  Light propagation through diffusive media can be described by the diffusion
equation in a space-time domain. Further, fluorescence can be described by a
system of coupled diffusion equations. This paper analyzes time-domain
measurements, which measure the temporal point-spread function (TPSF), at a
boundary of such diffusive media with a given source and detector. We focus on
the temporal position of the TPSF maximum, which we refer to as the peak time.
Although some unique properties of solutions of this system have been
numerically studied, we give a mathematical analysis of peak time, providing
proof of the existence, uniqueness, and the explicit expression of the peak
time. We clearly show the relationship between the peak time and the object
position in a medium.
