Evaluation of helical computed tomography scan parameters for vascular imaging.

Helical x-ray computed tomography (hCT) is the volume acquisition of image data in which the data set represents a series of projections obtained in a helical spatial distribution. Data acquisition times and spatial resolution are determined largely by two user defined parameters--collimation and pitch. Each combination of pitch and collimation results in a slice profile which can be considered as the apparent slice thickness. Careful selection of pitch and collimation allows the acquisition of up to 20 cm of axial data in a breathold or first pass of bolus contrast material. This makes helical CT an excellent technique for angiographic (CTA) studies. Additionally, slices can be reconstructed at any interval to reduce partial volume averaging, minimizing the beading artifacts seen in maximum intensity projections (MIP) when interslice resolution is poor. We have evaluated a range of collimation and pitches for clinical CTA. Additionally, three reconstruction filters were included to determine the optimal frequency (spatial) properties for these techniques. Objective and subjective analyses were performed. We found pitches of up to 1.5:1 to be acceptable for all collimations. For a given scan length (z direction), selection of thinner collimation and higher pitch resulted in better SNR and observer scoring for angiographic presentations.