Ultrasound monitoring of in vitro radio frequency ablation by echo decorrelation imaging.

Objective: The purpose of this study was to test ultrasound echo decorrelation imaging for mapping and characterization of tissue effects caused by radio frequency ablation (RFA).

Methods: Radio frequency ablation procedures (6-minute duration, 20-W power) were performed on fresh ex vivo bovine liver tissue (n = 9) with continuous acquisition of beam-formed ultrasound echo data from a 7-MHz linear array. Echo data were processed to form B-scan images, echo decorrelation images (related to rapid random changes in echo waveforms), and integrated backscatter images (related to local changes in received echo energy). Echo decorrelation and integrated backscatter values at the location of a low-noise thermocouple were assessed as functions of temperature. Echo decorrelation and integrated backscatter images were directly compared with ablated tissue cross sections and quantitatively evaluated as predictors of tissue ablation and overtreatment.

Results: Echo decorrelation maps corresponded with local tissue temperature and ablation effects. Consistent echo decorrelation increases were observed for temperatures above 75 degrees C, whereas integrated backscatter maps showed a nonmonotonic temperature dependence complicated by acoustic shadowing, with high variance at large temperature elevations. In receiver operating characteristic curve analysis of echo decorrelation and integrated backscatter maps as predictors of local tissue ablation, echo decorrelation performed well (area under the receiver operating characteristic curve [AUROC] = 0.855 for ablation and 0.913 for overtreatment), whereas integrated backscatter performed poorly (AUROC < 0.6).

Conclusions: Echo decorrelation imaging can map tissue changes due to RFA in vitro, with local echo decorrelation corresponding strongly to local tissue temperature elevations and ablation effects. With further development and in vivo validation, echo decorrelation imaging is potentially useful for improved image guidance of clinical RFA procedures.