Um, if you're a brain in a vat, then any "brain" you perceive in the real world like on a "real world" MRI is nothing but a fictitious sensory perception that the vat is effectively tricking you into thinking is your brain. If you're a brain in a vat, you have nothing to tell you that what you perceive as your brain is actually really your brain. It may be hard to implement the brain in the vat scenario, but when implemented, its absolutely undetectable.