Incredibly common for computers attached to expensive pieces of hardware to use very old operating systems. Even in very high tech places with a lot of money to throw around.
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.
