MRI anatomy of the cerebellum (T2-weighted images)
When analyzing T2-weighted images (T2-WI) in the sagittal projection, one can examine in detail the median structure of the cerebellum — its vermis (vermis). It consists of a series of consecutive structural elements. From anterior to posterior and superior to inferior, the following lobules are visualized:
- Lingula (lingula)
- Central lobule (lobulus centralis)
- Culmen (culmen)
- Declive (declive)
- Folium (folium)
- Tuber (tuber)
- Pyramis (pyramis)
- Uvula (uvula)
- Nodulus (nodulus)
An important anatomical landmark in this region is the superior medullary velum (superior medullary velum), which forms the roof of the fourth ventricle.
When evaluating the cerebellum in the axial projection (at the level of the pons and internal acoustic meatus), special attention should be paid to the dentate nuclei (nuclei dentati). Due to physiological iron deposition in these gray matter structures, they produce a characteristic change in the MR appearance, manifesting as decreased signal intensity on T2-WI.
Structures of the midbrain (Mesencephalon)
On axial T2-WIs, the midbrain demonstrates several critically important zones. The cerebral aqueduct (Sylvian aqueduct, aqueductus cerebri) runs through its center. Directly posterior to it lies the tectal plate (tectal plate), consisting of the superior and inferior colliculi. It is from here that the trochlear nerve (cranial nerve IV, n. trochlearis) originates — it is unique in being the only cranial nerve that exits from the dorsal (posterior) surface of the brainstem.
Located more ventrally are the cerebral peduncles (pedunculi cerebri), which represent massive bundles of ascending and descending tracts connecting the cerebral hemispheres with lower structures.
Among the grey matter structures, the red nuclei (nuclei rubri) and substantia nigra (substantia nigra) stand out. Like the dentate nuclei of the cerebellum, they are easily identified due to a low signal on tomograms (resulting from iron accumulation). Also passing here are limbic fibers from the hypothalamus and the septal area — the medial forebrain bundle (medial forebrain bundle), which terminates in the reticular formation (a loose cluster of neurons).
Structure of the pons (Pons)
Topographically and functionally, the pons is divided into two main parts:
- Anterior part (Basis pontis, basis pontis): It has a characteristic bulbous convexity. It is formed by massive bundles of transversely oriented fibers heading toward the large middle cerebellar peduncles (pedunculi cerebellares medii). Interspersed among these transverse fibers are scattered longitudinal bundles of corticospinal tracts (which previously divided upon exiting the midbrain). Also visualized here are the trigeminal nerves (cranial nerve V, n. trigeminus) running anteriorly toward Meckel's cave (Meckel cave).
- Posterior part (Pontine tegmentum, tegmentum pontis): Forms the floor of the upper half of the fourth ventricle. This zone contains crucial gray matter accumulations — the cranial nerve nuclei (pairs V through VIII).
It should be noted that the superior and middle cerebellar peduncles are anatomically separated by a space filled with cerebrospinal fluid — this is the inferior continuation of the ambient cistern.
Medulla oblongata (Medulla oblongata)
On axial slices, the contour of the medulla oblongata has a specific relief formed by three paired prominences. The following are located sequentially on each side:
- Pyramids (pyramides) — where the corticospinal tracts that passed through the pons reconvene.
- Olives (olivae).
- Inferior cerebellar peduncles (pedunculi cerebellares inferiores).
Anatomically, the medulla oblongata is divided into two functional zones. The lower "closed" part begins at the level of the spinal nerve roots of the first cervical segment (C1) and contains the central canal within, which continues into the spinal cord. The upper "open" part adjoins the lower half of the fourth ventricle, participating in the formation of its floor.
Briefly: answers to topical questions
Which anatomical structures form the roof of the fourth ventricle?
The roof of the fourth ventricle is formed by several structures giving it a tent-like shape.
Main elements:
- Superior medullary velum (velum medullare rostrale / superius / superior medullary velum) — stretched between the superior cerebellar peduncles.
- Superior cerebellar peduncles — participate in forming the roof together with the superior velum.
- Inferior medullary velum (velum medullare caudale / inferius).
- Choroid plexus of the fourth ventricle (tela choroidea ventriculi quarti) — is a rudimentary remnant of the posterior wall of the primary brain vesicle.
Which brainstem regions are connected to the cerebellum by its superior, middle, and inferior peduncles?
The cerebellum is connected to the brainstem by three pairs of peduncles, each associated with a specific region.
| Peduncles | Latin name | Connection with the brainstem |
|---|---|---|
| Superior | pedunculi cerebellares rostrales (superiores) | Midbrain |
| Middle | pedunculi cerebellares medii | Pons |
| Inferior | pedunculi cerebellares caudales (inferiores) | Medulla oblongata |
Where is Meckel's cave anatomically located, and what does it contain?
Meckel's cave (cavum trigeminale / Meckel cave) is anatomically located within a double fold of the inner layer of the dura mater, the wall of which is derived from it.
This cavity contains the trigeminal (Gasserian) ganglion. It is also visualized that the trigeminal nerves (n. trigeminus, cranial nerve V) run anteriorly into this cavity.
Which gray matter nuclei are located within the midbrain structures?
The following gray matter nuclei are located within the midbrain structures:
- Red nuclei (nuclei rubri) and substantia nigra (substantia nigra).
- Oculomotor nerve nucleus (cranial nerve III) and its accessory autonomic Edinger-Westphal nucleus.
- Trochlear nerve nucleus (nucleus n. trochlearis, cranial nerve IV) — located in the tegmentum at the level of the inferior colliculi.
- Mesencephalic nucleus of the trigeminal nerve (cranial nerve V).
- Interstitial nucleus of Cajal (interstitial nucleus of Cajal) and rostral interstitial nucleus of the medial longitudinal fasciculus (rostral interstitial nucleus of the MLF) — located in the dorsal rostral part.
To memorize the cerebellar vermis lobules in order (anterior to posterior): Lingula, Central lobule, Culmen, Declive, Folium, Tuber, Pyramis, Uvula, Nodulus.
Class and exam questions
Why do some brainstem and cerebellar nuclei appear dark on T2-WI?
The decrease in signal intensity in the dentate and red nuclei, as well as in the substantia nigra, is associated with the natural physiological deposition of iron in these tissues.
What forms the roof of the IV ventricle?
Anatomically, the roof of the fourth ventricle is formed by a structure called the superior medullary velum.
What is the unique feature of the exit of the IV cranial nerve?
The trochlear nerve (n. trochlearis) is the absolute only cranial nerve that exits the brainstem from its dorsal (posterior) surface, originating immediately caudal to the tectal plate.
Where do the corticospinal tracts run at the level of the pons?
In the basis pontis, the fibers of the corticospinal tracts run as scattered bundles among transversely oriented fibers directed toward the middle cerebellar peduncles.
What else to study on this topic
- Imaging features of Meckel's cave and the trigeminal nerve
- Anatomy and boundaries of the ambient cistern
- Topography of the cranial nerve nuclei (V, VI, VII, VIII pairs) in the pontine tegmentum
- Limbic system of the midbrain: the role of the reticular formation
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