Delbert Yip

Delbert Yip

Medical student at the University of British Columbia.

Notes on Neuroradiology Jan 27

5 minute read

Published:

The bright signal seen on diffusion-weighted imaging (DWI) of acute infarcts persists for approximately 10 to 14 days

This occurs because:

  1. On DWI, signal intensity increases with restricted diffusion of water molecules.
  2. In acute infarcts, ischemia results in cytotoxic edema, which leads to:
    1. cellular swelling
    2. decreased extracellular space
    3. decreased water diffusion
    4. increased signal intensity on DWI
  3. These changes are the primary reason for the bright DWI signal for the first 5-7 days.
  4. After 5-7 days, the signal increasingly results from altered T2 properties of infarcted tissues, i.e. “T2 shine-through”

Apparent diffusion coefficient (ADC) maps can be used to distinguish between old and new infarcts:

  • Due to T2-shine-through, a bright DWI signal may be up to 10-14 days old.
  • On ADC maps, signal intensity also increases with reduced diffusion of water, but it is sensitive to the rate of reduction.
  • Hence, as edema and ischemic changes stabilize, signal intensity on ADC progressively decreases after the onset of ischemia
  • A hypointense area on ADC is likely less than 7-10 days old.
  • However, the average ADC only stays hypointense for 4-7 days.
  • After 7-10 days, the ADC normalizes and may become bright again.

Persistent areas of hypodensity in the brain after an infarct may result from gliosis or brain necrosis replaced by CSF.

After an infarct, brain tissue undergoes necrosis and degeneration:

  • Astrocytes and other glial cells proliferate to form a glial scar
  • The scar appears:
    • Hyperintense on T2-weighted and FLAIR MRI
    • Hypointense on CT due to being less dense than regular brain tissue
    • Along the borders of the area of tissue destruction, i.e. encephalomalacia
  • Over time, the necrotic tissue is reabsorbed and replaced by CSF
  • These cavitary lesions appear:
    • Hyperintense on T2-weighted MRI
    • Hypointense on T1- and FLAIR MRI
    • Hypointense on CT because CSF is less dense than brain tissue
  • With time, there may be ex vacuo enlargement of adjacent CSF spaces, including sulci and parts of the ventricles.

Simple definitions of T1- vs T2-weighted MRI

The intensity of T1 and T2 MRI scans is primarily determined by how protons in different tissues interact with the magnetic field and radiofrequency (RF) pulses. These interactions in turn are largely driven by their:

  • Water and fat content
  • T1 and T2 relaxation times
    • T1 relaxation is the time for proton spins to relax along the longitudinal plane
    • T2 relaxation is the time for proton spins to relax in the transverse plane
    • TR (repetition time) is the interval between 2 successive RF pulses
    • TE (echo time) is the time from RF pulse onset to measurement of the MRI signal
  • Proton density: all things equal, tissues with higher proton density will produce a stronger signal, e.g. air, bone, and calcifications appear dark due to a relative absence of protons

DWI MRI allows visualization of acute cerebral infarcts earlier than conventional MRI

DWI uses strong magnetic gradients to measure the diffusion of water protons in brain tissue. As mentioned above, a decrease in the diffusion of water causes an increase in signal intensity on DWI. This is helpful for identifying pathologies that restrict the movement of water,

  • e.g. in acute ischemia, water moves into and is ‘trapped’ within cells, which then swell, causing contraction of the extracellular space.
  • hence, normal tissue appears dark on DWI because water molecules can move more freely

DWI’s main advantage over conventional MRI (e.g. T1, T2, FLAIR) is that the latter rely on the overall water content in tissues and subsequent changes in proton relaxation times.

  • These changes can take several hours to occur, which is why conventional MRI is relatively insensitive to ischemia in the first few hours.
  • In contrast, DWI can detect the changes associated with cytotoxic edema within 30 minutes of stroke onset.

Use of CT for detecting hemorrhage versus ischemia

CT is generally used for detecting hemorrhages, but is not as helpful in visualizing acute ischemia:

  • Use in hemorrhage
    • Fresh blood appears hyperdense on CT due to the high density of iron in hemoglobin. This makes hemorrhage and fresh blood clots readily distinguishable from surrounding brain tissue on CT. Hence, CT is the preferred way to diagnose intra-cerebral hemorrahge (ICH)
    • Other advantages: Fast, Widely available, Cheap
  • Use in ischemia
    • As with MRI, early ischemic changes may not be visible on CT until several hours, or even days, have passed after stroke. These changes include the obscuration or loss of prominent gray matter structures, e.g. lentiform nucleus, insular ribbon.
    • Ischemia can cause ionic edema, but the resulting increase in water content only causes subtle X-ray attenuation (hypointensity).
    • Small infarcts may never be visible on CT.
    • Contrast is not helpful because it is used to visualize areas of increased vascularity, whereas ischemia results from decreased perfusion.

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