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We have examined the temporal characteristics of the optical properties of blood undergoing laser-induced photocoagulation during long pulse (10 ms) 532-nm irradiation. The time-domain optical properties were probed at 532 nm, 594 nm, 633 nm and 1064 nm using a newly developed pump-probe technique in a double integrating sphere apparatus. During the 10 ms illumination period, blood evolves from liquid to a liquid blood-coagulum mixture to a system at the liquid/vapor transition in an essentially adiabatic manner. As with previous studies, a sharp rise in the 532 nm signal remitted from the sample can be linked to the onset of coagulation and a concomitant increase in scattering caused by microscopic coagulum particles. We also observe a subsequent decay in this remittance and, at sufficiently high radiant exposures, acoustic and visual transients indicating the onset of microvaporization. Probing the sample at the other wavelengths, we show that the optical properties of the system display highly complex behavior in multiple time frames. We believe that this rich behavior results from the interplay of: i) a time/temperature-dependent red-shift in the absorption spectrum of the oxy-hemoglobin chromophore, ii) coagulation dynamics occurring on at least two distinct time and length scales, and, iii) the creation of at least one new chemical species possessing a different absorption spectrum to that of oxy-hemoglobin. The thermal properties of the system were measured in a time- and spatially-resolved manner using a newly developed technique, and modeled using finite-element analysis incorporating the effects of time-dependent changes in the absorption coefficients of the blood, and phase changes representing coagulation and the liquid/vapor transition. Cross-correlating the optical and thermal studies, we show that the temporal properties of the 532 nm and 633 nm remittance signals can potentially be used to develop a sensitive real-time probe of the onset of coagulation, which in turn will lead to accurate dosimetry during clinical procedures. We also show that the three features of the coagulation highlighted above have profound implications for the design of lasers for vascular therapeutic applications.
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