Background and objective: This in vivo study examines the validity of using fluorescence measurements of laser-induced release of temperature-sensitive, liposome-encapsulated dye for real-time monitoring of temperature and for prediction of tissue thermal damage.
Study design/materials and methods: An in vivo study is performed in rat liver after i.v. injection of liposomes loaded with a fluorescent dye and i.v. injection of indocyanine green (ICG) for diode laser potentiation. Temperature-sensitive liposomes (DSPC: Di-Stearoyl-Phosphatidyl-Choline) are loaded with 5,6-carboxyfluorescein (5,6-CF). These liposomes (1.5 ml solution) and ICG (1.5 ml solution-5mg/kg) are injected in adult male wistar rats. Two hours later, the liver is exposed and irradiated with a 0.8 W diode laser using pulses lasting from 1-6s (fluence ranging from 16-98 J/cm2). Simultaneously, the fluorescence emission is analysed with an ultrahigh sensitivity intensified camera.
Results: The fluorescence intensity I(F) increases linearly from 18 J/cm2 up to 75 J/cm2. These fluences correspond to surface temperatures between 42 degrees C and 65 degrees C. The measurements appear to be highly reproducible. In this temperature range, the accuracy is +/- 3 degrees C. The maximum intensity is observed immediately after the laser is switched off. A decrease of the fluorescence intensity (27% in 20 minutes) is observed due to the 5,6-CF clearance. However, the ratio I(F)/I(BCK) (I(BCK): background fluorescence intensity) remains almost stable over this period of time and the determination of the temperature is still possible with good accuracy even 20 minutes after laser irradiation.
Conclusion: Real-time temperature monitoring by using fluorescence measurement of laser-induced release of liposome-encapsulated dye is clearly demonstrated. This procedure could conceivably prove useful for controlling the thermal coagulation of biological tissues.