Analysis of tissue changes, measurement system effects, and motion artifacts in echo decorrelation imaging

Abstract

Echo decorrelation imaging, a method for mapping ablation-induced ultrasound echo changes, is analyzed. Local echo decorrelation is shown to approximate the decoherence spectrum of tissue reflectivity. Effects of the ultrasound measurement system, echo signal windowing, electronic noise, and tissue motion on echo decorrelation images are determined theoretically, leading to a method for reduction of motion and noise artifacts. Theoretical analysis is validated by simulations and experiments. Simulated decoherence of the scattering medium was recovered with root-mean-square error less than 10% with accuracy dependent on the correlation window size. Motion-induced decorrelation measured in an ex vivo pubovisceral muscle model showed similar trends to theoretical motion-induced decorrelation for a 2.1 MHz curvilinear array with decorrelation approaching unity for 3-4 mm elevational displacement or 1-1.6 mm range displacement. For in vivo imaging of porcine liver by a 7 MHz linear array, theoretical decorrelation computed using image-based motion estimates correlated significantly with measured decorrelation (r = 0.931, N = 10). Echo decorrelation artifacts incurred during in vivo radiofrequency ablation in the same porcine liver were effectively compensated based on the theoretical echo decorrelation model and measured pre-treatment decorrelation. These results demonstrate the potential of echo decorrelation imaging for quantification of heat-induced changes to the scattering tissue medium during thermal ablation.

Echo Decorrelation Imaging of Rabbit Liver and VX2 Tumor during In Vivo Ultrasound Ablation

“…Compensation using eqn (1) resulted in significantly reduced artifactual echo decorrelation in non-ablated regions for focused exposures, unfocused exposures and all exposures combined in liver, indicating that substantial motion-induced decorrelation occurred in these experiments, as has been observed previously (Hooi et al 2015; Subramanian et al 2014). However, Figure 5 illustrates that prediction performance declined only slightly for both focused and unfocused exposures as the inter-frame time increased, indicating that effects of motion-induced decorrelation on ablation prediction were relatively small for inter-frame times as large as 0.85 s. Motion may still degrade ablation prediction performance by introducing imprecise image registration; this effect would be greater for focused exposures because of the smaller ablated region.…”

“…Artifactual decorrelation caused by respiration-induced tissue displacement and deformation, as well as electronic noise, may limit ultrasound monitoring of liver cancer ablation (Hooi et al 2015). Compensation using eqn (1) resulted in significantly reduced artifactual echo decorrelation in non-ablated regions for focused exposures, unfocused exposures and all exposures combined in liver, indicating that substantial motion-induced decorrelation occurred in these experiments, as has been observed previously (Hooi et al 2015; Subramanian et al 2014).…”

“…This relatively low spatial resolution might explain the smaller differences observed between decorrelation in non-ablated versus ablated regions in VX2 tumor. Hooi et al (2015), using simulated echo decorrelation images, reported that a correlation window size on the order of 1/6th the expected lesion size was most appropriate for estimating physical changes in the ablated tissue, whereas relatively small error was achieved over a wider range of window sizes, approximately 1/20th to 1/2 the lesion size. On the basis of these considerations, the 1.00-mm correlation window size employed here was nearly optimal for unfocused ablation, for which the approximate optimal window size estimated from simulated beams was about 1.67 mm, but was suboptimal for focused ablation, for which the approximate optimal window size was 0.02 mm (Fosnight 2015).…”

“…Echo decorrelation imaging is expected to successfully predict thermal ablation in a clinical setting, as echo decorrelation is believed to map thermally induced tissue microstructural changes as well as transient gas activity (Hooi et al 2015). Such microstructural changes include cellular swelling, microvascular changes, protein denaturation and tissue damage caused by water vaporization during thermal ablation (Gravante et al 2011), as well as changes in fat cells at the melting points of saturated fatty acids, approximately 43°C–69°C (Timberlake, 2014).…”

“…Estimates of artifactual cumulative decorrelation obtained from corresponding sham trials, denoted Δ sham , were used to construct corrected cumulative decorrelation, denoted Δ corrected , from uncorrected cumulative decorrelation, denoted Δ uncorrected , using the equation (Hooi et al 2015) $${\mathrm{\Delta }}_{\text{corrected}}false(\mathrm{boldr},tfalse)=\frac{{\mathrm{\Delta }}_{\text{corrected}}-{\mathrm{\Delta }}_{\text{sham}}}{1-{\mathrm{\Delta }}_{\text{sham}}}$$…”