Elsevier

Urology

Volume 95, September 2016, Pages 67-71
Urology

Endourology and Stones
Ultrasonography Significantly Overestimates Stone Size When Compared to Low-dose, Noncontrast Computed Tomography

https://doi.org/10.1016/j.urology.2016.06.002Get rights and content

Objective

To evaluate the differences between low-dose noncontrast computed tomography (NCCT) and renal ultrasound (US) in the identification and measurement of urinary calculi.

Materials and Methods

A retrospective review was conducted at 3 institutions of patients evaluated for flank pain with both renal US and NCCT, within 1 day of one another, from 2012 to 2015. Stone presence and size were compared between imaging modalities. Stone size was determined by largest measured diameter. Stones were grouped into size categories (≤5 mm, 5.1-10 mm, and >10 mm) based on NCCT and compared with US. Statistical analysis was performed using 2-sided t tests.

Results

One hundred fifty-five patients received both a renal US and NCCT within 1 day. In 79 patients (51.0%), both US and NCCT identified a stone for size comparison. Fifty-eight patients (37.4%) had a stone visualized on NCCT but not on US, and 2 patients (1.3%) had a stone documented on US but not seen on NCCT. The average NCCT size of the stones missed on US was 4.5 mm. When comparing the average largest stone diameter for US (9.1 mm) vs NCCT (6.9 mm), US overestimated stone size by 2.2 mm (P < .001). US overestimated stone size by 84.6% for stones ≤5 mm, 27.1% for stones 5.1-10 mm, and 3.0% for stones >10 mm.

Conclusion

US significantly overestimated stone size and this was most pronounced for small (≤5 mm) stones. The potential for systematic overestimation of stone size with standard US techniques should be taken into consideration when evaluating endourologic treatment options.

Section snippets

Materials and Methods

After obtaining institutional review board approval, we performed a multicenter retrospective review of patients who obtained renal US and NCCT imaging within 1 day at 3 academic institutions (University of Vermont Medical Center, Massachusetts General Hospital, Dartmouth-Hitchcock Medical Center) between 2012 and 2015.

Inclusion was limited to adult patients >18 years of age. Only formal radiologic US, not bedside-US, were included. CT images were obtained using a low-dose stone protocol with

Results

One hundred fifty-five patients were included in the analysis. Stone size data were analyzed from 79 patients who received both renal US and NCCT within the 1 day time period and in whom both US and NCCT detected a stone. There were 60 patients where a stone was only identified on 1 imaging modality and these were not included for the size comparison analysis. Specifically, 58 patients (37.4%) had a stone visualized on NCCT but not on US, and 2 patients (1.3%) had a stone documented on US but

Comment

There has been increasing interest in the ideal imaging modality to diagnose and monitor nephrolithiasis. NCCT has become the gold standard imaging modality for the evaluation of flank pain. The sensitivity and specificities of NCCT approach 98%-100%.15 Additional advantages include the accurate detection of ureteral calculi and the ability to identify alternative diagnoses that may require more immediate intervention. Equally important is the ability of NCCT to accurately measure stone size

Conclusion

The current ability of US to identify and accurately measure urinary stones is inferior compared to NCCT. Ultrasonography, however, is safe, avoids exposure to ionizing radiation, and may be a useful initial screening modality. Future algorithms need to be developed to decide which patients require additional NCCT imaging to facilitate their urologic care.

References (20)

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Financial Disclosure: Brian Eisner is owner of the Ravine Group, and a Consultant for Boston Scientific, Bard, Retrophin, and Olympus. The remaining authors declare that they have no relevant financial interests.

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