Ultrasound Delta CBE Imaging: A New Approach Based on Local Energy Subtraction to Localization of the HIFU Focal Spot Using Changes in Backscattered Energy

Purpose: High-intensity focused ultrasound (HIFU) is a promising non-invasive technique for thermal ablation of tumors. Positioning the focal point of the HIFU accurately prior to the procedure is crucial to the success of the treatment. A change in backscattered energy (CBE) in ultrasound images has been shown to allow visualization of thermal information and can be used to locate the focal spot of HIFU prior to ablation. In CBE imaging, however, tailing artifacts may exist below the focal point of HIFU to hinder the identification of the HIFU focal spot. Methods: This study proposed ultrasound delta CBE (DCBE) imaging that reduces CBE artifacts by local energy subtraction between measured and the reference envelope images. Phantom experiments were performed for validation of the proposed method. A HIFU system operating at a frequency of 2.12 MHz was used to heat phantoms, which were imaged with a clinical ultrasound scanner equipped with a 3-MHz convex transducer for analysis of CBE and DCBE data. Results: The results showed that the DCBE value increases monotonically with temperature (correlation coefficient = 0.90). Particularly, DCBE imaging can identify the HIFU focal spot, suppress tailing artifacts, and increase the contrast between the focal and artifact zones by 8 dB in comparison with conventional CBE imaging. Conclusion: Based on this study, DCBE imaging may be an effective method of locating HIFU focal points through ultrasound backscattered energy.