Radiation Exposure - How much is too much?

I was wondering if anyone tries to keep track of all their radiation exposure? One of the things that bothers me, is that there is no central "person" or "doctor" or anyone really keeping track of how much radiation I'm being exposed to. Each doctor makes a decision to perform a CT or MRI in isolation, without having information at hand about how many other radioactive tests I've had done. I counted them up, and in the past 18 months I've had 8 CT scans (most of those were multiple parts of the body such as chest/abdomen/pelvis so this is really 24 CTs), 3 MRIs, 3 Xrays. I also had a kidney function test, which I think was a very high dose of radiation, since it was something like a movie being made using a CT scanner like device (or it might have been an xray like machine).

The second thing I'm wondering, is is it total number of radiation that you receive that is bad, or is it radiation in a specific time frame. If they are spread out over a long period of time, does that help? Worse if squished into a small time frame?

Thoughts?

My tests were spread over 4-5 different hospitals and emergency rooms, and that's one of the things that worries me, that a doctor at each institution might not be aware I've had all those others done.

Todd