10:48 Nov 5 

I asked one of my friends who's working in the hospital, and here is his reply:

In general, washout in this case is "faded vision" as seen in MRI/Scan after a contrast is injected. To make it less complicated, if the radiologist wanted to see if there is a lesion in the person's liver, (bening or malignant) a contrast usually a gadolinium will be administered intravenously. In this case, the radiologist will able to see the lesion because there will be a rim to the center of the lesion ("peripheral washout" sign) after administration of contrast material.
MY friend suggested "冲蚀“； but I finally followed the original translation of "门静脉相的消退” （as I was proofing), so Shirley's answer is more close.

Do all of you have any further suggestion? Thank you so much!