Technical aspects of helical (spiral) CT.

Care must be taken to optimally select settings for collimation, table feed, and reconstruction interval for successful helical CT and associated special application such as CT angiography. Thus, prospective planning is critical for optimal 3-D imaging. Generally, one wishes to minimize all of these parameters to maximize longitudinal resolution; however, one must be aware of the tradeoffs associated with these choices. Pixel noise increases with decreasing collimation. Limiting the table feed to a value equal to the collimation (pitch of 1:1) limits broadening of the effective slice thickness associated with the table motion but also limits the scan coverage. Generally, one may accept a table feed up to twice the collimation (pitch of 2:1) on modern equipment with current software. Reconstruction intervals should also be selected to satisfy the clinical need. When multiplanar and 3-D imaging is desired, the reconstruction interval should be selected such that at least two slices are generated per table increment.