Time and Frequency domain approaches for the characterization of hydroxyapatite granules by ultrasounds

The aim of the experimental analysis was to investigate the nature of subresolution targets, such as Hydroxy Apatite (HA) granules, in order to assess their detectability within a tissue mimicking material. Microcalcifications under 1mm are of clinic interest in mammary pathologies, and we considered HA granules with mean diameters ranging from 64 to 651 μm. Being the features of microcalcifications important indicators in the mammographic screening programs, we did target those microcalcifications to evaluate some diagnostic feasibility, complementary with respect to traditional B-mode imaging. The experimental characterization was conducted with a single element 5 MHz unfocused transducer and a single element 10.7 MHz focused transducer. "Ad hoc" Agar phantoms were scanned by means of a translation stage with a bidirectional repeatability of 2 μm. The acquired signals produced by the ultrasounds backscattered by the microgranules were processed in the frequency and time domains. The results showed a suitable agreement with the Faran theory for the backscatter from spherical targets, both in the frequency domain, when considering the amplitude spectrum of the received echoes, and in the time domain, when Signal-To-Noise and Contrast-To-Noise ratios of such echoes were computed.