Detecting laser induced thermal activity in tissue-a Doppler ultrasound approach

There is a growing need for a real-time, noninvasive technique to determine the extent and geometry of treated tissue during thermal therapy. We have developed an analog-digital hybrid system which measures the phase and amplitude of the ultrasonic echoes returned from a heated tissue at multiple depths. Two distinct types of echo modulations were observed in our in-vitro studies using beef muscle irradiated with an 810 nm diode laser. Type 1 modulation showed small and slow variations in amplitude and phase, whereas type 2 modulation demonstrated much larger and faster variations in both amplitude and phase. Our study suggests that type 1 modulation is consistent with tissue coagulation, whereas type 2 modulation is associated with tissue evaporation and explosive events.