Quantitative measurements of porphyrin pigments in tissues via photoacoustic spectroscopy

We investigated the possibility of using photoacoustic spectroscopy as an analytical technique for the quantitative measurement of injected porphyrins in tissues. Samples of liver excised from treated (i.e., injected 24 h in advance with 100 mg/kg Photofrin II) mice and control mice were lyophilized and reduced to powder; then, about 16-mg powder samples were compacted to equal volumes inside the photoacoustic cell. Amplitude and phase spectra were measured in the range 280-760 nm. From these data we computed the photoacoustic absorbance spectra; they were suitably normalized in order to account for differences in hemoglobin concentration among the livers of different mice. The absorbance difference spectra (treated minus control) were computed in the region of wavelengths above 450 nm, where porphyrins and hemoglobin exhibit major differences. Finally, by estimating the value of the thermal diffusion length of the powdered sample and those of the extinction coefficients of the most relevant Photofrin II components in the spectral region considered, we were able to evaluate the local drug concentration and determined a value (260 micrograms/g of wet tissue) that is in the range expected for the dose of Photofrin II injected.