Dynamic Analysis of Laser Ablation of Biological Tissue by Optical Coherence Tomography

Abstract

Laser ablation is widely used in optical material engineering but also in clinical medicine. Actually, it has been used for evaporation and cutting of biological tissue in surgical operations; for example, the refractive surgery of cornea (Trokel et al. 1983; Puliafito et al. 1985) and the surgery of vascular (Isner et al. 1987). In particular, various types of CW and pulsed lasers have been considered for removal of hard dental tissues. Laser ablation may potentially provide an effective method for removal of caries and hard dental tissues with minimal thermal and mechanical damage to surrounding tissue. An important issue is quantitatively determining the dependence of tooth ablation efficiency or the ablation rate on the laser parameters such as repetition rate and energy of laser pulses. Up to now, the measurement has been made by observation of the cross section of the tissue surface, using a microscope or SEM, after cutting and polishing of a tissue sample (Esenaliev et al. 1996). This sort of process is cumbersome and destructive. On the other hand, shape of the tissue surface may change gradually with time after irradiation of laser pulses. The deformation of tissue surface is due to dehydration. The surrounding tissue may also suffer serious damage from laser ablation if the laser fluence is too high. Therefore, in-situ observation of the cross section of tissue surface is strongly required. A very promising candidate for such an in-situ observation is the so-called optical coherence tomography (OCT) (Huang et al. 1991). The OCT is a medical diagnostic imaging technology that permits in-situ, micron-scale, tomographic cross-sectional imaging of microstructures in biological tissues (Hee et al. 1995; Izatt et al. 1996; Brezinski et al. 1996). At present, in the practical OCT, a super luminescent diode (SLD) is used as the light source for the lowcoherence interferometer, providing the spatial resolution of 10 to 20 m along the depth. Therefore, the OCT is potential for monitoring of the surface change during tissue ablation with micrometer resolution. Boppart et al have first demonstrated OCT imaging for observation of ex vivo rat organ tissue (Boppart et al. 1999). Alfrado et al have demonstrated thermal and mechanical damage to dentin by sub-microsecond pulsed IR lasers using OCT imaging (Alfano et al. 2004). We have also demonstrated an effective method for the in situ observation of laser ablation of biological tissues based on OCT (Haruna et al. 2001; Ohmi et