High-intensity focused ultrasound (HIFU) is a therapeutic modality that can produce coagulative necrosis in biological tissue, noninvasively. This technique requires the determination of the focus of a HIFU transducer before treatment. We investigated a method to localize the focus based on the change in radio-frequency (RF) signal at HIFU intensity levels below the threshold for tissue damage. 2-D RF data were collected during the HIFU exposure at subablative intensity levels in ex vivo chicken tissue. The HIFU intensity was then increased to induce necrotic lesions in tissue, visualized as hyperecho in B-mode images, and the location of hyperecho was used to represent physical lesions, as established in our prior studies. Results showed that focal RF amplitude increased immediately after HIFU started at subablative intensity. The focus was clearly visualized in a 2-D map of the average rate of change (ARC) of RF amplitude. The 2-D difference between focus locations based on RF data and hyperecho was 1.92 +/- 1.02 mm (mean +/- standard deviation), with no statistically significant difference (axial: p = 0.50, lateral: p = 0.07, paired t -test). Histological analysis was also performed to examine structural change in tissue after HIFU exposure. The results indicated that ARC map could be used to determine the focus of a HIFU transducer at subablative intensity, and it has the potential to be used for HIFU treatment targeting.
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