The Transportation Security Administration was under pressure to establish rules regarding airport body scanners, and well, the Department of Homeland Security's weighed in. Now the Advanced Imaging Technologies (AIT) using Automatic Target Recognition (ATR) will be mandatory in certain cases. Slashgear notes that prior to this the scanners were opt-in, and one could go through a contactless, non-imaging scan instead. That option will exist, but security agents can insist on mandatory screening "for some passengers." The argument the DHS gives (PDF) is that these scanners are more capable of detecting prohibited, non-metallic items that could be hidden under a few layers of clothing than a metal detector wand would be.
Of course, there are lingering privacy concerns over previous clothing-penetrating scanners. The Department writes that the new ATR system replaces photographs of the person in the machine with a generic body, indicating where possible foreign objects are on the image. If any appear, that's when a further physical search will be conducted. Automatic Target Recognition apparently can't store any images and the DHS says that the AIT gear "does not generate or retain an underlying image of the individual." Moreover, the physical scans will only target the area on a person's body where it's indicated the contraband is. If there are multiple alerts, a full screening might take place. So that's settled at least.
Compared to so many other government documents, this one is actually pretty short and readable. If you want to take advantage of that, hit the source link below.