Analytical and Computational Tools for Long-Pulse Photoacoustics

光声信号产生技术是一种在无创医学断层成像中具有巨大潜力的技术。利用这种技术，在样品上使用短脉冲激光能量。吸收的能量产生小的温度上升，这反过来又引起由于热膨胀而引起的样品内部压力的变化。该压力充当超声波源，该超声波可以由位于样品外部的超声检测器检测到。由于血液和周围组织吸收光能的方式存在很大差异，这提供了一种寻找组织中异常的方法。因此，该方法适用于微血管系统的成像、肿瘤检测或组织表征。 生物医学光声的一般领域在很大程度上是基于使用非常短的激光脉冲。短脉冲的方法提供了计算的优势，但也有许多困难与仪器和辐射暴露的限制。为了克服这些困难，已经出现了一种方法，该方法使用线性频率调制的光波而不是短脉冲来产生超声波。这种技术被称为频域光声。使用这种方法，使用较长的脉冲，脉冲光声的基础上的许多建模和计算方法必须重新检查。最近，雷达的脉冲压缩技术已经利用频域光声来实现，由此所获取的声信号与所生成的光信号互相关。这种方法已经成功地证明了实验，但许多分析和计算的问题仍然是脉冲压缩雷达尚未进行分析或优化的光声应用。 拟议研究将解决的问题是：频域光声可以保留哪些脉冲光声建模和计算方法？如何有效地利用脉冲压缩雷达方法来产生基于频域光声测量的图像？探测（输入）波形是否可以设计为实现（i）更好的信噪比（ii）更多的信息（iii）对图像重建有用的信息？ 这项研究的目标是开发光声雷达成像建模和计算方法，这些方法有望对例如人类乳房中的癌性病变进行灵敏的诊断。改进的建模和计算方法，除了更好的波形设计，需要实现这些诊断和成像目标，并因此形成拟议的研究的目标。

Photoacoustic signal generation is a technique which has demonstrated great potential for non-invasive medical tomography. With this technique, a short-pulse of laser energy is used on a sample. The absorbed energy produces a small temperature rise, which in turn induces a change in pressure inside the sample due to thermal expansion. This pressure acts as a source of ultrasound waves which can be detected by ultrasound detectors positioned outside the sample. Since there is a large difference between how blood and surrounding tissue absorb the light energy, this provides a way to look for anomalies in the tissue. This approach is thus suitable for imaging of the micro-vascular system, tumour detection or tissue characterization. The general field of biomedical photoacoustics has largely been based on using very short laser pulses. The short-pulse approach offers computational advantages but also many difficulties with instrumentation and radiation exposure limits. To overcome these difficulties, a methodology has emerged that uses linearly frequency modulated waves of light instead of short pulses to generate the ultrasound wave. This technique is referred to as frequency domain photoacoustics. With this approach that uses longer pulses, many of the modelling and computation approaches on which pulsed photoacoustics is based must be re-examined. More recently, pulse compression techniques of radar have been implemented with frequency domain photoacoustics, whereby the acquired acoustic signal is cross-correlated with the generating light signal. This methodology has been successfully demonstrated experimentally, however many analytical and computational questions remain as pulse-compression radar has not been analyzed or optimized for a photoacoustic application. The questions that the proposed research will address are: Which modelling and computation approaches of pulsed photoacoustics can be retained for frequency domain photoacoustics? How can pulse-compression radar approaches be used effectively to produce images based on frequency domain photoacoustic measurements? Can probing (input) waveforms be designed to achieve (i) a better signal-to-noise ratio (ii) more information (iii) information that is useful for the reconstruction of image? The goal of this research is the development of photoacoustic radar imaging modelling and computational methodologies which hold the promise for sensitive diagnostics of cancerous lesions in, for example, a human breast. Improved modelling and computational approaches, in addition to better waveform design are required to achieve these diagnostic and imaging objectives, and as such form the goals of the proposed research.