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Aneurysms arising from the cavernous segment of the internal carotid artery have several distinctive imaging features that can be recognized in MR images. Cavernous carotid aneurysms may present with cranial nerve abnormalities or with evidence of a carotid cavernous fistula. While cavernous carotid aneurysms typically are confined to the parasellar region they may rarely extend into the sella turcica and simulate a pituitary mass. In the case of an aneurysm involving the cavernous sinus or sella, MRI typically shows a "flow void" or lack of signal within a portion of the aneurysm. It may also show artifacts related to the pulsatile flow of blood within the aneurysm. These appear as multiple ghosts of the vascular structure appearing along one axis of the image [FIG 14]. In cases with thrombus within an aneurysm, MR shows tissue replacing the flow void. This tissue may have multiple lamina of variable signal intensity that represents blood products in various stages of degradation. A dark circumferential band of low signal intensity due to the presence of hemosiderin may be identified in partially thrombosed aneurysms [FIG 15]. Craniopharyngiomas, although benign and slow growing neoplasms, are difficult to remove surgically. Characterized by epithelial lined cysts, they are thought to arise from remnants of Rathke's pouch. The cysts contain fluid that is thick and dark in color, like motor oil. They occur in children and young adults, in the suprasellar region, the sella or in both regions. A second peak of incidence occurs during the fifth decade of life. CT or MR shows these tumors as inhomogeneous lesions, in most cases, with both cystic and solid components. The solid portions usually have contrast enhancement [FIG 16]. The cystic portions may have signal intensity greater than brain on T2 weighted images and low signal intensity on T1 weighted images. Because of the effects of large protein molecules within the cyst, the signal intensity on T1 weighted images may be increased with respect to brain. Globular calcifications, which commonly occur in the solid portions of the mass, have high attenuation values in CT images but provide little or no signal in MR images [FIG 17]. They may elevate the optic chiasm or encroach on the base of the brain.
Rathke's cleft cysts are epithelial lined cysts that contain a proteinaceous fluid. Because of the variable amount of protein in fluid, the signal intensity in the cyst may vary. In T1 weighted images, the signal intensity may resemble that of CSF or may be hyperintense to CSF and brain. Since they arise from the craniopharyngeal duct, an epithelial structure connecting the nasopharynx with the third ventricle that normally involutes during fetal development, Rathke's pouch cysts may be found in the sella, in the sphenoid bone or in the suprasellar cistern. Rarely a persistent bony canal may be documented on imaging studies marking the site of the craniopharyngial duct [FIG 18]. The most common location is in the pars intermedia of the pituitary gland anterior to the infundibulum. They are common incidental finding on MR images or at autopsy. They rarely cause symptoms, by compressing the optic chiasm, pituitary gland or third ventricle. MR shows these lesions as well circumscribed round or oval masses, with a rim of contrast enhancement. The cyst contents may vary from serous to proteinaceous, which alters the MR appearances [FIG 19]. Serous fluid containing cysts tend to parallel CSF signal intensity while proteinaceous cysts are intrinsically bright on T1 and low signal on T2 weighted images [FIG 20]. The MR appearance resembles that of a craniopharyngioma. One MR pattern that is more consistent with a Rathke's pouch cyst than a craniopharyngioma is a non-enhancing low signal intensity nodule within the cyst MR [FIG 20].
Hamartomas are non-neoplastic developmental lesions. Hamartomas of the tuber cinereum usually present in early childhood with one or more sign of hypothalamic disorder such as precocious puberty, gelastic seizures, rapid change in weight or hyperactivity. MR typically shows a homogeneous sessile mass iso-intense to gray matter on T1 and T2 weighted images and anatomically related to the hypothalamus. Because sagittal images are the most effective for demonstrating hypothalamic hematomas, MR outperforms CT for their detection. They do not enhance after the administration of intravenous contrast medium [FIG 21]. Surgery may be indicated in some cases with seizures.
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