Transparent Mice

I'm currently taking a course on "Multi-Modality Molecular Imaging in Living Subjects" which covers MRI, CT, Optical, and PET imaging. The first topic was optical (fluorescence and bioluminescence) imaging which, in a way, is easiest. Certain molecules give off light, either when exposed to specific other molecules or when excited with light. You then look for this light to see where the molecule of interest is. The problem is, tissue isn't transparent to light. Red light gets through the easiest, which can be seen by shining a flashlight through your hand.