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Can Ultrasonography be used to guide the diagnosis and management of nephrolithiasis?
Troy R. Larson, BS1, Natalia Hernandez, MD2, Brian Eisner, MD2, Jullet Han, BS3, Vernon M. Pais, Jr., MD3, Kevan M. Sternberg, MD1.
1University of Vermont Medical Center, Burlington, VT, USA, 2Massachusetts General Hospital, Boston, MA, USA, 3Dartmouth–Hitchcock Medical Center, Lebanon, NH, USA.

Non-Contrast Computed Tomography (NCCT) has the highest sensitivity and specificity for detection of nephrolithiasis and the greatest accuracy for determination of stone size. Renal ultrasound (US) may have advantages over NCCT insofar as it is lower cost with no ionizing radiation. However, the ability to make judgements about endourologic intervention for stones based on US has not been proven. Herein we compare findings on NCCT and US which were obtained within 1 day of one another in order to evaluate the concordance between the two studies.
A retrospective review was conducted at three academic institutions of patients who were evaluated for flank pain with both renal US and NCCT from 2012-2015. Patients receiving both imaging modalities within 1 day were included. Imaging was obtained through both the emergency department and outpatient settings. Stone presence and size were reviewed and compared between imaging modalities. Stone size was determined by largest measured diameter. Stones were then grouped into 3 size categories (1-5mm, 6-10mm, and >10mm) based on NCCT measurement and compared with US. Statistical analysis was performed using 2-sided t-tests.
155 patients received an US and NCCT within a 1 day period and in 79 patients (51.0%), both US and CT identified a stone for size comparison. When comparing the average largest stone diameter for US (9.08mm) vs. NCCT (6.92mm), US overestimated stone size by 2.16mm (p<0.001). US overestimated stone size by 106.1% for stones 1-5 mm, 63.6% for stones 6-10 mm, and 27.3% for stones > 10 mm (Table 1).
NCCT Measured Stone Diameter (mm)Average Diameter on NCCT (mm)Average Diameter on US (mm)Frequency US overestimate Average US size overestimate (mm)
1-5mm- n=283.917.2182.1% (23/28)4.15 (106.1%)
5-10mm- n =386.988.9052.6% (20/38)4.44 (63.6%)
>10mm- n=1313.2113.6138.5% (5/13)3.60 (27.3%)

Renal ultrasound significantly overestimated stone size when compared to Non-Contrast Computed Tomography. This was most pronounced in the evaluation of small (1-5mm) and intermediate (6-10mm) sized stones. While ultrasound offers a radiation free means of evaluating and monitoring nephrolithiasis, it may not always provide adequate information on stone size to correctly inform urologic management. The potential for systematic over-
estimation of stone size with standard ultrasound techniques should be taken into consideration when evaluating treatment options.

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