It seems more likely that brain tissue absorbs microwaves rather than reflect them back toward the source.
However perhaps bone tissue might reflect them.
Ultrasound reflects from internal tissue. It's a common medical procedure. Dolphins are able to emit ultrasound and detect the echo from another animal, thereby visualizing its internal organs, sick or injured condition, etc.
There is magnetic resonance imaging (MRI).
There is CAT scan although the description doesn't mention microwaves.