In the 63 page document linked on CNET, (as far as I saw) none of these scanners are currently slated to be on our northern border. The "estimated locations" indicate that there will be two in CA, two in AZ, two in NM, and and nine in TX.
Maybe they are looking at densities and not the image..With the car moving and the top down image, it's basically a three dimensional scan..Rutheford Backscatter Spectometry will tell you the density of something if you know the thickness of whatever you're shooting the alpha particles through..I dont' know if that's true when you're shooting them through the mishmash of materials in a car- I don't know how that works if they are going through a steel door and through explosives and back out a steel door again...I failed my second semester of AP Chem
Edit and disclaimer: I most likely am talking completely out of my ass. I bombed the nuclear chemistry section on my final too. Literally the only thing I am certain of is that backscatter spectometry will tell you the density of something if you know the thickness..I do not know how applicable that is.