# Refractive index tomography with structured illumination

## TL;DR

This paper presents a new method for 3D imaging of complex refractive index using structured illumination microscopy, enabling high-resolution, label-free imaging of biological cells through computational reconstruction.

## Contribution

It introduces a reinterpretation of structured illumination microscopy as a superposition of angled illuminations for coherent imaging, facilitating 3D refractive index tomography.

## Key findings

- Successfully reconstructed 3D RI of cancer cells
- Demonstrated compatibility with standard SI microscopy setup
- Achieved sub-diffraction resolution in RI imaging

## Abstract

This work introduces a novel reinterpretation of structured illumination (SI) microscopy for coherent imaging that allows three-dimensional imaging of complex refractive index (RI). To do so, we show that coherent SI is mathematically equivalent to a superposition of angled illuminations. It follows that raw acquisitions for standard SI-enhanced quantitative-phase images can be processed into complex electric-field maps describing sample diffraction under angled illuminations. Standard diffraction tomography (DT) computation can then be used to reconstruct the sample 3D RI distribution at sub-diffraction resolutions. We demonstrate this concept by using a SI-quantitative-phase imaging system to computationally reconstruct 3D RI distributions of human breast (MCF-7) and colorectal (HT-29) adenocarcinoma cells. Our experimental setup uses a spatial light modulator to generate structured patterns at the sample and collects angle-dependent sample diffraction using a common-path, off-axis interference configuration with no moving components. Furthermore, this technique holds promise for easy pairing with SI fluorescence microscopy, and important future extensions may include multimodal, sub-diffraction resolution, 3D RI and fluorescent visualizations.

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Source: https://tomesphere.com/paper/1702.03595