[ars]

You could solve that by scanning across the skin, and immersing the side of the head and the detector in some kind of fluid that helps light transmission (avoids reflections on the skin), so you don't actually have to touch the skin.

[metalman]

inserting a medical light bulb in the pattients mouth will decrease the distance the light has to travel, and presumably it could be a high intensity "flash" tuned to conditions with very precisely known timing

[mpreda]

In the nose as well.

[saltcured]

Suddenly, the idea of aliens sticking probes into various orifices takes on a new light...