The sellar and parasellar region is a complex and relatively small intersection of endocrine, neural, vascular, meningeal, and skeletal structures. Various types of mass lesions, inflammatory processes, cysts, and congenital lesions are found in and around the sella Turcica. The clinical presentation of sellar-parasellar lesions is extremely variable. Pituitary dysfunctions, visual field defects, hydrocephalus, and intracranial mass effect are some of the most common signs and symptoms. Significant advances have been made in neuroimaging techniques that have influenced the diagnostic studies of patients with lesions in or around the sella. In this chapter, we review the imaging characteristics of the most common lesions in and around the sella with special emphasis on their MRI appearance.
Normal Anatomy of the Sellar-Parasellar Region
The anterior wall and the floor of the sella consist of thin cortical bone. Its thickness depends on the degree of pneumatization of the underlying sphenoid sinus. Pneumatization of the sphenoid bone varies from several millimeters to a few microns and sometimes is absent. The bone between the carotid and the sphenoid sinus is very thin and can be incomplete in one third of patients. The chiasmatic sulcus is a depression of variable depth, spanning the distance between the cranial openings of the optic canals. It is bounded posteriorly by the tuberculum sellae. The tuberculum sellae is a transverse ridge that forms the anterior margin of the pituitary fossa. The prominence of the tuberculum sellae is related to the shape of the chiasmatic sulcus.
The anterior and lateral borders of the carotid sulcus are bounded by the anterior clinoid process. Occasionally there is a bony protuberance medial to the carotid artery, the middle clinoid process. Ossification of the intraclinoid bridge between the anterior and middle clinoid processes leads to the formation of the anomalous caroticoclinoid canal.
The floor of the sella Turcica extends from the tuberculum to the dorsum sellae. It is concave and round in shape. The cortical bone lining the lateral parts of the floor is termed the lamina dura. The lamina dura continues posteriorly on to the dorsum sellae. The dorsum sellae is the posterior boundary of the sella and is formed by two lateral struts topped by a horizontal strut. The lateral aspect of the dorsum sellae is grooved by the sixth nerve. The posterior clinoid processes are the lateral and superior extensions of the dorsum sellae.
The roof of the sella is formed by a dural reflection, the diaphragma sella. It is perforated by a foramen for passage of the pituitary stalk. Above the pituitary fossa is the suprasellar cistern, which contains the optic nerves, chiasm, optic tracts, the circle of Willis, and the infundibulum ( Figure 7-1 ).
The sella accommodates the pituitary gland, which occupies about 80% of the fossa. The pituitary gland consists of two distinct structures. The adenohypophysis or anterior lobe makes up approximately 75% of the gland and includes three parts: pars distalis, tuberalis, and intermedialis. The neurohypophysis or posterior lobe includes the infundibular process, neural stalk, and median eminence of the hypothalamus. The pituitary gland varies in size and shape, especially in craniocaudal dimension, depending on age and sex.
The paired cavernous sinuses lie on both sides of the sella ( Figure 7-2 ). The internal carotid artery passes between the dural leaves of the cavernous sinus, giving off some small branches. The plexus of sympathetic nerves that accompanies the cavernous carotid artery and the sixth nerve course within the dural envelope of the medial cavernous sinus wall. The lateral wall of the cavernous sinus contains the third, fourth, and the first branch of the fifth cranial nerves. The maxillary nerve, second branch of the fifth cranial nerve, passes through the inferior wall of the cavernous sinus.
Various radiological imaging techniques are available to study the sellar region. With the availability of high resolution axial imaging studies such as CT and MRI, some of the traditional techniques used in imaging the sella region are of historic interest only. However, in selected situations more traditional radiological techniques still hold some value.
Assessment of size and configuration of the sella Turcica is possible by a single lateral skull roentgenogram. In the past, it was customary to obtain multiple plain skull films in different planes. However, little useful information can be gathered from plain examinations other than lateral projection. On plain films, important bony elements are identified, the size and configuration of the sella Turcica is assessed, calcifications in or around the sella are detected, and pneumatization of the sphenoid sinus can be determined. In obtaining the correct lateral film, it is essential that a true lateral projection be got hold as indicated by the superimposition of the anterior clinoid processes. Rotated films may cause the erroneous impression of a false double floor of the sella. Intrasellar masses may cause enlargement of the sella with preservation of cortical bone. There is uniform enlargement with deepening of the floor, thinning, and posterior displacement of the dorsum sellae. This displacement results in an increase in the distance between the tuberculum sellae and the posterior clinoid processes. The cortical bone lining the floor is destroyed with increasing intrasellar pressure. The anterior clinoid processes are often elevated and eroded on the undersurface. Uniform enlargement of the sella is seen in pituitary tumors and empty sella syndrome. It is seen less commonly in intrasellar aneurysms and craniopharyngiomas. Hypothyroidism, hypogonadism, neurofibromatosis, and oxycephaly are some of the rare pathologies that may cause homogeneous enlargement of the sella.
Parasellar meningiomas, chordomas, internal carotid artery aneurysms, epidermoid cysts, and all suprasellar mass lesions can cause erosion of the dorsum sellae. Fibrous dysplasia, ossifying fibromas, some meningiomas, osteomas, osteochondromas, and chondromas may cause local bony sclerosis and change in the bony nature of the sella.
Hydrocephalus and raised intracranial pressure can result in alteration of the sella Turcica profile. Sellar cortical erosion, enlargement of the sella, and anterior or posterior clinoid process erosions are some of these alterations.
Routine and advanced CT modalities are widely used to detect sellar and parasellar lesions. These techniques are also used as guidance during transsphenoidal surgery. In patients who cannot undergo MRI, such as patients with a pacemaker, CT is the preferred neurodiagnostic procedure. In addition, there are instances in which CT imaging may be superior to MRI. CT is helpful in depicting bony anatomy and erosions, intralesional calcifications, the nasal mucosa, and the shape and pneumatization of the sphenoid sinus and thickness of the sellar floor.
Detailed volumetric assessment and the pattern and configuration of the sphenoid sinus aeration are enabled by high resolution helical CT and is important information to have before transsphenoidal surgery. The sphenoid sinus can be divided into three types based on the extent of the pneumatization. The sellar type of sphenoid sinus is well pneumatized. The presellar type is characterized by a lower degree of pneumatization with the sphenoid sinus localized to the anterior part of the sella floor. The choanal type of sphenoid sinus is characterized by a very low degree of pneumatization or absent pneumatization. In these cases the floor of the sella is thicker than 10 mm and consists of cancellous bone. Extensive drilling of the thick bone forming the floor of the sella may be required during surgery to expose the sella in such cases.
Better imaging of the sella region is obtained with modern multidetector-row CT, which allows multiplane 0.5-mm slice thickness images. Dynamic serial CT studies of the sella can detect microadenomas. The CT features of several intrasellar or juxtasellar lesions, including pituitary adenomas, craniopharyngiomas, pituitary hyperplasias, meningiomas, distant metastatic masses, chordomas, lymphomas, astrocytomas, cysticercosis, tuberculomas, and cysts of different etiology, have been detailed in numerous publications. Suprasellar or parasellar invasion of sellar masses can also be evaluated by CT scan. The addition of CT cisternography can be used for rapid evaluation of spontaneous or postoperative CSF fistula. Determining the exact site of the fistula is of paramount importance to enhance the chance of successful dural repair in patients with CSF fistula.
Magnetic Resonance Imaging
MRI is the preferred neurodiagnostic study in evaluating the sellar/parasellar region and most of the pathologies encountered in this area. *
* References , , , , , .Sensitivity and specificity of MRI are higher than any other method used to detect sellar-parasellar pathologies. MRI is also useful for following patients after surgery. During transsphenoidal surgery, the extent of resection can be effectively assessed with intraoperative MRI.
Standard protocol for imaging of the sella consists of precontrast and postcontrast T1-weighted (T1WI) axial, sagittal, and coronal sections, and T2-weighted (T2WI) coronal sections. Gadolinium-chelated compounds are used as a contrast medium. T1WI axial and coronal proton density and T2WI fast spin-echo (FSE) sequences with or without gadolinium can be added as supplementary sequences. Dynamic MRI studies are key to diagnosing microadenomas often not well visualized on routine sequences. A modified T1-weighted FSE method is one of the most preferred sequences for dynamic study. Faster and delayed sequences can be used to demonstrate early and late components of lesions.
High-field MR imagers operating at a 3 Tesla or higher are currently available for clinical applications. They provide a higher signal-to-noise ratio, higher spatial resolution, more detailed anatomical depiction, better visualization of parasellar structures, and invasion of adjacent structres.
Pituitary gland and adenoma The height of the pituitary gland is the most variable dimension. Mean adult values are 4.2 mm in women and 3.5 mm in men. In young women, the height of the pituitary can reach to 10 mm. The pituitary morphology and diameters change with age and sex. As mentioned, the pituitary height increases during puberty and after the fifth decade, begins to get smaller, especially in women. The convex upper shape seen in young females generally flattens after the fifth decade. It is important to differentiate this normal convexity from an adenoma.
Although indistinguishable in neonates and pregnant women, the lobes of the pituitary gland are easily recognized on MRI in most studies (see Figure 7-1 ). The anterior lobe is similar in signal intensity to cerebral white matter on all pulse sequences. The posterior lobe is distinctly hyperintense on T1WI and seems to lose its hyperintensity gradually with aging. The normal pituitary stalk is hypointense relative to neurohypophysis and the optic chiasm on T1WI (see Figure 7-1 ). The stalk enhances intensely after IV gadolinium administration, although its central area remains unenhanced because of the caudal extension of the infundibular recess. The caliper of the stalk is generally equal to the caliper of the basilar artery. A stalk-to-basilar artery ratio greater than or equal to 1 is evaluated as an abnormal finding. With the increasing use of advanced imaging techniques, abnormalities or normal anatomical and physiological variations of the pituitary gland are commonly found in asymptomatic patients (“pituitary incidentalomas”). Pituitary enlargement without endocrine abnormality is often a normal variation. This entity must be recognized to avoid unnecessary surgery. Serial MRI examination of these patients shows no further change in size of the pituitary. MRI can usually distinguish physiological pituitary hypertrophy from tumors and infiltrating diseases. Dynamic MRI studies of these patients show homogeneous uptake with a normal pattern and no abnormal foci.
Pituitary adenomas are the most common intrasellar pathology. They account for 10% to 15% of all intracranial tumors. Adenomas are classified by size. Microadenomas ( Figure 7-3 ) are smaller than 10 mm in diameter whereas macroadenomas ( Figure 7-4 ) are larger than 10 mm. Coronal precontrast and postcontrast MRI sequences are the most sensitive and specific techniques for detecting pituitary adenoma. Microadenomas are commonly hypointense relative to the gland on T1WI, but sometimes they can be seen as isointense. Microadenomas generally do not have the same rate of contrast enhancement as the normal pituitary gland (see Figure 7-3 ) on the early-phase of postcontrast T1-weighted studies. Because of the differential rates of contrast enhancement, images must be taken immediately after contrast administration. Most adenomas appear as a hypointense area within the intensely enhanced gland on images taken immediately after injection of gadolinium (see Figure 7-3 ). Dynamic contrast-enhanced scans may be required to visualize some microadenomas. Faster sequences can show early components of enhancement in microadenomas not visible on slower sequences. On delayed postcontrast-sequences, adenomas are seen as hyperintense to a normal gland. Their appearance on T2WI pregadolinium or postgadolinium MRI is variable.
Lateral deviation of the infundibulum, focal upward convexity, and asymmetrical downward bulging of the gland are secondary signs of microadenomas helpful for diagnosis.
Delineation of the extent of macroadenomas is clearly demonstrated on MRI ( Figure 7-4 ). Macroadenomas are generally isointense or hypointense relative to gray matter on T1WI, but isointense or hyperintense on T2WI. Cystic, necrotic, or hemorrhagic components with fluid-fluid levels may be seen ( Figure 7-5 ). Most macroadenomas enhance with gadolinium, usually to a lesser degree than the gland. Macroadenomas also display variable signal intensity on T2WI. Chiasmal compression, encirclement of the carotid arteries, and hemorrhage in the adenoma can be diagnosed easily by MRI (see Figures 7-4 and 7-5 ). Macroadenomas have the potential for multidirectional extension. These extensions can be classified according to their directions seen on MR images: superior, inferior, parasellar, anterior, and posterior (SIPAP). Some macroadenomas may infiltrate adjacent tissues. Approximately 10% of pituitary adenomas invade the cavernous sinus. MRI has great sensitivity and specificity in depicting invasiveness and the encasement of the internal carotid arteries (see Figure 7-4 ). The most specific signs of cavernous sinus invasion on MR images are intracavernous carotid artery encasement and the obliteration of the lateral venous compartment.
High signal intensity on T1WI in the posterior lobe of the pituitary gland is considered to represent neurosecretory granules that consist of ADH-neurophysin complex packaged within a phospholipid membrane. Patients with central diabetes insipidus have been shown to lack this high signal intensity. Any process that blocks the transport of ADH from the hypothalamus to the posterior lobe can cause the accumulation of high signal intensity in the median eminence proximal to the blockage. This blockage may be caused by an adenoma. After surgical removal of the adenoma, this high signal intensity of the posterior lobe should be displaced toward the internal portion of the sella.
The transsphenoidal approach is the preferred operation for most pituitary adenoma. Midsagittal MR scans are helpful to planning an operation by showing a line of vision for the transsphenoidal route. MRI is also valuable in the assessment of the extent of resection postoperatively. However, it can be difficult to evaluate the extent of resection on MRI due to coexistent postoperative changes such as blood products, implanted material, surgically induced meningeal sellar enhancement, and residual tumor. Postoperative early MR scans must be obtained within 48 hours after surgery to detect a remnant tumor. T1WI enhanced and nonenhanced, coronal and sagittal, and T2WI axial 3-mm slices should be obtained. Fat suppression technique could be applied on T1WI and T2WI sequences to distinguish between hemorrhage, fat, and the posterior lobe of the pituitary gland. The dynamic enhanced method is the most sensitive to detection of a residual adenoma. An intermediate MRI study done 3 months after surgery is most helpful in assessing for possible residual tumor. Suprasellar remnants could descend to the intrasellar compartment; postoperative hemorrhage and fluids are reabsorbed, so MRI interpretation is more accurate after this interval. Late follow-up studies are useful to detect progression of remnants or frank de novo recurrences.
Pituitary apoplexy Early diagnosis and treatment of pituitary apoplexy is important to reverse its high morbidity and potential mortality. Differentiation of hemorrhage, infection, and hypophysitis can be done by MRI. Acute hemorrhage within the pituitary gland appears isointense on T1 and hypointense on T2 images. In the subacute stage, methemoglobin appears hyperintense on T1 (see Figure 7-5 ) and can be either hypointense due to intracellular methemoglobin or hyperintense due to extracellular methemoglobin on T2 images. Clinical apoplexy can occur in the setting of bland pituitary infarction. This phenomenon can be detected by using diffusion MRI.
Craniopharyngioma Craniopharyngiomas occur most commonly in the suprasellar compartment ( Figure 7-6, A-F ), but can also have primary intrasellar extension. Craniopharyngiomas have a quite variable radiological appearance because of their different histological features. They may be cystic, solid, or cystic and solid. Cyst formation is seen in 70% to 85% of these lesions. The cystic portions of craniopharyngiomas typically have a density similar to that of CSF on CT or may be hypodense because of high cholesterol content. On MRI, the cystic component is often hypointense on T1 images, but can also display high signal intensity due to its cholesterol or methemoglobin content. Most of craniopharyngioma cysts appear hyperintense on T2 images (see Figure 7-6, A ). Calcifications can be seen in 70% to 90% of pediatric, and 30% to 50% of adult patients and CT (see Figure 7-6, F ) is more accurate and sensitive for detecting calcifications than MRI. Additional T1 images can be obtained with inversion recovery (IR) pulse, and gradient-recalled-echo (GRE) sequences to confirmation of diagnosis. The IR-T1 images may also differentiate hemorrhagic from proteinaceous fluid. The GRE-T1 images are helpful to differentiate high-signal intensity fluid from hemorrhage content as well. Solid parts, nodules, and cyst walls enhance after contrast medium administration (see Figure 7-6, C and E ). The suprasellar components of craniopharyngioma can induce edema along the optic tracts. This feature is useful to distinguish craniopharyngiomas from macroadenomas. Enlargement of the sella, sinus opacification, fluid level, and bony erosions are also seen in craniopharyngiomas.