BACKGROUND: In many cases, uric acid calculi may be treated with oral dissolution therapy and procedures can be avoided. We analyzed the appearance of uric acid calculi on computed tomography (CT) scout films to investigate the frequency and determinants of visibility.
METHODS: 28 patients with 100% uric acid calculi underwent 4-16 multidetector computed tomography (KVP 100, slice thickness 2.5mm). Two radiologists evaluated CT scout films and axial CT scans from a picture archiving and communicating system (PACS). Axial CT images were evaluated for calculus position, axial dimensions, and Hounsfield Unit density (HU). CT scout films were evaluated for presence or absence of urinary tract calculi. Statistical analysis was performed using student’s t-test.
RESULTS: Uric acid calculi were visible on 16/28 (57%) of CT scout films. Mean CT HU of radiopaque and radiolucent uric acid calculi were similar (431 versus 453, p = 0.65). Mean axial diameter was significantly greater for radiopaque calculi than in radiolucent calculi (1.25 cm versus 0.67 cm, p = 0.03). Location of radiopaque calculi was as follows: kidney 7/16 (43%), proximal ureter 5/14 (31%), distal ureter 2/16 (13%), bladder 2/16 (13%). The smallest visible calculus was 5mm diameter. Three radiopaque calculi were 5mm in diameter, 6 radiopaque calculi were between 5-10mm in diameter, and 7 radiopaque calculi were >1cm in diameter.
CONCLUSIONS: A majority of uric acid calculi are visible on CT scout film. Visibility was related to size, but not Hounsfield Unit density. Visible uric acid calculi may appear at any point along the urinary tract, and may be as small as 5mm in diameter. When planning treatment, urologists should be aware that calculi which are radiopaque on CT scout film may be composed of 100% uric acid.