Arachnoid Cyst

KEY FACTS

TERMINOLOGY

• Intraarachnoid cerebrospinal fluid (CSF)-filled sac that does

not communicate with ventricular system

IMAGING

• General findings

○ Sharply demarcated round/ovoid extraaxial cyst

○ Isodense/isointense with CSF

• Location

○ Middle cranial fossa (50-60%)

○ Cerebellopontine angle (10%)

○ Suprasellar (10%)

○ Miscellaneous (10%) (convexity, quadrigeminal)

• MR

○ Isointense with CSF on all sequences

○ Suppresses completely with FLAIR

○ No restriction on DWI

○ CISS, FIESTA: Use to delineate cyst wall, adjacent

structures

○ 2D cine phase contrast: Use to detect communication

between arachnoid cyst (AC), adjacent subarachnoid

space

TOP DIFFERENTIAL DIAGNOSES

• Epidermoid cyst

• Other nonneoplastic cysts (e.g., porencephalic)

• Chronic subdural hematoma

• Subdural hygroma

CLINICAL ISSUES

• Most common congenital intracranial cystic abnormality

• 1% of all intracranial masses

• Can be found at any age (75% in children)

• Usually an incidental finding

DIAGNOSTIC CHECKLIST

• FLAIR, DWI best for AC vs. epidermoid

(Left) Graphic depicts the

middle cranial fossa arachnoid

cyst (AC). Note the arachnoid

﬈ splits and encloses CSF. The

middle fossa is expanded, and

the overlying bone is thinned.

Note that the temporal lobe

﬇ is displaced posteriorly.

(Right) Submentovertex view

shows autopsied brain with

incidental finding of middle

cranial fossa AC. Note

"splitting" of arachnoid ﬇

containing a large CSF

collection (drained during

removal). The temporal lobe is

displaced posteriorly ﬈, and

the middle fossa is expanded.

(Courtesy J. Townsend, MD.)

(Left) Sagittal T1WI MR shows

a classic middle fossa

arachnoid cyst ﬈. Note the

expansion of the greater

sphenoid wing st and

posteriorly displaced temporal

lobe ﬇. (Right) Composite of

axial T2, FLAIR, postcontrast

T1WI, and coronal T1 C+

images shows the scalloped

margins characteristic of ACs.

ACs are CSF-like on T2WI ﬈,

suppress completely on FLAIR

﬊, remodel the adjacent

calvaria ﬉, and do not

enhance.

Cysts, and Disorders

Brain: Pathology-Based Diagnoses: Neoplasms,

161