"I need a Magnetic Resonance Image of my knee"

OR

"I need to have an MRI on my knee."

In the United States, we customarily use the initialism, MRI. Many people probably don't even know that it stands for "magnetic resonance imaging." I would say "I need to have an MRI of my knee."

I use an rather than a because "MRI," when spoken aloud, begins with a vowel sound.

I can't explain why I say "of my knee" when Richard suggests "on my knee." Both are possible.