From what I have read I'm sure MRI machines use liquid helium because the superconducting magnet needs to be very cold and liquid nitrogen isn't cold enough. LN

From what I have read I'm sure MRI machines use liquid helium because the superconducting magnet needs to be very cold and liquid nitrogen isn't cold enough. LN2 was used as a sort of buffer/insulator but has been replaced by cryocooler cooled dewar. Not that I know much more I just skimmed The Basics of MRI: http://www.cis.rit.edu/htbooks/mri/chap-9/chap-9.htm http://www.cis.rit.edu/htbooks/mri/chap-9/chap-9.htm