Ren Yaguang, Zhang Ying, He Honghui, Liu Liangjian, Wu Xiaojun, Song Liang, Liu Chengbo
Research Laboratory for Biomedical Optics and Molecular Imaging, CAS Key Laboratory of Health Informatics, Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China.
School of Mechanical and Electrical Engineering, Xi'an University of Architecture and Technology, Xi'an, China.
Quant Imaging Med Surg. 2022 Apr;12(4):2238-2246. doi: 10.21037/qims-21-658.
Photoacoustic computed tomography (PACT) is a fast-developing biomedical imaging modality and has immense potential for clinical translation. It utilizes laser excitation and acoustic detection to achieve high spatial resolution and considerable imaging depth in biological tissues. Current PACT primarily treats the absorption coefficient of tissues as a scalar variable while reconstructing the image, which limits its use for anisotropic evaluation of the tissues. Thus, by incorporating polarized imaging methods to evaluate anisotropy, applications of PACT can be further enhanced. So far, dichroism-sensitive PACT has been suggested for polarization detection of biological tissues. However, this approach is unsuitable for intraoperative imaging, since high-power spatial light is needed for excitation, which is dangerous and inconvenient to operate. Thus, there is a need to develop a polarized PACT system suitable for clinical use.
Herein, we have proposed a specially designed handheld polarized PACT (HP-PACT) system, which was designed to promote intraoperative anisotropy detection of biological tissues. Excitation light was delivered by an optical fiber and reshaped by a compact set of lenses at the output end of the optical fiber. A polarizer was applied to generate linearly polarized light, and the polarization direction was adjusted by simply rotating the half-wave plate. Photoacoustic imaging (PAI) using excitation with several different polarization directions was carried out. Optical axes and the structure of the anisotropic objects were obtained using the principle of polarization detection with the PAI.
We experimentally demonstrated the performance of HP-PACT by imaging both the polarized and unpolarized plastic films. The results showed that HP-PACT can successfully detect the direction of the optical axes of polarized plastic films and has the ability to image at different depths. When linearly polarized light with different polarization directions was used as excitation, PAI studies on a highly anisotropic bovine tendon and relatively low anisotropic mouse leg showed the structural differences between the 2 tissues. The quantified degrees of anisotropy of the bovine tendon and mouse legs were 0.6 and 0.3, respectively.
The proposed HP-PACT is able to determine the anisotropic substances' optical axes and distinguish anisotropic substances from isotropic ones. Thus, HP-PACT has the potential for intraoperative diagnosis and treatment of anisotropic tissues, including nerves and tendons.
光声计算机断层扫描（PACT）是一种快速发展的生物医学成像模式，在临床转化方面具有巨大潜力。它利用激光激发和声检测在生物组织中实现高空间分辨率和可观的成像深度。当前的PACT在重建图像时主要将组织的吸收系数视为标量变量，这限制了其在组织各向异性评估中的应用。因此，通过结合偏振成像方法来评估各向异性，可以进一步拓展PACT的应用。到目前为止，已提出利用对二向色性敏感的PACT进行生物组织的偏振检测。然而，这种方法不适用于术中成像，因为激发需要高功率空间光，既危险又操作不便。因此，需要开发一种适用于临床的偏振PACT系统。
在此，我们提出了一种专门设计的手持式偏振PACT（HP-PACT）系统，旨在促进生物组织术中各向异性检测。激发光通过光纤传输，并在光纤输出端由一组紧凑的透镜进行重塑。使用偏振器产生线偏振光，通过简单旋转半波片来调整偏振方向。利用具有不同偏振方向的激发光进行光声成像（PAI）。根据PAI的偏振检测原理获取各向异性物体的光轴和结构。
我们通过对偏振和非偏振塑料薄膜成像，实验证明了HP-PACT的性能。结果表明，HP-PACT能够成功检测偏振塑料薄膜的光轴方向，并具有在不同深度成像的能力。当使用具有不同偏振方向的线偏振光作为激发光时，对高度各向异性的牛肌腱和相对低各向异性的小鼠腿部进行的PAI研究显示了这两种组织之间的结构差异。牛肌腱和小鼠腿部的各向异性量化程度分别为0.6和0.3。
所提出的HP-PACT能够确定各向异性物质的光轴，并区分各向异性物质和各向同性物质。因此，HP-PACT在包括神经和肌腱在内的各向异性组织的术中诊断和治疗方面具有潜力。
