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  • Title: Reconstructed helical CT scans: improvement in z-axis resolution compared with overlapped and nonoverlapped conventional CT scans.
    Author: Kasales CJ, Hopper KD, Ariola DN, TenHave TR, Meilstrup JW, Mahraj RP, Van Hook D, Westacott S, Sefczek RJ, Barr JD.
    Journal: AJR Am J Roentgenol; 1995 May; 164(5):1281-4. PubMed ID: 7717248.
    Abstract:
    OBJECTIVE: This in vitro study was designed to assess the z-axis resolving capabilities of reconstructed helical CT scans obtained with various imaging parameters versus those of conventional CT scans and the effect of decreasing slice index on the z-axis resolution of helical CT. MATERIALS AND METHODS: A z-axis line-pair phantom was imaged using conventional nonoverlapped CT scans, conventional CT scans that overlapped by 50%, and helical CT scans with pitches of 1.0 and 1.5. All helical images were reconstructed at comparable slice indexes (image indexes 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 reconstructed images were obtained to allow standardized visualization of line pairs. The reconstructed images were reviewed separately by 10 radiologists. RESULTS: The overall z-axis resolution of reconstructed helical 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 improved z-axis resolution with decreasing slice index. No statistically significant improvement in z-axis resolution could be determined by the observers for 1.5-pitch sequences with decreasing slice index. CONCLUSION: The use of helical CT with a pitch of 1.0 or 1.5 and an increased slice index can improve the z-axis resolution of reconstructed images when 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 images (as used in CT angiography and three-dimensional imaging) by reducing partial volume artifacts while affording faster scanning at a reduced skin-surface radiation dose.
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