Abstract: The present invention provides a novel technique for collective tomography (i.e. analyzing many cells at once), without staining, using one 3D model representing the entire cell population, enabling high-throughput tomography. There is provided a tomographic technique for analyzing a sample containing a population of biological cells. In this technique, at least one tomographic projection may come from another cell in the cell population, and each cell in the population may contribute one or more projections to the final 3D refractive-index map.