The explanation of the fiber optic probe method signal by means of the time-resolved photography for laser ablation bubble

It is important to develop a practical technique for monitoring the ablation interaction at the fiber tip during contact laser ablation, because ablation interaction monitoring may offer useful information about ablation qualities and tissue types (normal aorta, fatty atheroumatous plaque, calcified plaque). In the case of pulsed Ho:YAG laser (λ=2.1 μm) ablation, the ablation bubble is generated at the fiber tip. Also the ablation bubble is indispensable for effective ablation, however excessive ablation bubble formation readily induces damage to the surrounding tissue. Therefore, real-time monitoring of the ablation bubble at the fiber tip is necessary. We have previously developed the fiber optic probe method to monitor the ablation bubbles. The backscattered light of the probe laser from the ablation area was monitored through the same fiber of which the Ho:YAG laser pulse was delivered. We measured the decay time of the backscattered light waveform. The theoretical explanation showed that the calculated bubble collapse time approximately coincided with the decay time of waveform. Since the ablation bubble collapse is strongly affected by mechanical properties of its adjacent tissues, the decay time suggested their surrounding tissue types. We demonstrated the possibility of tissue characterization by this probe method. In order to know the characteristic of the observed backscattered light to understand our probe method capability for tissue characterization, we performed comparative measurements for the ablation bubble by means of time-resolved photography and the probe method