Evaluation of carotid artery stenosis with multisection CT and MR imaging: influence of imaging modality and postprocessing

Abstract

Background and purpose: We prospectively evaluated the influence of different imaging techniques (time-of-flight MR angiography [TOF-MRA], contrast-enhanced MR angiography [CE-MRA], multisection CT angiography [CTA]) and postprocessing methods (maximum intensity projection [MIP], multiplanar reformation [MPR]) on carotid artery stenosis grading.

Materials and methods: Fifty patients (34 men, 16 women) with symptomatic stenosis of the internal carotid artery were examined with a 16-section spiral CT and a 1.5T MR unit. Two MRA techniques were applied: 3D-TOF and CE-MRA. MPR was used for postprocessing with all modalities; MIP was used only with MRA. Four readers measured and calculated the percentage diameter stenosis independently according to NASCET criteria. The Wilcoxon test was used to measure interobserver variability, and the Friedman test was used to test the null-hypothesis of equality of the modalities.

Results: The hypothesis for global equality was rejected (P < .001). TOF-MRA and CTA assessed with MPR showed the highest concordance (difference, 0.6%; confidence interval [CI], -3.0, 4.3%), and CE-MRA with MIP and CTA showed the lowest concordance in stenosis grading (difference, 7.0%; CI, 3.4, 10.6%). MPR resulted in lower degrees of stenosis than MIP for both MRA sequences, although not statistically significant (CE, -3.0%; CI, -6.6, 0.6%; TOF, -2.2%; CI, -5.8, 1.4%). When only studies with good or excellent image quality were considered, the differences decreased, but the trends remained.

Conclusion: Stenosis grading is dependent on the examination method and postprocessing technique. CTA and TOF-MRA evaluated with MPR revealed highest concordance.

Fig 1.

Variability of stenosis values between 2 observers for the different imaging and evaluation techniques assessed by scatterplots and ICC values showing excellent agreement between the observers.

Fig 2.

Comparison of modalities by differences between stenosis values. Pairs of modalities that cross the zero-line are not considered significantly different. In cases of local signal intensity loss, the grade of stenosis was assigned as 90%. On average, stenosis values obtained with CE-MRA and MIP postprocessing were 7% higher than those with CTA. Stenosis values of TOF-MRA with MPR postprocessing were comparable with CTA.

Fig 3.

Stenosis values for CTA, CE-MRA, and TOF-MRA evaluated with the MPR method are represented graphically in scatter plots (upper row) and Bland-Altman-plots (lower row). Confidence intervals of the difference in stenosis values are given as gray bands. Highest agreement is found for CTA and TOF-MRA. In cases of local signal loss, the grade of stenosis was assigned as 90%.

Fig 4.

Stenosis of the left ICA. Local signal intensity loss at CE-MRA (A) with distal enhancement, but residual signal intensity is detectable with TOF-MRA (B: TOF-MIP) and CTA (C, “scout” MIP; D, transverse MPR at site of minimal lumen; E, transverse MPR at reference site beyond poststenotic dilation). Signal intensity is fading at the edge of the scan volume (B), leading to artificial lumen reduction on MIP images (B).