T2 washout

T2 washout is a phenomenon encountered on diffusion weighted imaging (DWI) which results in DWI images (e.g. b = 1000) appearing normal despite abnormal ADC maps.

For the phenomenon to occur a particular combination of ADC and T2 signal intensity is required.

• increased T2 signal
• facilitated diffusion (high ADC values) 

Remembering that the signal of diffusion weighted images is the result of both diffusion and T2 signal, it becomes apparent that if the amount of expected T2 shine through from the increased T2 signal is balanced by elevated ADC values (which would result in lower DWI signal intensity), then DWI signal may appear normal .

The phenomenon is most commonly observed with vasogenic edema both surrounding tumors and during the subacute phase of ischemic stroke.

T2 washout should not be confused with related phenomena in ischemic stroke; early DWI reversal, seen acutely (typically 3-6 hours) and ADC pseudonormalization seen somewhat later (around 1 week post stroke) but before T2 washout, which occurs between 10-15 days typically.