It's free if you sign up for the appropriate study at the National Institute of Health in Bethesda, Maryland.
Search engine of studies
Actually, I'm sorry. It's not free. They pay you to come in and do their study(typically 70-80bux per hour).
I work there and we have to ensure that the part of you we want to look at is "normal".
Thus there is extensive medical testing done on each of our subjects to make sure they are okay.
Only then can we use their data.
I've been paid to do brain MRI's of all types, knee MRI's, almost every blood test under the sun...
And I dont even get the chance to do as many as I would like.