Angle monitoring of directional energy deposition for catheter-based ultrasound thermal therapy using fitted changes in ultrasound backscatter energy imaging: ex vivo investigation

Objective: Catheter-based ultrasound (CBUS) can treat deep tumors precisely with angle control of directional energy deposition and spatial control of ablation zones. To address the low sensitivity and contrast of conventional ultrasound imaging during monitoring of CBUS thermal therapy, multi-spatiotemporal (MST) compounding changes in ultrasound backscatter energy (CBE) method have been proposed in our previous work. However, there is no investigation of monitoring of angle or direction of energy deposition during CBUS therapy to ensure proper targeting and safety. This study aimed to investigate the feasibility of angle monitoring of energy deposition of CBUS using the fitted CBE method. Methods: Ex vivo samples (n=9) of degassed beef liver were ablated using a directional CBUS applicator (transducer 1.5 mm OD x 10 mm, 180 degrees energy pattern, Catheter OD=2.4 mm, fc=8MHz, power=12W) at 3 treatment rotation angles of 0 degrees, 180 degrees, 270 degrees. Radiofrequency (RF) data were recorded by Ultrasonix MDP system with an endorectal probe (BPC8-4). Ground truths of ablation angle were obtained from the subsequent lesion sectioning and measurement. Three components of absolute, positive, and negative CBE and MSTCBE maps were calculated from the RF data, which were further fitted to enhance the angle estimation robustness. The ablation angles were estimated from the center position of CBUS to the maximum values of the fitted CBE and MST-CBE maps. Results: According to the ground truths of nine groups, with maximum and minimum removed to reduce the influence on statistical analysis, the components of positive fitted CBE and positive MST-CBE maps had the smallest average values of absolute errors in two method groups of CBE and MST-CBE, respectively, which is 6.5 +/- 7.3 degrees and 10.3 +/- 11.2 degrees, respectively. Conclusion: These ex vivo investigations illustrated that the fitted CBE imaging can monitor the positions and angles of maximum changes of energy deposition, which facilitates the direction and spatial control for precise thermal therapy using CBUS.