Technical Note: Performance comparison of ultra-high-resolution scan modes of two clinical computed tomography systems.

Purpose: Two ultra-high-resolution (UHR) computed tomography (CT) scanners are clinically available. One is achieved by a CT system with a 0.25 mm × 160 row detector (Detector-UHR), whereas 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 microdisk phantom were used for measuring the modulation transfer function (MTF), noise power spectrum (NPS), and slice sensitivity profiles (SSP), respectively, at CTDI of 20 mGy. Images with minimal slice thicknesses were reconstructed by filtered back projection methods. System performance functions (SPF ) based on the prewhitening theorem were calculated by dividing MTF by NPS measurements. The ideal observer's detectability index (d' ) 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: System performance function (SPF ) results showed that Comb-UHR has a 70% dose efficiency compared to RS and provides better than twofold SPF compared to Detector-UHR. The d' values of Detector-UHR, Comb-UHR, and RS were 6.5, 14.7, and 16.0, respectively. Although 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.