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The progression of disease is certainly accompanied by biochemical changes. Since early detection promises a greater efficacy for therapeutic intervention, non-invasive biomedical optics may offer the opportunity for detecting biochemical changes thereby improving prognosis by providing early diagnosis. Magnetic resonance imaging (MRI) is an example of a modality that has successfully monitored the relaxation of spin states of paramagnetic nuclei in order to provide biomedical imaging and biochemical spectroscopy of tissues. In this paper, we discuss an optical analog of MRI, called fluorescence lifetime imaging. instead of monitoring the relaxation of a spin state, lifetime imaging depends upon monitoring the relaxation of an electronically activated state which is brought about by the absorption of a photon. Figure 1 is the Jablonski diagram outlining the electronic transitions which occur following the absorption of light. Relaxation from the activated state to the ground state can occur via either non-radiative or fluorescent decay, depending upon the local environment. The mean time between the events of the absorption of an excitation photon and the radiative relaxation process which produces a fluorescent photon is known as the lifetime, (tau) , of the activated state. Typically, endogenous fluorophores have lifetimes on the order of nanoseconds while exogenous compounds have lifetimes ranging from sub nanosecond to tens of nanoseconds. If the spin state of the electron in the activated state is 'flipped' (referred to as a 'intersystem crossing') then radiative relaxation requires extra time since paired electrons of the same spin are forbidden. Consequently, the resulting phosphorescence lifetime is on the order of milliseconds. The lifetime of the activated probe is dependent on its environment much the same as the time for relaxation of the spin states of paramagnetic nuclei in MRI and NMR depends upon local environment. There are two mechanisms responsible for reducing the lifetime of an activated probe: (1) energy transfer from the activated state to a donor molecule(s), and (2) collisional quenching of the activated state. For an activated probe experiencing collisional quenching, the measured lifetime, (tau) , can be used to determine metabolite or quencher concentration [Q], from the empirical Stern-Volmer relationship. Consequently, a map of lifetime provides a map of metabolite or quencher concentration. In this paper, we report computational experiments which point to the feasibility of the optical analog to MRI: fluorescence lifetime imaging in tissues and other scattering media.
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