Comparison of digitally reconstructed radiographs generated from axial and helical CT scanning modes: a phantom study

Digitally reconstructed radiographs (DRRs) play a vital role for verifying patient position for many radiotherapy treatments. As DRRs are generated from CT scans, image quality may be affected by the scanning mode (axial or helical). The aim of this study was to investigate the high and low contrast resolution and the spatial linearity in DRRs as a function of CT scanning mode and to highlight the significance of this variation, if any. A commercial CT phantom (Fluke Biomedical Model 76-417) was scanned with a Siemens Somatom Sensation 4 CT scanner using six variations of field of view, scanning mode and helical pitch. The image quality of the DRR's produced from the scans was evaluated in terms of high contrast resolution using the modulation transfer function, low contrast resolution using Image Quality Score method and spatial linearity. The results indicated that the high contrast resolution for the axial mode was comparable to that for the helical modes. The low contrast resolution with axial scanning was comparable to scans performed with a helical pitch of 1 but deteriorated at pitches greater than 1. The field of view was not found to impact the low contrast resolution. When changing scanning parameters the impact on DRR quality should be considered. For DRRs, particularly where visualisation of low contrast structures is desired, a helical scan with a pitch of 1 is recommended.