Retrospective comparison of measured stone size and posterior acoustic shadow width in clinical ultrasound images

Abstract

Purpose

Posterior acoustic shadow width has been proposed as a more accurate measure of kidney stone size compared to direct measurement of stone width on ultrasound (US). Published data in humans to date have been based on a research US system. Herein, we compare these two measurements in clinical US images.

Methods

Thirty patient image sets where computed tomography (CT) and US images were captured less than one day apart were retrospectively reviewed. Five blinded reviewers independently assessed the largest stone in each image set for shadow presence and size. Shadow size was compared to US and CT stone sizes.

Results

Eighty percent of included stones demonstrated anacoustic shadow; 83% of stones without a shadow were ≤ 5 mm on CT. Average stone size was 6.5 mm ± 4.0 on CT, 10.3 mm ± 4.1 on US, and 7.5 mm ± 4.2 by shadow width. On average, US overestimated stone size by 3.8 mm ± 2.4 based on stone width ( p < 0.001) and 1.0 mm ± 1.4 based on shadow width (p < 0.0098). Shadow measurements decreased misclassification of stones by 25% among three clinically relevant size categories (≤ 5 mm, 5.1 – 10 mm, > 10 mm), and by 50% for stones ≤ 5 mm.

Conclusions

US overestimates stone size compared to CT. Retrospective measurement of the acoustic shadow from the same clinical US images is a more accurate reflection of true stone size than direct stone measurement. Most stones without a posterior shadow are ≤ 5 mm.