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For intracorporeal CO2 laser surgery, the laser beam is usually delivered through long focal length optics coupled to an operating microscope or an endoscope. While the target tissue is in focus for viewing, the power density in the spot of the beam can be affected by defocusing or irradiating tissue under a small angle of incidence. When the beam is used to drill a channel (e.g. transmyocardial revascularization, TMR), the power density along the beam will determine the shape of the channel. The area for effective ablation was studied for an ultra-pulsed CO2 laser beam in combination with devices accommodating optics with focal lengths of 120 to 450 mm. The position of the ablation threshold along the waist of the beam in water and crater depths in (model) tissue were determined in relation to pulse energy (1 to 250 mJ) and angle of incidence. The crater formation during ablation of the model tissue and lateral thermal effects were recorded using fast photography and a thermal-imaging method based on Schlieren techniques. Using Gaussian beam theory, the ablation area in the beam of these optical systems was calculated. For the highest energies, the ablation area extended over a length up to 60 mm resulting in the formation of channels of similar length within several pulses. In the waist of the beam, the channels were only 100 - 300 micrometers with minimal thermal effects laterally. Away from the focus, more pulses were needed, larger diameter channels were formed and thermal effects became more pronounced. The theoretical predicted ablation area was in correspondence with of the measurements. For the beam delivery devices studied, tissue effects are along the `depth of focus' of viewing due to the relatively long `ablation waist' of the focused laser beam. However, for superficial applications, the depth of the narrow ablation craters is hard to appreciate and tissues in the depth can easily be perforated. Ablation is more controlled using larger spot sizes (> 0.5 mm) and delivering intermitted pulses with energies above ablation threshold.
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