Coherence of Ultrasound Radiofrequency Echoes from the Liver Estimated Using Multi-Taper Calculation.

Acoustic scattering from the microvasculature in the liver is frequently modeled by a quasi-periodic distribution of point scatterers. The received echoes from such an arrangement of scatterers has random but correlated phase at frequency components corresponding to the average spacing of the quasi-periodic scatterers. The extent of the correlation between different frequency components is measured by calculating signal coherence. To date in ultrasound, estimators for coherence have used windowed short-time FFTs of radiofrequency data where the windows selected have been chosen ad hoc. Using receiver operating characteristic analysis (ROC), we show that Slepian sequences provide an estimator which more reliably distinguishes between liver and ablated tissue in an ex vivo setting than estimators using the Hann, Hamming, or Blackman-Harris window. For a gate length of 7 mm or 21 wavelengths we achieve an area under the ROC curve of 0.93 with Slepian sequences for coherence calculations.