The PI-methods for Helical Cone-Beam Tomography

Abstract. Next generation helical cone-beam CT will feature pitches around 80 mm. It is predicted that reconstruction algorithms to be used in these machines with still rather modest cone angles may not necessarily be exact, but rather have an emphasis on simplicity and speed. The PImethods are a family of non-exact algorithms, all of which are based on complete data capture with a detector collimated to the Tam-window followed by rebinning to obliquely parallel ray geometry. The non-exactness is identified as inconsistency in the space invariant one-dimensional ramp-filtering step. It is shown that this inconsistency can be reduced resulting in significant improvement in image quality and increased tolerance for higher pitch and cone angle. A short theoretical background for the PI-methods is given but the algorithms themselves are not given in any detail. A set of experiments on mathematical phantoms illustrate (among other things) how the amount of artefacts grow with increasing cone angles. 1