ALIGNMENT OF ELECTRON TOMOGRAPHIC SERIES BY CORRELATION WITHOUT THE USE OF GOLD PARTICLES

Electron tomography requires that a tilt series of micrographs be alig ned and that the orientation of the tilt axis be known. This has been done conveniently with Sold markers applied tc, the surface of a speci men to provide easily accessible information on the orientation of eac h tilt projection. Where gold markers are absent, another approach to alignment must be used. A method is presented here for aligning tilt p rojections without the use of markers, utilizing correlation methods, The technique is iterative, drawing principally on the work of Dengler [Ultramicroscopy 30 (1989) 337], and consists of computing a low reso lution back projection image from which computed tomographic projectio ns can be generated. These in turn serve as reference images for the n ext alignment of the tomographic series. An initial alignment must be made before the first back projection, and this is done following the method of Guckenberger [Ultramicroscopy 9 (1982) 167] for translationa l alignment and by common lines analysis [Liu et al., Ultramicroscopy 58 (1995) 393] for identification of the tilt axis. Four tomographic s eries of a biological nature were aligned and analyzed, and the method has pr oven to be both accurate and reproducible for the data present ed here.