Spatial resolution and radiation dose of a 64-MDCT scanner compared with published CT urography protocols

The objective of our study was to compare the spatial resolution and effective dose from 64-MDCT with several published CT urography protocols.

A phantom containing 1-, 2-, or 4-mm cylindric channels to simulate ureters with 0.25- to 3-mm plugs to simulate ureteral filling defects or ureteral diverticula was imaged using eight helical CT urography protocols. Computed radiography (CR) was also performed. Coronal maximum-intensity-projection images were created and, with the CR image, were evaluated independently by two genitourinary radiologists. Spatial resolution was evaluated by scoring each abnormality as present, visible; or as absent, not visible. Effective dose estimates for 11 CT urography protocols, including the radiographs obtained in the CT urography protocol, were calculated using published Monte Carlo organ dose coefficients.

All ureteral abnormalities detected on CR were detected on the highest-spatial-resolution reconstruction using the evaluated 64-MDCT system. The smallest filling defect identified by both was 0.25 mm. Three 0.25-mm filling defects were not detected using the evaluated 16-MDCT system. The 4-MDCT system protocols showed the poorest performance. The range of effective doses for the evaluated CT urography protocols was 20.1-66.3 mSv. The number of phases, anatomic coverage per phase, and scanning parameters all contributed to this variation in dose.

The evaluated 64-MDCT system showed detection accuracy identical to that of CR. Limiting anatomic coverage for specific phases and combining phases can reduce dose for multiphase protocols by up to a factor of 2 relative to early (circa 2000) 4-MDCT.