Fig. 11.1
Normal anatomy is shown above. (a) Sagittal T1. (b) Coronal T2. Legend: Red arrow: pituitary anterior lobe (adenohypophysis), Orange: pituitary posterior lobe (neurohypophysis), Blue: optic chiasm, Purple: pituitary stalk, Brown: suprasellar cistern, Green: internal carotid arteries running inside the cavernous sinuses
The dimensions of the pituitary gland may vary according to age and gender, averaging 3–8 mm in height, 12 mm in width, and 8 mm anteroposterior. It may increase in size as a result of physiological hormones, mainly in girls and pregnant women. At female puberty, it may measure 10 mm in height; for boys, the normal height of a pituitary gland can be up to 8 mm. During pregnancy, the pituitary gland can show a more significant growth, measuring up to 12 mm, with the maximum height reached immediately postpartum [2].
The pituitary gland vascularization is provided mainly from the hypophyseal portal venous system. Branches from the internal carotid artery and the superior and inferior hypophyseal arteries also exist. Because of this mixed combination of arterial and venous supply, differential rates of enhancement of the pituitary gland can be seen during dynamic scans after bolus administration of an intravenous contrast agent (Fig. 11.2).
Fig. 11.2
Microadenoma. T1-weighted coronal images obtained dynamically after the bolus injection of intravenous gadolinium (a–d); the time interval between each sequence is 20 s. It is possible to see the gradual enhancement of the gland starting from its top part until it is fully complete. Note a small area of hypoperfusion at the right blue arrow, corresponding to a nodule, that is better seen at the last sequence.
Magnetic resonance imaging (MRI) is an excellent method for studying the sella turcica [3]. The anterior and posterior lobes of the pituitary gland can be distinguished. Whereas the anterior lobe appears with signal intensities similar to normal white matter in all sequences, the posterior lobe shows hyperintensity on T1, making it very easy to be identified even on non-contrast examinations. After the administration of intravenous (IV) contrast, both anterior and posterior lobes exhibit intense enhancement (Fig. 11.3).
Fig. 11.3
Normal neurohyposhisis anatomy. T1-weighted sagittal images pre- and post-contrast. The arrow points to the spontaneous bright neurohypophysis, located at the sella turcica posteriorly. After the injection of IV gadolinium, the entire gland exhibits an intense enhancement
The exception to this signal pattern is seen in newborns, where the anterior lobe exhibits higher signal intensity on T1 images [4] (Fig. 11.4). This signal intensity usually lasts until age 2 months and begins decreasing until the anterior lobe shows hypointensity, just like in adults, sometime between the 4th and 6th months.
Fig. 11.4
Normal anatomy of newborns. (a) T1 Sagittal pre-contrast. Anterior lobe showing hyperintensity on sagittal T1-weighted image. (b) T1 sagittal post-contrast injection. Normal enhancement of the anterior lobe as well as the pituitary stalk
Imaging Techniques
The best modality used for evaluating the sella turcica is the MRI. It generates images with high resolution, enabling a proper identification of the sella turcica and the pituitary gland and its parts, thus allowing the visualization of lesions [5].
Images are usually performed in two planes, coronal and sagittal. The coronal plane is great for identifying the sella turcica, pituitary gland, cavernous sinus, internal carotid arteries, and the suprasellar cistern. Because of the typical T1 hyperintensity, the sagittal plane is good for confirming the location of the posterior lobe of the pituitary gland inside the sella turcica. Some pituitary microadenomas can show hyperintensity in T2 images, so a coronal T2 image is commonly included in all protocols.
The paramagnetic contrast-enhanced images can increase the sensitivity for detecting small microadenomas. As adenomas exhibit different contrast-enhancement rates, dynamic images should be taken immediately after the injection of the contrast medium [6]. The adenomas usually show less enhancement than a normal gland around it during the first seconds. As the image contrast begins to dissipate, the visual difference between the adenoma and the rest of the normal gland can disappear. As this time can vary with different adenomas, this is the reason that dynamic enhanced images obtained after the injection of the contrast medium can increase the sensitivity for detecting microadenomas (Fig. 11.5).
Fig. 11.5
Post-contrast dynamic acquisition. (a–d) Coronal T1 weighted images acquired dynamically after the injection of IV contrast. The same microadenoma from this figure (blue arrow) shows less enhancement than the rest of the pituitary gland
Many services suggest scanning T1-weighted images in coronal and sagittal planes. If no lesion is detected, the IV contrast is injected and coronal dynamic T1-weighted images are immediately obtained. After the dynamic images, conventional T1-weighted spin-echo images are scanned. A T2-weighted coronal sequence can be performed to evaluate lesions at the suprasellar cistern, optic chiasm extrinsic compressions, or hypothalamic lesion.
Congenital Abnormalities
Pituitary Gland Hypoplasia
Short stature and growth hormone failure are common indications for sella turcica evaluation. Many patients also have anterior pituitary hormone deficiencies [7]. MRI can show some of these findings: small sella turcica, small anterior pituitary gland lobe, absence of T1-weighted hyperintensity related to the neurohypophysis, absent or hypoplastic distal pituitary stalk, and atopic neurohypophysis located close to the proximal pituitary stalk (Fig. 11.6). Other midline malformations such as Chiari I malformation, optic nerve hypoplasia, and medial deviation of the carotid arteries can be present.
Fig. 11.6
Pituitary gland hypoplasia. (a) Sagittal T1-weighted image pre-gadolinum and (b) post-gadolinium injection shows a small sella turcica, with a small pituitary gland and the absence of the characteristic T1 bright spot related to the neurohypophysis
“Empty Sella Turcica”
The term “empty sella turcica” refers to a pituitary gland that is present but flattened, with the sella turcica filled with CSF (Fig. 11.7). In some cases, the sella can become enlarged due to CSF pulsation. This finding is usually incidental, and has little or no clinical significance [8]. When symptoms are present, patients can exhibit visual field loss, CSF rhinorrhea or endocrine dysfunction. Other non-specific complaints such as headache, memory loss, or dizziness can be seen. In some cases, the above-mentioned findings can be related to idiopathic intracranial hypertension (IIH), also known as benign intracranial hypertension or pseudotumor cerebri.
Fig. 11.7
Empty sella turcica. (a) Sagittal T1-weighted image showing an enlarged sella turcica filled with cerebralspinal fluid (CSF). The pituitary gland is flattened (arrow) against the floor. (b) A female patient with idiopathic intracranial hypertension. Besides the empty sella, the MRI shows a prominent subarachnoid space around the optic nerves (posterior arrow) and flattening of the posterior sclera (anterior arrow), characterizing papilloedema
Tumors
Pituitary Microadenoma
The pituitary microadenomas represent 10–15% of all intracranial neoplasms and are the most common sella turcica tumor. Almost all pituitary tumors are benign adenomas. A nodule measuring up to 10 mm is called a microadenoma, and when it is larger than 10 mm, it is considered a macroadenoma [9]. Adenomas can still be classified as functional or nonfunctional, depending on the presence or absence of hormonal activity. Approximately 25% of all adenomas will be nonfunctioning tumors, and 75% will present clinical symptoms, depending on the kind of hormone secreted. Usually, functional adenomas are discovered early because of the signs and symptoms of hypersecretion hormone. Some large tumors can also be discovered when they cause dysfunction due to normal pituitary gland or chiasm compression.
Among functional tumors, 50% will be prolactinomas, which secrete prolactin, leading to galactorrhea, amenorrhea, infertility, loss of libido, and impotence. These symptoms can be more discrete in men and postmenopausal women. After prolactinomas, adenomas that produce growth hormone and ACTH are the next most prevalent functional tumors.
While functional tumors can be detected early, non-functional tumors can be discovered when compressing or invading adjacent structures. They can lead to visual disturbance when compressing the optic chiasm, and headache due to the third ventricle and hydrocephalus or cranial nerve palsy and the cavernous sinus is invaded.
On MRI, the pituitary adenoma can show as a subtle hypointensity lesion on T1-weighted images compared to a normal gland signal. When old blood is present, the nodule can appear hyperintense on T1. On T2 images, one-third to one-half of microadenomas will be hyperintense (Fig. 11.8). Most are isointense to normal gland. Other image findings can be seen in microadenomas: unilateral contour deformities and contralateral deviation of the pituitary stalk.
