Standardization of carotid ultrasound.

Standardization of Carotid Ultrasound 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. Percent diameter stenosis51183[12square root (1/velocity ratio)] as a reasonable calculation of stenosis severity from the intrastenotic velocity/distal velocity ratio?