High-resolution Phase Contrast Imaging Research Articles

Synthesis of rod-shaped Au-Cu intermetallic nanoparticles and SERS detection

Intermetallic phases in Au-Cu nanoparticles has been synthesized chemically in liquid phase. The heat-treatment of these nanoparticles at 180 °C for 1 h in solution phase led to the formation of Au-Cu alloy nanoparticles with nearly spherical shapes and monodispersed size of ∼10 nm. Subsequent heat-treatment at 300 °C for 30 min of these Au-Cu alloy nanoparticles not only revealed structural transformation to intermetallic phases (tetragonal AuCu (tP4) and cubic Cu3Au (cP4)) but also shape change occurs from spherical to rods having average aspect ratio ∼3.0. The structural determination at the particle has been carried out through nano-beam electron diffraction coupled with simulation of electron diffraction patterns and high resolution phase contrast images. These Au–Cu intermetallic nanoparticles exhibit an excellent surface enhanced Raman spectroscopic activity with methylene blue compared to that of Au-Cu alloy nanoparticles.

Evidence for spinodal decomposition in Ti-15Mo quenched alloy using transmission electron microscopy

Electron diffraction effect associated with nano-scale periodic modulations observed in a quenched Ti-15 wt%Mo alloy within ω phase forming regime has been investigated in this study by analyzing selected area diffraction patterns and high-resolution phase contrast images. Crystallographic direction of the side bands in electron diffraction associated with the periodic modulation of wavelength 1.3 nm is established as [001]β which is an elastically soft direction for the body-centered cubic lattice. Characteristics of the side bands in the diffraction patterns, such as the correlation of spacing of satellite spots with real-space modulation wavelength and the constancy in the direction of satellites and modulations are interpreted as the proof of phase separation by spinodal decomposition. The striations associated with spinodal decomposition form only in Mo-enriched regions of dimensions ˜ 20 nm distributed throughout the β matrix. Accordingly, in this study, two varieties of compositional modulations that appear in the matrix are experimentally illustrated: (1) Mo-depleted and Mo-enriched domains of 10–20 nm size, and (2) The anisotropic compositional modulation by spinodal decomposition in [001]β direction with a modulation wavelength of 1.3 nm occurring in Mo-enriched pockets. It is confirmed that the structural modulations and ω phase formation occurs only in the regions without striations (Mo-lean regions). By analyzing the diffraction spots, it is also shown that the ω phase does not undergo any phase separation by spinodal decomposition.

Wave-propagation simulation and dark-field computed tomography imaging study to elucidate the contrast-loss problem in X-ray diffraction-based transcranial brain imaging

It is crucial to overcome the skull-based contrast loss of cerebral structures in the development of high-resolution X-ray phase-contrast transcranial brain imaging. The present study aimed to determine the consequence of microporous skull-based scattering and its correlation with the contrast loss in a dark-field imaging-computed tomography transcranial brain imaging setup and to suggest methods for visualizing interior structures in analyzer-based X-ray transcranial brain CT imaging. Virtual-skull brain phantoms that mimic the microporous structure of the skull and internal structures including gray/white matter, glioma edema, and the central necrosis of glioma were prepared for wave-propagation simulation of dark-field computed tomography imaging. Reconstruction of refraction angle images and sinogram analysis of angle deviation showed microporous skull-mediated increases in the right-left angle mismatch and out-of-range values in curve fitting with a rocking curve, which was correlated with the contrast loss for internal structures. Correcting the angle mismatch and artificially widening the rocking curve both restored contrast to the internal structures in the reconstructed images. In conclusion, the contrast loss in analyzer-based X-ray diffraction for transcranial brain imaging was related to increases in microporous skull-mediated angle mismatches and out-of-range values when calculating refraction angles using the analyzer rocking curve. This contrast loss may be resolved by using refraction angle measurements from an analyzer crystal with a wider rocking curve or by using an algorithm such as a neural network that can distinguish the left/right refraction angle for a given intensity in a rocking curve.

Quantitative Assessment of Degenerative Cartilage and Subchondral Bony Lesions in a Preserved Cadaveric Knee: Propagation-Based Phase-Contrast CT Versus Conventional MRI and CT.

The aim of this study was to quantitatively assess hyaline cartilage and subchondral bone conditions in a fully preserved cadaveric human knee joint using high-resolution x-ray propagation-based phase-contrast imaging (PBI) CT and to compare the performance of the new technique with conventional CT and MRI. A cadaveric human knee was examined using an x-ray beam of 60 keV, a detector with a 90-mm2 FOV, and a pixel size of 46 × 46 μm2. PBI CT images were reconstructed with both the filtered back projection algorithm and the equally sloped tomography method. Conventional 3-T MRI and CT were also performed. Measurements of cartilage thickness, cartilage lesions, International Cartilage Repair Society scoring, and detection of subchondral bone changes were evaluated. Visual inspection of the specimen akin to arthroscopy was conducted and served as a standard of reference for lesion detection. Loss of cartilage height was visible on PBI CT and MRI. Quantification of cartilage thickness showed a strong correlation between the two modalities. Cartilage lesions appeared darker than the adjacent cartilage on PBI CT. PBI CT showed similar agreement to MRI for depicting cartilage substance defects or lesions compared with the visual inspection. The assessment of subchondral bone cysts showed moderate to strong agreement between PBI CT and CT. In contrast to the standard clinical methods of MRI and CT, PBI CT is able to simultaneously depict cartilage and bony changes at high resolution. Though still an experimental technique, PBI CT is a promising high-resolution imaging method to evaluate comprehensive changes of osteoarthritic disease in a clinical setting.

Periodic Giant Polarization Gradients in Doped BiFeO3 Thin Films.

The ultimate challenge for the development of new multiferroics with enhanced properties lies in achieving nanoscale control of the coupling between different ordering parameters. In oxide-based multiferroics, substitutional cation dopants offer the unparalleled possibility to modify both the electric and magnetic properties at a local scale. Herein it is demonstrated the formation of a dopant-controlled polar pattern in BiFeO3 leading to the spontaneous instauration of periodic polarization waves. In particular, nonpolar Ca-doped rich regions act as spacers between consecutive dopant-depleted regions displaying coupled ferroelectric states. This alternation of layers with different ferroelectric state creates a novel vertical polar structure exhibiting giant polarization gradients as large as 70 μC cm-2 across 30 Å thick domains. The drastic change in the polar state of the film is visualized using high-resolution differential phase-contrast imaging able to map changes in ferroelectric polarization at atomic scale. Furthermore, a periodic distortion in the Fe-O-Fe bonding angle suggests a local variation in the magnetic ordering. The findings provide a new insight into the role of doping and reveal hitherto unexplored means to tailor the functional properties of multiferroics by doping engineering.

High-resolution X-ray phase-contrast imaging with a grating interferometer

High-resolution X-ray imaging is a promising technique for studies of biological structures on a micron-scale. Conventional X-ray imaging is limited by its poor soft tissue contrast. X-ray phasecontrast imaging has the potential to significantly improve the biological contrast in terms of the refractive index variation. In this paper, we analyze an X-ray grating interferometer we set up with a micro-focus X-ray source and a flat panel detector. The system utilizes the geometric magnification for high-resolution imaging. After our initial calibration, the visibility function reaches about 10%. The experiments demonstrate that the spatial resolution of the system is about 10 μm, which helps reveal features invisible with X-ray attenuation-based imaging.

Synchrotron radiation X-ray in-line phase-contrast imaging with highly symmetric source

In available synchrotron radiation facilities all over the world, the size of electron beam bunch has distinct difference between the horizontal direction and the vertical direction at the cross section vertical to electron moving direction. It will cause low symmetry of source size in horizontal and vertical directions, and result in big difference in the spacial resolution and phase contrast between these two directions in synchrotron radiation X-ray in-line phase contrast imaging. The superior resolution and phase contrast of one direction will couple with the inferior ones of the other direction in actual object imaging, this interaction will degrade both spacial resolution and phase contrast of the whole image and lead to poor image quality. In this paper, a four-knife white beam slit is used in Beijing synchrotron radiation facility (BSRF) 4W1A beamline to adjust source size in horizontal and vertical directions and restrict the point spread function (PSF) of source, the source size is limited from 1.23 mm (horizontal)×0.46 mm (vertical) to 0.35 mm×0.35 mm. By this way, a highly symmetric effective X-ray source is obtained. The symmetry of source size is improved from 2.7 to about 1. Although the total X-ray flux loses 99.95%, the X-ray flux density is only reduced to 22%. It because total flux is taken account of the expanded spot size due to divergence, while flux density is mainly related to effective source size, which is limited not severely as expanded spot size by white beam slit. The flux density reduction by this method is not a critical defect in high resolution X-ray phase contrast imaging and can be improved by other ways. The polyamide fiber net is imaged using X-ray in-line phase contrast imaging at 4W1A, BSRF. Diameter of fiber is about 50 microns, X-ray energy is 13 keV, the effective pixel size is 0.325 micron, the experiment images are obtained at object-to-detector distances of 4, 10, 16, 36, 100, 200 and 300 mm with source-to-object distance of about 43 m. The results show that when source size in horizontal direction is limited from 1.23 to 0.35 mm (symmetry of source size is about 1 at this condition), the maximal phase contrast of image is advanced from 0.057 to 0.35. It means the density sensitivity is promoted about 5 times than in ordinary source size conditions. The spacial resolutions in different source size symmetry conditions are also primary analyzed based on Rayleigh criterion. The result shows that spacial resolution can be improved notably in horizontal direction by limiting horizontal source size to a nearly symmetric source size condition. Our work demonstrates that the spacial resolution and phase contrast can be promoted at the same level by source size symmetry improved, which markedly improved the imaging quality in synchrotron radiation X-ray in-line phase-contrast imaging. It will have great benefit to high-resolution X-ray in-line phase contrast imaging. The reasons of these improvements are analyzed and developments in future are also discussed.

Measurement of atomic electric fields and charge densities from average momentum transfers using scanning transmission electron microscopy

This study sheds light on the prerequisites, possibilities, limitations and interpretation of high-resolution differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM). We draw particular attention to the well-established DPC technique based on segmented annular detectors and its relation to recent developments based on pixelated detectors. These employ the expectation value of the momentum transfer as a reliable measure of the angular deflection of the STEM beam induced by an electric field in the specimen. The influence of scattering and propagation of electrons within the specimen is initially discussed separately and then treated in terms of a two-state channeling theory.A detailed simulation study of GaN is presented as a function of specimen thickness and bonding. It is found that bonding effects are rather detectable implicitly, e.g., by characteristics of the momentum flux in areas between the atoms than by directly mapping electric fields and charge densities. For strontium titanate, experimental charge densities are compared with simulations and discussed with respect to experimental artifacts such as scan noise. Finally, we consider practical issues such as figures of merit for spatial and momentum resolution, minimum electron dose, and the mapping of larger-scale, built-in electric fields by virtue of data averaged over a crystal unit cell. We find that the latter is possible for crystals with an inversion center. Concerning the optimal detector design, this study indicates that a sampling of 5mrad per pixel is sufficient in typical applications, corresponding to approximately 10×10 available pixels.

0217: Is high temporal resolution achievable for pediatric cardiac acquisitions during several heart beats? Illustration with cardiac phase contrast cine-MRI

During pediatric cardiac Cine-MRI, data acquired during cycles of different lengths must be combined. Feinstein’s model is used to project multiple cardiac cycles of variable lengths into a mean cycle. 1/The temporal errors during Feinstein’s projection were computed in 306 cardiac cycles fully characterized by tissue Doppler imaging with 6-phase analysis (from a population of 7 children and young adults). 2/The effects of these temporal errors on tissue velocities were assessed by simulating a typical tissue phase mapping acquisition and reconstruction. 3/Myocardial velocities curves, extracted from high-resolution phase-contrast cine images, were compared for the 6 volunteers with lowest and highest heart rate variability, within a population of 36young adults. 1/The mean of temporal misalignments was 30ms over the cardiac cycle but reached 60ms during early diastole. 2/During phase contrast MRI simulation, early diastole velocity peaks were diminished by 5cm/s leading to virtual disappearance of isovolumic relaxation peaks. 3/The smoothing and erasing of isovolumic relaxation peaks was confirmed on tissue phase mapping velocity curves, between subjects with low and high heart rate variability (p=0.05). Feinstein cardiac model creates temporal misalignments that impair high temporal resolution (notably for phase contrast cine imaging) in a population of high heart rate variations such as in paediatry (figure next page). Abstract 0217 – Figure: Velocity curves with/without heart rate variation

Is High Temporal Resolution Achievable for Paediatric Cardiac Acquisitions during Several Heart Beats? Illustration with Cardiac Phase Contrast Cine-MRI.

BackgroundDuring paediatric cardiac Cine-MRI, data acquired during cycles of different lengths must be combined. Most of the time, Feinstein’s model is used to project multiple cardiac cycles of variable lengths into a mean cycle.ObjectiveTo assess the effect of Feinstein projection on temporal resolution of Cine-MRI.Methods1/The temporal errors during Feinstein’s projection were computed in 306 cardiac cycles fully characterized by tissue Doppler imaging with 6-phase analysis (from a population of 7 children and young adults). 2/The effects of these temporal errors on tissue velocities were assessed by simulating typical tissue phase mapping acquisitions and reconstructions. 3/Myocardial velocities curves, extracted from high-resolution phase-contrast cine images, were compared for the 6 volunteers with lowest and highest heart rate variability, within a population of 36 young adults.Results1/The mean of temporal misalignments was 30 ms over the cardiac cycle but reached 60 ms during early diastole. 2/During phase contrast MRI simulation, early diastole velocity peaks were diminished by 6.1 cm/s leading to virtual disappearance of isovolumic relaxation peaks. 3/The smoothing and erasing of isovolumic relaxation peaks was confirmed on tissue phase mapping velocity curves, between subjects with low and high heart rate variability (p = 0.05).ConclusionsFeinstein cardiac model creates temporal misalignments that impair high temporal resolution phase contrast cine imaging when beat-to-beat heart rate is changing.

3D Investigation of Fatigue Crack Morphology and Crack Growth of Iron-Based Materials via Synchrotron X-Ray CT

The present paper addresses a 3D investigation of a complicated fatigue crack profile and crack growth behavior in cast Hadfield high manganese steel by in-situ X-ray computed tomography (CT) experiments. In-situ loading experiments were performed on fatigue pre-cracked samples at the X-ray beamline BL13W1 of Shanghai synchrotron radiation facility in China, and high-resolution phase contrast imaging technique was applied to obtain the 3D images. Based on the rendered 3D images at varied loading levels, various crack features and the interactions of the fatigue crack with casting pores were identified and analyzed.

High-Resolution Phase-Contrast Imaging at XFELs

We built an x-ray microscope for the Matter in Extreme Conditions (MEC) endstation of the LCLS, which is especially adapted to the XFEL environment, and used the setup to carry out different experiments based on magnified x-ray phase-contrast imaging. The method allows one to image a sample with high spatial resolution and high sensitivity to small density changes within a sample. The xray microscope is based on a set of Beryllium compound refractive lenses (Be-CRLs) creating a secondary x-ray source closely in front of a sample. The sample is positioned in the divergent x-ray beam and a CCD detector records a magnified image of the illuminated area at a larger distance further downstream [cf. Fig. 1 a)]. In a pump-probe scheme an optical laser hits the sample from the side, which initiates a shock wave propagating into the material, and the XFEL pulse probes the state of the material shortly after. In Figs. 1 b) – 1 e) a series of measured phase-contrast images recorded on Kapton at diff t delay times between the optical and XFEL pulse is shown. Larger features such as the position of the shock front and structures behind it can be observed directly in the image. However, due to diffraction effects it is necessary to evaluate the phase-contrast images numerically using phase retrieval methods in order to obtain high-resolution images of the sample. In addition, a quantitative analysis requires a precise knowledge of the illumination wave fi to disentangle features already present in the incoming x-ray beam from the actual object. Beam characterization was carried out using scanning coherent x-ray microscopy (ptychography). The method yields the wave fi distribution of the nano-focused XFEL beam with high spatial resolution (cf. Fig. 2) [3, 4]. In this contribution we report on current activities to implement high-resolution x-ray imaging at the Linac Coherent Light Source (LCLS) [5]. We demonstrate these new capabilities at the example of imaging of shock waves in different materials, outline the analysis procedure and discuss limitations related to fluctuations in beam intensity and position, bandwidth of the XFEL beam [6] and dispersive properties of the x-ray optics.

Are human peripheral nerves sensitive to X-ray imaging?

Diagnostic imaging techniques play an important role in assessing the exact location, cause, and extent of a nerve lesion, thus allowing clinicians to diagnose and manage more effectively a variety of pathological conditions, such as entrapment syndromes, traumatic injuries, and space-occupying lesions. Ultrasound and nuclear magnetic resonance imaging are becoming useful methods for this purpose, but they still lack spatial resolution. In this regard, recent phase contrast x-ray imaging experiments of peripheral nerve allowed the visualization of each nerve fiber surrounded by its myelin sheath as clearly as optical microscopy. In the present study, we attempted to produce high-resolution x-ray phase contrast images of a human sciatic nerve by using synchrotron radiation propagation-based imaging. The images showed high contrast and high spatial resolution, allowing clear identification of each fascicle structure and surrounding connective tissue. The outstanding result is the detection of such structures by phase contrast x-ray tomography of a thick human sciatic nerve section. This may further enable the identification of diverse pathological patterns, such as Wallerian degeneration, hypertrophic neuropathy, inflammatory infiltration, leprosy neuropathy and amyloid deposits. To the best of our knowledge, this is the first successful phase contrast x-ray imaging experiment of a human peripheral nerve sample. Our long-term goal is to develop peripheral nerve imaging methods that could supersede biopsy procedures.

The influence of Cs/Cc correction in analytical imaging and spectroscopy in scanning and transmission electron microscopy

Aberration correction in scanning/transmission electron microscopy (S/TEM) owes much to the efforts of a small dedicated group of innovators. Leading that frontier has been Prof. Harald Rose. To date his leadership and dynamic personality has spearheaded our ability to leave behind many of the limitations imposed by spherical aberration (Cs) in high resolution phase contrast imaging. Following shortly behind, has been the development of chromatic aberration correction (Cc) which augments those accomplishments. In this paper we will review and summarize how the combination of Cs/Cc technology enhances our ability to conduct hyperspectral imaging and spectroscopy in today׳s and future computationally mediated experiments in both thin as well as realistic specimens in vacuo and during in-situ/environmental experiments.

Accelerated radial Fourier-velocity encoding using compressed sensing

PurposePhase Contrast Magnetic Resonance Imaging (MRI) is a tool for non-invasive determination of flow velocities inside blood vessels. Because Phase Contrast MRI only measures a single mean velocity per voxel, it is only applicable to vessels significantly larger than the voxel size. In contrast, Fourier Velocity Encoding measures the entire velocity distribution inside a voxel, but requires a much longer acquisition time. For accurate diagnosis of stenosis in vessels on the scale of spatial resolution, it is important to know the velocity distribution of a voxel. Our aim was to determine velocity distributions with accelerated Fourier Velocity Encoding in an acquisition time required for a conventional Phase Contrast image. Materials and MethodsWe imaged the femoral artery of healthy volunteers with ECG – triggered, radial CINE acquisition. Data acquisition was accelerated by undersampling, while missing data were reconstructed by Compressed Sensing. Velocity spectra of the vessel were evaluated by high resolution Phase Contrast images and compared to spectra from fully sampled and undersampled Fourier Velocity Encoding. By means of undersampling, it was possible to reduce the scan time for Fourier Velocity Encoding to the duration required for a conventional Phase Contrast image. ResultsAcquisition time for a fully sampled data set with 12 different Velocity Encodings was 40min. By applying a 12.6–fold retrospective undersampling, a data set was generated equal to 3:10min acquisition time, which is similar to a conventional Phase Contrast measurement. Velocity spectra from fully sampled and undersampled Fourier Velocity Encoded images are in good agreement and show the same maximum velocities as compared to velocity maps from Phase Contrast measurements. ConclusionCompressed Sensing proved to reliably reconstruct Fourier Velocity Encoded data. Our results indicate that Fourier Velocity Encoding allows an accurate determination of the velocity distribution in vessels in the order of the voxel size. Thus, compared to normal Phase Contrast measurements delivering only mean velocities, no additional scan time is necessary to retrieve meaningful velocity spectra in small vessels.

High-resolution and high-sensitivity phase-contrast imaging by focused hard x-ray ptychography with a spatial filter

We demonstrate high-resolution and high-sensitivity x-ray phase-contrast imaging of a weakly scattering extended object by scanning coherent diffractive imaging, i.e., ptychography, using a focused x-ray beam with a spatial filter. We develop the x-ray illumination optics installed with the spatial filter to collect coherent diffraction patterns with a high signal-to-noise ratio. We quantitatively visualize the object with a slight phase shift (∼λ/320) at spatial resolution better than 17 nm in a field of view larger than ∼2×2μm2. The present coherent method has a marked potential for high-resolution and wide-field-of-view observation of weakly scattering objects such as biological soft tissues.

First application of liquid-metal-jet sources for small-animal imaging: High-resolution CT and phase-contrast tumor demarcation

Small-animal studies require images with high spatial resolution and high contrast due to the small scale of the structures. X-ray imaging systems for small animals are often limited by the microfocus source. Here, the authors investigate the applicability of liquid-metal-jet x-ray sources for such high-resolution small-animal imaging, both in tomography based on absorption and in soft-tissue tumor imaging based on in-line phase contrast. The experimental arrangement consists of a liquid-metal-jet x-ray source, the small-animal object on a rotating stage, and an imaging detector. The source-to-object and object-to-detector distances are adjusted for the preferred contrast mechanism. Two different liquid-metal-jet sources are used, one circulating a Ga∕In∕Sn alloy and the other an In∕Ga alloy for higher penetration through thick tissue. Both sources are operated at 40-50 W electron-beam power with ∼7 μm x-ray spots, providing high spatial resolution in absorption imaging and high spatial coherence for the phase-contrast imaging. High-resolution absorption imaging is demonstrated on mice with CT, showing 50 μm bone details in the reconstructed slices. High-resolution phase-contrast soft-tissue imaging shows clear demarcation of mm-sized tumors at much lower dose than is required in absorption. This is the first application of liquid-metal-jet x-ray sources for whole-body small-animal x-ray imaging. In absorption, the method allows high-resolution tomographic skeletal imaging with potential for significantly shorter exposure times due to the power scalability of liquid-metal-jet sources. In phase contrast, the authors use a simple in-line arrangement to show distinct tumor demarcation of few-mm-sized tumors. This is, to their knowledge, the first small-animal tumor visualization with a laboratory phase-contrast system.

High‐resolution cell outline segmentation and tracking from phase‐contrast microscopy images

Accurate extraction of cell outlines from microscopy images is essential for analysing the dynamics of migrating cells. Phase-contrast microscopy is one of the most common and convenient imaging modalities for observing cell motility because it does not require exogenous labelling and uses only moderate light levels with generally negligible phototoxicity effects. Automatic extraction and tracking of high-resolution cell outlines from phase-contrast images, however, is difficult due to complex and non-uniform edge intensity. We present a novel image-processing method based on refined level-set segmentation for accurate extraction of cell outlines from high-resolution phase-contrast images. The algorithm is validated on synthetic images of defined noise levels and applied to real image sequences of polarizing and persistently migrating keratocyte cells. We demonstrate that the algorithm is able to reliably reveal fine features in the cell edge dynamics.

X-ray Angle-Resolved Computed Tomography Using an Asymmetric Analyzer Crystal

Because X-ray angle-resolved computed tomography is sensitive to the X-ray phase gradients produced by a sample, it has much higher sensitivity than the conventional techniques based on the absorption contrast. To improve this technique, we introduced an asymmetrically cut crystal for the analyzer, which functions as either a magnifier or demagnifier. For image magnification at a wavelength of 0.0766 nm, we utilized an asymmetric Si(220) crystal (θB=11.5°), the diffraction plane of which was inclined by 8° from the crystal surface. The magnification ratio, m, was 5.47 and improvement of spatial resolution was successfully observed. In the image demagnification experiment, another asymmetric Si(220) crystal (m=0.49) was used, and cross-sectional images were successfully reconstructed for a sample larger than the viewing field of the X-ray charge coupled device (CCD) image sensor. The asymmetric analyzer opens up new ways, for example, to improve the spatial resolution of pixel array detectors (PADs) for high-sensitivity and high-resolution phase-contrast X-ray imaging.

Applications of TEM imaging, analysis and electron holography to III-nitride HEMT devices

This paper reviews some of our recent studies of III-nitride high electron mobility transistor (HEMT) devices using advanced electron microscopy methods. Sample preparation protocols have been developed that can routinely provide thin, electron-transparent specimen regions of uniform thickness extending across entire HEMT devices. The use of focused-ion-beam (FIB) thinning facilitated access to specific device regions, although structural damage and imaging artifacts can result unless suitable precautions are taken during milling to minimize ion-beam damage. The extent of gallium-ion sidewall implantation during FIB milling, and surface damage caused by deposition of Pt protective layers, have been assessed. As-processed device structures have been examined by conventional diffraction contrast imaging as well as high-resolution phase contrast imaging, while nanospectroscopy and nanoscale elemental mapping have been used to measure local variations in chemical composition. Annealing of Ti/Al/Ni/Au ohmic contacts for AlInN/AlN/GaN devices lead to the formation of TiN contact inclusions that are invariably located at mixed-type threading dislocations originating from the underlying GaN layers. Some preliminary observations of device structures after extended periods of operation and after device failure have also been made. The technique of off-axis electron holography has been used to quantify two-dimensional electrostatic fields within cross-sectioned devices with nanometer-scale resolution. Polarization fields of 6.9 MV/cm and a two-dimensional electron gas of ∼2.1 × 10 13/cm 2 have been measured for an AlInN/AlN/GaN heterostructure. Methods suitable for in situ biasing of HEMT samples during electron holography observations have also been explored.