Cutting effects induced by 2 μm laser radiation of cw Tm:YLF and cw and Q-switched Ho:YAG lasers on ex-vivo tissue

Background and objectives

Laser radiation in the 2 μm wavelength region is well-absorbed by water and has good transmittance through commercially available, low-OH quartz optical fibers which are generally acknowledged to have great potential for medical application in endoscopic and open surgery. Medical laser systems in this wavelength range are flashlamp- or laser-pumped pulsed Ho:YAG lasers and continuous wave (cw) thulium (Tm)-doped fiber lasers. This paper presents the laser–tissue effects on an ex-vivo tissue model using an innovative diode-pumped cw Tm:YLF laser and cw and Q-switched Ho:YAG laser.

Materials and methods

The diode-pumped air-cooled Tm:YLF laser (IAP RAS prototype) emits cw light at 1909 nm with an output power of up to 20 W and an optic-to-optic efficiency of more than 41%. The Ho:YAG laser (IAP RAS prototype) is pumped by the radiation of a Tm:YLF laser and can operate at 2090 nm in cw or Q-switched mode with an average power of up to 10 W. The laser beam quality of both lasers is about M² < 1.3 and can be easily coupled in low-OH optical fibers.

Laser–tissue interaction experiments were performed using porcine kidney and liver tissue. Single-spot and cutting experiments were performed in a reproducible set-up. In the single-spot experiments, a fixed distance was used between fiber and tissue surface of d = 5 mm and energies were applied in the 10–200 J range. The cutting experiments were performed with the same laser parameters but in contact mode (d = 0 mm) using a constant scanning velocity of $v=1\text{mm/s}$ of the linear fiber movement. Macroscopic and histological evaluations were performed.

Results

The tissue effect showed precise and reproducible ablation. The ablation depth depended on the applied power. Interestingly, the histological findings showed that the thickness of the coagulation zone in lateral and axial plane was nearly constant at 1 ± 0.5 mm in each direction.

Conclusion

The presented lasers showed more flexibility for surgical approaches. The induced tissue effects showed a very high reproducibility in ablation and in coagulation for both radial and axial planes. The constant small size of these effects may show the potential for precise laser-assisted surgical preparation.