Purpose: To retrospectively determine whether the nephrographic phase of CTU is reliable for the detection of renal calculi.
Patients and methods: IRB approved this study and informed consent was waived. Two radiologists retrospectively reviewed CTU examinations of 359 patients referred for evaluation of painless hematuria between 2015-2016. All examinations included unenhanced, nephrographic and delayed excretory phases. The examinations were reviewed using both soft tissue and angiographic windows. The nephrographic phase images were reviewed first followed by the “gold standard” unenhanced phase images. Number, size and location of stones were documented as well as the phases and CT window in which these stones could be seen. In case of discrepancy between the two readers, consensus was made with discussion. Sensitivity and specificity for renal stone disease was calculated on a per-kidney basis.
Results: Eighty-five stones were found in 50 patients on the unenhanced phase images. There were 33 males and 17 women (median age 60.3 years, IQR 52-70). Stone size ranged from 1.5 mm to 16 mm (mean 4.15mm). Among the patients who had renal calculi the mean number of stones per patient was 1.7 (SD 1.1). In the nephrographic phase, 80% and 82% of the calculi were identified in the soft tissue window respectively, and the angiographic window respectively. Sensitivity, specificity, PPV and NPV were 82.9%, 98.7%, 87.1% and 98.3% for the soft tissue window and 86.9%, 100%, 100% and 98.8% for the angiographic window respectively. All stones larger than 5 mm were detected.
Conclusion: The nephrographic phase of CTU is reliable for the detection of renal calculi. To reduce exposure to radiation in these patients, eliminating the unenhanced phase should be considered.
