Giant PICA Aneurysm

Kathleen A. Sorensen, MD; Georgianne M. Snowden, MD

Disclosures

Appl Radiol. 2007;36(11):66-70. 

In This Article

Discussion

An aneurysm that measures >2.5 cm in diameter is termed a giant intracranial aneurysm.[1,2,3,4] They reportedly represent approximately 2.5% to 13% of all intracranial aneurysms[1,3] and occur more commonly in women.[1,4] Aneurysms that originate from the PICA are especially rare.[5] Giant intracranial aneurysms more commonly present secondary to mass effect rather than to subarachnoid hemorrhage, as is the case in the smaller versions.[1,2,4] Examples of previously reported presentations include cranial nerve palsies,[5,6] dysphagia,[7] homonymous hemianopsia,[8] seizures,[9,10] and obstructive hydrocephalus,[2,11,12,13] as in the current case.

Our patient presented with symptoms related to obstructive hydrocephalus and underwent 2 separate MRIs. On both occasions, the studies were interpreted as a fourth ventricular mass causing the hydrocephalus. Differential diagnoses that were given included those mentioned previously. The patient subsequently underwent surgery to remove this mass. Aneurysm was never considered in the diagnosis prior to surgery.

Since most giant aneurysms are associated with the extradural internal carotid artery, the middle cerebral artery, or the basilar artery,[4] "masses" occurring within these regions should include aneurysm in the differential diagnosis. Imaging features and characteristics of giant intracranial aneurysms have been well described. These were present in the current case and should be investigated when a mass is identified within the head, especially within regions in which giant aneurysms are more commonly found.

Giant intracranial aneurysms that are partially thrombosed exhibit both a multilayered thrombosed portion and a patent vessel lumen.[1,4] The thrombosed portion of the aneurysm is characterized by mixed heterogeneous signal on MRI. Layers of clot of differing ages result in a multilaminated appearance (Figure 1). A patent lumen with rapidly flowing blood will result in a signal void that may be visualized on T1- or T2-weighted images[1,2] (Figure 2). Flow void occurs because protons do not remain within the selected slice long enough to produce a signal when 90° and 180° degree pulses are given.[3] Slow flow or turbulence with the vessel can occur and can result in heterogeneous signal.[2]

With the administration of gadolinium, the patent lumen of the aneurysm along with the parent vessel can be visualized (Figures 3 and 4). Gadolinium can also help to identify a patent lumen with slow flow. An extremely helpful clue to the vascular etiology of these lesions is the phase-ghosting phenomenon.[6] Phaseghosting artifact occurs secondary to the pulsation present within the vessel and should indicate the vascular nature of the mass. Although it was not performed in the current case, MR angiography can further characterize the lesion. MR angiography may, however, fail to adequately characterize the size and flow of the aneurysm secondary to slow intraluminal flow.[3,4]

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