Although luminal stenosis percentage is the most commonly used clinical measure of plaque risk, it is not at all clear how to measure it in the first place, as evidenced by the two different systems adopted by the two major clinical trials in this area, ECST and NASCET. ECST determined the fractional stenosis by using the luminal diameter at the point of maximal stenosis at angiography as the numerator and the projected diameter of the artery at that same
point were it to be undiseased as the denominator. NASCET however used the luminal diameter of the undiseased internal carotid artery above the diseased portion as the denominator (Fig. 1). It has recently been shown that ECST measurements have a linear correlation with plaque risk whereas NASCET measurements need to be transformed before showing such a direct relationship [38].
A third method of using the diameter of the undiseased common carotid artery as the denominator has also been investigated and was thought by some to be superior to the two previous measurements [62], [49]. There remains no consensus in the literature, thus far, as to the gold standard measurement that should be adopted when assessing luminal stenosis using X-ray angiography.
One further factor, not controlled for in any an-giographic study is one of projection. As shown in Fig. 1, an angiographic measure of stenosis is based on a 2D projection in the longitudinal direction of the point of maximal stenosis. Unless the luminal morphology is entirely round, this point of maximal stenosis should be better considered to be described by not one measurement, describing a circle but by two, describing an ellipse. Although more recent advances in X-ray angiography have addressed this problem with the advent of 3D rotational angiography, this is at the expense of significantly increased radiation exposure to both the patient and the radiologist. Increasingly, luminal morphology has been shown to be critical in the biomechanics of plaque rupture and thus should be taken into consideration in any model of plaque rupture risk [30], [4], [55], [66], [56], [51].
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