Diffusion-weighted imaging in acute ischemic stroke
Citation, DOI & article data
Diffusion-weighted imaging (DWI) is a commonly performed MRI sequence for the evaluation of acute ischemic stroke and is very sensitive in the detection of small and early infarcts. Conventional MRI sequences (T1WI, T2WI) may not demonstrate an infarct for 6 hours, and small infarcts may be hard to appreciate on CT for days, especially without the benefit of prior imaging.
DWI is used to denote the entire sequence performed to acquire and calculate b=0 and b=1000 images, and apparent diffusion coefficient (ADC) maps. DWI is often, if not universally, also used to denote b=1000 images. For the purpose of this article, DWI will be used interchangeably with b=1000.
Increased DWI signal in ischemic brain tissue is usually observed within a few minutes after arterial occlusion and is primarily due to cytotoxic edema resulting from a cascade that begins with depletion of ATP and failure of the sodium-potassium transmembrane pump. Imaging changes progress through a stereotypic sequence of DWI increase and ADC reduction due to initial intracellular shift of water (cytotoxic edema), replenishment of depleted extracellular fluid (ionic edema) and eventual apoptosis and/or liquefactive necrosis.
For a general discussion of the pathogenesis and radiographic features please refer to ischemic stroke.
Reported sensitivity ranges from 88-100% and specificity ranges from 86-100%. Only in a small minority of cases (~7%) imaging of patients with subsequently demonstrated ischemic strokes is diffusion-weighted imaging negative 6. This occurs in one of three contexts 6:
- posterior fossa infarcts (most common)
- small volume infarcts
- early imaging <6 hours
The appearance of DWI/ADC depends on the timing.
Acute (0-7 days)
- ADC value decreases with maximal signal reduction at 1 to 4 days
- marked hyperintensity on DWI (a combination of T2 and diffusion weighting), less hyperintensity on exponential images, and hypointensity on ADC images
- subsequently, the release of inflammatory mediators from ischemic brain tissue leads to vasogenic edema with extravasation of water molecules from blood vessels to expand the interstitial space, where water molecule diffusion is highly unrestricted
- early DWI reversal (a.k.a. diffusion lesion reversal) can occur, most frequently with reperfusion, but this rarely alters the size of the eventual infarct and is probably a 'pseudoreversal' 3-5.
Subacute (1-3 weeks)
ADC pseudonormalization occurs in the second week (7-15 days)
- ADC values rise and return to near baseline
- irreversible tissue necrosis is present despite normal ADC values
- DWI remains hyperintense due to T2 shine through
- after 2 weeks ADC values continue to rise above normal parenchyma and the region appears hyperintense 2
Chronic (>3 weeks)
- ADC values increase
- DWI signal low - despite T2 hyperintensity because of the overwhelmingly facilitated ADC values DWI signal decreases (T2 washout)
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