Hi Dear, An MRI is a series of detailed picture of your brain, as they were at the time the MRI was done. They are valid as long as no other changes occur. Basically, if somebody has an MRI today that is normal, and have a stroke or hernia tomorrow, obviously the result will change (You never know).
Undo an action. Press Ctrl+Z or click (Undo) in the ribbon until you've fixed your mistake. You can't undo some actions, such as clicking commands on the File tab or saving a file. If you can't undo an action, the Undo command is dimmed.
The simplest form of redo is flip-undo, in which using undo after undoing redoes the undone action. In this case, the program flips back and forth between two states when the undo button is pressed. In a more typical redo model, there are separate undo and redo buttons. The redo can be used for each undo action performed.