MRI Sequences: T1, T2, FLAIR, and DWI Explained
Core MRI Physics: The Basics
Magnetic Resonance Imaging (MRI) is a cornerstone of modern neuroimaging. Unlike CT, which relies on X-ray attenuation, MRI harnesses the magnetic properties of hydrogen protons in the body. By applying specific Radiofrequency (RF) pulses and manipulating gradient fields, radiologists can generate exquisite soft-tissue contrast.
T1-Weighted Imaging (T1WI)
T1WI is universally characterized as the "Anatomy Sequence" because its contrast perfectly mirrors gross anatomical dissection.
- Fluid (CSF, Water): Appears completely black (hypointense).
- Subcutaneous Fat & Bone Marrow: Appears completely white (hyperintense).
- Clinical Utility: Superb for defining gray/white matter boundaries, evaluating congenital malformations, and serving as the baseline for Gadolinium contrast administration.
💉Contrast Pearl
Gadolinium exclusively shortens the T1 relaxation time, causing any vascular or pathological tissue with blood-brain barrier breakdown (e.g., tumors, active MS plaques) to glow bright white on T1WI post-contrast.
T2-Weighted Imaging (T2WI)
T2WI is the quintessential "Pathology Sequence." Since the vast majority of human pathology (edema, inflammation, tumors) is inherently water-rich, T2WI makes pathology literally glow.
- Fluid (CSF, Water): Appears brilliant white (hyperintense).
- Fat: Appears white (but slightly less bright than on T1).
- Bone Cortex, Air, Flowing Blood: Appears black (signal void).
FLAIR (Fluid-Attenuated Inversion Recovery)
FLAIR is functionally a T2-weighted sequence where the signal from free water (CSF) has been mathematically completely nullified.
- CSF in Ventricles/Sulci: Appears definitively black.
- Pathological Fluid (Edema, Gliosis, Demyelination): Appears white.
💡Diagnostic Pearl
FLAIR is the most sensitive sequence in neuroradiology for detecting subtle parenchymal edema, Multiple Sclerosis (MS) plaques, and juxtacortical lesions that would otherwise be hidden by bright contiguous CSF on a standard T2 sequence.
DWI (Diffusion-Weighted Imaging)
DWI is arguably the single most critical sequence for acute stroke imaging. It measures the microscopic "Brownian" random motion of extracellular water molecules.
- Acute Ischemic Stroke: Within minutes of an infarction, the Na/K pump fails, causing cells to swell (cytotoxic edema). This physically traps extracellular water, "restricting" its normal random diffusion.
- Appearance: Restricted diffusion appears brutally bright white on the DWI trace and strictly dark on the corresponding ADC (Apparent Diffusion Coefficient) map.
- Abscess vs Tumor: DWI perfectly distinguishes complex brain abscesses (which heavily restrict diffusion due to viscous pus) from necrotic ring-enhancing tumors (which do not restrict).
Related Pathologies & Review
Understanding these sequences is vital for interpreting the following classical presentations:
- Hyperacute Stroke: Positive (white) on DWI, but completely normal on T1 and T2/FLAIR.
- Meningioma: Isointense on T1, intensely enhancing post-Gadolinium with a classic "dural tail".
- Multiple Sclerosis: Periventricular hyperintense ovoid plaques ("Dawson's Fingers") best appreciated on sagittal FLAIR.
High Yield Facts
💡FRCR / MD Prep Pearl
Always review the DWI sequence first when evaluating for acute neurological deficits. DWI can be positive within 30 minutes of symptom onset, whereas T2 and FLAIR may remain falsely negative for 6-12 hours.