Posterior fossa

Gerhard van der WesthuizenMedical officer (3 Military Hosp)Department of Radiology

Posterior fossa - Outline

• Calvarium▫Posterior skull base

• Brainstem anteriorly▫Midbrain, pons and medulla

• Cerebellum posteriorly▫2 Hemispheres and midline vermis

• Divided into:▫Mesencephalon (midbrain)▫Rhomboencephalon (pons, medulla and cerebellum)

• Cerebral aquaduct and fourth ventricle• CSF cisterns containing vertebrobasilar arteries and

veins

Posterior skull base• Formed by posterior temporal and occipital

bones• Anterior - Dorsum sellae medially, petrous ridges

laterally• Posterior - Groove for transverse sinus on

occipital bone• Transmits CN 7-12, medulla oblangata and

jugular veins• Multiple foramina and fissures

Posterior skull base -Foramina• Internal acoustic meatus

▫ Porus acusticus – CN VII & VIII, labyrinthine artery• Jugular foramen

▫ Pars nervosa - anteromedial CN IX, Jacobson’s nerve and inferior petrosal sinus

▫ Pars vascularis - posterolateral Jugular bulb, CN X & XI, Arnold’s nerve, posterior meningeal

artery, meningeal branch of ascending pharyngeal artery• Hypoglossal canal

▫ CN XII• Stylomastoid foramen

▫ CN VII• Foramen magnum

▫ Medulla oblangata, CN XI and vertebral arteries

Brainstem and cerebellum

Brainstem

•Midbrain▫Connects pons and cerebellum with

forebrain•Pons

▫Relays information from brain to cerebellum

•Medulla▫Relays information from spinal cord to

brain

Midbrain (Mesenchephalon)• “Butterfly-shaped”, passes through tentorium cerebelli

• 3 Main parts:▫ Cerebral peduncles

White matter tracts - Corticospinal, corticobulbar & corticopontine tracts

▫ Tegmentum CN nuclei: III – Level of superior colliculus;

IV – Level of inferior colliculus Accessory oculomotor (Edinger-Westphal)

Gray matter nuclei Substantia nigra - Motor planning, eye movement, reward seeking, learning and

addiction Red nucleus – Relay and control centre of cortiomotor impulses. Periaquaductal gray matter – Pain and defensive behaviour

White matter tracts Spinothalamic Medial and lateral lemniscus Somatosensory Medial longitudinal fasciculus – Vestibulo-ocular and optokinetic reflexes

Midbrain▫ Tectum

Superior colliculus ( visual pathway) Inferior colliculus (auditory pathway)

Cerebral aquaduct passes between tectum and tegmentum

CSF cisterns associated with midbrain Ambient – Lateral, CN IV Quadrigeminal – Posterior, CN IV Interpeduncular – Anterior, CN III.

Connections: Superior – Cerebral hemispheres, basal ganglia and thalami Posterior – Cerebellum via superior cerebellar peduncle (brachium

conjuntivum) Inferior – Pons

Blood supply via vertebrobasilar circulation Perforating branches of basilar, SCA, PCA.

Axial T2

Axial T2

Axial T2

Axial T2

Axial T1

Pons•Relays info from brain to cerebellum.•Middle cerebellar peduncle – Brachium pontis•Bulbous midportion of brainstem•Two main parts:

▫Ventral pons – White matter tracts continuous with cerebral peduncles and medullary pyramids.

▫Dorsal tegmentum– CN nuclei, gray matter nuclei and white matter tracts. Continuation of midbrain tegmentum superiorly and medullary tegmentum inferiorly.

Pons

•Tranverse fibres make up bulk•Dorsal surface forms rostral half of 4th

ventricle.•Adjacent CSF cisterns:

▫Prepontine – CN V & VI ▫CP angle – CN VII & VIII

•Blood supply▫Medial branches SCA, perforating branches

of basilar artery, thalamoperforator arteries.

Pons• CN nuclei:

▫ V – Throughout brainstem and upper cord. Bulk of motor and sensory in pons. Enters and exits at level of midlateral pons

▫ VI – In pontine tegmentum, near midline, anterior to fourth ventricle. Exits anterior at ponto-medullary junction

▫ VII – Ventrolateral aspect of pons Motor, superior salivatory, solitary tract Exits laterally at ponto-medullary junction

VIII – Vestibular along floor of 4th ventricle Cochlear on lateral surface of inferior cerebellar peduncle

Exits at ponto-medullary junction, posterior to VII

Axial T1

Axial T1

Axial T2

Axial T2

Axial T2

Axial T2

Axial T2

Cor T2

CPA

IAM

IAM

Medulla• Caudal part of brainstem composed of gray

matter formations, CN nuclei IX – XII and white matter tracts.

• Between pons and spinal cord.• 4th ventricle and cerebellum posteriorly• Connected to cerebellum via inferior

cerebellar peduncle (restiform body).• 2 Main parts:

▫Ventral – olive and pyramidal tract▫Dorsal tegmentum – CN nuclei and white

matter tracts

Medulla•Ventral medulla:

•Pyramid▫Paired; anterior surface; midline ventral median

fissure▫Ipsilateral corticospinal tracts prior to

decussation•Olive

▫Lateral to pyramids, venterolateral sulcus (pre-olivary) and posterolateral sulcus (post-olivary)

▫Inferior olivary complex of nuclei

Medulla• Dorsal tegmentum:

• Multiple white matter tracts.• Gracile and cuneate tubercles

▫Lower aspect of dorsal medulla▫Nuclei gracilis(medial) ; cuneatus (lateral)

• Fourth ventricle terminates in caudal medulla.• Blood supply:

▫Distal vertebral arteries▫PICA ▫Anterior spinal artery

Medulla• CN nuclei:

▫ IX – Upper and mid medulla (nucleus ambiguus, solitary tract nucleus and inferior salivatory nucleus.

Exits medulla in postolivary sulcus above X

▫ X – Upper and mid medulla (nucleus ambiguus, solitary tract and dorsal vagal nucleus)

Exits postolivary sulcus between IX and XI

▫ XI – Lower nucleus ambiguus and spinal nucleus Exits postolivary sulcus inferior to X

▫ XII – Mid medulla, hypoglossal eminence in 4th ventricle

Exits anterior medulla in pre-olivary sulcus

Axial T2

Axial T2

Axial T2

Axial T2

Cerebellum•Function: Integrates coordination and fine-

tuning of movement and regulation of muscle tone.

•2 Hemispheres and midline vermis•Three surfaces – superior,inferior and anterior•Divided into 3 lobes and 9 lobules by

transverse fissures.•3 Cerebellar peduncles•Cortical gray matter, central white matter and

4 paired deep gray nuclei.

Cerebellum

•Adjacent CSF cisterns▫CPA cistern▫Cisterna magna▫Quadrigeminal plate cistern▫Superior cerebellar cistern

•Blood supply▫SCA▫AICA▫PICA

Cerebellum – Lobes and lobules• 2 Hemispheres seperated by shallow median groove

superiorly and deep grove inferiorly.• Midline posterior cerebellar notch lodges the falx

cerebelli.• Devided into lobules by fissures

▫ Horizontal fissure – Deepest, divides into sup & inf vermis.▫ Primary fissure – Divides superior surface, small anterior

lobe and larger posterior lobe.▫ Posterolateral fissure – Between posterior lobe and flocculo-

nodular lobe.• 3 Lobes with 9 lobules:

▫ Anterior – Lingula, central lobule, culmen▫ Posterior – Declive, folium, tuber, pyramid, uvula▫ Flocculo-nodular – Nodule

Cerebellum – Lobes and lobules

Vermis lobules Associated hemispheric lobules

Superior vermis:

Lingula Wing of lingula

Central lobule Wing of central lobule Anterior

Culmen Quadrangular lobule lobe

Primary fissure

Declive Simple lobule

Folium Superior semilunar lobule

Horizontal fissure

Inferior vermis: Posterior

Tuber Inferior semilunar lobule lobe

Pyramid Biventral lobule

Uvula Tonsils

Posterolateral (dorsolateral) fissure

Nodule Flocculus Flocculo-nodular lobe

Lobules

“Like cats catch dogs for the party up north”

Deep cerebellar nuclei• Fastigial :

▫ Medial group (vermis)▫ Antigravity muscle groups

• Globose: ▫ Posterior intermediate group Ipsilateral

• Emboliform: motor activity▫ Anterior intermediate group (Rubrospinal)

• Dentate: Lateral group, largest nucleus▫ Ipsilateral motor activity (Corticospinal)

Deep cerebellar nuclei

Sag T2

Sag T2

Sag T2

Cor T2

Cor T2

Cor T2

Axial T1

Axial T1

Axial T1

Axial T1

Arterial supply – Vertebrobasilar system•V4 segments of bilateral vertebral

arteries enters through foramen magnum.•Courses superomedially posterior to

clivus•Unites – forms basilar artery.•Terminates into 2 posterior cerebral

arteries in interpeduncular/suprasellar cistern above dorsum sellae

Arterial supply – Vertebrobasilar system• Branches:

▫ Vertebral artery segment V4 Meningeal branch Anterior and posterior spinal arteries Perforating branches to medulla PICA (largest branch)

Lateral, hemispheric branches, inferior vermian artery▫ Basilar

Pontine and midbrain perforating branches Labyrinthine artery AICA

Lateral and medial branches. SCA

Perforating, marginal and hemispheric branches, superior vermian artery▫ PCA’s

Terminal branches of BA. Perforating – Posterior thalamoperforating, thalamogeniculate Choroidal – Medial posterior, lateral posterior Cortical branches – Anterior & posterior temporal Two terminal trunks - Medial: Medial occipital, parieto-occipital, calcarine, posterior

splenial - Lateral: Lateral occipital, temporal

Variants- Persistent trigeminal artery

Variants – Persistent hypoglossal artery

Variants – Proatlantal intersegmental artery

Venous drainage• Three major drainage systems:

▫ Superior (galenic) group Drains into vein of Galen, 3 major veins Precentral cerebellar – single, between lingula and central

lobule Superior vermian – originates near declive, course superiorly

over culmen Anterior pontomesencephalic – anterior to pons and midbrain;

in relation to basilar artery▫ Anterior (petrosal) group

Petrosal vein – in CPA, tributaries from cerebellum, pons and medulla

▫ Posterior (tentorial) group Inferior vermian veins – Paired, paramedian. Curves

posterosuperiorly under pyramids and uvula

References • Netter, F.H. (2011). Atlas of Human Anatomy,

5th ed. Philadelphia: Saunders Elsevier• Ryan, S., McNicholas, M., Eustace, S. (2011).

Anatomy for diagnostic imaging, 3rd ed. London: Saunders Elsevier

• Butler, P., Mitchell, A.W.M., Ellis, H. (1999). Applied Radiological Anatomy. Cambridge: Cambridge University Press

• Harnsberger,H.R., Osborn, A.G., (2006). Imaging anatomy – Brain, head and neck, spine, 1st ed. Utah: Amirsys