Analysis of three-dimensional echo decorrelation and integrated backscatter imaging during ex vivo radiofrequency ablation

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Radiofrequency ablation experiments were performed in ex vivo bovine liver (N = 14) using a clinical ablation system (RITA 1500X generator and StarBurst probe, Angiodynamics). Three-dimensional (3-D) ultrasound images were acquired as complex beamformed echo volumes from a Siemens Acuson SC2000 scanner with a 4Z1c matrix array. Echo decorrelation images, depicting echo changes between sequential volumes (interframe time 18 ms), and integrated backscatter (IBS) images, depicting local changes in echogenicity relative to baseline, were computed in 3-D at 11 s intervals throughout each treatment. To assess potential for real-time prediction of tissue ablation, cumulative 3-D echo decorrelation and integrated backscatter images were compared to reconstructed ablation zones, segmented from optically scanned tissue sections. Both echo decorrelation and IBS provided good prediction of local ablation using receiver operator characteristic (ROC) curve analysis (area under curve 0.91 for echo decorrelation normalized to global echo energy; >0.85 for all parameters), with accuracy depending weakly on correlation window size. While ablation regions were predicted fairly well (Dice coefficients >0.6), ablation zone volume was not consistently predicted by thresholded parameter maps. Both echo decorrelation and IBS showed weak but statistically significant correlation with simultaneous, co-located tissue temperatures measured by four thermocouples on the RF probe.