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Original Research |
1 Department of Radiology, Mayo Clinic, Rochester, MN.
2 Present address: Department of Radiology, Baylor University Medical Center,
3500 Gaston Ave., Dallas, TX 75246.
Received March 21, 2006;
accepted after revision October 4, 2006.
Address correspondence to K. F. Layton
(klayton{at}americanrad.com).
Abstract
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MATERIALS AND METHODS. Seventy-seven carotid arteries were evaluated in 63 patients suspected of having a high-grade carotid stenosis, and all vessels were evaluated with contrast-enhanced MRA. Two subgroups were created, and the specificity of MRA was calculated for each group using digital subtraction angiography (DSA) as the gold standard. Group 1 included 44 vessels classified as high-grade stenosis on sonography and all were evaluated with DSA. To test our hypothesis, group 2 included the 44 carotid arteries from group 1 plus 33 carotid arteries classified as normal or minimally narrowed on sonography and MRA.
RESULTS. In group 1, the specificity of MRA for accurately confirming a high-grade stenosis was 29% for contrast-enhanced maximum-intensity-projection (MIP) images alone and 75% for contrast-enhanced axially reformatted source images as compared with DSA. When the 33 normal arteries from group 2 were added to the data set, the specificities increased to 70% and 89%, respectively.
CONCLUSION. The calculated specificity of MRA as a confirmatory test for high-grade carotid stenosis is highly dependent on the proportion of normal carotid arteries included in the calculation. Based on our results, the specificity of MRA reported in the literature has likely been overstated because of spectrum bias.
Keywords: biomedical statistics • cardiovascular imaging • carotid arteries • carotid artery stenosis • MR angiography • neuroimaging
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As a screening test, the sensitivity of carotid Doppler sonography is generally optimized at the expense of specificity. If screening sonography reveals a carotid stenosis, additional confirmatory imaging is often performed to improve the specificity of the diagnostic testing. Because it is noninvasive, MRA has replaced digital subtraction angiography (DSA) as the confirmatory test for diagnosing high-grade carotid stenoses in many practices [2-4]. The prime objective of implementing MRA in this clinical scenario is to confirm that vessels identified on screening sonography as severely stenosed are, in fact, severely stenosed. In other words, MRA is used to identify vessels falsely identified on sonography as severely stenosed. In that clinical scenario, the true-negative rate—that is, specificity—should be determined on the basis of a population of vessels previously identified as abnormal on screening sonography.
The common use of MRA as a confirmatory test is based on previous reports that suggest a high specificity for accurately excluding a suspected high-grade carotid artery stenosis [5-11]. However, the most typical study design in previous reports includes both carotid arteries from all patients, irrespective of whether the carotid artery was identified as normal or abnormal on screening sonography. We suspect that this practice of including both carotid arteries is done to increase the number of vessels to satisfy statistical goals, but we submit that such a practice does not reflect the common clinical scenario and falsely elevates the apparent specificity, or true-negative rate, of MRA as a confirmatory test. The addition of normal or nearly normal vessels (true-negative cases) can be expected to falsely elevate the specificity of MRA as a confirmatory test because calculation of specificity is heavily dependent on the number of true-negative cases [12-17].
In the current study, we tested the hypothesis that inclusion of a substantial proportion of normal carotid arteries in a study population will falsely elevate the specificity of MRA for confirming a high-grade carotid artery stenosis. On the basis of those findings, we make recommendations regarding appropriate reporting of studies of the sensitivity of MRA or of any noninvasive confirmatory test reported in the future.
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Three experienced neuroradiologists retrospectively reviewed the MRA studies and were blinded to sonography and DSA results. Each reviewer separately evaluated the contrast-enhanced maximumintensity-projection (MIP) images alone and the contrast-enhanced source images. The contrast-enhanced source images were acquired in the coronal plane and reformatted in the axial plane. The reviewers recorded the percent diameter of stenosis for both contrast-enhanced MRA data sets in accordance with NASCET criteria after reviewing the images on a computer workstation. The final value was the mean value from the three reviewers. A cut point of 70% or greater was used to designate a high-grade stenosis.
To determine what effect the addition of normal carotid arteries has on the calculation of specificity, the carotid arteries from these 63 patients were divided into two groups. Group 1 was the clinically relevant vessels that were being evaluated before an anticipated intervention, either CEA or stenting. Group 1 included only those vessels for which the initial screening carotid sonography findings suggested a high-grade (70-99%) carotid artery stenosis. The 44 carotid arteries in group 1 were evaluated on sonography, contrast-enhanced MRA, and DSA.
Group 2 was created to test our theory that including a large number of clinically irrelevant normal carotid arteries in a study population will skew the results for calculated specificity. In group 2, we added 11 vessels that were designated as normal or mildly stenosed (0-39%) on sonography, contrast-enhanced MRA, and DSA to the carotid arteries in group 1. Also added to group 2 were 22 contralateral carotid arteries that were normal or minimally narrowed on contrast-enhanced MRA. Although DSA was not performed in these 22 contralateral carotids arteries, they were assumed to be normal or mildly stenosed because no false-positives were encountered in our study population with contrast-enhanced MRA findings that suggested a normal or mildly stenosed vessel.
Specificity calculations were performed on both groups for the contrast-enhanced MRA MIP images and contrast-enhanced axially reformatted source images.
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The specificity of the contrast-enhanced MIP images was 29% (7/24) in the clinically germane group 1 and 70% (40/57) in group 2; these results show significantly elevated apparent specificity when normal vessels are included in the calculation. The specificity of contrast-enhanced axially reformatted source images was 75% (18/24) in group 1 and was 89% (51/57) in group 2. In both groups, the contrast-enhanced axially reformatted source images were more specific than the MIP images alone for confirming high-grade carotid stenosis.
Of the 59 carotid arteries that were evaluated using carotid sonography (44 in group 1 and 15 in group 2), there were 24 false-positives, 20 true-positives, 13 true-negatives, and two false-negatives compared with the reference standard of DSA. The calculated sensitivity and specificity of sonography in our population was therefore 91% and 35%, respectively.
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Based on our results, the specificity of contrast-enhanced MRA MIP images is very poor when applied to carotid arteries screened as positive for stenosis on sonography. Indeed, for arteries identified as high-grade stenosis on sonography, the specificity of contrast-enhanced MIP images for confirming high-grade stenosis was worse than a flip of a coin (i.e., < 50%). Among 24 vessels falsely identified as severely stenosed on sonography when using DSA as the reference standard, contrast-enhanced MIP images correctly showed only seven of these vessels as less than 70% stenosed. The contrast-enhanced MRA axially reformatted source images performed better than the MIP images, but still showed falsely elevated specificity when normal and near-normal arteries were included in the calculation.
In previous studies, researchers have reported MRA as a highly specific test for evaluating high-grade cervical carotid artery stenosis [10-17]. In a previous study by Huston et al. [10] evaluating 2D time-of-flight MRA, a technique that is widely known to overestimate carotid artery stenosis, the specificity decreased from 80% to 18% when the normal carotid arteries were removed from the calculation. Similarly, the specificity of MRA decreased from 82% to 57% in reports by Anson et al. [11] and Kallmes et al. [12] after the normal carotid arteries had been excluded. The results from these studies are in line with our data and confirm that the specificity of MRA is falsely elevated if a large number of normal carotid arteries are used to calculate the true-negative rate.
Our study addresses the particular question of MRA as a confirmatory test for carotid artery stenosis after an initial positive result on sonography. This clinical scenario is different from evaluating MRA as an initial screening examination. In this regard, we do not dispute the specificity calculations from previous articles that evaluated MRA as a screening test. However, we disagree with the subsequent application of these results when using MRA as a confirmatory test. The most appropriate method for assessing the utility of MRA as a confirmatory test is to apply it only to those carotid arteries determined to have a high-grade stenosis on initial screening sonography.
Screening tests should have high sensitivity to limit the false-negative cases, even at the expense of false-positive results [19]. A second, or confirmatory, test should have high specificity to accurately identify false-positive cases resulting from the screening test. Given this logic, we caution the wide application of MRA as a confirmatory test because the tendency for MRA to "overcall" stenoses is widely known [20]. We submit that the current state-of-the-art MRA may function well as a screening test but that its value as a confirmatory test remains suspect.
There are several limitations to our study. This study was retrospective and evaluated only patients suspected of having a highgrade carotid stenosis. DSA was considered the gold standard for this study even though 22 vessels in group 2 did not undergo DSA evaluation. The ability of contrast-enhanced MRA to accurately show a carotid artery as normal is widely accepted; therefore, we are confident that those 22 vessels added to group 2 were indeed normal. To prove our hypothesis was correct, it was necessary to add these clinically irrelevant vessels to group 2 to show specificity would become falsely elevated.
Although the commonly accepted threshold for a high-grade stenosis is 70%, it is possible that stenoses in the 50-70% range are important in certain situations [18]. We designated 70% and greater stenoses as positive and those less than 70% as negative. If future investigations suggest a definitive role for CEA or stenting in patients with stenoses less than 70%, the specificity of MRA will need to be reevaluated in this subset of patients. It was not the purpose of our analysis to determine the accuracy of MRA but rather to confirm a suspected source of bias in the MRA evaluation of carotid stenosis. Although it is true that DSA is an invasive test that is associated with complications, DSA is a relatively safe examination when performed by experienced operators and still has a place in the evaluation of patients with suspected carotid artery stenosis [21].
As MRA and CT angiography techniques continue to improve, these techniques may prove to be more specific noninvasive confirmatory tests for evaluating carotid artery stenosis. In addition, 64-MDCT angiography with bone-removing techniques has shown considerable promise [22]. Advances in MRA continue with 3.0 T and novel sequences offering higher spatial resolution [23]. Prospective studies comparing the sensitivity and specificity of CT angiography versus MRA should be performed to investigate whether CT angiography is a more specific confirmatory test. If confirmation of a high-grade stenosis suspected on sonography is necessary before intervention, practitioners should consider these results when deciding whether to proceed with DSA.
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