On the use of the Structure Factor Model to understand the measured backscatter coefficient from concentrated cell pellet biophantoms

Quantitative ultrasound technique utilizes the amplitude and frequency dependence of the backscatter coefficient (BSC) to estimate the size and concentration of scatterers. QUS techniques rely on theoretical scattering models to fit the BSC from biological tissues to an estimated BSC using an appropriate model. Franceschini and Guillermin (J. Acoust. Soc. Am. 132(6) 3735-7347 2012) recently showed that the Structure Factor Model (SFM) was the more suitable theoretical scattering model for dealing with concentrated medium, such as densely packed cells in tumors. The aim of the present study is to use the SFM to understand and identify the scattering sites in concentrated cell pellet biophantoms. The biophantoms consisted of a suspension of human erythromyeloblastoid leukemia (K562) cells trapped in a mixture of plasma and thrombin. The biophantoms had identical cells (cell radius r_c around 6.44 μm) but had different cell volume fractions φ_c of 0.03, 0.06, 0.18 and 0.30. Ultrasonic backscatter measurements were made from frequencies from 10 MHz to 42 MHz using an ultrasound scanner (Vevo 770, Visualsonics, Toronto, Canada) equipped with the RMV 710 and 703 probes. In order to understand these four measured BSC behaviors, one cell (composed of a nucleus and of a cytoplasm) was assumed to scatter as an effective and single fluid sphere. An optimization procedure was then performed to minimize a cost function, which is the average over frequency of the difference between the sum of four measured BSC and the sum of four estimated BSC using the SFM. Two parameters were estimated: the effective scatterer radius and the corresponding effective impedance contrast. The goodness of the fit for the cell concentration ranging from 0.03 to 0.18 shows that the SFM is sufficient to explain the frequency-dependance and amplitude of the measured BSC from concentrated cell pellet biophantoms. Using the SFM and K562 cell pellet biophantoms, we foun- that the main cellular structure responsive for scattering in the frequency range 10-42 MHz is the entire cell.