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. Author manuscript; available in PMC: 2011 Dec 25.
Published in final edited form as: Methods Enzymol. 2010;482:167–183. doi: 10.1016/S0076-6879(10)82006-3

Figure 1.

Figure 1

The IHRSR cycle, reproduced from (Egelman, 2007b). A reference volume (top) is used to generate 90 reference projections, where each involves a 4° azimuthal rotation of the reference volume. The actual angular increment (4° in this example) depends upon the diameter of the object (D) and the expected resolution (d). The reference projections (90 in this case) are cross-correlated against the actual image segments. The highest correlation determines the azimuthal orientation of the image in question, as well as providing the in-plane rotation and translation needed to bring it into register with the reference projection. The aligned images are then used in a back-projection to generate a three-dimensional reconstruction (lower left corner). This volume is searched for the helical screw operator (the coupled rotation and axial translation) that minimizes the variance between the actual volume and a symmetrized version of the volume. This screw operator is then imposed on the reconstruction to generate a helically symmetric volume used as a new reference (top) for the next cycle of the procedure.