# Refractive index measurements of multicellular tumour spheroids using optical coherence tomography: dependence on growth phase and size

## TL;DR

This study measures the refractive index of multicellular tumour spheroids using optical coherence tomography to optimize imaging protocols and improve confocal microscopy imaging depth in cancer research.

## Contribution

It introduces a simple spectral domain OCT method for RI measurement of spheroids and demonstrates how matching immersion media enhances imaging depth.

## Key findings

- ScaleView-A2 matching medium improves confocal imaging depth.
- Refractive index varies with spheroid growth phase.
- OCT effectively measures tissue optical properties.

## Abstract

Knowledge of optical properties, such as the refractive index (RI), of biological tissues is important in optical imaging, as they influence the distribution and propagation of light in tissue. To accurately study the response of cancerous cells to drugs, optimised imaging protocols are required. This study uses a simple custom-built spectral domain optical coherence tomography (OCT) system to conduct RI measurements of multicellular spheroids, three-dimensional \textit{in-vitro} culture systems, of the cell line HCT116. The spheroid RIs are compared to study the effect of growth time. To improve confocal microscopy imaging protocols, two immersion media (glycerol and ScaleView-A2) matching the spheroid RIs were trialled, with the aim to reduce the RI mismatch between the spheroid and the immersion medium and thus improving imaging depth with confocal microscopy. ScaleView-A2 (n = 1.380) aided in achieving greater depths of imaging of the multicellular spheroids under confocal microscopy. This improvement in imaging depth confirmed the utility of our RI measurements, proving the promising outlook of OCT as a complementary tool to microscopy in cancer research.