Lasers Med Sci DOI 10.1007/s10103-014-1604-6
ORIGINAL ARTICLE
Optical feedback-induced light modulation for fiber-based laser ablation Hyun Wook Kang
Received: 22 January 2014 / Accepted: 28 May 2014 # Springer-Verlag London 2014
Abstract Optical fibers have been used as a minimally invasive tool in various medical fields. However, due to excessive heat accumulation, the distal end of a fiber often suffers from severe melting or devitrification, leading to the eventual fiber failure during laser treatment. In order to minimize thermal damage at the fiber tip, an optical feedback sensor was developed and tested ex vivo. Porcine kidney tissue was used to evaluate the feasibility of optical feedback in terms of signal activation, ablation performance, and light transmission. Testing various signal thresholds demonstrated that 3 V was relatively appropriate to trigger the feedback sensor and to prevent the fiber deterioration during kidney tissue ablation. Based upon the development of temporal signal signatures, full contact mode rapidly activated the optical feedback sensor possibly due to heat accumulation. Modulated light delivery induced by optical feedback diminished ablation efficiency by 30 % in comparison with no feedback case. However, long-term transmission results validated that laser ablation assisted with optical feedback was able to almost consistently sustain light delivery to the tissue as well as ablation efficiency. Therefore, an optical feedback sensor can be a feasible tool to protect optical fiber tips by minimizing debris contamination and delaying thermal damage process and to ensure more efficient and safer laser-induced tissue ablation.
Keywords Optical feedback . IR detector . Laser ablation . Thermal damage
H. W. Kang (*) Department of Biomedical Engineering and Center for Marine-Integrated Biomedical Technology (BK21 Plus), Pukyong National University, 45 Yongso-ro, Nam-gu, Busan 608-737, South Korea e-mail: [email protected]
Introduction Optical fibers have been used to photothermally treat a variety of disease or disorders in medical fields, including dermatology, gynecology, neurology, and urology [1–6]. As laser technology has been advancing, a wide range of wavelengths as well as power levels have been attempted with fibers to improve clinical outcomes. Depending on anatomical features and clinical goals, a number of fiber tips have been designed and evaluated such as ball-shaped, tapering, side-firing, and diffusing types [7]. Particularly, surgical fibers used for laser prostate ablation have been utilized to deliver high optical power of up to 200 W to the tissue lesions of interest [8]. Previous clinical studies quantitatively demonstrated severe deterioration of fiber tips in terms of laser energy delivery during laser prostatectomy [9, 10]. Due to the reattachment of ablated debris to the fiber tip surface, fiber transmission dramatically decreased down to 20 % after an hour delivery of 80 W [9]. The degradation rate was even further aggravated with the delivery of higher power of 120 W, finally causing the tested fiber to fail [10]. The decreased transmission led to inefficient ablation and undesirable coagulation in tissue, eventually prolonging operation time and postoperative patient discomfort such as dysuria and urinary retention. During laser surgery, catastrophic fiber failure could induce irreversible damage to the surrounding tissue and even bladder perforation [10, 11]. Additionally, our unpublished simulation study also verified that as thermal nucleates, the tissue debris attached to the fiber tip considerably increased temperature by more than 1,400 K, which is higher than glass transition temperature (i.e., 927 K for quartz). In turn, excessive heat accumulation can primarily be concentrated on the tip, which initiates and aggravates melting or devitrification of the fiber tip, leading to unfavorable fiber failure that may shatter the glass tip and eventually require
Lasers Med Sci
additional procedure or surgery to eliminate the materials that remained in prostate or bladder with a cystoscope. In an attempt to minimize or prevent fiber deterioration during laser surgery, many fiber manipulation techniques have been investigated. Sweeping a surgical fiber during laser treatment at a constant speed and angle could maximize ablation efficacy as well as minimize thermal injury to tissue and fiber surface [12–14]. In addition, maintaining the fiber distance between fiber and tissue surface at 1~3 mm could give rise to consistent ablation performance due to almost invariable irradiance, thus reducing a probability of debris reattachment to the fiber surface [15]. In the face of the recommended techniques, fiber damage and eventual failure have often occurred in clinical situations due to complexities of surgical procedures and surgeons’ skill as well as pertaining experience. Thus, there is still an imminent need to prevent the fiber tip deterioration process in advance with minimal dependence upon human and environmental factors. In the current study, an optical feedback sensor was developed and tested to sense the onset of thermal damage at the fiberglass cap during laser ablation and eventually to obstruct deterioration process associated with intense interaction between fiber and tissue. The sensor was designed to detect infrared (IR) emissions generated from the fiber glass cap during the irradiation and to instantly modulate the transmitted laser light for cooling the tip. The feasibility of optical feedback during fiber-based laser surgery was evaluated ex vivo to identify any variations in ablation performance along with light transmission.
Materials and methods Porcine kidney tissue was used for ex vivo fiber testing, in that optical properties of glandular components are well known [7]. The kidney tissue was achieved from a local grocery store and sectioned in the area of 3×3 cm2. Prior to tissue experiments, all the prepared samples were stored at 4 °C to minimize dehydration and morphological deformation. As a light source, a customized 532-nm laser system was employed to irradiate the kidney tissue for laser-induced ablation, and during the tests, each sample was maintained at around room temperature. The laser system delivered a power of 100 W through a straight-firing 600-μm core-diameter fiber (FT600EMT, Thorlabs, NJ, USA) with a protective glass cap onto the tissue surface. In order to achieve the prompt fiber protection during tissue ablation, a customized mechanical shutter was installed on the optical pathway (Fig. 1). The shutter was connected to a photodetector (PDA20H, Thorlabs, NJ, USA) with an IR band-pass filter (2~5 μm), which could detect any IR emissions generated from the glass cap due to undesirable heat accumulation at the fiber tip during tissue ablation. Once the IR signals sensed by the photodetector went above the predetermined signal threshold, the detector
immediately could trigger the shutter to block the incoming laser light. Thus, the intermittent light modulation could allow the fiber glass cap to cool down until the amplitude of the detected IR signals became lower than that of the threshold. Preliminarily, the maximum amplitude of IR signals generated during laser ablation of kidney tissue was measured around 5.2 V with a photodetector. Thus, various levels of the signals (from 1 to 5 V with 1-V increment) were evaluated to identify the appropriate range of the triggering threshold for activating optical feedback and minimizing thermal damage at the fiber tip. All the detected signals were acquired with a digital oscilloscope (DPO3054, Tektronix, OR, USA). For tissue ablation, a kidney specimen was placed in a tissue holder (Fig. 1). Each sample was covered with a plastic plate with a 1×1.5-cm2 aperture to firmly secure the tissue. During the tests, the entire holder was submerged in saline (room temperature) to emulate clinical situations. A computercontrolled XYZ stage was used to predetermine various physical distances between tissue surface and fiber tip in order to reflect three surgical techniques such as sweeping (2 mm), close (1 mm), and full contact mode (0 mm). Ablation speed was fixed at 4 mm/s, which was often implemented by surgeons [12]. The temporal profile of the acquired IR signals was also measured and evaluated. Ablation performance was qualitatively and quantitatively compared without and with optical feedback to predict any differences in ablation outcomes. Postexperimentally, each ablated tissue was crosssectioned and imaged with a digital camera. Then, physical dimensions of coagulated and carbonized lesions were measured at various locations (i.e., 3, 5, 6, 7, and 9 o’clock) with ImageJ (National Health Institute, Bethesda, MD, USA). Ablation volume was also estimated by integrating all the crosssectional areas measured from a specimen. Each condition was run five times (N=5), and a Student’s t test was used for statistical analysis (p
ORIGINAL ARTICLE
Optical feedback-induced light modulation for fiber-based laser ablation Hyun Wook Kang
Received: 22 January 2014 / Accepted: 28 May 2014 # Springer-Verlag London 2014
Abstract Optical fibers have been used as a minimally invasive tool in various medical fields. However, due to excessive heat accumulation, the distal end of a fiber often suffers from severe melting or devitrification, leading to the eventual fiber failure during laser treatment. In order to minimize thermal damage at the fiber tip, an optical feedback sensor was developed and tested ex vivo. Porcine kidney tissue was used to evaluate the feasibility of optical feedback in terms of signal activation, ablation performance, and light transmission. Testing various signal thresholds demonstrated that 3 V was relatively appropriate to trigger the feedback sensor and to prevent the fiber deterioration during kidney tissue ablation. Based upon the development of temporal signal signatures, full contact mode rapidly activated the optical feedback sensor possibly due to heat accumulation. Modulated light delivery induced by optical feedback diminished ablation efficiency by 30 % in comparison with no feedback case. However, long-term transmission results validated that laser ablation assisted with optical feedback was able to almost consistently sustain light delivery to the tissue as well as ablation efficiency. Therefore, an optical feedback sensor can be a feasible tool to protect optical fiber tips by minimizing debris contamination and delaying thermal damage process and to ensure more efficient and safer laser-induced tissue ablation.
Keywords Optical feedback . IR detector . Laser ablation . Thermal damage
H. W. Kang (*) Department of Biomedical Engineering and Center for Marine-Integrated Biomedical Technology (BK21 Plus), Pukyong National University, 45 Yongso-ro, Nam-gu, Busan 608-737, South Korea e-mail: [email protected]
Introduction Optical fibers have been used to photothermally treat a variety of disease or disorders in medical fields, including dermatology, gynecology, neurology, and urology [1–6]. As laser technology has been advancing, a wide range of wavelengths as well as power levels have been attempted with fibers to improve clinical outcomes. Depending on anatomical features and clinical goals, a number of fiber tips have been designed and evaluated such as ball-shaped, tapering, side-firing, and diffusing types [7]. Particularly, surgical fibers used for laser prostate ablation have been utilized to deliver high optical power of up to 200 W to the tissue lesions of interest [8]. Previous clinical studies quantitatively demonstrated severe deterioration of fiber tips in terms of laser energy delivery during laser prostatectomy [9, 10]. Due to the reattachment of ablated debris to the fiber tip surface, fiber transmission dramatically decreased down to 20 % after an hour delivery of 80 W [9]. The degradation rate was even further aggravated with the delivery of higher power of 120 W, finally causing the tested fiber to fail [10]. The decreased transmission led to inefficient ablation and undesirable coagulation in tissue, eventually prolonging operation time and postoperative patient discomfort such as dysuria and urinary retention. During laser surgery, catastrophic fiber failure could induce irreversible damage to the surrounding tissue and even bladder perforation [10, 11]. Additionally, our unpublished simulation study also verified that as thermal nucleates, the tissue debris attached to the fiber tip considerably increased temperature by more than 1,400 K, which is higher than glass transition temperature (i.e., 927 K for quartz). In turn, excessive heat accumulation can primarily be concentrated on the tip, which initiates and aggravates melting or devitrification of the fiber tip, leading to unfavorable fiber failure that may shatter the glass tip and eventually require
Lasers Med Sci
additional procedure or surgery to eliminate the materials that remained in prostate or bladder with a cystoscope. In an attempt to minimize or prevent fiber deterioration during laser surgery, many fiber manipulation techniques have been investigated. Sweeping a surgical fiber during laser treatment at a constant speed and angle could maximize ablation efficacy as well as minimize thermal injury to tissue and fiber surface [12–14]. In addition, maintaining the fiber distance between fiber and tissue surface at 1~3 mm could give rise to consistent ablation performance due to almost invariable irradiance, thus reducing a probability of debris reattachment to the fiber surface [15]. In the face of the recommended techniques, fiber damage and eventual failure have often occurred in clinical situations due to complexities of surgical procedures and surgeons’ skill as well as pertaining experience. Thus, there is still an imminent need to prevent the fiber tip deterioration process in advance with minimal dependence upon human and environmental factors. In the current study, an optical feedback sensor was developed and tested to sense the onset of thermal damage at the fiberglass cap during laser ablation and eventually to obstruct deterioration process associated with intense interaction between fiber and tissue. The sensor was designed to detect infrared (IR) emissions generated from the fiber glass cap during the irradiation and to instantly modulate the transmitted laser light for cooling the tip. The feasibility of optical feedback during fiber-based laser surgery was evaluated ex vivo to identify any variations in ablation performance along with light transmission.
Materials and methods Porcine kidney tissue was used for ex vivo fiber testing, in that optical properties of glandular components are well known [7]. The kidney tissue was achieved from a local grocery store and sectioned in the area of 3×3 cm2. Prior to tissue experiments, all the prepared samples were stored at 4 °C to minimize dehydration and morphological deformation. As a light source, a customized 532-nm laser system was employed to irradiate the kidney tissue for laser-induced ablation, and during the tests, each sample was maintained at around room temperature. The laser system delivered a power of 100 W through a straight-firing 600-μm core-diameter fiber (FT600EMT, Thorlabs, NJ, USA) with a protective glass cap onto the tissue surface. In order to achieve the prompt fiber protection during tissue ablation, a customized mechanical shutter was installed on the optical pathway (Fig. 1). The shutter was connected to a photodetector (PDA20H, Thorlabs, NJ, USA) with an IR band-pass filter (2~5 μm), which could detect any IR emissions generated from the glass cap due to undesirable heat accumulation at the fiber tip during tissue ablation. Once the IR signals sensed by the photodetector went above the predetermined signal threshold, the detector
immediately could trigger the shutter to block the incoming laser light. Thus, the intermittent light modulation could allow the fiber glass cap to cool down until the amplitude of the detected IR signals became lower than that of the threshold. Preliminarily, the maximum amplitude of IR signals generated during laser ablation of kidney tissue was measured around 5.2 V with a photodetector. Thus, various levels of the signals (from 1 to 5 V with 1-V increment) were evaluated to identify the appropriate range of the triggering threshold for activating optical feedback and minimizing thermal damage at the fiber tip. All the detected signals were acquired with a digital oscilloscope (DPO3054, Tektronix, OR, USA). For tissue ablation, a kidney specimen was placed in a tissue holder (Fig. 1). Each sample was covered with a plastic plate with a 1×1.5-cm2 aperture to firmly secure the tissue. During the tests, the entire holder was submerged in saline (room temperature) to emulate clinical situations. A computercontrolled XYZ stage was used to predetermine various physical distances between tissue surface and fiber tip in order to reflect three surgical techniques such as sweeping (2 mm), close (1 mm), and full contact mode (0 mm). Ablation speed was fixed at 4 mm/s, which was often implemented by surgeons [12]. The temporal profile of the acquired IR signals was also measured and evaluated. Ablation performance was qualitatively and quantitatively compared without and with optical feedback to predict any differences in ablation outcomes. Postexperimentally, each ablated tissue was crosssectioned and imaged with a digital camera. Then, physical dimensions of coagulated and carbonized lesions were measured at various locations (i.e., 3, 5, 6, 7, and 9 o’clock) with ImageJ (National Health Institute, Bethesda, MD, USA). Ablation volume was also estimated by integrating all the crosssectional areas measured from a specimen. Each condition was run five times (N=5), and a Student’s t test was used for statistical analysis (p
