Influencing factors in the comparison of renal stone sizemeasurements between CT scan and ultrasound

Ultrasound is widely used to detect renal stones. A no-contrast CT scan is the most accurate modality for stone examination. Considering that determining the size of a stone is the main criterion in treatment planning, the purpose of this study was to compare ultrasound and non-contrast CT scans in determining renal stone size.

In this cross-sectional study, 140 patients were assessed from June to October 2017 at Namazi Hospital of Shiraz University of Medical Sciences. A radiologist compared the CT scans of patients with their ultrasound images. The ultrasound and CT scan results in terms of stone size were assessed. The correlation rate of stone size in ultrasound and CT scans in different subgroups was assessed by categorizing the stone size into three subgroups. The impact of stone location and the CT window on measuring stone size were evaluated.

The mean stone size measured by ultrasound and CT scan was 11.23±5.26 and 9.48±4.7mm, respectively, which was significantly different. The highest rate of correlation was observed in the stones>10mm (81%) and then in 5-10mm (69.3%). The lowest agreement was observed in stones <5mm (37.5%).

Using ultrasonography as a screening test or a primary test for detecting the renal stones is recommended. However, considering the importance of a precise assessment of stone size in choosing the treatment method and the limitations of sonography in the evaluation of small stones, a CT scan should be considered as a standard diagnostic tool if there is no contraindication. In CT images, the application of BW might underestimate the stone size in comparison to SW. Considering the results of previous studies, it is better to use magnified BW to assess the stone size on CT scan images. Although the axial view in a CT scan is more routine, for measuring the stone size it is better to use coronal or sagittal views together with axial views.