Specific Imaging Findings

Small aneurysms arising from intracranial arteries are rarely seen on routine CT or MR brain imaging, however CTA and MRA are reliable modalities for detection and evaluation of these lesions. Aneurysms without intraluminal thrombus are round to oval structures that are slightly hyperdense on CT and flow-void dark on T2WI, contiguous with the parent artery, and densely enhancing with contrast. Large (> 1 cm) and giant (> 2.5 cm) aneurysms are characteristically round, densely enhancing extra-axial masses hyperdense on CT and flow-void dark on MRI. Additional flow effects include a bright linear “jet” of flow within the dark signal void and a band of pulsation artifact radiating along the phase-encoding direction, accentuated on post-contrast images. Common peripheral calcifications are reliably detected with CT but may not be identified on MRI. Endoluminal turbulent flow is responsible for frequent lack of flow-related hyperintensity and hence decreased conspicuity on MRA without contrast. Aneurysms with intraluminal thrombus are characteristically laminated with bands of varying signal intensities, frequently with “onion skin” appearance. The bands correspond to thrombus and are usually eccentric, with signal intensities depending on the stage of hemoglobin degradation products – typically at least a portion is T1 bright and T2 dark. The thrombosed parts do not enhance with contrast. Complete thrombosis is identified as high T1 and T2 signal filling the lumen without any flow effects. Presence of either flow-void on pre-contrast T1WI, contrast enhancement, or pulsation artifact provides a definite confirmation of patency. Giant aneurysms may be surrounded by vasogenic edema in the adjacent brain.