单个病毒的X射线3D成像

自从1957 年确立肌红蛋白的结构以来,研究人员利用X射线晶体学确定了数以万计蛋白、核酸和其他生物分子的结构,并由此展开了分子功能研究。这种方法要求被研究的分子必须可以形成晶体,当X射线通过晶体时会发生衍射,由此可获得结构信息。但是,很多重要的分子无法结晶,因此不能用这种方法来研究。这就是X射线晶体学的“阿喀琉斯之踵”。科学家们正在寻找重建分子结构的新方案：他们希望只需要将单分子注入到自由电子激光(XFEL)的强激光束中即可通过X 射线衍射信息重建分子结构。由瑞典Uppsala 大学Hajdu 领导的研究小组报道了一项关键进展：如果利用随机取向的单个病毒获得大量的衍射图样,那么就有可能重建病毒的三维结构。     

High-resolution cardiac imaging using an interleaved 3D double slab technique

A three-dimensional (3D) gradient-echo sequence with interleaved double-slab excitation was developed and optimized for the requirements in pediatric cardiac imaging. For this purpose high contrast between blood and myocardium signal should be obtained without the use of contrast agents. An acceptable measuring time for a large region examined with high spatial resolution should be achieved as well, especially with regard to the small structures of the heart and vessels of infants. The presented approach works with gradient moment nulling and a short echo time of 5.5 ms resulting in generally high signal intensity and only minor signal losses due to turbulent flow. The sequence allows simultaneous ECG-gated recording of two separately excited slabs with small thickness (10 mm) and with a distance of several centimeters between them. Thus, common effects of presaturation in 3D imaging can be avoided, although a relatively short measuring time is achievable. In order to get a 3D data set with good signal homogeneity of blood and of the other structures across a large volume of interest several double-slab measurements with suitable positions must be performed. The latter aspect is especially important for postprocessing techniques as multiple planar reconstruction and maximum intensity projection. Examples of applications of the new technique and appropriately postprocessed images are presented allowing demonstration even of subtle cardiac malformations.     

High-resolution bone microstructure imaging based on ultrasonic frequency-domain full-waveform inversion

The main challenge in bone ultrasound imaging is the large acoustic impedance contrast and sound velocity differences between the bone and surrounding soft tissue. It is difficult for conventional pulse-echo modalities to give accurate ultrasound images for irregular bone boundaries and microstructures using uniform sound velocity assumption rather than getting a prior knowledge of sound speed. To overcome these limitations, this paper proposed a frequency-domain fullwaveform inversion(FDFWI) algorithm for bone quantitative imaging utilizing ultrasonic computed tomography(USCT).The forward model was calculated in the frequency domain by solving the full-wave equation. The inverse problem was solved iteratively from low to high discrete frequency components via minimizing a cost function between the modeled and measured data. A quasi-Newton method called the limited-memory Broyden–Fletcher–Goldfarb–Shanno algorithm(L-BFGS) was utilized in the optimization process. Then, bone images were obtained based on the estimation of the velocity and density. The performance of the proposed method was verified by numerical examples, from tubular bone phantom to single distal fibula model, and finally with a distal tibia-fibula pair model. Compared with the high-resolution peripheral quantitative computed tomography(HR-p QCT), the proposed FDFWI can also clearly and accurately presented the wavelength scaled pores and trabeculae in bone images. The results proved that the FDFWI is capable of reconstructing high-resolution ultrasound bone images with sub-millimeter resolution. The parametric bone images may have the potential for the diagnosis of bone disease.     

High-resolution 3D MR spectroscopic imaging of the prostate at 3 T with the MLEV-PRESS sequence

A 3 T MLEV-point-resolved spectroscopy (PRESS) sequence employing optimized spectral-spatial and very selective outer-voxel suppression pulses was tested in 25 prostate cancer patients. At an echo time of 85 ms, the MLEV-PRESS sequence resulted in maximally upright inner resonances and minimal outer resonances of the citrate doublet of doublets. Magnetic resonance spectroscopic imaging (MRSI) exams performed at both 3 and 1.5 T for 10 patients demonstrated a 2.08+/-0.36-fold increase in signal-to-noise ratio (SNR) at 3 T as compared with 1.5 T for the center citrate resonances. This permitted the acquisition of MRSI data with a nominal spatial resolution of 0.16 cm3 at 3 T with similar SNR as the 0.34-cm3 data acquired at 1.5 T. Due to the twofold increase in spectral resolution at 3 T and the improved magnetic field homogeneity provided by susceptibility-matched endorectal coils, the choline resonance was better resolved from polyamine and creatine resonances as compared with 1.5 T spectra. In prostate cancer patients, the elevation of choline and the reduction of polyamines were more clearly observed at 3 T, as compared with 1.5 T MRSI. The increased SNR and corresponding spatial resolution obtainable at 3 T reduced partial volume effects and allowed improved detection of the presence and extent of abnormal metabolite levels in prostate cancer patients, as compared with 1.5 T MRSI.     

Review on applications of synchrotron-based X-ray techniques in materials characterization

Synchrotron radiation (SR), as a result of its high-intensity, brilliant, monochromatic, and collimated beams, is becoming one of the most crucial components of research in various fields of materials science such as nanomaterials, biomaterials, and energy materials. SR-based characterization methods can be employed to analyze different systems such as powders, thin films, and bulk forms having complex crystalline or amorphous structures. In this review, peculiarities of SR are briefly explained. Moreover, various techniques carried out utilizing this instrument for material characterization such as X-ray powder diffraction, grazing-incidence X-ray diffraction, small/wide-angle X-ray scattering, X-ray absorption spectroscopy, different techniques of X-ray imaging, X-ray photoelectron spectroscopy, and X-ray microprobes/nanoprobes are presented. As a result, by shedding light on the advantages of SR and its superiority to the equivalent laboratory experiments, researchers are recommended to exploit the capabilities of this invaluable tool in their materials characterization.     

High-resolution magnetic imaging by local tunneling magnetoresistance

We give an overview over our recent efforts of high-resolution magnetic imaging using scanning tunneling microscopy with a ferromagnetic tip. Magnetic sensitivity is obtained on the basis of local tunneling magnetoresistance between a soft magnetic tip and the sample. The magnetisation of the tip is switched periodically with a small coil, leading to variations of the tunneling current due to the tunneling magnetoresistance effect. These variations are detected with a lock-in amplifier to separate spin-dependent parts from the topographic parts of the tunneling current such that the topography and the magnetic structure of the sample can be recorded simultaneously. Crucial for this method is to avoid mechanical vibrations of the tip, that may also lead to variations in the tunneling current. Exemplary studies of polycrystalline Ni and the closure domain pattern of Co(0001) are presented, showing high contrast at acquisition times as low as 3 ms/pixel and a lateral resolution of the order of 1 nm. Further it is demonstrated that besides topography and magnetisation, also local information about the magnetic susceptibility can be obtained. Received: 28 April 2000 / Accepted: 15 May 2000 / Published online: 7 March 2001     

Direct 3D imaging of Al70.4Pd21Mn8.6 quasicrystal local atomic structure by X-ray holography

Inverse x-ray fluorescence holography was used to explore the local atomic order of a nearly perfect quasicrystal with composition Al70. 4Pd21Mn8.6. We have demonstrated the possibility of direct 3D imaging of the atomic decoration in a quasicrystal. We have obtained the average 3D environment of selected coordination shells around the Mn atoms. These results open the way to obtaining further and more complete information about the various coordination shells in complex materials by measuring multiple energy x-ray holograms at different sites.     

High-resolution X-ray diffraction study of the cubic-to-tetragonal transition in BaTiO3

A drastic change is found in the diffraction pattern of a single-crystal specimen of BaTiO₃. At and just below the transition temperature, the original single peak of 004 of the tetragonal phase splits or collapses into a group of small peaks. They appear around the positions of 004 and 400, and probably correspond to different orientations of grains and domains. The positions and relative intensities of the peaks randomly vary with time and temperature. These features are observed for both heating and cooling processes.     

Volumetric q-space imaging by 3D diffusion-weighted MRI

High b-value diffusion magnetic resonance imaging (MRI) enables us to detect far smaller architectures, by using q-space analysis, than the resolution in conventional MRI. Average displacement, one of the q-space parameters, quantitatively reflects architecture size and is very useful in observing small changes in microstructures in vivo (e.g., neurodegeneration, tumor heterogeneity, and others). Diffusion-weighted imaging (DWI) is performed by a two-dimensional (2D) multislice method; however, due to finite slice thickness and slice gap, there is a partial-volume effect that makes it difficult to detect the net q-space signal. On the other hand, three-dimensional (3D) MRI, having the advantages of very thin slice thickness and no slice gap (contiguous slices), allows volumetric evaluation acquired in a small isotropic voxel, as compared to 2D multislice imaging. Little is known about the isotropic high-resolution 3D DWI application to q-space analysis. In this study, we have developed and implemented a high b-value 3D DWI sequence, applied q-space analysis to study the reliability of high b-value 3D DWI and obtained a microscopic analytical map with isotropic high resolution and less contamination.     

3D atomic imaging by internal-detector electron holography

A method of internal-detector electron holography is the time-reversed version of photoelectron holography. Using an energy-dispersive x-ray detector, an electron gun, and a computer-controllable sample stage, we measured a multiple-energy hologram of the atomic arrangement around the Ti atom in SrTiO3 by recording the characteristic Ti Kα x-ray spectra for different electron beam angles and wavelengths. A real-space image was obtained by using a fitting-based reconstruction algorithm. 3D atomic images of the elements Sr, Ti, and O in SrTiO3 were clearly visualized. The present work reveals that internal-detector electron holography has great potential for reproducing 3D atomic arrangements, even for light elements.     

Application of pulse acoustic microscopy technique for 3D imaging bulk microstructure of carbon fiber-reinforced composites

Impulse acoustic microscopy technique is applied for 3D imaging of bulk microstructure of composite materials. Short pulses of focused high-frequency ultrasound have been employed for layer-by-layer imaging of internal microstructure of carbon fiber-reinforced composite (CFRC) laminates. The method provides spatial resolution of 60 microm and in-depth resolution of 80 microm, approximately. Echo signals reflected from structural units--plies, fiber bundles; and microflaws form acoustic images of microstructure at different depth inside samples. The images make it possible to see ply arrays, packing of bundles in plies; binding material distribution over the specimen body. They reveal failure of interply adhesion, buckling of single plies and fiber bundles, internal defoliations and disbonds, voids in the specimen body. The series of successive images offer outstanding possibilities to reconstruct the bulk structure, to estimate local variations of properties, topological and geometrical characteristics of structural components. The imaging technique has been applied to study different types of fiber packing--unidirectional, cross-ply and woven laminates. Mechanisms of ultrasonic contrast for diverse elements in acoustic images of CFRC laminate bulk microstructure and structural defects are discussed.     

Assessment of 3D inhomogeneous microstructure of highly alloyed aluminium foam via dual energy K-edge subtraction imaging

X-ray microtomography was used to evaluate the inhomogeneous characteristics of newly-developed Al–Zn–Mg foam. Using the synchrotron K-edge subtraction technique, a highly heterogeneous distribution of Zn was quantified three-dimensionally (3D) in the cell wall of as-cast foam. Time-resolved analysis of the concentration evolution revealed a tendency to a homogeneous Zn distribution as solution time prolonged. This was accompanied by a declined variation in hardness measurement. Other microstructural features after solution treatment, such as number and size distribution of micropores, were also characterised. By utilising various quenching rates, the inhomogeneities in microstructure and compression properties inside the foam were also clarified. Thus, element-sensitive tomography provides a novel solution for the 3D/4D analysis in the study of foams.     

3D structure of dendritic and hyper-branched macromolecules by X-ray diffraction

The atomic ordering in dendritic and hyper-branched macromolecules has been determined by X-ray diffraction. The approach of the atomic pair distribution function technique has been used due to the lack of 3D periodicity in these polymeric materials. Dendrimers are found to possess a semi-regular structure riddled with nanosize cavities. The cavities are joined into channels connecting dendrimer's surface and core. In contrast, hyper-branched polymers are rather irregular at the atomic scale and with less accessible interior.     

Label-free 3D imaging of microstructure, blood, and lymphatic vessels within tissue beds in vivo

This Letter reports the use of an ultrahigh resolution optical microangiography (OMAG) system for simultaneous 3D imaging of microstructure and lymphatic and blood vessels without the use of an exogenous contrast agent. An automatic algorithm is developed to segment the lymphatic vessels from the microstructural images based on the fact that the lymph fluid is optically transparent. An OMAG system is developed that utilizes a broadband supercontinuum light source, providing an axial resolution of 2.3 μm and lateral resolution of 5.8 μm, capable of resolving the capillary vasculature and lymphatic vessels innervating microcirculatory tissue beds. Experimental demonstration is performed by showing detailed 3D lymphatic and blood vessel maps, coupled with morphology, within mouse ears in vivo.     

High-resolution time-resolved contrast-enhanced 3D MRA by combining SENSE with keyhole and SLAM strategies

Sensitivity encoding (SENSE) was combined with keyhole and selective line acquisition mode (SLAM) techniques to acquire a time series of images during contrast passage. The acquisition speed of the dynamic time frames was improved by a factor of 8 in total. The high spatial frequencies were sampled during the steady state and combined with the dynamic time frames to construct a series of high-resolution time-resolved contrast-enhanced 3D images. Filtered temporal correlation analysis was used to separate the arteries and veins.     

3D molecular imaging SIMS

Thin monolayer and bilayer films of spin cast poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (PHEMA), poly(lactic) acid (PLA) and PLA doped with several pharmaceuticals have been analyzed by dynamic SIMS using SF₅⁺ polyatomic primary ion bombardment. Each of these systems exhibited minimal primary beam-induced degradation under cluster ion bombardment allowing molecular depth profiles to be obtained through the film. By combing secondary ion imaging with depth profiling, three-dimensional molecular image depth profiles have been obtained from these systems. In another approach, bevel cross-sections are cut in the samples with the SF₅⁺ primary ion beam to produce a laterally magnified cross-section of the sample that does not contain the beam-induced damage that would be induced by conventional focussed ion beam (FIB) cross-sectioning. The bevel surface can then be examined using cluster SIMS imaging or other appropriate microanalysis technique.     

3D imaging of evaporating fuel droplets by stereoscopic PIV

A gun-type burner is a widely used oil burner for industrial and domestic applications. The oil is pressure-atomized and mixed with air generating a recirculating, swirling flow. Because of the surrounding flame, fuel droplets evaporate, being difficult to obtain information on droplets’ dynamics. Several laser techniques have been applied to this burner for spray diagnosis. PDA provides information about droplet size and velocity but can say little about the instantaneous spatial structures in the flow. Planar laser techniques as PIV can describe the 2D instantaneous spatial structures, but cannot provide information about the 3D structures in the flow. Then Stereoscopic PIV was applied. This technique allows us to measure the full 3D velocity vector map in a whole fluid plane. This paper has a double purpose. Firstly, to visualize the 3D structures which are present in the burner; secondly, to show that Stereoscopic PIV is an applicable technique for the diagnosis of an evaporating spray.