Tomographic flow cytometry by digital holography

High-throughput single-cell analysis is a challenging task. Label-free tomographic phase microscopy is an excellent candidate to perform this task. However, in-line tomography is very difficult to implement in practice because it requires a complex set-up for rotating the sample and examining the cell along several directions. We demonstrate that by exploiting the random rolling of cells while they are flowing along a microfluidic channel, it is possible to obtain in-line phase-contrast tomography, if smart strategies for wavefront analysis are adopted. In fact, surprisingly, a priori knowledge of the three-dimensional position and orientation of rotating cells is no longer needed because this information can be completely retrieved through digital holography wavefront numerical analysis. This approach makes continuous-flow cytotomography suitable for practical operation in real-world, single-cell analysis and with a substantial simplification of the optical system; that is, no mechanical scanning or multi-direction probing is required. A demonstration is given for two completely different classes of biosamples: red blood cells and diatom algae. An accurate characterization of both types of cells is reported, despite their very different nature and material content, thus showing that the proposed method can be extended by adopting two alternate strategies of wavefront analysis to many classes of cells. Combining digital holographic analysis with numerical wavefront analysis enables the structures of different classes of cells to be imaged in detail. Tomographic phase microscopy is promising for high-throughput, single-cell analysis, but it is difficult to implement as it requires a complex setup for rotating samples. Now, Pietro Ferraro and co-workers in Italy have performed tomographic phase microscopy of red blood cells and diatoms by exploiting their random rolling motion as they flow along a microfluidic channel. Their technique can reconstruct complete three-dimensional images of cell structures from a series of holograms obtained without scanning the laser beam. The scheme can identify morphological abnormalities due to anemia and other medical conditions in red blood cells as well as capture the complex shapes of diatoms. It can also determine important cell biomarkers.