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 issue with the sudo command is that it lets you delete files that are normally protected and there are some file that if you delete, your whole computer won't work. Instructions form apple.com sites are pretty trusted. Hackers aren't really the worry with the sudo command.