MRI anatomy of the pituitary gland
On magnetic resonance images, the pituitary gland appears as a structure in which the anterior and posterior lobes are clearly differentiated. An interesting anatomical feature is that in women, this gland may completely fill the space of the hypophysial fossa. In such cases, the upper margin of the gland acquires a characteristic convex shape.
The posterior lobe, or neurohypophysis, exhibits a unique radiological sign on T1-weighted images (T1WI). Due to the presence of a large number of neurosecretory granules, the posterior lobe demonstrates a hyperintense (visually high, bright) MR signal. In clinical radiology, this phenomenon is well known as the “pituitary bright spot” (pituitary bright spot). In addition, both the glandular tissue itself and its stalk possess the ability to intensely enhance with administered contrast agent, which is important to consider during diagnostic MRI studies.
Ventricular system of the brain
The ventricular system of the brain is a complex network of interconnected cavities. All these spaces are continuously filled with cerebrospinal fluid, also known as CSF. Embryologically, the ventricular system is a direct derivative of the neural tube cavity. The ventricles are lined from the inside by a specific layer of modified glial cells called ependyma.
During embryonic development, each part of the brain forms its own region of the ventricular system:
- The telencephalon (telencephalon) gives rise to the lateral ventricles.
- The diencephalon (diencephalon) gives rise to the third ventricle.
- The midbrain (midbrain) develops into the cerebral aqueduct.
- The brainstem (brainstem) ultimately transforms into the fourth ventricle.
Lateral ventricles (lateral ventricles) consist of several parts: the frontal horn, central part (body), occipital (posterior) horn, and temporal horn. The anatomical junction of the body, occipital, and temporal horns is called the trigone (trigone). On the posteromedial wall of this trigone, a deep impression from the calcarine sulcus (calcarine fissure) is identified, which is termed the calcar avis (calcar avis). The lateral ventricles communicate with the anterosuperior part of the third ventricle via the interventricular foramina of Monro (interventricular foramina of Monro).
Third and fourth ventricles
Third ventricle (third ventricle) appears as a midline slit-like cavity. Its lateral walls are formed by massive accumulations of the gray matter of the thalami. In the center of this ventricle, the thalami contact each other, forming the interthalamic adhesion (massa intermedia), where nerve fibers also decussate. The anteroinferior and inferior boundaries of the third ventricle are represented by hypothalamic structures.
The posteroinferior part of the third ventricle gives rise to the cerebral aqueduct (cerebral aqueduct of Sylvius). It passes through the midbrain and opens directly into the cavity of the fourth ventricle.
The fourth ventricle (fourth ventricle) has a characteristic rhombus shape. Its floor, known as the rhomboid fossa, is formed by structures of the pons (pons) and medulla oblongata (medulla oblongata). The roof is formed by the cerebellum, and the apex of this roof is called the fastigium (fastigium).
CSF outflow from the fourth ventricle into the subarachnoid space occurs through three openings:
- Two lateral apertures of Luschka (foramina of Luschka).
- A single median posterior aperture of Magendie (foramen of Magendie).
Inferiorly, the fourth ventricle continues into the central canal of the spinal cord, which is typically obliterated in adults.
Medial structures of the temporal lobe (limbic system)
When analyzing T2-weighted tomograms, the medial regions of the temporal lobe can be studied in detail.
On axial slices, the uncal recess of the temporal horn of the lateral ventricle can be seen curving medially. This recess serves as an important anatomical landmark: it separates the anteriorly located amygdala (amygdala) from the posteriorly located hippocampal head (hippocampal head). Meanwhile, the uncus (uncus) occupies a distinct medial position.
In coronal projection (viewing from anterior to posterior), the picture changes:
- Anterior slices clearly visualize the amygdala, uncus, and the free margin of the tentorium cerebelli (tentorium cerebelli).
- Posterior slices demonstrate the hippocampal head. The uncal recess of the temporal horn acts as a reliable divider here: superior to it lies the posterior part of the amygdala, and inferior to it lies the hippocampal head itself.
Briefly: answers to topical questions
Which hormones are secreted into the bloodstream from the posterior pituitary?
Two peptide neurohormones, produced in the hypothalamus, are secreted into the bloodstream from the posterior pituitary (neurohypophysis).
- Antidiuretic hormone (ADH, vasopressin) inhibits urine production in the kidneys and increases blood pressure.
- Oxytocin acts selectively on the smooth muscle of the uterus, enhancing its contraction.
The hormones themselves are not synthesized in the posterior lobe; it only stores them and subsequently releases them into the systemic circulation.
What anatomical structures form the walls of the third ventricle?
The walls of the third ventricle are formed by diencephalic structures and derivatives of the embryonic neural tube.
- Thalami — their massive gray matter forms the lateral walls.
- Hypothalamus (hypothalamus) forms the floor, anteroinferior and inferior boundaries, as well as the anterior lateral walls.
- Lamina terminalis (lamina terminalis) forms the anterior wall of the ventricle.
In the center of the ventricle, the thalami communicate via the interthalamic adhesion (massa intermedia).
Which bony structures form the walls of the hypophysial fossa?
The walls of the hypophysial fossa (fossa hypophysialis) are formed by the structures of the sella turcica, located on the superior surface of the body of the sphenoid bone (os sphenoidale).
- Dorsum sellae (dorsum sellae) forms the posterior wall.
- Tuberculum sellae (tuberculum sellae) forms the anterior boundary of the fossa.
Inferiorly, the fossa is bounded by the body of the sphenoid bone itself, inside which lies the sphenoid air sinus.
Where exactly is cerebrospinal fluid produced within the ventricular system?
Cerebrospinal fluid (CSF) is produced by the choroid plexuses of the ventricular system.
- Choroid plexuses (choroid plexuses) are located in all four ventricles of the brain (right and left lateral, third, and fourth).
- Ependyma — CSF secretion is carried out directly by the ependymal cells covering these choroid plexuses.
From the internal CSF-filled space, the fluid then flows out into the subarachnoid space.
Which anatomical structures pass through the cavernous sinus lateral to the pituitary gland?
The following neurovascular structures pass through or closely adjoin the area of the cavernous sinus (sinus cavernosus):
- Internal carotid artery (a. carotis interna) curves posteriorly and superiorly within the cavernous sinus, forming the carotid siphon.
- Oculomotor nerve (n. oculomotorius, CN III).
- Trochlear nerve (n. trochlearis, CN IV).
- Abducens nerve (n. abducens, CN VI).
- Trigeminal nerve (n. trigeminus, cranial nerve V), its 1st and 2nd divisions, and the trigeminal ganglion (ganglion trigeminale) are described in topographic relations with the cavernous sinus region.
Cranial nerves III, IV, and VI closely adjoin the 1st and 2nd divisions of the trigeminal nerve and the internal carotid artery.
To memorize CSF outflow openings: foramina of Luschka are Lateral (two of them), foramen of Magendie is Median (midline, one).
Class and exam questions
What causes the pituitary "bright spot" phenomenon on MRI?
This phenomenon appears on T1-weighted images in the region of the posterior lobe (neurohypophysis). A hyperintense (high) signal occurs due to the presence of a large number of neurosecretory granules in this zone.
Which structures form the floor and roof of the fourth ventricle?
The floor of the fourth ventricle (rhomboid fossa) is formed by the pons and medulla oblongata. The roof is formed by cerebellar structures, and its apex is called the fastigium.
How does the uncal recess of the temporal horn aid in MRI navigation?
On T2-WI, it serves as a reliable landmark separating the structures of the limbic system. On axial slices, it separates the amygdala anteriorly from the hippocampal head posteriorly, while on posterior coronal slices, it separates the posterior part of the amygdala (superiorly) from the hippocampal head (inferiorly).
What is the «calcar avis» in the anatomy of the lateral ventricles?
The calcar avis is a characteristic elevation on the posteromedial wall of the lateral ventricle atrium. It is formed by the deep penetration of the calcarine fissure.
What else to study on this topic
- Features of the differentiation of the anterior and posterior pituitary lobes in various scanning modes
- Pattern of contrast enhancement by the pituitary tissue and stalk
- Interthalamic adhesion and decussation of nerve fibers within the third ventricle cavity
- Process of age-related obliteration of the central canal of the spinal cord
- Relationship between the uncus structures and the free edge of the tentorium cerebelli
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