Variability in measurement of extracranial internal carotid artery stenosis as displayed by both digital subtraction and magnetic resonance angiography : an assessment of three caliper techniques and visual impression of stenosis

The degree of stenosis in the extracranial internal carotid artery helps predict the risk of an individual suffering subsequent cerebrovascular ischemic events. Different techniques have evolved to measure stenosis from angiograms, leading to some confusion and a call for the adoption of a single technique. To help choose the most reliable technique, this study assessed observer variability in reporting carotid stenosis for four different techniques, from both digital subtraction (DSA) and MR angiograms (MRA). Three of the techniques used caliper measurements; the fourth was the visual impression of stenosis. From a total of 137 angiograms, caliper measurements were possible on 105 DSAs and 74 MRAs. Measurements from these angiograms were made by two independent observers on two separate occasions to assess interobserver and intraobserver variation in reporting. For DSA, the variability in reporting and the number of clinically significant differences arising as a result were similar for each of the four techniques. While the typical measurement errors for each of the techniques studied were on the order of +/- 5%, each technique produced some sizable individual differences for the same angiogram, with resultant wide 95% limits of agreement. Observer variability for reporting MRA was generally a little greater than for DSA. Compared with the caliper techniques, the visual impression of stenosis technique performed well, particularly for MRA. Although observer variability in reporting can be considerable, no important differences were found among the different techniques widely used for measuring carotid stenosis.