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Optic nerve and optic chiasm gliomas occur in two varieties: a non-invasive, indolent neoplasm that is typically found in children and an invasive neoplasm that is found in adults. The former is frequently associated with neurofibromatosis type I. Both forms of the tumor produce a well-defined expansion of the optic chiasm and/or the optic nerves. Optic nerve and chiasmal gliomas in children usually have homogeneous signal intensity iso-intense or hyper-intense to gray matter with no contrast enhancement. The chiasmal gliomas of adults have greater inhomogeneity and often have contrast enhancement. These often extend into the hypothalamus or involve the hypothalamus primarily [FIG 22].
Sarcoidosis may involve the leptomeninges of the brain, producing granulomas on the pituitary stalk, optic chiasm or lesions of the cerebral parenchyma. The granulomas demonstrate marked contrast enhancement. One of the most typical appearances is a thickened contrast enhancing infundibulum. Lymphoma, metastatic carcinoma and Langherhan's cell histiocytosis X may produce this finding as well [FIG 23].
MR usually demonstrates characteristic findings in pituitary apoplexy, characterized by the sudden onset of headache, hypotension or visual disturbance. Since pituitary apoplexy usually occurs as a result of rapid degeneration or hemorrhage within a macroadenoma, MR shows a mass within the sella with inhomogenous signal intensity. The MR appearance depends on the stage of degeneration or the state of the blood products within the tumor. Typically within the tumor is a region containing high signal intensity on T1 and/or T2 weighted images. It may have a region of low signal intensity on T2 weighted images and isointensity on T1 weighted images if relatively fresh blood containing deoxyhemoglobin is present. In some cases, it may show only a region of high signal intensity on T2 weighed mages suggesting the presence of a necrotic region. The MR scans show the relationship of the mass to the optic chiasm and cavernous sinuses. During pregnancy, prolactin-producing cells in the adenohypophysis undergo hyperplasia, rendering the gland more susceptible to infarction from fluctuations in blood supply. Blood pressure fluctuations associated with delivery may result in postpartum hemorrhagic necrosis of the pituitary gland. MR typically shows a small pituitary gland, when an enlarged gland is the rule. It may show the effect of blood products such as deoxyhemoglobin or methemoglobin on the signal intensity of the gland. In these cases, the gland may have abnormally high signal intensity on T1 and T2 weighted images or abnormally low signal intensity on T2 weighted images. The size of the sella turcica remains normal, however, differentiating Sheehan's syndrome from a hemorrhagic adenoma. The commonest tumors that metastasize to the sellar or parasellar region are breast and lung carcinoma. Patients with intrasellar metastases may present with diabetes insipidus, endocrine abnormalities or visual difficulties related to the effect on the optic chiasm. MR demonstrates replacement of the normal pituitary gland and often the adjacent sphenoid bone and cavernous sinus with enhancing tissue. MR may show thickening of the infundibulum when it is involved. The enhancement pattern may be inhomogeneous. The imaging appearance of metastases to the sella is not specific [FIG 24].
Defects in the diaphragma sellae may allow herniation of the subarachnoid space into the sella causing the gland to become compressed against the dorsal surface of the sella. This condition is a common incidental imaging finding. However an "empty sella syndrome" has been described in patients who suffer from a combination of headache, pituitary dysfunction and visual disturbance [FIG 25]. In cases with an empty sella, MR shows the pituitary fossa completely or largely filled with fluid having the signal intensity of CSF. The pituitary stalk coursing directly from the hypothalamus to the flattened gland can be identified in cases of empty sell, but not in cases of arachnoid cyst in the suprasellar cistern.
Langerhans Cell HistiocytosisLangerhans Cell Histiocytosis This granulomatous disease process may be unifocal or multifocal. The multifocal form more commonly involves the brain and hypothalamic/pituitary axis during the course of the disease. The classical clinical triad of diabetes insipidus, exophthalmos and lytic bone lesions occurs in a fraction of cases. Granulomas form in the subarachnoid space that cause abnormal thickening and enhancement of the infundibulum and hypothalamus. If a sufficient portion of cells in the supraoptic and paraventricular nuclei are affected then diabetes insipidus ensues. MRI studies in the coronal and sagittal plane are best suited to make the diagnosis and show abnormal thickening and enhancement of the infundibulum and hypothalamus [FIG 26]. If diabetes insipidus is present it may be manifested on MRI by disappearance of the posterior pituitary bright spot. Lytic calvarial lesions and enhancing parenchymal and meningeal lesions of the brain may also occur.
Suprasellar arachnoid cyst is a well studied but uncommon subtype of arachnoid cysts that are more frequently encountered in the middle fossa, over the convexity or in the cerebellar-pontine angle cistern. These lesions usually present during childhood with manifestations of hydrocephalus. Visual difficulties or hypopituitarism may also result from the compressive effect on the optic chiasm or hypothalamus and infundibulum. Arachnoid cysts in the suprasellar region develop from an imperforate membrane of Liliequist. The constant upward flow of CSF in the prepontine cistern results in the gradual formation of a diverticulum that causes mass effect on the third ventricle and adjacent structures. The appearance is best illustrated in the sagittal plane on MRI that shows a thin walled CSF filled diverticulum herniating upward displacing the floor of the third ventricle, the pons and midbrain [FIG 27]. The signal characteristics of arachnoid cysts follow CSF on fluid attenuated inversion recovery (FLAIR) and diffusion weighted images (DWI) allow distinction from epidermoid tumors and other cysts occurring in this location.
Epidermoid cysts are slow growing tumors composed of squamous cells and keratinaceous debris that arise from epithelial inclusions occurring during neural tube closure. The frequency of occurrence in the parasellar region is surpassed only by that in the cerebellar-pontine angle cistern. Epidermoid cysts are typically located off-midline. These lesions grow slowly and insinuate themselves into surrounding neurovascular structures. Compressive effects on the optic chiasm, hypothalamus and infundibulum or cranial nerves usually cause symptoms of visual disturbance, diabetes insipidus, hypopituitarism or cranial nerve palsy. On imaging studies, epidermoids mimic arachnoid cysts by their close approximation of CSF on CT images and T1 & T2 signal intensity on MRI [FIG 28]. The FLAIR and DWI pulse sequences show high signal intensity and are essential to make the distinction from arachnoid cysts. Calcification and contrast enhancement are rare and usually occur at the periphery of lesions.
Dermoid cysts occur more often in the pediatric population. They arise in the midline most commonly in the vermis or fourth ventricle but also occur in the suprasellar region. The histological composition is derived from squamous epithelium and dermal appendages. Although dermoid tumors may mimic epidermoids or arachnoid cysts on imaging exams, they frequently have heterogeneous high signal on T1 weighted images due to lipid content. Areas of calcification may be seen as signal voids on MRI studies [FIG 29].
Germinoma and Teratoma These germ cell neoplasms most frequently occur during childhood and young adulthood. The most common location is the pineal region or posterior third ventricle followed by the suprasellar region. Metachronous lesions may occur in the pineal gland and suprasellar cistern. Pineal germinomas have a male predominance that is not observed in the suprasellar location. Because these lesions are not encapsulated they tend to be locally invasive and may spread by CSF flow to the ventricles and basal cisterns. Involvement of the hypothalamus causes the most common clinical presentation of diabetes insipidus but suprasellar invasion also causes optic atrophy, diplopia, hypopituitarism or hydrocephalus. Magnetic resonance imaging of germ cell tumors shows them to be slightly hypointense to brain parenchyma on T1 and isointense to gray matter on T2 weighted sequences. There is robust enhancement following administration of gadolinium [FIG 30].
Ectopic location of the pituitary gland is frequently accompanied by multiple endocrine deficiencies. Children may present because of growth delay. The MRI appearance is characteristic. The "bright spot" or neurohypophysis is found in the floor of the third ventricle and is absent from the sella turcica. The sella turcica may also be small and the infundibular stalk is absent. The differential diagnosis of a suprasellar bright spot includes dermoid tumor, thrombosed aneurysm or lipoma. Distinction from dermoid tumor or lipoma can be confirmed by images obtained using fat saturation [FIG 31].
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