Quantifying the ultrasonic properties of cells during apoptosis using time resolved acoustic microscopy

Time resolved acoustic microscopy was used to determine the thickness, sound velocity, acoustic impedance, density, bulk modulus and attenuation of cells before and after apoptosis. Measurements of the cellular properties at 375 MHz before and after the onset of structural changes associated with apoptosis showed an increase in the thickness from 13.6 ± 3.1 ¿m to 17.3 ± 1.6 ¿m, and the attenuation from 1.08 ± 0.21 dB/cm/MHz to 1.74 ± 0.36 dB/cm/MHz. The other parameters were similar within experimental error. Acoustic images at 1.2 GHz indicated a clear delineation between the nucleus and cytoplasm for apoptotic cells, a feature not observed at lower frequencies. Rapid temporal variations were observed within apoptotic cells as indicated by changes in the spatial distribution of regions of increased backscatter within the cell. These results indicate structural changes that alter the cellular ultrasound backscatter are occurring during apoptosis on time scales on less than a minute.