Radiological anatomy of brain (click this link )
As an infarct evolves(chronic infarct)
- progressively lower in density on CT (higher in signal on T2-weighted images)
- well defined over the next few weeks
- approaching the density of CSF.
- As the mass effect resolves and the infarcted tissue is resorbed
- adjacent sulci and ventricle will enlarge.
- focal areas of cystic encephalomalacia
- surrounding parenchymal change due to gliosis.
Virchow-Robin (VR) spaces
1.Virchow-Robin (VR) spaces surround the walls of vessels as they course from the subarachnoid space through the brain parenchyma. Small VR spaces appear in all age groups. With advancing age, VR spaces are found with increasing frequency and larger apparent sizes. Small VR spaces ( 2 mm) appear in all age groups. With advancing age, VR spaces are found with increasing frequency and larger apparent size ( 2 mm)
2.the signal intensity of VR spaces is identical to that of cerebrospinal fluid with all magnetic resonance imaging sequences
3.Dilated VR spaces typically occur in three characteristic locations
Type I VR
spaces appear along the lenticulostriate arteries entering the basal ganglia through the anterior perforated substance.
Type II VR
spaces are found along the paths of the perforating medullary arteries as they enter the cortical gray matter over the high convexities and extend into the white matter.
Type III VR
spaces appear in the midbrain
DD
- lacunar infarctions
- cystic periventricular leukomalacia
- multiple sclerosis
- cryptococcosis
- mucopolysaccharidoses
- cystic neoplasms
- neurocysticercosis
- arachnoid cysts
- neuroepithelial cysts
Lacunar infarctions
small infarctions lying in deeper noncortical parts of the cerebrum and brainstem. They are caused by occlusion of penetrating branches that arise from the middle cerebral, posterior cerebral, and basilar arteries and less commonly from the anterior cerebral and vertebral arteries .
Sites of predilection are the basal ganglia, thalamus, internal and externalcapsule, ventral pons, and periventricular white matter
Lacunar infarctions tend to be larger than VR spaces and often exceed 5 mm
In contrast to VR spaces, lacunar infarctions are generally not symmetric
wedge-shaped holes are more likely to be lacunar infarctions
Lacunar infarctions can be differentiated from VR spaces by signal intensity characteristics
The Fisher Grade classifies the appearance of subarachnoid hemorrhage on CT scan
| Grade | Appearance of hemorrhage |
|---|---|
| 1 | None evident |
| 2 | Less than 1 mm thick |
| 3 | More than 1 mm thick |
| 4 | Diffuse or none with intraventricular hemorrhage or parenchymal extension |
This scale has been modified by Claassen and coworkers, reflecting the additive risk from SAH size and accompanying intraventricular hemorrhage (0 - none; 1 - minimal SAH w/o IVH; 2 - minimal SAH with IVH; 3 - thick SAH w/o IVH; 4 - thick SAH with IVH)
Brain herniation syndromes
Parts of
the brain can herniate down over these firm dural folds (falx cerebri and
tentorium cerebelli )or through the hole of the foramen magnum..
The
intracranial pressure can be increased by a "mass effect". These can
include brain
tumors, hematomas (large bruises), and abscesses (pockets of
infection).
There are
four main types of brain herniation syndromes. These include the cingulate,
central, uncal, and cerebellar tonsillar herniations described below:
-Subfalcine
(or cingulate) herniation:
A section
of brain herniates under the falx cerebri. This can cause compression of the
anterior cerebral artery.
-Downward
Transtentorial (or central) herniation:
The
thalamic area herniates down over the tentorial notch. This can lead to decorticate
posturing during which the individual's body is in an extended position but the
arms and wrists flex in response to pain.
The
paramedian arteries that branch off of the basilar artery may rupture due to
excessive stretching. This will cause a characteristic brain bleed known as
Duret hemorrhages. The result is usually fatal.
-Temporal
Transtentorial (or uncal) herniation:
The
medial part of the temporal lobe herniates down over the tentorial notch. This
can lead to pressure on the brainstem and decerebrate posturing, often
beginning unilaterally and progressing to involve both sides. Decerebrate
posturing is full extension of the arms, legs, and back.
Uncal
herniation can also create dilated pupils on the same side as the lesion. This
is due to stretching of cranial nerve III.
Paresis
(slight or partial paralysis) may also be present on the ipsilateral side (same
side as the lesion). However, this is known as a false localizing sign because
it is actually due to compression damage on the side opposite the lesion to an
area known as the crux cerebri. This area is where the bulbar and corticospinal
tracts run on their way to the spinal cord.
-Cerebellar
Tonsillar herniation:
Part of
the cerebellum herniates through the foramen magnum. Neck
stiffness, known as nuchal rigidity, is a common finding. Irregular
respiration can quickly lead to cessation of breathing due to direct pressure
on the medulla.
There are
several types of brain herniation syndromes, each with specific signs and
symptoms that provide important clues to an accurate and timely diagnosis.
Anatomy of PNS (click this link)
Keros classification of olfactory fossa
The Keros classification is a method of classifying the depth of the olfactory fossa.
The ethmoid labyrinth is covered by the fovea ethmoidalis of the frontal bone and separates the ethmoidal cells from the anterior cranial fossa.
The very thin, horizontal cribriform plate (lamina cribrosa) of the ethmoid bone is bounded laterally by the vertical lateral lamella. The lateral lamella joins the cribriform plate to the fovea ethmoidalis.
In adults, the olfactory recess is a variable depression in the cribriform plate that medially is bounded by the perpendicular plate and laterally by the lateral lamella. It contains olfactory nerves and a small artery.
The depth of the olfactory fossa is determined by the height of the lateral lamella of the cribriform plate. Keros in 19621, classified the depth into three categories.
- type 1 : has a depth of 1 - 3 mm (26.3% of population)
- type 2 : has a depth of 4 - 7mm (73.3% of population)
- type 3 : has a depth of 8 - 16mm (0.5% of population)
The type 3 essentially exposes more of the very thin cribriform plate to potential damage from trauma, tumour erosion, CSF erosion (in benign intracranial hypertension) and local nasal surgery or orbital decompression. 2
Thin bone in the skull base, especially the cribriform plate, is susceptable to erosion, encephalomeningocoele herniation and CSF leaks
The most common sites of erosion or defects in the skull base are
- cribriform plate (51%)
- sphenoid lateral pterygoid recess (31%)
- ethmoid roof (8%)
- perisella
- inferolateral or pterygoid recesses
OSTEO MEATAL COMPLEX
White cerebellum sign
White cerebellum sign is encountered when there is diffuse decrease in density of cerebral parenchyma, with relatively increased attenuation of thalamus, brainstem and cerebellum. This sign indicates irreversible brain damage and has a very poor prognosis. Thats the reason, its quite vital for the radiologist to understand this sign.
There are different theories proposed for this sign
- Raised intracranial tension causes partial venous obstruction resulting in distension of deep medullary veins.
- Preferential flow to posterior circulation
- Transtentorial herniation partially relieving the increased ICT, and thus increase perfusion of central structures.
It is seen in
- head injury
- birth asphyxia
- drowning
- status epilepticus
- bacterial meningitis
- encephalitis
- post-cardiac arrest
“Mount Fuji sign”
The “Mount Fuji sign” on CT scans is useful in discriminating tension pneumocephalus from non-tension pneumocephalus.
Indeed, the characteristic separation of the frontal lobes is not found in patients with non-tension pneumocephalus.
Tension pneumocephalus occurs most commonly after neurosurgical evacuation of a subdural hematoma, but has also been reported as a result of skull base surgery, paranasal sinus surgery, posterior fossa surgery in the sitting position, or head trauma.
Tension pneumocephalus is characterized by an increased pressure of air within the subdural space. The increased pressure of air is assumed to be due to a check-valve mechanism, in which the air enters into the subdural space by a defect in the skull bone, but egress of air is blocked by an obstruction. Tension pneumocephalus leads to extraaxial mass effect and subsequent compression of the frontal lobes. The presence of air between the frontal lobes suggests that the pressure of the subdural air exceeds the surface tension of cerebrospinal fluid between the frontal lobes
Cavum septum pellucidum & Cavum vergae
Cavum velum interpositum
