There's something in rhetoric that we call the principal of charity. When arguing with someone, we try to figure out what was meant based on what was said, and arguing against our best guess at what someone's best argument is instead of assuming that everyone always means what they say and never makes a mistake in communication.

It is possible that the person you replied to does not understand the difference between Cat, Pet, and fMRI. This is in fact a very easy mistake to make for anyone who doesn't work in medicine. So reconsider their comment with that in mind. If we substitute CT scan for fMRI, does that change your response?