Multifiber excimer laser catheter design strategies for various medical applications

For the XeC1 Excimer laser (308 nm, 1 15 ns), special design multifiber laser catheters were developed and theoretically as well as experimentally evaluated. Monte Carlo simulations showed that the penetration depth of 308 nm XeC1 excimer light varied from 50 to 200 im for fiber diameters from 50 to 550 pm and larger. Tissue ablation is expected to be restricted to this irradiated area. In order to ablate larger tissue areas, a flexible bundle of fibers is used introducing gaps in the irradiance distribution due to dead space rn-between the individual fibers. For a cladding/core ratio of 0 to 0.25 , the dead space varies from 10 to 50 % independent of fiber diameter. However, the gaps in the irradiance distribution will be filled in by lateral scattering at the tissue surface if the fiber diameter equals the penetration depth of the laser light. By placing the fibers at preferential positions or guiding the light through selected individual fibers or clusters of fibers, selective tissue areas can be ablated. Based on these concepts multifiber catheters were developed for a unique neurosurgery bypass procedure and for urethra stricture surgery. Real-time, close-up, high speed video imaging showed that tissue ablation mechanism of these catheters is predominately governed by explosive short-life vapor bubbles fragmenting the tissue to small particles. These vapor bubbles might induce unwanted lateral damage. In order to temper the ablation process, laser energy was delivered in 8 pulses divided over 8 sectors of a multifiber catheter (multiplexing), keeping the same fluence instead of one pulse addressing the all fibers at once. This resulted in smaller vapor bubbles, expected to cause less lateral tissue damage. Using multifiber bundles, special design laser catheters enable preferential and precise tissue ablation for unique applications while the explosive tissue ablation might be controlled to minimize lateral damage. The special design multifiber catheters have successfiully been used during patient treatment in neurosurgery and in urology.