In Reply: Identification of the Distal Dural Ring and Definition of Paraclinoid Aneurysms According to Bony Landmarks on 3-Dimensional Computed Tomography Angiography: A Cadaveric and Radiological Study.

Abstract

To the Editor: We greatly appreciate this interest in our work, and sincerely thank the authors for their comments.1 Surgical exploration is clearly the only way to determine the true relationship between an aneurysm and the distal dural ring (DDR). We thus could only statistically validate the DDR plane method using surgery. This provided statistical limits for our group of 27 patients without selection bias. Because this surgical group consisted only of intradural or transitional aneurysms because our practice does not treat asymptomatic extradural aneurysms surgically, we therefore could not demonstrate the applicability of the DDR plane method for extradural aneurysms using surgical exploration. Because of this limitation, we could only evaluate the DDR plane method using the T2-weighted magnetic resonance imaging-magentic resonance angiography fusion technique for extradural cases but could not use these patients for statistical validation. We agree that it is not realistic to have a diagnostic method with 100% accuracy, and certainly our small sample size is insufficient for high-powered statistical results with a narrow CI. To achieve a 95% CI with a 10% margin of error, we would need to evaluate more than 40 patients with transitional aneurysms within a population of 200 surgically treated paraclinoid aneurysms. Such a large number of cases is unrealistic even within a multicenter study. Anatomic studies and our cadaveric dissections confirmed that the DDR is attached to the optic strut (OS) superior border anteriorly.2-4 In previous radiological studies, it was found that using the OS as a reference point can be inaccurate for localizing paraclinoid aneurysms in medial or posterior locations.5 These findings can be explained by the fact that the DDR slopes downward in the medial and posterior directions and is not parallel to the planum sphenoidale.3 For this reason, we believe that our circumferential 3-dimensional DDR model is more accurate. As we all know, the geometric determination of a plane requires 3 noncollinear points. Thus, any points that are coplanar with the DDR could be used. For example, 3 points could be selected from the readily identifiable OS superior border. The problem is that the manipulations that could be made using the 3-dimensional computed tomography angiography reconstruction software at our institution (Vitrea version 6.7, Vital Images Inc, Minnetonka, Minnesota) were limited to the drawing of lines for defining a plane. We were unable to find a way to create a plane with this software that was defined by 3 points. As a result, we used 4 coplanar points around the DDR to define the imaginary plane. An important aspect for our selection of which points to use is that the opticocarotid elevation, the superior border of the OS, and the intersection between internal carotid artery (ICA) and anterior clinoid process (ACP) are located in 1 continuous line representing DDR bony attachments anterolaterally (Scerback et al,4 Figure 2); thus, any of these points can be selected to create the DDR plane. We selected 4 distant points to define the plane. These were the following: the anterior clinoid point in which the ICA intersects with the inferior border of the ACP (the intersection point is on the inferomedial border of the ACP; however, this may not be sufficiently clear on all images because of the 2-dimensional view); the lateral point of the OS superior border adjunct to the ACP attachment; the medial point of the optico-carotid elevation near the tubercullum sellae; the posterior point of the posterior clinoid process base. If some of these bony landmarks are blocked from view by the ICA, or more likely by a giant aneurysm, bone-subtraction computed tomography angiography can aid with imaging these points. The semitransparent view option for imaging vascular structures enables visualization of the DDR bony landmarks. The semitransparent viewing mode is also very useful for visualizing the parts of transitional aneurysms that are hidden by bony structures. In conclusion, we greatly appreciate the interest in this work and sincerely hope that readers will find our DDR plane method approach useful. Disclosures The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.