Propagation of Light in Tissue and Imaging

光在组织中传播的理论模型考虑了光的散射和吸收，预测了再发射光的表面强度的空间分布。该模型允许所引入的光和由嵌入的发色团生成的荧光的再发射，所述发色团是组织固有的或引入组织中的。一系列的表面强度分布的测量允许使用逆分析技术的组织的三维结构的重建。原型仪器已经开发出来，用于捕获表面图像进行分析。激光扫描系统在感兴趣区域中的一系列位点处将光引入组织中，并且所发射的光通过二向色滤光器进行光学过滤并成像到冷却的电荷耦合检测器上。使用这种仪器的测量嵌入在一个高度散射混浊介质的荧光标记物产生了极好的重建的理论预测和这些已知的网站的实验测量之间的协议。该技术正在发展为非侵入性方法，以诊断Sjorgen综合征中患病的唾液腺，并确定乳腺癌活检的前哨淋巴结。通过扩展仪器以捕获光谱的近红外区域中的图像，以及通过使用新型红外化合物作为定位在组织内的期望部位处的探针，可以识别组织内的更深结构。 DBEPS与NCI和NICHD合作，探索了纳米晶体作为血管造影造影剂的用途。这些纳米晶体，得自Nomadics，Inc.，具有直径约为5 nm的半导体芯（CdMnHgTe），并涂覆有牛血清白蛋白（BSA）。这些颗粒的大小和组成使它们在近红外（NIR）中发出荧光，在该光谱范围内，水以及大多数组织发色团的吸收率最低。通过将少量纳米晶体静脉注射到小鼠体内，我们已经证明了使用CdTeMnHg-BSA纳米晶体作为近红外范围内的无毒荧光血管造影造影剂的能力。在由皮肤、脂肪和/或骨组成的组织中，位于1-2 mm深度处的直径约为100微米的血管已经被可视化。对于任何体模或体内实验，在时间尺度上纳米晶体没有显著的光漂白或降解。 偏振光的使用已经与NICHD和NCI一起被探索作为跟踪组织结构变化的方法。偏振摄影提供了区分皮肤表面下发展的隐藏结构的潜力，例如由X射线辐射引起的纤维化。可变角度偏振照明系统将激光引导到无胸腺小鼠的皮肤表面上。使用偏振敏感检测器分析散射光（以使直接反射光最小化的角度捕获），以确定由光和组织的相互作用引起的偏振变化。正常无胸腺小鼠和X射线照射小鼠皮肤的偏振度有明显差异。

A theoretical model of light propagation in tissue, which accounts for scattering and absorption of light, predicts the spatial profile of the surface intensity of re-emitted light. The model allows for both re-emission of the introduced light and fluorescent light generated by an embedded chromophore either inherent to or introduced in the tissue. Measurement of a series of surface intensity profiles allows reconstruction of the three-dimensional structure of the tissue using inverse analytical techniques. Prototype instrumentation has been developed to capture surface images for analysis. A laser scanning system introduces light into the tissue at a series of sites in the region of interest and the emitted light is optically filtered through a dichroic filter and imaged on to a cooled charge coupled detector. Measurements using this instrumentation of fluorescent markers embedded in a highly scattering turbid medium yielded excellent agreement between the reconstructed theoretical prediction and the experimental measurement of these known sites. The techniques are being developed as non-invasive methods to diagnose diseased salivary glands in Sjorgen's syndrome and to identify sentinel nodes for biopsy in breast cancer. Identification of deeper structures within the tissue is possible by extending the instrumentation to capture images in the near infra-red region of the spectrum and by the use of novel infra-red compounds to act as probes localized at desired sites within the tissue. DBEPS, in collaboration with NCI and NICHD, has explored the use of nanocrystals as an angiographic contrast agent. These nanocrystals, obtained from Nomadics, Inc., have semiconductor cores (CdMnHgTe) approximately 5 nm in diameter, and are coated with bovine serum albumin (BSA). The size and composition of these particles cause them to fluoresce in the near-infrared (NIR), the spectral range in which the absorbance of water as well as most tissue chromophores is lowest. By injecting small amounts of nanocrystals intravenously into mice, we have demonstrated the ability to use the CdTeMnHg-BSA nanocrystals as a non-toxic, fluorescent, angiographic contrast agent in the NIR range. Blood vessels on the order of about 100 micometers in diameter, located at a depth of 1-2 mm in tissues consisting of skin, fat, and/or bone have been visualized. There was no significant photobleaching or degradation of the nanocrystals over the time scale for any of the phantom or in vivo experiments. The use of polarized light has been explored in conjunction with NICHD and NCI as a method to track changes in tissue structure. Polarized photography offers the potential of distinguishing hidden structures developed below the skin surface such as fibrosis resulting from X-ray radiation. A variable angle polarized illumination system directed laser light on the surface of the skin of an athymic mouse. The scattered light (captured at an angle to minimize directly reflected light) was analyzed using a polarization sensitive detector for changes in polarization resulting from the interaction of the light and the tissue. Clear differences were observed for the degree of polarization between the skin of normal athymic mice and mice irradiated with X-rays.