Reply:

We thank Drs. Schulte-Altedorneburg and Ahlhelm for their interest in our work “Correlation of Carotid Stenosis Diameter and Cross-Sectional Areas with CT Angiography”¹ and our other related works.^2–4 Their concerns over detection of “moderate” stenosis, quantification of absolute stenosis versus relative stenosis, and the accuracy of measurement are important issues that relate to all imaging techniques and statistical methods of carotid stenosis quantification.

All methods of carotid stenosis quantification are relatively flawed, despite the imaging technique (ie, CT angiography [CTA], MR angiography [MRA], duplex sonography, or conventional angiography). It has been shown that severe carotid stenosis (≥70% stenosis according to North American Symptomatic Carotid Endarterectomy Trial [NASCET]) can be reliably detected by all the various imaging techniques.^5,6 On the other hand, the detection of moderate carotid stenosis (50%–69% stenosis according to NASCET) has been more challenging for all imaging techniques.^5,6 This point is again demonstrated by our study, showing a relatively greater variability in the correlation of carotid stenosis diameter and cross-sectional area in patients with 50%–69% stenosis (corresponding to absolute millimeter measurement of the narrowest residual stenosis between 2.2 and 1.4 mm, respectively). Nonetheless, our study proved that the ability to predict the cross-sectional area from the narrowest stenosis diameter on CTA was excellent, with an r² value of 0.76 (taking all data into account from no stenosis through severe stenosis).¹

The ambiguity of these “moderate” carotid stenoses does not end with quantification. The ability to detect a consistent benefit of carotid endarterectomy for these patients has also proved highly challenging. The NASCET concluded that the benefit of carotid endarterectomy was less in patients with 50%–69% stenosis and did not exist for some subgroups such as women and patients with multiple risk factors.⁷ In its initial report, the NASCET⁸ had already shown the greatest benefit of endarterectomy in patients with 90%–99% stenosis, medium benefit in patients with 80%–89%, and less in those with 70%–79%. For the 50%–69% group, NASCET found even less benefit, if any at all.⁸

Quantification of carotid stenosis—either by absolute millimeter measurements, relative area reduction, or other various ratio calculations—is only part of the diagnostic picture. Characterization of the carotid plaque may ultimately prove to be predictive of ipsilateral stroke risk in addition to degree of stenosis, despite absolute luminal measurement or the relative luminal reduction. MR imaging provides noninvasive methods of qualifying carotid plaque.⁹ CTA shows some promise in the qualification of plaques as well, showing plaques as being fatty or calcified or having varying densities.⁴

Halo artifacts and indistinct edge definition are a challenge in CTA, as they are with other angiography techniques whether performed by MRA, digital subtraction angiography (DSA), or old film-screen techniques. This is inherent in x-ray physics as well in that of digital imaging display. There is inherent limitation to how much magnification one can do to carry out a measurement because magnification will blur the edges. For NASCET, measurements obtained showed an extremely high kappa of consistency,⁸ despite this limitation. In our study, we placed our measurement calipers half-way between the visualized attenuated CTA contrast luminogram edge and the outer halo,^1–4 mimicking measurements acquired in NASCET, which then used a jeweler's eye piece to look at stenosis on angiographic films and DSA.⁸

CTA is now the preferred angiographic technique at many sites to quantify carotid stenosis due to the lack of stroke risk, ease of standardization of CTA, the quick time for the examination (seconds to acquire images from arch to vertex), and the high-quality data produced. It is understandable that those in favor of duplex sonography carotid imaging could be concerned about the capabilities of carotid CTA. Duplex sonography is an excellent screening technique to detect carotid plaque within a narrow window in the neck, with correlations to percentage stenosis from angiography that generally have rather wide numeric ranges. Due to the stroke risk and the resource-intensive nature of conventional angiography, the decision to perform endarterectomy is based on sonography data in some centers, without more accurate angiographic measures. Carotid duplex sonography scanning, however, is also relatively labor-intensive, requiring very highly skilled technologists to achieve accuracy. Adding orbital and transcranial Doppler is required if some distal information of the intracranial circulation is desired.

CTA can be performed within a few seconds without stroke risk and with excellent visualization of all vessels from arch to vertex. Compared with MRA, sonography, and conventional angiography, CTA is the fastest, is easily standardized (between patients, scanners, and technologists), and provides high-resolution images of the intracranial/extracranial vessels as well as the surrounding soft tissues. Additionally, CTA has no stroke risk and demands little time of labor-intensive resources.