Hierarchical wavelets projection matching for orientation determination of low contrast electron cryomicroscopic images of icosahedral virus particles

Electron cryomicroscopic images of virus particles, which contain structural signals at high spatial frequencies, have very low contrast. This makes the determination of the particles orientations used for three-dimensional reconstruction very difficult. In this paper we propose a new method, based on wavelet filtering and the weighted-projection matching between projections from an initial model and the raw particle images. The implementation of this method is performed hierarchically by grouping the model´s projections into classes of similar orientations in order to reduce the processing time. The raw particle image is compared by correlation method, with centers of classes and then with all members of the best class, to find the best match with the projections in wavelet space. The orientation of the projection giving the highest correlation coefficient is assigned to the raw particle. Our results show that a significant improvement is obtained, in terms of accuracy and speed, for the initial orientation estimates in the low contrast virus particle images