I don't think Mythbusters had the budget to deal with a possible result of "bullet gets sucked directly into the insanely expensive machinery, destroying it"
Plus the danger of fluid helium escaping within seconds.
I suspect "MRI-strength electromagnet" falls into a different budget category than "medical grade and certified imaging system", including but not limited to "MRI removed from hospital and junked because it's no longer maintained / useful / accurate enough". They are pretty resourceful