It appears to be the microwave based technology. The radio-activity you are thinking about (X-rays) would require a emitter on the other side of the subject.
This simply looks through the first couple layers of the subject. Depending upon the energy behind the waves, these are no more harmful than your cell phone, calculator, laptop, PC, FM radio or high voltage transmission lines that criss-cross our country delivering power to every home in the USA.
No, I based it on this from the excerpt: "the same backscatter x-ray scanner technology ..." and that doesn't sound like microwaves. I think you are thinking of the naughty-bits scanner for people and this is the technology for luggage and other inanimate objects. After all, we're not talking about a few layers of clothing but rather vehicle and truck bodies. I believe the backscatter sensing is what lets the emitter be in front and the receiver is reading reflections.