My mom had an interesting experience in an x-ray backscatter machine a couple weeks ago.

She has a lot of scar tissue UNDER the skin of her belly from decades of insulin injections. (no visible scars)

The backscatter radiation penetrated below the level of her skin and reflected off those subdermal scars, setting off an alert on my mom. She got a pat-down of her torso before they would let her through.

There's a bit more penetrating radiation in that procedure than we might think.