Development of electro-optically tuneable Fabry-Pérot ultrasound sensors for high speed biomedical photoacoustic imaging

光声成像（PA）是一种新兴的生物医学成像方式，它依靠吸收短光脉冲在组织内产生超声波。这些波传播到皮肤，在那里时间分辨PA信号被传感器阵列检测到。然后使用图像重建算法获得高分辨率（数十微米）的三维图像。PA成像结合了许多强大的属性，例如多尺度成像能力，从单细胞分辨率（使用PA显微镜）到使用PA断层扫描的微米分辨率（毫米深度数十微米到厘米深度数百微米），以及在血管化软组织中的强对比度，而其他方式如MRI， x射线CT和超声缺乏灵敏度。它是非侵入性的，并且结合了纯光学成像方式的优点，即光谱特异性，以及传统超声成像的优点，即高空间分辨率。虽然压电超声探测器的应用最为广泛，但当应用于表面、高分辨率PA成像时，它们有明显的缺点，因为所需的小有源元件尺寸导致低声灵敏度。相比之下，光学超声探测器，如法布里-普氏干涉仪（FPI）传感器已被证明提供非常小的，衍射有限的元件尺寸，高声学灵敏度，近均匀的频率响应（dc-100MHz），以及有效的后向模式PA成像的光学透明度。基于FPI传感器的全光PA扫描仪无疑为厘米深度的高分辨率3d成像设定了标准。它的成像性能使得在临床前研究中获得令人信服的大脑、皮肤和肿瘤的血管系统图像成为可能。然而，目前的扫描仪设计有一个显著的局限性：与基于压电探测器阵列的最先进的扫描仪相比，成像速度较低。该项目旨在通过开发用于快速生物医学PA成像的电光可调谐FPI传感器来克服这一限制，通过满足以下目标：1)开发和合成电光（EO）可调谐聚合物间隔，2)开发EOFPI传感器的制造方法，3)开发用于EOFPI控制和并行读出的仪器，以及4)使用并行检测的高帧率PA成像演示。这有望为FPI传感概念实现的成像速度提供一个阶跃变化。最终，这项技术可以提供实时体积，高分辨率的PA成像，这将为广泛的临床前和临床应用打开大门，如神经功能成像和血流成像。在这个项目中，目的是演示基于eofpi的PA成像的原理验证及其随之而来的成像速度的提高。

Photoacoustic (PA) imaging is an emerging biomedical imaging modality that relies on the absorption of short optical pulses to generate ultrasound waves within the tissue. These waves propagate to the skin where time-resolved PA signals are detected by transducer arrays. High resolution (tens of microns) 3-D images are then obtained using image reconstruction algorithms. PA imaging combines a number of powerful attributes, such as multiscale imaging capabilities ranging from single cell resolution (using PA microscopy) to micron resolution using PA tomography (tens of microns at mm depths to hundreds of microns at cm depths) and strong contrast in vascularised soft tissues where other modalities such as MRI, x-ray CT, and ultrasound lack sensitivity. It is non-invasive and combines the advantages of purely optical imaging modalities, i.e. spectral specificity, with those of conventional ultrasound imaging, i.e. high spatial resolution. While piezoelectric ultrasound detectors are the most widely used, they have distinct drawbacks when applied to superficial, high resolution PA imaging since the required small active element size results in low acoustic sensitivity. By contrast, optical ultrasound detectors such as Fabry-Pérot interferometer (FPI) sensors have been shown to provide very small, diffraction-limited element sizes, high acoustic sensitivity, near-uniform frequency response (dc-100MHz), and optical transparency for efficient backward mode PA imaging. The all-optical PA scanner based on the FPI sensor has arguably set the standard for high resolution, 3-D imaging to cm depths. Its imaging performance has enabled the acquisition of compelling images of the vasculature in the brain, the skin, and tumours in preclinical studies. However, the current design of the scanner has one significant limitation: low imaging speed compared to state-of-the-art scanners based on piezoelectric detector arrays. This project aims to overcome this limitation by developing electro-optically tuneable FPI sensors for fast biomedical PA imaging by meeting the following objectives: 1) development and synthesis of electro-optically (EO) tuneable polymer spacers, 2) development of methods for the fabrication of EOFPI sensors, 3) instrumentation development for EOFPI control and parallelised readout, and 4) demonstration of high frame rate PA imaging using parallelised detection. This is expected to provide a step change in imaging speed achievable with the FPI sensing concept. Ultimately, this technology could provide real time volumetric, high resolution PA imaging, which would open the door to a broad range of preclinical and clinical applications, such as neuro-functional imaging and blood flow imaging. In this project, the aim is to demonstrate the proof-of-principle of EOFPI-based PA imaging and its attendant increase in imaging speed.