Lasers in Surgery and Medicine 46:718–725 (2014)
Fabrication of Novel Bundled Fiber and Performance Assessment for Clinical Applications Changhwan Kim, MS,1 Myung Jin Jeon, MS,1 Jin Hyang Jung, MD,2 Jung dug Yang, MD,3 Hoyong Park, MD,2 Hyun Wook Kang, PhD,4** and Ho Lee, PhD1* 1 School of Mechanical Engineering, Kyungpook National University, Daegu 702-701, Korea 2 Department of surgery, School of Medicine, Kyungpook National University, Daegu 702-210, Korea 3 Department of Plastic and Reconstructive Surgery, School of Medicine, Kyungpook National University, Daegu 702-210, Korea 4 Department of Biomedical Engineering and Center for Marine-integrated Biomedical Technology (BK21 Plus), Pukyong National University, Busan 608-737, Korea
Background and Objectives: During laser vaporization of benign prostate hyperplasia (BPH), high precision of optical fiber handling is pivotal to minimize any postoperative complications. The aim of the study was to evaluate the feasible applications of a bundled fiber to treat BPH by directionally and selectively manipulating laser light onto the targeted tissue. Methods: A bundled optical fiber, consisting of four sidefiring fibers, was fabricated to selectively emit laser beams in from one to four directions. Both transmission efficiency and light distribution were qualitatively and quantitatively characterized on the bundled fiber. In terms of interstitial application of the proposed fiber with 1064 nm on porcine liver tissue, the extent of thermal denaturation was estimated and compared at various laser parameterizations and for different directions of light. Results: From the laser source to the fiber tip, the fabricated fiber device demonstrated a total light transmission of 52%. Due to internal light reflection, a secondary beam was emitted backward from the fiber tip and was responsible for 25% of the transmission loss. According to tissue testing, the extent of tissue denaturation generally increased with laser power, irradiation time, and number of light directions. The geometrical shape of thermal coagulation correlated well with the direction of light emission. Thermal damage to the glass tube occurred during excessive heat accumulation generated by continuous irradiation. Conclusions: The proposed fiber can be beneficial for laser vaporization of BPH by providing a selective light direction irradiation along with minimal thermal damage. Further studies will extend the applicability of the bundled fiber to treat tubular tissue structure. Lasers Surg. Med. 46:718–725, 2014. ß 2014 Wiley Periodicals, Inc. Key words: laser-induced damage; fiber optics; laserassisted prostatectomy; endoscopy surgery INTRODUCTION Clinical laser application has been studied and employed in various medical fields, such as surgical treatment ß 2014 Wiley Periodicals, Inc.
(photorefractive keratectomy, laser lithotripsy, laser prostatectomy, dental cavity treatment, and cosmetic surgery [1–9]) and diagnostic imaging (optical coherence tomography, photoacoustic imaging, and NIR imaging [10–12]). Particularly, endoscopic surgical procedures with lasers often require application of optical fibers in order to precisely deliver optical energy to the target tissue and to minimize any adverse injury to the peripheral region. Considerable effort have been made to apply various optical fibers specifically designed for laser therapy, such as for the treatment of varicose vein, laser prostatectomy, photodynamic therapy (PDT), and laser lipolysis, in which the direction and spatial distribution of laser light are typically contingent upon the anatomical geometry of the targeted tissue and the surgical outcomes of interest [13– 18]. The typical configurations for the designed optical fibers include end-firing, side-firing, diffusing, tapered, and conical fibers [19]. In particular, both cone shaped and diffusing fibers can uniformly and simultaneously radiate laser light in all directions and these characteristics can be instrumental in improving therapeutic outcomes for varicose vein laser treatment and PDT. However, it is often difficult to achieve selective control over the direction of the laser light during laser-induced surgery. End-firing optical fibers designed for uniform light distribution are rarely applied for surgical treatments that require selective control over the direction of the laser
Conflict of Interest Disclosures: All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and none were reported Contract grant sponsor: National Research Foundation; Contract grant number: 2009–0091571. Correspondence to: Ho Lee, PhD, School of Mechanical Engineering, Kyungpook National University, Daegu, 702-701, Korea. E-mail: [email protected] Correspondence to: Hyun Wook Kang, PhD, Department of Biomedical Engineering, Pukyong National University, Busan 608-737, Korea. E-mail: [email protected] Accepted 21 July 2014 Published online 30 August 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/lsm.22284
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light. Instead, side-firing and twister fibers can be made to radiate only in one direction away from the fiber’s axis, eventually allowing directionally selective tissue vaporization during laser prostatectomy [20–23]. As a means of achieving efficient vaporization of benign prostate hyperplasia (BPH), various clinical laser systems including quasi-cw Q-switched lasers (532 nm) and diode lasers (940, 980, and 1318 nm) have been manufactured and clinically applied. Recently, the 120-W 532-nm GreenLightTM HPS laser has been used to increase tissue vaporization efficiency in terms of strong light absorption of hemoglobin [20–24]. During the laser vaporization of BPH, rotation of the side-firing fibers is often required in order to irradiate different positions, such as dorsal and ventral regions. Thus, fiber handling techniques, such as control of the sweep angle, sweep speed, and distance between the target tissue and the fiber tip are large contributors to the clinical outcomes of laser-induced prostatectomy along with the tissue properties (optical, thermal, and mechanical) and laser configurations. Surgical urologists are often required to possess high technical precision and dexterity for optical fiber handling in order to avoid perioperative and post-operative complications, including dysuria, hemorrhage, stricture, and perforation. Particularly, during the procedure, the moving speed of a side-firing fiber should be carefully chosen to maximize vaporization efficiency of target tissue at the appropriate working distance, while avoiding any tissue perforation caused by highly localized energy deposition where the location is difficult for the application of light [22–27]. If both the direction and the power of laser irradiation can be selectively controlled with optical fiber configurations, selective laser surgery would help enhance surgical performance and efficacy as well as eventually minimize the perioperative and post-operative complications. In an attempt to control beam direction and to selectively irradiate in multiple directions without any fiber handling, a bundled optical fiber was designed, developed, and evaluated quantitatively. Thus, the aim of the current study was to assess the feasible application of the proposed bundled fiber for safe and efficient prostatectomy in directional tissue treatment by manipulating the distribution light on tissue.
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METHODS AND MATERIALS Fiber Manufacturing Manufacturing a bundled optical fiber consisted of two steps: one was the polishing of each individual fiber to have an angled (408) tip structure for side-firing, and the other was the integration of the four side-firing fibers into a single bundled fiber. For the polishing process, four multimode fibers with core/clad diameter of 300/330mm (NA ¼ 0.22, FIP300330370, Polymicro Technologies) were used. Initially, both the buffer and the jacket of each multi-mode fiber were stripped, and the tips of the fibers were cleaved. Each cleaved optical fiber was cleaned with a cleaning paper and acetone. After cleaning the fibers, they were individually mounted in a fiber chuck, which in turn, was fixed in a polishing jig that was designed to form an angled surface. The angle of the fiber tip, with respect to the optical axis, was set to 408 to ensure the total internal reflection at the air-core interface [19]. Lapping sheets with 1 and 5 mm grain sizes were used to polish the fibers for side-firing. Then, an ultrasonic cleaner was applied to the angled surface of each polished fiber. An image of the polished fiber tip is presented in Figure 1(a) and 1(b). In order to examine the direction of light propagation, each fiber was then coupled with a He:Ne laser (l ¼ 632 nm), and the beam emission profile was qualitatively observed. The distal end of each fiber was positioned flat on white paper, and the reflected light from the paper was captured with a digital camera (EOS 50D, Canon, Japan) in a dark room. Figure 1(c) presents the beam emission pattern of the fabricated side-firing fiber in air. The emission from the side-firing fiber clearly demonstrated the directional emission as the emission beam was reflected at 728 relative to the fiber axis (Fig. 1(c)). Light transmission efficiency of each side-firing fiber was evaluated with 1064 nm laser light (continuous wave mode). Both the output power of the transmitted light through each polished fiber and the input power from the laser cavity (input power) were measured and compared to quantitatively identify any transmission loss. Figure 1(d) demonstrates an experimental set-up of measuring the transmission power from the fiber. The 1064 nm laser light was coupled into the proximal
Fig. 1. Images of side-firing fiber: (a) polished fiber tip (side view), (b) polished fiber tip (front view), (c) beam propagation, and (d) measurement set-up with side-firing fiber.
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end of the fiber tip. Three laser power levels of 1, 5, and 10 W were tested for 5 minutes. Figure 2 illustrates the assembly process of a bundled fiber with four side-firing fibers. To maintain the angle of the side-directional beam emission and protect the fiber tip, a closed end glass tube was employed. By considering chemical and thermal resistance as well as optical transmittance for visible and near-IR wavelengths, the tube was made of quartz in size of 2 cm, 1 mm, and 0.4 mm (length, inner diameter, and thickness, respectively); thus, the outer diameter of the assembled fiber added up to be 1.8 mm. The four subcomponent fiber tips were carefully inserted into the tube 1 cm away from the entrance and were perpendicularly secured by a custom-made fiber jig system. For fine and precise adjustments, the position of the fiber tips was visually confirmed with a microscope while they were being slightly rotated or moved back and forth by the fiber jig system. In turn, the four side-firing fibers became orthogonally aligned in the glass tube. After the fiber alignment, individual beam emission was qualitatively inspected with He:Ne laser light. Finally, biocompatible epoxy adhesive (EPO-TEK 353ND, Epoxy Technology, MA) was used to glue the fibers and the tube together to prevent any possible misalignment of the fiber tips during tissue experiments. The epoxy was also applied to the entrance of the tube twice to ensure the complete sealing against any contamination and then cured for 8 hours at room temperature. The beam emission profile of a bundled fiber was qualitatively examined with a digital camera after coupling a He:Ne laser into the fiber. Figure 3 presents a measurement set-up to characterize the bundled fiber in terms of light distribution. The bundled fiber tip was placed inside a quadrangular pyramid-shaped fixture with grid pattern written on the inner wall. Thus, the fixture with the grid pattern was used to promptly inspect and identify the direction and position of the emitted laser light after fiber alignment. A digital camera was positioned at the base side of the fixture to capture the beam profile
Fig. 2. Illustration of assembly process for bundled fiber.
Fig. 3. Characterization of bundled fiber: (a) schematic diagram of beam profile measurement and (b) spatial beam distribution from bundled fiber.
image for further adjustment of the four side-firing fibers (Fig. 3(a)). Figure 3(b) demonstrates a captured image of the spatial beam distribution out of the bundled fiber as four equally-positioned dots on the surface of the pyramid fixture. The irradiation pattern in the orthogonal direction induced an elliptical beam spot. The main angle from the optical axis of the fiber was 728 (Fig. 1(c)), and the beam divergence was approximately 158, which was indirectly estimated from a cross-sectional view of the beam irradiation pattern. Experimental Set-up A 25-W custom-made diode pumped solid state (DPSS) laser system with wavelength of 1064 nm in cw mode was employed as an irradiating source for laser-induced damage tests on soft tissue. Figure 4 illustrates a schematic diagram of the laser beam coupling into the integrated bundled fiber. Firstly, the laser light was divided into four quarter-power beams using beam
Fig. 4. Schematic diagram of laser beam arrangement.
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splitters (BSW26, Thorlabs, NJ), and the divided laser beams were coupled into the proximal end of the individual side-firing fiber tip via four convex lenses. After the fiber coupling and beam alignment, the output power from an individual fiber was measured with a power meter to validate its transmission performance in comparison with laser input power. For reference, an end-firing fiber was also used to estimate fiber transmission efficiency. The average difference in transmission efficiency among the four side-firing fibers was around 2.3%. The delivery of the individual quarter-power laser beam was selectively controlled with a blocking plate that consisted of an aluminum plate treated by anodization in black color. The blocking plate was manually moved to block the beam path in front of the convex lens and to control the beam emission directions. Porcine liver tissue from a local slaughterhouse was used as a target sample in order to investigate various patterns of laser-induced coagulation with a bundled fiber. In order to reflect the physical environments for laser prostatectomy, the proposed fibers were evaluated in terms of interstitial application. Figure 5 shows a schematic diagram of interstitial laser coagulation experiments on the liver tissue. The tissue was cut into pieces of approximately 10 15 3 cm3 (width, length, and height). The samples were then placed on a 3-cm thick insulating stage prior to laser irradiation in order to prevent heat loss from the bottom of the sample. A custom-made guiding needle with an inner diameter of 3 mm was used to position the bundled fiber tip inside the tissue samples. The guiding needle was firstly inserted into the tissue at 6 cm below the surface, and the optical fibers were threaded into the tissue along the needle until the fiber tips reached the distal end of the needle. The guiding needle was then completely retracted from the sample. Upon removal, the laser light was irradiated through the fiber onto the tissue. Four laser power and duration cohorts were tested for the bundled fiber (i.e. 2 W per direction for 60 seconds, 2 W per direction for 120 seconds, 3 W per direction for 60 seconds, and 3 W per direction for 120 seconds). For each test condition, three types of the laser emission were applied to evaluate the directional capability of the integrated bundle fiber for
laser coagulation; uni-directional, bi-directional, and quadra-directional emission emanating from the bundled fiber tip. After laser irradiation, the guiding needle was reinserted into the tissue, and then the bundled fiber tip, along with the guiding needle, was carefully removed. The tested tissue samples were horizontally dissected with a surgical scalpel along the A-A’ direction (Fig. 5), which demonstrated the maximum degree of tissue coagulation, and was imaged with a digital camera. Thermal denaturation on the tissue was quantitatively evaluated in terms of coagulation (discoloration visible as a whitish color) and carbonization (discoloration visible as a black color). Thickness of coagulation and carbonization surrounding the irradiated tissue was circumferentially measured 10 times with Image J (National Health Institute, MD) and compared for various test conditions. For statistical analysis, a Students’ t-test was performed and a P-value of less than 0.05 represented statistical significance.
Fig. 5. Schematic diagram of laser ablation experiment on liver
Fig. 6. Transmission efficiency at various input power levels (1, 5,
tissue.
and 10 W).
RESULTS Figure 6 shows the transmission efficiency of a sidefiring fiber in comparison with an end-firing fiber. Regardless of power, the overall transmission efficiencies of the end-firing and side-firing fibers were approximately 80% and 60%, respectively, which agreed well with the previous study [28]. In comparison to the end-firing fiber, the side-firing fiber experienced an additional 25% transmission loss. Given the same input power, the percent difference of transmission power among the side-firing fibers was less than 15%. The total transmission of the complete system (including the beam splitters, mirrors, lenses, and bundled fiber) was measured to be approximately 52% due to loss by all the side-firing fibers, specular reflection within the glass tube and the optics. Figure 7 presents cross-sectional images of interstitially laserinduced damage within liver tissue at various conditions of power, irradiation time, direction, and energy per direction. A hole in the middle of the tissue represented the location of the bundled fiber inserted into the tissue during laser coagulation. For all the irradiation conditions, the coagulation and carbonization zones were validated by discoloration in the tissue (i.e. whitish color for coagulation
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Fig. 7. Typical image of laser-induced damage region (bar ¼ 1 cm).
and black for carbonization). Overall, the coagulation region was nearly symmetrically distributed outward from the carbonization zone, and the degree of coagulation gradually decreased with radial distance from the fiber. In several cases (i.e. each of the 4-directions independently at 2 W for 2 minutes and in all directions simultaneously at 3 W for 1 minute and 2 minutes), slight tissue vaporization occurred, resulting in a physical cavity, and the wall of each cavity was circumscribed by a dark carbonized region. Then, each dark cavity was surrounded by coagulative necrosis (i.e. whitish color). Mostly, the geometrical shape of the coagulation zone was well correlated with the direction of light emission. Regardless of irradiation direction and laser power, the direction of thermal damage zone (i.e. coagulation and carbonization) was clearly described in the tissue, corresponding to the beam irradiation direction under the same testing conditions. In the case of a single direction, a hole with thin coagulation zone was created with 2-W laser power (i.e. 120 and 240 J/direction). Yet, the application of the higher power of 3 W (i.e. 180 and 360 J/direction) showed asymmetrical distribution of tissue coagulation along with minimal carbonization: one side of the coagulated tissue was thicker than the other. For bi-directional cases, both 180 and 360 J/direction (i.e. 3-W condition) evidently presented two opposite direction of the laser-induced coagulation. On the other hand, both 120 and 240 J/
direction (i.e. 2 W condition) still exhibited symmetrical tissue coagulation with minimal carbonization. Lastly, quadra-directional cases with higher total energy delivery (i.e. 180, 240, and 360 J/direction) markedly demonstrated the four-directional zone of coagulation as well as carbonization. However, the lower total energy delivery (120 J/direction) created circular and relatively thick coagulation in the tissue, compared with the other conditions. The circular shape of laser-induced damage for the quadra-directional configuration with 2 W/direction for 1 minute irradiation was similar to the coagulation shape induced by the bi-directional configuration with 2 W/ direction for 2 minutes irradiation. The physical thickness of interstitial laser coagulation was measured and compared for all tested conditions in Figure 8. Overall, irrespective of power and irradiation time, the coagulation thickness increased with the number of directions except for the condition with 3 W and 2 minutes (i.e. 2 W and 1 minute: 0.6 1.1, 3.2 0.7, and 5.0 0.4 mm for the uni-, bi-, and quadra-directional cases). A statistically significant difference was found among all the directions (P < 0.001). In addition, longer irradiation induced thicker coagulation for all directions at both 2 W and 3 W. Given the equivalent irradiation time, higher laser power (3 W) generated a thicker laser-induced damaged zone than that with lower power (2 W). Although coagulation thickness mostly increased with total laser
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Fig. 8. Depth of coagulated zone per direction for various laser settings (power, irradiation time, and direction).
Fig. 9. Images of bundles fiber: (a) before irradiation, (b) after
energy, 3 W with delivery of 180 J notably entailed more coagulative necrosis than 2 W with delivery of 240 J (i.e. bidirection: 4.6 0.5 mm for 240 J vs. 7.1 1.1 mm for 180 J; P < 0.001). In the case of the total energy delivery from 180 to 360 J, the quadra-directional cases induced an almost equivalent degree of tissue coagulation (i.e. 8.6 1.1 mm for 180 J vs. 8.1 0.6mm for 360 J; P ¼ 0.1). It was also noted that coagulation thickness became saturated for bi- and quadra-direction at 360 J (i.e. 8.6 1.1 mm for bi-direction vs. 8.1 0.6 mm for quadra-direction; P ¼ 0.1). Both the bi-directional configuration with 240 J and the quadra-directional one with 120 J yielded comparable distributions of laser-induced denaturation. However, the difference in coagulation thickness between the two configurations was statistically significant (i.e. 4.6 0.5 mm for bi-directional vs. 5.0 0.4 mm for quadradirectional; P < 0.001). Figure 9 shows images of a bundled fiber under various conditions: preirradiation (Fig. 9(a)), post-irradiation with no carbonization observed (Fig. 9(b)), post-irradiation with carbonization observed (Fig. 9(c)), and a magnified image of the carbonization observed after cleaning (Fig. 9(d)). Before the irradiation, hardly any damage was observed on the glass tube of the bundled fiber (Fig. 9(a)). Under the 2-W laser power condition, while the glass tube surface still showed no surface deformation, it became opaque and there was discoloration (i.e. reddish) of the proximal end of the fiber (Fig. 9(b)). Once the irradiation power increased to 3 W, the glass tube of the bundled fiber was covered with black soot (i.e. carbonized debris) (Fig. 9(c)) and partial physical damage (i.e. devitrification) was observed exclusively on the surface of the glass tube even after cleaning the fiber tip (Fig. 9(d)). Similar phenomenon was also found with the 2-W quadra-directional configuration for the 2-minutes irradiation case. The output power of bundled fiber with the damaged glass tube was up to 17.5% lower than that with the undamaged glass tube. DISCUSSION A bundled fiber was assembled with four side-firing fibers that emitted laser light in a lateral direction. Although the fiber fabrication satisfied the conditions to
irradiation with no carbonization observed, (c) after irradiation with carbonized surface, and (d) magnified image of carbonized fiber surface after cleaning.
create total internal reflection, 25% of the rays was reflected backward and created a secondary beam (Fig. 1 (c)), which may be attributed to an incident angle smaller than the critical angle. In addition, since an air gap existed between the fiber and the glass tube, partial specular reflection (i.e. Fresnel reflection) of the primary beam could have occurred at the internal surface of the glass tube. Due to energy loss by the secondary beam, undesirable tissue damage took place in the opposite direction and became palpable particularly under an increased power condition (Fig. 7). Thus, in order to minimize secondary beam effects, further investigations will focus on polishing fiber-end surface with various angles and shapes, identifying the primary reasons for beam loss, and configuring the optimal fabrication conditions for side-firing fibers. Anti-reflection coating will also be applied to the other side of the glass tube to reduce Fresnel reflection, so the secondary light can be reflected again into alignment with the primary beam. Directionality of beam irradiation was determined by the selection of side-firing fibers in the bundled fiber. In the case of lower power application under short irradiation time (i.e. 2 W for 1 minute), the direction of the beam irradiation was hardly recognizable while plainly generating a circular pattern of tissue coagulation (Fig. 7). However, once the irradiation time (i.e. 2 minutes) or the applied power (i.e. 3 W) or both were increased, the selected direction of the laser light vividly appeared on the tissue surface in the form of discolored coagulation (Fig. 7). In addition, thicker coagulative necrosis, as observed particularly with bi-/quadra-emission evidenced that the beam irradiation was selectively controllable and as a result, collective tissue coagulation was created (Figs. 7 and 8). However, since the beam direction was manually predetermined with a block plate during the experimental studies, further designs of automatic selection devices should be considered such as electromechanical shutters along with an energy feedback system and automatic fiber coupler. Accordingly, during laser prostatectomy, urological surgeons can promptly
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manipulate the direction of laser light contingent upon the location of the targeted tissue while being confirmed by cystoscopic observation. In spite of passive light irradiation, the current feasibility results still implicate that the bundled fiber could control the direction of the laser irradiation as well as physical shape of tissue coagulation at the distal end of the fiber. In turn, the potential benefit of the bundled fiber application could include selective laser treatment on asymmetrical lesion without rotating the fiber tip. In fact, during a laser vaporization procedure of BPH, a high power 532-nm light is delivered to prostatic issue through a side-firing fiber, and urological surgeons often rotate either a cystoscope or a side-firing fiber to treat the ventral region in the prostate, which requires high fiber handling skill (i.e. fiber sweeping) and adversely prolongs the treatment time [22–27]. Moreover, misalignment or misfiring of the high power laser with a side-firing fiber could lead to immediate tissue perforation in the bladder neck area, ultimately resulting in a major surgery. Currently, the application of side-firing fibers for BPH treatment can be associated with protracted operation time due to single spot irradiation. Hence, the application of the bundled fiber with selectable beam can diminish the chance of unexpected tissue damage due to careless fiber handling and enhance surgical performance with a more precise fiber handling technique. Furthermore, the developed bundled fiber currently consisted of four side-firing fibers, which could protract the entire manufacturing process. To achieve high reproducibility of bundled fibers, a newly customized fiber jig system combined with optical imaging are under development to readily and accurately position each fiber tip for orthogonal alignment and to facilitate production process with high quality. Thermal damage was observed on the surface of the glass tube when tissue carbonization occurred (Fig. 9(c) and (d)). In addition, carbonization was often observed on the peripheral surface of the tissue and followed by coagulation (Fig. 7). Typically, a superficial carbonization layer can absorb most of the incident optical energy and establish a thermal barrier to principally accumulate excessive heat within that layer [29–31]. Thus, significant light absorption as well as minimal heat diffusion could be responsible for the observed thermal damage in the peripheral tissue region and glass devitrification on the glass tube (Fig. 9). In an attempt to identify the effect of tissue denaturation on irradiation pattern and fiber durability, spatial energy distribution from the bundled fiber should also be examined as a function of irradiation time and power. Due to the complexities in coagulation geometry along the bundled fiber, the current study minimally estimated volumetric variations in the denatured tissue. For a more accurate comparison, optical imaging or direct tissue dissection methods will be implemented to calculate the total volume of irreversible structural changes in tissue after laser irradiation. To prove the feasibility of beam manipulation with a bundled fiber, the current study employed a near IR wavelength of 1064 nm in cw mode, which was consider-
ably longer than thermal relaxation time. It can be conceived that rapid heat accumulation along with weak light absorption in tissue led to adverse formation of tissue carbonization. Notably, carbonization should be avoided during clinical surgery to promote the healing process with minimal infection and inflammation [28,29]. In an attempt to avoid any carbonization, various wavelengths, such as 532, 980, and 2120 nm will be tested in pulsed mode for safe laser prostatectomy [22–27]. Hence, strong light absorption and pulse duration shorter than the required thermal relaxation time (i.e. 150 ms for 532 nm) are expected to prevent any adverse heat effects in tissue and, in turn, to preserve the glass tube from thermal damage. CONCLUSION The current study demonstrated that the bundled fiber was able to control the shape and direction of the laserinduced thermal denaturation by adjusting the light emission direction without fiber handling. The proposed fibers with selectable beams can achieve precise fiber handling during laser vaporization of BPH as well as help avoid unfavorable tissue damage to minimize clinical complications. Further investigations will be performed to optimize the fiber manufacturing process as well as to enhance the tissue ablation performance with a variety of wavelengths and pulse durations. ACKNOWLEDGMENT This work was supported by National Research Foundation grant funded by the Korea Government (MEST: No. 2009–0091571). REFERENCES 1. Sakimoto T, Rosenblatt MI, Azar DT. Laser eye surgery for refractive errors. Lancet 2006;367:1432–1447. 2. Felipe AF, Agahan ALD, Cham TL, Evangelista RP. Photorefractive keratectomy using a 213 nm wavelength solid-state laser in eyes with previous conductive keratoplasty to treat presbyopia: Early results. J Cataract Refract Surg 2011;37: 518–524. 3. Yaqub Y, Suarez J, Jenkins LA. LASER endovascular atherectomy with secondary stenting of technically challenging calcified celiac trunk stenosis. Catheter Cardio Inte 2011; 78:301–303. 4. Van Den Berg JC, Pedrotti M, Canevascini R, Chimchila Chevili S, Giovannacci L, Rosso R. Endovascular treatment of in-stent restenosis using excimer laser angioplasty and drug eluting balloons. J Cardiovasc Surg 2012;53(2):215–222. 5. Takazawa R, Kitayama S, Tsujii T. Successful outcome of flexible ureteroscopy with holmium laser lithotripsy for renal stones 2 cm or greater. Int J Urol 2012;19(3):264–267. 6. Bach T, Xia SJ, Yang Y, Mattioli S, Watson GM, Gross AJ, Herrmann TRW. Thulium: YAG 2 mm cw laser prostatectomy: Where do we stand? World J Urol 2010;28(2):163–168. 7. Ehlers V, Ernst CP, Reich M, Ka¨mmerer P, Willershausen B. Clinical comparison of gluma and Er:YAG laser treatment of cervically exposed hypersensitive dentin. Am J Dent 2012; 25(3):131–135. 8. Zhao XL, Fu ZJ, Xu YG, Zhao XJ, Song WG, Zheng H. Treatment of lumbar intervertebral disc herniation using carm fluoroscopy guided target percutaneous laser disc decompression. Photomed Laser Surg 2012;30(2):92–95. 9. Kunishige JH, Katz TM, Goldberg LH. Friedman PM. Fractional photothermolysis for the treatment of surgical scars. Dermatol Surg 2010;36(4):538–541.
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complications. A cooperative study of 13 participating in institutions evaluating 3,885 patients. J Urol 1989;141:243– 247. Rajabhandharaks D, Kang HW, Oh J. Angular effect of optical fiber movement on endoscopic laser prostatectomy. Lasers Surg Med 2012;44:653–663. Bach T, Muschter R, Sroka R, Gravas S, Skolarikos A, Herrmann TRW, Bayer T, Knoll T, Abbou CC, Janetschek G, Bachmann A, Rassweiler JJ. Laser Treatment of benign prostatic obstruction: Basics and physical differences. Eur Urol 2012;61:317–325. Choi B, Tabatabaei S, Bachmann A, Collins E, de la Rosette J, Sancha FG, Muir G, Reich O, Woo H. Greenlight HPS 120-W laser for benign prostatic hyperplasia: Comparative complications and technical recommendations. Eur Urol Suppl 2008;7:384–392. Osterberg EC, Kauffman EC, Kang HW, Koullick E, Choi BB. Optimal laser fiber rotational movement during photoselective vaporization of the prostate in a bovine ex-vivo animal model. J Endourol 2011;25(7):1209–1215. Muir G, Sancha FG, Bachmann A, Choi B, Collins E, de la Rosette J, Reich O, Tabatabaei S, Woo H. Techniques and training with GreenLight HPS 120-W laser therapy of the prostate: Position paper. Eur Urol Suppl 2008;7:370– 377. Kang HW. Characterization on ablation performance of various surgical fibers. Lasers Med Sci 2014;29:273–277. van Swol CFP, Verdaasdonk RM, van Vliet RJ, Molenaar DG, Boon TA. Side-firing devices for laser prostatectomy. World J Urol 1995;13:88–93. Bown SG. Phototherapy of tumors. World J Urol 1983;7: 700–709. Thomsen S, Pearce JA. Thermal damage and rate processes in biologic tissues. In: Welch AJ, van Gemert MJC, editors Optical-Thermal Response of Laser Irradiated Tissue. New York: Springer; 2011. Chapter 13. 487–550. Verdaasdonk RM, Borst C. van Gemert MJC, Explosive onset of continuous wave laser tissue ablation. Phys. Med. Biol 1990;35(8):1129–1144.
Fabrication of Novel Bundled Fiber and Performance Assessment for Clinical Applications Changhwan Kim, MS,1 Myung Jin Jeon, MS,1 Jin Hyang Jung, MD,2 Jung dug Yang, MD,3 Hoyong Park, MD,2 Hyun Wook Kang, PhD,4** and Ho Lee, PhD1* 1 School of Mechanical Engineering, Kyungpook National University, Daegu 702-701, Korea 2 Department of surgery, School of Medicine, Kyungpook National University, Daegu 702-210, Korea 3 Department of Plastic and Reconstructive Surgery, School of Medicine, Kyungpook National University, Daegu 702-210, Korea 4 Department of Biomedical Engineering and Center for Marine-integrated Biomedical Technology (BK21 Plus), Pukyong National University, Busan 608-737, Korea
Background and Objectives: During laser vaporization of benign prostate hyperplasia (BPH), high precision of optical fiber handling is pivotal to minimize any postoperative complications. The aim of the study was to evaluate the feasible applications of a bundled fiber to treat BPH by directionally and selectively manipulating laser light onto the targeted tissue. Methods: A bundled optical fiber, consisting of four sidefiring fibers, was fabricated to selectively emit laser beams in from one to four directions. Both transmission efficiency and light distribution were qualitatively and quantitatively characterized on the bundled fiber. In terms of interstitial application of the proposed fiber with 1064 nm on porcine liver tissue, the extent of thermal denaturation was estimated and compared at various laser parameterizations and for different directions of light. Results: From the laser source to the fiber tip, the fabricated fiber device demonstrated a total light transmission of 52%. Due to internal light reflection, a secondary beam was emitted backward from the fiber tip and was responsible for 25% of the transmission loss. According to tissue testing, the extent of tissue denaturation generally increased with laser power, irradiation time, and number of light directions. The geometrical shape of thermal coagulation correlated well with the direction of light emission. Thermal damage to the glass tube occurred during excessive heat accumulation generated by continuous irradiation. Conclusions: The proposed fiber can be beneficial for laser vaporization of BPH by providing a selective light direction irradiation along with minimal thermal damage. Further studies will extend the applicability of the bundled fiber to treat tubular tissue structure. Lasers Surg. Med. 46:718–725, 2014. ß 2014 Wiley Periodicals, Inc. Key words: laser-induced damage; fiber optics; laserassisted prostatectomy; endoscopy surgery INTRODUCTION Clinical laser application has been studied and employed in various medical fields, such as surgical treatment ß 2014 Wiley Periodicals, Inc.
(photorefractive keratectomy, laser lithotripsy, laser prostatectomy, dental cavity treatment, and cosmetic surgery [1–9]) and diagnostic imaging (optical coherence tomography, photoacoustic imaging, and NIR imaging [10–12]). Particularly, endoscopic surgical procedures with lasers often require application of optical fibers in order to precisely deliver optical energy to the target tissue and to minimize any adverse injury to the peripheral region. Considerable effort have been made to apply various optical fibers specifically designed for laser therapy, such as for the treatment of varicose vein, laser prostatectomy, photodynamic therapy (PDT), and laser lipolysis, in which the direction and spatial distribution of laser light are typically contingent upon the anatomical geometry of the targeted tissue and the surgical outcomes of interest [13– 18]. The typical configurations for the designed optical fibers include end-firing, side-firing, diffusing, tapered, and conical fibers [19]. In particular, both cone shaped and diffusing fibers can uniformly and simultaneously radiate laser light in all directions and these characteristics can be instrumental in improving therapeutic outcomes for varicose vein laser treatment and PDT. However, it is often difficult to achieve selective control over the direction of the laser light during laser-induced surgery. End-firing optical fibers designed for uniform light distribution are rarely applied for surgical treatments that require selective control over the direction of the laser
Conflict of Interest Disclosures: All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and none were reported Contract grant sponsor: National Research Foundation; Contract grant number: 2009–0091571. Correspondence to: Ho Lee, PhD, School of Mechanical Engineering, Kyungpook National University, Daegu, 702-701, Korea. E-mail: [email protected] Correspondence to: Hyun Wook Kang, PhD, Department of Biomedical Engineering, Pukyong National University, Busan 608-737, Korea. E-mail: [email protected] Accepted 21 July 2014 Published online 30 August 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/lsm.22284
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light. Instead, side-firing and twister fibers can be made to radiate only in one direction away from the fiber’s axis, eventually allowing directionally selective tissue vaporization during laser prostatectomy [20–23]. As a means of achieving efficient vaporization of benign prostate hyperplasia (BPH), various clinical laser systems including quasi-cw Q-switched lasers (532 nm) and diode lasers (940, 980, and 1318 nm) have been manufactured and clinically applied. Recently, the 120-W 532-nm GreenLightTM HPS laser has been used to increase tissue vaporization efficiency in terms of strong light absorption of hemoglobin [20–24]. During the laser vaporization of BPH, rotation of the side-firing fibers is often required in order to irradiate different positions, such as dorsal and ventral regions. Thus, fiber handling techniques, such as control of the sweep angle, sweep speed, and distance between the target tissue and the fiber tip are large contributors to the clinical outcomes of laser-induced prostatectomy along with the tissue properties (optical, thermal, and mechanical) and laser configurations. Surgical urologists are often required to possess high technical precision and dexterity for optical fiber handling in order to avoid perioperative and post-operative complications, including dysuria, hemorrhage, stricture, and perforation. Particularly, during the procedure, the moving speed of a side-firing fiber should be carefully chosen to maximize vaporization efficiency of target tissue at the appropriate working distance, while avoiding any tissue perforation caused by highly localized energy deposition where the location is difficult for the application of light [22–27]. If both the direction and the power of laser irradiation can be selectively controlled with optical fiber configurations, selective laser surgery would help enhance surgical performance and efficacy as well as eventually minimize the perioperative and post-operative complications. In an attempt to control beam direction and to selectively irradiate in multiple directions without any fiber handling, a bundled optical fiber was designed, developed, and evaluated quantitatively. Thus, the aim of the current study was to assess the feasible application of the proposed bundled fiber for safe and efficient prostatectomy in directional tissue treatment by manipulating the distribution light on tissue.
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METHODS AND MATERIALS Fiber Manufacturing Manufacturing a bundled optical fiber consisted of two steps: one was the polishing of each individual fiber to have an angled (408) tip structure for side-firing, and the other was the integration of the four side-firing fibers into a single bundled fiber. For the polishing process, four multimode fibers with core/clad diameter of 300/330mm (NA ¼ 0.22, FIP300330370, Polymicro Technologies) were used. Initially, both the buffer and the jacket of each multi-mode fiber were stripped, and the tips of the fibers were cleaved. Each cleaved optical fiber was cleaned with a cleaning paper and acetone. After cleaning the fibers, they were individually mounted in a fiber chuck, which in turn, was fixed in a polishing jig that was designed to form an angled surface. The angle of the fiber tip, with respect to the optical axis, was set to 408 to ensure the total internal reflection at the air-core interface [19]. Lapping sheets with 1 and 5 mm grain sizes were used to polish the fibers for side-firing. Then, an ultrasonic cleaner was applied to the angled surface of each polished fiber. An image of the polished fiber tip is presented in Figure 1(a) and 1(b). In order to examine the direction of light propagation, each fiber was then coupled with a He:Ne laser (l ¼ 632 nm), and the beam emission profile was qualitatively observed. The distal end of each fiber was positioned flat on white paper, and the reflected light from the paper was captured with a digital camera (EOS 50D, Canon, Japan) in a dark room. Figure 1(c) presents the beam emission pattern of the fabricated side-firing fiber in air. The emission from the side-firing fiber clearly demonstrated the directional emission as the emission beam was reflected at 728 relative to the fiber axis (Fig. 1(c)). Light transmission efficiency of each side-firing fiber was evaluated with 1064 nm laser light (continuous wave mode). Both the output power of the transmitted light through each polished fiber and the input power from the laser cavity (input power) were measured and compared to quantitatively identify any transmission loss. Figure 1(d) demonstrates an experimental set-up of measuring the transmission power from the fiber. The 1064 nm laser light was coupled into the proximal
Fig. 1. Images of side-firing fiber: (a) polished fiber tip (side view), (b) polished fiber tip (front view), (c) beam propagation, and (d) measurement set-up with side-firing fiber.
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end of the fiber tip. Three laser power levels of 1, 5, and 10 W were tested for 5 minutes. Figure 2 illustrates the assembly process of a bundled fiber with four side-firing fibers. To maintain the angle of the side-directional beam emission and protect the fiber tip, a closed end glass tube was employed. By considering chemical and thermal resistance as well as optical transmittance for visible and near-IR wavelengths, the tube was made of quartz in size of 2 cm, 1 mm, and 0.4 mm (length, inner diameter, and thickness, respectively); thus, the outer diameter of the assembled fiber added up to be 1.8 mm. The four subcomponent fiber tips were carefully inserted into the tube 1 cm away from the entrance and were perpendicularly secured by a custom-made fiber jig system. For fine and precise adjustments, the position of the fiber tips was visually confirmed with a microscope while they were being slightly rotated or moved back and forth by the fiber jig system. In turn, the four side-firing fibers became orthogonally aligned in the glass tube. After the fiber alignment, individual beam emission was qualitatively inspected with He:Ne laser light. Finally, biocompatible epoxy adhesive (EPO-TEK 353ND, Epoxy Technology, MA) was used to glue the fibers and the tube together to prevent any possible misalignment of the fiber tips during tissue experiments. The epoxy was also applied to the entrance of the tube twice to ensure the complete sealing against any contamination and then cured for 8 hours at room temperature. The beam emission profile of a bundled fiber was qualitatively examined with a digital camera after coupling a He:Ne laser into the fiber. Figure 3 presents a measurement set-up to characterize the bundled fiber in terms of light distribution. The bundled fiber tip was placed inside a quadrangular pyramid-shaped fixture with grid pattern written on the inner wall. Thus, the fixture with the grid pattern was used to promptly inspect and identify the direction and position of the emitted laser light after fiber alignment. A digital camera was positioned at the base side of the fixture to capture the beam profile
Fig. 2. Illustration of assembly process for bundled fiber.
Fig. 3. Characterization of bundled fiber: (a) schematic diagram of beam profile measurement and (b) spatial beam distribution from bundled fiber.
image for further adjustment of the four side-firing fibers (Fig. 3(a)). Figure 3(b) demonstrates a captured image of the spatial beam distribution out of the bundled fiber as four equally-positioned dots on the surface of the pyramid fixture. The irradiation pattern in the orthogonal direction induced an elliptical beam spot. The main angle from the optical axis of the fiber was 728 (Fig. 1(c)), and the beam divergence was approximately 158, which was indirectly estimated from a cross-sectional view of the beam irradiation pattern. Experimental Set-up A 25-W custom-made diode pumped solid state (DPSS) laser system with wavelength of 1064 nm in cw mode was employed as an irradiating source for laser-induced damage tests on soft tissue. Figure 4 illustrates a schematic diagram of the laser beam coupling into the integrated bundled fiber. Firstly, the laser light was divided into four quarter-power beams using beam
Fig. 4. Schematic diagram of laser beam arrangement.
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splitters (BSW26, Thorlabs, NJ), and the divided laser beams were coupled into the proximal end of the individual side-firing fiber tip via four convex lenses. After the fiber coupling and beam alignment, the output power from an individual fiber was measured with a power meter to validate its transmission performance in comparison with laser input power. For reference, an end-firing fiber was also used to estimate fiber transmission efficiency. The average difference in transmission efficiency among the four side-firing fibers was around 2.3%. The delivery of the individual quarter-power laser beam was selectively controlled with a blocking plate that consisted of an aluminum plate treated by anodization in black color. The blocking plate was manually moved to block the beam path in front of the convex lens and to control the beam emission directions. Porcine liver tissue from a local slaughterhouse was used as a target sample in order to investigate various patterns of laser-induced coagulation with a bundled fiber. In order to reflect the physical environments for laser prostatectomy, the proposed fibers were evaluated in terms of interstitial application. Figure 5 shows a schematic diagram of interstitial laser coagulation experiments on the liver tissue. The tissue was cut into pieces of approximately 10 15 3 cm3 (width, length, and height). The samples were then placed on a 3-cm thick insulating stage prior to laser irradiation in order to prevent heat loss from the bottom of the sample. A custom-made guiding needle with an inner diameter of 3 mm was used to position the bundled fiber tip inside the tissue samples. The guiding needle was firstly inserted into the tissue at 6 cm below the surface, and the optical fibers were threaded into the tissue along the needle until the fiber tips reached the distal end of the needle. The guiding needle was then completely retracted from the sample. Upon removal, the laser light was irradiated through the fiber onto the tissue. Four laser power and duration cohorts were tested for the bundled fiber (i.e. 2 W per direction for 60 seconds, 2 W per direction for 120 seconds, 3 W per direction for 60 seconds, and 3 W per direction for 120 seconds). For each test condition, three types of the laser emission were applied to evaluate the directional capability of the integrated bundle fiber for
laser coagulation; uni-directional, bi-directional, and quadra-directional emission emanating from the bundled fiber tip. After laser irradiation, the guiding needle was reinserted into the tissue, and then the bundled fiber tip, along with the guiding needle, was carefully removed. The tested tissue samples were horizontally dissected with a surgical scalpel along the A-A’ direction (Fig. 5), which demonstrated the maximum degree of tissue coagulation, and was imaged with a digital camera. Thermal denaturation on the tissue was quantitatively evaluated in terms of coagulation (discoloration visible as a whitish color) and carbonization (discoloration visible as a black color). Thickness of coagulation and carbonization surrounding the irradiated tissue was circumferentially measured 10 times with Image J (National Health Institute, MD) and compared for various test conditions. For statistical analysis, a Students’ t-test was performed and a P-value of less than 0.05 represented statistical significance.
Fig. 5. Schematic diagram of laser ablation experiment on liver
Fig. 6. Transmission efficiency at various input power levels (1, 5,
tissue.
and 10 W).
RESULTS Figure 6 shows the transmission efficiency of a sidefiring fiber in comparison with an end-firing fiber. Regardless of power, the overall transmission efficiencies of the end-firing and side-firing fibers were approximately 80% and 60%, respectively, which agreed well with the previous study [28]. In comparison to the end-firing fiber, the side-firing fiber experienced an additional 25% transmission loss. Given the same input power, the percent difference of transmission power among the side-firing fibers was less than 15%. The total transmission of the complete system (including the beam splitters, mirrors, lenses, and bundled fiber) was measured to be approximately 52% due to loss by all the side-firing fibers, specular reflection within the glass tube and the optics. Figure 7 presents cross-sectional images of interstitially laserinduced damage within liver tissue at various conditions of power, irradiation time, direction, and energy per direction. A hole in the middle of the tissue represented the location of the bundled fiber inserted into the tissue during laser coagulation. For all the irradiation conditions, the coagulation and carbonization zones were validated by discoloration in the tissue (i.e. whitish color for coagulation
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Fig. 7. Typical image of laser-induced damage region (bar ¼ 1 cm).
and black for carbonization). Overall, the coagulation region was nearly symmetrically distributed outward from the carbonization zone, and the degree of coagulation gradually decreased with radial distance from the fiber. In several cases (i.e. each of the 4-directions independently at 2 W for 2 minutes and in all directions simultaneously at 3 W for 1 minute and 2 minutes), slight tissue vaporization occurred, resulting in a physical cavity, and the wall of each cavity was circumscribed by a dark carbonized region. Then, each dark cavity was surrounded by coagulative necrosis (i.e. whitish color). Mostly, the geometrical shape of the coagulation zone was well correlated with the direction of light emission. Regardless of irradiation direction and laser power, the direction of thermal damage zone (i.e. coagulation and carbonization) was clearly described in the tissue, corresponding to the beam irradiation direction under the same testing conditions. In the case of a single direction, a hole with thin coagulation zone was created with 2-W laser power (i.e. 120 and 240 J/direction). Yet, the application of the higher power of 3 W (i.e. 180 and 360 J/direction) showed asymmetrical distribution of tissue coagulation along with minimal carbonization: one side of the coagulated tissue was thicker than the other. For bi-directional cases, both 180 and 360 J/direction (i.e. 3-W condition) evidently presented two opposite direction of the laser-induced coagulation. On the other hand, both 120 and 240 J/
direction (i.e. 2 W condition) still exhibited symmetrical tissue coagulation with minimal carbonization. Lastly, quadra-directional cases with higher total energy delivery (i.e. 180, 240, and 360 J/direction) markedly demonstrated the four-directional zone of coagulation as well as carbonization. However, the lower total energy delivery (120 J/direction) created circular and relatively thick coagulation in the tissue, compared with the other conditions. The circular shape of laser-induced damage for the quadra-directional configuration with 2 W/direction for 1 minute irradiation was similar to the coagulation shape induced by the bi-directional configuration with 2 W/ direction for 2 minutes irradiation. The physical thickness of interstitial laser coagulation was measured and compared for all tested conditions in Figure 8. Overall, irrespective of power and irradiation time, the coagulation thickness increased with the number of directions except for the condition with 3 W and 2 minutes (i.e. 2 W and 1 minute: 0.6 1.1, 3.2 0.7, and 5.0 0.4 mm for the uni-, bi-, and quadra-directional cases). A statistically significant difference was found among all the directions (P < 0.001). In addition, longer irradiation induced thicker coagulation for all directions at both 2 W and 3 W. Given the equivalent irradiation time, higher laser power (3 W) generated a thicker laser-induced damaged zone than that with lower power (2 W). Although coagulation thickness mostly increased with total laser
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Fig. 8. Depth of coagulated zone per direction for various laser settings (power, irradiation time, and direction).
Fig. 9. Images of bundles fiber: (a) before irradiation, (b) after
energy, 3 W with delivery of 180 J notably entailed more coagulative necrosis than 2 W with delivery of 240 J (i.e. bidirection: 4.6 0.5 mm for 240 J vs. 7.1 1.1 mm for 180 J; P < 0.001). In the case of the total energy delivery from 180 to 360 J, the quadra-directional cases induced an almost equivalent degree of tissue coagulation (i.e. 8.6 1.1 mm for 180 J vs. 8.1 0.6mm for 360 J; P ¼ 0.1). It was also noted that coagulation thickness became saturated for bi- and quadra-direction at 360 J (i.e. 8.6 1.1 mm for bi-direction vs. 8.1 0.6 mm for quadra-direction; P ¼ 0.1). Both the bi-directional configuration with 240 J and the quadra-directional one with 120 J yielded comparable distributions of laser-induced denaturation. However, the difference in coagulation thickness between the two configurations was statistically significant (i.e. 4.6 0.5 mm for bi-directional vs. 5.0 0.4 mm for quadradirectional; P < 0.001). Figure 9 shows images of a bundled fiber under various conditions: preirradiation (Fig. 9(a)), post-irradiation with no carbonization observed (Fig. 9(b)), post-irradiation with carbonization observed (Fig. 9(c)), and a magnified image of the carbonization observed after cleaning (Fig. 9(d)). Before the irradiation, hardly any damage was observed on the glass tube of the bundled fiber (Fig. 9(a)). Under the 2-W laser power condition, while the glass tube surface still showed no surface deformation, it became opaque and there was discoloration (i.e. reddish) of the proximal end of the fiber (Fig. 9(b)). Once the irradiation power increased to 3 W, the glass tube of the bundled fiber was covered with black soot (i.e. carbonized debris) (Fig. 9(c)) and partial physical damage (i.e. devitrification) was observed exclusively on the surface of the glass tube even after cleaning the fiber tip (Fig. 9(d)). Similar phenomenon was also found with the 2-W quadra-directional configuration for the 2-minutes irradiation case. The output power of bundled fiber with the damaged glass tube was up to 17.5% lower than that with the undamaged glass tube. DISCUSSION A bundled fiber was assembled with four side-firing fibers that emitted laser light in a lateral direction. Although the fiber fabrication satisfied the conditions to
irradiation with no carbonization observed, (c) after irradiation with carbonized surface, and (d) magnified image of carbonized fiber surface after cleaning.
create total internal reflection, 25% of the rays was reflected backward and created a secondary beam (Fig. 1 (c)), which may be attributed to an incident angle smaller than the critical angle. In addition, since an air gap existed between the fiber and the glass tube, partial specular reflection (i.e. Fresnel reflection) of the primary beam could have occurred at the internal surface of the glass tube. Due to energy loss by the secondary beam, undesirable tissue damage took place in the opposite direction and became palpable particularly under an increased power condition (Fig. 7). Thus, in order to minimize secondary beam effects, further investigations will focus on polishing fiber-end surface with various angles and shapes, identifying the primary reasons for beam loss, and configuring the optimal fabrication conditions for side-firing fibers. Anti-reflection coating will also be applied to the other side of the glass tube to reduce Fresnel reflection, so the secondary light can be reflected again into alignment with the primary beam. Directionality of beam irradiation was determined by the selection of side-firing fibers in the bundled fiber. In the case of lower power application under short irradiation time (i.e. 2 W for 1 minute), the direction of the beam irradiation was hardly recognizable while plainly generating a circular pattern of tissue coagulation (Fig. 7). However, once the irradiation time (i.e. 2 minutes) or the applied power (i.e. 3 W) or both were increased, the selected direction of the laser light vividly appeared on the tissue surface in the form of discolored coagulation (Fig. 7). In addition, thicker coagulative necrosis, as observed particularly with bi-/quadra-emission evidenced that the beam irradiation was selectively controllable and as a result, collective tissue coagulation was created (Figs. 7 and 8). However, since the beam direction was manually predetermined with a block plate during the experimental studies, further designs of automatic selection devices should be considered such as electromechanical shutters along with an energy feedback system and automatic fiber coupler. Accordingly, during laser prostatectomy, urological surgeons can promptly
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manipulate the direction of laser light contingent upon the location of the targeted tissue while being confirmed by cystoscopic observation. In spite of passive light irradiation, the current feasibility results still implicate that the bundled fiber could control the direction of the laser irradiation as well as physical shape of tissue coagulation at the distal end of the fiber. In turn, the potential benefit of the bundled fiber application could include selective laser treatment on asymmetrical lesion without rotating the fiber tip. In fact, during a laser vaporization procedure of BPH, a high power 532-nm light is delivered to prostatic issue through a side-firing fiber, and urological surgeons often rotate either a cystoscope or a side-firing fiber to treat the ventral region in the prostate, which requires high fiber handling skill (i.e. fiber sweeping) and adversely prolongs the treatment time [22–27]. Moreover, misalignment or misfiring of the high power laser with a side-firing fiber could lead to immediate tissue perforation in the bladder neck area, ultimately resulting in a major surgery. Currently, the application of side-firing fibers for BPH treatment can be associated with protracted operation time due to single spot irradiation. Hence, the application of the bundled fiber with selectable beam can diminish the chance of unexpected tissue damage due to careless fiber handling and enhance surgical performance with a more precise fiber handling technique. Furthermore, the developed bundled fiber currently consisted of four side-firing fibers, which could protract the entire manufacturing process. To achieve high reproducibility of bundled fibers, a newly customized fiber jig system combined with optical imaging are under development to readily and accurately position each fiber tip for orthogonal alignment and to facilitate production process with high quality. Thermal damage was observed on the surface of the glass tube when tissue carbonization occurred (Fig. 9(c) and (d)). In addition, carbonization was often observed on the peripheral surface of the tissue and followed by coagulation (Fig. 7). Typically, a superficial carbonization layer can absorb most of the incident optical energy and establish a thermal barrier to principally accumulate excessive heat within that layer [29–31]. Thus, significant light absorption as well as minimal heat diffusion could be responsible for the observed thermal damage in the peripheral tissue region and glass devitrification on the glass tube (Fig. 9). In an attempt to identify the effect of tissue denaturation on irradiation pattern and fiber durability, spatial energy distribution from the bundled fiber should also be examined as a function of irradiation time and power. Due to the complexities in coagulation geometry along the bundled fiber, the current study minimally estimated volumetric variations in the denatured tissue. For a more accurate comparison, optical imaging or direct tissue dissection methods will be implemented to calculate the total volume of irreversible structural changes in tissue after laser irradiation. To prove the feasibility of beam manipulation with a bundled fiber, the current study employed a near IR wavelength of 1064 nm in cw mode, which was consider-
ably longer than thermal relaxation time. It can be conceived that rapid heat accumulation along with weak light absorption in tissue led to adverse formation of tissue carbonization. Notably, carbonization should be avoided during clinical surgery to promote the healing process with minimal infection and inflammation [28,29]. In an attempt to avoid any carbonization, various wavelengths, such as 532, 980, and 2120 nm will be tested in pulsed mode for safe laser prostatectomy [22–27]. Hence, strong light absorption and pulse duration shorter than the required thermal relaxation time (i.e. 150 ms for 532 nm) are expected to prevent any adverse heat effects in tissue and, in turn, to preserve the glass tube from thermal damage. CONCLUSION The current study demonstrated that the bundled fiber was able to control the shape and direction of the laserinduced thermal denaturation by adjusting the light emission direction without fiber handling. The proposed fibers with selectable beams can achieve precise fiber handling during laser vaporization of BPH as well as help avoid unfavorable tissue damage to minimize clinical complications. Further investigations will be performed to optimize the fiber manufacturing process as well as to enhance the tissue ablation performance with a variety of wavelengths and pulse durations. ACKNOWLEDGMENT This work was supported by National Research Foundation grant funded by the Korea Government (MEST: No. 2009–0091571). REFERENCES 1. Sakimoto T, Rosenblatt MI, Azar DT. Laser eye surgery for refractive errors. Lancet 2006;367:1432–1447. 2. Felipe AF, Agahan ALD, Cham TL, Evangelista RP. Photorefractive keratectomy using a 213 nm wavelength solid-state laser in eyes with previous conductive keratoplasty to treat presbyopia: Early results. J Cataract Refract Surg 2011;37: 518–524. 3. Yaqub Y, Suarez J, Jenkins LA. LASER endovascular atherectomy with secondary stenting of technically challenging calcified celiac trunk stenosis. Catheter Cardio Inte 2011; 78:301–303. 4. Van Den Berg JC, Pedrotti M, Canevascini R, Chimchila Chevili S, Giovannacci L, Rosso R. Endovascular treatment of in-stent restenosis using excimer laser angioplasty and drug eluting balloons. J Cardiovasc Surg 2012;53(2):215–222. 5. Takazawa R, Kitayama S, Tsujii T. Successful outcome of flexible ureteroscopy with holmium laser lithotripsy for renal stones 2 cm or greater. Int J Urol 2012;19(3):264–267. 6. Bach T, Xia SJ, Yang Y, Mattioli S, Watson GM, Gross AJ, Herrmann TRW. Thulium: YAG 2 mm cw laser prostatectomy: Where do we stand? World J Urol 2010;28(2):163–168. 7. Ehlers V, Ernst CP, Reich M, Ka¨mmerer P, Willershausen B. Clinical comparison of gluma and Er:YAG laser treatment of cervically exposed hypersensitive dentin. Am J Dent 2012; 25(3):131–135. 8. Zhao XL, Fu ZJ, Xu YG, Zhao XJ, Song WG, Zheng H. Treatment of lumbar intervertebral disc herniation using carm fluoroscopy guided target percutaneous laser disc decompression. Photomed Laser Surg 2012;30(2):92–95. 9. Kunishige JH, Katz TM, Goldberg LH. Friedman PM. Fractional photothermolysis for the treatment of surgical scars. Dermatol Surg 2010;36(4):538–541.
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