Dependence of ultrasound echo decorrelation on local tissue temperature during ex vivo radiofrequency ablation

Abstract

This study investigates echo decorrelation imaging, an ultrasound method for thermal ablation monitoring. The effect of tissue temperature on the mapped echo decorrelation parameter was assessed in radiofrequency ablation experiments performed on ex vivo bovine liver tissue. Echo decorrelation maps were compared with corresponding tissue temperatures simulated using the finite element method. For both echo decorrelation imaging and integrated backscatter imaging, the mapped tissue parameters correlated significantly but weakly with local tissue temperature. Receiver operating characteristic (ROC) curves were used to assess the ability of echo decorrelation and integrated backscatter to predict tissue temperature greater than 40, 60, and 80 °C. Significantly higher area under the ROC curve (AUROC) values were obtained for prediction of tissue temperatures greater than 40, 60, and 80 °C using echo decorrelation imaging (AUROC = 0.871, 0.948 and 0.966) compared to integrated backscatter imaging (AUROC = 0.865, 0.877 and 0.832).

Figure 1

(a) Schematic diagram of the in vitro RFA experiments. (b) Experimental setup for RFA experiments. (c) Ultrasound B-mode image showing tips of the RFA electrode (yellow circle) and two thermocouples (red circles) 9 mm (right) and 11 mm (left) away from the RF electrode.

Figure 2

Time-dependent echo decorrelation and temperature for a representative treatment. (a) Instantaneous echo decorrelation near the RFA probe, without temporal averaging. (b) Corresponding running-averaged, log₁₀-scaled echo decorrelation (blue line) and cumulative decorrelation (green line). (c) Simulated tissue temperature at the same location.

Figure 3

Hybrid echo decorrelation images and tissue temperature for a representative RFA treatment. (a) After 15 s treatment. (b) After 20 s treatment. (c) 60 s treatment. The red crosses indicate the location of the two thermocouples.

Figure 4

Parametric images, simulated temperatures, and tissue histology for three representative RFA treatments. (a) Hybrid echo decorrelation images, with dashed lines (yellow, red, and black) representing boundaries predicted using optimum decorrelation thresholds for temperatures greater than 40, 60, and 80 °C. (b) Hybrid integrated backscatter images, with dashed lines (yellow, red, and black) representing boundaries predicted using optimum integrated backscatter thresholds for 40, 60, and 80 °C. (c) Temperature profiles simulated using the tissue physical parameters estimated by UKF with equal-temperature contours at 40, 60, and 80 °C represented by dashed yellow, red, and black lines respectively. (d) Scanned tissue sections, with dashed black lines representing lesion boundaries.

Figure 5

Scatter plots of mapped image parameters plotted against simulated tissue temperature for all sampled spatiotemporal points in the 15 RFA experiments. (a) Running-average, log₁₀-scaled echo decorrelation. (b) Running-average, decibel-scaled integrated backscatter.

Figure 6

ROC curves for prediction of RFA effects. (a) Prediction of tissue temperatures greater than 40, 60, and 80 °C using echo decorrelation. (b) Prediction of tissue temperatures greater than 40, 60, and 80 °C using integrated backscatter. (c) Prediction of lesion boundaries segmented from gross tissue histology.

Figure 7

Scatter plots comparing measured to predicted ablated areas. (a) Areas predicted using echo decorrelation. (b) Areas predicted using integrated backscatter.