No abstract

Assessing the proportion of biological cells in a volume of interest undergoing structural changes, such as cell death, using high-frequency ultrasound (20-100 MHz), requires the development of a theoretical model of scattering by any arbitrary cell ensemble. A prerequisite to building such a model is to know the scattering by a single cell in different states. In this paper, a simple model for the high-frequency acoustic scattering by one cell is proposed. A method for deducing the backscatter transfer function from a single, subresolution scatterer is also devised. Using this method, experimental measurements of backscatter from homogeneous, subresolution polystyrene microspheres and single, viable eukaryotic cells, acquired across a broad, continuous range of frequencies were compared with elastic scattering theory and the proposed cell scattering model, respectively. The resonant features observed in the backscatter transfer function of microspheres were found to correspond accurately to theoretical predictions. Using the spacing of the major spectral peaks in the transfer functions obtained experimentally, it is possible to predict microsphere diameters with less than 4% error. Such good agreement was not seen between the cell model and the measured backscatter from cells. Possible reasons for this discrepancy are discussed.

It has been previously shown that high frequency ultrasound (20 -100 MHz) can be used to detect cellular structure changes in tissues and cell ensembles. However, the changes seen in the backscattered ultrasound intensity and frequency spectrum are not fully understood. In this paper we attempt to better understand the nature of these changes by examination of the backscatter power spectra from cell ensembles (in pellet form) that have undergone two different types of cell death: by exposure to the chemotherapeutic cisplatin and by withdrawal of nutrients (decay). Three different ultrasound transducers were used, centered at 20MHz and 40MHz. In both death pathways, an increase of the midband fit of 10-12dB was measured, and there were significant changes in the spectral slopes. Furthermore, our initial analysis of the backscatter from single cells and polystyrene microspheres demonstrates the potential of the technique to assess scatterer size.

In a previous experiment [Baddour et al., J. Acoust. Soc. Am. 117(2), 934–943 (2005)] it was shown that it is possible to deduce the ultrasound backscatter transfer function from single, subresolution cells in vitro, across a broad, continuous range of frequencies. Additional measurements have been performed at high frequencies (10–65MHz) on cells with different relative nucleus sizes. It was found that for cells with a nucleus to cell volume ratio of 0.50, the backscatter response was better modeled as an elastic sphere. For the cells in which the ratio was 0.33, the backscatter showed good agreement with the theoretical solution for a fluid sphere.

It has previously been demonstrated in tissuemimicking phantoms and in tissue that envelope statistics of US backscatter are affected by changes in the scatterer properties [1-4, 32, 37]. At higher frequencies the wavelength of the US begins to approach the size of cells and cellular components and at this scale the envelope statistics of HFUS backscatter become more sensitive to structural changes within cells. To investigate the relation between the envelope statistics and cell structure, experiments were performed in vitro. The physical meaning of the fit parameters was evaluated by investigating HFUS backscatter from suspensions of various concentrations of two different cell lines of different sizes.