Hypodense Extra-Axial Mass(es)

Bronwyn E. Hamilton, MD

DIFFERENTIAL DIAGNOSIS

Common

Arachnoid Cyst
Subdural Hematoma, Chronic
Post-Operative Epidural Fluid, Effusion, Fat, or Air
Pneumocephalus

Less Common

Neurocysticercosis
Lipoma
Pineal Cyst
Schwannoma
Craniopharyngioma
Epidural Hematoma
Epidermoid Cyst
Rathke Cleft Cyst

Rare but Important

Extra-Axial Empyema
Arachnoid Granulations, Dural Sinuses
Dermoid Cyst
Neurenteric Cyst (and Other Epithelial Cysts)

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Hypodense extra-axial masses can often be best characterized using FLAIR & DWI MR
Wide window CT settings are important for differentiating air, fat, & water densities
Contrast-enhancement is key for differentiating cystic neoplasm & most infectious etiologies from benign or developmental cysts

Helpful Clues for Common Diagnoses

Arachnoid Cyst
- Round/oval CSF density extra-axial mass
- May remodel adjacent calvarium
- 50-60% middle cranial fossa; 10% CPA
- DWI, FLAIR MR differentiate from epidermoid cyst
Subdural Hematoma, Chronic
- Hypodense, lentiform subdural collection(s)
- Can be uni- or bilateral
- May be loculated, septated
- Can have mixed density fluid-fluid layers
- Enhancement along dural margins & septations common
Post-Operative Epidural Fluid, Effusion, Fat, or Air
- Fat/muscle used to repair craniofacial defects may have “mass-like” appearance
- Post-operative fluid collections often contain blood products &/or protein resulting in hypodense collections
- Surgical or traumatic arachnoid tear may permit CSF to collect in subdural space
Pneumocephalus
- Typically related to trauma or post-surgical
- Air may become trapped & expand resulting in “tension pneumocephalus”
  - “Mount Fuji sign”: Subdural air separates/compresses frontal lobes, creating widened interhemispheric space between frontal lobe tips that mimics silhouette of Mount Fuji

Helpful Clues for Less Common Diagnoses

Neurocysticercosis
- Convexity subarachnoid spaces most common location
- Commonly involves basal cisterns
- Racemose form less common: “Grape-like” cystic masses in basal cisterns
- Imaging varies with stage & host response
Lipoma
- Well-delineated, lobulated, fat density, extra-axial mass
- 40-50% along interhemispheric fissure
  - Pericallosal & cisternal locations are common
  - Perisylvian location may be associated with seizures
- Midline lipomas should prompt search for other abnormalities
  - Callosal dysgenesis
  - Azygous anterior cerebral artery
  - Aneurysms
Pineal Cyst
- Homogeneous fluid-filled pineal mass
- 25% have associated calcification
- Rare enhancement along rim or in adjacent compressed pineal gland
Schwannoma
- Most common CPA mass (85-90%)
- Enhancing mass with extension into internal auditory canal (“ice cream on cone”)
- Intratumoral cysts in about 20% of cases
- Associated arachnoid cysts rare
Craniopharyngioma
- Calcified, cystic/solid suprasellar mass in a child
- Rim &/or solid portions enhance
Epidural Hematoma
- Extra-axial biconvex lesion
- Usually hyperdense; late subacute/chronic or rapid acute bleeding (“swirl sign”) may be partially hypodense
Epidermoid Cyst
- Lobulated, insinuating CSF density mass with potential deformity of surrounding structures
- DWI MR hyperintensity differentiates from other lesions (arachnoid cyst, cystic mass)
Rathke Cleft Cyst
- Sellar/suprasellar cystic mass with intracystic nodule
- No calcification or enhancement

Helpful Clues for Rare Diagnoses

Extra-Axial Empyema
- Subdural much more common than epidural empyema
- Peripherally enhancing extra-axial lesion
- DWI can help differentiate from other more benign lesions
- 15% of cases have both epidural & subdural components
- Complication of paranasal sinus disease & bacterial meningitis
Arachnoid Granulations, Dural Sinuses
- Fluid signal cysts in or near dural sinuses
- No enhancement
- Scalloping of inner calvarium is common
Dermoid Cyst
- Fat &/or calcifications are key to diagnosis
- Commonly midline location
- Look for pathognomonic “fat droplets” in ruptured dermoid cysts
Neurenteric Cyst (and Other Epithelial Cysts)
- Neurenteric cyst: Round/lobulated nonenhancing, slightly hyperintense to CSF mass in posterior fossa, typically anterior to pons/medulla
- Epithelial cysts not adequately differentiated by imaging: Characterized by histologic wall make-up
- Internal signal depends on contents