Wonder how much control the computer has over the machine. MRIs are heavy beasts. An uncontrolled quenching or erratic or unexpected protocol could injure or even kill.
An air-gap might indeed be inconvenient, but it doesn't need full network access just to send images. It can send the images to an up-to-date system over a "data diode" network.
How hard would it be to put a supported windows on the network right next to the XP box and transfer files via flash drives?