Tomographic Reconstruction of Label Images from a Few Projections

Our aim is to produce a tessellation of space into small voxels and, based on tomo-graphic projections, assign to each one of them a label from a small predetermined set. The traditional approach first assigns to each voxel a gray value based on the projections and then thresholds this gray value image to obtain the label image. There are problems with this approach. For example, in electron microscopy of macromolecules at high resolution (small voxels) the ranges of atom densities corresponding to different labels greatly overlap, and so the label image has to be of low resolution in order to be reliable. Another difficulty is that, due to the destructive nature of the electron microscope, only a few projections can be taken. We propose to overcome such difficulties by first postulating some low level prior knowledge (in electron microscopy this would be based on the general nature of macromolecules) regarding the underlying distribution of label images, and then estimating directly a particular label image based on this prior distribution together with the projections. Here we report on our early experiments aimed at evaluating this approach.