An arachnoid cyst is a benign cyst located within the intracranial or spinal subarachnoid space that contains cerebrospinal fluid. The majority are asymptomatic, but some can cause symptoms by compressing surrounding structures through gradual enlargement. Magnetic resonance imaging is used to identify arachnoid cysts and distinguish them from other cystic lesions based on their cerebrospinal fluid-like signal and lack of enhancement or solid components. While most remain asymptomatic, surgical treatment may be considered for symptomatic cysts to relieve mass effect.

2. Cerebral cyst
Khazaei mojtaba , fellowship of
neurovascular intervention

3. Arachnoids' cyst
relatively common.
benign and asymptomatic.
There are within the intracranial compartment.
(most common) and the spinal canal.
located within the subarachnoid space .
contain CSF.

4. Arachnoids' cyst
• majority are sporadic.
• seen with increased frequency.
in mucopolysaccharidoses (as are perivascular
spaces).

5. Clinical presentation of Arachnoids' cyst
• Approximately 5% of patients experience
symptoms.
• result of gradual enlargement resulting in mass
effect.
• This results in either direct neurological
dysfunction or distortion of normal CSF pathways
resulting in obstructive hydrocephalus.
• Sellar/suprasellar, quadrigeminal,
and cerebellopontine angle arachnoid cysts were
more likely to be symptomatic.

6. Radiographic features
of arachnoid cyst
• can occur anywhere within the central nervous
system.
• most frequently (50-60%) located in the middle
cranial fossa.
• retrocerebellar location accounts for 30-40%
• suprasellar cistern
• within the ventricles
• posterior fossa
– cisterna magna (need to be distinguished from a mega
cisterna magna)
– cerebellopontine angle (need to be distinguished from
an epidermoid cyst)
• spinal canal

7. MRI of arachnoid cyct
• follow CSF on all sequences,
• isplacement of surrounding structures
• no solid component, no enhancement
• Phase contrast imaging: determine if the cyst
communicates with the subarachnoid space
• high resolution sequences such as CISS
& FIESTA help to delineate cyst wall and
adjacent anatomic structures.

8. Treatment and prognosis
of arachnoid cyst
• majority remain asymptomatic throughout
life.
• If they are deemed to be causing symptoms,
then surgery can be contemplated.
• rare complication is spontaneous rupture in
the subdural space.

9. Differential diagnosis of arachnoid cyst
• enlarged CSF space (e.g. mega cisterna magna)
• epidermoid cyst
– often shows a heterogeneous/dirty signal on FLAIR
– restricted diffusion
– more lobulated
– tend to engulf adjacent arteries and cranial nerves
• subdural hygroma/chronic subdural hemorrhage
– do not typically show CSF signal intensity on MRI , can have an enhancing membrane
• cystic tumors: often will have a solid/enhancing component and be intra-axial
– pilocytic astrocytoma
– Hemangioblastoma
• non-neoplastic cysts
– neurenteric cyst
– neuroglial cyst
– porencephalic cyst
– often follow a history of trauma or stroke
– surrounded by gliotic brain
• neurocysticercosis
– small cyst, usually multiple when in the subarachnoid space

10. Arachnoid cyst
Arachnoid cyst of the middle cranial fossa. Note the remodelling of the
lateral wall of the right middle cranial fossa on bone window CT.

11. Arachnoid cyst - parafalcine
MRI images demonstrate a left posterior
parafalcine cyst overlying the precuneus that
follows CSF signal on all sequences. Features
are those of an arachnoid cyst.

12. Arachnoid cyst - posterior fossa
MRI through the posterior fossa demonstrates a
large right-sided extra-axial CSF intensity mass
lesion. It follows CSF on all sequences, including
FLAIR. There is significant mass effect on the
adjacent cerebellar tissue and remodelling and
expansion of the adjacent skull is evident.
High resolution T2 images (FIESTA) demonstrates
that this lesion is bounded by a very thin
membrane, best seen bulging across the midline
towards the left.
Sagittal images demonstrate upward bowing and
thinning of the corpus callosum suggesting
hydrocephalus.

13. Arachnoid cysts causing proptosis
Large middle cranial
fossa arachnoid
cyst expanding the bone
and deforming the
posterolateral wall of the
orbit resulting in
proptosis. The arachnoid
cyst extends inferiorly into
the pterygoid process
(open arrow).

14. Arachnoid cyst - middle cranial fossa
A region of CSF intensity which expands the
middle cranial fossa and displaces the
temporal lobe posteriorly. There is no
restricted diffusion, no solid component and
no enhancement. Features are characteristic of
an arachnoid cyst.

15. Arachnoid cyst – spinal Female with LBP
The thoracic spinal cord (C) is displaced anteriorly by a
CSF intensity space which on axial imaging in fact has
slightly higher and more homogeneous signal than one
normal sees in thoracic CSF.

16. Arachnoid cyst - posterior fossa
Large posterior fossa arachnoid cyst (note it
follows CSF signal on both T1 and T2) with
mass effect.

17. Arachnoid cyst of the posterior fossa with hydrocephalus
Large posterior cranial fossa CSF intensity space consistent with
an arachnoid cyst displaces the cerebellum anteriorly resulting
in marked obstructive hydrocephalus. Note the ballooning of
the third ventricular recesses and the upward displacement of
the corpus callosum.

18. Arachnoid cyst - cerebellopontine angle
Right cerebellopontine angle CSF density space with mass-
effect on the adjacent cerebellar hemisphere and
remodelling of the overlying bone consistent with
an arachnoid cyst. The only viable differential is that of an
epidermoid cyst which is far less likely given the very
homogeneously exactly-CSF density and the presence of
bony remodelling absent in epidermoid cysts. An MRI would
be required to categorically confirm this.

19. Arachnoid cyst – suprasellar cont..
MRI demonstrates a CSF intensity space distorting the optic chiasm and
pituitary infundibulum (pushing them forwards and upwards). A thin
membrane can be seen (best on sagittal and axial T2 weighted images)
invaginating upwards into the third ventricle, splaying the septum pellucidum
and even bulging into the left foramen of Munro. The lateral ventricles are
dilated, in keeping with obstructive hydrocephalus, clearly long standing
(note large size and lack of transependymal edema).

20. Arachnoid cyst - suprasellar
Anterior cerebral arteries and
basilar artery tip are seen as
flow voids (red arrows); optic
chiasm (yellow arrow); pituitary
infundibulum (orange arrow);
septum pellucidum (green
dotted lines); arachnoid cyst
(blue dotted line).
A third ventriculostomy was
performed, and the floor of the
cyst was also opened into the
interpeduncular cistern.
Prominent CSF pulsation can be
seen continuously from the
third ventricle to the
prepontine cistern.
Note the reduced distortion on
the optic chiasm and pituitary
infundibulum.

21. Arachnoid cyst – frontal lob with overlying bony remodelling

22. Arachnonid cyst
Large extra-axial CSF-density centered on the
middle cranial fossa. Mass effect onto the
adjacent brain.

23. Arachnoid cyst with headache
A large cystic
lesion that
follows CSF
signal on all
pulse
sequences
representing
an arachnoid
cyst.

24. Arachnoid cyst distorting the third ventricle
Case Discussion
Hydrocephalus was secondary to an obstructive third ventricle
cyst.VP shunt tube in situ.
The MR appearance suggests a suprasellar arachnoid
cyst displacing the chiasm and infundibulum anteriorly however
on endoscopy the arachnoid cyst was (apparently) actually
intraventricular in location, within the third ventricle.

25. Case Discussion
CT and MR images demonstrate a hemorrhagic arachnoid cyst

26. Arachnoid cyst - cerebellopontine angle
Right sided vertigo ?
CSF intensity cystic lesion in the right
cerebellopontine angle. It follows CSF
signal in all the sequences. The lesion
does not show restricted diffusion,
excluding an epidermoid cyst.
Cerebellar hemispheric compression
(sagittal series) and distortion of the
brain stem with anterior
displacement and stretching of the
VII/VIIIth and XIIth nerves.

27. Supravermian arachnoid cyst 50 year old F
Headaches. Unexplained weakness of the right arm.
Homogeneous lesion between the upper
vermis and the cerebellar tentorium with
homogeneous CSF signal on all sequences, a
thin (non-discernible) wall and slight
indentation of the vermian surface.

28. Arachnoid cyst
motor vehicle accident.
no intra axial or extra axial hemorrhage.
no calvarial fracture.
large extra axial CSF attenuating lesion is
seen occupying the left middle cranial fossa
splaying the sylvian fissure and displacing
the frontal, parietal and temporal lobes.
secondary mass effect is seen in the form of
effacement of left cerebral hemispheric
sulci, mild midline shift to right, effaced
ipsilateral lateral ventricle.
no transtentorial herniation.

29. Arachnoid cyst
headache
Well circumscribed thin walled extra-axial CSF
density mass in the right frontal area with
remodelling effect on the adjacent bone. The
remaining of the study is unremarkable.

30. Retro-cerebellar arachnoid cyst
A large right retro-cerebellar extra-axial cyst is seen
compressing the right cerebellar hemisphere and
elevating the tentorium being insinuated between
the occipital lobes. It is seen following CSF signal on
all sequences with facilitated diffusion. It is seen
measuring 3.2X7.7X6.5 cm along its axial and
cranio-caudal dimensions.
Annotated image showing the membrane of the
arachnoid cyst (red arrows). Remodelling of the
skull is also present (blue arrow head).
cystoperitoneal shunt

31. Posterior fossa arachnoid cysts can be classified into
four subgroups as follow:
1a: midline inside the 4th ventricle treated by
shunting or endoscopic fenestration
1b: midline outside the 4th ventricle, treated by
surgery
2a: off-midline within the cerebellopontine angle
treated by surgery or endoscopic fenestration
2b: off-midline either retro-cerebellar or
intracerebellar treated by surgery

32. Quadrigeminal cistern arachnoid cyst
There is a well-circumscribed, rounded lesion with imperceptible
walls located at the level of quadrigeminal cistern following CSF
signal in all pulse sequences without evidence of soft tissue
component. It is exerting mass effect over the third ventricle and
aqueduct of sylvius with resultant moderate dilatation of the third
and lateral ventricles.

33. Epidermoid cyst - cerebellopontine angle

34. Mega cisterna magna
Large retrocerebellar space that follows CSF signal on all sequences. Normal
cerebellar vermis.
Case Discussion
Incidental mega cisterna magna. Images at higher levels confimed the presence of
a normal posterior fossa and vermis distinguishing it from a Dandy-Walker
malformation.

35. Neuroglial cyst