Performance of semiautomatic assessment of carotid artery stenosis on CT angiography: clarification of differences with manual assessment

BACKGROUND AND PURPOSE:Semiautomated methods for ICA stenosis measurements have the potential to reduce interobserver variability and to speed up its analysis. In this study, we estimate the precision and accuracy of a semiautomated measurement for carotid artery stenosis degree and identify and explain differences compared with the manual method.MATERIALS AND METHODS:In this retrospective study involving 90 patients, 2 observers determined the stenosis degree twice, with both the semiautomated and the manual method. Intra- and interobserver correlations were calculated for both methods. The accuracy was estimated by comparing average semiautomated with manual measurements. The semiautomated stenosis calculations were performed using either the minimal or maximal intersection at the reference site. Individual cases with large differences in measurement were retrospectively inspected by 3 observers.RESULTS:Intra- (R = 0.93, 0.96) and interobserver (R = 0.98) correlations for the semiautomated method were excellent and exceeded the manual performance correlations (R = 0.87, 0.86). The semiautomated measurements correlated well with the manual measurements (R = 0.87), with high specificity of 96% and lower sensitivity of 63%. Large differences were caused by misinterpretations of the semiautomated method associated with calcified plaques, resulting in overestimations of the minimal diameter, underestimation of stenosis degree, and incorrect centerlines. The effect of using the minimal diameter at the reference position resulted in a small, but significant, underestimation of the stenosis degree by the semiautomated method.CONCLUSIONS:The semiautomated method showed an excellent reproducibility and good correlation with manual measurements with a high specificity and lower sensitivity for detecting a significant stenosis. Erroneous semiautomatic stenosis measurements were associated with the presence of calcium.

Atherosclerotic stenosis of the ICA may lead to neurologic symptoms and is an important risk factor for ischemic stroke. Large randomized trials determined that CEA is beneficial for recently symptomatic patients with a severe (70%–99%) stenosis.^1–3 In the trials with symptomatic patients, a higher degree of stenosis was associated with increased benefit from surgery. Therefore, precise assessment of the degree of stenosis is crucial for decisions on CEA. Currently, CTA is increasingly used to measure the degree of carotid artery stenosis.⁴Determining the degree of carotid stenosis on CTA, according to the NASCET method, is tedious and may lead to clinically important differences.^5,6 Reading CTA studies requires some familiarity with postprocessing techniques, such as MPR. Semiautomated methods have been developed and introduced in the market to overcome the drawbacks of these measurements.^7–11 The potential advantages of such a system, such as the acceleration of measurements and reduced interobserver variability, have been widely acknowledged; however, the diagnostic value has not been sufficiently determined. Several studies have shown excellent intra- and interobserver variability,^7–13 yet the diagnostic accuracy and the cause of deviations of semiautomatic measurements have received little attention.The aim of this study was to validate semiautomated carotid stenosis measurements by comparison with a standard manual method.^14–16