Abstract
The reconstruction problem for a combined parallel beam (PB) and cone beam (CB) imaging geometry has been addressed. The general algorithm (CB-FBP) of Defrise and Clack (see Conf. Records 1993 Nucl. Sci. Symp. (San Francisco, 1993) (New York: IEEE) p. 1590-4, 1994) has been applied to this geometry and shown to provide accurate images as expected. A second algorithm specifically tailored to the PB-CB geometry was developed. It uses the general principles of the CB-FBP method to combine a shift-variant filtering of the PB data with a standard reconstruction of the CB data using the algorithm of Feldkamp et al. (see J. Opt. Soc. Am, vol. 1, p. 612-9, 1984). This 'mixed' algorithm has the advantage of fewer interpolation steps, thereby reducing reconstruction time and providing more accurate reconstructions. The algorithms were applied to noiseless data from a computer-generated 3D Shepp phantom. Both the CB-FBP algorithm and the mixed algorithm successfully combine the CB and the PB data to correct the well known artefacts observed when reconstructing CB data acquired with a circular orbit. The mixed algorithm is about twice as fast as CB-FBP and results in better image quality, due to decreased discretization errors. However, the two algorithms yield comparable image quality when applied to a disc phantom measured with a two-headed SPECT system.