Limitations to Ultrasound in the Detection and Measurement of Urinary Tract Calculi
Received 4 September 2009; accepted 10 December 2009. published online 08 March 2010. Corrected Proof
Objectives
To evaluate differences in stone measurement using computed tomography (CT) and ultrasound (US). Axial unenhanced helical CT is thereference-standard imaging modality for the assessment of urinary tract calculi; however, US is also commonly used. Differences in stonemeasurement using these techniques are poorly described and contributors to measurement error remain unknown.
Methods
All patients at our institution undergoing both abdominal CT and renal US less than 1 month apart since June 2004 were reviewed. Solitary renal calculi were identified on both CT and US in all cases.
Results
We identified 71 calculi in 60 patients. Compared with CT, US overestimated stone size, an effect that was more pronounced with smaller calculi. The mean stone measurement on CT was 7.4 ± 4.4 mm and on US it was 9.2 ± 4.5 mm (P = .018). For stones ≤5 mm, US measurements were a mean of 1.9 ± 1.2 mm greater than CT (P <.001). US and CT measurements were discordant for 60% of stones ≤5 mm. Discordance was associated with US measurement of skin-to-stone distance (P = .018), but not body mass index (P = .189) or location within the urinary tract (P = .161). Review of the literature revealed that US has a pooled sensitivity and specificity of 45% and 94%, respectively, for the detection of ureteric calculi and 45% and 88%, respectively, for renal calculi.
Conclusions
US overestimates stone size in urolithiasis, a finding that may have implications for stone management. Discordance in stone measurement varies with size and is greatest in stones ≤5 mm. US measurement of skin-stone-distance is an important determinant of error in US measurement of renal calculi.
Department of Urology, St. Michael's Hospital, University of Toronto, Toronto, Canada
Reprint requests: R. John D'A. Honey, M.D., M.A., F.R.C.S.C., Division of Urology, St. Michael's Hospital, 61 Queen St. E, Ste. 9-103Q, Toronto, ON, M5C 2T2
This study was presented at the American Urological Association Annual Meeting, Chicago, Illinois, 2009.
This study was presented as a poster at the Canadian Urological Association Annual Meeting, Toronto, Ontario, 2009.
ABSTRACT