Normal Anatomy and Pituitary Embryology

Identification of abnormalities in the sellar and parasellar regions requires a basic familiarity with both the normal anatomy and embryologic development of the pituitary gland. The pituitary gland is composed of 2 different parts with embryonically distinct development: the anteriorly located adenohypophysis and the posteriorly located neurohypophysis. The adenohypophysis is thought to derive from Rathke pouch, which separates during embryologic development from the oral ectoderm, then migrates upwards toward the infundibulum, a downward extension from the hypothalamus. The infundibulum represents the budding neurohypophysis that eventually extends inferiorly to the sella to form the posterior pituitary proper. The adenohypophysis is made up of 3 parts: the pars tuberalis, the pars intermedia, and the pars distalis. The pars tuberalis is located in the pituitary stalk where it surrounds the neurohypophyseal axons extending from the hypothalamus to the posterior pituitary proper. The pars distalis is the largest, most anterior portion of the adenohypophysis, and the pars intermedia is the portion located directly in contact with the neurohypophysis.

In both preterm and term newborns, the anterior lobe of the pituitary (adenohypophysis) is characteristically T1 hyperintense, and gradually becomes T1 isointense to the pons by 6 to 8 weeks of life. The normal craniocaudal dimension in children younger than 12 years of age is less than 6 mm and the upper surface should be flat or slightly concave. In pregnancy, during lactation, and during puberty, the upper margin can become convex superiorly and pituitary heights up to 12 mm can be normal. In men and postmenopausal women, the normal pituitary craniocaudal dimension is less than 8 mm and, in young menstruating girls and women, less than 10 mm. The anterior pituitary takes up about 70% of the volume of the gland and has a homogenous appearance on both precontrast and postcontrast sequences. The posterior pituitary has a characteristic intrinsic T1 hyperintensity, which is also known as the posterior pituitary bright spot. The infundibulum (or pituitary stalk) is widest at its origin at the hypothalamus and tapers to its distal attachment at the level of the pituitary gland. Both the adenohypophysis and neurohypophysis, including pituitary stalk, show strong contrast enhancement. At the insertion of the pituitary stalk, a focal hyperenhancement may be seen that matches the distal component of the portal venous system that connects the adenohypophysis with the hypothalamus.

Normal Anatomy and Pituitary Embryology

Identification of abnormalities in the sellar and parasellar regions requires a basic familiarity with both the normal anatomy and embryologic development of the pituitary gland. The pituitary gland is composed of 2 different parts with embryonically distinct development: the anteriorly located adenohypophysis and the posteriorly located neurohypophysis. The adenohypophysis is thought to derive from Rathke pouch, which separates during embryologic development from the oral ectoderm, then migrates upwards toward the infundibulum, a downward extension from the hypothalamus. The infundibulum represents the budding neurohypophysis that eventually extends inferiorly to the sella to form the posterior pituitary proper. The adenohypophysis is made up of 3 parts: the pars tuberalis, the pars intermedia, and the pars distalis. The pars tuberalis is located in the pituitary stalk where it surrounds the neurohypophyseal axons extending from the hypothalamus to the posterior pituitary proper. The pars distalis is the largest, most anterior portion of the adenohypophysis, and the pars intermedia is the portion located directly in contact with the neurohypophysis.

In both preterm and term newborns, the anterior lobe of the pituitary (adenohypophysis) is characteristically T1 hyperintense, and gradually becomes T1 isointense to the pons by 6 to 8 weeks of life. The normal craniocaudal dimension in children younger than 12 years of age is less than 6 mm and the upper surface should be flat or slightly concave. In pregnancy, during lactation, and during puberty, the upper margin can become convex superiorly and pituitary heights up to 12 mm can be normal. In men and postmenopausal women, the normal pituitary craniocaudal dimension is less than 8 mm and, in young menstruating girls and women, less than 10 mm. The anterior pituitary takes up about 70% of the volume of the gland and has a homogenous appearance on both precontrast and postcontrast sequences. The posterior pituitary has a characteristic intrinsic T1 hyperintensity, which is also known as the posterior pituitary bright spot. The infundibulum (or pituitary stalk) is widest at its origin at the hypothalamus and tapers to its distal attachment at the level of the pituitary gland. Both the adenohypophysis and neurohypophysis, including pituitary stalk, show strong contrast enhancement. At the insertion of the pituitary stalk, a focal hyperenhancement may be seen that matches the distal component of the portal venous system that connects the adenohypophysis with the hypothalamus.

Rathke Cleft Cyst

Rathke cleft cysts are benign cysts arising from remnants of Rathke cleft (lumen between anterior and posterior walls of Rathke pouch) that fails to involute during development. The walls of the cysts are lined by columnar or cuboidal epithelium. They are not uncommon in the general population and found in about 4% of routine autopsies. Compared with the adult population, they are less common and smaller in size in children, typically measuring less than 2 mm in children younger than 9 years of age, likely due to their natural history of slow expansion over time. They are often asymptomatic and found incidentally, especially when small. When larger, they may cause symptoms from mass effect, including headache, visual symptoms, and pituitary dysfunction. They may rarely bleed and cause pituitary apoplexy-like symptoms.

On imaging, Rathke cleft cysts are centered between the anterior and posterior lobes of the pituitary gland and are either entirely intrasellar; intrasellar with suprasellar extension; and, uncommonly, entirely suprasellar ( Fig. 1 ). With suprasellar extension, they are located along the anterior aspect of the infundibulum. Their T1 signal intensity varies from hypointense (serous cystic contents) to more hyperintense with increasing proteinaceous or mucoid contents of the cyst. There is no solid enhancing tissue and the walls of the cyst do not enhance but there may be pseudoenhancement of the walls when enhancing pituitary gland is stretched over the cyst walls (claw sign). A characteristic nonenhancing T1 hyperintense nodule is present in 44% to 77% of Rathke cleft cysts.

Rathke cleft cyst. Sagittal noncontrast T1-weighted image ( A ) demonstrates a slightly T1 hyperintense cystic lesion centered between the anterior and posterior lobes of the pituitary gland, which does not enhance and becomes hypointense to the normally enhancing surrounding pituitary gland after contrast administration ( B ).

Features differentiating Rathke cleft cysts from other cystic pituitary lesions, in particular craniopharyngiomas (CPs) and cystic adenomas, include its sharply demarcated appearance with homogenous signal intensity, lack of a solidly enhancing component, and characteristic location between the anterior and posterior lobes of the pituitary. In the setting of a CP, normal pituitary gland can often be identified inferior to the tumor, and there is often calcification as well as tumor wall enhancement. Pituitary adenomas are centered within the adenohypophysis and cystic components have thick, irregular, enhancing walls. Finally, the cytokeratin expression pattern from pathologic specimens can differentiate Rathke cleft cysts from CPs because the former express cytokeratins 8 and 20, whereas the latter do not.

In young children, small cysts (<3 mm) are sometimes seen incidentally in the pars intermedia region and often referred to as pars intermedia cysts. These are also derived from embryologic remnants of the Rathke cleft and do not require follow-up imaging in asymptomatic individuals.

Craniopharyngioma

CPs are the most common pediatric intracranial tumor of nonglial origin. They account for about 5% of intracranial tumors in children and up to 50% of pediatric suprasellar tumors. There are 2 main histologic subtypes: the adamantinomatous and the squamous-papillary. Adamantinomatous CP has a bimodal age distribution, with most cases arising in the pediatric age group and a smaller peak in older adults (>65 years), whereas papillary CP occurs mostly in adults older than 50 years of age. The prevailing theory for pathogenesis of CPs is the embryogenetic theory, whereby adamantinomatous CPs arise from epithelial remnants of Rathke pouch along the craniopharyngeal duct, which is the pathway of migration of Rathke pouch from the stomodeum (embryologic precursor of the mouth) to the infundibulum. The squamous papillary subtype is thought to occur by squamous metaplasia of cells derived from the pars tuberalis of the adenohypophysis. Clinical symptoms arise from either local mass effect or increased intracranial pressure, and include headache, visual disturbance, and endocrine abnormalities, including decreased growth rate and polydipsia or polyuria.

CPs are most commonly both intrasellar and suprasellar in location (53%–75%), followed by purely suprasellar (20%–41%), and least commonly purely intrasellar (5%–6%). Although characteristic imaging features have been described for adamantinomatous and squamous-papillary CP subtypes, some investigators maintain that there are no reliable distinguishing features between them. In general, adamantinomatous CPs are described as lobulated multicystic tumors with enhancing walls and calcifications, typically with a solid enhancing portion also present ( Fig. 2 ). The cyst contents are T1 hyperintense in about 33% of cases due to high protein content or methemoglobin. The squamous-papillary subtype, in contrast, is described as more spherical and predominantly solid. If cystic components are present, they are typically T1 hypointense and calcifications are atypical. This subtype can sometimes arise in ectopic locations, such as the floor of the third ventricle, the posterior fossa, or the nasopharynx. For presurgical planning, CPs can be classified into groups based on their relationship to the optic chiasm as either sellar (sellar ± suprasellar extension without mass effect on the chiasm), prechiasmatic (centered anterior to the optic chiasm with posterior or superior displacement of the optic chiasm), or retrochiasmatic (centered posteriorly with anterior displacement of the optic chiasm). In addition, their relationship to the hypothalamus, third ventricle, and vessels of the circle of Willis is important to document in the report.

Two patients with CP. The first patient ( A – C ) has a predominantly solid sellar-suprasellar mass with internal calcifications, which are seen best on the axial CT image ( A ). The mass is predominantly T2 isointense to hypointense and there are few T2 hyperintense cystic components along the left aspect of the mass ( B ). The solid components show enhancement ( C ). The second patient ( D – F ) has a predominantly cystic sellar-suprasellar mass with internal calcification ( D ), intrinsic T1 hyperintensity of the cyst contents ( E ), and a thin rim of postcontrast enhancement ( F ). Both lesions result in posterior and superior displacement of the optic chiasm.

Although these tumors are benign, malignant transformation can rarely occur after irradiation. Rarely, CPs may spread along the surgical path or more distantly through leptomeningeal spread. Adamantinomatous CPs are histologically benign but biologically aggressive and have a tendency to invade surrounding structures, which often prevents gross total resection. For squamous-papillary CPs, gross total resection is typically curative.

Pituitary Adenoma

Although common in the adult population, pituitary adenomas are rare in children, and comprise less than 3% of supratentorial tumors in this age group. On the other hand, pituitary adenomas are quite common in the setting of a purely intrasellar enhancing lesion, so should probably be considered as a diagnostic possibility, even in the pediatric age group. In children, most are of the functional variety, with prolactinomas accounting for about half of those, followed by corticotropin-secreting tumors and growth hormone–secreting tumors. Prolactinomas are seen most commonly in female patients and tend to present with galactorrhea and amenorrhea. Nonfunctioning adenomas tend to present when they are larger and produce mass effect on adjacent structures, which can result in headache and visual disturbances.

Imaging appearance of microadenomas (<10 mm in size) and macroadenomas (>10 mm in size) is identical to that of the adult population. Prolactin and growth hormone–secreting microadenomas tend to be located more laterally in the adenohypophysis, whereas corticotropin, thyroid-stimulating hormone, and gonadotropin-secreting microadenomas tend to be located more medially. Microadenomas often show delayed enhancement relative to the normal adjacent pituitary gland, with peak enhancement occurring after about 1 to 4 minutes. This differential enhancement pattern can be taken advantage of in detection of otherwise difficult to identify microadenomas by using thin section coronal T1-weighted dynamic precontrast and postcontrast imaging. On unenhanced sequences, microadenomas are typically isointense or hypointense on T1-weighted images compared with the normal surrounding gland ( Fig. 3 ). On T2-weighted imaging, their appearance is more variable but tends to be bright (80% of prolactinomas) and those tend to be softer tumors that are easier to resect than those that are T2 dark. Macroadenomas often extend beyond the confines of the pituitary gland, most commonly into the suprasellar cistern (80%), with less common extension into the cavernous sinus, sphenoid sinus, or dorsum sella (see Fig. 3 ). Their signal is often more heterogeneous than that of microadenomas and is related to cystic, necrotic, and hemorrhagic components (see Fig. 3 ). If possible, it is important to try to identify any normal-appearing remaining pituitary tissue because its preservation is the goal during surgery to prevent pituitary insufficiency.

Microadenoma in 2 patients ( A – D ) and macroadenoma in a third patient ( E–F ). The microadenomas demonstrate slight intrinsic T1 hypointensity on the T1 precontrast images ( A , C ), and are hypoenhancing relative to the normal surrounding pituitary enhancement on the T1 postcontrast images ( B , D ). The macroadenoma extends into the suprasellar cistern and exerts mass effect on the optic chiasm ( E , F ); is heterogeneous in signal intensity, with a focal area of intrinsic T1 hyperintensity superiorly compatible with proteinaceous or hemorrhagic contents ( E ); and demonstrates extension into the patient’s left cavernous sinus, best seen on the postcontrast sequence ( F ). Normal-appearing pituitary gland is seen along the floor of the sella to the patient’s right ( F ).

Pituitary Hyperplasia

Pituitary hyperplasia is defined as a nonneoplastic polyclonal absolute increase in the number of 1 or more adenohypophyseal cell subtypes, which results in enlargement of the gland. This may be physiologic as, for example, during puberty, pregnancy, or during lactation. Pathologic pituitary hyperplasia, on the other hand, is present when the gland enlarges beyond what is considered normal as predicted by the patient’s age, gender, and physiologic state. In pregnancy and in the postpartum state, for example, it is normal to see physiologic enlargement of the pituitary gland height to up to approximately 12 mm. Pathologic pituitary hyperplasia is most commonly encountered in the setting of end-organ deficiency, including thyroid insufficiency and primary hypogonadism. Thyroid insufficiency and lack of thyroxine hormone results in overproduction of hypothalamic thyrotropin-releasing hormone, which in turn leads to thyrotroph hyperplasia in the pituitary gland and increased thyroid-stimulating hormone production. In addition, prolactin releasing cells may be stimulated and these patients may present clinically due to symptoms related to hyperprolactinemia.

On imaging, the normal pituitary gland in children should be less than 6 mm in height and in young menstruating female patients less than 10 mm in height. During puberty, and in pregnant as well as lactating patients, pituitary heights up to 12 mm can be normal ( Fig. 4 ). In both physiologic and pathologic pituitary hyperplasia, the pituitary gland demonstrates diffuse symmetric enlargement and, especially in pathologic cases, the gland can extend far into the suprasellar cistern, even causing mass effect on the prechiasmatic optic nerves and optic chiasm. In contrast to macroadenomas, there is no remodeling of the sella, and the gland will always show homogenous postcontrast enhancement. On precontrast and postcontrast T1-weighted images, as well as on T2-weighted images, the gland will be homogenous in signal (see Fig. 4 ).

Physiologic hyperplasia ( A , B ) and empty sella ( C , D ). Sagittal ( A ) and coronal ( B ) T1-weighted images show homogenous T1 isointense enlargement of the pituitary gland, with extension into the suprasellar cistern in this 12-year-old adolescent. Sagittal ( C ) and coronal ( D ) T1-weighted images in another patient show thinned pituitary gland along the floor of the sella with CSF signal occupying the remainder of the sella.

Empty Sella

The empty sella, defined as a pituitary gland measuring 2 mm or less in height with cerebrospinal fluid (CSF) occupying greater than half of the sella (see Fig. 4 ), is rare in children, with an incidence of about 1% in patients with a normal hypothalamic-pituitary axis. Primary empty sella occurs when CSF enters the sella through a rent in the sellar diaphragm and may or may not be associated with increased intracranial pressure. Secondary empty sella occurs as a result of injury to the pituitary itself, for example, in the setting of pituitary apoplexy, or following surgery or radiation treatment. In adults, the empty sella is commonly seen in older, obese, or hypertensive patients, and may often be asymptomatic. In children, conversely, an empty sella is more likely associated with clinical symptoms and endocrinopathies, especially growth hormone deficiency, hypogonadism, or multiple pituitary hormone deficiency. In children with known endocrinopathy, empty sella may be seen in up to 68% of patients. Thus, the presence of an empty sella in the pediatric population should prompt endocrinologic and ophthalmologic evaluation.

Langerhans Cell Histiocytosis

LCH is a rare idiopathic disorder characterized by proliferation of Langerhans cell histiocytes, resulting in the formation of granulomas and can be found in any organ system. The clinical course of LCH is variable, ranging from a solitary lytic bone or skin lesions with complete remission to a multisystem disorder with potentially lethal outcome. The peak incidence is between the ages of 1 to 4 years but LCH can present at any age from the newborn period to the geriatric age group. Intracranial involvement most commonly occurs in the form of a lytic bone mass or diabetes insipidus. In multicentric LCH, the hypothalamus and infundibulum are involved in up to 20% of patients and, in those cases, diabetes insipidus and absence of the posterior pituitary bright spot are also typically present. When involved, the infundibulum becomes focally or diffusely thickened (typically > 3.5 mm if proximal, or >2 mm if distal) and enhances avidly. The pituitary gland itself may also be involved in up to about 10% of patients with central nervous system (CNS) involvement, with imaging demonstrating infiltration and enlargement of the gland.

Neither absence of the posterior pituitary bright spot nor thickening of the infundibulum is specific to LCH, and there is significant imaging overlap with germinomas and the other entities discussed in this section ( Fig. 5 ). However, there may often be additional findings to suggest the diagnosis of LCH. For example, calvarial involvement with lytic punched-out bone lesions in the skull, including the frontal, parietal, and mastoid portion of the temporal bones, is common, as are involvement of the orbit and facial bones. Additional findings that may be encountered include prominent Virchow-Robin spaces, white matter parenchymal changes with a leukoencephalopathy-like pattern, and gray matter signal changes in the cerebellar dentate nucleus and basal ganglia.

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