Accuracy and precision of automated kidney stone detection on CT.

backgroundWhile computed tomography (CT) is the preferred imaging modality for kidney stone detection and measurement, quantifying the accuracy and precision of stone size metrics is needed, particularly with the growing use of automated image analysis tools to measure kidney stone size.

methodsA phantom study was conducted using 120 kidney stones of various known maximum diameters (ranging from 1.4 to 9.9 mm) and compositions (60 calcium, 30 uric acid, and 30 mixed calcium and uric acid). The stones were placed in an anthropomorphic phantom and CT images were reconstructed with slice thicknesses of 1, 3, or 5 mm and processed using an in-house quantitative Stone Analysis Software (qSAS) to detect each stone and determine its maximum diameter. Separately, we also assessed the agreement between qSAS and a radiologist in determining the maximum diameter of 45 kidney stones on CT images (1-mm slice) from patients.

resultsStone detection frequencies by qSAS for stones in phantoms were 100%, 93%, and 86% for 1-, 3-, and 5-mm CT slice thicknesses, respectively. The proportion of qSAS-detected stones with a diameter error > 1 mm was 5%, 28%, and 64% for 1-, 3-, and 5-mm CT slice thickness, respectively. The CT-measured diameters on 1-mm images were accurate regardless of stone composition. However, diameters with thicker slices trended toward underestimation in uric acid stones and overestimation in calcium and mixed stones. Stone diameter by a radiologist was relatively unbiased but had limited precision with a diameter error > 1 mm occurring in 51% compared to qSAS.

conclusionsThis study quantifies the improved precision with an automated assessment of stone diameter on CT as well as the bias and imprecision that remains across different slice thicknesses and stone compositions.