Angularly-resolved light scattering is useful for computing organelle size metrics of a cell due to its sensitivity to scatterer size and refractive index contrast. Unfortunately, the cell itself acts as a larger scatterer and contributes its own angular signature. For an adherent cell on a coverslip immersed in standard media with a refractive index close to that of water, we have found that the cell:media refractive index contrast can contribute significant scattering at angular deflections below twenty degrees. This whole-cell scattering, highly dependent on the cell’s shape and size, is challenging to distinguish from the desired organelle scattering signal. This degrades the accuracy with which organelle size information can be extracted from the angular scattering signal. To address the whole-cell contribution, we manipulate the refractive index of the immersion medium by mixing it with a water-soluble, biocompatible, high-refractive-index liquid. By minimizing the refractive index contrast between the cytoplasm and modified medium, this approach physically reduces the amount of whole-cell scattering. We demonstrate this technique on live cells, using a Fourier phase microscope to obtain the complex field of the sample and using Fourier transform light scattering to compute the angular scattering. Results show significant reduction of the whole-cell contribution, indicating the potential of this method for improving the estimates of organelle size distributions in single cells.
The substantial refractive index contrast between a cell’s cytoplasm and traditional immersion media scatters enough light to obscure the angular scattering from cellular organelles and to reduce contrast in quantitative phase images (QPI). To reduce the whole-cell scattering, we mix cell media with a biocompatible, high-refractive-index liquid to closely match the cytoplasm’s refractive index. We demonstrate with live single-cell images that this enables isolation of organelles’ angular scattering, which will improve angular scattering-based estimates of organelle size. We also explain why index-matching enhances phase contrast within a cell’s QPI and propose a digital method for obtaining this result.
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