Real-Time Spatiotemporal Control of High-Intensity Focused Ultrasound Thermal Ablation Using Echo Decorrelation Imaging in ex Vivo Bovine Liver

Abstract

The ability to control high-intensity focused ultrasound (HIFU) thermal ablation using echo decorrelation imaging feedback was evaluated in ex vivo bovine liver. Sonications were automatically ceased when the minimum cumulative echo decorrelation within the region of interest exceeded an ablation control threshold, determined from preliminary experiments as -2.7 (log-scaled decorrelation per millisecond), corresponding to 90% specificity for local ablation prediction. Controlled HIFU thermal ablation experiments were compared with uncontrolled experiments employing two, five or nine sonication cycles. Means and standard errors of the lesion width, area and depth, as well as receiver operating characteristic curves testing ablation prediction performance, were computed for each group. Controlled trials exhibited significantly smaller average lesion area, width and treatment time than five-cycle or nine-cycle uncontrolled trials and also had significantly greater prediction capability than two-cycle uncontrolled trials. These results suggest echo decorrelation imaging is an effective approach to real-time HIFU ablation control.

Keywords: Echo decorrelation imaging; High-intensity focused ultrasound; Real-time control; Thermal ablation.

Figure 1

Experimental setup. (a) 64 element image-treat transducer aligned with ex vivo bovine liver during the in vitro experiment, (b) Schematic of the experimental setup, (c) The developed real-time feedback control algorithm flow chart, (d) Timing diagram of the treatment scenario.

Figure 2

GUI of the implemented Qt-based C++ application used for the HIFU thermal ablation control and imaging; the controlling ROI is bounded by a yellow line.

Figure 3

Ablation prediction sensitivities and specificities for preliminary uncontrolled HIFU exposures in ex vivo liver (N=13) plotted vs. echo decorrelation threshold. The blue line represents the chosen ablation control threshold of −2.7 (log₁₀-scaled echo decorrelation per ms).

Figure 4

Means and standard errors of minimum (Δ_min) and average (Δ_avg) cumulative echo decorrelation values vs. ROI size for the controlled group; the blue line represents the ablation control threshold.

Figure 5

Representative vital-stained histologic sections and hybrid echo decorrelation/B-mode images for controlled and uncontrolled HIFU trials. (I) Controlled group. (II) 2-cycle group. (III) 5-cycle group. (IV) 9-cycle group. Column (a) shows representative treatments at the average acoustic intensity, while column (b) shows representative treatments at the maximum acoustic intensity employed. White lines indicate tissue boundaries segmented in the B-mode images, while yellow dashed lines show contours of the control threshold (−2.7 log₁₀-scaled echo decorrelation per millisecond) in the echo decorrelation maps. In the tissue sections, red and green boundaries indicate the segmented tissue and ablated regions. Lesion dimensions (width/depth/area) are close to reported means for each group.

Figure 6

Statistical results. (a) ROC curves showing the performance of the echo decorrelation imaging as an ablation predictor for each group. (b) Means and standard errors of ablated lesion width; (c) lesion area; (d) lesion depth. (*p ≤ 0.05; **p ≤ 10⁻²; ***p ≤ 10⁻³)