Do you really expect "appetite center" to lit up when shown logo of nappies?
Right now it is simply Pavlov's dog - sime images are associated with food, some don't. Compare logos of providers of "non-healthy" food to some healthy food - vegetables, fruits - then you'll have something to talk about, but I bet you'll find no difference.

First, this frequency range is absorbed by water very, very effectively (fractions of a milimiter is enough to shut it out completely) and man is mainly water. So it can see just under the skin. Then - resolution. 390 GHz has wavelength of about a millimeter - not much to see in a human tooth. At last - power. 2 microwatts is not much, it is not much at all. There is no very sensitive detectors without cryogenic cooling, so ...
We have some time to figure out is terahertz radiation really safe.