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Multipixel assessment of fluorescence uptake and lifetime in the detection of heterogeneous tissue volumes

The objective of this work is to identify an imaging modality which can best locate heterogeneous tissue volumes when designer contrast agents are used. We used a multi- pixel, homodyne, frequency-domain photon migration detection system to acquire images of 830 nm fluorescent heterogeneities immersed within a tissue-simulating phantom that contained 0.5 percent Intralipid solution. An expanded beam of 25 mW, 778 nm light modulated at 100 MHz illuminated the phantom surface. Specifically, we monitor fluorescence average intensity, modulation amplitude, phase, and modulation ratio resulting from micromolar concentrations of indocyanine green and DTTCI embedded within tissue- mimicking, highly scattering media. The results indicate that under conditions of perfect uptake, only phase and modulation distinguish dye solutions that possess equivalent fluorescence yield but unequal lifetime when both heterogeneities are located 0.5 cm from the illumination surface. Enhanced phase contrast was observed for fluorescent solutions with short lifetimes located within a surrounding of longer lived fluorophore and visa versa. These results have important implications for the development of contrast agents whose lifetimes depend on the local biochemical environment.