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Letters to the Editor |
,
Mario Alerci, MD, 
,
Rolf Wyttenbach, MD, ,
Pier Luigi Pedrazzi, MD, 
and
Augusto Gallino, MD*
Divisions of Cardiology and Vascular Medicine,* Radiology, and Neurology, Ospedale San Giovanni, Bellinzona, Switzerland
Department of Cardiology, University Hospital Zurich, Rämistrasse 100, 8091 Zurich, Switzerland. e-mail: roberto_corti@hotmail.com
Editor:
We read with great interest the article by Dr Hirai and colleagues, which was published in the December issue of Radiology (1). Findings from the study confirmed the usefulness of multiplanar reconstruction (MPR) for the precise definition of arterial stenosis in carotid artery disease. The study was performed by using single-section spiral computed tomographic (CT) technique. We, however, raise some concerns about some of the statements reported in the article.
We were surprised by the authors’ claims that "no studies have been published in which MPR images at 3D [three-dimensional] CT angiography were used to assess the luminal morphology of the internal carotid artery and to measure the carotid artery stenosis." We reported in 1998 about the use of spiral CT with maximum intensity projection (MIP) and MPR to assess the degree of stenosis in extracranial cerebral arteries by comparing it with duplex ultrasonography (2). Similarly, in 1998, Sugahara et al (3) reported about a comparison of MPR and MIP images with conventional angiograms, and Dr Hirai was a co-author of the article.
The technique used in our study differed slightly from that used by Dr Hirai and colleagues in the present study. We would like, however, to point out that our single-section CT technique allowed a volume of acquisition, which included the entire carotid arterial district (extracranial and intracranial segments of the carotid circulation), by using the following imaging parameters: 3-mm section thickness, 5-mm table speed, 1-mm reconstruction index, and a 1-second scanning rotation time. The data acquisition time was less than 1 minute. In addition, their conclusions are in complete agreement with the aforementioned previous reports, which should have been considered. For instance, several groups are currently clinically using MPR images in cases in which MIP images are not conclusive for adequate definition of the degree of stenosis.
An important advantage of MPR images is the ability to choose curvilinear reconstruction plans that skip vessel wall calcifications, rendering difficult a precise definition of the lumen. Several authors have previously propagated the use of shaded-surface display. This reconstruction modality, although highly appreciated by surgeons for planning endarterectomy because of the 3D display of vessel anatomy, has limitation in cases of severe calcification. All propagated postprocessing software for shaded-surface display has been shown to be very time consuming, without adequately increasing the quality of the measurement of the lumen (4–6). In contrast, MIP image reconstruction is easier and faster, allowing adequate evaluation of the carotid artery stenosis in noncalcified nontortuous vessels.
Dr Hirai and colleagues raise an important question: Is the measurement of carotid stenosis accurately measured in the vessel long axis (even with MPR images) or is cross section needed? In this context, the authors correctly highlighted the important variability in plaque distribution and shape, which leads to possible underestimation of the stenosis with use of a limited number of projections (as is the case with conventional angiography). Other investigators (7) have recently raised this concern and, therefore, some have proposed use of shaded-surface display or volume-rendering techniques in combination with cross-sectional images (4,5,8).
The authors reported that the MPR images were analyzed in conjunction with the corresponding 3D CT angiograms. They do not specify the criteria for selecting the arterial segments; however, one would assume that MPR images were centered in the stenotic segments detected on 3D CT angiograms. In fact, a complete analysis of the carotid vessel by using cross-sectional images would be very time consuming, if performed with 1-mm collimation. It would be, therefore, of great interest to know if the authors have data about time consumption and if they could provide some data about the difference between MPR analysis using cross-sectional and curvilinear reconstruction along the vessel lumen based on their long experience with this technique.
Intuitively, the combination of 3D CT angiograms and selected use of MPR images at the level of luminal narrowing appears to be an appealing approach that may also provide important information about plaque composition and remodeling and could easily be applied in clinical setting. A similar approach is under evaluation for the clinical application of high-resolution imaging of atherosclerotic plaques by using magnetic resonance (MR) imaging (9). The same approach could solve some of the problems in the quantification of plaque composition in coronary arteries. In fact, plaque imaging in the coronary circulation is hampered by cardiac and respiratory motion, as well as by the extension and tortuosity of the coronary tree. Combined approaches with angiographic methods (multisection CT or MR angiography) could serve as a guide to selectively target high-resolution plaque-imaging modalities.
REFERENCES
Department of Radiology, Amakusa Medical Center, 854-1 Kameba, Hondo, Kumamoto 863-0046, Japan*
Department of Radiology, Kumamoto University School of Medicine, Kumamoto, Japan. e-mail: toshinor@beige.ocn.ne.jp
We thank Dr Corti and colleagues for their interest and comments regarding our article (1), and we appreciate the opportunity to discuss and clarify several points of interest for the readers.
First, we apologize for not citing in our article the article by Corti and colleagues (2). We carefully read their article. They stated that the degree of stenosis for spiral CT was assessed in most cases with the MIP technique and that the MPR technique was applied only in cases of heavily calcified lesions. They did not describe in detail how and what kind of MPR images were used, but one would assume that they mainly used the curvilinear reconstruction images in selected cases. In contrast, we applied cross-sectional MPR images perpendicular to the longitudinal axis of the internal carotid artery in all cases and measured two axes of the arterial lumen to calculate the stenotic ratio. Therefore, one could state that our study was the first in which MPR images at 3D CT angiography were systematically used to assess the luminal morphology of the internal carotid artery and to measure the carotid artery stenosis. Although Sugahara et al (3) reported about the usefulness of CT angiography with MIP, MPR, and source images in vascular intervention in steno-occlusive diseases, the carotid lesions were not included in the study. Therefore, we did not cite the study in our article.
We used both volume-rendering and MPR techniques in all cases. The criteria for selecting the arterial segments on MPR images in our study were as follows: To evaluate luminal morphology of extracranial internal carotid diseases, cross-sectional MPR images perpendicular to the longitudinal axis of the internal carotid artery were selected in the stenotic and its distal nonstenotic segments detected at volume-rendering 3D CT angiography. Sagittal or coronal MPR images were also interactively used to select the cross-sectional images on a monitor. It usually takes only a few minutes to select the two cross-sectional images. Since the curvilinear MPR technique was not available in our study, we are not able to provide data regarding the technique.
There are various display techniques in CT angiography. The techniques include axial, MIP, MPR, shaded-surface display, and volume rendering. Although all of these techniques are valuable for the display of CT angiographic data, it has not been established which technique or which combination of techniques is the best method to measure extracranial carotid stenosis. In the article in which a phantom-simulating vascular stenosis was used (4), the volume-rendering technique was reported to be accurate for measuring stenosis. Since the luminal morphology on MPR images perpendicular to the longitudinal axis of the internal carotid artery at the most stenotic area showed a wide range of shapes in our study, we recommend that combined volume-rendering 3D angiograms and selected cross-sectional images with the MPR technique should be obtained when the maximum stenosis of the extracranial internal carotid artery is measured.
REFERENCES
This article has been cited by other articles:
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  L. Saba, G. Caddeo, R. Sanfilippo, R. Montisci, and G. Mallarini Efficacy and Sensitivity of Axial Scans and Different Reconstruction Methods in the Study of the Ulcerated Carotid Plaque Using Multidetector-Row CT Angiography: Comparison with Surgical Results AJNR Am. J. Neuroradiol., April 1, 2007; 28(4): 716 - 723. [Abstract] [Full Text] [PDF]
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