We developed a medical surgical system with function variability and flexible delivery of dual-wavelength infrared fiber laser that simultaneously oscillated 3 and 2 micrometers radiation. We studied the function variability (cutting/coagulation) of this fiber laser using a specially designed filter which contained two adjacent areas with different transmission for 2 micrometers radiation. When the power of 3 micrometers radiation was set to 0.6 W and the power of 2 micrometers radiation was changed from 0 W to 0.77 W using this filter at the beam traveling speed of 0.25 mm/s, the cutting groove formed in myocardium tissue increased from 0.80 mm to 0.89 mm and the coagulation layer thickness increased from 0.41 mm to 0.85 mm. We successfully developed a filter system that could continuously control coagulation layer thickness with cutting capability constant. We studied a flexible beam delivery using a single crystal sapphire fiber. The transmission efficiency per meter for 3 and 2 micrometers radiation using a 200-micrometers core-diameter sapphire fiber was 58.0% and 60.4%, respectively. The bending loss at 3-cm loop radius was 5.6%. The high flexibility and low bending loss of the sapphire fiber might make it possible to apply the dual-wavelength infrared fiber laser to endoscopic treatment. In summary, these results may encourage the possibility for practical use of the fiber laser.
We theoretically investigated variable-function (cutting/coagulating) characteristics of the continuous wave 3 μm, 2 μm cascade Ho3+:ZBLAN fiber laser using 3D heat-conduction calculation with finite element method. We have modified a commercial-available simulator in order to calculate heat conduction and thermal ablation process in soft tissue. In this calculation we considered specific heat rise due to the thermal denaturation of protein and volume shrinkage caused by temperature elevation. Beam profile, beam traveling speed, output power, and absorption coefficient were employed to describe the laser beam. The configuration of cutting groove and temperature distribution were calculated by varying the power ratio of the two wavelengths. Coagulation layer was defined as the region that was over 60°C for 1 second because we found that birefringence loss in porcine myocardium observed by a polarizing microscope occurred on this temperature history. When we increased the power ratio of 2 micrometers radiation to the total power of 0.9 W from 0% to 100% at the traveling speed of 0.5 mm/s, the incision depth decreased form 1.45 mm to 0.25 mm, while the coagulation layer thickness increased from 0.17 mm to 0.70 mm. We experimentally performed laser cutting on the same condition by our calculation using extracted porcine myocardium and compared this experimental results with the calculated results. We demonstrated that the incision depth and coagulation layer thickness estimated by our calculation indicated good agreement with the experimental results within 20% differences regarding the function variability by 3 μm/2 μm light mixing.
We studied coagulation layer controlled incision with newly developed continuous wave 2 micrometer, 3 micrometer cascade oscillation fiber laser in vitro. Since this laser device simultaneously oscillates 2 micrometer and 3 micrometer radiation, we could change tissue interaction by arranging power ratio of 2 micrometer to 3 micrometer radiation. About one watt of total irradiation power with various power ratios was focused to extracted fresh porcine myocardium or anesthetized rabbit on an automatic moving stage to obtain line incision. Macro photograph and microscopic histology were used to observe tissue interaction phenomenon. The incised specimen showed that precise cutting groove with thin coagulation layer was attained by a 3 micrometer based radiation, meanwhile addition of 2 micrometer radiation to 3 micrometer radiation made coagulation layer thicker. A heat conduction simulator using finite-element method was used to qualitatively explain obtained coagulation layer thickness. This precise incision with controllable side coagulation layer may effective to control bleeding during incision, for instance, for skin, liver, and kidney incisions. Pure continuous wave radiation of 2 micrometer and 3 micrometer may eliminate stress wave induced tissue damage which is frequently found in Ho:YAG and/or Er:YAG tissue interactions. Moreover, sapphire fiber might offer flexible power delivery to this new laser to establish endoscopic application and/or to improved beam handling.
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