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.

FIG. 1.

(Color online) Diagrams and photos of experimental setup for the no-strain configuration [(a) and (b)] and strain configuration [(c) and (d)] showing the indenter, bovine skeletal muscle, mounting apparatus, and imaging transducer position.

FIG. 2.

A representative cross-sectional B-scan from the acquired volumetric image of bovine muscle tissue modeling the human pubovisceral muscle. The ROI used for echo decorrelation computations is shown by a superimposed rectangle.

FIG. 3.

(Color online) Simulated echo decorrelation images of a random medium with a lesion represented by a region of decoherence with average SNR of 20 dB. Top left: Logarithmically scaled decorrelation map without tissue motion and with 20 dB SNR. Top right: Decorrelation map corrupted by tissue motion with 20 dB SNR. Center left: Decorrelation map after compensation for tissue motion and noise. Center right: Actual decoherence of the medium. Bottom: Cross sections through echo decorrelation images of the simulated lesion for all four cases, along the segment at azimuth 0 mm spanning the range 10–30 mm.

FIG. 4.

(Color online) RMS error between echo decorrelation and actual scattering medium decoherence for simulations with varying displacements and SNR values with and without compensation. Top left: RMS error vs SNR for a motionless medium. Remaining panels: RMS error vs displacement in the elevation (x), azimuth (y), and range (z) directions for noiseless echo data.

FIG. 5.

(Color online) Normalized RMS error between simulated echo decorrelation maps and actual decoherence for four lesion size parameters α as a function of the correlation window size parameter σ.

FIG. 6.

(Color online) Measured and theoretical echo decorrelation caused by tissue motion in an ex vivo model of the human pubovisceral muscle. The theoretical decorrelation is shown with a solid line with open circle markers for the no strain case in both panels and is shown with a dashed line with star markers for the strain case in the right panel. Also in both panels is the measured decorrelation (mean ± standard deviation) shown by a dashed-dotted line with triangle markers for the no-strain case and by a dashed-dotted line with inverted triangle markers for the strain case. Left: Echo decorrelation caused by motion in the elevation (x) direction. Right: Echo decorrelation caused by tissue indentation in the range (z) direction.

FIG. 7.

(Color online) Measured vs theoretical motion-induced echo decorrelation for paired pulse- echo images of swine liver in vivo. The mean and standard deviation of the decorrelation values from each measured and theoretical echo decorrelation image are shown.

FIG. 8.

(Color online) Echo decorrelation images of swine liver undergoing RFA in vivo. Each panel shows a maximum-amplitude projection of the log-scaled echo decorrelation during the ablation treatment, superimposed on the corresponding B-scan image. Left: Uncompensated images for frames 30 (early in treatment), 50 (mid-treatment), 70 (late in treatment), and 110 (post- treatment). Right: Images compensated based on motion- and noise-induced decorrelation estimated from a sham data set taken before the ablation treatment. The RFA probe tip is indicated by the dashed circle in the upper left panel.