The effect of overabundant projection directions on 3D reconstruction algorithms

The experimental process of collecting images from macromolecules in an ele ctron microscope is such that it does not allow for prior specification of the angular distribution of the projection images. As a consequence, an une ven distribution of projection directions may occur. Concerns have been rai sed recently about the behavior of 3D reconstruction algorithms for the cas e of unevenly distributed projections. It has been illustrated on experimen tal data that in the case of a heavily uneven distribution of projection di rections some algorithms tend to elongate the reconstructed volumes along t he overloaded direction so much as to make a quantitative biological analys is impossible. In answer to these concerns we have developed a strategy for quantitative comparison and optimization of 3D reconstruction algorithms. We apply this strategy to quantitatively analyze algebraic reconstruction t echniques (ART) with blobs, simultaneous iterative reconstruction technique s (SIRT) with voxels, and weighted backprojection (WBP). We show that the e longation artifacts that had been previously reported can be strongly reduc ed. With our specific choices for the free parameters of the three algorith ms, NMP reconstructions tend to be inferior to those obtained with either S IRT or ART and the results obtained with ART are comparable to those with S IRT, but at a very small fraction of the computational cost of SIRT. (C) 20 01 Academic Press.