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生物组织中的吸收体受到脉冲激光辐射时,基态分子吸收光子能量跃迁到激发态,发生非辐射跃迁产生的热能使吸收体温度升高,发生热弹性膨胀,继而产生光声信号。被体外的超声探测器捕获后重建出的光声成像能反映生物体内吸收体的空间分布。其中产生的光声信号振幅可以表示为[15]:
$$ p = I\mu {\eta _{{\rm{th}}}}\varGamma $$ 式中:
$I$ 为激发光的光强;$\;\mu $ 为组织的光吸收系数;${\eta _{{\text{th}}}}$ 为热转换效率;$\varGamma$ 为 Grueneisen 参数,与组织的弹性模量和比热容有关。脉宽为纳秒或皮秒的短脉冲激光作为激发源时,单发脉冲持续的时间远远小于生物组织的热弛豫时间,此时热量被禁闭在光照范围之内,即满足热禁闭条件,吸收体会产生瞬间温升,产生波前速度更大的热弹波,使得伴随产生的光声信号振幅增强。所以选择的激发光源的脉宽在纳秒级别,以保证更高的信号转化效率。所提出的光声成像系统装置如图1(a)所示,其中激光器的波长为532 nm,脉宽为10 ns,实验时利用的重复频率为10 kHz (DTL-314QT, 脉冲调Q,俄罗斯)。激光先经过空间针孔滤波系统滤掉杂散光,再经过分束棱镜(BS)分成两束光,其中一束光作用于光电二极管用于监控激光能量,另一束光经过物镜(NA=0.1)聚焦到样品表面,产生的光声信号被自制的中空聚焦换能器(UT,中心孔直径为3 mm,焦距为8 mm,中心频率为30 MHz,100%带宽)接收,然后再通过信号放大器(AMP, LNA-650, RF BAY)被高速采集卡(DAQ, M3 i.4110)采集,结合二维扫描平台和Matlab软件重建出样品相应的光声成像。
图 1 (a)光声成像系统装置图;(b)系统横向分辨率;(c)系统轴向分辨率
Figure 1. (a) Photoacoustic (PA) imaging system experimental setups; (b) System lateral resolution; (c) System axial resolution
实验系统的横向分辨率是通过成像锐利的手术刀锋边缘来测量的。通过扫描锐利刀锋边缘得到光声信号的边缘扩散函数(ESF),进一步求导得到线扩散函数(LSF),最后测得系统横向分辨约为20 μm,如图1(b)所示。轴向分辨率的测量通过对直径为20 μm的钨丝进行x-z截面成像得到,如图1(c)所示,系统的轴向分辨率约为75 μm。在实验过程中,系统的电动平移台 (LS2-25T, 简成光电,中国)快轴速度设置为10 mm/s,慢轴的步距为10 μm,所以扫描10 mm×10 mm的成像区域所需时间大约为16.7 min,后续可以提高激光的重复频率,实现更快的成像速度。
Applications of photoacoustic technology in brain tissue imaging (invited)
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摘要: 基于激光诱导超声机制的光声成像技术结合了光学成像的高对比度和超声成像的深穿透性,能无标记、非侵入反映生命体内源性吸收物质的分布,适合啮齿类动物模型全脑的即时成像。为了证明光声技术在脑科学研究和脑疾病监测中的应用,搭建了光声显微成像系统,其空间分辨率可达几十微米,有效成像深度可达1 mm以上,并以APP/PS1转基因阿尔茨海默症(Alzheimer’s disease, AD)模型小鼠和野生型WT小鼠为研究对象,从脑组织切片、离体全脑和活体全脑三个层面探究了光声成像在表征AD鼠和WT鼠脑结构变化和血管网络的能力,证明了光声技术在研究脑疾病发展过程中监控脑结构变化和脑血管网络特征的巨大潜力,可以为诸多脑科学研究和神经退行性疾病发展机制提供更深入的见解。Abstract: Photoacoustic imaging technology based on laser-induced ultrasound mechanism combines the high contrast of optical imaging and the deep penetration of ultrasound imaging, which can reflect the distribution of endogenous absorbents in living organisms in a label-free and non-invasive way, especially suitable for real-time imaging of the whole brain of rodent models. In order to prove the application of photoacoustic imaging technology in brain science research and brain disease monitoring, a photoacoustic microscopic imaging system with spatial resolution of tens of microns and effective imaging depth of more than 1 mm was constructed. Taking APP/PS1 transgenic Alzheimer’s disease (AD) model mice and WT mice as research objects, the ability of photoacoustic imaging in characterizing the brain structure changes and vascular network of AD mice and WT mice was explored from three levels of brain tissue slices, in vitro whole brain and in vivo whole brain. It demonstrates the great potential of photoacoustic imaging technology in monitoring brain structural changes and cerebrovascular network characteristics during the development of brain diseases, which can provide deeper insights into many brain science studies and the development mechanism of neurodegenerative brain diseases.
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Key words:
- photoacoustic imaging /
- brain imaging /
- Alzheimer's disease /
- photoacoustic angiography
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图 2 (a) WT鼠和AD鼠大脑组织和脑冠状切面的光学照片(蓝色虚线标注了冠状切面所在位置);(b) WT鼠和AD鼠脑组织冠状切面的光声成像;(c) WT鼠脑皮层区域光声信号幅值分布统计图;(d) AD鼠脑皮层区域光声信号幅值分布统计图
Figure 2. (a) Optical photographs of WT and AD rat brain tissue and coronal sections (blue dashed lines indicating the location of the coronal section); (b) Coronal PA imaging of WT and AD mice brain; (c) Statistical graph of amplitude distribution of PA signal in cortex of WT mouse; (d) Statistical graph of amplitude distribution of PA signal in cortex of AD mouse
图 3 (a) WT鼠和AD鼠脑组织的光学照片;(b) WT鼠和AD鼠脑组织的光声成像;(c) WT鼠和AD鼠脑组织沿图(a)蓝色虚线位置的冠状面光声成像;(d) WT鼠和AD鼠脑组织深度为0.6 mm处横断面光声成像
Figure 3. (a) Optical photograph of brain tissue of WT and AD mice; (b) PA imaging of WT and AD mice brain tissue; (c) Coronal PA imaging of WT and AD mice brain tissue along the blue dotted line in (a); (d) Transverse PA imaging at a depth of 0.6 mm in WT and AD mouse brain tissue
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