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Letters to the Editor |
Departments of Diagnostic Radiology* and Vascular Surgery, Friedrich-Alexander-University Erlangen-Nürnberg, Maximiliansplatz 1, D-91054 Erlangen, Germany; e-mail: franz.fellner@idr.imed.uni-erlangen.de
Editor:
We read with great interest the article by Dr Elgersma and co-workers in the August 2000 issue of Radiology (1). In this article, they discussed multidirectional imaging of internal carotid artery stenosis, comparing three-dimensional time-of-flight (TOF) magnetic resonance (MR) angiography with rotational and conventional digital subtraction angiography (DSA). In this important and well-conducted study, Dr Elgersma and colleagues conclude that MR angiography does not cause underestimation of carotid stenoses, but rather, that stenoses are underestimated at conventional DSA with two or three projections. This is attributable to the fact that MR angiography provides a greater number of projection images and thus reliably shows asymmetric stenoses. This also correlates with our clinical experience. As the authors stress, the area of the stenosis is indeed rarely concentric (2), as we have recently seen in our investigations of 20 endarterectomic specimens with MR microscopy.
However, we would like to draw the attention of Dr Elgersma and colleagues to the fact that the MR angiographic technique used in this study is not ideal. At 9 minutes, the sampling time appears relatively long, particularly in view of the fact that only the carotid bifurcation is visualized. Shorter sampling times have now become possible with similar resolution by using TOF techniques (3). Furthermore, MR investigations should not show only the carotid bifurcation but also the entire field of supraaortic arteries, from the aortic arch to the circle of Willis. We cannot endorse the authors’ statement that contrast material–enhanced MR angiography (which they do not specify in more detail) is limited by venous superimposition of the jugular veins. This contradicts with our own experience (3) with high-spatial-resolution contrast-enhanced MR angiography with an elliptical-centric k-space order. However, we agree with Dr Elgersma and colleagues insofar as the spatial resolution currently possible with contrast-enhanced MR angiography does not reliably permit accurate stenosis measurement at the carotid bifurcation. Thus, we currently use two MR angiographic sequences: high-spatial-resolution three-dimensional TOF turbo MR angiography for the carotid bifurcation (sampling time, 3 minutes 22 seconds) and contrast-enhanced MR angiography with an elliptical-centric k-space order for the entire vascular field (3).
Irrespective of this, Dr Elgersma and his co-workers have conducted an extremely important study in which the problems of conventional DSA in two or three planes were critically examined. During evaluation of the value of MR angiography, the traditional standard, conventional DSA, was also examined, and multidirectional imaging was presented as decisively advantageous compared with conventional DSA. Furthermore, ultrasonography (US) does not seem to be suitable as a standard, either. First, performance of US depends to a large extent on the observer, and, second, it is not absolutely reliable even in the hands of experienced observers (4–6). Therefore, we critically view the practice of electing endarterectomy solely on the basis of US findings.
Hence, correctly performed MR angiography seems to be developing into the standard in the diagnosis of carotid stenoses, and this new standard is not invasive!
REFERENCES
and
Willem P. T. M. Mali, MD, PhD*
Departments of Radiology* and Vascular Surgery, University Hospital Utrecht, Heidelberglaan 100, 3584 CX Utrecht, the Netherlands; e-mail: o.e.h.elgersma@azu.nl
We thank Dr Fellner and his colleagues for their response. They state that new hardware and software recently became available, allowing performance of high-spatial-resolution contrast-enhanced MR angiography and three-dimensional TOF MR angiography with shorter acquisition times. We are aware of the fact that, with the ongoing technical developments, MR angiographic techniques do change. In the past 4 years, we have compared intraarterial DSA with MR angiography in a large clinical trial (1) involving 400 patients with carotid disease. This trial did not allow us to change the three-dimensional TOF imaging protocol. We choose this protocol because it can be performed with commercially available machines and is therefore widely applicable. Furthermore, the carotid bifurcation is by far the most relevant region of interest, and, therefore, MR investigations do not necessarily have to include the entire vascular tree.
Besides the three-dimensional TOF protocol, we have also implemented other techniques, such as contrast-enhanced MR angiography, by using state-of-the-art MR imaging techniques. In many articles (2–6), including that of Dr Fellner and colleagues, it is reported that these recently introduced techniques are promising. However, these studies are hampered by small study populations and either a lack of a standard or patient selection bias. For contrast-enhanced MR angiography to become a standard of reference, it needs to be evaluated more extensively.
Our preliminary results in 50 patients with carotid disease show high agreement between contrast-enhanced MR angiography and DSA. Although flow-related artifacts were minimized at contrast-enhanced MR angiography compared with three-dimensional TOF MR angiography, sensitivity and specificity for identifying a severe carotid stenosis were similar. Likewise, the overestimation of carotid stenosis at contrast-enhanced MR angiography was comparable to that of three-dimensional TOF MR angiography. Therefore, the type of imaging protocol used is not relevant and does not affect our findings and conclusions.
Ideally, new three-dimensional techniques should be compared with a three-dimensional standard of reference, such as three-dimensional rotational angiography. When new three-dimensional contrast-enhanced MR angiographic techniques are validated, by using conventional DSA as the standard of reference, identical projections of the carotid arteries should be used to allow accurate comparison. Otherwise, the greater number of projections will most certainly cause overestimation of carotid stenosis at MR angiography compared with DSA.
REFERENCES
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