Comparison of measurements of the nucleus-to-cytoplasmic ratio in MCF-7 cells using ultra-high frequency photoacoustic microscopy and imaging flow cytometry

Abstract

A cell’s nucleus-to-cytoplasm ratio (N:C) can be used as a metric in histology for staging malignancy. Current techniques used for N:C assessment are time-consuming, low-throughput, and lack 3-D structure assessment. In this study, we assess the N:C ratio of MCF-7 cells using an ultra-high frequency acoustic/photoacoustic (PA) microscope and compare measurements to an imaging flow cytometer (IFC). MCF-7 cells were stained with the DRAQ-5 nuclear dye to facilitate production of fluorescence and PA signals. A PA microscope (PAM) consisting of a 375 MHz transducer and a pulsed 532 nm laser focused through a 10X optical objective was used for cell measurements. Cell and nucleus diameters of 37 stationary MCF-7 cells were obtained by fitting the power spectrum of backscattered ultrasound pulses and emitted PA waves, respectively, to well-established theoretical models. An Amnis ImageStreamX® IFC acquired brightfield/fluorescence images of cells and their nuclei, respectively. Masking and gating techniques were used to obtain the cell and nucleus diameters for 2004 MCF-7 cells from the IFC. For both systems, the N:C ratio was calculated as the ratio ofthe nucleus diameter to total cell diameter. The average cell and nucleus diameters from PAM were 15.2 ± 3.5 μm and 10.2 ± 3.5 μm, respectively, and were in good agreement with the average cell and nucleus diameters of 18.7 ± 2.6 μm and 12.6 ± 1.9 μm measured by the IFC. The N:C ratios were in excellent agreement, calculated to be 0.68 ± 0.19 and 0.68 ± 0.09 for PAM and IFC, respectively. This study demonstrates the ability of PAM to measure the N:C ratio of cells which in excellent agreement with IFC assessments.