快速荧光寿命显微成像技术及其在活体应用的研究进展(特邀)特邀综述
1 引言
光学显微镜的发明打开了细胞生物学说的大门。在众多光学显微成像技术中,荧光显微成像技术以高时空分辨率、高特异性、非侵入及原位探测等优势,已成为生命科学研究中不可或缺的工具。荧光是指荧光分子处于基态的电子在吸收光子能量后,从基态跃迁到激发态,经过短暂的弛豫过程之后,再通过辐射跃迁回到基态所发出的光[1]。描述荧光特性的参量包括荧光强度、荧光光谱、荧光寿命及荧光偏振,根据这些参量在荧光分子和非荧光背景之间的差异,能够形成高对比度的图像。但是,由于荧光的光致发光特性,荧光的表征容易受到成像系统影响,其中包括光源功率和探测器响应等因素,使得实现定量的生物医学检测变得颇具挑战。
荧光寿命是与时间有关的一个参量,表示电子在激发态平均停留的时间。通常情况下,停止激发荧光分子后,在t时刻的荧光强度服从指数衰减[2],表达式为
式中:ai为第i个荧光组分的相对权重;τ是荧光寿命,是荧光从初始强度I0衰减至原来的1/e所需的时间,这个时间取决于荧光分子的类型及其所处的微环境,与激发光的强度、荧光分子的浓度和光漂白效应等因素无关。因此,荧光寿命显微成像(FLIM)是一种非常适用于定量表征微环境的工具,已在细胞内多种生理参数的高灵敏检测上得到广泛应用,这些参数包括温度[3]、离子浓度[4-5]、极性[6]、氧气浓度[7]、蛋白质浓度[8-10]、黏度[11-12]、磷脂膜张力[13-14]和pH值[15-16]等。此外,FLIM在临床癌症诊断领域显示出巨大潜力,通过测量正常和肿瘤组织中像烟酰胺腺嘌呤二核苷酸(NADH)和黄素腺嘌呤二核苷酸(FAD)这样生物内源性辅酶的荧光寿命的差异,可以辨别肿瘤边界和评估肿瘤的发展阶段[17-21]。
随着对生命科学研究的深入,人们越来越希望能在保持动物正常生理状态稳定的条件下表征细胞或组织的微环境,从而探索细胞组织的功能信息与生命活动之间的关系。通过结合多光子成像、近红外成像及内窥成像等技术,FLIM能够在活体动物中对一定深度的组织进行成像[22-24]。如
2 从硬件方面提升FLIM速度
荧光寿命的测量是FLIM技术的关键,根据测量方式的不同,FLIM技术可以分为两大类:1)时域(TD)法,如
2.1 TCSPC-FLIM
TCSPC是目前应用最广泛的荧光寿命测量技术,它可以和扫描成像或者宽场成像相结合。以其结合传统共聚焦扫描技术为例,通过使用高重复频率的脉冲激光,对样品进行逐点扫描,每个像素点所产生的单光子信号被光电倍增管(PMT)或雪崩光电二极管(APD)等高灵敏探测器收集,通过计时模块分析光子到达探测器的时间,并将结果记录在“到达时间-光子数”直方图中。不过,分析过程会有一段无法记录的死区时间,这是探测模块在记录一个脉冲后,到可以记录下一个新脉冲之前需要的时间间隔,通常为几十到几百纳秒。在这段时间内,其他的光子不会被记录。另外,为了防止堆叠效应影响荧光衰减曲线的准确性,需要把光子的计数率保持在一个较低的水平,大概是激发脉冲数的1%~5%[31]。因此,为了获取足够数量的光子来精准拟合荧光寿命,每个像素点需要通过多次扫描累计上千个光子[26]。这就使得TCSPC-FLIM技术具有优异的时间分辨能力,但同时也意味着其成像速度较慢,通常需要至少30 s才能得到一幅FLIM图像,这在很大程度上限制了它在活体成像中的应用。
近年来,随着微电子工艺的不断改进,发展出许多基于时间数字转换器(TDC)的TCSPC模块,这些模块具有更短的死区时间。同时,探测技术也在不断更新,出现了混合型PMT[32]、单光子雪崩二极管(SPAD)阵列[33]、硅光电倍增管(SiPM)[34]及超导纳米线单光子探测器(SNSPD)[35]等超快响应探测器,这些硬件方面的革新使得快速FLIM成为可能。例如,德国PicoQuant公司[36]研制的TCSPC采集卡(TimeHarp 260 Nano),其死区时间小于2 ns,结合死区时间极短的混合型PMT,能够在相邻的脉冲周期里测量多个光子,如
图 3. 优化探测方式提升TCSPC-FLIM速度。(a)使用更短死区时间的采集模块实现单脉冲周期内记录多个光子[36];(b)使用光子“自旋器”缩短采集卡的死区时间[37];(c)使用并行阵列探测模块提升光子计数率[38]
Fig. 3. Imaging speed improving of TCSPC-FLIM by optimizing detection methods. (a) Using a collection module with a shorter dead time to record multiple photons within a single pulse cycle[36]; (b) using a photon'Spinner' to shorten the dead time of the capture card[37]; (c) using parallel array detection modules to improve photon counting rate[38]
在对生物样品进行成像时,感兴趣区域(ROI)通常只占整个视场的一部分,并且这些区域常常呈现不规则的形状。本团队[39-40]开发了一种快速FLIM技术,如
图 4. 优化扫描方式提升TCSPC-FLIM速度。(a)基于2D-AOD实现任意形状快速扫描[39];(b)利用多焦点阵列实现快速扫描[41];(c)利用线激发实现快速扫描及切片样品的成像结果[44]
Fig. 4. Imaging speed improving of TCSPC-FLIM by optimizing scanning methods. (a) Implementing fast scanning with arbitrary shapes based on 2D-AOD[39]; (b) using a multifocal array to achieve fast scanning[41]; (c) using line excitation to realize fast scanning, and the imaging results of two slices shown on right[44]
2.2 TG-FLIM
TG-FLIM是一种宽场荧光寿命测量技术,通过脉冲光源和门控探测器的配合,控制荧光信号的获取时间和探测频率。具体操作是在脉冲激光激发后不同时刻选通成像,获取一系列荧光强度图像来记录荧光衰减。这里的获取时间包含两个方面:相对于激发脉冲的延迟和单次探测的门宽(即探测器的曝光时间)。目前,延迟和门宽一般可达到纳秒级别或更短,而探测频率可达103 Hz以上。在单帧的门宽内,TG-FLIM可以多次获取脉冲激发后相同时间延迟的荧光信号;并在不同帧之间设置不同的时间延迟,从而根据不同时间延迟帧的强度变化解析每个像素的荧光寿命。与TCSPC-FLIM类似,TG-FLIM同样需要高重复频率的脉冲光源及对荧光强度衰减曲线进行拟合;但TG-FLIM对单组分的荧光寿命进行测量时理论上只需要记录两个不同时间延迟的荧光强度图像,是各类FLIM技术中速度最快的。
在探测模块方面,基于各种技术的TG-FLIM之间存在一些差别,主要体现在成像器件的选择和门控的实现方式上。TG-FLIM常用的成像器件包括增强型电荷耦合器件(ICCD)和SPAD阵列。ICCD是由CCD相机和门控光学图像增强器(GOI)结合而成的,GOI负责将微弱的荧光信号转换为电信号,通过电子放大进而轰击荧光靶面得到增强的光信号,此光信号传输至CCD被接收。这一过程包含光-电-光信号的两次转换,通过外部的调制电路可以控制GOI,以实现门控功能。与基于TCSPC-FLIM的多点SPAD阵列不同,TG-FLIM中的SPAD阵列用于面探测,因此需要更多的像素,门控信号作用于SPAD的偏置电路,使SPAD在指定时刻响应入射光。近期报道的SPAD阵列,像素数达到500×500[51]或512×512[52-53],配合互补金属氧化物半导体(CMOS)器件,能够以高达17.9 ps的时间分辨率和最短0.99 ns的门宽实现TG-FLIM测量。半导体技术的不断进步大大提高了sCMOS的灵敏度和速度,sCMOS也可以用于TG-FLIM。然而,它的响应速度相比CCD和SPAD还是较慢,因此门控信号通常不直接作用于感光器件,而是控制光信号调制器件(如电光调制器[54]或GOI[55]),使荧光信号在门宽时间内照射到sCMOS感光面,从而实现窄门宽和高探测频率。sCMOS具有全局曝光和卷帘曝光两种模式,通常使用全局曝光模式进行TG-FLIM测量,此时每个像素均以相同的脉冲时间延迟探测门宽内的荧光信号;而在卷帘曝光模式下,不同行像素之间存在曝光时间延迟,接近行像素的读出时间(约数10 μs),而这已经远超过门宽时间,将导致门宽内仅有个别行像素能够成像,无法实现快速的宽场全画幅荧光寿命成像。如
图 5. 基于光片技术的TG-FLIM系统[55]。(a)系统示意图;(b)对活体斑马鱼的成像结果
Fig. 5. Schematic of TG-FLIM system based on light-sheet[55]. (a) Schematic of the system; (b) imaging results of a living zebrafish
2.3 PS-FLIM
近年来,发展出一种直接且快速的荧光寿命测量方法:在样品被单个脉冲光激发后,利用高采样率和宽带宽的数据采集模块(如数字示波器)直接记录荧光衰减曲线,该方法称为PS-FLIM[56]。为了有效实施这个方法,需要满足以下条件:1)待测样品的荧光信号必须足够强,能够高于系统噪声,确保记录到的信号包含了可识别的样品信息;2)探测器必须具备高灵敏度和宽带宽,这样才能精确捕捉到强度的细微变化,并能够清楚地区分不同时间点的荧光信号,对单次激发后的荧光衰减曲线有良好的响应。在PS-FLIM中常用的一种探测器是MCP-PMT,其特点是有很短的脉冲响应时间,一般百皮秒的响应时间就足以满足纳秒级荧光寿命的测量需求;3)数据采集系统需要非常高的采样率,针对纳秒级的荧光寿命,根据采样定理,采样率应达到109 Hz以上才能有效记录荧光衰减曲线,这对成像系统的硬件提出了很高的要求。与TCSPC和TG方法一样,PS-FLIM所用的激发光源脉宽应远小于样品的荧光寿命,但是由于单次激发后即可获得荧光衰减曲线,因此光源的重复频率在103 Hz量级即可。由于PS-FLIM不适用于宽场成像而是采用单点探测,它特别适合与内窥光纤结合使用,以进行快速的活体荧光寿命测量,在临床疾病诊断中发挥着重要作用[57-59]。
2.4 SC-FLIM
使用条纹相机(SC)作为探测器,可以实现高时间分辨率的FLIM,在SC的二维光电阴极上,一个维度用于反映时间信息,另一个维度则记录空间信息或者光谱信息[60-61],其基本原理如
本团队[66]开发了一种基于一对检流振镜的双光子2D SC-FLIM技术,其能够测量百皮秒至数微秒的荧光寿命,时间分辨率为50 ps。通过引入双螺旋点扩散函数工程和三维定位追踪技术,从单个快照同时获取多个粒子在微米级深度范围内的空间运动和荧光寿命信息,沿X、Y、Z方向的平均定位精度分别达26 nm、35 nm和53 nm,时间分辨率为103 ps。此外,本团队[67]还研究了巨噬细胞吞噬外源颗粒过程中微环境的动态变化,如
图 6. 用于快速成像的SC-FLIM技术。(a)基于双螺旋点扩散函数工程的单粒子追踪FLIM[67];(b)基于压缩感知的DMD空间编码技术实现高速FLIM[68]
Fig. 6. SC-FLIM technique for rapid imaging. (a) Single particle tracking FLIM based on double helix point spread function engineering[67]; (b) implementation of high-speed FLIM using compressed sensing-based DMD spatial encoding technology[68]
2.5 FD-FLIM
荧光信号的产生频率与激发光的信号同步,但是会有幅度的减小和相位的延迟。根据这一原理,Wang等[69]提出了一种基于调制系数M、角频率为ω和相位延迟Δφ来计算荧光寿命的频域荧光寿命显微成像技术(FD-FLIM)。这种方法对设备要求不高,常用连续的激光器作为光源,对激发光进行正弦调制,并解调测得的荧光信号从而得到荧光寿命。需要注意的是,测得荧光寿命与调制频率成反比,所以测量不同荧光寿命时需要使用不同的调制频率。此外,发光二极管(LED)[70]和脉冲激光器[71]也可以用来提供FD-FLIM的光源。使用LED时也要进行正弦调制,而处理脉冲光源则有所不同,对于固定重复频率的脉冲激发光,产生的荧光信号既有直流分量也有交流分量,与激发光混合之后产生差频信号,从中可解调出荧光寿命信息。FD-FLIM可以与点扫描成像[72]或宽场成像[73-75]结合使用,根据不同需要选择探测器。点扫描FD-FLIM通常使用PMT和光电二极管(PD)作为探测器,其中PMT检测荧光信号,PD获取激发光波形,两路波形由采集卡获取后计算荧光寿命。在宽场成像模式下,FD-FLIM需要获得传感器上每个像素的响应曲线来确定每个像素的荧光寿命,结合高频调制(106 Hz量级),传统的CCD或CMOS无法直接使用,因此宽场FD-FLIM通常采用具有门控功能的高时间分辨率传感器。最近,Yahav等[70]使用MCP和CCD作为探测模块,如
图 7. 调制LED光源的宽场FD-FLIM[70]。(a)光路示意图;(b)不同比例荧光素-甘油混合溶液的荧光强度成像结果;(c)对应的荧光寿命成像结果
Fig. 7. Widefield FD-FLIM based on modulation of LED[70]. (a) Schematic of the system; (b) fluorescence intensity images of Fluorescein-Glycerol (Fl-Gly) solutions with different ratios; (c) corresponding fluorescence lifetime images
3 从算法方面提升FLIM 速度
3.1 校正运动伪影
上述各种FLIM技术都有其独特之处,但通常难以同时实现高的荧光寿命分辨精度和快速成像。因此,在进行活体FLIM时,除了权衡选择合适的FLIM技术外,还可以通过校正运动伪影来实现长时间稳定成像。运动伪影是指在成像期间活体样品的心跳、呼吸或肢体运动等原因造成的图像扭曲。当图像帧率较高时,样品运动主要在帧间引入伪影,这对于需要多帧叠加获取图像堆栈的应用是不利的,比如采集多帧荧光强度图像后获取单帧荧光寿命图像;而当帧率较低时,除了帧间伪影,单帧图像内也会出现失真,导致最终结果进一步恶化。需要特别指出的是,TCSPC-FLIM在获取单帧图像时通常需要更长时间,这使得它对运动伪影更敏感,由此引起的误差会降低测量精度。相比之下,TG-FLIM、PS-FLIM和FD-FLIM速度较快,测量精度相对较低,所以运动伪影对它们的影响相对较小。
采用外部固定的手段,如麻醉、夹持等[76],能够有效降低成像活体样品时的运动伪影,从而减小后续图像配准的难度。如
图 8. 通过后期图像处理校正活体FLIM的运动伪影。(a)基于归一化互相关算法的帧间伪影校正[77];(b)基于Lucas-Kanade框架的帧间和帧内伪影校正[78]
Fig. 8. Correcting motion artifacts for in vivo FLIM through post-processing. (a) Inter frame artifact correction based on normalized cross-correlation algorithm[77]; (b) inter frame and intra frame artifact correction based on the Lucas-Kanade framework[78]
3.2 优化荧光寿命分析算法
最小二乘法(LS)是TD-FLIM中常用的荧光衰减曲线拟合算法,需要累积足够数量的光子才能准确分析荧光寿命,这是限制成像速度的主要因素。因此,改进荧光寿命数据的分析算法也能间接提升成像速度。近年来,发展了一些其他算法,能够减少荧光衰减曲线拟合对光子数的需求,包括极大似然估计(MLE)[81-82]、贝叶斯分析[83-84]及压缩感知[85]等。最近,本团队[86]提出了交替下降条件梯度(ADCG)分析法,将荧光寿命分析看作稀疏逆问题,交替进行全局条件梯度和非凸局部搜索,实现快速收敛。该方法适用于仅45个光子数的极端条件,尤其当存在高水平噪声时,具有更好的准确性和荧光寿命精度,如
图 9. ADCG算法流程和光子数为45时的拟合结果[86]
Fig. 9. Flowchart of ADCG algorithm and the fitting results with photon number of 45[86]
3.3 深度学习
随着计算机算力的快速提升,深度学习(DL)也逐渐在间接提升FLIM速度方面发挥重要作用,主要分为以下3种方式。
1)替代传统数据分析中耗时的迭代过程。作为一种数据驱动方法,DL通过对原始数据执行多次非线性变换以提取高级特征,从而进行分层表示学习,可直接输入原始数据映射得到相应的荧光寿命结果。Wu等[90]提出了一种基于人工神经网络(ANN)的FLIM方法,构建了由1个输入层、1个输出层和2个隐藏层组成的多层感知机(MLP)。输入层用于向MLP传递测得的荧光衰减数据,其节点数量取决于数据采集系统的时间通道数,输出层具有4个神经元映射双指数衰减曲线的荧光寿命参数。结果表明,ANN-FLIM可以提供相当甚至更好的图像质量,并且数据分析速度比传统的LS算法快180倍。然而,该方法忽略了仪器响应函数(IRF)对荧光衰减曲线的影响。Yao等[91]设计了大量模拟数据集,先采用压缩感知算法处理时间分辨的单像素数据集,再通过基于卷积神经网络(CNN)结构的方法进行荧光强度和寿命图像重构训练,其图像生成速度比传统拟合方法快了约7000倍,并且在低光子数(<100)情况下也具有可定量性。如
图 10. 3类间接减少FLIM成像时间的DL算法。(a)使用3D-CNN[92](左)或1D-CNN[93](右)替代传统衰减曲线拟合过程;(b)采用LLE和NIII子网络对低光子数生成高光子数[96];(c)基于图像分辨率增强的网络框架[97]
Fig. 10. Three types of DL algorithms contribute to reduce FLIM processing time. (a) Traditional curve fitting is substituted by either a 3D-CNN[92] (left) or a 1D-CNN[93] (right) framework; (b) LLE and NIII sub networks are utilized to produce data with high photon counts from low photon counts[96]; (c) DL algorithms are employed to improve imaging resolution[97]
2)使用低光子数原始数据生成高光子数数据。为了在光子匮乏的条件下(50光子数/像素)生成高质量的FLIM图像,Chen等[94]发展了一种名为flimGANE(generative adversarial network estimation)的网络框架,基于对抗生成网络生成图像的思想,将低光子计数的荧光衰减直方图输入到Wasserstein GAN中直接生成高光子计数的荧光衰减直方图,然后使用估计器计算双指数荧光寿命参数。在生成512×512像素的FLIM图像时,flimGANE比LS算法快258倍,比MLE快2800倍。然而,整个flimGANE的训练比较耗时,最长达500 h。此外,当FLIM系统发生变化时,IRF也会发生变化,相应需要重新训练神经网络,导致通用性降低。最近,已有不少研究尝试减少这种劣势。Zang等[95]提出了一种使用极限学习机(ELM)的方法,该方法在训练阶段无需反向传播过程,所以能提供更快的训练速度,支持任何系统配置的在线网络训练。Xiao等[96]提出了一种基于DL的极少光子数荧光寿命成像方法,由局部寿命估算(LLE)和神经隐式图像插值(NIII)两个子网络构成,如
3)对低空间分辨率荧光寿命图像进行图像增强。如
DL算法已能在FPGA和智能手机等边缘计算平台上实现,促进了智能便携式FLIM设备的开发[98]。
表 1. 几种基于不同DL网络的快速FLIM技术性能对比
Table 1. Performance comparison of several fast FLIM technologies based on DL
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4 FLIM技术在活体中的应用
如前所述,FLIM通过利用内源性或外源性荧光物质,能够高灵敏地对细胞和组织的微环境进行量化分析。随着仪器硬件和分析算法的共同发展,如今FLIM技术的应用已经不限于离体样品,而是能够在活体水平提供高时空分辨的图像,给生物医学基础研究和临床疾病诊断等领域的发展提供了重要支持。
4.1 生物医学基础研究
生物电信号在脑神经元之间的传导机制是脑科学研究中的一个热门且是具有挑战性的话题。一方面因为Ca2+探针的响应速度远远赶不上生物电信号的传导速度;另一方面,现有的膜电位探针在进行荧光强度成像时,还面临着灵敏度低、易光漂白等问题。因此,利用荧光寿命来对膜电位探针进行在体定量表征具有重要意义,这对FLIM速度提出了更高的要求。Bowman等[54,100]发展了一种基于电光调制器的快速TG-FLIM方法,通过时空复用对荧光进行偏振延迟调制和分光,实现在单帧图像中获取不同时间门的荧光信号。通过该技术,他们能够对活体果蝇单个脑神经元的动作电位进行高灵敏度成像,成像速度达1 kHz,且荧光寿命分辨率优于5 ps。他们还观察到,当果蝇受到直接机械刺激时,能产生持续毫秒量级的动作电位尖峰[101],这展示了快速FLIM技术在脑科学研究中的应用潜力。Zhang等[71]发展了一种基于模拟信号处理的双光子点扫描FD-FLIM,能够实时显示荧光强度、荧光寿命和phasor plot的图像结果。他们将PMT探测到的荧光信号一分为四,与经过四步相移(间隔0.5π)的脉冲激光信号进行混频,再从中解调差频项,得到荧光寿命信息,如
图 11. “即时”FLIM技术的原理和对活体小鼠脑神经元成像的结果[71]
Fig. 11. Schematic of instant FLIM technique and its in vivo imaging results of neurons in mouse brain[71]
通过颅骨固定的方式减少运动伪影,速度较慢的TCSPC-FLIM也能够实现活体脑成像。Gómez等[102]对阿尔茨海默病(AD)转基因小鼠的脑皮层进行双光子FLIM,phasor plot分析结果表明,在淀粉样(Aβ)蛋白周围的内源性NADH和脂褐素荧光寿命与野生型小鼠有明显差异,暗示其可以作为评估AD的指标之一。如果使用高亮度的外源荧光探针标记,能够显著缩短成像时间,TCSPC-FLIM则有可能获取更高分辨率的图像。如
图 12. TCSPC-FLIM用于活体小鼠成像。(a)AD小鼠的脑血管和Aβ斑块成像[103];(b)脑皮层神经元细胞内葡萄糖浓度的定量表征[104];(c)脑血管的三光子FLIM[105];(d)基于内窥显微技术的小鼠多脏器成像[106]
Fig. 12. TCSPC-FLIM for living mice. (a) Imaging brain vessels and Aβ plaque in AD mice[103]; (b) quantitative characterization of intracellular glucose concentration in cortical neurons[104]; (c) three photon FLIM of cerebral vessels[105]; (d) imaging of different organs in mice based on microendoscopy[106]
4.2 临床疾病诊断研究
NADH和FAD等内源性荧光团的荧光寿命能反映其状态、浓度及结合蛋白质的比率,与细胞的呼吸作用密切相关。不同于正常细胞,癌细胞的主要产能方式为糖酵解,这会改变NADH和FAD的荧光寿命,从而为肿瘤的诊断提供一种辅助的无标记检测方法。加利福尼亚大学戴维斯分校的Marcu团队长期开展FLIM在临床的应用研究,通过表征生物组织自发荧光寿命的差异,实现手术过程中的病变组织实时鉴别,该技术已经用于脑肿瘤[107]、头颈癌[108]、口咽癌[59]、口腔扁平苔藓[109]和甲状旁腺[58]诊断等。他们将带有渐变折射率(GRIN)透镜的光纤束与TG-FLIM系统结合,用于术中脑瘤边界的辅助诊断,并且展示了NADH荧光寿命在人脑胶质瘤和正常脑组织中的显著性差异。虽然该显微系统具有较大的成像视场(4 mm),但为了获取足够准确的荧光寿命信息,需要设置29个“时间门”(门宽和间隔均为0.5 ns),获取一张FLIM图像需要约2 min,所以术中需要通过外加稳定装置的方式减少运动伪影的影响[110]。随后,该团队使用皮秒脉冲紫外光纤激光器作为光源,发展了多光谱时间分辨荧光光谱显微系统(ms-TRFS)[111],采用基于多芯内窥光纤的PS-FLIM方法对生物组进行逐点测量,获取并显示荧光寿命编码结果的速度大于10 frame/s[112]。他们将ms-TRFS整合到机器人外科手术操作系统中,利用多个光谱通道的荧光寿命信息优化机器人对口腔癌的视觉评估功能[113],该系统对原位癌、淋巴组织上方原位癌以及正常组织具有很好的辨别能力。为了便于医生在术中徒手操纵ms-TRFS光纤探头扫描病灶,并提高稀疏采样数据的配准精度和可视化连贯性,他们在软件中引入基于DL的图像分割、运动校正及插值图像重构,如
图 13. FLIM在临床术中肿瘤诊断的应用。(a)基于PS-FLIM的肿瘤边界识别[108];(b)基于TCSPC-FLIM的高分辨肿瘤细胞识别[115];(c)基于5-ALA探针标记的FD-FLIM大视场快速成像[117]
Fig. 13. Applications of FLIM for intraoperative tumor diagnosis. (a) Recognition of tumor boundary based on PS-FLIM[108]; (b) high-resolution recognition of tumor cells based on TCSPC-FLIM[115]; (c) fast FD-FLIM with a large field-of-view based on 5-ALA labeling[117]
除了通过内源荧光实现脑肿瘤的临床诊断之外,利用外源造影剂的荧光寿命也能实现对癌变区域的高对比度识别。5-氨基乙酰丙酸(5-ALA)是脑瘤术中导航常用的造影剂,经过肿瘤组织的代谢之后生成具有红色荧光的原卟啉IX(PpIX),从而特异性识别肿瘤。对于荧光强度较弱的肿瘤组织边缘,通过获取PpIX的荧光寿命能够减少组织自发荧光的干扰[116],从而更准确地判断切除区域。Reichert等[117]在外科显微镜中引入点扫描FD-FLIM对人脑组织中的PpIX进行成像,获取6.5 mm
5 结束语
FLIM技术不仅拥有荧光显微成像的高分辨率和高特异性,还可以定量分析荧光团的微环境和相互作用,因此成为了生命科学研究中强有力的工具。然而,如何在保持其准确的荧光寿命分辨能力的同时提高成像速度,是FLIM长期面临的挑战,也限制了其在活体研究中的应用。综述了时域和频域FLIM技术是如何通过硬件与算法的优化实现快速成像的,并讨论每种方法的重要进展及其在活体成像中的应用。目前,这些研究通常基于单一尺度和有限的寿命范围,但未来实际应用需要跨尺度和宽量程的FLIM技术。跨尺度成像可灵活适应不同的模式动物,同时满足切换大视场-低分辨和小视场-高分辨的需求;而宽的寿命量程可以测量更多种类的荧光团,实现多生理参量的同步定量分析。另一方面,扩大成像深度是FLIM在活体应用中急需解决的问题。尽管多光子荧光显微技术能实现约1~2 mm的成像深度,但仍不足以满足活体应用的需求。结合生物相容性好的近红外荧光探针或者利用组织光透明化技术,有望进一步提升成像深度。近年来,基于单根多模光纤的高分辨内窥显微成像技术日趋成熟,其微创性和深穿透能力有望继续扩展FLIM技术在临床研究中的应用。此外,人工智能在图像处理方面的应用也将推动临床活检成像技术的革新。
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Article Outline
林方睿, 王义强, 易敏, 张晨爽, 刘丽炜, 屈军乐. 快速荧光寿命显微成像技术及其在活体应用的研究进展(特邀)[J]. 激光与光电子学进展, 2024, 61(6): 0618005. Fangrui Lin, Yiqiang Wang, Min Yi, Chenshuang Zhang, Liwei Liu, Junle Qu. Research Progress on Fast Fluorescence Lifetime Imaging Microscopy and Its in vivo Applications (Invited)[J]. Laser & Optoelectronics Progress, 2024, 61(6): 0618005.