(Stroke. 1999;30:1286-1295.)
© 1999 American Heart Association, Inc.
Letters to the Editor |
Standardization of Carotid Ultrasound
Institute of Applied Physiology and Medicine, Seattle, Washington
To the Editor:
The article by Ranke et al1 in the February issue of Stroke represents an excellent advancement in Doppler grading of carotid artery stenosis. Since many surgeons now depend on Doppler ultrasound for decisions for carotid endarterectomy because of the costs and dangers of angiography and the cost and resolution problems of magnetic resonance imaging, it is of paramount importance that the accuracy of vascular laboratories be improved. Use of the continuity principle with Doppler ultrasound offers the greatest hope of correlating Doppler hemodynamics with angiographic morphology if velocities representative of the mean velocity can be consistently measured. The initial trial of this principle2 was disappointing due to the problems with the unknown angle of continuous-wave Doppler and the limited resolution of then-current angiography. Also, without biplane angiography the shape of the stenotic lumen, which greatly affects the relationship of velocity ratios, was not known. Our analysis of the results of Ranke and coworkers1 indicates a systematic asymmetry of the lumen exists, resulting in the intrastenotic/distal velocity ratio increasing the estimated severity by 18% over that expected if the stenosis in the lumen cross-section progressed in a symmetrical way.
To deal with the angle problem, we have found that handheld 2 megahertz pulse wave Doppler probes, interrogating the internal carotid artery stenosis and distal segment with a focal distance of 4 cm, agree well with velocity ratios from color Doppler imaging. Handheld pulse-wave Doppler ensures a consistent low angle and reaches the internal carotid artery well beyond turbulence. Our results, preoperative to carotid endarterectomy, indicate that many laboratories are overestimating the severity of carotid stenosis, leading to some unjustified surgery.
The authors' answers to the following questions will be helpful in
exploring the velocity ratios. What was the maximum resolution of their
angiograms? How did they deal with the shrinking size of the distal
internal carotid artery in cases of preocclusive stenosis? What
is their 95% confidence interval for the range of data in their Figure 1
? Can they provide their formula for calculating the percent
stenosis over the range of velocity ratios? Can they confirm
the formula.
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Percent diameter stenosis=118x[1-square root (1/velocity ratio)] as a reasonable calculation of stenosis severity from the intrastenotic velocity/distal velocity ratio?
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References
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Ranke C, Creutzig A, Becker H, Trappe H-J.
Standardization of carotid ultrasound: a hemodynamic
method to normalize for interindividual and interequipment variability.
Stroke. 1999;30:402–406.
[Abstract/Free Full Text] -
North American Symptomatic Carotid
Endarterectomy Trial (NASCET) Steering Committee.
North American Symptomatic Carotid
Endarterectomy Trial: methods, patient
characteristics, and progress. Stroke. 1991;22:711–720.
[Abstract/Free Full Text]
Response
Department of Cardiology and Angiology, University Hospital Herne, Ruhr-University Bochum, Herne, Germany
Center of Internal Medicine, Department of Angiology
Center of Radiology, Department of Neuroradiology, Hannover Medical School, Hannover, Germany
Key Words: carotid artery
diseases • hemodynamics • ultrasonography
where a=116.6 (R2=0.92, standard error of the estimate=9.2%).
Thus, we can confirm Dr Spencer's formula as a reasonable calculation
of diameter stenosis from the mean velocity ratio. Our digital
subtraction angiography unit was based on a 1024x1024 matrix with a
maximum resolution of 3.5 line pairs/mm. None of our patients had a
preocclusive stenosis with a collapsed distal
lumen,1 3 but 3 patients had stenosis diameter of
>90% compared with the luminal diameter cranially, according to the
NASCET definition. In preocclusive carotid stenosis, both
angiographic percentage calculation based on the distal carotid
diameter and the intrastenotic velocity/distal velocity ratio
will underestimate the degree of stenosis when the distal
carotid diameter is reduced. With the NASCET method, such lesions are
classified angiographically as 95%.2 Classification as
reduced or not reduced is sometimes difficult and can lead to observer
variability with use of the NASCET stenosis
definition.4 In 1979, Spencer and Reid1 first
described reduced Doppler frequencies downstream to a tight
stenosis. Carotid Doppler offers the opportunity for
hemodynamic analysis. Evaluation of downstream
velocity together with the velocity ratio could increase
diagnostic accuracy in preocclusive stenoses with
reduced flow. In our study, a mean velocity ratio >10 in combination
with a mean velocity <0.26 m/s in the high cervical carotid artery
increases sensitivity for detection of >90% stenosis to 100%
with a specificity of 90%. The 95% confidence interval for the
correlation of angiographic stenosis with predicted
stenosis values using the mean velocity ratio (Figure 1
in our
article) is plotted in Figure 2.
References
-
Spencer MP, Reid JM. Quantitation of
carotid stenosis with continuous-wave (C-W) Doppler
ultrasound. Stroke. 1979;10:326–330.
[Abstract/Free Full Text] - North American Symptomatic Carotid Endarterectomy Trial (NASCET) Steering Committee. North American Symptomatic Carotid Endarterectomy Trial: methods, patient characteristics, and progress. Stroke. 1991;22:711–720.
- Fox AJ. How to measure carotid stenosis. Radiology. 1993;186: 316–318.
- Rothwell PM, Gibson RJ, Slattery J, Sellar RJ, Warlow CP for the European Carotid Surgery Trialists' Collaborative Group. Equivalence of measurements of carotid stenosis. A comparison of three methods on 1001 angiograms. Stroke. 1994;25:2435–2439.[Abstract]
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