Towards Understanding the Nature of High Frequency Ultrasound Backscatter from Cells and Tissues: an Investigation of Backscatter Power Spectra from Different Concentrations of Cells of Different Sizes

During cell death a series of structural changes occur within the cell. We have shown that cell ensembles and tissues undergoing structural changes associated with various cell death pathways can be detected using high-frequency ultrasound. In our effort to better understand the nature of backscatter from collections of cells (which emulate tissues), we have collected raw RF backscatter data from cells of two different sizes in solutions for a series of concentrations or in pellet form. Human acute myeloid leukemia cells (AML-5, ~10µm in diameter) and transformed prostate cells (~25-30µm in diameter) were imaged either in suspension or in pellet form. Images and radiofreqeuncy data were acquired using a VS40B ultrasound instrument (VisualSonics Inc., Toronto, Ont) and 20MHz, 30MHz and 40MHz transducers with -6dB bandwidths approaching 100%. The cells were either imaged in degassed phosphate buffered solution in which their volumetric fraction increased from 0.0025% to 2%, or in pellet form by using a swinging bucket centrifuge. It was found that the backscatter power (as measured by the mid-band fit) increased by ~ 3 dB for both cell types in dilute solutions for which the volumetric concentration was doubled for a specific range of cell concentrations (which was dependent on cell size). In pellet form the backscatter power from the prostate cell pellets was ~12-14dB greater than the AML cell pellets. A comparison of the spectral slopes also strongly suggests a change in the scattering source contributions when the cells are in pellets: the spectral slope was negative for all concentrations for prostate cells imaged at 40MHz, but positive when measured in pellets. This is consistent an increased contribution to the backscatter of smaller sized scatterers (such as the cell nucleus) that manifests itself only when the cells are in pellets but not in solution. These data will be compared to theoretical predictions and their significance discussed.