ADC pseudonormalization

ADC pseudonormalization is a normal phase encountered in the subacute stage of ischemic stroke and represents an apparent return to normal healthy brain values on ADC maps which does not, however, represent true resolution of ischemic damage.

ADC pseudonormalization is seen typically around 1 week following ischemic stroke and is thought to be due to a combination of cell wall breakdown and increase of extracellular edema (both of which result in facilitated diffusion - increased ADC values) ². At some point, these processes, combined with residual true abnormal restricted diffusion due to cellular swelling, result in ADC values ostensibly returning to those of a normal, healthy brain. 

It is important to realize that at the point that ADC values normalize, DWI images (e.g. b = 1000 s/mm²) will continue to show increased signal due to T2 shine-through. It is only later (around 10-15 days) that DWI pseudonormalization occurs, when ADC values are higher than normal, offsetting the gradually waning T2 signal. 

Note: ADC pseudonormalization should not be confused with early DWI reversal (a.k.a. diffusion lesion reversal), which is seen early in the course of ischemic infarction, particularly in the setting of reperfusion therapy. Nor should it be confused with T2 washout seen a little later.