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MDCTA of Carotid Plaque Degree of Stenosis: Evaluation of Interobserver Agreement

¹ Department of Science of the Images, Policlinico Universitario, s.s. 554 Monserrato, Cagliari 09045, Italy.
² Institute of Radiology of the University of Cagliari, 46 Hospital St., Cagliari 09126, Italy.

Received May 20, 2007; accepted after revision July 27, 2007.

 
WEB This is a Web exclusive article.

Address correspondence to L. Saba (lucasaba{at}tiscali.it).

Abstract

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OBJECTIVE. Atherosclerotic disease of the carotid arteries is one of the most important causes of stroke. Our objective was to evaluate the interobserver agreement in the measurement of the degree of carotid plaque stenosis by using MDCT angiography (MDCTA) and the effects produced using different window parameters and by the different types of plaque.

MATERIALS AND METHODS. From June 2005 to June 2006, we retrospectively evaluated 215 patients (151 men, 64 women) who underwent MDCTA for the study of carotid arteries. In all patients we measured degree of stenosis, applying the criteria of the North American Symptomatic Carotid Endarterectomy Trial (NASCET). Each patient was studied independently by two observers. We used three window settings for the measurements. We grouped the measurements according to the type of plaque. Obtained data were then analyzed to calculate the interobserver agreement and the kappa value.

RESULTS. Kappa values for the degree of stenosis evaluation were 0.696, 0.79, and 0.775 for window settings 1, 2, and 3, respectively. The best agreement was observed in the assessment of fatty plaque, whereas the presence of calcification produced disagreement.

CONCLUSION. We observed a very good interobserver agreement in the evaluation of degree of stenosis using MDCTA with the application of specific visualization parameters. Our data suggested that MDCTA can provide reproducible values.

Keywords: angiography • carotid arteries • CT angiography • interobserver agreement • MDCTA • vascular system

Introduction

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Stroke is a dramatic medical problem; in fact, when considered separately from other cardiovascular diseases, stroke ranks third among all causes of death after heart disease and cancer [1]. In the United States every year, about 700,000 people experience a new or recurrent stroke and, on average, someone has a stroke every 45 seconds [1]. The estimated direct and indirect costs of stroke for 2006 are $57.9 billion.

Several studies [2-4] have shown that carotid artery degree of stenosis is a critical parameter in the evaluation of stroke risk because the risk of ischemic stroke distal to an atherothrombotic carotid stenosis increases with the degree of stenosis and can be markedly reduced with endarterectomy. Recently, new parameters [5-10] other than degree of stenosis have been shown to be important markers for the stratification of the risk of stroke, although the degree of stenosis is still considered the leading parameter for choosing a specific therapeutic option.

Numerous imaging techniques are used in the evaluation of carotid artery degree of stenosis: angiography, MRI, sonography, and CT. Today, MDCT angiography (MDCTA) sensitivity for the evaluation of degree of stenosis may be compared with that of angiography, although MDCTA has fewer risks [10, 11-18]. In particular, MDCTA sensitivity for stenosis of 70-99% in the internal carotid artery, using angiography as the reference standard, can achieve excellent results [14-19]. One of the difficulties in evaluating an imaging method is its reproducibility because that represents the data variability in a specific observation. Reproducibility is well addressed by the calculation of interobserver agreement.

The purpose of this study was to evaluate interobserver agreement in the measurement the degree of stenosis of carotid plaque using MDCTA in order to assess the reproducibility of this technique.

Materials and Methods

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Patient Population
We retrospectively studied 215 patients (151 men, 64 women; mean age, 68 years; age range, 31-86 years) and a total of 430 carotid arteries using an MDCT scanner. Examinations were performed between June 2005 and June 2006. None of the studied patients showed a contraindication to IV injection of iodinated contrast material. A history of previous symptomatic ischemic episodes was found in 89 patients (41.4%); 126 patients (58.6%) were previously asymptomatic. According to our department procedures, MDCTA is performed when a previous color Doppler sonographic examination has shown a stenosis of more than 50% or plaque alteration.

View larger version (112K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A —Fatty plaque (average attenuation, 39 H) in 68-year-old man. MDCT angiography axial images obtained using window settings 1 (A), 2 (B), and 3 (C). Plaque that produced stenosis in left internal carotid artery is indicated by arrowhead. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

View larger version (106K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B —Fatty plaque (average attenuation, 39 H) in 68-year-old man. MDCT angiography axial images obtained using window settings 1 (A), 2 (B), and 3 (C). Plaque that produced stenosis in left internal carotid artery is indicated by arrowhead. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

View larger version (103K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C —Fatty plaque (average attenuation, 39 H) in 68-year-old man. MDCT angiography axial images obtained using window settings 1 (A), 2 (B), and 3 (C). Plaque that produced stenosis in left internal carotid artery is indicated by arrowhead. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

MDCTA Technique
After being informed of the type of investigation, each patient was asked to sign an informed consent for contrast administration. All MDCTA examinations were performed with a 4-MDCT scanner (Somatom Mx8000, Siemens Medical Solutions). Arterial enhancement was provided by IV administration, in an antecubital vein through an 18- to 20-gauge IV catheter, of 110-130 mL of nonionic iodinated contrast material (Ultravist 370 [iopromide], Bayer HealthCare [formerly Schering] and Iomeron 350 [iomeprol], Bracco) at an injection rate of 4-6 mL/s with a power injector (single syringe pump). We used a variable delay time ranging from 11 to 18 seconds, and the table speed was 3 mm/s with a collimation of 3 mm. CT technical parameters included matrix, 512 x 512; field of view, 12-19 cm; mA, 180-210; kV, 120; and section thickness, 1.6 mm. Scanning started at the seventh cervical vertebra and proceeded as far cephalad as possible, always including the carotid siphon. Axial source images were reconstructed in 1.6-mm increments, and the field of view was 11-14 cm. Total coverage was 14-22 cm. Subsequently, obtained data were processed, using a workstation, to create multiplanar reconstruction (MPR) and maximum-intensity-projection (MIP) postprocessed images.

Image Analysis and Measurement of Stenosis
First, two radiologists in consensus rated the quality of each examination on a 5-point scale (1 = poor, 5 = excellent), and cases in which image quality was 2 or less were excluded from this study. We graded stenosis of each carotid artery according to the following classification based on the North American Symptomatic Carotid Endarterectomy Trial criteria; IV, (50-69% stenosis; Va, 70-84% stenosis; Vb, 85-99% stenosis; and VI, occluded. To quantify the degree of stenosis using MDCTA, two radiologists independently reviewed oblique axial images perpendicular to the centerline of the internal carotid artery lumen, and the value was calculated by comparing the diameter of the stenosed segment with the most distal normal segment in which no stenosis was present. We used three window settings to assess the best agreement for each segment: window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H. We grouped degree of stenosis agreement for each type of plaque. We used axial scans and classified carotid plaque into three groups, as described in a previous work [9]: fatty (soft) plaque, a plaque with attenuation < 50 H; mixed (intermediate) plaque, plaque with attenuation of 50-119 H; and calcified plaque, plaque with attenuation >120 H.

For measuring attenuation in Hounsfield units, a circular or elliptic region-of-interest cursor in the predominant area of plaque was used [20], and areas showing contamination by contrast material or calcification not contributing to the stenosis were carefully avoided. We also excluded regions of beam hardening in calcified areas. Radiologists took the measurements at the point of major stenosis.

View larger version (93K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A —Mixed plaque (average attenuation, 57 H) in 64-year-old woman. MDCT angiography axial images obtained using window settings 1 (A), 2 (B), and 3 (C). Plaque that produced stenosis in left internal carotid artery is indicated by arrowhead. Note some calcific eccentric spots in this mixed plaque. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

View larger version (86K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B —Mixed plaque (average attenuation, 57 H) in 64-year-old woman. MDCT angiography axial images obtained using window settings 1 (A), 2 (B), and 3 (C). Plaque that produced stenosis in left internal carotid artery is indicated by arrowhead. Note some calcific eccentric spots in this mixed plaque. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

View larger version (87K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2C —Mixed plaque (average attenuation, 57 H) in 64-year-old woman. MDCT angiography axial images obtained using window settings 1 (A), 2 (B), and 3 (C). Plaque that produced stenosis in left internal carotid artery is indicated by arrowhead. Note some calcific eccentric spots in this mixed plaque. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

View larger version (102K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A —Calcified plaque (average attenuation, 976 H) in 63-year-old man. MDCT angiography axial images obtained using window settings 1 (A), 2 (B), and 3 (C). Residual lumen in right carotid bifurcation is indicated by arrowhead. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

View larger version (93K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B —Calcified plaque (average attenuation, 976 H) in 63-year-old man. MDCT angiography axial images obtained using window settings 1 (A), 2 (B), and 3 (C). Residual lumen in right carotid bifurcation is indicated by arrowhead. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

View larger version (92K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3C —Calcified plaque (average attenuation, 976 H) in 63-year-old man. MDCT angiography axial images obtained using window settings 1 (A), 2 (B), and 3 (C). Residual lumen in right carotid bifurcation is indicated by arrowhead. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

Results

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Among the 215 patients (430 arteries) available, 19 patients were excluded because of an image quality rating of 2 (in 12 patients we observed swallowing artifacts and in seven, inadequate contrast opacification). The mean age of the 196 remaining patients was 67 years (range, 31-84 years) and 69% were men.

Interobserver agreement for the degree of stenosis evaluation for the window setting 1 (window width preset at 500 H, level of 150 H) was 74.49%. The resulting kappa value was 0.696 (95% CI, 0.645-0.748) and the weighted kappa was 0.839; these values indicated good agreement. Interobserver agreement for the degree of stenosis evaluation for the window setting 2 (window width preset at 750 H, level of 200 H) was 82.4%. The resulting kappa value was 0.79 (0.745-0.835) and the weighted kappa was 0.893; these values indicated good to very good agreement. Interobserver agreement for the degree of stenosis evaluation for the window setting 3 (window width preset at 850 H, level of 300 H) was 81.12%. The resulting kappa value was 0.775 (0.729-0.821) and the weighted kappa was 0.882; these values indicated good agreement.

View larger version (84K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4A —Maximum-intensity-projection postprocessed images of calcified plaque (average attenuation, 976 H) in 63-year-old man. MDCT angiography obtained using window settings 1 (A), 2 (B), and 3 (C). Residual lumen in right carotid bifurcation is indicated by arrowhead. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

View larger version (87K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4B —Maximum-intensity-projection postprocessed images of calcified plaque (average attenuation, 976 H) in 63-year-old man. MDCT angiography obtained using window settings 1 (A), 2 (B), and 3 (C). Residual lumen in right carotid bifurcation is indicated by arrowhead. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

View larger version (87K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4C —Maximum-intensity-projection postprocessed images of calcified plaque (average attenuation, 976 H) in 63-year-old man. MDCT angiography obtained using window settings 1 (A), 2 (B), and 3 (C). Residual lumen in right carotid bifurcation is indicated by arrowhead. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

View larger version (69K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A —Multiplanar reconstruction postprocessed images of calcified plaque (average attenuation, 976 H) in 63-year-old man. MDCT angiography images obtained using window settings 1 (A), 2 (B), and 3 (C). Residual lumen in right carotid artery bifurcation is indicated by arrowhead. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

View larger version (70K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B —Multiplanar reconstruction postprocessed images of calcified plaque (average attenuation, 976 H) in 63-year-old man. MDCT angiography images obtained using window settings 1 (A), 2 (B), and 3 (C). Residual lumen in right carotid artery bifurcation is indicated by arrowhead. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

View larger version (75K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5C —Multiplanar reconstruction postprocessed images of calcified plaque (average attenuation, 976 H) in 63-year-old man. MDCT angiography images obtained using window settings 1 (A), 2 (B), and 3 (C). Residual lumen in right carotid artery bifurcation is indicated by arrowhead. Window setting 1, window width preset at 500 H, level of 150 H; window setting 2, window width preset at 750 H, level of 200 H; and window setting 3, window width preset at 850 H, level of 300 H.

Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
A correct, reproducible method for evaluating carotid artery stenosis is fundamental in order to provide the best information for planning the proper therapy. In fact, degree of stenosis is considered the leading parameter in the choice of the therapeutic option. In the past years, angiography has been thought to represent the gold standard in the evaluation of carotid artery stenosis. Angiography, however, is an invasive technique with a risk of catheter-induced complications [21], and it has a high cost compared with other techniques. Moreover, the interobserver variability of this technique, although widely accepted as reliable, is debatable [22].

Identification of degree of stenosis can be performed using noninvasive techniques such color Doppler sonography, MRI, and MDCTA. Color Doppler sonography is commonly performed to screen patients with possible carotid artery disease, but the accuracy of sonography is suboptimal for assessing degree of stenosis [23]; in fact, with the use of only sonography, several critical errors can occur and the number of false-negatives for stenosis > 70% can be high [23]. Moreover, numerous studies underscore that interobserver variability of color Doppler sonography in analysis of degree of stenosis is suboptimal [24-26].

MRI shows a good sensitivity in determining degree of stenosis; authors reported a median sensitivity of 93% and median specificity of 88% for assessing high-grade stenosis of the carotid arteries [27]. However, MRI has a high variability between observers [28]. Previous studies have reported CT interobserver agreement [12, 17, 29, 30] to be 82-95% ( = 0.92).

In this study, our purpose was to assess which visualization parameters would provide the best interobserver agreement. If we consider the three settings used in our analysis, window setting 1 (500 and 150 H) produced the worst result, with agreement of 75.5% ( = 0.696). This window setting showed suboptimal results in the assessment of all plaque types and provided the poorest performance in the evaluation of calcified plaque ( = 0.527). We obtained better results with window settings 2 (750 and 200 H) and 3 (850 and 300 H), which showed agreement rates of 82.4% and 81.1%, respectively ( = 0.79 and 0.76).

When analyzing these data, one should remember, first, the search for the most appropriate window setting is key in the assessment of carotid plaque because different settings may result in different results. The important parameters are window level and width. We observed that the larger the width, the better the visibility; larger width provides better visibility than a higher level. Increased width is especially useful for assessing calcified plaque because it enables one to clearly distinguish calcium from endoluminal contrast medium.

If we consider the agreement between studied plaque type and window setting used, we found that the best agreement resulted from the evaluation of fatty plaque using window setting 2 ( = 0.913). Worst results were obtained for assessing calcified plaque, which were kappa values of 0.527, 0.649, 0.668 for window settings 1, 2, and 3, respectively. We noticed also that the best result in evaluating degree of stenosis of calcified plaque was inferior to the worst result obtained by the same assessment of all other plaque types. With reference to our data, we can say that, generally, the presence of calcified plaque produces disagreement.

We want also to underscore that window settings intend to provide the best visualization of endoluminal contrast in the plaque in calcified, fatty, and mixed types. Using high flow rates (4-6 mL/s) with a concentration of 370 mg I/mL, we obtained values for endoluminal opacification of 350-550 H, so the window level settings we reported refer to this Hounsfield endoluminal value. On the other hand, if we use lower flow rates and lower concentrations, we think window level and width settings may consequently be inferior as well.

In conclusion, our results show how a window width of 750 H and a level of 200 H allow higher agreement in evaluating the degree of carotid stenosis in fatty or mixed plaques, whereas a window width and level of 850 and 300 H should be used to assess calcified plaque. The best use of all visualization parameters allows agreement values that range between good and very good.

References

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