Analysis of ultrasound backscatter from ensembles of cells and isolated nuclei

We have previously shown that the intensity of the ultrasound backscatter from cells ensembles undergoing apoptosis increases and shifts in their normalized power spectra are detected when compared to the backscatter from non-apoptotic cells. The etiology of these changes is unknown. During apoptosis many cellular changes occur, perhaps the most striking being the condensation and subsequent fragmentation of the cell nucleus. In this set of experiments have exposed either whole Acute Myeloid Leukemia (AML) cells or nuclei isolated from AML cells to different ionic strengths known to induce specific and reproducible cellular and nuclear changes. Ultrasound images and rf backscatter data were collected and analyzed at the different ionic strengths, and electron micrographs were made. Exposing cells to higher ionic strengths increased the ultrasound backscatter by 12 dB, but exposing the nuclei to the same experimental conditions decreased the backscatter by 23 dB. Furthermore, while the spectral slopes of the rf backscatter were similar for cells and nuclei at physiological saline, at increased concentrations the slope increased for the nuclei but decreased for the cells. The paper discusses the implications and significance of the findings. In conclusion, disruptions in cell and nuclear structure induced by exposure to strong ionic environments can greatly alter the ultrasound backscatter signal characteristics.