Signal analysis for the estimation of mechanical parameters of viable cells using GHz-acoustic microscopy

The current study aimed on the method development for the estimation of mechanical properties of individual biological cells. Samples investigated ultrasonically were kept under culture conditions and no chemical markers have been applied in order to not alter the cells chemical conditions. For the estimation of mechanical properties namely thickness and sound velocity ultrasonic waves in the GHz frequency range have been used. The major benefit of applying acoustic GHz-microscopy is the contact-free and therefore non-destructive mode of operation combined with a resolution of approx. 1 ?m. Modulated but otherwise unprocessed radio frequency (rf) echo signals of HeLa cells have been acquired. Signals were processed using a numerical deconvolution technique which has proven to be reliable and robust in low-frequency ultrasonic applications. Echo positions of the cell membrane and substrate have been estimated. From the time of flight (TOF) of the cell membrane versus the TOF of the substrate outside the cell the local cell thickness was derived. Cell thickness and the TOF inside a cell lead to the estimation of the local sound velocity. For verification cell thickness has been measured by confocal laser scanning microscopy. Individual cells showed varying thickness values ranging from 5 ?m to 15 ?m depending on the cell topography. Sound velocities however varied between 1600 m/s and 1800 m/s. Deviations of thickness estimates between laser scanning microscopy and ultrasonic GHz-microscopy are in acceptable agreement. It was noticed that the cell-thickness estimation shows a higher accuracy in regions of the cell nucleus vs. the surrounding cytoplasm. That fact may be attributed to the much smaller cellular thickness in extra-nuclear regions of adhering cells. Thickness values obtained by laser scanning microscopy showed an artifact at the cells nuclear region, however, values obtained at both edges of the nuclei agree well with the ultrasonically obtained results. T- - he current study contributes towards the estimation of intra-cellular changes of individual cells during chemo-therapy and the development of a low-frequency technique for an in-vivo monitoring of treatment responses in cancer therapy.