Sub-cellular quantitative optical diffraction tomography with digital holographic microscopy
Digital holographic microscopy (DHM) is an interferometric technique, providing quantitative mapping of the phase shift induced by semi- transparent microscopic specimens, such as cells, with sub-wavelenght resolution along the optical axis. Thanks to actual PCs and CCDs, DHM provides nowadays cost-effective instruments for real-time measurements at very high acquisition rates, with sub-micron transverse resolution. However, DHM phase images do not reveal the three-dimensional (3D) internal distribution of refractive index, but a phase shift resulting from a mean refractive index (RI) integrated over the cellular thickness. Standard optical diffraction tomography (ODT) techniques can be efficiently applied to reveal internal structures and to measure 3D RI spatial distributions, by recording 2D DHM phase data for different sample orientations or illumination beam direction, in order to fill up entirely the Ewald sphere in the Fourier space. The 3D refractive index can then be reconstructed, even in the direct space with backpropagation algorithms or from the Fourier space with inverse Fourier transform. The presented technique opens wide perspectives in 3D cell imaging: the DHM-based micro-tomography furnishes invaluable data on the cell components optical properties, potentially leading to information about organelles intracellular distribution. Results obtained on biological specimens will be presented. Morphometric measurements can be extracted from the tomographic data, by detection based on the refractive index contrast within the 3D reconstructions. Results and perspectives about sub- cellular organelles identification inside the cell will also be exposed.