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1 2001

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Zastosowanie czujników światłowodowych w precyzyjnych zabiegach terapii laserowej

Gryz G., Podniesiński D., Zając A.

Metrology and Measurement Systems

2001

Vol. 8, nr 1

75-90

W pracy zaproponowano sposób wykorzystania światłowodowych czujników z modulacją strumienia promieniowania do pomiaru w czasie rzeczywistym wybranych parametrów optycznych tkanek biologicznych. Przedstawione zostały propozycje rozwiązań konstrukcyjnych oraz wyniki laboratoryjnych badań układów do pomiaru współczynnika absorpcji i zmian współczynnika rozproszenia wybranych tkanek biologicznych. Zaproponowano koncepcję układu do obiektywnej oceny zmian parametrów optycznych tkanki w trakcie zabiegu laserowego. proponowane rozwiązania charakteryzują się kompatybilnością z laserową aparaturą medyczną. Przedstawiono przykładowe aplikacje w medycznym sprzęcie laserowym wykorzystującym źródła promieniowania z zakresu widmowego 1000-3000 nm (lasery: neodymowy Nd:YAG [lambda] = 1060 nm, holmowy CTH:YAG [lambda] = 2080 nm,erbowy: Er:YAG [lambda] = 2,94 nm). Zaprezentowano wstępne wyniki badań laboratoryjnych prezentowanych układów pomiarowych. Określono zakres widmowy, w którym każdy z czujników może być zastosowany.

In this paper two methods of tissues optical parameters measuring which are important for laser therapy are presented. Both of them can be applied in real time measurement in typical medical laser equipment. Fiber sensors are applied in proposed solutions in order to estimate objectively the values of tissue parameter important in laser therapy and to measure them in real time. These very important for laser therapy tissues optical parameters are: - absorption coefficient, - scattering coefficient. Knowledge of these parameters allows us to determine the spatial energy distribution of laser radiation in tissues. To make this precisely we should know both this parameters. Because of individual difference in basic tissue parameters appearing in different organs and patients the current control of the tissue parameters in operation area is necessary at the moment of the operation. These requirements caused that we tried to elaborate the methods of the measurement of these parameters in real time. Taking into account the necessity of adjusting of the measurement procedures with clinical practice some problems have appeared which are as follows: - the construction of sensors should not cause any changes in using of the medical equipmenmt as well as in its handling, - the measurement of tissues optical parameters have to be realized in the operating area and sufficiently fast in order not to disturb the operator work, - the sensors have to be coupled with an operation tool - leading optical fiber which delivers the laser beam to the operation area, - measurement procedure has to be insensitive for manual operation. The conditions mentioned above can be easily fulfilled when the fiber sensors will be used to measure the both optical parameters. The scheme of sensor for the measurement of the absorption coefficient is shown in Fig. 2. In this solution the typical approach fiber sensor was used. Its technical parameters were adjusted to ensure enough sensitivity of the method for relative large distances between the sensor ans the tissue surface. The method of the measurement of the effective tissue absorption coefficient based on the detection of the moment of the surface damage threshold was described. An output signal detected in the fiber sensor is shown in Fig. 4. The results of selected tissue absorption coefficient measurements are presented in Table 3. The proposed method can be applied in connection with using of the laser sources strongly absorbed in tissues. Nowadays in clinical practice this is a group of NIR laser sources - mainly CTN:YAG lasers, Er:YAG lasers and other laser sources emitting in 2000-3000 nm spectral range. Taking into account the specific process of laser beam interaction with tissues in this spectral range the proposed method allows us to control precisely the depth of interaction region. Consequently, more precise operation is possible which is especially important in microsurgery. Direct measurement of the absolute value of the tissue scattering coefficient is very complicated and it seams to be impossible to accomplish such a method for the tissues being alive during the laser therapy. However applying fiber sensors shown in Fig. 5 it is possible to determine of variations of the scattering coefficient, which are caused by laser power during the laser beam interaction. Temporal changes of this parameter resulted from heating of the tissue are utilized in our approach. In temperature range from normal to the coagulation point the scattering coefficient increases a few times to achieve the maximum value for coagulated tissue. Further tissues temperature increasing does not influence on the value of the scattering coefficient and it remains constant. This effect is useful for objective diagnostics of the tissue coagulation point. Experimental verification of this method has been carried out and we obtained total confirmation of the idea presented in this paper. Some examples of the output signals from the sensor versus tissue temperature are presented in Fig. 8. The presented methods and the fiber sensors may be successfully applied for automatic control of the laser energetic parameters in laser therapy. The scheme of such a laser system utilizing the real time methods of measuring basic tissue parameters is suggested in Fig. 11.