I bring up background just to point out that radiation is very natural and our bodies evolved in a radioactive world and have repair mechanisms that appear to be totally capable of handling low doses relevant to these machines. I can see that that's confusing to a degree.

I may be mistaken, I believe you're confusing milli-Sieverts (mSv) with micro-Sieverts (μSv). The machines are spec'd at 0.25 μSv/scan, or 0.00025 mSv.