Brain MRI

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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

MR Imaging (for Rule-Out CVA or TIA)

  • MRI Brain with DWI (without contrast) AND
  • Cervical vascular imaging (ACEP Level B in patients with high short-term risk for stroke):[1]
    • MRA brain (without contrast) AND
    • MRA neck (without contrast)
      • May instead use Carotid CTA or US (Carotid US slightly less sensitive than MRA)[2] (ACEP Level C)

Contrast only needed if concern for malignancy/mass

MRI Modalities

T1 Weighted Imaging

T1 Brain.png

  • 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

T2 Brain.png

  • 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)

FLAIR Brain.png

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

Diffusion Weighted Imaging (DWI)

DWI Brain.png

  • 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

References

  1. ACEP Clinical Policy: Suspected Transient Ischemic Attackfull text
  2. Nederkoorn PJ, Mali WP, Eikelboom BC, et al. Preoperative diagnosis of carotid artery stenosis. Accuracy of noninvasive testing. Stroke. 2002;33:2003-2008.