(Stroke. 1996;27:95-100.)
© 1996 American Heart Association, Inc.
Articles |
Cerebrovascular Disease Assessed by Color-Flow and Power Doppler Ultrasonography
Comparison With Digital Subtraction Angiography in Internal Carotid Artery Stenosis
From the Departments of Neurology and Neuroradiology (G.K.), Ernst Moritz Arndt University Greifswald (Germany).
Correspondence to Christof Kessler, MD, Department of Neurology, Ernst Moritz Arndt University of Greifswald, Ellernholzstrasse 1-2, D-17487 Greifswald, Germany.
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Abstract |
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 Top Abstract IntroductionSubjects and Methods Results Discussion References |
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Background and Purpose An understanding of carotid atherosclerosis is important to further our knowledge regarding the etiology of cerebral ischemia, and therefore it is necessary to accurately visualize carotid stenosis. The purpose of the present study was to compare different imaging techniques to determine their advantages and disadvantages in the diagnosis and quantification of middle- and high-grade internal carotid artery stenosis. In particular, we were interested in evaluating the effectiveness of the new ultrasound technique power Doppler.
Methods Fifty-four patients with greater than 50% extracranial internal carotid artery stenosis, as determined by continuous-wave Doppler, were recruited prospectively to serve as subjects. All subjects were examined with color-flow Doppler, power Doppler, and digital subtraction angiography to enable visualization of carotid stenosis and plaque surface morphology.
Results Thirty-four middle-grade stenoses (50% to 69%), 32 high-grade stenoses (70% to 99%), and 7 complete occlusions of the internal carotid artery were diagnosed with the use of digital subtraction angiography. Power Doppler visualized stenosis significantly more frequently and accurately than color-flow Doppler. Color-flow Doppler tended to overestimate and underestimate in patients with both middle- and high-grade stenosis. Power Doppler was superior to both color-flow Doppler and angiography with regard to differentiation of plaque surface morphology.
Conclusions This study demonstrates that power Doppler is an important, noninvasive imaging technique that has several advantages over color-flow Doppler in diagnosing carotid artery stenosis and visualizing plaque surface. Power Doppler, used in concert with other ultrasound techniques, should enable a more accurate detection and treatment of cerebrovascular disease.
Key Words: angiography • carotid artery disease • carotid stenosis • Doppler • ultrasonics
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Introduction |
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TopAbstractIntroductionSubjects and Methods Results Discussion References |
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The relationship of carotid atherosclerosis to cerebral ischemia has been the focus of many previous studies.1 2 3 Characteristics of carotid stenosis such as plaque morphology and surface structure, echogenicity, absolute size, and the degree to which the vessel lumen is narrowed have been considered to be important pathogenic mechanisms for cerebral ischemia. To obtain a better understanding of the exact role played by carotid stenosis in the etiology of stroke and to facilitate therapeutic decisions, it is imperative to be able to visualize clearly and quantify the extent of carotid atherosclerosis.
Multiple ultrasonographic techniques have been used that enable the description and quantification of extracranial ICA stenosis.4 5 6 7 8 9 10 11 12 For example, methods used routinely include the spectral interpretation of the CWD signal, measurement of the maximum systolic blood flow velocity with the use of CWD or pulsed-wave Doppler, and more recently CFD.13 14 15 16 17 With the introduction of the PD imaging system we have a new, complementary technique, which is still in the early stages of technical development and clinical evaluation.18 19 20 The image produced by the PD system resembles a map that indicates the density of red blood cells in the vessels, as well as the number of red blood cells per unit volume of tissue. Whereas this technique provides no information concerning the direction or velocity of blood flow, it is essentially angle independent and free of artifacts such as aliasing.
The present investigation was undertaken to determine the advantages and limitations of PD in the diagnosis and quantification of middle- and high-grade ICA stenosis. Data derived from this technique were compared with those from CFD and DSA. A method that enables a more complete visualization of ICA stenosis and plaque characteristics should provide a better basis for determining the exact relationship between ICA plaque morphology and cerebral ischemia.
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Subjects and Methods |
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TopAbstractIntroductionSubjects and Methods Results Discussion References |
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Patients
Fifty-four patients were examined in the Neurosonology Laboratory of the Department of Neurology, University of Greifswald. Twelve patients had experienced a cerebral transient ischemic attack, 16 had experienced an acute ischemic stroke, and 26 patients had been referred by their primary care physician for routine ultrasound examination. All patients included in the study were found to have greater than 50% extracranial ICA stenosis, as detected by a CWD examination. The diagnosis of acute ischemic stroke was made on the basis of a long-term motor or sensory loss, whereas transient ischemic attack was inferred on the basis of a sensorimotor deficit that resolved in less than 24 hours. The patients ranged in age from 45 to 84 years; mean age was 63.4±5.2 years for the 35 male patients and 67.8±4.1 years for the 19 female patients. The patients were recruited prospectively and consecutively for the study after neurovascular screening. All patients volunteered to participate in the study after being informed that they would be examined with the use of CFD and PD ultrasonography, and they gave their written consent.
CWD Ultrasonography
Bidirectional CWD ultrasound was used to
screen patients
initially for inclusion in the study. This examination was used to
determine the presence of carotid stenosis and as the basis for
differentiating degree of stenosis (middle versus high grade).
All patients were examined with a 4.0-MHz probe (DWL, Multi-DopX). The
differentiation of high- or middle-grade stenosis in the
plaque area was estimated by analysis of the frequency
spectrum, as described elsewhere.21 Middle-grade
stenosis was defined as 50% to 69% stenosis, and
high-grade stenosis was defined as that equal to or greater
than 70%.
CFD- and PD-Assisted Duplex Imaging
The carotid arteries were
examined in all patients with the use
of a Masters ultrasound device (Diasonics) with a 7.5-MHz linear array
and pulsed-wave transducer that enabled superimposed,
simultaneous color-encoded (PD and CFD) blood flow
information to be obtained. The CFD and PD examinations were performed
with the patient's head in a sideways position, focusing on the
longitudinal and transverse views of the ICA at the level of the
narrowest part of the stenosis. The stenosis was first
visualized with B mode, and measurements were immediately made with CFD
and PD. In this way, the former ICA lumen as well as the residual lumen
could be delineated. The amount of luminal reduction was determined as
the percentage of both cross-sectional area and longitudinal
diameter reductions, to yield two independent estimates of luminal
reduction (Fig 1
).
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The evaluation of plaque surface was conducted by an investigator who was blind to the results from the CWD examination. A plaque was characterized as smooth if the surface did not show any disruptions and as irregular on the basis of echo reflections showing ulcerations or discontinuity in the plaque surface.
Angiography
Selective DSA of the carotid arteries was
performed in all 54
patients with a minimum of two projections. The measurement of ICA
diameter reduction was performed with the use of the linear-based
method used in the European Carotid Surgery Trial.22
The order with which carotid artery examinations were made was as follows: CWD, followed by either CFD or PD, and DSA. The CFD and PD duplex sonographic examinations were performed blindly with regard to the DSA and CWD findings. The examination interval between duplex sonography and DSA ranged from 1 to 48 days (mean, 10.5 days). The CFD-assisted duplex imaging was performed by a different individual than the one who performed the CWD sonography or the DSA. The imaging data were evaluated by independent investigators, and in instances of disagreement the case was eliminated. Interobserver reliability was 95%.
Data Analysis
The data were analyzed with the use of one-way
ANOVA
(F), and post hoc comparisons were made with the Newman-Keuls test.
Linear relationships were demonstrated with the use of Pearson's
product-moment correlation (r). Frequency
analyses were made with the 
2 test. The
data are expressed as mean±SD.
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Results |
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TopAbstractIntroductionSubjects and Methods Results Discussion References |
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On the basis of DSA, of the 54 patients, 34 middle-grade ICA stenoses (50% to 69%), 32 high-grade stenoses (70% to 99%), and 7 ICA occlusions were diagnosed. Of the 66 total cases of carotid stenosis (stenosis was detected in the left and right arteries in 19 patients), 15 (22.7%) and 8 (12.0%) were unable to be visualized adequately with CFD and PD, respectively, because of extensive calcification (Fig 1
). PD was able to
visualize
stenosis significantly more frequently than was CFD
(2 [df=1]=8.33,
P<.01). Because of the limited resolution in B-mode in differentiating between the stenotic and normal (original) vessel lumen, quantification of the stenosis in cross section could be determined in only 45 cases (68.2%) compared with 58 cases (87.9%) reliably visualized in longitudinal sections. There was no difference in the percent stenosis between cross versus longitudinal sections, as determined by PD, and these measurements were highly correlated (r=.82, P<.001).
ICA Stenosis
The mean percent stenosis for those cases of
middle-grade stenosis, as detected by CFD, PD, and DSA, was
57.1±16.3%, 56.4±7.3%, and 55.0±5.1%, respectively.
Because of
the tendency for CFD to overestimate and underestimate
stenosis, deviation scores were calculated (eg, percent
stenosis DSA minus percent stenosis CFD). The mean
percent difference in stenosis, as determined relative to DSA,
was 11.2±13.3% for CFD and 5.2±5.9% for PD. This difference
between
CFD and PD, with regard to their similarity to DSA in the detection of
middle-grade stenosis, was significant (F[1, 45]=3.98,
P=.05). The predictive relationship between the three
techniques is depicted in Figs 2 and 3.
CFD and DSA (r=.23, P>.05; Fig 2)
were not
significantly correlated with one another, whereas CFD and PD
(r=.63, P=.002) and PD and DSA
(r=.51,
P=.01; Fig 3) were significantly correlated
with one another
with regard to estimates of middle-grade stenosis.
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In those cases of
high-grade stenosis, there was no
difference between the three techniques with regard to their estimates
of stenosis. The mean percent stenosis for CFD was
80.7±12.7%, and the values were 80.7±11.6% and 83.1±8.6%
for PD
and DSA, respectively. Despite the similarity between estimates of
stenosis when these three techniques were used, CFD tended to
underestimate and overestimate stenosis compared with DSA.
Analysis of deviation scores revealed that PD was significantly
closer to DSA than CFD in estimates of high-grade stenosis
(F[1, 40]=5.50, P=.024). PD agreed with DSA
significantly
more often than did CFD (2
[df=1]=15.88, P<.001). PD agreed
with DSA in
87% of the cases, whereas CFD agreed in only 62.5%. Figs 4 and
5 depict the predictive relationship
between DSA, PD, and CFD with regard to estimates of stenosis.
The three techniques were significantly correlated with one another
(CFD-DSA: r=.62, P=.004, Fig 4;
CFD-PD:
r=.65, P=.003; PD-DSA: r=.93,
P<.0001, Fig 5).
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Plaque Surface Morphology
In those cases of middle-grade
stenosis, the
three techniques were equally effective with regard to their ability to
visualize plaque surface. PD visualized plaque surface in 88% of the
cases compared with 77.3% for CFD and 74% for DSA. CFD was highly
correlated with the other techniques with regard to plaque surface
visualization (CFD-DSA: r=.77, P=.0006;
CFD-PD:
r=.69, P<.0001), whereas there was a
nonsignificant correlation between PD and DSA (r=.54,
P=.085). Based on PD visualization, 26% of the plaques were
smooth surfaced, whereas 74% had an irregular surface.
As seen in Fig
6, PD was superior to CFD or DSA with
regard to differentiation of plaque surface morphology in those cases
of high-grade stenosis (2
[df=2]=12.05, P<.0001). In 90% of
the
cases, PD was able to detect the plaque surface compared with only 72%
and 73% for CFD and DSA, respectively. Nevertheless, these techniques
were highly correlated with one another in their ability to visualize
plaque surface morphology (CFD-DSA: r=.42,
P=.009; CFD-PD: r=.53, P=.001;
PD-DSA:
r=.51, P=.001). PD visualized an irregular
plaque
surface in 87.5% of the cases and 12.5% smooth-surfaced plaques
in the remaining cases.
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Discussion |
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TopAbstractIntroductionSubjects and Methods Results Discussion References |
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In the present study we compared three methods used to visualize ICA atherosclerosis. We found that compared with DSA, PD was better able to detect both middle- and high-grade carotid stenosis than CFD. Moreover, in 23% of the cases of ICA stenosis, because of extensive calcification CFD was unable to make any determination of stenosis. This compares with a figure of only 12% with PD. Although there were no significant differences in the mean estimates of percent stenosis between the three techniques, CFD consistently overestimated and underestimated stenosis compared with DSA. Furthermore, PD proved to be superior to both DSA and CFD with regard to visualization of plaque surface morphology. Taken together, these data indicate that PD is an improvement in the noninvasive diagnosis and measurement of ICA disease.
CFD ultrasonography has been shown previously to be a useful, noninvasive method for visualization of carotid plaques and quantification of ICA stenosis.5 13 23 24 25 26 This technique is based on the mean Doppler frequency shift and is therefore able to detect the changes in blood velocity as it moves through a given volume of tissue. An inherent limitation of this method is that there is a tendency for noise to overwhelm the flow signal, particularly if the gain is too high or the Doppler display threshold too low. As a result, there is a loss in precision, as was evident in our study. In addition, this technique is angle dependent and prone to artifacts such as aliasing, thereby reducing its accuracy.
PD is an ultrasound technique based on the integrated Doppler power spectrum.18 20 The hue and brightness of the color signal reflect the strength of the Doppler signal, which is related to the number of red blood cells per unit volume, as the blood courses through the vessel. Because PD is essentially angle independent, it is relatively free of artifactual bias. The similarity in the estimates of stenosis between PD and DSA, as demonstrated in the present study, suggests that PD is a method that complements other techniques but has an enhanced ability to visualize ICA plaque morphology and stenosis.
In our study there are several probable explanations for the
superiority of PD over CFD (Table). One of the most
important advantages that PD has over CFD is that in PD noise can be
assigned a homogenous background, even when the gain is increased
greatly over the level at which noise begins to obscure the CFD image.
In contrast, in CFD noise is seen as random color, totally obscuring
the information-containing signal. Even minimal noise in CFD can be
misinterpreted as a signal for blood flow velocity. Because of the
inherent limitations of CFD, in our study we were better able to
visualize plaque surface morphology and residual vessel lumen using
PD.
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Another advantage that PD has over CFD is that it is essentially angle independent, so PD is able to display blood flow even when the mean Doppler frequency shift is zero. This advantage is illustrated in the present study by the ability of PD to detect more cases of high-grade stenosis than CFD. Moreover, our measurements of the residual vessel lumen with PD were consistently closer to those obtained with DSA. These findings support those of earlier studies that used CFD, which classified stenosis based on the measurement of the residual vessel lumen contrasted with the color-flow signal ("flow lumen").
In the selection of a technique for diagnosing and measuring carotid atherosclerosis, there are additional factors to weigh, and as a result CFD may be superior to PD in certain respects. For example, because PD is more sensitive to tissue motion, it can be overwhelmed by flash artifacts, which is problematic in areas of the vessel with large amounts of tissue motion. In addition, CFD provides both directional and velocity information, whereas PD does not. In neurovascular imaging, there are well-known instances in which aliasing actually helps to localize areas of high velocity for Doppler recording.5 17 23 Moreover, directional information can help identify areas of flow reversal within the vessel. For these reasons, despite the tendency of CFD in our study to overestimate and underestimate stenosis, the mean degree of stenosis measured by this method was remarkably similar to DSA.
In summary, this study demonstrates that PD is an important new technique for imaging of the cerebral vasculature. However, the goal of the study was not to promote PD as the new gold standard for imaging. As other authors have previously suggested, we believe that angiographic methods suffer from their inability to depict the outer vessel boundary, whereas ultrasound techniques are unable to consistently visualize the residual vessel lumen.27 28 29 30 31 32 Instead, we believe that the importance of this study is to propose PD as a complementary imaging method with several distinct advantages over CFD. PD is an angle-independent method with a greater range than CFD, which enables the noninvasive diagnosis of ICA stenosis. In concert, these techniques should advance the treatment of cerebrovascular disease.
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Selected Abbreviations and Acronyms |
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Received June 22, 1995; revision received July 27, 1995; accepted September 25, 1995.
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