Correlation of CT scan versus plain radiography for measuring urinary stone dimensions.

OBJECTIVES: To correlate the measured dimensions of urinary stones from spiral non-contrast computerized tomography (CT) with that of plain radiography (KUB).

METHODS: The transverse diameter as reported on CT was compared to the measured transverse diameter on KUB for 61 stones. The transverse and craniocaudal dimensions on CT were then re-measured for 30 urinary stones and again compared to the re-measured values for KUB. The craniocaudal dimension on CT was determined by measuring the stone on reconstructed coronal CT images. Measurements between imaging modalities were blinded and performed consecutively by a dedicated investigator.

RESULTS: The mean transverse size of the stones on the initial CT report was 6.0 mm +/- 2.8 mm versus 5.6 mm +/- 2.3 mm on KUB (paired t-test, p = 0.05, 95% CI difference between the means -1.3 to 0.5). The stones were categorized in transverse size ranges of 1.0 mm to 5.0 mm, > 5.0 mm to 10.0 mm, and > 10.0 mm. A total of 14 stones failed to be put into the same size categories by the two methods. The largest difference in measurements was 5 mm. In the second analysis, where the CT dimensions were re-measured, the mean transverse dimension on CT was 4.5 mm +/- 2.1 mm versus 4.7 mm +/- 2.0 mm on plain radiography (paired t-test, p = 0.06, 95% CI difference between the means -0.02 to 0.6). Mean craniocaudal dimension of the stones on CT was 7.4 mm +/- 3.2 mm versus 6.0 mm +/- 2.7 mm on plain radiography (paired t-test, p = 0.0001, 95% CI between the means -2.0 to -0.9). When the stones were categorized in transverse size ranges of 1.0 mm to 5.0 mm, >5.0mm to 10.0mm, and >10.0mm, CT and KUB agreed for 30/30 stones.

CONCLUSIONS: In this study, the initially reported CT transverse values were found to be significantly different from measured KUB values; moreover, large differences of up to 5 mm were found between the measurements. With fastidious measurement of stone dimensions on both CT and KUB, we found that the transverse dimension of stones measured by the two imaging modalities were similar. The craniocaudal measurements of the stones were found to be significantly different on CT versus KUB, with CT measurement being 1.4 mm larger on average.