一种有效提高太赫兹时域光谱装置成像空间分辨率的方法

太赫兹波成像技术一个最显著的制约因素是其有限的空间分辨率。提出通过在样品前加小孔的方法来提高传统太赫兹时域光谱装置成像的空间分辨率。采用在样品前约2 mm处加直径为0.5 mm小孔的方法使成像的空间分辨率从1.276 mm提高到0.774 mm,提高0.502 mm,约39％。通过这个简单的方法在传统的太赫兹时域光谱成像装置上实现了空间分辨率从毫米量级到亚毫米量级的提高。聚乙烯板上直径为1 mm的小孔被作为成像的研究对象,分别采用传统的太赫兹时域光谱装置对样品进行直接成像和在样品前约2 mm处加直径为0.5 mm的小孔后对样品成像两种方式,并采用损失成像中信噪比较好的能量损失成像,对比两种方式得到的样品的太赫兹像,结果显示聚乙烯板上小孔的边界加小孔后成像比不加小孔直接成像明显清晰。证实了在样品前加小孔可以有效的提高太赫兹成像系统的空间分辨率。从理论上对通过在样品前加小孔提高系统空间分辨率的方法进行了分析,指出小孔尺寸越小,系统的空间相干长度越大,空间分辨率越高,但同时太赫兹信号的强度会相应减小。该方法可以简单有效的提高太赫兹时域光谱装置成像的空间分辨率,从而进一步拓展太赫兹谱成像技术的应用领域。     

Investigation of noise properties in grating-based x-ray phase tomography with reverse projection method

The relationship between noise variance and spatial resolution in grating-based x-ray phase computed tomography(PCT) imaging is investigated with reverse projection extraction method, and the noise variances of the reconstructed absorption coefficient and refractive index decrement are compared. For the differential phase contrast method, the noise variance in the differential projection images follows the same inverse-square law with spatial resolution as in conventional absorption-based x-ray imaging projections. However, both theoretical analysis and simulations demonstrate that in PCT the noise variance of the reconstructed refractive index decrement scales with spatial resolution follows an inverse linear relationship at fixed slice thickness, while the noise variance of the reconstructed absorption coefficient conforms with the inverse cubic law. The results indicate that, for the same noise variance level, PCT imaging may enable higher spatial resolution than conventional absorption computed tomography(ACT), while ACT benefits more from degraded spatial resolution. This could be a useful guidance in imaging the inner structure of the sample in higher spatial resolution.     

A novel acquisition-reconstruction algorithm for surface magnetic resonance imaging

In U-shaped, hand-size magnetic resonance surface scanners, imaging is performed along only one spatial direction, with the application of just one gradient (one-dimensional imaging). Lateral spatial resolution can be obtained by magnet displacement, but, in this case, resolution is very poor (on the order of some millimeters) and cannot be useful for high-resolution imaging applications. In this article, an innovative technique for acquisition and reconstruction of images produced by U-shaped, hand-size MRI surface scanners is presented. The proposed method is based on the acquisition of overlapping strips and an analytical reconstruction technique; it is capable of arbitrarily improving spatial lateral resolution without either using a second magnetic field gradient or making any assumptions about the imaged sample extension. Numerical simulations on synthetic images are reported demonstrating the method functionalities. The presented method also makes it possible to use U-shaped, hand-size MRI surface scanners for high-resolution biomedical applications, such as the imaging of skin lesions.     

Signal generation in an isotropic medium in scanning electron acoustic microscope

Based on the research in Ref. [5][Materials Science and Engineering, 1989; A122： 57 63], an improved model of heat source is set up, the different modes of Lamb wave in an isotropic sample generated by a chopped electron beam at frequency f are obtained with integral transform and normal function expansion method, and the output signal of PZT coupled at the back surface of the sample is found out. The generation mechanism of SEAM （Scanning Electron Acoustic Microscopy） signal is discussed. It shows that the SEAM is a near field imaging technique with high spatial resolution and its best lateral spatial resolution is about 2√2α （α is the radius of the focused electron beam）. Some of experimental results of SEAM images are presented in the paper and it shows that the spatial resolution of SEAM is better than 0.5 μm and smaller than the thermal diffusion length of the sample. Therefore the character of near field imaging in SEAM is also proved experimentally.     

Effects of Pumping Sizes on THz Radiation Based on Ultrashort Light Pulse Optical Rectification for High Spatial Resolution T-Ray Imaging

We present our experimental studies on the effects of the pumping sizes on THz radiation based on ultrashort light pulse optical rectification for high spatial resolution T-Ray imaging. Our experiments show that high spatial resolution T-ray imaging requires both thin THz emitter and sample, and rigorous tolerance of the gap between the sample and the emitter, as well as small pumping size which usually much smaller compared with THz wavelength. Such a small pumping size results in dramatic decrease of the THz wave power, which originates from strong diffraction of THz wave, the depolarization of the focused tightly pumping beam, the spatial filtering of the emitter exit-surface, and the strong phase-mismatching between the pumping and the high spatial Fourier components of the THz signal, rather than two-photon absorption.     

Three-dimensional NMR microscopy: improving SNR with temperature and microcoils.

It is widely held that the spatial resolution achievable by NMR microscopic imaging is limited in biological systems by diffusion to approximately 1-5 microns. However, these estimates were developed for specific imaging techniques and represent practical rather than fundamental limits. NMR imaging is limited by the signal-to-noise ratio (SNR). Diffusion effects on spatial resolution can be made arbitrarily small in principle by increasing the gradient strength. The exponential signal attenuation from random spin motion in a gradient, however, will reduce the signal far below the noise level when the voxel size is reduced much below 5 microns. Two factors can be optimized to improve the SNR: (1) the inductive linkage between microscopic samples and the detection apparatus and (2) the temperature of the rf probe. In this work, the filling factor was optimized using inductors with diameters less than 1 mm. It is furthermore shown that probe circuit cooling results in significant improvements in SNR, whereas cooling of the preamplifier is of little value when proper noise matching between the resonant circuit and preamplifier is accomplished. Using three-dimensional Fourier imaging techniques, we have obtained images of single-cell organisms with spatial resolution of approximately 6 microns. Practical limitations include mechanical stability of the apparatus, thermal shielding between the sample and probe, and the magnetic susceptibility of the sample.     

Pattern projection for subpixel resolved imaging in microscopy

In this paper, we present a new approach providing super resolved images exceeding the geometrical limitation given by the detector pixel size of the imaging camera. The concept involves the projection of periodic patterns on top of the sample, which are then investigated under a microscope. Combining spatial scanning together with proper digital post-processing algorithm yields the improved geometrical resolution enhancement. This new method is especially interesting for microscopic imaging when the resolution of the detector is lower than the resolution due to diffraction.     

时域两维荧光寿命显微测量研究

提出了一种新的时域两维荧光寿命显微测量技术,建立了一套荧光寿命成像显微系统,介绍了这种测量技术的数据处理方法。用标准样品对该系统进行了测试,实验表明,该系统的时间分辨率为2ps,在放大倍率为100倍的情况下,该系统的空间分辨率为8um。如果在现有的设备下采用更细的网格板和微位移系统,那么该系统的空间分辨率可小于1um.     

Coherent x-ray diffraction imaging with nanofocused illumination

Coherent x-ray diffraction imaging is an x-ray microscopy technique with the potential of reaching spatial resolutions well beyond the diffraction limits of x-ray microscopes based on optics. However, the available coherent dose at modern x-ray sources is limited, setting practical bounds on the spatial resolution of the technique. By focusing the available coherent flux onto the sample, the spatial resolution can be improved for radiation-hard specimens. A small gold particle (size <100 nm) was illuminated with a hard x-ray nanobeam (E=15.25 keV, beam dimensions approximately 100 x 100 nm2) and is reconstructed from its coherent diffraction pattern. A resolution of about 5 nm is achieved in 600 s exposure time.     

Beyond the Rayleigh criterion: grating assisted far-field optical diffraction tomography

We propose an optical imaging system, in which both illumination and collection are done in far field, that presents a power of resolution better than one-tenth of the wavelength. This is achieved by depositing the sample on a periodically nanostructured substrate illuminated under various angles of incidence. The superresolution is due to the high spatial frequencies of the field illuminating the sample and to the use of an inversion algorithm for reconstructing the map of relative permittivity from the diffracted far field. Thus, we are able to obtain wide-field images with near-field resolution without scanning a probe in the vicinity of the sample.     

Wide-field coherent anti-Stokes Raman scattering microscopy with non-phase-matching illumination

We have developed and tested a wide-field coherent anti-Stokes Raman scattering (CARS) microscopy technique, which provides the simultaneous imaging of an extended illuminated area without scanning. This method is based on the non-phase-matching illumination of a sample and imaging of a CARS signal with a CCD camera using conventional microscope optics. We have identified a set of conditions on the illumination and imaging optics, as well as on sample preparation. Imaging of test objects proved high spatial resolution and chemical selectivity of this technique.     

微焦点源X射线相衬成像技术

 相衬成像方法利用硬X射线对低密度弱吸收物质成像,可获得高衬度图像。用菲涅尔衍射理论分析了X射线图像的形成机理。在频域中根据光学传递函数,对物像距离、样品空间频率等对图像相位衬度的影响进行了分析。分辨率和衬度是决定图像可见度的两个依据,分辨率主要依赖于光源的空间相干性,空间相干性又决定于源点尺寸,而时间相干性（单色性）是一个不重要的影响因子。利用多色微焦点源实现了X射线相衬成像技术,获得了有价值的相衬图像,如低原子序数低密度泡沫材料的硬X射线相衬图像,与吸收衬度成像相比,其图像质量得到了很大提高,能观察到泡沫材料的细微结构,分辨率可达μm量级。     

Localization microscopy (SPDM) facilitates high precision control of lithographically produced nanostructures

Nanoscale resolution in material sciences is usually restricted to scanning electron beam microscopes. Here we present a procedure that allows single molecule resolution of the sample surface with visible light. Highlighting the performance we used electron beam lithography to generate highly regular nanostructures consisting of interconnected cubes. The samples were labeled with Alexa 647 dyes. The spatial organization of the dyes on nanostructured surfaces was localized with single molecule resolution using localization microscopy. This succeeded also in an absolute spatial calibration of the localization method applied (spectral precision distance microscopy/SPDM). The findings will contribute to the field of product control for industrial applications and long-term fluorescence imaging.     

同步辐射硬X射线衍射增强峰位成像CT研究

对于主要由轻元素组成的生物、医学样品,利用衍射增强成像技术可以观察到常规吸收成像无法观察到的内部微观结构,因而衍射增强成像具有较高的衬度和空间分辨率.选用苍蝇作为实验样品,在北京同步辐射装置上首次开展了衍射增强峰位成像CT实验.重建出的样品断层像显示其分辨率达到了几十μm水平.对于将衍射增强成像技术应用于生物和医学等领域具有重要意义.     

A constrained Gauss-Seidel method for correction of point spread function effect in MR spectroscopic imaging

Magnetic resonance spectroscopic imaging is limited by a low signal-to-noise ratio, so a compromise between spatial resolution and examination time is needed in clinical application. The reconstruction of truncated signal introduces a Point Spread Function that considerably affects the spatial resolution. In order to reduce spatial contamination, three methods, applied after Fourier transform image reconstruction, based on deconvolution or iterative techniques are tested to decrease Point Spread Function contamination. A Gauss-Seidel (GS) algorithm is used for iterative techniques with and without a non-negative constraint (GS+). Convergence and noise dependence studies of the GS algorithm have been done. The linear property of contamination was validated on a point sample phantom. A significant decrease of contamination without broadening the spatial resolution was obtained with GS+ method compared to a conventional apodization. This post-processing method can provide a contrast enhancement of clinical spectroscopic images without changes in acquisition time.     

二维圆周光综合孔径阵的优化排列及其成像特性研究

光综合孔径成像的原理是应用干涉原理在空间频率域中进行采样,并通过傅里叶反变换或其他数值变换方法得到空间分辨力远高于单个孔径成像系统的目标图像。由若干个相同的小孔径在二维圆周上优化排列组成的综合孔径成像系统可以在二维空间频率域中实现较为均匀分布的、具有无冗余度的采样点覆盖,为高质量实时成像提供了一个有效的途径。运用模拟退火算法对由7～16个子孔径组成的二维圆周综合孔径阵列进行优化排列。并依据光学衍射成像原理,从空域和频域两个方面详细分析了二维圆周上优化排列与均匀排列光综合孔径阵的成像特性。对7～16个子孔径组成的光综合孔径的仿真结果表明：无论是在空域还是频域上,子孔径直径增大、孔径数目增多以及综合孔径阵的优化排列都是有利于提高成像质量的。但综合后的子孔径的直径的增大,虽然能获得极高的角分辨力,却并不利于成像质量的提高。     

Imaging and small spot analysis with ESCA

Existing theory indicates that photoelectron spectrometers equipped with spherical double-focusing electrostatic analyzers and position-sensitive detectors may be used to obtain ESCA images. Our experiments, performed with a McPherson ESCA-36 equipped in this manner, show that the theory is correct. A one-dimensional image of the sample area seen by the analyzer through the entrance slit is reproduced at the detector and may be displayed on a CRT. The addition of a specimen manipulator and a variable power supply allows two-dimensional images to be photographically recorded. The spatial resolution obtainable without instrument modification is approximately 250 μm. Small spot analysis and line profiling are also obtainable, with spatial resolution comparable to the imaging resolution.     

Magnetic resonance imaging of isolated single liposome by magnetic resonance force microscopy

Magnetic resonance imaging (MRI) is very useful spectroscopy to visualize a three-dimensional (3D) real structure inside the sample without physical destruction. The spatial resolution of the readily available MRI spectrometer is, however, limited by a few ten to hundreds of microns due to a technological boundary of generating larger magnetic field gradient and to the insensitivity inherent to the inductive signal detection. Magnetic resonance force microscopy (MRFM) is new alternative MRI spectroscopy which is anticipated to significantly surpass the conventional MRI in both resolution and sensitivity. We report two imaging experiments on our MRFM spectrometer operated at room temperature and in vacuum approximately 10(-3)Pa. One is for approximately 20 microm liposome membrane labeled entirely by a nitroxide imaging agent and the other for approximately 15 microm DPPH particles, both are nearly the same size as that of human cell. The reconstructed images at spatial resolution approximately 1 microm were in satisfactory agreement with the scanning electron microscope images. The potential capability of visualizing intrinsic radicals in the cell is suggested to investigate redox process from a microscopic point of view.     

Quadrature phase-shifting interferometer using spatial carrier

We propose a high-speed phase measurement using a phase calculation algorithm in a quadrature phase-shifting interferometer (QPI), which is applied to tilted fringes. Although the spatial carrier method is useful for a high-speed measurement with a single imaging sensor, the spatial resolution in conventional phase calculation algorithms is reduced owing to the use of three or more phase-shifted data at different sample points. Phases in the QPI method can be calculated with only two quadarture phase-shifted data extracted from the tilted fringes before and after a phase change of the interferometer. Thus, the proposed method can suppress the reduction of the spatial resolution compared with that in conventional methods, and makes it possible to measure a phase at high-speed. The principle and experimental results of this technique are presented.     

X射线背光成像的实现

为了实现对惯性约束聚变的诊断,获取聚爆过程中高温等离子体X射线能谱信息和内爆靶丸的二维空间分辨信息,利用晶体的布拉格衍射特性设计制作了球面晶体分析器,晶体弯曲半径为125 mm。为了验证球面晶体的空间分辨能力,搭建了背光成像平台进行了背光成像实验,石英球面晶体为衍射核心元件,接收装置IP板得到了Cu靶的二维空间分辨信息,基于石英球面晶体的成像平台得到的空间分辨率约为100 μm。