What is a Fluid Attenuated Inversion Recovery (FLAIR) sequence in Magnetic Resonance Imaging (MRI)?

Fluid Attenuated Inversion Recovery (FLAIR) Sequence in MRI

Fluid Attenuated Inversion Recovery (FLAIR) is an MRI pulse sequence that suppresses cerebrospinal fluid (CSF) signal while enhancing the visibility of pathological lesions, particularly those adjacent to CSF spaces such as periventricular and cortical/juxtacortical regions.

Technical Principles

• FLAIR combines T1 and T2-weighting techniques with a specific inversion recovery pulse that nullifies the CSF signal 1
• The sequence uses:
  • Long inversion time (TI) to allow CSF magnetization to reach the null point
  • Long repetition time (TR)
  • Long echo time (TE)
  • Typically acquired in 2D axial plane, though 3D acquisitions are increasingly available 1

Clinical Applications

Multiple Sclerosis (MS)

• Significantly improves detection of MS lesions, particularly:
  • Periventricular lesions
  • Juxtacortical and cortical lesions that may be missed on conventional T2-weighted images 1, 2
• Part of standardized MS imaging protocols 1
• Detects up to 9.6 cortical/juxtacortical lesions per MS patient on average, compared to only 26% visible on T2-weighted and 14% on T1-weighted sequences 2

Brain Tumors

• Included in standardized brain tumor imaging protocols 1
• Enhances visualization of tumor margins and infiltration, particularly near ventricles
• Recommended timing: acquired after contrast injection but before post-contrast T1-weighted images 1

Cerebrovascular Disease

• Superior for detection of acute and subacute infarcts
• In hemorrhagic lesions, FLAIR can detect extracellular methemoglobin within 2 weeks of clinical event 1

Leptomeningeal Disease

• Highly sensitive for detecting leptomeningeal metastases
• Combined with contrast-enhanced T1-weighted sequences, provides optimal detection of leptomeningeal abnormalities 1

Cavernous Malformations

• Helps identify recent hemorrhage from cavernous malformations 1

Technical Considerations

Acquisition Parameters

• Recommended slice thickness:
  • 3 mm with no interslice gap on 3T scanners
  • 4 mm with no interslice gap on 1.5T scanners 1
• Echo train length (ETL) between 8-16 is optimal 1
• Acquisition time is longer than conventional sequences but can be reduced with newer techniques

Limitations and Pitfalls

• Lower signal-to-noise ratio (SNR) compared to standard T2-weighted images 1
• CSF suppression may be incomplete when CSF contains blood products or high protein content 3
• May produce flow-related artifacts, particularly in posterior fossa and anterior temporal lobes 1
• Partial saturation FLAIR (PS-FLAIR) can reduce acquisition time but may compromise lesion-to-white matter contrast 4

Advanced Applications

• 3D FLAIR acquisition allows:
  • Multiplanar reformatting
  • Improved quantification of lesion volumes
  • Reduced flow artifacts compared to 2D sequences 1
• FLAIR can be combined with susceptibility-weighted imaging (SWI) to create "FLAIR*" which enhances detection of iron-containing tissue and small veins 1

Clinical Implementation

• FLAIR is now a standard component of brain MRI protocols
• Particularly valuable when evaluating:
  • White matter diseases (especially MS)
  • Cortical/subcortical lesions
  • Periventricular pathology
  • Leptomeningeal disease
• In some clinical settings, FLAIR may eventually replace conventional T2-weighted sequences for routine brain screening 5

Comparison with Other Sequences

• Superior to conventional T2-weighted sequences for periventricular and cortical lesions due to CSF signal suppression
• Less sensitive than T2-weighted sequences for posterior fossa lesions
• Complements T1-weighted sequences (pre and post-contrast) for comprehensive lesion characterization
• May be combined with diffusion-weighted imaging (DWI) for improved detection of acute ischemic changes 6