3-DIMENSIONAL RECONSTRUCTION FROM RANDOM PROJECTIONS - ORIENTATIONAL ALIGNMENT VIA RADON TRANSFORMS

A method for alignment of projections with unknown projecting directio ns towards a three-dimensional reference has been developed. The techn ique has been applied to the three-dimensional reconstruction from ima ges of a frozen-hydrated preparation of 50S ribosomal subunits from Es cherichia coli recorded in the electron microscope. The algorithm as u sed here combines the Single Exposure Conical Reconstruction Technique (SECReT) with a three-dimensional orientation search. The algorithm a llows for the refinement of a reconstruction obtained with SECReT by r efinement of the true projection angles, and by the inclusion of proje ctions with a priori unknown random orientation. With model data it is demonstrated that the algorithm works reliably even for signal-to-noi se ratios lower than 1.