Echo decorrelation imaging for quantification of tissue structural changes during ultrasound ablation

Abstract

Echo decorrelation imaging is a pulse-echo method that maps millisecond-scale changes in backscattered ultrasound signals, potentially providing real-time feedback during thermal ablation treatments. Decorrelation between echo signals from sequential image frames is spatially mapped and temporally averaged, resulting in images of cumulative, heat-induced tissue changes. Theoretical analysis indicates that the mapped echo decorrelation parameter is equivalent to a spatial decoherence spectrum of the tissue reflectivity, and also provides a method to compensate decorrelation artifacts caused by tissue motion and electronic noise. Results are presented from experiments employing 64-element linear arrays that perform bulk thermal ablation, focal ablation, and pulse-echo imaging using the same piezoelectric elements, ensuring co-registration of ablation and image planes. Decorrelation maps are shown to correlate with ablated tissue histology, including vital staining to map heat-induced cell death, for both ex vivo ablation of bovine liver tissue and in vivo ablation of rabbit liver with VX2 carcinoma. Receiver operating characteristic curve analysis shows that echo decorrelation predicts local ablation with greater success than integrated backscatter imaging. Using artifact-compensated echo decorrelation maps, heating-induced decoherence of tissue scattering media is assessed for ex vivo and in vivo ultrasound ablation by unfocused and focused beams.