NEUROIMAGING / MAGNETIC RESONANCE

Created 24/05/2021, last revision 22/04/2023

Diffusion-weighted imaging (DWI) is a common MRI sequence for evaluating acute ischemic stroke. Increased DWI signal in ischemic brain tissue is usually observed within a few minutes after arterial occlusion.

• ischemia leads to diffusion restriction (water molecules remain in the cells) due to the energetic failure of Na⁺/K⁺ membrane pumps
• the intensity of each image element (voxel) on the DWI sequence reflects the degree of water diffusion ⇒ restricted diffusion depicted on DWI is a sensitive indicator of acute ischemia
  • the acute lesion with restricted diffusion is hyperintense on DWI and hypointense on the ADC map (the lower the value, the greater the restriction)
    
  • CT or conventional MRI sequences (T1, T2) may not reveal the lesion in the first hours after the stroke onset
    
• DWI is acquired within 2 minutes and is less susceptible to motion artifacts than other sequences
• diffusion defect is not stroke-specific; it was described in many other neurological disorders (see table below)
• always assess all MRI sequences (DWI, ADC map, T1, T2) and exclude artifacts (e.g., T2 shine through phenomenon)
• when assessing DWI images, consider the following:
  • location and shape of the lesion (does it correspond to a vascular territory?)
  • is the DWI lesion hyperintense diffusely, centrally, or peripherally?
  • does a bright signal on DWI correspond to a dark signal on the ADC map?
  • is the lesion isolated or multifocal (multiple territories x symmetric or even diffuse involvement?)
  • is edema present (hyperintense on T2/FLAIR)? – typically in the tumors and abscesses
  • does the lesion show post-contrast enhancement  (T1C+)?

DWI in acute stroke

• DWI visualizes the impaired (restricted) diffusion of water molecules (or protons) caused by the energetic failure of Na⁺/K⁺ membrane pumps
• it is highly sensitive and specific (88-100%) for detecting acute cerebral infarction within minutes of its onset, with a maximum of 4-6 hours
• acute ischemia appears hyperintense (bright) on DWI (b factor around 1000 s/mm²) and hypointense (dark) on calculated ADC (apparent diffusion coefficient) maps
  • ADC <620×10-3 mm/s is likely to identify ischemia
  • after recanalization, there is often an increase of signal on ADC maps in the ischemic area  (visible on DWI and FLAIR)    
• the extent of the DWI lesion in acute stroke corresponds approximately to the size of the probably irreversibly affected tissue
  • however, the reversibility of DWI changes (early DWI reversal) with early reperfusion has been repeatedly described
• DWI allows the differentiation of acute, subacute, and chronic lesions
  • initially bright DWI signal decreases within a few days to become hypointense in later stages
    
  • low signal on the ADC map increases in the subacute stage with temporal pseudo-normalization of the ADC map during the second week; in the chronic stage, ADC values remain increased
• DWI allows assessment of ischemic penumbra (DWI/FLAIR and DWI/PWI mismatch)
• DWI changes are not specific to ischemia – they can occur in any transport disorder (edema), such changes are often reversible, and lesions are not hypointense on the ADC map) – see the table above

• artifacts – DWI depends on the T2 signal
  • increased T2 signal can lead to T2 shine through and T2 washout
  • decreased T2 signal leads to the T2 blackout phenomenon (hypointense DWI)
    • e.g., iron deposition
    • bleeding
    • infections (abscess, toxoplasmosis, aspergillosis)
    • some metastases

DWI/FLAIR mismatch (DFM)

DWI/PWI mismatch

ADC pseudonormalization

Early DWI reversal

• it occurs in the early phase of stroke during successful reperfusion (within 3-6 hours after stroke onset) [Pham, 2015]
• actual regression is relatively rare; more common is DWI pseudonormalization  (fluctuation) or only partial regression

T2 shine through

T2 washout

T2 blackout

• the actual DWI signal (given by ADC values) is reduced by the presence of a low  (hypointense) T2 signal ⇒ DWI is hypointense (dark)
• the T2 blackout phenomenon is present in hematomas, degenerative diseases with iron accumulation, etc.