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 METHODS
A. Experiments
In 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 (1272
cm 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 Simulation
Monte 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 decreasing
with 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. RESULTS
A. Experiments
Histologic 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 。

The depth of these welds was significantly greater than that produced in both the 1-mm- and 2-mm-diameter spot studies(p<0.001).The lateral zone of thermal denaturation near the epidermis measured m μ40150± similar to that measured for the 2-mm-diameter spot study Significantly more thermal denaturation was measured in the middle layers of the dermis for the 4-mm-diameter spot(m μ40150±,n=7), than for the 2-mm-diameter spot (m μ2570±,n=7)(p<0.001) The thermal denaturation gradient was less, however, when moving from the upper to middle layers of the dermis for the 4-mm-diameter spot than for the 2-mm-diameter spot. The use of a larger spot diameter therefore resulted in not only deeper, but also more uniform welds.
Experiments performed with a 6-mm-diameter spot resulted in full-thickness welds of m μ1900in depth. During surgery, however, the tissue surrounding the weld site began to
redden,blanch, and eventually burn. Due to concerns over the welfare of the animals, these experiments were terminated before statistically significant quantitative data could be obtained.
It should be noted that in some places where there were significant tinctoral and textural changes to the collagen fibers that made the tissue appeared completely denatured, significant collagen birefringence was often still present. In such cases,we measured to the edge of the region of tinctoral and textural change. This observation suggests that one should be careful when using birefringence as a tool for quantifying the extent of thermal denaturation in tissue.
B. Monte Carlo Simulations
It shows the effect of laser spot diameter on the absorption profile. absorption is plotted as a function of the depth into the tissue along the ink layer for an incident beam with a Gaussian beam profile. The absorption at the surface(d=0.0mm) is due to direct absorption of the incident radiation by the ink layer as well as absorption of backscattered radiation. The absorption immediately beneath the surface increases with increasing laser spot diameter because a larger number of photons are backscattered into the ink layer as the spot diameter increases. The distribution of photon absorption along the ink layer with depth in the tissue becomes more uniform at larger spot diameters. The use of a large diameter laser spot results in less scattering of radiation out of the initially collimated laser beam, resulting in a higher effective penetration depth. The depth at which the absorption drops to of the absorption at m is 1.0, 1.2, 1.6, and 2.0 mm for the 1.0-, 2.0-, 4.0-, and 6.0-mm-diameter spots,respectively. These results compare well with the thermal denaturation depths achieved in the experimental studies.
附录B 中文译文
激光的组织接合：激光束光斑尺寸和研究概况
摘要：
本文主要研究在激光缝合肌肉组织时激光光斑的热变性对缝合效果的评估。

将实验白鼠背部去毛切开2cm长的全层切口，用夹子在切口边缘附近固定。

进
行缝合的是采用连续波1.06μm Nd：YAG激光器扫描辐射对切口产生100ms脉冲。

1，2，4和6 mm激光光斑直径进行了研究，其能量分别为1，4，16，36 W。

辐照度维持为127
2
cm
W。

本文对蒙特卡洛模拟也不断进行研究。

激光光斑的大
小以及光束中光子的分布在内部被吸收。

激光光斑直径是不同的，从1到6mm。

高斯，平顶，双高斯，双平顶光束剖面进行了研究。

实验结果表明，1 ，2 ，4 和6mm直径的斑点产生的热变性的平均深度分别为570，970，1470和1900m。

蒙特卡罗模拟表明，光子吸收最均匀分布是通过使用大口径双平顶光束。

关键词：激光生物医学应用，激光材料加工应用，激光缝合，蒙特卡罗方法，光的传播。

材料和方法
A实验
在表面切口内部缝合是在不断进行辐照，调查在缝合时各种激光光斑尺寸效应的热变性程度（1，2，4，6mm直径的半高宽）。

把成年白鼠（哈特利，年龄7-8周，体重400-500g）用化学脱毛剂脱毛（奈尔，卡特 - 华莱士公司，纽约，NY）。

每个豚鼠被麻醉用阿托品（0.05 mg/kg），氯胺酮（30 mg/kg），甲苯噻嗪（2 mg/kg）腹腔注射给药。

把1％利多卡因与1:10万肾上腺素作为局部麻醉用在每个切口部位。

用15号手术刀在平行的脊柱进行了2cm长的全层切口。

拿微吸管将大约2-5升墨汁（印度Rapidograph黑色油墨，3080- F，100纳米颗粒的直径，酸值INoor，Bloomsbury区，新泽西州）分别适用于伤口边缘。

将试验品俯卧放置在实验台，为手术准备。

夹子被用来在缝合时放在附近的切口边缘固定。

缝合的进行是用连续波（CW），Nd：YAG激光（李激光，型号703T）发光1.06m
μ
辐射成600m
μ芯径光纤（托尔实验室，牛顿，新泽西州）。

用电机驱动（新港，尔湾，加利福尼亚州）沿伤口控制缝合速度，有效地生产的100ms长的脉冲激光束的轴。

扫描每个缝合点，光束在缝合点每次扫描10s停止一次。

为了尽量减少缝合面积以外的皮肤收到热损伤，每个切口结束位置都防止金属板来阻挡。

实
验是在恒定光照下（127
2
cm
W的）比较激光光斑直径1，2，4，6ms，激光输
出功率分别为1，4，16，36 W [在全最大（FWHM）全宽]。

通过让光束通过200μm
直径的针孔来衡量光束剖面，所有光斑直径约为高斯分布。

传递到每块肌肉组织的能量与功率计（MolectronPOWERMAX5100，波特兰，OR）连接。

给出了实验配置染料辅助激光焊接皮肤使用。

总结了这项研究的激光参数。

接合后，实验白鼠是安乐死于过量的钠pentobarbitol （戊巴比妥，雅培公司，北芝加哥，IL ）。

背侧皮肤，包括表皮和真皮，是用手术刀切除，然后切片。

样品处理采用标准组织学技术，包括10％的福尔马林存储，与分级醇，二甲苯，石蜡包埋，切片，HE 染色和hemotoxylin 和处理。

每个激光光斑直径和光束最少进行七次实验。

6mm 直径的光斑用于实验，发现伤口部位有非常明显的烧伤，因而停止实验。

热变性测量是用透射光显微镜（Nikon ，日本）配合（普林茨，日本）线性偏光片。

热变性是指从缝合中心在三个不同深度的方向延伸：乳突状真皮，中真皮，以及真皮基地。

深度观察每一个变化的记录和皮肤的厚度为获得一个完全缝合后接合处最小的部分。

测量研究组织的热变性，观察点。

统计分析是进行实验的数据。

用方差来确定激光光斑大小群体之间的热变性测量统计学意义。

B.蒙特卡罗模拟
蒙特卡罗模拟是调查各种斑的大小（1 – 6mm 直径）和光束剖面（高斯与平顶，单与双光束）对吸收的辐射分布的影响。

所有模拟均可以运行使用公共领域的代码。

一些变化的蒙特卡洛代码，用来适应与此应用程序使用它的几何形状。

首先，因为在组织垂直油墨层打乱了圆柱形对称假设在原程序，数据存储在直角坐标，而不是圆柱和卷积程序，不用于产生激光光束。

光束剖面，而是创建使用一个随机数发生器，大量的光子被用来创建所需的光束。

第二，垂直油墨层建模为一个无限延伸，从皮肤表面下100m 厚度的真皮作为吸收基础。

为实验测量油墨的吸收系数，被3500cm ，即使对焊缝组织学分析表明有横向从40到100米不等厚度组织变量染色，由于油墨层厚度比越大概率是一个光子可以穿过油墨层可以忽略不计，并假设是无限的，是合理的。

三，皮肤被塑造为一个单一的假设真皮组织层，表皮和皮下组织有类似真皮的光学特性。

最后，即使组织的光学特性是已知的温度依赖性，与最初的真皮散射系数随温度的增加温度低于60℃，然后在较高的温度急剧下降和皮肤吸收系数随着温度的降低，在这个模型的光学性能均假定是静态的。

他的假设，避免了完整的光热模型，将导致对真皮层的光子在穿透深度略有低估。

白鼠皮肤的在一个1.06m μ波长光学性质没有得到很好的特点。

人类，猪，鼠真皮层的光学性能，因此。

从几个来源汇编。

在蒙特卡罗模
拟中使用的光学特性。

请注意，在辐照实验，辐照度举行了1272W 不变。

对
于模拟照射，超过全宽，每束半最大平均照度为常数（10个1mm 直径的面积光子）。

在组织网格单元尺寸定为100m ，及组织（长宽深）的尺寸分别为1.0cm ，1.0cm ， 0.5cm 。

该组织厚度，部分选择的基础上的知识，人类的皮肤可能会比白鼠皮肤较厚，厚度范围从1到4mm 。

模拟是运行在一台Pentium133MHz 的PC 计算机上运行微软的Windows95（微软，微软，WA ）（美光，南帕，ID ） III 。

结果
A.实验
组织学分析表明，只有浅焊缝均达到使用1毫米直径的激光照射面积。

热变性，观察到570±100μm （平均±S.D.，n=7）或30%的平均厚度为1900±200μm 真皮％的深度。

热变性外侧切口被限制在μm 30100±附近的组织表面。

形象的一个焊接用1mm 直径点创建。

当激光光斑直径增加2毫米，热变性，观察到了真皮层，中间层。

热变性扩展到μm 210970±（n=7）（p<0.001）或皮肤厚度平均深度的 50%。

这个深度显着大于显着的横向热变性μm 40240±（n=7）（p<0,001），也是在皮肤面测量比为1mm 直径当场1mm 直径的现场实现。

增加光斑直径4mm ，导致在焊接用的μm 1901470±（n=7），或皮肤厚度平均深度的80%。

研究表示这些接合处深度均显着高于1mm 和2mm 直径的。

在附近的表皮热变性外侧区测量μm 40150±类似为2mm 直径的实地考察测量（p<0.001）。

在附近的表皮热变性外侧区测量类似于为2mm 直径的点，显着更多的热变性测定为4mm 直径的真皮斑在中间层（μm 2570±,N =7）但是，当移动从上层为4mm 直径点的真皮层中比为2mm 直径的位置，测量的研究比为2mm 直径点（μm 2570±,N =7）（P<0.001）热变性梯度较小。

一个较大的光斑直径，因此使用不仅造成了更深，也更均匀的焊缝。

在1900m μ深度全层接合造成6mm 直径的点进行实验。

然而在手术中，在焊接现场周围组织开始变红，变白，并最终烧毁。

由于对动物福利的关注，这些实验被终止。

但前期的量化数据可以得到。

应当指出，在一些地方存在显着tinctoral 和结构变化的胶原纤维，使组织出现了完全变性，显着的胶原蛋白往往是仍然存在双折射. 在这种情况下，我们测到的tinctoral 和结构变化区域的边缘。

这一观察表明，应该谨慎使用作为量化的热变性在组织范围内的双折射。

B.蒙特卡罗模拟
显示了激光光斑直径对吸收轮廓的效果。

这种吸收由高斯光束入射组织时被绘制出来。

在表面（d=0.0mm ）的这一辐射吸收是由直接辐射吸收油墨层以及反向散射辐射构成。

这种吸收随着激光光斑直径表面增加导致更多的光子数和油墨层散射的光斑直径的增大。

吸收的光子深入该组织分布均匀的油墨层时光斑直径较大。

这种大直径光斑的应用导致了反射的辐射减小和一个更高的穿透程度。

光斑的直径为1.0，2.0，4.0和6.0毫米时，在其中的吸收深度下降，分别为1.0，
1.2, 1.6,
2.0m 。

在这个实验中，比较热能性问题取得较好的结果。