RECONSTRUCTED HELICAL CT SCANS - IMPROVEMENT IN Z-AXIS RESOLUTION COMPARED WITH OVERLAPPED AND NONOVERLAPPED CONVENTIONAL CT SCANS

OBJECTIVE. This in vitro study was designed to assess the z-axis resol ving capabilities of reconstructed helical CT scans obtained with vari ous imaging parameters versus those of conventional CT scans and the e ffect of decreasing slice index on the z-axis resolution of helical CT . MATERIALS AND METHODS. A z-axis line-pair phantom was imaged using c onventional nonoverlapped CT scans, conventional CT scans that overlap ped by 50%, and helical CT scans with pitches of 1.0 and 1.5. All heli cal images were reconstructed at comparable slice indexes (image index es of 2.0, 1.0, and 0.5 mm for pitch = 1.0, and image indexes of 3.0, 1.5, and 0.75 mm for pitch = 1.5), Midline coronal and sagittal recons tructed images were obtained to allow standardized visualization of li ne pairs. The reconstructed images were reviewed separately by 10 radi ologists. RESULTS. The overall z-axis resolution of reconstructed heli cal CT scans equaled or exceeded that of nonoverlapped conventional CT scans in all cases and equaled that of 50% overlapped conventional CT scans in 75% of cases. The 1.0-pitch helical sequences showed improve d z-axis resolution with decreasing slice index, No statistically sign ificant improvement in z-axis resolution could be determined by the ob servers for 1.5-pitch sequences with decreasing slice index. CONCLUSIO N. The use of helical CT with a pitch of 1.0 or 1.5 and an increased s lice index can improve the z-axis resolution of reconstructed images w hen compared with nonoverlapped conventional CT and frequently equals the resolution of 50% overlapped conventional CT, This improvement in z-axis resolution should improve the appearance of reconstructed image s (as used in CT angiography and three-dimensional imaging) by reducin g partial volume artifacts while affording faster scanning at a reduce d skin-surface radiation dose.