Radiation in CT scans, x-ray/radiographies and PET scans (I): the scientific view

This is the first of a series of 3 posts describing:

1. The value and cost/harm of medical tests involving radiation, purely from a scientific perspective. This post.
2. Other factors affecting the prescription of these tests.
3. What to do if a doctor recommends such a test.

So, let's center this post around the scientific facts. The following table lists typical radiation levels for some of these imaging tests as well as other sources of radiation. Notice that these include mainly x-ray, CT, PET and variations of these. MRI and ultrasound do NOT use ionizing radiation and are considered, in principle, safe:

1. Natural background: 2.4mSv/year
2. Air travel crew: 3mSv/year 
3. Radiation worker federal limit: 50mSv/year
4. Dental radiography: 0.01mSv
5. Chest radiography: 0.1mSv
6. Mammography: 0.7mSv
7. PET/SPECT : 7mSv 
8. Chess CT: 8mSv
9. Pelvic/abdomen CT: 10mSv
10. CT Angiography: 15mSv
11. 50% of cases die in 30 days: 3Sv

Notice that:

1. Just by living, we do get a decent level of radiation (see #1)
2. That there is a very wide range between tests. For instance, a pelvic CT radiation is like 100 chest x-rays (this number is actually probably conservative).

Question comes on the relation between radiation dose and cancer. The following link can provide some guidance:
http://www.xrayrisk.com/calculator/calculator.php

Notice that:

1. Risk factors are extrapolations/guesses.
2. A ballpark is about 0.1% increase in cancer risk for a 10mSv exposure of a 30 year old person.
3. The younger the higher the risk.
4. Risk is additive. The more tests you get in your lifetime, the higher the cancer risk.

The actual dose of the test can vary even for the same test. For CT, to exactly compute the dose from a given test (once the test is done and the data of the test obtained from the radiologist), one can follow this example:
http://answers.google.com/answers/threadview/id/742075.html

So, the above would give us the "cost", the "risk" of performing such a test, but what are the benefits? In this context, we all have heard the expression "the benefits outweigh the risks". There are many conditions out there and talking about the benefits or the need of medical imaging would be plain impossible, but basically, one could say that the bigger the radiation the test carries, the better the image and the bigger the benefits, with a CT or a PET scan providing beautiful images and information. Notice that pure information does not imply need for the test. We may already know the reason for the disease (from some other diagnosis) and all what the test would do would be to confirm the diagnosis. Or we could take the path of treating the likely disease without 100% confirmation from a CT scan, and if that solves the problem, then just move on (saving with this approach the radiation and cancer risk). Ultimately, it is for the doctor to explain the benefits and alternatives of such test to the patient and for the patient to decide (see the 3rd post on this topic).

Let's move on now to other factors that may not be necessarily explained by the doctor and that do affect the prescription of imaging tests, at least in US.

Update (3/23/2013). I got across this alert from the Joint Commission  on this topic. May be useful reading...