Response to Letter by Bladin et al
We thank Dr Bladin et al for their interest in our work, “Carotid Stenosis Index Revisited With Direct CT Angiography Measurement of Carotid Arteries to Quantify Carotid Stenosis”1 and our related works.2–4
We agree that the optimal method to quantify carotid stenosis remains somewhat controversial. All methods of carotid stenosis quantification are relatively flawed, despite the imaging modality or the statistical technique. Nonetheless, the attempts to improve this quantification have all played an important role in our understanding of carotid disease and measurement methodology.
Bladen et al correctly state our message that carotid stenosis should be directly measured and individualized. We agree that we could have measured the widest portion of the carotid bulb, instead of at the level of tightest luminal stenosis. Yet, the carotid stenosis index (CSI)5 does not involve measurement of the carotid bulb at all. Instead the CSI method relied on a presumed “fixed anatomic relationship”5 between the common carotid artery (CCA) and the carotid bulb to provide an estimation of the widest point of the carotid bulb. This “fixed anatomic relationship” is far from fixed, with other authors reporting standard deviations ranging from ±0.09 to ±0.19.6 The CSI authors also report that estimations of the carotid bulb via measurement of the CCA is more accurate, because the CCA is easier to measure, is disease free and has less anatomic variation.5 With the high quality data from CTA, all vessels can be viewed and measured with the same ease, atherosclerotic disease can be identified (with qualification of plaque content), and anatomic variation is easier to identify because one CTA examination provides data for the entire neck vasculature.
CTA gives high resolution imaging of all arteries as well as the soft tissue details of the arterial wall.4 The outside arterial wall can be identified consistently, despite claims to the otherwise. The wall is well demarcated from surrounding peri-arterial fat, and from different densities of intraluminal plaque. There are technical choices that need to be considered to properly evaluate the arteries as we have described.1–4 Specifically, this involves the rewindowing of images using a digital PACS system, rather than interpretation from filmed images.
For now, carotid quantification methods will most likely remain individualized by regional and individual healthcare centers and physicians, depending on the mix of available technologies, resources and local expertise. One of the goals of our work was to introduce CTA as yet another method to quantify carotid stenosis. CTA has become the preferred angiographic modality at our center, and many others, because of its lack of stroke risk, ability to directly measure in millimeters, ease of standardization of CTA, the quickness of the examination (seconds to acquire images from the aortic arch to vertex), low demand of labor-intensive resources, and the high quality data produced.
Catheter angiography is no longer the “gold standard” in identifying carotid stenosis. Current CTA techniques allow for direct quantification and visualization of the neck vasculature from the arch through vertex in only a few seconds. CTA technology is readily available, can be performed by a single qualified technologist, and provides high quality data that was previously only available through catheter angiography, however, without risk of stroke.
Carotid ultrasound remains a very commonly used technique to visualize carotid disease, and is considered accurate to detect disease despite its narrow window of visualization within the neck. Formulae like CSI were introduced as attempts to allow derivation of carotid stenosis degree without evaluation of the entire length of the internal carotid artery. Nonetheless, these techniques are all indirect and strive to emulate data previously only available by catheter angiography. To continue to promote the indirect techniques that were developed over a decade ago and aspire to catheter angiography, is a step backwards.
Despite the imaging modality or the quantification methodology, it is important to realize that quantification of carotid stenosis is only part of the complexity of carotid disease and the risk of subsequent ipsilateral stroke. In addition to the most accurate and direct quantification of carotid stenosis, the characterization of the carotid plaque may ultimately prove as important or even more important to the prediction of ipsilateral stroke risk. MRI provides important data concerning qualification of carotid plaque.7 CTA also shows promise in plaque quantification as well by showing plaque density as fatty, calcified or with varying densities.1 The field of carotid imaging has come a long way since the CSI introduction,5 and we look forward to the continuing advances of the future.
Bartlett ES, Walters TD, Symons SP, Fox AJ. Carotid stenosis index revisited with direct CT angiography measurement of carotid arteries to quantify carotid stenosis. Stroke. 2007Feb; 38: 286–291.
Bartlett ES, Walters TD, Symons SP, Fox AJ. Quantification of carotid stenosis on CT angiography. AJNR. 2006; 27: 13–19.
Bartlett ES, Walters TD, Symons SP, Fox AJ. Diagnosing carotid stenosis near-occlusion by using CT angiography. AJNR. 2006; 27: 632–637.
Bartlett ES, Symons SP, Fox AJ. Correlation of carotid stenosis diameter and cross-sectional areas with CT angiography. AJNR. 2006; 27: 638–642.
Bladin CF, Alexandrov AV, Murphy J, Maggisano R, Norris JW. Carotid stenosis index: a new method of measuring internal carotid artery stenosis. Stroke. 1995; 26: 230–234.
Moody AR, Murphy RE, Morgan PS, Martel AL, Delay GS, Allder S, MacSweeney ST, Tennant WG, Gladman J, Lowe J, Hunt BJ. Characterization of complicated carotid plaque with magnetic resonance direct thrombus imaging in patients with cerebral ischemia. Circulation. 2003; 107: 3047–3052.