Single-particle averaging from electron microscope images strongly depends on alignment. Most alignment procedures are based on cross:correlation of an initial reference image with the rest of the population, leading to a clear pattern dependence. Among the different approaches that have been proposed to minimize this problem, one of the most widely used is the so-called iterative reference-free alignment algorithm (RFAA), proposed by Penczek et al. [Ultramicroscopy 40 (1992) 33]. To avoid the pattern dependence shown by the initial 'random approximation' step of this method, we propose a variant of the algorithm that is more independent of the input order of the initial images and which could substitute the random initialization of the RFAA.
Marco, S., Chagoyen, M., De La Fraga, L. G., Carazo, J. M., & Carrascosa, J. L. (1996). A variant to the “random approximation” of the reference-free alignment algorithm. Ultramicroscopy, 66(1–2), 5–10. https://doi.org/10.1016/S0304-3991(96)00083-6