Technical Note: Performance Comparison of Ultra-High-Resolution Scan Modes of Two Clinical Computed Tomography Systems.
PURPOSE
Two ultra-high-resolution (UHR) CT scanners are clinically available. One is achieved by a CT system with a 0.25 mm × 160 row detector (Detector-UHR), while the other is with a 0.6 mm × 32 row detector with in-plane comb filtering in a dual source CT (Comb-UHR). We compared radiation dose efficiencies of the two UHR modes to that of a routine scan mode (RS), using physical image quality measures for an assumed condition of abdominal CT angiography (CTA).


METHODS
A wire phantom, a 300-mm cylindrical water bath phantom, and a micro-disk phantom were used for measuring the modulation transfer function (MTF), noise power spectrum (NPS), and slice sensitivity profiles (SSP), respectively, at CTDIvol of 20 mGy. Images with minimal slice thicknesses were reconstructed by filtered back projection methods. System performance functions SPF2 based on the pre-whitening theorem were calculated by dividing MTF2 by NPS measurements. The ideal observer's detectability index (d'2 ) was also estimated for a task corresponding to a 1-mm diameter vessel. Furthermore, a bar-pattern phantom placed in a water phantom resembling an adult abdomen was scanned, and the visibility of the bars was observed.


RESULTS
SPF2 results showed that Comb-UHR has a 70% dose efficiency compared to RS and provides better than 2-fold SPF2 compared to Detector-UHR. The d'2 values of Detector-UHR, Comb-UHR, and RS were 6.5, 14.7, and 16.0, respectively. Though the bar-pattern phantom images were consistent with the SPF results, bar widths < 1.0 mm for Detector-UHR and < 0.75 mm for Comb-UHR and RS were not resolved.


CONCLUSIONS
Though both the UHR modes exhibited system performances extending to 2.6 cycles/mm, they appeared not to be advantageous compared to RS in the conditions applicable to abdominal CTA, because of their insufficient dose efficiencies.