In addition to the radiation, which can also induce cancer, you don't want to go scanning for things without a good reason. The false positive rates would increase a lot and you'd end up worrying a bunch of people for no good reason. Not to mention, you'd then have to have secondary biopsies or scans for these people, and those are not without risk.
Let's say that you had a low-radiation source imaging device that only had a 0.01% chance of causing cancer (I don't know the actual rates, so this is just a hypothetical). If you only get the scans when needed, you might only have 10,000 people getting the scan in a year. That means you'd have roughly 1 radiation induced cancer, and that would be in a patient that was already undergoing some form of treatment. Now let's say that you open up the criteria for who can get a scan. So instead of the 10,000, you now have 1,000,000 people. Now you have 100 people that will get a radiation induced cancer. While each of them has a low risk, the risk for the population is still too high to warrant unrestricted testing.
MRI's are a different beast, but you still have the risks of worrying a bunch of people that may not have anything wrong with them. And then there are the costs involved - both in terms of $$$ and time. For each healthy person getting a prophylactic MRI, a sick person would have to wait.
Let's say that you had a low-radiation source imaging device that only had a 0.01% chance of causing cancer (I don't know the actual rates, so this is just a hypothetical). If you only get the scans when needed, you might only have 10,000 people getting the scan in a year. That means you'd have roughly 1 radiation induced cancer, and that would be in a patient that was already undergoing some form of treatment. Now let's say that you open up the criteria for who can get a scan. So instead of the 10,000, you now have 1,000,000 people. Now you have 100 people that will get a radiation induced cancer. While each of them has a low risk, the risk for the population is still too high to warrant unrestricted testing.
MRI's are a different beast, but you still have the risks of worrying a bunch of people that may not have anything wrong with them. And then there are the costs involved - both in terms of $$$ and time. For each healthy person getting a prophylactic MRI, a sick person would have to wait.