Analytical and Computational Tools for Long-Pulse Photoacoustics

光声信号产生技术在非侵入性医学层析成像方面显示出巨大的潜力。利用这种技术，在样品上使用激光能量的短脉冲。吸收的能量会产生一个小的温升，这反过来又会由于热膨胀而引起样品内部压力的变化。这种压力作为超声波的来源，可以被放置在样品外部的超声波探测器检测到。由于血液和周围组织吸收光能的方式有很大不同，这为寻找组织中的异常提供了一种方法。因此，这种方法适用于微血管系统的成像、肿瘤检测或组织定征。 生物医学光声学的一般领域在很大程度上是基于使用非常短的激光脉冲。短脉冲方法提供了计算上的优势，但在仪器和辐射暴露限制方面也存在许多困难。为了克服这些困难，出现了一种使用线性频率调制的光波而不是短脉冲来产生超声波的方法。这种技术被称为频域光声学。对于这种使用较长脉冲的方法，必须重新检查脉冲光声学所基于的许多建模和计算方法。最近，雷达的脉冲压缩技术已经用频域光声学实现，其中采集的声信号与产生的光信号互相关。这一方法已经在实验上得到了成功的证明，但由于脉冲压缩雷达尚未针对光声应用进行分析或优化，因此许多分析和计算问题仍然存在。 拟议的研究将解决的问题是：哪些脉冲光声学的建模和计算方法可以保留用于频域光声学？如何有效地使用脉冲压缩雷达方法来基于频域光声测量产生图像？探测(输入)波形能否设计成(I)更好的信噪比(Ii)更多的信息(Iii)对图像重建有用的信息？ 这项研究的目标是发展光声雷达成像模型和计算方法，有望对例如人类乳房的癌症病变进行灵敏的诊断。为了实现这些诊断和成像目标，除了更好的波形设计外，还需要改进的建模和计算方法，因此形成了拟议研究的目标。

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.