FLAIR Hyperintense CSF

Bronwyn E. Hamilton, MD

DIFFERENTIAL DIAGNOSIS

Common

Subarachnoid Hemorrhage, NOS
Intraventricular Hemorrhage
Meningitis
MR Artifacts, Magnetic Susceptibility
MR Artifacts, Flow-Related
MR Artifacts, Patient-Related
Metastases, Meningeal
Ventriculitis

Less Common

Gadolinium in CSF due to
- Blood-Brain Barrier Leakage
- Chronic Renal Failure
Cerebral Ischemia-Infarction, Acute

Rare but Important

Dermoid Cyst (Ruptured)
Moyamoya

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Important to separate artifact from true pathology
FLAIR hyperintensity on MR is usually nonspecific
Correlation with clinical history & prior imaging essential (i.e., CT for hemorrhage)
Gadolinium in CSF may also cause FLAIR hyperintensity (i.e., renal failure)

Helpful Clues for Common Diagnoses

Subarachnoid Hemorrhage, NOS
- Common causes: Trauma, aneurysm rupture
  - Other vascular malformations can result in subarachnoid hemorrhage (SAH)
  - Location may help determine etiology
- FLAIR is more sensitive but less specific than CT for SAH
Intraventricular Hemorrhage
- Common causes: Trauma, hypertensive hemorrhage, aneurysm, or AVM rupture
- Any parenchymal hemorrhage may rupture into ventricle
- Ventricles normal in size at presentation
Meningitis
- Diffuse more common than focal hyperintensity
- Meningeal enhancement typical
- May be complicated by hydrocephalus, ventriculitis, abscess, vasculitis
- Remains a clinical-laboratory diagnosis
MR Artifacts, Magnetic Susceptibility
- Regionally adjacent metal, blood, air-bone interfaces causes FLAIR hyperintensity
- Distorts local magnetic field, altering null point for fluid (T1), resulting in inappropriate high signal
- Often seen close to aerated frontal sinuses & temporal bones
- Common surrounding aneurysm clips
MR Artifacts, Flow-Related
- CSF flow artifacts are common in basal cisterns, foramen of Monro, aqueduct, & 4th ventricle
- Periodic artifacts extending outside skull in phase encoding direction is diagnostic
- Usually absent on spin echo sequences (T1, T2); helpful to confirm artifact
MR Artifacts, Patient-Related
- Diffuse FLAIR hyperintensity
- Common etiologies: Head motion, Propofol, 50% or greater supplemental oxygen (4-5x ↑ signal with 100% O₂)
Metastases, Meningeal
- Usually due to cellularity &/or increased protein content within CSF
- May be focal or diffuse
- Meningeal enhancement typical
- Adjacent bone changes common
- Breast & lung most common distant primary tumors
Ventriculitis
- Ventriculomegaly with debris level
- DWI bright & ventricular enhancement
- Complication of meningitis, abscess, ventricular catheter

Helpful Clues for Less Common Diagnoses

Blood-Brain Barrier Leakage
- Etiologies include: Infection/inflammation, ischemia, tumor
  - Cerebritis, posterior reversible encephalopathy syndrome (PRES) may cause BBB leak
  - Acute/subacute stroke (poor prognostic sign suggests hemorrhagic transformation)
  - Neoplasms uncommon, usually with delayed imaging
- Gadolinium accumulates in CSF due to BBB leakage
- May cause focal or diffuse FLAIR hyperintensity & enhancement
Chronic Renal Failure
- Increased FLAIR related to delayed gadolinium clearance from circulation
- May augment other pathologic causes of FLAIR hyperintensity
- Usually seen with delayed imaging (may also be seen in normal patients)
Cerebral Ischemia-Infarction, Acute
- May see hyperintense CSF related to vessel occlusion or slow flow
- “Dot sign” related to occluded MCA branches in Sylvian fissure
- Enhancement related to slow flow

Helpful Clues for Rare Diagnoses

Dermoid Cyst (Ruptured)
- Fat-containing lesions are FLAIR bright from T1 shortening effects
- T1 foci in subarachnoid spaces pathognomonic
Moyamoya
- Progressive narrowing of distal ICA & proximal circle of Willis with collaterals, anterior > posterior circulation
- “Ivy sign”: Bright FLAIR signal related to slow-flowing engorged pial vessels, thickened arachnoid membranes
  - More commonly seen in frontal & parietal lobes
- Leptomeningeal enhancement (contrast “ivy sign”)

Other Essential Information

Causes of pathologic FLAIR hyperintense CSF: Blood, elevated protein, or cells
FLAIR hyperintensity can be due to T2 prolongation or T1 shortening
“Fast” FLAIR can cause artifactual FLAIR hyperintensity

Alternative Differential Approaches

FLAIR hyperintensity with enhancement: Meningitis, metastases, ventriculitis, blood-brain barrier leakage, chronic renal failure, acute ischemia, moyamoya

SELECTED REFERENCES

1. Morris JM et al: Increased signal in the subarachnoid space on fluid-attenuated inversion recovery imaging associated with the clearance dynamics of gadolinium chelate: a potential diagnostic pitfall. AJNR Am J Neuroradiol. 28(10):1964-7, 2007

2. Stuckey SL et al: Hyperintensity in the subarachnoid space on FLAIR MRI. AJR Am J Roentgenol. 189(4):913-21, 2007

3. Cianfoni A et al: Artifact simulating subarachnoid and intraventricular hemorrhage on single-shot, fast spin-echo fluid-attenuated inversion recovery images caused by head movement: A trap for the unwary. AJNR Am J Neuroradiol. 27(4):843-9, 2006

4. Frigon C et al: Supplemental oxygen causes increased signal intensity in subarachnoid cerebrospinal fluid on brain FLAIR MR images obtained in children during general anesthesia. Radiology. 233(1):51-5, 2004

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