激光束光斑尺寸和研究概况
Lar Tissue Welding: Lar Spot Size and Beam Pro?le Studies Abstract ：
This paper evaluates the effect of lar spot diameter and beam pro?le on the shape of the thermal denaturation zone produced during lar tissue welding. 2-cm-long full-thickness incisions were made on the epilated backs of guinea pigs in vivo. India ink was ud as an absorber and clamps were ud to appo the incision edges. Welding was performed using continuous-wave 1.06-μm , Nd:YAG lar radiation scanned over the incisions to produce 100-ms puls. Lar 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 lar spot size and beam pro?le on the distribution of photons absorbed in the tissue. The lar spot diameter was varied from 1 to 6 mm. Gaussian, ?at-top, dual Gaussian, and dual ?at -top beam pro?les were studied. The experimental results showed that 1-, 2-, 4-, and 6-mm-diameter spots produced thermal denaturation to an av
erage 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 ?at -top beams.
Index Terms — Denaturation, lar biomedical applications, lar materials-processing applications, lar welding, Monte Carlo methods, optical propagation.
MATERIALS AND METHODS
A. Experiments
In vivo welding of skin incisions was performed at constant irradiance to investigate the effect of various lar 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 intra
peritoneal injection. 1% lidocaine with 1:100000 epinephrine was ud 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 ud to temporarily appo the incision edges during welding.
Welding was performed with a continuous-wave (CW), Nd:YAG lar (Lee Lar, Model 703T) emitting 1.06m μ radiation that was coupled into a 600m μ -core diameter optical ?ber (Thor Labs, Newton, NJ). A stepper-motor-driven translation stage (Newport, Irvine, CA) scanned the lar beam along the axis of the weld site at speeds that effectively produced 100-ms-long puls. 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-re?ecting metal plates placed on each end of the incision blocked the beam. Experiments were performed at constant irradiance (1272
cm w ) comparing lar spot diameters of 1, 2, 4,
and 6 mm [full-width at full-maximum(FWHM)], with lar output powers of 1, 4, 16, and 36 W, respectively. The beam pro?le, 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 con?guration ud for dye-assisted lar skin welding and summarizes the lar parameters for this study.
After welding, the anesthetized guinea pig was euthanized with an intracardiac overdo of sodium pentobarbitol (Nembutal, Abbott Laboratories, North Chicago, IL). The dorsal skin, including epidermis and dermis, was excid with a scalpel and then ctioned. Samples were procesd using standard histological techniques, including storage in 10% formalin, processing with graded alcohols and xylenes, para?n embedding, ctioning, and hemotoxylin and eosin staining. A minimum of ven samples was procesd for each lar spot diameter and beam pro?le. 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) ?t with crosd 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 ba of the dermis. The depth to which one obrved 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 obrved.
Statistical analyzes were conducted on the histological data. ANOV A was ud to determine statistical signi?cance of thermal denaturation measurements between lar spot size groups.
B.Monte Carlo Simulation
Monte Carlo simulations were run to investigate the effect of various spot sizes (1–6-mm diameters) and beam pro?les (Gaussian versus ?at-top and single versus dual beam) on the distribution of absorbed radiation. All simulations were run using code available over t
he public domain . Several changes were made in the Monte Carlo code to adapt it for u with the geometry of this application. First, becau 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 ud to generate the lar beam pro?le. The beam pro?le was, instead, created using a random number generator ; a large number of photons was ud to create the desired beam pro?le. Second, the vertical ink layer was modeled as an in?nite absorber extending from the skin surface to the ba of the dermis with a uniform thickness of 100 m. The experimentally measured absorption coef?cient 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 in?nite is reasonabl e.
