Yep they are xray based. Your point is valid, and everyone should opt out. May cause the check to be longer since the xray line goes so fast, but isnt that why we get there 2hrs early? No nude pictures just a picture of your skeleton and a huge dose of xray energy to your body. They produce false positives as well. They are mainly looking for objects that are not metal based. E.g. Plastic knife. Bag of sugar. Choking harards. Besides the opt out they pat you down with a paper strip and run it through a machine to determine that you dont have explosive chemicals on you. I know this because i went on a date with a chinese national who was getting her Ph.D from SDSU and worked on those machines. She said she was an only child and was told by dad to do this. She wanted to be a lawyer. I got pulled aside once for having a ceramic plate in my carry on.