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Effective scatterer size estimates in HT-29 spheroids at 55 MHz and 80 MHz

Spheroids provide a three-dimensional in-vitro model of cell-to-cell interaction and basic structure. High-frequency ultrasound studies of spheroids show good image contrast between viable rims and necrotic cores, but spectral analyses have not been performed. This study examines colorectal HT-29 spheroids using high-frequency ultrasound at 55MHz (n=5) and 80MHz (n=4) to determine changes in spectral parameters as a function of transducer frequency and location (core vs rim). Histology shows low cell density within the core and a tightly packed rim, which forms a smooth shell. Statistically significant differences were found in the midband fit, spectral slope and effective scatterer diameter estimate as a function of imaging frequency and spheroid location. Effective scatterer diameter estimates (12.7 ± 0.4 μm at 55MHz and 6.6 ± 0.5 μm at 80MHz) agree closely with histological estimates (12.9 ± 0.3 μm for the cell and 7.0 ± 0.3 μm for the nucleus) suggesting different primary scatterers at the two frequencies.