Can I track a biological process in a living animal, non-invasively?

Following a biological process inside a living, intact animal without repeated invasive sampling.

References

• Contag, C.H., Spilman, S.D., Contag, P.R., Oshiro, M., Eames, B., Dennery, P., Stevenson, D.K., Benaron, D.A. "Visualizing Gene Expression in Living Mammals Using a Bioluminescent Reporter." Photochemistry and Photobiology 66, no. 4 (1997): 523–531.