Scientists love to to study the brain functions of people while they are performing various tasks, such as playing a musical instrument. Considering the configurations of fMRI scanners, this is not so easy to do. But a team from the Schulich School of Music at McGill University in Montreal have designed a special cello that can be played inside a scanner! It's small, without the wide body of traditional cellos, and it has no metal parts, which are forbidden in a highly-magnetic scanner.
“An opto-acoustic cello has been designed to investigate the neural correlates of cello performance using functional magnetic resonance imaging (MRI). Through the design of specialized optical sensors, for the first time, we are able to synchronously capture a cellist’s acoustic performance and musical gestures within the MRI scanner.”
The designers released a paper on the project. No, they haven't done any scanning of cello players yet; the paper is about designing the instrument. But those studies are possible now. Read more about the instrument at Improbable Rsearch.
I think in general, a lot of MRI related equipment is avoids any metal when possible to make it clear it is safe anyway. Having worked around strong magnets before (not MRI machines though), I've seen a few minor accidents and problems involving stuff that "should" have been safe, but was not properly tested beforehand.