It is the anatomic border between the intracavernous and supracavernous internal carotid artery (ICAs), and the DDR is an important landmark to differentiate an intradural paraclinoid aneurysm from an extradural cavernous sinus aneurysm. Paraclinoid aneurysms carry a risk for subarachnoid hemorrhage (SAH), and treatment can be rendered. On the other hand, cavernous sinus aneurysms have limited or no risk for SAH and are usually followed up in asymptomatic patients.
The carotid cave is a small intradural recess at the posteromedial aspect of the DDR, and it has been observed in two thirds of cadavers in 2 studies.
This recess causes difficulty in differentiating intradural from extradural aneurysms at the medial side of the ICA.
3D-MR cisternography with use of constructive interference in steady state (3D-CISS) can provide high-resolution images with good contrast between the CSF and solid structures.4,5 However, solid structures such as blood vessels, cranial nerves, and bones are difficult to identify only by 3D-CISS imaging, which imposes a limit on evaluating aneurysms by 3D-CISS imaging. 3D time-of flight (3D-TOF) MRA can demonstrate the artery as a high signal intensity, and this technique is widely applied to evaluate intracranial aneurysms.
Recent advances in 3D workstations have facilitated fusing the 2 images of different datasets. Fused CT and MR images have been used for surgical planning, and fused MRA and 3D cisternography images have been applied to analyze the intracranial blood vessels.
Fusion of 3D-CISS and MRA images will theoretically reveal the CSF space and artery, and the boundary where the ICA penetrates the dura mater should be visualized 1).
Clinoid aneurysms arise between the proximal and distal carotid distal dural rings.
High-resolution thin-cut CTA is a fast and crucial tool for diagnosing paraclinoid aneurysms. The optic strut OS serves as an effective landmark in CTA source images for distinguishing between intradural and extradural paraclinoid aneurysms. The distal dural ring (DDR) is supposed to be located 2 mm above the base of the OS in axial planes 2).