Evaluation of virtual unenhanced CT obtained from dual-energy CT urography for detecting urinary stones

Abstract

Objective: The aim of our study was to determine if virtual unenhanced CT (VUCT) is equivalent to unenhanced CT (UCT) for detecting urinary stones.

Methods: Our institutional review board approved this retrospective study, which was compliant with the Health Insurance Portability and Accountability Act. A total of 80 stones were detected in 32 patients among 146 consecutive patients undergoing dual-energy CT urography. The number and size of stones were recorded on nephrographic VUCT (NVUCT) and excretory VUCT (EVUCT) images, respectively. UCT was a reference of standard for the number and size of stones. Image quality of VUCT was qualitatively assessed using a five-point scale. Repeated-measures analysis of variance with post-test was used for statistical analysis.

Results: 62 stones in 29 patients were detected on NVUCT and 59 stones in 27 patients were detected on EVUCT. The size of stones detected on NVUCT or EVUCT was significantly smaller compared with stones on UCT (p<0.05). The size of stones detected on UCT, NVUCT and EVUCT ranged from 1.4 to 19.2 mm (mean, 4.6 mm), 0 to 19.2 mm (mean, 3.6 mm) and 0 to 18.7 mm (mean, 3.6 mm), respectively. 18 stones were missed on NVUCT and 21 were missed on EVUCT. The sizes ranged from 1.4 to 3.2 mm (mean, 2.1 mm) and 1.4 to 3.2 mm (mean, 2.2 mm) on UCT, respectively. VUCT was inferior to UCT regarding image quality (p<0.05).

Conclusion: VUCT missed a significant number of small stones probably owing to poor image quality compared with UCT. Subsequently, VUCT cannot replace UCT for detecting urinary stones.

Figure 1

56-year-old male with a left calyx stone. (a) Excretory axial CT image shows that the abdominal organs are almost entirely covered even with a small detector (arrows) measuring 32 cm in diameter. The body mass index of this patient is 28 kg m^-2. (b) Unenhanced axial CT image (left) shows a left calyx stone (arrow) of 2.5 mm. However, this stone measures 2 mm on nephrographic virtual unenhanced axial CT image (right). In addition, the edge of the stone is somewhat unclear. Left posterior pararenal fascia (arrowheads) is clearer on unenhanced CT image than on virtual unenhanced CT image.

Figure 2

74-year-old male with a left calyx stone. Unenhanced axial CT image (left) shows a left calyx stone (arrow) measuring 2.7 mm in diameter. However, this stone is not seen on excretory virtual unenhanced axial CT image (right).

Figure 3

44-year-old male with a right calyx stone. Excretory axial CT image (left) shows an upper polar calyx (arrowheads) that is filled with excreted contrast material. No stone is seen on virtual unenhanced CT image (middle) as a small amount of the contrast material (arrowheads) is still seen despite subtraction of iodine from the excretory CT image. However, a small calyx stone (arrow) measuring 2 mm in diameter is seen on unenhanced axial CT image (right).