The lone women would diddle it up until they reached "the pre-orgasmic stage," at which point they would inform the scientists over an intercom (which while not in use for communication was presumably feeding in an endless stream of R. Kelly songs, such as "My Pony," "Bump and Grind" and "Gotham City"). After some pre-orgasmic images were taken, the women would resume diddling until climax, which the MRI machine would also record.
The researchers were not the kind of people who thought that this situation would make maintaining an erection difficult, and as such "did not foresee" that sex in a sterile metal tube surrounded by a makeshift curtain and a room full of scientists would be a major boner slayer for nearly every male involved. There was only one guy who had no problems, and the scientists chalked this up to his "artistic commitment" to the project; he and his partner were both a) involved in and dedicated to the experiment from day one and b) amateur street acrobats and therefore "trained and used to performing under stress." So if nothing else, we already know that this experiment will answer most unanswered questions in the field of Amateur Street Gymnasts Who Like to Fuck in Metal Tubes.
Truly an underappreciated field of study.
The experiment's purpose is only ever vaguely described, and never really goes beyond "We want to see what it looks like when we make people have sex in an MRI machine." Their findings are always along the lines of "People don't seem to like having sex in MRI machines, except, again, unlicensed street acrobats." As stated in this video (which begins with the most amazing non sequitur in the history of the world), the experiment was of interest to specialists and "laypersons with an interest in reproductive anatomy" (read: the Internet).