>The vast majority of MRI machines used today use superconducting magnets made from niobium-titanium (NbTi), which becomes superconducting at 9.2 degrees above absolute zero. This is well below the boiling point of any other coolant, making liquid helium the only practical option for cooling the magnets.

Well, this is part of it. The other issue is that the superconducting phase diagram has two limits: the transition temperature Tc and the upper critical magnetic field Hc. The magnetic field limit is generally highest at absolute zero and drops steeply with temperature. Even for the superconductors with Tc as high as 120 K the Hc at 20 K will be much less than the Hc at 4 K. So in order to make powerful superconducting magnets you need helium regardless of what superconductor you use, since nothing has broken this pattern.