Abstract

Tissue fluorescence, whether from endogenous of exogenous probes, provides an opportunity for interrogation on the basis of both structure and function. However, to date there has been little understanding of the localization of the origin of the signal for fluorescent and phosphorescent signals re-emitted from tissues. In this paper present finite-element, probability, and preliminary experimental frequency-domain measurements that show that the origin of re-emitted signals depends on the lifetime of the optical probes. For example, our results show that the phosphorescent signal resulting from systemic administration of a long-lived porphyrin compound may arise from the tissue-air interface rather than from deep within tissues. In addition, the signal arises predominantly where the product of the excitation fluence and the optical-probe concentration is the highest. Since the excitation fluence is highest at the tissue-air interface in reflectance (or endoscopic) measurements, the signal will predominantly arise from this area. This restricts the use of long-lived optical probes as contrast agent-in biomedical optical imaging. When fluorescent or phosphorescent re-emission is employed as a means to locate a diseased tissue by means of noninvasive optical measurements, the apparent location of the tissue volume will depend not only on the differences between the optical probe concentration in the diseased volume and the surrounding tissues, but also on the lifetime. In summary, the short-lived optical probes are desirable for fluorescence spectroscopy and as contrast agents for biomedical optical imaging in tissues.

© 1995 Optical Society of America