Technical Comparison of Helical and Non-Helical(Axial) Scanning in Prospectively ECG-Triggered

Technical Comparison of Helical and Non-Helical(Axial) Scanning in Prospectively ECG-triggered Imaging

Introduction Helical and non-helical scanning are widely used in coronary CT imaging. Non-helical scanning has a device-dependent disadvantage, with misalignment artifacts occurring noticeably in axial mode. One of the causes is thought to be the physical properties. This study investigated the physical properties of helical and axial modes by using static phantoms. Methods Prospectively ECG-triggered helical and axial modes were evaluated in the following studies with use of a SOMATOM Definition AS+ scanner (Siemens AG, Erlangen, Germany). 1. We scanned a micro-disk phantom to obtain the slice sensitivity profile along the z-axis (SSPz). In the axial mode, scanning was extended by an additional 15 mm on both the anode and cathode sides. (helical ;120 kV, effective tube current-time product ;100 mAs, 0.3 s/rot, pitch ; 0.18, axial ;120 kV, effective tube current-time product ;100 mAs, 0.3 s/rot) 2. We scanned two slabs of a water phantom to obtain the standard deviation (SD) of the CT numbers. (helical ;120 kV, effective tube current-time product ;120 mAs, 0.3 s/rot, pitch ;0.18, axial ;120 kV, effective tube current-time product ;120 mAs, 0.3 s/rot) 3. A mock blood vessel phantom (φ3 mm,CT-number ; 300 Hounsfield unit) was scanned in helical and axial (2 slabs) modes three times each at the center of the acquisition field of view (FOV) and off center in a 10 cm FOV. The quality of volume-rendering (VR) images and multi planar reconstruction (MPR) images was evaluated by 4 clinical radiologists, each with 11 to 21 years of experience, with use of the following 3-point scale: Score 3 ; no misalignment artifacts , Score 2 ; misalignment artifacts discernible only by their boundaries, and Score 1 ; clearly noticeable misalignment artifacts. Mean data were expressed as the mean ± standard deviation. Mann-Whitney’s U test was employed for comparisons between the 2 groups. Values at p<0.05 were considered to indicate statistical significance in instances. Statistical analyses were performed with SPSS for Windows(v.15.0.J,Chicago, IL, USA) Results and Discussion The results of micro-disk phantom scanning showed that, in axial mode, the full width at half maximum (FWHM) at the isocenter varied in the anode-to-cathode direction between 0.579 and 0.619 mm when the reconstructed slice thickness was 0.6 mm and between 0.799 and 0.874 mm when the reconstructed slice thickness was 0.75 mm (Table 1). These results were interpreted as indicating that an increase in the z-axis beam width affected the effective focal spot size. The reason FWHM was less than 0.6 mm when the reconstructed slice thickness was 0.6 mm in axial mode is thought to be the effect of oversampling by the focusing system.(z-Sharp Technology) The results of water phantom scanning showed that, when scanning was done in axial mode with a reconstructed slice thickness of 0.6 mm, the SD was reduced by about 20% around the slab center and also was reduced along seams, but no SD reduction was observed when the reconstructed slice thickness was 0.75 mm (Fig 1). Visual evaluation of mock vessel phantom images detected some misalignment artifacts occurring in both modes because of their ECG-gated synchronization. Axial mode had a lower evaluation than did helical mode, with the detailed evaluation results (Table 2) for 0.6 mm reconstructed slice thickness showing statistically significant differences between axial and helical modes for both MPR and VR images (p = 0.0002 and p = 0.0355, respectively) at the center of the FOV and off center in a 10 cm FOV (p < 0.0001 and p = 0.0356, respectively). Both MPR and VR did not differ significantly between axial and helical modes for images with 0.75 mm reconstructed slice thickness at the center of the FOV (p = 0.2327 and p = 0.4834) and off center in a 10 cm FOV (p = 0.0696 and p = 0.0979), respectively. Conclusion Although the use of a reconstructed slice thickness of 0.75 mm somewhat reduced the z-axis resolution, it also reduced the SD, and the occurrence of misalignment artifacts did not differ significantly between axial and helical modes.