c has already passed through the patient 18 if the collimator assembly 36 is positioned to obtain one slice such as a 3 mm slice the collimated x ray fan beam 38 impinges on one detector array 21 if however the collimator assembly 36 is positioned to obtain two equal width slices such as two thin slices one half of the collimated x ray fan beam 38 impinges on the first detector array 20 while the second half of the collimated x ray fan beam 38 impinges on the second detector array 21 the collimator assembly 36 may also be shifted to a second position such that the x ray fan beam 38 may pass from the second x ray inlet aperture 66 to the second x ray outlet aperture 67 with the center of the x ray fan beam 38 being denoted by reference line f u2032 the second x ray inlet aperture 66 may be a different size than the first x ray inlet aperture 64 while the second x ray outlet aperture 67 may be a different size than the first x ray outlet aperture 65 thus different sized slices may be accommodated depending on the position of the collimator assembly 36 that is the collimator assembly 36 may be in a first position to obtain one intermediate slice when the x ray fan beam 38 passes through the first x ray inlet aperture 64 and the first x ray outlet aperture 65 a second position to obtain two thin slices when the x ray fan beam 38 passes through the first x ray inlet and outlet apertures.