Correlation of carotid stenosis diameter and cross-sectional areas with CT angiography

Abstract

Purpose: Carotid stenosis quantification traditionally uses measurements of narrowest stenosis diameter. The stenotic carotid lumen, however, is often irregularly shaped. New PACS workstation tools allow for more precise calculation of carotid geometry. We compare the narrowest stenosis diameter with 2D area stenosis measurements, with the hypothesis that the narrowest diameter is a good predictor of the more precise area measurement.

Methods: Two neuroradiologists evaluated 178 stenosed carotids in a blinded protocol. Carotid artery bulb stenosis was identified on axial CT angiography and measured in millimeters at its narrowest diameter. An AGFA Impax 4.5 Volume Tool (VT) using Hounsfield units was used to estimate the cross-sectional area of the contrast luminogram. Pearson correlation coefficients were calculated between the millimeter stenosis and the VT area, as well as between the VT area and the calculated area (radius based on narrowest diameter). Regression analysis was performed with the VT area and narrowest diameter datasets.

Results: Excellent interobserver correlation (correlation coefficients, 0.71-0.85; 2-tailed significance = .01) permitted averaging of measurement data. There is excellent correlation between the VT area and the narrowest diameter (correlation coefficient, 0.88; n = 176). The VT area was generally greater than the calculated area by an average of 2.77 mm2. There was excellent correlation between the VT area and the calculated area (correlation coefficient, 0.87; n = 176). Regression analysis shows the ability of the diameter measurements to predict corresponding area stenosis.

Conclusion: Although some carotid stenoses are irregularly shaped and noncircular, measurement of the narrowest stenosis is a reasonably reliable predictor of the cross-sectional area.

Fig 1.

Quantification of a symmetric right carotid bulb stenosis. A, Axial CTA source image showing symmetric stenosis of the right carotid bulb (white arrow). B, Axial magnification of the symmetric right carotid bulb stenosis with measurement calipers (calipers marked [A], showing measurement of 0.21 cm [2.1 mm]). C, Axial magnification of the symmetric right carotid bulb stenosis with the AGFA Impax 4.5 VT measuring the cross-sectional 2D area of 0.02 cm² (2.0 mm²).

Fig 2.

Quantification of an asymmetric right carotid bulb stenosis. A, Axial CTA source image showing asymmetric stenosis of the right carotid bulb (white arrow), with partially calcified posterior carotid bulb wall. B, Axial magnification of the asymmetric right carotid bulb stenosis with measurement calipers placed at the region of narrowest stenosis (calipers marked [A], showing measurement of 0.19 cm [1.9 mm]). C, Axial magnification of the asymmetric right carotid bulb stenosis with the AGFA Impax 4.5 VT measuring the cross-sectional 2D area of 0.03 cm² (3.0 mm²). Note that the narrowest diameter is slightly smaller than on Fig 1, though the area is slightly larger than on Fig 1.

Fig 3.

Quantification of an irregular, asymmetric carotid bulb stenosis with calcification. A, Axial CTA source image showing an asymmetric stenosis (white arrow). B, Axial magnification of the asymmetric carotid bulb stenosis with measurement calipers placed at the region of narrowest stenosis (calipers marked [A], showing measurement of 0.27 cm [2.7 mm]). C, Axial magnification of the asymmetric carotid bulb stenosis with the AGFA Impax 4.5 VT measuring the cross-sectional 2D area of 0.14 cm² (14.0 mm²).

Fig 4.

Quantification of a diminutive residual carotid bulb lumen. A, Axial CTA source image showing a diminutive left residual carotid bulb lumen with poor contrast filling (white arrow). B, Axial magnification of the residual carotid bulb lumen with measurement calipers placed at the region of narrowest stenosis (calipers marked [A], showing measurement of 0.11 cm [1.1 mm]). C, Axial magnification of the residual carotid bulb lumen with the AGFA Impax 4.5 VT. The VT could not accurately measure the cross-sectional 2D area of the lumen because there was very little contrast-filling of the diminutive carotid bulb. The difference in the HUs between the contrast-filled lumen and the surrounding tissues was not great enough for the VT to accurately measure the lumen.

Fig 5.

Quantification of a diminutive residual carotid bulb lumen. A, Axial CTA source image showing a diminutive right residual carotid bulb lumen (white arrow). B, Axial magnification of the residual carotid bulb lumen with measurement calipers placed at the region of narrowest stenosis (calipers marked [A], showing measurement of 0.10 cm [1.0 mm]). C, Axial magnification of the residual carotid bulb lumen with the AGFA Impax 4.5 VT, showing the cross-sectional 2D area of 0.02 cm² (2.0 mm²). The HUs between the contrast-filled lumen and the surrounding tissues was great enough for the VT to accurately measure the lumen, despite the small lumen size.

Fig 6.

Pearson correlation between the calculated stenosis area (based upon the narrowest diameter) and the measured cross-sectional VT area of stenosis (mm²). The calculated area showed a trend of underestimating the area in comparison to the measured area. This trend is not surprising, because the calculated area is based upon the narrowest stenosis, which does not account for noncircular stenoses. Nonetheless, there was excellent correlation between these 2 methods of area quantification (correlation coefficient = 0.87; n = 176).

Fig 7.

Pearson correlation between the mean carotid stenosis narrowest diameter (mm) and the mean cross-sectional VT stenosis area (mm²). There was excellent correlation between the 2 stenosis quantification measures at 0.88 (n = 176). This supports the use of the narrowest diameter measurement to quantify carotid bulb stenosis, in lieu of the more precise cross-sectional area measurement. A regression curve was plotted over the data with an R² value of 0.76, which indicates that the narrowest diameter has an excellent predictive ability to estimate the cross-sectional area (as defined by the VT).

Fig 8.

Pearson correlation between the mean carotid stenosis narrowest diameter (mm) and the mean calculated stenosis area (mm²). As anticipated, there was perfect correlation between the 2 methods of stenosis quantification at 1.0 (n = 176). A regression curve was plotted over the data, showing that the ability to predict the calculated area from the stenosis diameter is perfect with an R² value of 1.0. This is expected, because the calculated area is based on the diameter measurements. The data provide a graphic example of the nonlinear relationship between the narrowest diameter and the area, which demonstrates that minimal changes in the diameter of the carotid lumen cause more dramatic changes in the area of the lumen.