Fluorescence optical tomography

Two one-speed radiation transport equations coupled by a dynamic equation for the distribution of fluorophore electronic states were used to model the migration of excitation photons and emitted fluorescent photons. The conditions for producing appreciable levels of the fluorophore in the excited state were studied, and we concluded that under the conditions applicable to tissue imaging, minimal saturation occurs. This simplified the derivation of the frequency response for a time-harmonic excitation source and of the imaging operator. Several factors known to influence the fluorescence response have been examined. Among these are the concentration, mean lifetime, and quantum yield of the fluorophore, and the modulation frequency of the excitatory source. The fluorescence source strength was calculated as a function of the mean lifetime and modulation frequencies in the 50-200 MHz range. The dependence of demodulation of the fluorescent signal on the above factors was also examined. Results showed that demodulation increases at longer lifetimes and higher modulation frequencies. In additional studies, tomographic imaging operators based on transport theory were derived for imaging fluorophore concentrations embedded in a highly scattering medium. Experimental data were collected by irradiating a cylindrical phantom containing one or two fluorophore-filled balloons with CW laser light. The reconstruction results show that good quality images can be obtained, with embedded objects accurately located.