Non-contact methodology to detect surface displacements for photoacoustic imaging reconstruction

在商业光声系统中，非电离激光脉冲传播通过介质生物材料或组织并加热目标（即生物材料中的光学吸收剂、组织血管中的血红蛋白）。由目标吸收的热量引起瞬时热弹性膨胀，这产生压力波，该压力波通过耦合介质传播到表面或皮肤，并且由超声换能器检测以形成目标的图像。使用这种耦合介质存在许多缺点。 首先，耦合介质可以散射和衰减光脉冲，从而减少产生的压力波，降低系统的信噪比（SNR）。第二，当压力波通过耦合介质传播时，耦合介质使压力波衰减。第三，耦合介质的使用是可能使实验设置复杂化的步骤，这是由于凝胶的低粘度和难以保持静止的事实，这在临床上使许多患者感到不方便、混乱并且延长了成像过程。在本计画中，将发展一种不使用耦合介质的更直接的光声成像系统。一束连续的激光束将聚焦在表面上。 表面的运动导致反射光束的多普勒频移，这可以使用干涉测量技术来检测。这一技术发展也将指导后续激光干涉光声系统的设计和建造。

In commercial photoacoustic systems, a non-ionizing laser pulse propagates through media biomaterials or tissue and heats a target (i.e. optical absorbers in biomaterials, hemoglobin in tissue blood vessels). The absorbed heat by the target causes a transient thermal elastic expansion, which creates a pressure wave that propagates to the surface or skin through a coupling medium and that is detected by an ultrasound transducer to form an image of the target. There are many disadvantages of using such a coupling medium. First, the coupling medium can scatter and attenuate the light pulse and thereby reduce the pressure wave generated, decreasing the signal to noise ratio (SNR) of the system. Second, the coupling medium attenuates the pressure wave as it propagates through the coupling medium. Third, the use of a coupling medium is a step that can complicate an experimental set-up due to the low viscosity of the gel and the fact that it is hard to keep in stationary, which clinically, many patients find inconvenient, messy, and lengthens the imaging procedure. In this project, a more direct photoacoustic imaging system will be developed without using a coupling medium. A continuous laser beam will be focused on the surface. The surface's motion causes a Doppler shift in the reflected beam which can be detected using interferometry techniques. This technological development will also guide the subsequent design and construction of a laser interferometry photoacoustic system.