Technique for measuring laser radiation intensity in biological tissues

AbstractLaser methods, such as laser-induced fluorescence diagnostics, photodynamic therapy (PDT), and hyperthermia, are finding increasing use in medicine. Irradiation can be performed both with and without contact on the tissue surface. In the case of contact irradiation, especially when laser radiation is introduced into biological tissue through an optical fiber, it is important to know the processes taking place at the irradiation fiber end. These processes affect diffusely reflected radiation which returns to the fiber. By analyzing backscattered radiation, we can evaluate the quality of the radiation procedure and the state of the fiber end. The objectives of this study were to develop a method and device for measuring backscattered radiation power and using this method, to determine the time and temperature ranges realized in PDT and hyperthermia.Light propagation is discussed in bent optical fibers. A technique is proposed for measuring laser radiation intensity in the optical fiber bend. Based on this technique, a system was developed for monitoring the laser radiation dose absorbed in biological tissues. We studied samples of bovine liver, muscle and brain tissues. Experiments were performed using a 675 nm, 100–2200 mW continuous wave semi-conductor laser. Laser radiation was delivered through a silica/polymer optical fiber. Data concerning the temperature and transmitted radiation intensity was acquired.Modeling of the light propagation in a bent optical fiber showed that the sensitivity of the method depends on the position of the photodetectors, but is independent of the loop number of the optical fiber. The results of experiments are presented using different types of biological tissues. We obtained the experimental dependencies of backward and transmitted radiation intensities and the temperature of the tissue surface in the irradiated region on the irradiation time measured with a flat-end fiber. The characteristic ranges of tissue heating caused by irradiation were determined for use in clinical practice.The optical parameters of biological tissues change with increasing temperature. This affects the intensity of transmitting radiation and diffuse radiation entering the fiber. The change in the backscattered radiation intensity greatly depend on the temperature of the irradiated area. The control of the irradiation of biological objects provides an efficient delivery of laser radiation to biological tissues and increases hyperthermia and PDT treatment effect.