Image quality.

Along with the other reasons already mentioned here, some TSA reps stated to Rep. Chaffetz when grilled on Capital Hill, that they buy both to keep the prices down. Those TSA reps and their 4th grade science kit have determined both machines are safe, so, they're giving some people cancer to keep the price down. That's the TSA for you.

The most central problem of all these techniques is that they are an inappropriate methodology for the task at hand (detecting explosives). They cannot distinguish an underwear bomb from an adult diaper, or a tampon from a hidden pack of prohibited material. They will, inevitably, produce millions of false positive results, and the passengers privacy will be violated to "resolve" these alarms. On the other hand, ETD is safe, non-invasive and actually does detect explosives.

^^^^

Except that as currently implemented, ETD swabbing produces untold numbers of false alarms on passengers who then must have their crotches rubbed. As far as we know, thousands and thousands have alarmed and been subjected to this treatment with NO incident of anyone being actually found carrying explosives. Even if there was one incident of success, such poor results argue against any reasonability of this test.

The currently used ETD equipment/protocol obviously has problems, and these should be addressed. Glycerin and nitroglycerin, for example, are very distinct molecules from a physico-chemical standpoint, and there are straightforward spectroscopic approaches to distinguish them. But the truth is the methodological approach of ETD is correct, and therefore there is a solution to the false positive problem of ETDs.

Whole body imaging, on the other hand, is not "fixable", because it is the incorrect methodological approach.

It sounds like (not surprisingly) TSA is doing it wrong. I get hit with the "random" ETD in Australian airports (CBR almost every time :rolleyes:, SYD domestic occasionally and SYD international once) and have never alarmed. Nor have I ever seen anyone alarm it in Australian airports.
+1000

agree. but the burden is on TSA to fix their d*mn implementation BEFORE subjecting people to whatever crotch/b**b massages they do in the private hut. If false positives were rare, well, nobody expects perfection in this life. But AS CURRENTLY implemented, these tests are wrong ALL the time or very close to it.

Now there's a phrase you don't want to see appear in a discussion of TSA's screening...

To make matters worse, Nitroglycerin is a ligitamate hart medication that many older people use on a daily basis.