1.Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China 2.Institute of Aeronautical Equipment, Guangdong Academy of Sciences, Zhuhai 519000, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61975131, 61775144, 61835009, 11774242), the Natural Science Foundation of Guangdong Province, China (Grant No. 2018A030313362), the Basic and Applied Basic Research Foundation of Guangdong Province, China (Grant No. 2019A1515110412), the Basic Research Project of Shenzhen, China (Grant Nos. JCYJ20170818141701667, JCYJ20170818144012025, JCYJ20170412105003520, JCYJ20170818142804605), and the Science and Technology Development Foundation of Guangdong Academy of Sciences, China (Grant Nos. 2018GDASCX-0804, 2020GDASYL-20200103144)
Received Date:30 April 2020
Accepted Date:04 September 2020
Available Online:16 January 2021
Published Online:05 February 2021
Abstract:Confocal laser scanning microscopy (CLSM) is a powerful imaging tool providing high resolution and optical sectioning. In its standard optical configuration, a pair of confocal pinholes is used to reject out-of-focus light. The diffraction limited resolution can be broken by reducing the confocal pinhole size. But this comes at the cost of extremely low signal-to-noise ratio (SNR). The limited SNR problem can be solved by image scanning microscopy (ISM), in which the single-point detector of a regular point-scanning confocal microscopy is substituted with an array detector such as CCD or CMOS, thus the two-fold super-resolution imaging can be achieved by pixel reassignment and deconvolution. However, the practical application of ISM is challenging due to its limited image acquisition speed. Here, we present a hybrid microscopy technique, named multifocal refocusing after scanning using helical phase engineering microscopy (MRESCH), which combines the double-helix point spread function (DH-PSF) engineering with multifocal structured illumination to dramatically improve the image acquisition speed. In the illumination path, sparse multifocal illumination patterns are generated by a digital micromirror device for parallel imaging information acquisition. In the detection path, a phase mask is introduced to modulate the conventional PSF to the DH-PSF, which provides volumetric information, and meanwhile, we also present a digital refocusing strategy for processing the collected raw data to recover the wild-filed image from different sample layers. To demonstrate imaging capabilities of MRESCH, we acquire the images of mitochondria in live HeLa cells and make a detailed comparison with those from the wide-field microscopy. In contrast to the conventional wide-field approach, the MRESCH can expand the imaging depth in a range from –1 μm to 1 μm. Next, we sample the F-actin of bovine pulmonary artery endothelial cells to characterize the lateral resolution of the MRESCH. The results show that the MRESCH has a better resolution capability than the conventional wide-field illumination microscopy. Finally, the proposed image scanning microscopy can record three-dimensional specimen information from a single multi-spot two-dimensional scan, which ensures faster data acquisition and larger field of view than ISM. Keywords:image scanning microscopy/ double-helix point spread function/ confocal microscopy
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2.1.MRESCH的光路设计
MRESCH的光路示意图如图1(a)所示, 在照明光路中, 波长为488 nm的激光经4f系统准直扩束后直接照射到DMD面板上, 其入射角与DMD面板呈24°; 接着, 激光经DMD调制后进入后续的4f系统, 其傅里叶面位置上的光阑可以有效阻止多余衍射级的光进入显微系统. 最后, 4f后焦面位置形成的聚焦点阵经过管镜和物镜缩小后重新聚焦到样品面上, 尺寸为原来的1/90. 本文实验中所使用的DMD面板的像素个数为1024 × 768, 像素尺寸为10.8 μm × 10.8 μm, 对应到样品面上的尺寸为120 nm × 120 nm. 在探测光路中, 样品发出的荧光经物镜(尼康, 60×, NA = 1.27水镜)和管镜收集后, 然后通过具有双螺旋相位片的4f系统, 最后成像在科学级互补金属氧化物半导体探测器(sCMOS, 滨松, ORCA Flash 4.0 V2)上. 其中, 双螺旋相位片放置在4f系统的傅里叶面位置, 与物镜的后焦面呈共轭关系, 双螺旋相位片能够对探测系统的传递函数进行调制, 将标准PSF转换为DH-PSF的形式. DH-PSF是一种特殊的三维点扩散函数, 常用于细胞内分子的三维定位和成像[15-18], 当分子的轴向位置发生改变时, DH-PSF的两个旁瓣会围绕着光轴进行旋转, 如图1(b)所示, 因此, 可以通过两个旁瓣之间的相对旋转角度来精确确定分子的轴向位置. 目前, DH-PSF相位片的设计有多种方法[19-22], 本文根据文献[19]中的方法进行设计, 并利用荧光珠样品来标定DH-PSF旋转角度$ \theta $与样品激发位置z的线性关系k, 如图1(c)所示, 经计算DH-PSF旋转180°对应样品的轴向位移Δz约为5.5 μm. 图 1 (a) MRESCH的光路; (b) DH-PSF在不同轴向位置的强度分布; (c) DH-PSF旋转角度与对应轴向位置的关系曲线 Figure1. (a) Optical configuration of MRESCH; (b) intensity distribution of the DH-PSF at different positions along z-axis; (c) relationship between the two lobe rotation angles of the DH-PSF and position of z-axis.
22.2.MRESCH的成像原理 -->
2.2.MRESCH的成像原理
在MRESCH系统中, 将特定的投影模式(见图2(a))载入DMD来产生周期分布的聚焦点阵对样品进行照明激发, 在样品面的强度分布如图2(b)所示. 当投影模式切换时, “on”像素的位置将沿红色轨迹进行移动(1 pixel/step), 实现样品的快速扫描. 其中单个激发荧光点在探测面上的强度分布可以表示为 图 2 (a) DMD上载入的投影模式; (b) 激发罗丹明染料样品探测到的荧光点阵分布; (c) 存在相位片的条件下, 激发罗丹明染料样品探测到的双螺旋荧光点阵分布 Figure2. (a) Project pattern of DMD; (b) the fluorescence image of the excitation foci in a uniform solution of Rhodamine 6G at the sample plane; (c) the fluorescence image of the excitation foci in a uniform solution of Rhodamine 6G at the sample plane with DH phase mask.