Parametric imaging of specular reflections and diffuse scattering of tissue from multi-directional ultrasound echo signal data

In this paper, an approach for the extraction of quantitative parameters from multi-directional ultrasound echo signal data for the differentiation and characterization of specular reflections and diffuse backscattering is presented and evaluated. Spatially resolved, tissue-characterizing parametric images are calculated by analyzing the envelope of echo signals from different insonation angles. Frames of echo signals, which are acquired with a limited-angle spatial compound imaging system, are scan-converted, and the envelope of echo signals at each pixel in the axial / lateral imaging plane is analyzed as a function of the insonation angle. Statistical first-order parameters are directly calculated from this data, and further parameters are derived from fitted model functions. The proposed concept has been evaluated by means of phantom measurements and in vivo measurements on skin with a 20 MHz high-frequency ultrasound (HFUS) system for limited-angle (up to +/-40deg) pulse-echo measurements. Results of measurements on a plastic tube, which is surrounded by a speckle phantom, show that specular reflections can be distinguished from diffuse backscattering. Furthermore, the surface orientation of specular reflectors is depicted in parametric images. In vivo images of skin show that specular reflections, for example at scar tissue and at the border between the dermis and the subcutaneous fat, can be detected and differentiated from their surrounding based on the proposed parameters.