There are many different types of medical imaging and they all pick up different things.
X-rays are like light rays, but they can pass through more stuff. Some of the x-ray's energy is blocked by bone, which is why you can see bones so clearly on x-ray scans.
In a recent study of almost 11 million young Australians, we showed those exposed to a CT scan before the age of 20 had a small increase in cancer risk in the years after exposure.
Reducing health-care waste relating to unnecessary tests has been a major priority for researchers, governments and health services for decades. But how do we change the behaviour of doctors?