Evaluation of the Bending Loss of the Hollow Optical Fiber for Application of the Carbon Dioxide Laser to Endoscopic Therapy ()
1. Introduction
Since carbon dioxide laser with the wavelength of 10.6 mm is absorbed strongly by water, which is contained about 70% in biological soft tissues, it is excellent in incision, hemostasis, coagulation, and vaporization of the soft tissues [1,2]. In fact, carbon dioxide laser is widely efficient for medical field, e.g., plastic surgery, cosmetic surgery, otolaryngology, and dentistry. Even in the endoscopic surgery, carbon dioxide laser had various promising application, so that many researchers developed new techniques for clinical treatment [3,4]. At the same time, several kinds of optical fibers transmitting midinfrared light have been developed. These optical fibers are classified broadly into three categories, i.e., glasses composed of fluoride or oxide, special glasses such as chalcogenide, and polycrystalline materials such as metal halide [5]. However, these optical fibers have problems in terms of the mechanical and chemical properties, toxicity, manufacturing cost, etc. Therefore, they could not be used for medical purposes. Recently, hollow optical fibers with a dielectric coating made of cyclic olefin polymer (COP) inside a cylindrical metal waveguide have been developed as shown in Figure 1 [6]. Since the core of the hollow optical fibers is air or other gases with a high transmittance at the wavelength of the carbon dioxide laser, extremely low absorption loss could be obtained [7]. Because the reflection loss at the end surface of the fiber and related damage do not occur, the hollow optical fibers are suitable to transmit high-power lasers. In addition, the hollow optical fiber shown in Figure 1 can simultaneously transmit carbon dioxide laser and visible aiming laser. Although it is possible to enhance the reflectance of the inner surface to nearly 100% by designing the COP layer thickness according to the wavelength of the transmitting laser light, the leaking loss to the outside of the fiber can not be eliminated in principle. Therefore, by bending the hollow optical fiber, the transmittance decreases due to the increase in the number of reflection per unit length and decrease in the reflectance in the fiber. When the hollow optical fiber is used in a clinical treatment with an endoscope, it is inferred that output laser power and therapeutic effect are changed due to the change in the transmittance of the hollow optical fiber caused by the bending of the endoscope. The purpose of this research is to quantitatively evaluate the change in the output laser power and therapeutic effect caused by bending the hollow optical fiber in a gastrointestinal endoscope.
2. Materials and Methods
2.1. Transmittance of the Hollow Optical Fiber with Bending of the Endoscope
In this research, a carbon dioxide laser system modified from a commercial laser system for dental treatment and so on (COM-2, J. Morita Manufacturing Corp., Kyoto, Japan) was used. The laser was operated in the continuous wave mode, and the maximum output power of the laser oscillator was 30 W. Two types of the hollow optical fiber (J. Morita Manufacturing Corp.) listed in Table 1 were used to deliver the carbon dioxide laser through a gastrointestinal endoscope (GIF-2T200, Olympus Corp., Tokyo, Japan) shown in Figure 2. A laser power meter (30-A-BB-18, Ophir Optronics, Israel) was used to measure the laser power.
The carbon dioxide laser, the visible aiming laser with a wavelength of 650 nm, and air were simultaneously transmitted through the hollow optical fiber. Input and output powers of the hollow optical fibers were measured under four different conditions as shown in Figure 3, and each condition was described as follows:
Figure 1. Schematic of the hollow optical fiber used in this research.
Table 1. Specifications of the hollow optical fibers.
I. The hollow optical fiber was kept straight without insertion to the endoscope.
II. The hollow optical fiber was inserted into the endoscope, and endoscope was kept straight.
III.In addition to the condition II, the middle part of the endoscope was gradually bent 90˚ with a radius r1 = 50 cm.
IV. In addition to the condition III, the head of the endoscope was steeply bent 90˚ with a radius r2 = 5 cm.
2.2. Dependence of the Output Laser Power and Incision Depth on the Bending of the Head of the Endoscope
By assuming the case where the endoscope is used in transoral surgery, the middle part of the endoscope was bent 90˚ with a radius r1 = 50 cm, and the tip of the hollow optical fiber L-1 was bent with various angles and radii r2 as listed in Table 2. To avoid the accidental damage to the endoscope due to the break of the hollow optical fiber during laser irradiation, the fiber was ejected about 15 cm from the head of the endoscope, and only the fiber was bent by using in-house guide plates made of foamed polystyrene in order to simulate the bending conditions of the head of the endoscope.
A segment of a porcine stomach was set on a motorized linear stage (SGSP20-20, SIGMA KOKI Co., Ltd., Tokyo, Japan) and moved at a constant speed of 1.0 mm/s during laser irradiation for a time of 20 s. The car-
Figure 2. The photographs of the endoscope and its head and instrument channel. The hollow optical fiber is steeply bent 130˚ just after the insertion to the instrument channel.
Figure 3. Schematics of the measurement of the transmittance of the hollow optical fibers for various bending conditions.
bon dioxide laser was irradiated vertically to the surface of the mucosa from a distance of 2 mm. The setting power of the carbon dioxide laser system was set at 3, 5, and 8 W, and laser power transmitted through the hollow optical fiber was measured under each condition. To simulate the in vivo environment, the sample surface was kept wet by pouring saline on the sample at a rate of 120 mL/h using a micro syringe pump (IC3100, AS ONE, Osaka, Japan). After laser irradiation, each sample was stored at −80˚C and was sliced to a thickness of 10 mm by using a cryostat microtome (CM1850, Leica Microsystems, Wetzlar, Germany) at a temperature of −20˚C. Then, each sample section attached a glass slide was stained with hematoxylin and eosin (HE) staining and observed using a high-resolution slide scanner (NanoZoomer 2.0 RS, Hamamatsu Photonics K. K., Shizuoka, Japan).
3. Results and Discussion
3.1. Transmittance of the Hollow Optical Fiber with Bending of the Endoscope
Figure 4 shows the transmittance of the hollow optical fibers under each condition. When the hollow optical fiber was inserted into the instrument channel of the endoscope (from the condition I to II), the transmittance of the fiber L-1, L-2, and S-1 decreased by 29%, 28%, and 24%, respectively. On the other hand, the transmittance was not significantly decreased by the bending of the endo-