Here are some machines that perform this kind of scanning:
http://www.dspguide.com/xraysys.htm
and one in particular, the SECURE 1000
http://www.dspguide.com/secure.htm
The SECURE 2000 scans people with X-rays as they walk down a corridor
sorta like in the movie, "Total Recall."
The same website has a FREE book on Digital Signal Processing for Scientists and Engineers which you can download (if you like to read books with lots of equations) at:
http://www.dspguide.com/pdfbook.htm
--VM44