Elsevier

Urology

Volume 76, Issue 2, August 2010, Pages 295-300
Urology

Endourology and Stone
Limitations to Ultrasound in the Detection and Measurement of Urinary Tract Calculi

This study was presented at the American Urological Association Annual Meeting, Chicago, Illinois, 2009.
https://doi.org/10.1016/j.urology.2009.12.015Get rights and content

Objectives

To evaluate differences in stone measurement using computed tomography (CT) and ultrasound (US). Axial unenhanced helical CT is the reference-standard imaging modality for the assessment of urinary tract calculi; however, US is also commonly used. Differences in stone measurement 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.

Section snippets

Material and Methods

We performed a retrospective review of imaging for renal and ureteric calculi at a single institution. Enrolment was limited to the period between June 2004 and December 2008. Data were abstracted from patient records and an independent review of all imaging was conducted by a Urologist. Institutional ethics approval was obtained for this study.

Inclusion was limited to patients aged ≥18 years, with the finding of a solitary renal or ureteral calculus on both US and noncontrast CT. Both CT and

Results

Data were analyzed for 60 patients with 71 renal or ureteric calculi (Table 1). In total, 56 renal and 15 ureteric stones were included. The mean interval between CT and US examinations was 9.6 ± 9.5 days. US was conducted as the initial examination in 68.3% of cases. Indications for US were follow-up of a known stone in 34 (56.7%) and as the initial imaging modality for the investigation of acute flank pain following presentation to the emergency department in 17 (28.3%). Additionally, in 8

Comment

Unenhanced axial CT has the benefit of providing rapid diagnosis with high sensitivity and specificities and is considered the gold standard imaging modality for the diagnosis of kidney stones.1 As knowledge of stone burden forms the basis of management decisions and guides clinical decision-making, accurate measurement of urinary tract calculi is essential. Now routinely performed with slice collimations of less than 3 mm, published sensitivity and specificities approach 98%-100%.14 At this

Conclusions

Overestimation of stone size may have important implications for patient counselling and may affect the choice of intervention. Our data indicate that US overestimates renal stone size, an effect that is particularly pronounced for stones ≤5 mm. Furthermore, compared with unenhanced CT, US has poor sensitivity for detecting stones in both the ureter and kidney. For these reasons, US should be considered of limited value in the work-up of urolithiasis. Management decisions should incorporate

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    This study was presented as a poster at the Canadian Urological Association Annual Meeting, Toronto, Ontario, 2009.

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