Comparison between sparse and concentrated scattering models for characterization of cell-pellet biophantoms and excised mouse tumors

Three scattering models were examined for characterizing in vitro biophantoms and ex vivo colon adenocarcinoma (HT29) mouse tumors in the 10-42 MHz and the 15-42 MHz frequency bandwidth, respectively. The Spherical Gaussian Model (SGM) and the Fluid-Filled Sphere Model (FFSM) that were examined are suitable for dealing with sparse media, whereas the Structure Factor Model (SFM) is adapted for characterizing concentrated media. Biophantoms results demonstrated the superiority of the SFM for all investigated volume fractions (i.e., from 0.006 to 0.30). In particular, the sparse SGM and FFSM underestimated scatterer size and overestimated acoustic concentration when the volume fraction was greater than 0.12. The SFM-based tumor results yielded scatterer radius and volume fraction in good agreement with the expected whole cell structures obtained from histological analysis (with relative errors less than 15%), whereas the sparse models gave inconsistent QUS estimates. These results suggest that the whole HT29 cells could be responsible for scattering. Moreover, the concentrated SFM provides better fit to the BSC data when compared with classical models (SGM, FFSM) for both experiments on cell-pellet biophantoms and mouse tumors. This study demonstrates that the SFM is the most-suitable model for densely packed scatterers, e.g., tissue.