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Helical reconstruction algorithms for an eight-row CT scanner

This paper presents new helical weighting algorithms associated with an eight-row multislice CT development. The five related methods underlying the reconstruction algorithms are described. For an eight-row scanner, the helical pitch useful range is 0 to 16. Pitches may be classified into "high-quality" modes, where for every line integral at least two measurements are acquired (0 to 8), and "high-speed" where at least one measurement of every relevant line integral is acquired (8 to 16). Also, helical weighting methods may rely exclusively on row-to-row interpolation, to improve temporal resolution at the expense of the slice sensitivity profile, or use conjugate ray interpolation for improved z-resolution. It is first shown that, for multislice systems in high-speed modes, or multislice systems with a large number of rows, the sampling varies considerably within the slice. To this variable z-sampling is associated a specific numerical instability risk that requires regularization. Two methods of handling these discontinuities are introduced. Further, an eight slice system is sensitive to cone-beam artifacts. Four specific methods of significantly reducing the image artifacts, without requiring a cone-beam backprojector, are described and discussed. Their effectiveness is advantageously compared to the traditional Feldkamp method as adapted to helical third-generation CT systems.