Arachnoid cyst MRI

A 32 y o male with left hemi facial pain for last 1 year. 

Here is his MRI Brain Axial FLAIR and Diffusion.

This MRI study of Brain shows a sharply demarcated left Cp angle cystic lesion. 

Extra axial, not communicating with ventricular system.
Isointense to csf, complete suppression of fluid signal on FLAIR. No septations or loculations.
No restricted diffusion on Dw images.
An associated hypoplastic left cerebellar hemisphere. No gliosis in adjacent brain parenchyma. No volume loss.
Mass effect is mild, decently displaces adjacent vessels and nerves without encasement. Left trigeminal nerve is stretched may be the cause of left hemi facial pain. Left 7-8th CN complex mildly displaced. No significant mass effect fourth ventricle to cause obstructive hydrocephalus.
Non enhancing on post contrast.

Imaging diagnosis : Left Cp angle Arachnoid Cyst.

Needs to be differentiated from its closest DDs;
1. Epidermoid cyst which typically shows restricted diffusion. Mass effect on adjacent parenchyma instead of hypoplasia, insinuates in cisternal spaces, encases nerves and vessels implies to its plasticity. Signal suppression on FLAIR may not be as complete as arachnoid cyst.
2. Porencephalic cyst show Gliosis in adjacent brain parenchyma and volume loss like ex vacuo dilatation of adjacent ventricle. May communicate with ventricular system. It’s a cortical area of Csf signals involves cortical grey white mater where as arachnoid is an extra axial lesion.
3. Neuroglial cyst is intra axial, rare, commonly seen in frontal lobes.

Related post :
Arachnoid-cyst parietal region - uncommon for site.
Epidermoid-vs-arachnoid-cyst

ARACHNOID CYST

AC belongs to a category of primary non neoplastic intracranial Cysts.
An intra arachnoid CSF-filled sac that does not communicate with ventricular system.
ACs contributes to ~ 1% of all intracranial masses and as incidental finding on imaging for seizure in ~ 2 % of cases.

Imaging wise diagnostic clue is sharply demarcated round/ovoid extra-axial cyst that follows CSF attenuation on CT or signal on all MRI pulse sequences. No restricted diffusion on DWI (vs Epidermoid cyst)

Most common location is middle cranial fossa (~50-60%), Cerebellopontine angle (~ 10%), Suprasellar arachnoid cyst (~10%), Cerebral convexity, quadrigeminal cistern (~10%)

Imaging findings
NECT: Hypodense with density same as that of Csf.
Calcification is not a feature.
Non enhancing on post contrast.
Hyperdense if intracyst hemorrhage present and is rare.
MRI
Iso intense to Csf on T1, T2w and FLAIR images. Complete signal suppression of FLAIR.
T2* GRE: No blooming unless hemorrhage present and is rare.
DWI, no restriction
T1 C+, doesn't enhance
MRA, cortical vessels displaced away from calvarium.

Associated abnormalities are scalloping of adjacent bony calvarium.
Hypoplastic adjacent part of brain for example temporal pole in case of middle cranial fossa ACs and Cerebellar hemisphere in cases of Posterior fossa ACs.

Pathogenesis 
AC is usually sporadic, non-syndromic, rarely familial.
Frontal, temporal embryonic meninges (endomeninx) fail to merge as sylvian fissure forms, remain separate, forming duplicated arachnoid. Proposed mechanism behind development of cyst are active fluid secretion by cyst wall, Slow distention by CSF pulsations, CSF accumulates by ball-valve mechanism.
Microscopic Features are wall consists of flattened but normal arachnoid cells. No inflammation, neoplastic cells.

Clinical presentation
Often asymptomatic and found incidentally
If symptomatic, symptoms vary with size, location of cyst. Headache, dizziness, sensorineural hearing loss, hemifacial spasm/tic.

ACs can be found at any age, 3-5 times more common in male.
Usually don't enlarge. If at all grows very slowly.

Treatment
Often requires no treatment.
May need endoscopic resection, Fenestration, Cystoperitoneal shunt.

Reference : Diagnostic imaging Osborn.