Sonographic Assessment of Carotid Artery Stenosis
Comparison of Power Doppler Imaging and Color Doppler Flow Imaging
Background and Purpose Power Doppler imaging (PDI) is a new ultrasound technique that, in contrast to color Doppler flow imaging (CDFI), generates intravascular color signals from the reflected echo amplitude depending mainly on the density of red blood cells. We evaluated the diagnostic significance of PDI compared with CDFI for the measurement of carotid stenosis and characterization of plaque surface.
Methods In 25 internal carotid artery stenoses, reduction of the intrastenotic lumen contrasted by blood density signals and color Doppler signals on longitudinal and transverse views was assessed for correlative evaluation. In addition, the peak systolic flow velocity of the Doppler spectrum was correlated with PDI and CDFI measurements.
Results PDI provided good visualization of the residual lumen in all stenoses, whereas displays on CDFI were inadequate in two calcified plaques. PDI revealed two ulcerative stenoses classified as smooth on CDFI. The correlation between PDI and CDFI was high for measurement of area stenosis (r=.93) and moderate for diameter stenosis (r=.73). Similarly, cross-sectional reduction on both imaging methods correlated more significantly with peak systolic flow velocity than diameter reduction.
Conclusions This pilot study suggests that PDI provides additional information for luminal measurement and characterization of plaque surface in complicated high-grade carotid stenosis. Because of the absent visualization of hemodynamics, PDI should be used in combination with CDFI.
During recent years, CDFI has been established in most neurovascular laboratories as a routine sonographic method for the assessment of extracranial arterial disease.1 2 3 4 5 CDFI displays vascular structures as well as the velocity and flow direction of red blood cells by mapping the Doppler frequency information. In contrast, PDI, the most recent sonographic development for vascular applications, generates intravascular color signals from the amplitude of the echo signal, which essentially depends on the density of red blood cells within the sample volume.
With the introduction of CDFI, measurement of the residual lumen of carotid stenosis in longitudinal and transverse views became more reliable than with conventional B-mode scans.6 7 However, plaque calcification remained a significant limitation for the assessment of the intrastenotic diameter and area reduction. Because an increased sensitivity to depict the continuity of blood flow and improved intravascular edge definition are claimed for PDI, we evaluated this new technique in comparison with CDFI for the measurement and characterization of ICA stenoses. Development and validation of new ultrasound techniques are particularly important in view of the ongoing discussion concerning the diagnostic capacity of ultrasound and the discrepant results of angiographic methods in measuring the degree of carotid stenosis that followed the publication of the two large endarterectomy trials.8 9 10 11 12 13
Subjects and Methods
Twenty asymptomatic outpatients (13 men, 7 women; mean age, 66 years; range, 46 to 80 years) with a total of 25 ICA stenoses of different degrees according to the criteria of CW Doppler sonography14 15 as a screening method were consecutively selected for the study. In brief, CW Doppler criteria were as follows: diameter reduction 40% to 59% (n=9), moderate local increase of peak blood flow velocity with poststenotic turbulence; diameter reduction 60% to 79% (n=7), marked increase of peak and mean velocity with poststenotic systolic deceleration; and diameter reduction 80% to 99% (n=9), more severe increase of peak and mean velocity and significant reduction of ipsilateral prestenotic and postenotic flow velocity; retrograde blood flow in the ophthalmic artery may occur.
Examinations were performed by means of a VST Master ultrasound scanner (Diasonics Ultrasound Inc) with the use of a 10-MHz linear array transducer for both CDFI and PDI. The technical principles of CDFI have been reported elsewhere.16 17 PDI uses the amplitude of the echo signal reflected from red blood cells regardless of their flow velocity and flow direction. Thus, in contrast to CDFI, the color display does not depend on the insonation angle and is free from aliasing phenomena. The greater the density of red blood cells within a given sample volume, the stronger the echo amplitude. A special filter system for blood-tissue discrimination, as well as addition of amplitudes of positive and negative shifts, increases the sensitivity for displaying the continuity of blood flow.18
Sequential perpendicular cross sections along the axis of the carotid artery and parallel longitudinal sections were displayed on CDFI and PDI. Delineation and measurement of the vessel wall and the minimal residual intrastenotic lumen at the same site contrasted by either color Doppler or blood density signals on longitudinal and transverse views were used to calculate the degree of stenosis as percentage of the maximal diameter and area reduction, respectively, for correlation analysis. The residual area was measured at exactly the same level of the stenosis by simply switching from CDFI to PDI display. In addition, the PSV of the PW Doppler frequency spectrum recorded at the site of maximal blood flow velocity on color flow imaging was correlated with the lumen measurements. Statistical evaluation consisted of simple regression analysis.
PDI provided sufficient displays in all 25 stenoses (100%), whereas visualization of the diameter and area reduction was inadequate on CDFI in 2 high-grade stenoses (8%) (Fig 1⇓). The correlation of measurements between CDFI and PDI was moderate for diameter stenosis (coefficient of regression r=.73) but was excellent for area stenosis (r=.93) (Fig 2⇓). The characterization of the plaque surface on CDFI and PDI was in agreement in 9 smooth, 7 irregular, and 3 ulcerative plaques (Fig 3⇓), whereas PDI diagnosed 2 ulcerative and 1 irregular stenoses classified as smooth on CDFI.
Correlation of CW Doppler sonography and PSV (PW Doppler spectrum) with both imaging methods was higher for measurements of area stenosis than for measurements of diameter stenosis on CDFI and PDI (Table⇓). The best agreement was found between PSV and area stenosis on CDFI (r=.88).
The results of the present pilot study suggest that PDI may provide additional information for luminal measurements in high-grade carotid stenosis with complex plaque composition and calcification since assessment of the diameter and area reduction was feasible in all stenoses on PDI, whereas CDFI did not visualize the residual intrastenotic lumen in 8%, a failure rate that agrees with previous studies of carotid stenosis reporting inadequate display quality of CDFI in 8% to 16%.4 5 6 7 In addition, the characterization of the plaque surface on PDI was improved in some stenoses, which is probably due to the different sonographic principles of PDI (see “Subjects and Methods”) providing better edge definition of the intravascular surface even in complicated atherosclerotic lesions. The results suggest that application of echo contrast medium, which enhances color Doppler signal intensity on CDFI and thereby improves the visualization of the “flow lumen” of high-grade carotid stenosis19 20 may not be mandatory for satisfactory sonographic evaluation if CDFI is combined with PDI.
In this series correlation was significant for measurement of stenotic area reduction on cross-sectional views of CDFI and PDI, as well as for the comparison of both imaging techniques with CW Doppler sonography and PSV of the PW Doppler spectrum. Corresponding results were reported in a recent study that demonstrated that color-assisted duplex sonography using PSV correlated well with planimetric measurements of surgical specimens of high-grade carotid stenoses.13 Unfortunately, the authors did not use measurements of area reduction on CDFI for their analysis. Nevertheless, on the basis of available data it may be postulated that sonographic measurement of the residual intrastenotic lumen on transverse views by means of PDI and CDFI probably comes closest to the “true anatomic” degree of stenosis.6 7 13
A recent international consensus meeting on the quantification of ICA stenosis came to the same conclusion.21 22 Apart from hemodynamic parameters, the jury suggested direct measurement of the area ratio on transverse CDFI as a valid criterion for the assessment of the degree of stenosis. Compared with angiography and the use of different diameter ratios, area stenosis on ultrasonography was considered to more accurately represent the anatomic reference.13
In contrast to CDFI, which provides color-coded hemodynamics in real time as well as color-assisted Doppler spectrum analysis, color signals on PDI contrast the lumen of the vessel without hemodynamic information about flow velocity and direction, thus generating displays similar to angiography. However, adjustment of adequate color signal intensity was difficult in some cases, leading to longer examination time.
PDI is a promising new sonographic technique for vascular imaging that improves the quantification and morphological characterization of high-grade carotid stenosis. However, PDI currently should be used only in combination with CDFI until its performance has been further evaluated in larger clinical trials in which angiography is used as another reference method.
Selected Abbreviations and Acronyms
|CDFI||=||color Doppler flow imaging|
|ICA||=||internal carotid artery|
|PDI||=||power Doppler imaging|
|PSV||=||peak systolic flow velocity|
- Received May 2, 1995.
- Revision received August 26, 1995.
- Accepted September 21, 1995.
- Copyright © 1996 by American Heart Association
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