激光束光斑尺寸和研究概况

Laser Tissue Welding: Laser Spot Size and Beam Profile Studies Abstract ：This paper evaluates the effect of laser spot diameter and beam profile on the shape of the thermal denaturation zone produced during laser tissue welding. 2-cm-long full-thickness incisions were made on the epilated backs of guinea pigs in vivo. India ink was used as an absorber and clamps were used to appose the incision edges. Welding was performed using continuous-wave 1.06-μm , Nd:YAG laser radiation scanned over the incisions to produce 100-ms pulses. Laser spot diameters of 1, 2, 4, and 6 mm were studied, with powers of 1, 4, 16, and 36 W, respectively. The irradiance remained constant at 1272cm W Monte Carlo simulations were also conducted to examine .the effe ct of laser spot size and beam profile on the distribution of photons absorbed in the tissue. The laser spot diameter was varied from 1 to 6 mm. Gaussian, flat-top, dual Gaussian, and dual flat -top beam profiles were studied. The experimental results showed that 1-, 2-, 4-, and 6-mm-diameter spots produced thermal denaturation to an average depth of 570, 970, 1470, and 1900 m, respectively. Monte Carlo simulations demonstrated that the most uniform distribution of photon absorption is achieved using large diame ter dual flat -top beams.Index Terms — Denaturation, laser biomedical applications, laser materials-processing applications, laser welding, Monte Carlo methods, optical propagation.MATERIALS AND METHODSA. ExperimentsIn vivo welding of skin incisions was performed at constant irradiance to investigate the effect of various laser spot sizes (1-, 2-, 4-, and 6-mm-diameter FWHM) on the extent of thermal denaturation at the weld site. Adult female albino guinea pigs (Hartley, age 7–8 weeks, weight 400–500 grams) were shaved then epilated with a chemical depilator (Nair,Carter-Wallace, Inc., New York, NY). Each guinea pig was anesthetized with atropine (0.05 mg/kg), ketamine (30 mg/kg), and xylazine (2 mg/kg) administered by intraperitoneal injection. 1% lidocaine with 1:100000 epinephrine was used as a local anesthetic at each incision site. 2-cm-long, fullthickness incisions were made parallel to the spine with a no.15 scalpel. Four incisions were made on the back of each guinea pig. Approximately 2–5 l of India ink (black India Rapidograph ink, 3080-F, 100-nm particle diameter, Koh-INoor, Bloomsbury, NJ) were applied to the wound edges with a micropipette. The animal was then placed prone on a translation stage, in preparation for surgery. Clamps were used to temporarily appose the incision edges during welding.Welding was performed with a continuous-wave (CW), Nd:YAG laser (Lee Laser, Model 703T) emitting 1.06m μ radiation that was coupled into a 600m μ -core diameter optical fiber (Thor Labs, Newton, NJ). A stepper-motor-driven translation stage (Newport, Irvine, CA) scanned the laser beam along the axis of the weld site at speeds that effectively produced 100-ms-long pulses. Seventy scans were made along each weld; the beam stopped at the end of the weld site for 10 s after each scan. To minimize thermal damage to the skin beyond the weld area, high-reflecting metal plates placed on each end of the incision blocked the beam. Experiments were performed at constant irradiance (1272cm w ) comparing laser spot diameters of 1, 2, 4,and 6 mm [full-width at full-maximum(FWHM)], with laser output powers of 1, 4, 16, and 36 W, respectively. The beam profile, as measured by scanning a 200- m-diameter pin hole across the beam, was approximately Gaussian for all spot diameters. The power delivered to the tissue was measured before each weld with a power meter (Molectron PowerMax 5100, Portland, OR). It shows the experimental configuration used for dye-assisted laser skin welding and summarizes the laser parameters for this study.After welding, the anesthetized guinea pig was euthanized with an intracardiac overdose of sodium pentobarbitol (Nembutal, Abbott Laboratories, North Chicago, IL). The dorsal skin, including epidermis and dermis, was excised with a scalpel and then sectioned. Samples were processed using standard histological techniques, including storage in 10% formalin, processing with graded alcohols and xylenes, parafin embedding, sectioning, and hemotoxylin and eosin staining. A minimum of seven samples was processed for each laser spot diameter and beam profile. The 6-mm-diameter spot study was discontinued after grossly obvious burns developed at the wound site.Thermal denaturation measurements were made using a transmission light microscope (Nikon, Japan) fit with crossed linear polarizers (Prinz, Japan). Thermal denaturation was measured laterally from the center of the weld site at three different depths: the papillary dermis, mid-dermis, and base of the dermis. The depth to which one observed denaturation was recorded and divided by the skin thickness to obtain the fraction of a full-thickness weld that was achieved. Measurements were made consistently to the point at which complete thermal denaturation of the tissue was observed.Statistical analyzes were conducted on the histological data. ANOV A was used to determine statistical significance of thermal denaturation measurements between laser spot size groups.B.Monte Carlo SimulationMonte Carlo simulations were run to investigate the effect of various spot sizes (1–6-mm diameters) and beam profiles (Gaussian versus flat-top and single versus dual beam) on the distribution of absorbed radiation. All simulations were run using code available over the public domain . Several changes were made in the Monte Carlo code to adapt it for use with the geometry of this application. First, because the vertical ink layer in the tissue disrupted the cylindrical symmetry assumed in the Original program, the data were stored in Cartesian rather than cylindrical coordinates and a convolution program was not used to generate the laser beam profile. The beam profile was, instead, created using a random number generator ; a large number of photons was used to create the desired beam profile. Second, the vertical ink layer was modeled as an infinite absorber extending from the skin surface to the base of the dermis with a uniform thickness of 100 m. The experimentally measured absorption coefficient for the ink, was 3500 cm. Even though histologic analysis of the welds showed variable staining of the tissue with a lateral thickness varying from 40 to 100 m, since the ink layer thickness was much greater than the probability that a photon could cross the ink layer was negligible, and the assumption that was infinite is reasonabl e.Third, the skin was modeled as a single dermal tissue layer with the assumption that the epidermis and subcutaneous tissue have optical properties similar to that of the dermis. Finally, even though the optical properties of tissue are known to be temperature-dependent, with the dermal scattering coefficient initially increasing with temperature for temperatures less than 60 C then decreasing sharply at higher temperatures and the dermal absorption coefficient decreasingwith increasing temperature , the optical properties in this model were assumed to be static. This assumption, which avoided a complete optical-thermal model, will result in a slight underestimation of the penetration depth of the photons in the dermis. The optical properties of guinea pig skin at a wavelength of 1.06m μ have not been well characterized. The optical properties for human, pig, and rat dermis were therefore. compiled from several sources. The optical properties used in the Monte Carlo simulations are listed in Table II. Note that in the experimental irradiations, the irradiance was held constant at 127 2cm W . For the simulated irradiations, the mean irradiance over the full-width, halfmaximum of each beam was constant (10 photons per 1-mm-diameter area). The grid element size in the tissue was fixed at 100 m, and the dimensions of the tissue (length width depth) were 1.0 cm 1.0 cm 0.5 cm, respectively. The tissue thickness was, in part, chosen based on the knowledge that human skin may be thicker than guinea pig skin, ranging in thickness from 1 to 4 mm. Simulations were run on a Pentium 133 MHz PC computer (Micron, Nampa, ID)running Microsoft Windows 95 (Microsoft, Redmond, WA) III. RESULTSA. ExperimentsHistologic analysis showed that only shallow welds were achieved using a 1-mm-diameter laser irradiation area. Thermal denaturation was observed only to a depth of 570±100m μ (mean ±S.D.,n=7) or 30% of the average dermal thickness of 1900±200m μ , see Table III. Thermal denaturation lateral to the incision was limited to m μ30100± near the tissue surface. An image of a weld created with a 1-mm-diameter spot is shown in.When the laser spot diameter was increased to 2 mm, thermal denaturation was observed down to the middle layers of the dermis, as show. The thermal denaturation extended to an average depth of m μ210970±(n=7)(p<0.001) or 50% of the dermal thickness. This depth was significantly greater than achieved with a 1-mm-diameter spot Significantly more la teral thermal denaturation was also measured at the surface of the skin, m, than for the 1-mm-diameter spot.Increasing the spot diameter to 4 mm resulted in welds with an average depth of m μ1901470±(n=7) , or 80% of the dermal thickness 。