Imaging anatomy of the sella and its content:
The sellar turcica is a saddle-shaped depression in the sphenoid bone.
It forms the caudal border of the pituitary gland.
This region contains critical neurovascular structures that directly cause disease processes or that are involved in the pathology as a result of the compact surroundings.
The area is bounded by sphenoid sinus anteroinferorly,
the paired cavernous sinuses laterally,
the suprasellar cistern and its contents,
diaphragma sellae and hypothalamus superiorly,
and the dorsum sella and brainstem posteriorly.[1]
Advances in imaging techniques have enabled visualization of vital neural and vascular structures involving the sellar and parasellar regions.
[2]
The primary component of the sella turcica is the sphenoid bone with a component from the basiocciput.
*Anteriorly: it is bounded by a bony ridge,
the tuberculum sella.
*Posteriorly: it is bounded by the dorsum sella and the posterior clinoid processes.
The anterior clinoid processes of the lesser wing of the sphenoid are lateral to the tuberculum sella and project posteriorly.
*The roof of the sphenoid sinuses form the floor of the sella,
and their architecture affects the floor of the sella and its reaction to pathology.
A dural extension defines the superior margin,
the diaphragma sella.
This lines the sella turcica,
envelopes the pituitary gland and forms the incomplete superior border.
*Laterally: there are the venous sinusoids of the cavernous sinues and middle clinoid processes are variably present.
The pituitary fossa holds the pituitary gland,
which is composed of the adenophysis and neurohypophysis.
The pars distalis,
pars intermedia and pars tuberalis form the adenophypophysis.
The neurohypophysis is made up of the pars nervosa,
infundibular stalk and the infundibula proper.
The size of the pituitary gland varies with age and gender: 6 mm in children,
8 mm in males and post-menopausal females,
10 mm in pre-menopausal females,
and up to 12 mm in pregnant lactating females. [3]
The hypothalamus and pituitary gland are connected by important neurovascular connections.
Axons of supra-optic and paraventricular nuclei of the hypothalamus traverse the infundibular stalk and extend into the neurohypophysis.
The secretary granules carrying vasopression and oxytocin appear as the "bright spot" of the posterior lobe of the pituitary gland on T1-weighted unenhanced magnetic resonance imaging (MRI).
Releasing and inhibiting factors produced in the neurons in the hypothalamus are transported to the adenohypophysis via the tuberhypophyseal neural tract and the hypophyseal portal system.
Bilateral cavernous sinuses extend from the petrous segment of the temporal bone to the orbit and contain cranial nerves III (oculomotor),
IV (trochlear),
V1 (ophthalmic division of the trigeminal nerve),
V2 (maxillary division of the trigeminal nerve) and VI (abducens).
The cavernous segment of the internal carotid arteries and their meningohypophyseal trunks travel through these paired duraperiosteal spaces.
The most medial structures within the cavernous sinuses are the internal carotid artery and cranial nerve VI.
The other forementioned cranial nerves travel along the lateral aspects.
The mandibular division of the trigeminal nerve (V3) lies external to the cavernous sinus and exits through foramen ovale vertically oriented beneath Meckel’s cave.
Bilateral Meckel’s caves are inferolateral to each of the cavernous sinuses.
Above the sellar region lies the suprasellar cistern.
Several critical structures traverse this area,
including the circle of Willis,
optic nerves and optic chiasm,
hypothalamus,
pituitary infundibulum,
and the infundibular and suprachiasmatic recesses of the third ventricle.
Imaging
*Computed tomography (CT) is useful in the delination of the osseous margins of the sella.
It is particularly helpful in evaluating the bony changes related to pathologic processes.
CT may be the only option in patients who cannot have an MRI examination (e.g.
those with pacemakers,
incompatible hardware and severe claustrophia).
Thin section 0.625 mm axial spirally acquired images can be reformatted into sagittal and coronal images.
The examination may be obtained without contrast for dedicated bony assessment.
[2]
*MRI provides detailed information about the contents of the sellar and parasellar regions.
It is the fundamental preoperative and postoperative imaging modality.
Sagittal and coronal images with a small field of view in thin sections (≤3 mm) are obtained through the sella turcica to include the parasellar structures,
including the suprasellar cistern,
cavernous sinuses,
Meckel's cave and hypothalamus.
Post-gadolinium enhanced sequences are obtained with fat saturation to improve contrast between pathology and the basicranium.
Generating differential diagnoses can be difficult because of the complexity of the structures in the sellar and suprasellar region.
Dividing this region into the pituitary fossa,
cavernous sinuses and the suprasellar cisterns can be helpful.
However,
many disease processes can involve multiple components of the sellar and parasellar region and it can sometimes be difficult to delineate the origin of large neoplasms and extensive disease processes.
Identification of normal structures,
such as the pituitary gland,
in relation to the pathology can be helpful to determine the etiology.