@JLeslie To my knowledge, there is no maximum dose. We operate by the principle of “As Low As Reasonably Achievable”. For each examination, there is a necessary minimum dose. If you are overweight, this dose increases exponentially. There is a threshold where if too great a dose is given it must be reported as a radiation accident, but that applies more to nuclear medicine than radiology.
Doctors get surprisingly little education on radiation, considering they are given licence to order imaging. In the US, it is estimated that around 20,000 people a year contract medically induced cancers (don’t quote me on that statistic, I can’t quite find my source). It is hard to get a figure of how many of these are preventable, but certainly this figure could be reduced with just a little more care. Of course many more lives are saved by imaging, so it is still worthwhile if intelligently requested.
No, I’m not American, I’m Australian. One of the things very wrong with the US health system is that some things aren’t regulated heavily enough. Some radiology practices in the US do whole body CT scans, just to see if something is wrong. Basically it is sponging off hypochondriacs, while giving them a 1.5–3% greater chance of contracting cancer during their lifetime. Such practices are illegal here. But again, we could always do more.