|III. Histopathologic correlation
- Cellular swelling (cytotoxic edema) with resultant restricted
diffusion is the first histologic change to be observed.
- Subsequently in the course of stroke evolution, the cytotoxic edema is
replaced by vasogenic edema (interstitial edema). Inflammatory cells
start to clean up the damaged cells and replace the lost neuronal
tissue with gliosis.
Cytotoxic and interstitial edema
Diffusion Weighted MRI and Conventional MRI
prediction of histopathologic changes
- DWI (Diffusion Weighted Imaging) and ADC (Apparent
Diffusion Coefficient) MRI sequences are very sensitive to diffusion
restriction and cytotoxic edema.
- Diffusion restriction is bright on DWI and dark on ADC map.
- DWI signal intensity is calculated from the following formula:
- This equation indicates that DWI signal has T2 contrast as part of the
- ADC map is a more accurate image to predict the diffusion restriction
as it represents the distribution of ADC throughout the brain without
the influence of T2 signal.
Evolution of infarction based on DWI, ADC map
- Changes on diffusion-weighted images and ADC map become apparent
within half to one hour of ictus.
- DWI SI peaks in 3-4 days while ADC map signal is lowest in 2-3
- Lovblad et al reported an 88% stroke detection sensitivity
using DWI within 6 hours. This contradicts the prior reported
sensitivity which approached 100%.
- Normalization of ADC signal by visual inspection after infarction
indicates an approximately 10 day old infarct with 88% sensitivity and
- The higher the SI is on DWI and the lower the intensity is on ADC in
hyperacute stroke (in the first few hours), the more severe the
- The majority of stroke lesions increase in volume on DWI with the
maximum volume achieved at 2–3 days.
Reversibility of diffusion restriction signal on DWI and ADC map
The initial reversible cytotoxic edema is a very brief period. There are
very few cases mentioned in the literature describing a positive DWI
stroke in which the DWI signal reversed to normal following thrombolytic
Pitfalls of Diffusion Restriction Imaging:
- False negative DWI/ ADC
Ranges between 2-11%. This is related to early scanning during which there
is hemodynamic derangement, but cytotoxic edema has not yet developed.
These patients are the ones which will benefit most from early
thrombolytic treatment. In these situations PWI is more accurate than
- False positive DWI/ ADC
This is related to causes of diffusion restriction other than vascular
disease and ischemia, including viral encephalitis in which intracellular
viruses cause cellular derangement and cytotoxic edema.
Conventional T2 and FLAIR (Fluid Attenuation Inversion Recovery)
These sequences are very sensitive to free extracellular (ECF) water.
They are excellent for imaging interstitial (vasogenic) edema and will
depict any hemorrhagic changes in the infarction, along with T1 sequence.
The vasogenic edema gradually progresses after the cytotoxic edema with
associated increase in T2 signal.
Lesions with SI FLAIR ratio relative to contralateral symmetrical area
of less than 1.37 have been found to be 36 hours old or less,
with 91% sensitivity and 83% specificity.