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The TSA systems (at least the backscatter X-ray) do use a focussed moving beam to build up an image. They rely on the continual movement of the beam to limit the dose.

There are a number of safety systems to prevent the machine leaving the beam on should the mechanism stick.

And of course they are operated by the finest trained experts with a comprehensive background in nuclear physics and the ability to react in the fraction of a second before a notifiable dose was delivered