(Stroke. 1996;27:2173-2177.)
© 1996 American Heart Association, Inc.
Articles |
the Departments of Neurology (V.Y.B., R.E.K.), Pathology (M.F.), and Surgery (T.P.), Louisiana State University Medical Center, Shreveport.
| Abstract |
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Methods We performed densitometric analysis of B-mode images of carotid plaques in nine patients scheduled for carotid endarterectomy. All patients had preoperative duplex color imaging and cerebral arteriography. The surgical specimen was analyzed histologically to determine the plaque components (soft plaque/organized thrombus, intraplaque hemorrhage/lipid deposition, fibrosis, and calcification). The specimen findings were correlated with the densitometric measurements to determine whether the density analysis would allow a reliable determination of the plaque substratum.
Results With 1.0 as a reference point for the moving column of blood, the mean acoustic densities (±SD) were as follows: organized thrombus, 1.8±0.5; intraplaque hemorrhage/lipid deposition, 5.15±0.9; fibrosis, 9.51±2.9; and calcification, 15.5±8.6.
Conclusions We conclude that densitometric evaluation allows differentiation of the various possible components of carotid plaque. The determination of plaque composition, based on density measurement, may provide information about its potential for thromboembolization.
Key Words: carotid artery diseases carotid endarterectomy densitometry diagnostic imaging ultrasonics
| Introduction |
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The degree of associated stenosis is of paramount importance, but even with mild to moderate stenosis, the more complex and irregular the plaque, the greater the likelihood of infarction.10 The European Carotid Plaque Study Group concluded that ultrasonic plaque characteristics that correlate with the histological composition of the plaque might affect decision making in reference to carotid endarterectomy in asymptomatic patients.11 Studies to date have indicated that carotid plaque ulceration12 13 14 as well as intraplaque hemorrhage and thrombus formation15 16 17 is associated with an increased risk of ipsilateral stroke.
The ultrasonic imaging of plaque is highly dependent on the quality of the scanner and the experience of the technician. The presence of "soft" (ie, noncalcified) plaque as well as partially organized or fresh thrombus formation can lead to an inaccurate interpretation. Partial absence of color Doppler flow, or altered flow characteristics within the vessel lumen, suggest the presence of a soft plaque or thrombus.18 19 20 Despite such a criterion, the sensitivity is limited. Furthermore, there is a potential for inaccurate interpretation because of normal variations in vessel anatomy that can alter the flow characteristics despite the absence of significant stenosis.
| Subjects and Methods |
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We used a high-resolution Acuson 128XP10v duplex scan with color Doppler imaging in all nine preoperative studies. We obtained multiple longitudinal, oblique, and cross-sectional B-mode views for each subject to assess carotid plaque composition by evaluating sonolucent and sonodense components. For each patient, the scan that allowed optimal visualization of the plaque was chosen for density analysis. The machine settings for depth gain compensation control were set as a straight line, and the only variable was the overall gain. The initial duplex image was printed on a color Sony Mavigraph printer with 256 levels of color intensity. This image was then reconstituted with a Hewlett-Packard ScanJet scanner interfaced with Image Pro Plus (Media Cybernetics) software. This resulted in a 12 bits-per-pixel (BPP) gray scale spectrum of 4096 possible shades, which represents an expanded array of ultrasonic densities of the analyzed tissues. After registration of the brightness of each plaque component in absolute BPP numbers, the relative values were calculated for each analyzed picture, with the reference point of blood density set as 1.0. Such densitometric analysis of the B-mode images allows enhanced differentiation of plaque composition.
Each surgical specimen was obtained en bloc and then sectioned to optimize correlation with the B-mode images. The plaques were examined by our collaborating histopathologist, who was advised about the sectioning preoperatively but was blinded to the ultrasound results. Plaque components that could be distinguished histologically included densely calcified plaque, fibrosis, intraplaque hemorrhage, intraplaque fatty deposition, and organized or fresh thrombus (soft) plaque.22 Because of similar ultrasonic densities among different histological plaque components, we grouped plaque components as follows: soft plaque/organized thrombus, intraplaque hemorrhage/fatty deposition, fibrosis, and densely calcified plaque.
We also performed a densitometric analysis of the moving column of blood by incorporating the color flow images into the same density-based gray scale that was used for plaque analysis. This type of analysis might allow differentiation of high-shear versus low-shear flow patterns within the carotid bifurcation.23
| Results |
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The relationship between ultrasonic density measurements and histological plaque composition is summarized in Table 2
. In patient 1, the plaque was termed fibrovascular by the histopathologist because of a combination of fibrous (collagenous) material with a microvascular component. On average, soft plaque/organized thrombus had a lower density than intraplaque hemorrhage/fatty deposition, which in turn had a lower density than fibrosis. Not unexpectedly, calcified plaque had the highest density measurement. When these plaque components were compared with the reference value of 1.0 for the moving column of blood, the corresponding mean ratios (±SD) were as follows: 1.8±0.5:1; 5.15±0.9:1; 9.51±2.9:1; and 15.5±8.6:1. The densitometric value of the calcified component of the plaque was the most variable, and it overlapped with the acoustic density of fibrous material. In one case (patient 4, Table 2
), the ultrasound examination and densitometric analysis failed to detect microscopic intraplaque hemorrhage. In another (patient 8, Table 2
), the ultrasound failed to demonstrate a fatty component of the plaque as well as a microscopic hemorrhage. These were the only instances of a discrepancy between ultrasound density versus histological analysis that we observed.
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Fig 1
is a histographic presentation of densitometric analysis of a complicated plaque with an irregular surface, compatible with ulceration, and associated thrombus along with corresponding histology. The mean density of soft plaque component was calculated as 382 BPP with a corresponding blood density of 193 BPP in absolute numbers.
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In Fig 2
, the effect of diffuse intimal thickening of the carotid vessel wall on the density profile is illustrated. Compared with the symmetrical double-peak pattern of the normal vessel walls (panel A), there are multiple peaks and valleys within the vessel walls of the artery with diffuse intimal thickening (panel B). Corresponding B-mode scans are illustrated in Fig 3
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Reconstitution of the original color-coded Doppler flow image into a broad-spectrum gray scale also allows derivation of a blood velocity profile of the components of the moving column of blood. This analysis is representative of the different velocity components within the vessel lumen rather than the actual density of the components. The comparison of a normal (laminar) flow pattern within the vessel lumen versus a turbulent flow pattern is illustrated in Fig 4
. The normal parabolic shape of the flow curve (A) is replaced by sharp fluctuations in the density profile of moving blood (B). This presumably reflects the eddying effect within a narrowed channel. The corresponding duplex scans are illustrated in Fig 5
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| Discussion |
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In a recent study of the circle of Willis with a 30-MHz intravascular ultrasound probe, with histological correlation, the sensitivity was 100% and the specificity 80% for calcium deposits and 83% and 75%, respectively, for fibrous tissue.25 Assessment of the correlation between ultrasonic carotid plaque appearance and histological examination was recently reported by Kagawa et al.26 The authors divided the plaques into seven ultrasonic categories according to sonolucent versus sonodense and homogeneous versus heterogeneous characteristics. They reported an accuracy of 93.8% for plaque characterization in the 68 vessels studied.
Digital densitometric analysis of plaque composition has been evaluated by el-Barghouty et al,27 but without histological correlation. Of 148 carotid plaques analyzed, 36% were associated with ipsilateral cerebral infarction by CT brain scan. They reported a rate of infarction of 55% in association with sonolucent plaques compared with 11% seen with sonodense plaques. They concluded that computerized plaque density analysis could identify carotid plaques associated with a higher risk of cerebral infarction. Mazzone et al28 differentiated "homogeneous" from "heterogeneous" plaques with similar methodology. They divided carotid plaques into three categoriessoft, fibrotic, and calcificand concluded that quantitative texture analysis is "feasible in man" and could have potential clinical application. In a radiofrequency-based ultrasound study of carotid plaque composition, the authors were able to reliably distinguish lipid, fibrotic, and calcific components based on histological correlation in 15 patients.29
Based on our initial nine cases, this method appears to provide reliable information about plaque composition. This commercially available software package can be interfaced with conventional duplex scans, and such image analysis may enhance our ability to predict the risk of stroke associated with different carotid plaque subtypes. This may be especially pertinent for patients with mild to moderate carotid stenosis or those subjects who are at higher risk for carotid endarterectomy, in which case the potential benefits must clearly outweigh risks.
At the present time, we do not believe that our densitometric technique allows differentiation of intraplaque hemorrhage from fatty material, but it appears to allow reliable differentiation of soft plaque/organized thrombus from intraplaque hemorrhage or fatty material, as well as from a predominantly calcified component. We did not observe a readily discernible relationship between the plaque morphology and/or composition and the degree of turbulent flow, but this might well be related to our limited patient series.
| Acknowledgments |
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| Footnotes |
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Received May 31, 1996; revision received September 3, 1996; accepted September 3, 1996.
| References |
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