| High Intensity Focused Ultrasound (HIFU) has been investigated as a non-invasive ablation method for morbid tissue in various diseases. It is of great importance to know where the focus would be in HIFU treatment planning to ensure that only the desired tissue is ablated while sparing normal tissue. We investigated a method for HIFU targeting based on the change in radiofrequency (RF) signals at pre-ablative HIFU intensity levels. In ex vivo experiment, HIFU was applied to chicken tissue at low intensity levels for 5 seconds, during which no tissue damage was observed. RF data were collected during this period and the amplitude change was analyzed. High intensity was then applied to induce lesion in tissue and hyperecho in B-mode images. Focus estimated from RF data was compared with that obtained from hyperechoic region. Results showed that RF amplitude of the focal region increased immediately after the start of HIFU exposure, even at pre-ablative intensity levels. The difference between focus locations based on RF data and B-mode images was 1.92 ± 1.02 mm (mean ± standard deviation). No statistically significant difference was found between focus coordinates in the axial and lateral dimensions (axial: p = 0.50 and lateral: p = 0.07). The increasing rate in RF amplitude was shown to be a signature of HIFU focus and could be used for HIFU treatment planning. |