In order to precisely recover fluorescence lifetimes from bulk tissues, one needs to employ complex light propagation models (e.g., the radiative transfer equation or a simpler yet consistent approximation, the diffusion equation) requiring knowledge of the tissue optical properties. This can be computationally expensive and therefore not practical in many applications. We present a novel method to estimate the fluorescence lifetimes of multiple fluorophores embedded in mice. By assuming that the photon diffusion does not significantly change the fluorescence decay slope, the light propagation is simply modeled as a time-delay during lifetime estimation. Applications of this approach are demonstrated by simulation, phantom data, and in vivo experiments.

In this paper, nebulized or intravenous cetuximab (also known as Erbitux) labeled with NIR dyes is administered in the lungs of the mouse and imaged using a time-domain fluorescence imaging system (Optix ® ). Time resolved measurements provide lifetime of the fluorescent probes. In addition, through time-of-flight information contained in the data, one can also assess probe localization and concentration distribution quantitatively. Results shown include suppression of tissue autofluorescence by lifetime gating and recovery of targeted and non-targeted distributions of cetuximab labeled with the NIR fluorophores.