[论文解读] One laser pulse generates two photoacoustic signals

Fei Gao, Xiaohua Feng+7 more|arXiv (Cornell University)|Feb 25, 2016

Photoacoustic and Ultrasonic Imaging参考文献 26被引用 6

一句话总结

本文提出了一种长激光脉冲诱导的双光声（LDPA）效应，即单个激光脉冲可产生两个不同的光声信号——第一个来自脉冲上升沿的热膨胀，第二个来自脉冲下降沿的热收缩。该方法利用了不完全的应力和热约束，通过热扩散和格鲁内森参数饱和实现幅度可调、波形反转的信号，为生物医学成像中的对比度增强提供了一种新颖的非线性光声方法。

ABSTRACT

Photoacoustic sensing and imaging techniques have been studied widely to explore optical absorption contrast based on nanosecond laser illumination. In this paper, we report a long laser pulse induced dual photoacoustic (LDPA) nonlinear effect, which originates from unsatisfied stress and thermal confinements. Being different from conventional short laser pulse illumination, the proposed method utilizes a long square-profile laser pulse to induce dual photoacoustic signals. Without satisfying the stress confinement, the dual photoacoustic signals are generated following the positive and negative edges of the long laser pulse. More interestingly, the first expansion-induced photoacoustic signal exhibits positive waveform due to the initial sharp rising of temperature. On the contrary, the second contraction-induced photoacoustic signal exhibits exactly negative waveform due to the falling of temperature, as well as pulse-width-dependent signal amplitude which is caused by the concurrent heat accumulation and thermal diffusion during the long laser illumination. An analytical model is derived to describe the generation of the dual photoacoustic pulses, incorporating Gruneisen saturation and thermal diffusion effect, which is experimentally proved. Lastly, an alternate of LDPA technique using quasi-CW laser excitation is also introduced and demonstrated for both super-contrast in vitro and in vivo imaging. Compared with existing nonlinear PA techniques, the proposed LDPA nonlinear effect could enable a much broader range of potential applications.

研究动机与目标

• 研究使用长激光脉冲而非传统短脉冲生成双光声信号。
• 理解单个激光脉冲产生两个不同光声信号的物理机制。
• 建立一个结合热扩散和格鲁内森参数饱和的解析模型，以解释双信号行为。
• 展示LDPA技术在体外和体内光声成像中实现超对比度的可行性。
• 探索准连续激光激发作为非线性光声传感中脉冲激发的替代方案的潜力。

提出的方法

• 本研究采用长方波形激光脉冲，在脉冲起始时诱导热膨胀，在脉冲衰减时诱导热收缩。
• 在激光脉冲的上升沿和下降沿分别测量光声信号，分别产生正向和负向波形。
• 推导出一个包含热扩散效应和格鲁内森参数饱和的解析模型，以描述双信号的产生。
• 通过比较不同脉宽下预测的信号幅度和波形与实测数据，对模型进行实验验证。
• 将LDPA技术扩展至准连续激光激发，实现类似连续波的运行并输出双信号。
• 表明信号幅度受脉宽影响，这是由于长时间照射下热积累与热扩散同时发生。

实验结果

研究问题

• RQ1 什么物理机制使得单个长激光脉冲能够产生两个光声信号？
• RQ2 热扩散和不完全的应力约束如何影响双光声信号的幅度和极性？
• RQ3 能否通过热扩散和格鲁内森参数饱和效应准确建模双信号行为？
• RQ4 脉宽对两个光声信号的幅度有何影响？
• RQ5 LDPA效应是否可适配于使用准连续激光激发的体外和体内光声成像中超对比度应用？

主要发现

• 第一个光声信号为正，源于激光脉冲起始时温度迅速上升引起的热膨胀。
• 第二个光声信号为负，源于脉冲衰减时温度下降引起的热收缩。
• 信号幅度与脉宽相关，这是由于长时间照射下热积累与热扩散同时发生。
• 结合热扩散和格鲁内森参数饱和的解析模型能准确预测双信号的波形和幅度。
• LDPA技术在使用准连续激光激发时，可在体外和体内环境中实现超对比度成像。
• 双信号效应归因于未满足的应力和热约束，使其与传统的短脉冲光声学相区别。