There are "small animal" MRI machines, which are mostly intended for research. The bores are really small: 55mm or so is not uncommon, so a mouse would fit, bu

There are "small animal" MRI machines, which are mostly intended for research. The bores are really small: 55mm or so is not uncommon, so a mouse would fit, but nothing much bigger. Other animals are usually scanned on a machine meant for humans (sometimes even the exact same ones, very early in the morning or late at night).