Determination of photosensitizer concentration in biological tissues from diffuse reflectance and fluorescence

Abstract

We present a fiber-optic device for noninvasive determination of the optical properties of biological tissues and photosensitizer concentration. The device developed can be used in two modes: the mode for detection of the spatial distribution of diffuse reflectance, and the fluorescence mode. We tested the device in vitro on specimens simulating tissue and on whole blood, and also in vivo on rats. We have shown that with additional modification and adaptation, the device can be used for in vivo monitoring of parameters important for photodynamic therapy.Introduction. Currently photodynamic therapy (PDT) is intensively used in clinics for treatment of cancer and other diseases [1]. The efficacy of PDT depends on the concentration of the photosensitizer and oxygen, their distribution in the tissue, the optical properties of the tissue, and the exposure dose. In this case, considerable attention has been focused on development and improvement of methods for monitoring these parameters during PDT [2, 3]. Methods based on measurement of the spatial distribution of diffuse reflectance and fluorescence are the simplest and the most economical [4,5]. We should point out that fluorescence spectroscopy is applied only in determination of the concentration of fluorescent materials, while from the spatial distribution of diffuse reflectance we can obtain data on the optical properties of biological tissues and also can determine the concentration of fluorescent chromophores in the tissues. Unfortunately, the theoretical approach based on the diffusion approximation that is used in this method has constraints on the optical properties of the tissues, and is not always effective for real biological specimens. Consequently, both methods have disadvantages, and so for practical application it is optimal to combine them.The aim of this work was to develop a fiber-optic device which would allow us to combine the methods based on measurement of the spatial distribution of diffuse reflectance and fluorescence for determining the photosensitizer (PS) concentration and also the optical properties of biological tissues.Materials and Methods. Figure 1 shows a block diagram of the fiber-optic system developed for noninvasive study of the optical properties of biological tissues and the photosensitizer concentration. Semiconductor laser photodiodes (λ = 650 nm and 635 nm) are used as the radiation source. Both laser diodes are used to detect diffuse reflectance. A laser photodiode with λ = 635 nm is used to excite fluorescence. In the fluorescence measurement, the diffuse reflectance signal at the excitation wavelength is attenuated by filters. During the measurement, using an optical fiber of thickness 200 μm with numerical aperture 0.22, laser radiation with λ = 650 nm and 635 nm is alternately delivered to the specimen.A characteristic feature and one of the advantages of this instrument is the signal detection system, which consists of a coherent optical fiber of width 5 mm, a modulator (which is a disk with slots of width 300 μm, rot...

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