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

Abstract

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 using US system. Herein, we compared these two measurements in clinical US images. Thirty patient image sets where computed tomography (CT) and US images were captured less than 1 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. Eighty percent of included stones demonstrated an acoustic shadow; 83% of stones without a shadow were≤5mm on CT. Average stone size was 6.5±4.0mm on CT, 10.3±4.1mm on US, and 7.5±4.2mm by shadow width. On average, US overestimated stone size by 3.8±2.4mm based on stone width (p<0.001) and 1.0±1.4mm based on shadow width (p<0.0098). Shadow measurements decreased misclassification of stones by 25% among three clinically relevant size categories (≤5, 5.1-10,>10mm), and by 50% for stones≤5mm. 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≤5mm.