共查询到20条相似文献,搜索用时 562 毫秒
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YU Bin PENG Xiang TIAN Jindong NIU Hanben DIAO Luhong & LI Hua . Key Laboratory of Optoelectronic Devices Systems of Ministry of Education Institute of Optoelec- tronics Shenzhen University Shenzhen China . State Key Laboratory of Precision Measuring Technology Instruments Tianjin University Tianjin China . Key Laboratory of Intelligent Information Processing Institute of Computing Technology Chinese Academy of Sciences Beijing China 《中国科学G辑(英文版)》2005,48(4):450-458
X-ray phase contrast imaging (XPCI) is a novel method that exploits the phase shift for the incident X-ray to form an image. For light elements such as carbon, hydrogen and oxygen, the phase-shift term can be up to 1000 times greater than the absorption term in the hard X-ray energy region. So XPCI has attracted much attention in recent years. Various methods for XPCI have been proposed and demonstrated on synchrotron devices and other X-ray sources[1―13], particularly the in-line metho… 相似文献
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Most papers in the field of optical phase retrieval either consider only the intensity (or amplitude) profile of the object under inspection (or scatterer location in the X-ray version), or a uniform tilt/rotation of the object beam. However, phase retrieval is able to recover the phase profile of the object (beam) as well, which theoretically makes the observable interference of phase retrieved object waves possible. In this paper we demonstrate this principle experimentally on centimeter sized deformable reflective objects (as large as 40 mm by 40 mm) and corresponding simulations are also presented. When the CCD camera is moved along the optical axis in the Fresnel region, the interference fringes of the displacement field have low contrast. On the other hand, when an imaging setup is built, and the camera moves near the image plane, high fringe contrast can be obtained. These fringes however suffer from some phase error. In our work the iterative modulus projection algorithm was used as a simply implementable phase retrieval method. 相似文献
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Effective receive beamforming in medical ultrasound imaging is important for enhancing spatial and contrast resolution. In current ultrasound receive beamforming, a constant sound speed (e.g., 1540 m/s) is assumed. However, the variations of sound speed in soft tissues could introduce phase distortions, leading to degradation in spatial and contrast resolution. This degradation becomes even more severe in imaging fatty tissues (e.g., breast) and with obese patients. In this paper, a mean sound speed estimation method where phase variance of radio-frequency channel data in the region of interest is evaluated is presented for improving spatial and contrast resolution. The proposed estimation method was validated by the Field II simulation and the tissue mimicking phantom experiments. In the simulation, the sound speed of the medium was set to 1450 m/s and the proposed method was capable of capturing this value correctly. From the phantom experiments, the −18-dB lateral resolution of the point target at 50 mm obtained with the estimated mean sound speed was improved by a factor of 1.3, i.e., from 3.9 mm to 2.9 mm. The proposed estimation method also provides an improvement of 0.4 in the contrast-to-noise ratio, i.e., from 2.4 to 2.8. These results indicate that the proposed mean sound speed estimation method could enhance the spatial and contrast resolution in the medical ultrasound imaging systems. 相似文献
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V. G. Kohna M. A. Orlovb 《Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques》2010,4(6):941-946
Computer simulation of an experiment on imaging transparent (phase) microobjects using the Zernike phase contrast method under
hard X-ray radiation has been performed. The beam parameters typical for synchrotron radiation sources of the third generation
were used in calculations. Both a refracting lens and a zone plate have been considered as a focusing element. The phase shifting
quarter-wave plate is located at a spot of the point source image. The results of calculations have shown that the method
can be successfully used for objects with the sizes greater than 0.1 μm along and 1 μm across the beam. It has been shown
that the contrast is caused not only by increasing the intensity within the shadow of the objects, but also by decreasing
the intensity in the area beyond the objects, which is necessary to retain the integral intensity. 相似文献
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We present a method for phase retrieval in propagation-based x-ray imaging, based on the contrast transfer and transport of intensity equation approaches. We show that the contrast transfer model does not coincide with the transport of intensity in the limit of small propagation distances, and we derive a new model that alleviates this problem. Using this model, we devise an algorithm to retrieve the phase from slowly varying samples that is valid beyond the limit of small distances. We show its utility by imaging in three dimensions a biological sample that causes both strong absorption and phase shift. 相似文献
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A variable-wavelength-based approach of phase retrieval for contrast transfer function based methods
Kashyap YS Agrawal A Sarkar PS Shukla M Roy T Sinha A 《Journal of synchrotron radiation》2010,17(6):799-803
X-ray phase-contrast imaging has emerged as an important method for improving contrast and sensitivity in the field of X-ray imaging. This increase in the sensitivity is attributed to the fact that, in the hard X-ray regime, the phase shift is more prominent as compared with the attenuation for materials having a low X-ray absorption coefficient. Among all the methods using the X-ray phase-contrast technique, in-line phase-contrast imaging scores over the other methods in terms of ease of implementation and efficient use of available X-ray flux. In order to retrieve the projected phase map of the object from the recorded intensity pattern, a large number of algorithms have been proposed. These algorithms generally use either the transport of intensity or contrast transfer function based approach for phase retrieval. In this paper it is proposed to use multiple wavelengths for phase retrieval using the contrast transfer function based formalism. 相似文献
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For weakly absorbing materials, image contrast can be enhanced by
phase contrast in formation. The effectiveness of the in-line phase
contrast technique relies on its ability to record intensity data
which contain information on the x-ray's phase shift. Four kinds of
approaches to the relationship between intensity distribution and
phase shift are reviewed and discussed. A micro-focal x-ray source
with high geometrical magnification is used to acquire phase
contrast images. A great improvement on image quality is shown and
geometrical parameters are modified for comparison between different
imaging positions. 相似文献
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Sticker M Pircher M Götzinger E Sattmann H Fercher AF Hitzenberger CK 《Optics letters》2002,27(13):1126-1128
Optical coherence microscopy (OCM) is capable of imaging the backscattering potential of a sample with high transversal and axial resolution. We report on a combination of OCM with a differential phase-contrast technique that permits imaging of the subwavelength optical path differences that occur between a narrow beam probing a sample and its surrounding. This technique allows small transversal refractive-index variations close to a selected interface to be seen. We report on the method and present first images of a test sample and a single cell layer. The cells act as phase objects; imaging the phase properties improves the contrast compared with that of intensity images. 相似文献
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通过对具有二维高斯型相位分布特征物体的X射线同轴相衬成像技术进行理论分析,提出了一种基于图像信噪比的成像系统优化设计方法。采用数值模拟的方式分别对亚微焦点源、激光驱动微焦点源和同步辐射源三种X射线源下的成像系统进行了优化设计,结果表明:相对于最大化衬度的系统优化设计方法,信噪比优化设计方法在较小降低衬度的前提下,可以较好地改善图像信噪比并得到更为合理的成像距离。 相似文献
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We propose a method to visualize a phase object using a locally-controllable image amplification scheme we developed earlier which is based on two-wave coupling in a bismuth silicon oxide (Bi12SiO20, or BSO) crystal. In this method the BSO crystal is used as a phase plate in the configuration of Zernike’s phase contrast imaging system. The principle of the phase-visualization with two-wave coupling is presented. An experimental result to verify the proposed method is shown. 相似文献
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A one-laser-shot measurement technique, which can characterize the nonlinear optical properties of the film prepared on the opaque substrates conveniently and exactly, is presented. This method is based on the reflection 4f coherent imaging system with phase objects. The proposed technique is examined in the case of a novel ultrathin film [CuPc(COONa)4]/PDDA coated on an opaque silicon substrate with 532 nm picosecond laser pulses. By measuring the reflected intensity, but not the transmitted one, we verify both nonlinear absorption and refraction of the coated thin film. This progress obviously ensures that nonlinearities caused by reflection of opaque surfaces are measurable. 相似文献
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A novel radio frequency (RF) field intensity mapping or imaging method using a composite NMR spin-echo sequence is proposed. A composite spin-echo RF pulse with 90 degrees y-180 degrees x-90 degrees y sequence makes phase change in the final image depending on the RF field intensity on the object. The resultant phase change or phase map can be used to obtain the actual RF flip-angle map for a given condition which includes the status of tuning and RF inhomogeneity, etc. Bloch equation has been solved numerically to obtain the effects of the RF field intensity as well as the main magnetic field inhomogeneity and the results are used for the mapping (imaging) of the RF field intensity. Phantom studies have been performed using a 1.5 Tesla whole body MRI system and the results are presented. 相似文献
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Over the last decade, the spatial resolution of transmission electron microscopes has been considerably improved thanks to the development of aberration correctors. At the same time, image interpretation has become easier as the influence of instrument aberrations on image intensity has been reduced for phase contrast imaging. New aberration-corrected microscopes now offer the possibility to extract both the structural and the chemical information from a quantitative analysis of the image's contrast, which is promising in many fields of materials science where knowledge of the chemical content at the atomic scale is crucial. However, appropriate imaging conditions must be used for a quantitative analysis of the image at the sub-angström scale. In this paper, we focus on the impact of chromatic and geometric aberrations on phase contrast and we compare the advantages offered by the few optimum imaging conditions proposed in the literature. Effects of residual aberrations are also considered while the influence of chromatic aberration correction in future Cs/Cc-corrected instruments is emphasized. A critical value of Cc is given depending on the instrumental parameters. Silicon carbide imaging using a Cs-corrected microscope is presented and illustrates the assessments derived from the theoretical study of residual aberration influence on phase contrast imaging. 相似文献
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Ollerenshaw J McClung RE 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2000,144(2):255-265
A magnetic resonance imaging method is presented for imaging of heterogeneous broad linewidth materials. This method allows for distortionless relaxation weighted imaging by obtaining multiple phase encoded k-space data points with each RF excitation pulse train. The use of this method, turbo spin echo single-point imaging-(turboSPI), leads to decreased imaging times compared to traditional constant-time imaging techniques, as well as the ability to introduce spin-spin relaxation contrast through the use of longer effective echo times. Imaging times in turboSPI are further decreased through the use of low flip angle steady-state excitation. Two-dimensional images of paramagnetic doped agarose phantoms were obtained, demonstrating the contrast and resolution characteristics of the sequence, and a method for both amplitude and phase deconvolution was demonstrated for use in high-resolution turboSPI imaging. Three-dimensional images of a partially water-saturated porous volcanic aggregate (T(2L) approximately 200 ms, Deltanu(1/2) approximately 2500 Hz) contained in a hardened white Portland cement matrix (T(2L) approximately 0.5 ms, Deltanu(1/2) approximately 2500 Hz) and a water-saturated quartz sand (T(2) approximately 300 ms, T(2)(*) approximately 800 microseconds) are shown. 相似文献
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We propose a modified design of the phase contrast filter to optimize the synthetic reference wave (SRW) in the generalized phase contrast method. The modified filter consists of a π-phase shifting disk surrounded by an annular ring to modulate the curvature of the intensity distribution generally associated with the SRW. We identify optimal filter parameters to yield a uniform intensity distribution within the working area. Performance tests where done for both periodic and arbitrary input phase patterns using the optimal filter parameters. Assessment of the quality of the projected intensity patterns suggests a more uniform intensity distribution at the output when using the modified filter. 相似文献