Oxygen sensing for in-vivo imaging by photoacoustic lifetime Probing

We have developed a new method to perform local measurements of fluorophore excited state lifetimes in turbid media without collecting the fluorescence emission. The method is based on a double pulse illumination where a first laser pulse excites the dye and then a second laser is used for photoacoustic probing of the transient absorption. The photoacoustic response generated by the probe pulse is recorded by an ultrasound receiver. Varying the time delay between excitation and probing allows for tracking the relaxation dynamics of the excited state. The method was validated by measuring the lifetime of an oxygen sensitive dye (Pt(II) octaethylporphine) solution at different concentrations of dissolved oxygen. The dye was excited with a 532 nm pulsed laser and the transient absorption at 740 nm was probed using a second pulsed laser system. The photoacoustics based results coincide with those obtained from simultaneous time-resolved fluorescent measurements. The method can be extended to photoacoustic lifetime imaging by using a receiver array instead of a single receiver. This opens unique possibilities for non-invasive, clinical functional imaging. For example, combined with oxygen sensitive dye, 3D imaging of tissue oxygenation could be developed for accurate diagnosis of cancer tumors, better planning of radiation therapy, and monitoring efficacy of treatment. Other potential applications include: in-vivo mapping of ion (e.g. Ca) concentration and dynamics and imaging of enzymes activity and metabolic functions, as well as environmental studies in turbid media.