Volumetric Assessment of the Carotid Bifurcation: An Alternative Concept to Stenosis Grading

Background: To design a volumetric method for the assessment of carotid atheromatosis (CA) based on computed tomography (CT) angiography and three-dimensional (3D) reconstructions; to analyze the accuracy and optimal threshold values to differentiate between equivalent degrees of severity by duplex scanning and CT angiography (two-dimensional maximum intensity projection [2D MIP]; internal carotid artery stenosis [ICS] < 50%; ICS > 50%); and to assess the method's suitability to detect progression of CA. Methods: Design: suitability and accuracy of a new diagnostic method. Population: 90 carotid bifurcations (45 patients) were assessed with duplex scanning and CT angiography, and reevaluated after 12 +/- 2months follow-up. Determinations: Assessment of internal carotid artery (ICA) stenosis degree with duplex scanning and 2D MIP CT angiography projections. Volumetric assessment of carotid bifurcation by CT angiography (contrast volume [mm 3] and density [Hounsfield units, H.U.] between 2-cm below and 1-cm above the anatomic bifurcation of the carotid artery [BifV], and its ratio with 1-cm segment of the common carotid artery [CCV]). Statistical analysis: descriptive statistics; intraobserver and interobserver agreement (Blande-Altman plot and intraclass correlation coefficient [ICC], accuracy of 3D volumetry and duplex scanning as referred to MIP 2D CT angiography as gold standard: sensitivity (Sens), specificity (Sp), kappa index, and receiver operating characteristic curves (ROCs). Results: Estimation of MIP 2D images (CT angiography) confirmed the findings of duplex scanning in 23 of 30 ICS < 50% and 48 of 53 ICS > 50% (Sens, 0.91; Sp, 0.77% kappa = 0.68). Three-dimensional volumetric assessment of carotid bifurcation showed an intraobserver and interobserver agreement with an ICC of 0.96 (95% confidence interval [CI], 0.904-0.985) and 0.94 (95% CI, 0.822-0.977), respectively. The BifV-to-CCV ratio was 5.2 +/- 1.8 in the ICS < 50% group versus 3.8 +/- 1.3 in the ICS > 50% group (P = 0.001). The optimal cutoff point of the BifV-to-CCV relationship was identified from the ROC curve in 4.1 (Sens, 0.75; Sp, 0.75; kappa, 0.46). At 12 months, a decrease of the average BifV with regard to the baseline value (475.45 [155.6] mm(3) x H.U. vs. 501.3 [171.9] mm(3) x H.U.; P = 0.04) was observed. CA progression was detected in 32 bifurcations (14 ICS < 50%; 18 ICS> 50%), with a reduced bifurcation volume of 137.8 (71.4) mm(3) x H.U.; P < 0.001. Conclusions: Volumetric assessment of carotid bifurcation is a new concept based on assessing plaque burden rather than its hemodynamic effect or maximum stenosis; thus, justifying its moderate accuracy with regard to ICS conventional ICA grading based on biplanar images. This method can be especially useful in plaque progression studies given its accuracy to detect minor changes in the arterial lumen.