Search

Some physical concepts of laser‐tissue interactions that occur in 308‐nm excimer laser angioplasty are addressed. Monte Carlo numerical computations were used to analyze the light fluence rate distributions resulting from finite diameter laser beams incident on tissue, as applied by fiber‐optic light delivery catheters. The fluence rate at the inside part of the tissue surface from a 0.2‐mm diameter fiber emitting 308‐nm light, is increased more than twice relative to the incident power density. The light fluence rate distribution inside the tissue spreads very little outside the incident beam diameter. Therefore, the distributions from different fibers in multifiber catheters will not overlap unless the fibers are very close together. The maximum fluence rate decreases with decreasing beam diameters. Ablation of tissue by a 308‐nm excimer laser delivery system in contact with the tissue resulted in a damage zone adjacent to the crater wall, due to expansion of the gaseous debris trapped under the tip of delivery system. In case of contact irradiation, the ablation was more efficient than in case of noncontact irradiation. Direct temperature measurements during excimer laser ablation by an infared (IR) camera showed that temperature accumulation will occur when a sequence of pulses is applied at frequencies of at least 5 Hz. The temperature rise above ambient under circumstances simulating clinical conditions is measured to be 66°± 7°C.

Laser angioplasty with modified fiber tips has become a common procedure for the recanalization of totally occluded peripheral arteries. We evaluated the contribution of optical, thermal, and mechanical effects to the mechanism of recanalization of various probes, theoretically and experimentally. Temperature behavior and tissue penetration were measured in relation to axial force exerted by metal laser probes, and optical contact probes coupled to continuous-wave and pulsed Nd:YAG lasers. Modified fiber tips only penetrated tissue when the contact surface of the probe exceeded a temperature threshold of about 225°C in the fatty tissue model used. Metal laser probes had to be insulated from a liquid environment to attain this temperature. Optical probes needed to have an absorbing layer of carbonized tissue particles to attain this temperature. Tissue penetration by modified fiber tips was force dependent, especially with optical probes. Since the diameter of the probe was larger than the ablative laser beam, the atraumatic probe had to distend soft tissues mechanically. Because the metal laser probes delivered their energy in all directions, undesired heating in the radial direction has to be reduced by motion. The recanalization mechanism of modified fiber tips will depend on the properties of the obstruction. Sometimes the probes will recanalize the obstruction without the use of laser energy (Dotter effect). Moderate heating of the tissue by either direct absorption of light, or by heat conduction from the heated surface of the probe, may help to remodel the obstruction. Tissue temperatures around 100°C will vaporize the water compound, and temperatures higher than 225°C will ablate the solid compounds of the tissue creating a channel. The present unsteerable probes will not penetrate heavily calcified obstructions. Calcific deposits may deflect the probe into the wall with the risk of perforation.