Acoustic shadowing in pediatric kidney stone ultrasound: a retrospective study with non-enhanced computed tomography as reference standard

Background The usefulness of acoustic shadowing as a feature of pediatric kidney stone ultrasound (US) may be underestimated. Objective The hypothesis was that the majority of stones in children have acoustic shadowing and that its specificity is high (>90%) in pediatric kidney stones. Materials and methods Our retrospective observational study included children who had undergone abdominal non-enhanced computed tomography (CT) for kidney stones in a pediatric renal stone referral centre between 2015 and 2016. US examinations prior to CT were retrospectively assessed for US features such as acoustic shadowing, twinkle artifact and stone size. These features were compared to CT as reference standard. Results Thirty-one patients (median age: 13 years, range: 1–17 years) with 77 suspected kidney stones were included. The median stone size was 5 mm (interquartile range [IQR]: 5 mm). For acoustic shadowing, sensitivity was 70% (95% confidence interval [CI] 56–80%) and specificity was 100% (95% CI 56–100%). All kidney stones with a diameter ≥9 mm demonstrated shadowing. Sensitivity for twinkle artifact was 88% (95% CI 72–96%), but specificity for twinkle artifact could not be calculated due to the lack of true negatives. All false-positive stones on US demonstrated twinkle artifact, but none showed shadowing. Conclusion Acoustic shadowing was demonstrated in the majority of pediatric kidney stones. Specificity was high, but this was not significant. Twinkle artifact is a sensitive US tool for detecting (pediatric) kidney calculi, but with a risk of false-positive findings.