that were thermal in origin, while avoiding effects due to tissue water boiling and acoustic cavitation. Arrays were formed in excised bovine liver. Under some exposure conditions, lesions were found to merge in front of the focal point, and failed to cover the desired volume. Using fine wire manganin-constantan thermocouples, temperature studies revealed a substantial rise in the temperature of surrounding untreated tissue. Cooling curves showed that it was necessary to allow surrounding tissue to cool for up to 2 min before ambient temperature was reached. By allowing the tissue to cool between exposures it was possible to form arrays of overlapping lesions thus successfully ablating the complete target region.