Can the CT planning image determine whether a kidney stone is radiopaque on a plain KUB?

Almost all kidney stones are CT positive. Before a CT scan can be done a CT planning image (CTI) is generated in order to select the exact scanning area. The CTI looks approximately like a normal kidney-ureter-bladder abdominal radiography (KUB) but with reduced quality. It has been used as a guide, assuming that if the kidney stone could be seen on the CTI the kidney stone also would be visible on a conventional plain KUB (radiopaque). From the perspective of diagnosis and treatment as well as follow-up it is of importance to know whether a kidney stone is radiopaque or not. The aim of this study was to evaluate whether the CTI actually can predict radiopacity. CT scans and corresponding KUB's were analysed in 76 consecutive kidney stone patients. The CT scan and the KUB were performed on the same day. All patients were examined with the same CT scanner (64 slice GE light speed VCT). Three radiologists evaluated the images in plenum. The following was recorded regarding the kidney stones: X-ray positive (radiopaque on KUB), CTI positive (radiopaque on CTI), location (a kidney, b upper two-thirds of ureter and c lower one-thirds of ureter including the bladder), size and Hounsfield units (HU). We also measured the patient's 'anterior-posterior depth' (APD) at the kidney stone level in axial plane, and whether the stone was homogeneous/inhomogeneous. 54 of the 76 patients (71%) had radiopaque stones on KUB. 43 (57%) of these also could be seen on the CTI, resulting in a positive predicting value (PPV) of 100% and a negative predictive value (NPV) of 67%. In the 54 KUB positive kidney stones the mean kidney stone diameter was 7 mm (2-30 mm), mean HU's 1,007 (294-1,782 HU), location: a:32, b:9 and c:13 patients. APD was mean 23.6 cm (13-39 cm). In the KUB positive and CTI negative kidney stones (11 patients) mean kidney stone diameter was 4 mm (2-9 mm), mean HU's 742 (294-1,253 HU), location: a:32, b:9 and c:13 patients. APD in this group was mean 26.1 cm (13-37 cm). If the kidney stone can be seen on the CTI it is also visible on a plain KUB (PPV 100%). The CTI do, however, underestimate the radiopacity of a stone on a plain KUB (NPV 67%). Kidney stone HU > 742, stone location in the kidney and proximal ureter and APD < 26 cm independently predict agreement between CTI and KUB with regard to radiopacity.