  Single particle electron cryomicroscopy (cryo-EM) allows for structures of
proteins and protein complexes to be determined from images of non-crystalline
specimens. Cryo-EM data analysis requires electron microscope images of
randomly oriented ice-embedded protein particles to be rotated and translated
to allow for coherent averaging when calculating three-dimensional (3D)
structures. Rotation of 2D images is usually done with the assumption that the
magnification of the electron microscope is the same in all directions.
However, due to electron optical aberrations, this condition is not met with
some electron microscopes when used with the settings necessary for cryo-EM
with a direct detector device (DDD) camera. Correction of images by linear
interpolation in real space has allowed high-resolution structures to be
calculated from cryo-EM images for symmetric particles. Here we describe and
compare a simple real space method, a simple Fourier space method, and a
somewhat more sophisticated Fourier space method to correct images for a
measured anisotropy in magnification. Further, anisotropic magnification causes
contrast transfer function (CTF) parameters estimated from image power spectra
to have an apparent systematic astigmatism. To address this problem we develop
an approach to adjust CTF parameters measured from distorted images so that
they can be used with corrected images. The effect of anisotropic magnification
on CTF parameters provides a simple way of detecting magnification anisotropy
in cryo-EM datasets.
