I read with interest the article by Samuels et al (1) regarding the measurement of intracranial arterial stenosis. I do not understand, however, why the authors, and the Warfarin-Aspirin Symptomatic Intracranial Disease method, calculate vessel stenosis by the formula: percent stenosis = [1 − (Dstenosis/Dnormal)] × 100, where Dstenosis = the diameter of the artery at the site of the most severe stenosis, and Dnormal = the diameter of the normal segment of the artery.
This calculation is a one-dimensional view. Most vessels are circular or elliptical, and the stenoses are frequently asymmetric when viewed in cross section. A vessel with 50% diameter stenosis in one plane is 50% stenotic. If it has 50% stenosis in two planes it is 75% narrowed. The following is the correct formula: percent stenosis = [1 − ((Dstenosis 1 × Dstenosis 2)/(Dnormal 1 × Dnormal 2))] × 100, where Dstenosis 1 and Dstenosis 2 = diameters of the stenotic segment in two planes, and Dnormal 1 and Dnormal 2 = diameters of the normal artery in two planes.
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