Influence of ablated tissue on the formation of high-intensity focused ultrasound lesions

In order to ablate tumours using high-intensity focused ultrasound (HIFU) it is necessary to irradiate the tumour with a confluent array of single ultrasound exposures. We have identified a phenomenon that we term lesion-to-lesion interaction, which occurs when the spatial separation of individual exposures is such that an existing lesion appears to affect the formation of a subsequent lesion. This article investigates the implications of this phenomenon for strategies to ablate large tissue volumes in the treatment of hepatic metastases. Experiments on pig and rat livers have been carried out using a focused ultrasound system with a frequency of 1.7 MHz, an in situ spatially averaged focal intensity (I(SAL)) of 133-658 W cm^-2 (I(sp) of 239-1185 W cm^-2) and an exposure duration of 5-15 s. The results show that there is interaction between lesions at spatial exposure separations that depend on the intensities and exposure durations used. As a result, either subsequent lesions form closer to the ultrasound source (if the focal peak of the ultrasound beam is placed deep inside the liver tissue) or their length is reduced (if the focal peak is near the liver surface). An explanation is suggested for this effect and a strategy for its avoidance during in vivo HIFU treatment is discussed.