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Friedrich-Alexander-University Erlangen-Nürnberg D-91054 Erlangen, Germany
I read with great interest the original article by Serfaty et al. [1] about the accuracy of three-dimensional gadolinium-enhanced MR angiography in the assessment of extracranial carotid artery disease. In this article the authors conclude that "used alone, 3D [three-dimensional] gadolinium-enhanced MR angiography is not accurate enough to replace conventional angiography in the evaluation of extracranial carotid arteries." This conclusion is correct regarding the low quality of the MR angiography sequences used in this study. However, it must not be misunderstood as a general statement concerning the value of MR angiography for the evaluation of extracranial carotid arteries, because the MR angiography sequence as applied in this study is not state of the art. The voxel size (2.34 x 1.17 x 2.00 mm3, with zero filling) of the contrast-enhanced MR angiography, and also the voxel size (1.56 x 0.98 x 1.4 mm3) of the unenhanced multiple overlapping thinslab acquisition time-of-flight MR angiography are not acceptable. Contrast-enhanced MR angiography techniques are available that have much better resolution, such as high-resolution contrast-enhanced MR angiography (which does, however, require measurement of circulation time) or contrast-enhanced MR angiography with elliptic centric k-space order [2].
Using the latter technique, we achieved a voxel size of 1.18 x 0.59 x 0.8 mm3 (with zero filling) for contrast-enhanced MR angiography [3]. When we compared this sequence with an unenhanced high-resolution turbo time-of-flight MR angiography (voxel size, 0.78 x 0.39 x 1.00 mm3), we found the time-of-flight technique, which provides a better resolution, to be slightly superior to contrast-enhanced MR angiography. Therefore, we combine contrast-enhanced MR angiography for imaging of supraaortic arteries from the aortic arch to the circle of Willis with a turbo time-of-flight MR angiography for dedicated visualization of the carotid bifurcation. This technique results in a high accuracy of grading stenoses in acceptable measurement times.
We agree with the authors regarding the need for improved spatial resolution, but we should be conscious that several authors have already taken this step [2, 3]. Therefore, it is not fair to conclude that contrast-enhanced MR angiography is not accurate enough to replace conventional angiography in the evaluation of extracranial carotid arteries. It may be that colleagues from radiology or clinical disciplines who are not familiar with the complex area of MR angiography techniques would not understand this phrase in the sense that MR angiography is not able to replace X-ray angiography at the moment. If sufficient MR angiography techniques are used on modern MR systems, then we may obtain results that would justify giving up invasive X-ray angiography in favor of MR angiography [3].
References
Hopital Cardiovasculaire et Pneumologique L. Pradel, 69500 Bron, France
We thank Dr. Fellner for his interest in our article [1] and for his comments. In response we note that the diversity of the MR scanners available in hospitals is a limitation in any clinical study today that compares the accuracy of an MR angiography technique with sonography or X-ray fluoroscopy technique. Although some institutions have the capability to perform MR angiography using more modern MR systems, others still use scanners with gradients that are 1-4 years older. The same diversity exists for the availability of sequences [2], reconstruction algorithms, and automatic gadolinium-injection systems [3]. Therefore, every study that compares an MR angiography technique with sonography or radiographic fluoroscopy is scanner-dependent and laboratory-dependent.
The goal of our article [1] was not to evaluate the most recent 3D gadolinium-enhanced MR angiography technique but to evaluate one 3D gadolinium-enhanced MR angiography technique that uses a sequence widely available throughout the world. Although our voxel size was limited, it was the best possible compromise between spatial resolution and temporal resolution, which determines reliability and reproducibility of the technique (i.e., without venous enhancement).
Our conclusion that "3D gadolinium-enhanced MR angiography is not accurate enough to replace conventional angiography in the evaluation of extracranial carotid arteries" should not be interpreted as a general statement that condemns the 3D gadolinium-enhanced angiography technique for the future but simply as a statement reflecting the lack of specificity of the technique during the time of our study. We believe this information is important for the numerous radiologists still working on similar scanners and with similar sequences, so that carotid artery surgeries that are not indicated may be avoided.
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