I did a search of the IEEE, alot of action in this arena. I found 14 papers on the subject, here is a title and abstract from one of them. Earliest work is 1992 and 1996, but alot more activity starting in 2001.
Here is one;
Three-dimensional millimeter-wave imaging for concealed weapon detection
Sheen, D.M.; McMakin, D.L.; Hall, T.E.;
Microwave Theory and Techniques, IEEE Transactions on
Volume 49, Issue 9, Sept. 2001 Page(s):1581 - 1592
Digital Object Identifier 10.1109/22.942570
Summary: Millimeter-wave imaging techniques and systems have been developed at the Pacific Northwest National Laboratory (PNNL), Richland, WA, for the detection of concealed weapons and contraband at airports and other secure locations. These techniques were .....