Reducing the effects of specular scatterers on QUS imaging using the generalized spectrum

Abstract

The effective scatterer diameter (ESD) and effective acoustic concentration (EAC) are quantitative ultrasound (QUS) imaging parameters that employ scattering models and spectral fit methods to characterize tissue microstructure. These methods work best when the region of interest (ROI) from which the parameters are derived contains uniform diffuse scatterers. In some tissues, specular scatterers (e.g., calcifications, blood vessels, etc.) can exist and cause decreases in the accuracy and precision of QUS parameter estimates based on diffuse scattering. In this study the generalized spectrum (GS) intercept parameter was used to detect echoes from specular scatterers. The signals corresponding to the specular scatterers were then removed in order to reduce the effect of specular scatterers on QUS estimates. Backscatter data from a simulated phantom, rat mammary tumors, and fresh beef liver samples that underwent elevations in temperature were analyzed to evaluate the effectiveness of using the GS intercept parameter. The ESD and EAC were estimated assuming a spherical Gaussian scattering model for each data block outlined in an ROI in the sample. The GS intercept parameter was estimated for each data block and used to sort data blocks and their corresponding QUS estimates were sorted into diffuse and specular scattering groups. Modified parametric images were then formed by using only the data blocks in the diffuse scattering group. For the simulated phantom, the exclusion of specular scatterers from the QUS estimates resulted in a reduction in ESD standard deviation of 66.4%. For the rat mammary tumors, the average reduction in ESD and EAC standard deviation was 17.1% and 24.8%, respectively. When monitoring the changes in ESD and EAC in beef liver samples versus temperature over the temperature range of 37 to 50 °C, the mean ESD and EAC values changed monotonically with temperature. By excluding the specular scatterers, ESD and EAC were observed to change by 25.4% and −40.3% respectively as opposed to 14.8% and −30.7% respectively when including specular scatterers. When excluding specular scatterers from QUS analysis, the precision of QUS estimates was improved and the sensitivity of QUS estimates to temperature changes was increased. These results suggest that the GS intercept parameter has the potential to reduce the effects of specular scatterers on diffuse scattering estimates and to improve QUS imaging.