Fruit and vegetables are pretty popular test objects: they sit nice and still, they have interesting structures (i.e., look cool), and they avoid ethics/safety

Fruit and vegetables are pretty popular test objects: they sit nice and still, they have interesting structures (i.e., look cool), and they avoid ethics/safety issues involved in scanning humans or animals. "Phantoms", test devices with specific and well-known properties, are used for more careful testing but produce works pretty well for a quick sanity check.