Abstracts & Commentary
Synopsis: CT angiography has high sensitivity and high negative predictive value for carotid disease.
Sources: Josephson, et al. Evaluation of Carotid Stenosis Using CT Angiography in the Initial Evaluation of Stroke and TIA. Neurology. 2004;63:457-460.; Marie, et al. Measuring Carotid Stenosis on Contrast-Enhanced Magnetic Resonance Angiography. Stroke. 2004;35:2083-2088.
Imaging of the carotid arteries is a recommended part of the diagnostic evaluation of patients with ischemic stroke or TIA. It has been demonstrated in several studies that carotid endarterectomy will benefit patients, particularly those with high-grade stenosis. The accepted gold standard for evaluation of carotid artery stenosis is catheter angiography. This however, has substantial risk and is expensive. Alternatives include MR angiography and CT angiography. These have the advantage of being non-invasive. In the present study to evaluate the accuracy of CTA, Josephson and colleagues compared the degree of stenosis found using CTA with digital subtraction angiography (DSA), in consecutive patients during a 3-year period. This was done from April 2000 to November 2002 at the University of California, San Francisco. Eight-one vessels were studied. The stenosis on the CTA of the internal carotid artery was measured in the axial plane at the point of maximum stenosis, and referenced to the distal cervical internal carotid by 2 blinded readers. Similarly, 2 blinded readers measured stenosis with the digital subtraction angiography studies. Josephson et al found that using a 70% cutoff value for stenosis CTA and DSA were in agreement in 78 of 81 vessels (96%). CTA was 100% sensitive, and the negative predictive value of a CTA demonstrating < 70% stenosis was 100%. Josephson et al conclude that CT angiography has high sensitivity and high negative predictive value for carotid disease.
A recent study has also compared DSA to contrast-enhanced magnetic resonance angiography (CEMRA). One hundred and sixty-seven symptomatic patients scheduled for DSA were prospectively recruited to undergo CEMRA. Josephson et al determined the severity of stenosis using 3 different methodologies, adopted by their North American Symptomatic Trial (NASCET) collaborators, the European Carotid Surgery Trial (ECST), and the common carotid (CC) methods. Measurements were each made in 284 vessels of the 142 included patients. Both the CEMRA and the DSA were analyzed in a blinded and randomized manner by 3 independent attending neuroradiologists. Josephson et al found that the NASCET method was the most predictive in measuring severe stenosis. The 3 different methods of examining stenosis however, did show reasonable intermodality correlation and agreement. With CEMRA however, Josephson et al determined a lower sensitivity for detection of severe stenosis with ECST, as compared to NASCET using DSA as the reference standard. The sensitivity for determining severe stenosis with CEMRA, with the NASCET analysis, was 93%. Overall, the diagnostic accuracy CEMRA for detection of severe stenosis as measured by NASCET was 93% sensitivity and 88% specificity. The positive predictive value was 72.5%, and negative predictive value 97.4%. The sensitivity in specificity therefore, appears to be less than that determined in the prior CTA study above.
CTA and CEMRA have major advantages over DSA in that they do not have the routine complication rate and high expense associated with DSA. The issue has been whether they have adequate sensitivity and negative predictive value. The present study suggests that this is the case with CTA. CEMRA was less sensitive and specific. Multi-detector CT scanners are continually improving in their sensitivity. General Electric has recently released a 64-slice scanner, which will have a considerable improvement in its overall ability to image the carotid artery. Even more sensitive CT scanners are under development. It is, therefore, highly likely that CTA will become the method of choice in evaluating carotid stenosis in the future. — M. Flint Beal, MD
Dr. Beal, Professor and Chairman; Department of Neurology; Cornell University Medical College New York, NY, is Editor of Neurology Alert.