Brain MRI

Background

• MRI uses magnetic fields and radiowaves to develop high definition imaging of the brain and excellent tissue contrast
• No radiation associated with imaging
• Ideal for looking at brain parenchyma and midbrain
• Contrast is commented on by signal intensity
  • Dark areas are hypointense
  • Bright areas are hyperintense

Ordering Studies

Ordering for Rule-Out CVA or TIA

• MRI Brain DWI and cervical vascular imaging predict short-term risk for stroke in patients presenting with suspected TIA. When feasible, physicians should obtain:
  • MRI with DWI/MRA in patients with high short-term risk for stroke (ACEP Level B)
  • Carotid US/CTA/MRA in patients with high short-term risk for stroke (ACEP Level B)
    • Carotid US is slightly less sensitive than MRA, but useful for carotid stenosis eval (ACEP Level C)

MRI Modalities

T1 Weighted Imaging

• Ideal for brain parenchyma
• With the addition of contrast, this can differentiate causes of inflammation
• Fluid is hypointense (similar to CT imaging)
• Methemoglobin, fat, and protein are hyperintense

T2 Weighted Imaging

• Highlights CSF
• Good for identifying tissue edema around pathologic areas
• Fluid is hyperintense (reverse of T1)
• Tissue tends to be more hypointense

Fluid Attenuation Inversion Recovery (FLAIR)

• Appears as T2 images with hypointense CSF
• Ideal for identifying tumors/GBS
• Also used to identify leptomeningeal enhancement in meningitis

Diffusion Weighted Imaging (DWI)

• A method of measuring the Brownian motion of water molecules
• Diffusion within the intracellular fluid, diffusion within extracellular fluid, and between these areas will differ depending on pathology
• Ideal for cellular swelling especially in acute ischemic stroke which will be hyperintense

Blood

Age of Blood	T1 Imaging	T2 Imaging
Hyperacute	Iso	Bright
Acute	Iso/Dark	Dark
1-3 Days	Bright	Dark
1-2 Wks	Bright	Bright
2-3 Wks	Iso/Dark	Dark

See Also

• MRI (main)