While many brain scans of musicians have been done in the research lab, it is virtually impossible to scan the brain of a musician on stage during performance. But Charles Limb has come closer than anyone else to replicating an actual performance situation in the lab.
Limb is a hearing specialist and surgeon at Johns Hopkins who also studies the way the brain creates. For the past several years he has been watching jazz piano players in an fMRI scanner to see what parts of the brain are involved as they improvise. This has led to further study of rappers in a scanner, all of which you can watch in this wonderful TEDx talk from 2010. (Limb continues to study jazz, rap and creativity and no doubt more insights will be coming in the future.)