Ultrafast electron crystallography: transient structures of molecules, surfaces, and phase transitions

The static structure of macromolecular assemblies can be mapped out with atomic-scale resolution by using electron diffraction and microscopy of crystals. For transient nonequilibrium structures, which are critical to the understanding of dynamics and mechanisms, both spatial and temporal resolutions are required; the shortest scales of length (0.1-1 nm) and time (10(-13) to 10(-12) s) represent the quantum limit, the nonstatistical regime of rates. Here, we report the development of ultrafast electron crystallography for direct determination of structures with submonolayer sensitivity. In these experiments, we use crystalline silicon as a template for different adsorbates: hydrogen, chlorine, and trifluoroiodomethane. We observe the coherent restructuring of the surface layers with subangstrom displacement of atoms after the ultrafast heat impulse. This nonequilibrium dynamics, which is monitored in steps of 2 ps (total change 相似文献   

High numerical aperture tabletop soft x-ray diffraction microscopy with 70-nm resolution

Light microscopy has greatly advanced our understanding of nature. The achievable resolution, however, is limited by optical wavelengths to approximately 200 nm. By using imaging and labeling technologies, resolutions beyond the diffraction limit can be achieved for specialized specimens with techniques such as near-field scanning optical microscopy, stimulated emission depletion microscopy, and photoactivated localization microscopy. Here, we report a versatile soft x-ray diffraction microscope with 70- to 90-nm resolution by using two different tabletop coherent soft x-ray sources-a soft x-ray laser and a high-harmonic source. We also use field curvature correction that allows high numerical aperture imaging and near-diffraction-limited resolution of 1.5lambda. A tabletop soft x-ray diffraction microscope should find broad applications in biology, nanoscience, and materials science because of its simple optical design, high resolution, large depth of field, 3D imaging capability, scalability to shorter wavelengths, and ultrafast temporal resolution.     

Four-dimensional ultrafast electron microscopy

Electron microscopy is arguably the most powerful tool for spatial imaging of structures. As such, 2D and 3D microscopies provide static structures with subnanometer and increasingly with ångstrom-scale spatial resolution. Here we report the development of 4D ultrafast electron microscopy, whose capability imparts another dimension to imaging in general and to dynamics in particular. We demonstrate its versatility by recording images and diffraction patterns of crystalline and amorphous materials and images of biological cells. The electron packets, which were generated with femtosecond laser pulses, have a de Broglie wavelength of 0.0335 Å at 120 keV and have as low as one electron per pulse. With such few particles, doses of few electrons per square ångstrom, and ultrafast temporal duration, the long sought after but hitherto unrealized quest for ultrafast electron microscopy has been realized. Ultrafast electron microscopy should have an impact on all areas of microscopy, including biological imaging.     

Breaking resolution limits in ultrafast electron diffraction and microscopy

Ultrafast electron microscopy and diffraction are powerful techniques for the study of the time-resolved structures of molecules, materials, and biological systems. Central to these approaches is the use of ultrafast coherent electron packets. The electron pulses typically have an energy of 30 keV for diffraction and 100-200 keV for microscopy, corresponding to speeds of 33-70% of the speed of light. Although the spatial resolution can reach the atomic scale, the temporal resolution is limited by the pulse width and by the difference in group velocities of electrons and the light used to initiate the dynamical change. In this contribution, we introduce the concept of tilted optical pulses into diffraction and imaging techniques and demonstrate the methodology experimentally. These advances allow us to reach limits of time resolution down to regimes of a few femtoseconds and, possibly, attoseconds. With tilted pulses, every part of the sample is excited at precisely the same time as when the electrons arrive at the specimen. Here, this approach is demonstrated for the most unfavorable case of ultrafast crystallography. We also present a method for measuring the duration of electron packets by autocorrelating electron pulses in free space and without streaking, and we discuss the potential of tilting the electron pulses themselves for applications in domains involving nuclear and electron motions.     

Infrared PINEM developed by diffraction in 4D UEM

The development of four-dimensional ultrafast electron microscopy (4D UEM) has enabled not only observations of the ultrafast dynamics of photon–matter interactions at the atomic scale with ultrafast resolution in image, diffraction, and energy space, but photon–electron interactions in the field of nanoplasmonics and nanophotonics also have been captured by the related technique of photon-induced near-field electron microscopy (PINEM) in image and energy space. Here we report a further extension in the ongoing development of PINEM using a focused, nanometer-scale, electron beam in diffraction space for measurements of infrared-light-induced PINEM. The energy resolution in diffraction mode is unprecedented, reaching 0.63 eV under the 200-keV electron beam illumination, and separated peaks of the PINEM electron-energy spectrum induced by infrared light of wavelength 1,038 nm (photon energy 1.2 eV) have been well resolved for the first time, to our knowledge. In a comparison with excitation by green (519-nm) pulses, similar first-order PINEM peak amplitudes were obtained for optical fluence differing by a factor of more than 60 at the interface of copper metal and vacuum. Under high fluence, the nonlinear regime of IR PINEM was observed, and its spatial dependence was studied. In combination with PINEM temporal gating and low-fluence infrared excitation, the PINEM diffraction method paves the way for studies of structural dynamics in reciprocal space and energy space with high temporal resolution.Since its invention in the 1930s by Knoll and Ruska (1), the electron microscope has become a powerful tool in the fields of physics, chemistry, materials, and biology. A great variety of techniques related to the electron microscope has been developed in image, diffraction, and energy space (2, 3), with the spatial and energy resolutions of the transmission electron microscope now reaching 0.5 Å with Cs corrector (4) and sub-100 meV with electron monochromators (5, 6), respectively.To these capabilities of spatial and energy resolution has been added the high resolution in the fourth dimension (time) by the development of four-dimensional ultrafast electron microscopy (4D UEM) (7–9), currently enabling nanoscale dynamic studies with temporal resolution that is 10 orders of magnitude better than the millisecond range of video-camera-rate recording in conventional microscopes. In 4D UEM, ultrafast time resolution is reached by using two separate but synchronized ultrashort laser pulses, one to generate a probing electron pulse by photoemission at the microscope cathode and the other to excite the specimen into a nonequilibrium state. The state of the specimen within the window of time of the probe pulse can be observed by recording the probe electron packet scattered from the specimen in any of the different working modes of the microscope, such as image and diffraction (10), energy spectrum (11), convergent beam (12), or scanning transmission electron microscopy (TEM) (13). Scanning the time delay between arrival of the pump and probe pulses at the specimen, which is controlled by a precise optical delay line, allows the evolution of the specimen to be traced.One of the important techniques developed in, and unique to, UEM is photon-induced near-field electron microscopy (PINEM) (14). PINEM has extended the capability of UEM to observation of light–electron interactions near nanostructures or at an interface, which offers exciting prospects for the study of dynamics of photonics and plasmonics at the nanometer scale (15). The three-body interaction of photon, electron, and nanostructure relaxes momentum conservation and leads to efficient coupling between photons and electrons (16). In PINEM, an ultrashort optical pulse is used to excite evanescent electromagnetic fields near a nanostructure or at an interface. When the probe electron packet is in spatiotemporal overlap with these evanescent or scatter fields, some of its electrons can absorb/emit one or more scattered photons and then be detected by their contributions to displaced energy peaks in the electron energy spectrum. These displaced peaks appear as discrete sidebands to the zero-loss peak at separations given by the photon energy (hν) of the pump optical pulse. When using energy filtering to select for imaging only those electrons gaining energy, the resulting PINEM image reflects the strength and topology of the excited near field around the nanostructure or interface.The PINEM technique has been used to detect the evanescent near field surrounding a variety of structures with different materials properties and different geometries, such as carbon nanotubes (14), silver nanowires (14, 17), nanoparticles (16, 18), cells and protein vesicles (19), and several-atoms-thick graphene-layered steps (20). In addition, focused-beam PINEM has been used in scanning TEM mode to obtain induced near-field distributions for a copper grid bar (21), a nanometer gold tip (22), and a silver nanoparticle at the subparticle level (21). In a recent publication, three pulses, two optical and one electron, were introduced into the arsenal of techniques to gate the electron pulse and make its width only limited by the optical-pulse durations (23). Numerous general theoretical treatments (24–28) have successfully described the phenomenon, with detailed treatments quantitatively reproducing many unique features of these multifaceted experimental observations (17, 20–22).Despite the growing body of PINEM studies, almost all previously published PINEM results were obtained in the image mode of the electron microscope using optical pulses with wavelengths of 500–800 nm. An exception is a single unresolved PINEM spectrum for 1,038-nm excitation published from this laboratory (25). Because the PINEM response of a material is governed by its optical properties and dimensions relative to the wavelength of light, excitation wavelength is an important parameter largely remaining to be explored experimentally.Here we report the development of IR PINEM using excitation at the wavelength of 1,038 nm (photon energy 1.2 eV). The spatial- and fluence-dependent behavior of well-resolved IR PINEM induced at the edge of a copper grid bar is examined by combining nanometer-scale convergent-beam electron diffraction and diffraction-mode detection for electron-energy spectroscopy with an unprecedented energy resolution down to 0.63 eV at 200 keV. Different e-beam size effects were compared for PINEM generated by green and IR pump pulses. The spatial dependence of IR PINEM at the interface was studied at low-pulse fluence (linear regime) and high-pulse fluence (nonlinear regime). Diffraction of a gold crystal film was observed using the energy-resolved PINEM electrons produced by interaction with the scatter field of the adjacent copper grid edge. Notably, substantial PINEM peak amplitudes were achievable at dramatically lower fluence for IR pulses than for green pulses, opening up a possible path for studies of photosensitive materials. This general accessibility of strong PINEM signals is of particular importance for our primary interest of ultrafast dynamics, for which PINEM photon gating has the potential to vastly improve temporal resolution.All PINEM experiments reported here were performed on the California Institute of Technology UEM-2 apparatus. The operation voltage on UEM-2 is 200 keV. The laser system used emits a train of ∼220-fs pulses with wavelength of 1,038 nm, set to operate at a repetition rate of 1 MHz. The laser output was frequency-doubled two successive times to provide the 259-nm pulses used to generate the electron packet (probe beam) at the 200-keV microscope photocathode source. The residual 1,038-nm and 519-nm optical pulses were each available for use as the PINEM pump beam to excite the near-field plasmons at the interface. All of the experiments were carried out with polarization set to be perpendicular to the interface and in the single-electron regime (8) to eliminate space-charge effects. In diffraction mode, a camera length of 920 mm and a spectrometer entrance aperture of 1 mm were used to obtain a small collection angle for better energy resolution.     

4D multiple-cathode ultrafast electron microscopy

Four-dimensional multiple-cathode ultrafast electron microscopy is developed to enable the capture of multiple images at ultrashort time intervals for a single microscopic dynamic process. The dynamic process is initiated in the specimen by one femtosecond light pulse and probed by multiple packets of electrons generated by one UV laser pulse impinging on multiple, spatially distinct, cathode surfaces. Each packet is distinctly recorded, with timing and detector location controlled by the cathode configuration. In the first demonstration, two packets of electrons on each image frame (of the CCD) probe different times, separated by 19 picoseconds, in the evolution of the diffraction of a gold film following femtosecond heating. Future elaborations of this concept to extend its capabilities and expand the range of applications of 4D ultrafast electron microscopy are discussed. The proof-of-principle demonstration reported here provides a path toward the imaging of irreversible ultrafast phenomena of materials, and opens the door to studies involving the single-frame capture of ultrafast dynamics using single-pump/multiple-probe, embedded stroboscopic imaging.In 4D ultrafast electron microscopy (UEM), ultrafast light pulses generate electron packets by photoemission at the cathode of an electron microscope, and these are used to probe a dynamic process initiated by heating or exciting the microscopic specimen with a second, synchronized ultrafast light pulse (1, 2). In conventional implementations, each pump pulse on the specimen is accompanied by one suitably delayed laser pulse on the cathode to generate one packet of electrons probing a single time point in the evolution of the specimen. A record of the full course of temporal evolution of the specimen is then constructed by repeating the experiment multiple times with variation of the delay time between the two light pulses, reading out a separate CCD image for each delay time. Thus, information about different time points in the dynamic response of the specimen is obtained from different excitation events. This implementation is ideally suited for a specimen that undergoes irreversible but sufficiently well-defined dynamics to allow a new specimen area to be used for each time point (Fig. 1A), or for a specimen that recovers fully to allow repeated identical excitations of the same area (Fig. 1B); see also Methodology. The applications of these two approaches are numerous, as highlighted in a recent review account of the work (3).Open in a separate windowFig. 1.Variant implementations of UEM. (A) Single-pulse UEM, which enables single-shot imaging of homogeneous specimens. (B) Stroboscopic UEM. (D) Multiple-cathode UEM. For comparison, we include, in C, the single-cathode, deflection method. See text for details.For the study of completely nonrepetitive dynamics, for example, a stochastic process in a heterogeneous sample that does not return to its initial configuration, a series of snapshots following a single excitation event can provide the only direct and detailed view of the evolution. Observing the effects of a single excitation pulse with video-mode imaging can currently reach millisecond-scale time resolution, far short of the time scale for many phenomena of interest in nanoscale materials science, chemistry, and physics. Nanosecond resolution has been reached (4) by combining one excitation pulse with a train of light pulses on a single cathode, with deflection of the imaging electrons after passing the specimen plane to direct each successive pulse to a new region of the detector (Fig. 1C). This nanosecond method has been successfully used with a high-speed electrostatic deflector array to obtain time sequences of irreversible and stochastic processes (5, 6).Here we demonstrate a technique that removes any limit on time resolution imposed by image deflection and in a single frame enables the capture of ultrafast phenomena. With this approach, it is possible to probe and distinctly record multiple time points in a dynamic process following a single initiation pulse. The probing electron packets are all generated by a single light pulse that impinges on multiple, spatially distinct, cathode surfaces (Fig. 1D). Time separations between packets in the electron-pulse train are adjusted by the cathode spatial and electrostatic configuration. In the present application, two electron packets, generated from two source locations at the same potential and separated in time by 19 picoseconds (ps), are recorded on each CCD frame after undergoing diffraction in a gold film following femtosecond heating. The packets originate from different cathode locations, pass through the same area of the specimen, and are recorded at distinct locations on the detector, thereby encoding two different time points in the evolution of the specimen. The results obtained provide the basis for exploration of expanded application of the multiple-cathode concept.     

Super-resolution fluorescence imaging of organelles in live cells with photoswitchable membrane probes

Imaging membranes in live cells with nanometer-scale resolution promises to reveal ultrastructural dynamics of organelles that are essential for cellular functions. In this work, we identified photoswitchable membrane probes and obtained super-resolution fluorescence images of cellular membranes. We demonstrated the photoswitching capabilities of eight commonly used membrane probes, each specific to the plasma membrane, mitochondria, the endoplasmic recticulum (ER) or lysosomes. These small-molecule probes readily label live cells with high probe densities. Using these probes, we achieved dynamic imaging of specific membrane structures in living cells with 30–60 nm spatial resolution at temporal resolutions down to 1–2 s. Moreover, by using spectrally distinguishable probes, we obtained two-color super-resolution images of mitochondria and the ER. We observed previously obscured details of morphological dynamics of mitochondrial fusion/fission and ER remodeling, as well as heterogeneous membrane diffusivity on neuronal processes.     

Scanning ultrafast electron microscopy

Progress has been made in the development of four-dimensional ultrafast electron microscopy, which enables space-time imaging of structural dynamics in the condensed phase. In ultrafast electron microscopy, the electrons are accelerated, typically to 200 keV, and the microscope operates in the transmission mode. Here, we report the development of scanning ultrafast electron microscopy using a field-emission-source configuration. Scanning of pulses is made in the single-electron mode, for which the pulse contains at most one or a few electrons, thus achieving imaging without the space-charge effect between electrons, and still in ten(s) of seconds. For imaging, the secondary electrons from surface structures are detected, as demonstrated here for material surfaces and biological specimens. By recording backscattered electrons, diffraction patterns from single crystals were also obtained. Scanning pulsed-electron microscopy with the acquired spatiotemporal resolutions, and its efficient heat-dissipation feature, is now poised to provide in situ 4D imaging and with environmental capability.     

Micro-computed tomography of the lungs and pulmonary-vascular system

Three-dimensional imaging of the intact lung and its vasculature is essential if the hierarchical and volumetric aspects of its structures and functions are to be quantitated. Although this is possible with clinical multislice helical CT scanners, the spatial resolution does not scale down adequately for small rodents for which cubic voxel dimensions of 50-100 microm are required. Micro-computed tomography (micro-CT) provides the necessary spatial resolution of 3D images of the intact thoracic contents. Micro-CT can provide higher resolution so that basic micro-architectural structures, such as alveoli, can be individually visualized and quantitated. Dynamic events, such as the respiratory and cardiac cycles, can be imaged at multiple time points throughout a representative cycle by coordinating the scan sequence (i.e., gating) to the cycle phase of a sequence of cycles. Fusion of the micro-CT image data with other image data, such as micro-SPECT or histology, can enhance the information content beyond the mainly structural information provided by micro-CT. Conventional attenuation-based X-ray imaging can involve significant X-ray exposures at high spatial resolutions, and this could affect the phenotype (e.g., via interstitial fibrosis) and genotype (e.g., via mutation), so its use in longitudinal studies using micro-CT may be limited in some cases. However, because of recent developments in which the phase shift or refraction of X-rays rather than attenuation is used, the X-ray exposure may be significantly reduced.     

Fast,background-free, 3D super-resolution optical fluctuation imaging (SOFI)

Super-resolution optical microscopy is a rapidly evolving area of fluorescence microscopy with a tremendous potential for impacting many fields of science. Several super-resolution methods have been developed over the last decade, all capable of overcoming the fundamental diffraction limit of light. We present here an approach for obtaining subdiffraction limit optical resolution in all three dimensions. This method relies on higher-order statistical analysis of temporal fluctuations (caused by fluorescence blinking/intermittency) recorded in a sequence of images (movie). We demonstrate a 5-fold improvement in spatial resolution by using a conventional wide-field microscope. This resolution enhancement is achieved in iterative discrete steps, which in turn allows the evaluation of images at different resolution levels. Even at the lowest level of resolution enhancement, our method features significant background reduction and thus contrast enhancement and is demonstrated on quantum dot-labeled microtubules of fibroblast cells.     

Kikuchi ultrafast nanodiffraction in four-dimensional electron microscopy

Coherent atomic motions in materials can be revealed using time-resolved X-ray and electron Bragg diffraction. Because of the size of the beam used, typically on the micron scale, the detection of nanoscale propagating waves in extended structures hitherto has not been reported. For elastic waves of complex motions, Bragg intensities contain all polarizations and they are not straightforward to disentangle. Here, we introduce Kikuchi diffraction dynamics, using convergent-beam geometry in an ultrafast electron microscope, to selectively probe propagating transverse elastic waves with nanoscale resolution. It is shown that Kikuchi band shifts, which are sensitive only to the tilting of atomic planes, reveal the resonance oscillations, unit cell angular amplitudes, and the polarization directions. For silicon, the observed wave packet temporal envelope (resonance frequency of 33 GHz), the out-of-phase temporal behavior of Kikuchi's edges, and the magnitude of angular amplitude (0.3 mrad) and polarization elucidate the nature of the motion: one that preserves the mass density (i.e., no compression or expansion) but leads to sliding of planes in the antisymmetric shear eigenmode of the elastic waveguide. As such, the method of Kikuchi diffraction dynamics, which is unique to electron imaging, can be used to characterize the atomic motions of propagating waves and their interactions with interfaces, defects, and grain boundaries at the nanoscale.     

Magnetic resonance imaging with an optical atomic magnetometer

We report an approach for the detection of magnetic resonance imaging without superconducting magnets and cryogenics: optical atomic magnetometry. This technique possesses a high sensitivity independent of the strength of the static magnetic field, extending the applicability of magnetic resonance imaging to low magnetic fields and eliminating imaging artifacts associated with high fields. By coupling with a remote-detection scheme, thereby improving the filling factor of the sample, we obtained time-resolved flow images of water with a temporal resolution of 0.1 s and spatial resolutions of 1.6 mm perpendicular to the flow and 4.5 mm along the flow. Potentially inexpensive, compact, and mobile, our technique provides a viable alternative for MRI detection with substantially enhanced sensitivity and time resolution for various situations where traditional MRI is not optimal.     

Coronary Flow Assessment Using 3-Dimensional Ultrafast Ultrasound Localization Microscopy

BackgroundDirect assessment of the coronary microcirculation has long been hampered by the limited spatial and temporal resolutions of cardiac imaging modalities.ObjectivesThe purpose of this study was to demonstrate 3-dimensional (3D) coronary ultrasound localization microscopy (CorULM) of the whole heart beyond the acoustic diffraction limit (<20 μm resolution) at ultrafast frame rate (>1000 images/s).MethodsCorULM was performed in isolated beating rat hearts (N = 6) with ultrasound contrast agents (Sonovue, Bracco), using an ultrasonic matrix transducer connected to a high channel–count ultrafast electronics. We assessed the 3D coronary microvascular anatomy, flow velocity, and flow rate of beating hearts under normal conditions, during vasodilator adenosine infusion, and during coronary occlusion. The coronary vasculature was compared with micro-computed tomography performed on the fixed heart. In vivo transthoracic CorULM was eventually assessed on anaesthetized rats (N = 3).ResultsCorULM enables the 3D visualization of the coronary vasculature in beating hearts at a scale down to microvascular structures (<20 μm resolution). Absolute flow velocity estimates range from 10 mm/s in tiny arterioles up to more than 300 mm/s in large arteries. Fitting to a power law, the flow rate–radius relationship provides an exponent of 2.61 (r² = 0.96; P < 0.001), which is consistent with theoretical predictions and experimental validations of scaling laws in vascular trees. A 2-fold increase of the microvascular coronary flow rate is found in response to adenosine, which is in good agreement with the overall perfusion flow rate measured in the aorta (control measurement) that increased from 8.80 ± 1.03 mL/min to 16.54 ± 2.35 mL/min (P < 0.001). The feasibility of CorULM was demonstrated in vivo for N = 3 rats.ConclusionsCorULM provides unprecedented insights into the anatomy and function of coronary arteries at the microvasculature level in beating hearts. This new technology is highly translational and has the potential to become a major tool for the clinical investigation of the coronary microcirculation.     

Atomic-scale dynamical structures of fatty acid bilayers observed by ultrafast electron crystallography

The structure and dynamics of a biological model bilayer are reported with atomic-scale resolution by using ultrafast electron crystallography. The bilayer was deposited as a Langmuir-Blodgett structure of arachidic (eicosanoic) fatty acids with the two chains containing 40 carbon atoms (approximately = 50 angstroms), on a hydrophobic substrate, the hydrogen terminated silicon(111) surface. We determined the structure of the 2D assembly, establishing the orientation of the chains and the subunit cell of the CH2 distances: a0 = 4.7 angstroms, b0 = 8.0 angstroms, and c0 = 2.54 angstroms. For structural dynamics, the diffraction frames were taken every 1 picosecond after a femtosecond temperature jump. The observed motions, with sub-angstroms resolution and monolayer sensitivity, clearly indicate the coherent anisotropic expansion of the bilayer solely along the aliphatic chains, followed by nonequilibrium contraction and restructuring at longer times. This motion is indicative of a nonlinear behavior among the anharmonically coupled bonds on the ultrashort time scale and energy redistribution and diffusion on the longer time scale. The ability to observe such atomic motions of complex structures and at interfaces is a significant leap forward for the determination of macromolecular dynamical structures by using ultrafast electron crystallography.     

Efficacy of 3D‐positron emission tomography/computed tomography for upper abdomen

Recent advancement in computed tomography (CT) enables us to obtain high spatial resolution image and made it possible to construct extensive high‐definition three‐dimensional (3D) images. But a lack of contrast resolution in CT alone is still remained problem. Meanwhile, as fluorodeoxyglucose‐positron emission tomography (PET) can visualize tumors in high contrast, we can create 3D images fusing the accumulation in tumors on PET/CT images. Such images can play the role of a “map of body” which makes it easy to understand the anatomical information before surgery. We also try to evaluate segmental liver function by using PET/CT fusion images. By using ¹¹C‐methionine PET/contrast‐enhanced CT, superior image quality compared to single photon emission computed tomography/CT can be obtained. CT, especially with contrast enhancement for obtaining anatomical imaging information plus PET for obtaining functional imaging information is a highly compatible combination, and adding these two types information will further increase clinical usefulness.     

4D visualization of embryonic, structural crystallization by single-pulse microscopy

In many physical and biological systems the transition from an amorphous to ordered native structure involves complex energy landscapes, and understanding such transformations requires not only their thermodynamics but also the structural dynamics during the process. Here, we extend our 4D visualization method with electron imaging to include the study of irreversible processes with a single pulse in the same ultrafast electron microscope (UEM) as used before in the single-electron mode for the study of reversible processes. With this augmentation, we report on the transformation of amorphous to crystalline structure with silicon as an example. A single heating pulse was used to initiate crystallization from the amorphous phase while a single packet of electrons imaged selectively in space the transformation as the structure continuously changes with time. From the evolution of crystallinity in real time and the changes in morphology, for nanosecond and femtosecond pulse heating, we describe two types of processes, one that occurs at early time and involves a nondiffusive motion and another that takes place on a longer time scale. Similar mechanisms of two distinct time scales may perhaps be important in biomolecular folding.     

Time resolved DCE-MRI of the kidneys: Evaluation of the renal vasculatures and tumors using F-DISCO with and without compressed sensing in normal and wide-bore 3T systems

Dynamic contrast-enhanced MR imaging (DCE-MRI) has been widely used for the evaluation of renal arteries. This method is also useful for tumor and renal parenchyma characterization. The very fast MRI may provide stable and precise information regarding vasculature and soft tissues. The purpose of this study was to evaluate the ability of DCE-MRI to assess renal vasculatures and tumor perfusions using Differential subsampling with Cartesian ordering with spectrally selected inversion recovery with adiabatic pulses (F-DISCO) with and without compressed sensing (CS) in normal and wide-bore 3T systems.Fifty-one patients who underwent DCE-MRI using F-DISCO with or without CS for evaluation of renal or adrenal regions were included. Image quality, artifacts, fat saturation, and selective visual recognition of renal vasculatures were assessed by using a 5-point scale. Tumor recognition was verified by using a 5-point scale of confidence level. Signal intensities of each structure were also measured.In all cases, the temporal resolution of each phase for DCE-MRI was 1.9 to 2.0 seconds. Image quality, artifacts, fat saturation, and selective visual recognition of vasculatures were all acceptable (mean score 4.2–4.9). The selective visualization of renal arteries and veins was successfully accomplished (mean score 4.0–4.9). Contrast media perfusion for renal vasculature, renal parenchyma, and tumors was also recognized.DCE-MRI for the evaluation of renal vasculatures and tumors using F-DISCO with or without CS can be performed with high temporal and spatial resolutions in normal and wide-bore 3T systems. This information can be obtained in a stable fashion throughout the dynamic contrast study. CS can additionally provide benefits that the total imaging time may be shorter than without CS.     

Ultrafast multiplanar determination of left ventricular hypertrophy in spontaneously hypertensive rats with single-shot spin-echo nuclear magnetic resonance imaging

OBJECTIVE: To determine whether left ventricular hypertrophy can be correctly evaluated in hypertensive rats with a new nuclear magnetic resonance (NMR) imaging modality that is relatively simple to operate and provides results of constant quality while offering a high signal-to-noise ratio.DESIGN Left ventricular mass as calculated from the NMR imaging analysis was compared with the actual left ventricular mass measured by gravimetry. METHODS: Single-shot ultrafast spin-echo (SSFSE) imaging of hearts of Wistar-Kyoto rats and spontaneously hypertensive rats was performed at 4 T. Left ventricular mass was determined by using Simpson's rule on stacks of images acquired in systole and diastole. RESULTS: SSFSE NMR imaging performed in systole or in diastole evaluated and quantified left ventricular hypertrophy in hearts of spontaneously hypertensive rats very similarly to gravimetry. The left ventricular mass as determined by NMR was in good accordance with the actual left ventricular weight (SEE: 30.39 and 35.86 mg for the systolic and diastolic NMR acquisitions, respectively). CONCLUSION: Using an SSFSE sequence, high-quality NMR images of the rat heart can be generated very reliably with sufficient contrast and temporal and spatial resolution, and allow precise, non-invasive and fast characterization of left ventricular hypertrophy in a hypertensive rat model.     

Multidetector row helical CT of the pancreas: value of three-dimensional images, two-dimensional reformations, and contrast-enhanced multiphasic imaging

Multidetector row helical computed tomography (MD‐CT) scanning is performed for the evaluation of pancreatic tumors. Three‐phase contrast study is performed using 2.5‐mm collimation, and the images are reconstructed at 1.25‐mm intervals. CT angiography and pancreatic duct images using two‐ or three‐dimensional techniques are reconstructed from the volumetric data. MD‐CT can perform multiphasic scanning rapidly with an optimal temporal window. CT angiography obtained with MD‐CT can delineate peripancreatic vasculature with high spatial resolution and sufficient vascular enhancement. Pancreatic duct images can provide important information in assessing pancreatic disease. MD‐CT has the potential to improve detection and preoperative assessment of pancreatic tumors.     

X-ray computed tomography and magnetic resonance imaging of the cardiovascular system

Both electron beam (or ultrafast) x-ray computed tomography and magnetic resonance imaging are developing cardiovascular imaging modalities that can provide high temporal and spatial resolution images of the beating heart and the great vessels in the outpatient setting. The three-dimensional registration of these images has facilitated numerous studies, showing that these devices are capable of quantitating cardiovascular anatomy, function, and blood flow. Continued research employing these methodologies has examined both applications that are complementary to more traditional noninvasive cardiology tools, such as two-dimensional echocardiography and radionuclide and perfusion imaging, as well as applications unique to computed tomography or magnetic resonance imaging. This review discusses progress made in x-ray computed tomography, focusing on the application of electron beam computed tomography and new applications of magnetic resonance imaging within the past 2 years. Specific comments are made regarding these applications, as well as the limitations of both with regard to general clinical applications.