We performed phase relaxation-time measurements of the accumulated photon echo for normal and cancerous tissue samples of human liver, that were stained with several kinds of dye. Each dye interacted selectively with a specific kind of biomolecule in the tissues, for example, DNA, proteins, or lipids. The cancerous tissues showed a tendency to have shorter relaxation times than the normal tissues had. The difference of relaxation time observed between the normal and the cancerous tissues depended on the dyes that were used for staining, and the difference became most clear when the tissues were stained with a cyanine dye derivative, i.e., YO-PRO3 Iodide, which has a high selectivity for DNA. We performed the relaxation-time mapping of the accumulated photon echo (photon-echo imaging) for a liver tissue sample stained by YO-PRO3 Iodide. The photon-echo imaging was successful in showing a region of a highly differentiated hepatocellular carcinoma in the tissue sample, while fluorescence-intensity mapping could not identify the cancerous region in the same tissue sample. These results indicated a possibility that the reduction of accumulated photon-echo relaxation time observed in the cancerous tissues was caused by a change of microscopic dynamics of DNA.
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