Internal carotid artery aneurysm

Neuroimaging of Internal Carotid Artery Aneurysm

On unenhanced computed tomography (CT) in the axial projection, an internal carotid artery aneurysm appears as a homogeneous mass lesion. It has intermediate density and causes characteristic expansion of the pituitary fossa of the sella turcica.

For precise verification of the diagnosis, CT angiography (CTA) with sagittal projections and 3D reconstructions is used. This method definitively confirms the high vascularity of the detected lesion and establishes its nature as an aneurysm of the cavernous segment of the internal carotid artery. The most critical clinical rule that every specialist must remember: this lesion falls into the category of "do not biopsy" lesions. Any attempts at biopsy are strictly prohibited due to the risk of massive, life-threatening hemorrhage.

Differential Diagnosis: Pituitary Macroadenomas and Microadenomas

Since an ICA aneurysm expands the sella turcica, it must first be differentiated from pituitary lesions.

Pituitary microadenomas measure less than 10 mm. On unenhanced T1-weighted MRI scans (T1WI), they may manifest as an asymmetrical enlargement of the anterior lobe of the gland. With intravenous contrast administration, they enhance more slowly and to a lesser degree than healthy tissue, making them well-defined. The tumor frequently abuts the cavernous sinus. If MRI findings are equivocal (e.g., in Cushing's disease), inferior petrosal or cavernous sinus venous sampling is performed for topical diagnosis (sensitivity over 90%).

Pituitary macroadenomas tend to cause ballooning of the pituitary fossa. They can grow suprasellarly, displacing and compressing the optic chiasm, as well as extending inferiorly into the sphenoid sinus and clivus. On CT, macroadenomas are hyperdense, and on MRI (T1WI) they are isointense to brain parenchyma. Contrast administration helps detect invasion into the cavernous sinus, which is a poor prognostic sign. A characteristic symptom of large and aggressive macroadenomas is the dural tail sign (dural tail), which is thickening and contrast enhancement of the adjacent dura mater resulting from venous stasis.

Diagnostic Challenges: Craniopharyngiomas and Cysts

An important distinguishing feature of craniopharyngiomas (tumors originating from Rathke's pouch remnants) from adenomas and aneurysms is that they generally do not expand the pituitary fossa until reaching giant proportions. These are suprasellar masses that cause symptoms of optic chiasm compression or intracranial hypertension (due to obstruction of the interventricular foramina of Monro). Two types are distinguished:

• Adamantinomatous: more common in children, contains large cysts that are hyperintense on T1WI.
• Papillary: more common in adults (peak incidence 50–59 years), predominantly solid, with cysts isointense to cerebrospinal fluid.

The solid components of craniopharyngiomas intensely enhance with contrast and show calcification.

Rathke's cleft cysts are located within the pituitary gland or near the infundibulum (infundibulum). Unlike craniopharyngiomas, they have thin walls that rarely enhance with contrast, and their fluid is frequently hyperintense on T1WI due to high protein content.

Parasellar Lesions Involving the ICA

Special attention should be given to parasellar meningiomas originating from the dura mater of the cavernous sinus, the dorsum sellae, or the diaphragma sellae. On MRI, these are intensely enhancing tumors that expand the cavernous sinus. They can encase (encase) and narrow the cavernous segment of the internal carotid artery, presenting clinically with cranial nerve palsies and visual disturbances.

Other pathologies include suprasellar meningiomas (extending anteriorly, causing "ballooning" of the sphenoid sinus), optic nerve gliomas (frequently associated with type 1 neurofibromatosis in children), and metastases (especially breast cancer), which cause thickening of the pituitary stalk and clinically manifest as diabetes insipidus.

Class and exam questions

What else to study on this topic

• Pituitary apoplexy: MRI presentation of acute hemorrhage into a macroadenoma.
• Pituitary carcinoma and its dissemination pathways along CSF spaces.
• Technique of selective venous blood sampling from the cavernous sinus.
• Optic nerve gliomas and their association with neurofibromatosis type 1.
• Differential diagnosis of diabetes insipidus: metastases, sarcoidosis, and histiocytosis.

Other topics in the Neuroimaging section

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