Improvement In Lateral Resolution Research Articles

Continuously moving table MRI with SENSE: Application in peripheral contrast enhanced MR angiography

An integration of SENSitivity Encoding (SENSE) with continuously moving table (CMT) MRI for extended field-of-view (FOV) acquisitions is described. In this work, the approach in which receiver coils are attached to the object and move in synchrony with the scanner table is considered. Technical issues dealing with the implementation of SENSE-CMT are addressed, including coil calibration, correction for non-uniform magnetic gradients, and specific reconstruction steps. An explanation of combining SENSE with gradient non-linearity correction is given, as the latter becomes necessary in CMT acquisitions where a large sampling FOV is used. It is hypothesized that SENSE can provide at least a 2-fold improvement in lateral spatial resolution compared to non-accelerated CMT acquisitions. The hypothesis is tested in phantoms, where the effectiveness of both SENSE and gradient non-linearity correction to improve spatial resolution is shown. The SENSE-CMT technique is further demonstrated in vivo with contrast-enhanced MR angiography of the peripheral vasculature.

Variable field of view for spatial resolution improvement in continuously moving table magnetic resonance imaging

An approach is described in which the field of view (FOV) along the Y (right/left) phase encoding direction can be dynamically altered during a continuously moving table (CMT) coronal acquisition for extended FOV MRI. We hypothesize that with this method, regions of the anatomy exhibiting significantly different lateral widths can be imaged with a matching local FOV(Y), thereby improving local lateral spatial resolution. k-space raw data from the variable-FOV CMT acquisition do not allow simple Fourier reconstruction due to the presence of a mixture of phase encodes sampled at different Deltak(Y) intervals. In this work, we employ spline interpolation to reregister the mixed data set onto a uniformly sampled k-space grid. Using this interpolation scheme, we present phantom and peripheral contrast-enhanced MR angiography results demonstrating an approximate 45% improvement in local lateral spatial resolution for continuously moving table acquisitions.

Recent developments and new strategies in scanning electron microscopy*

In addition to improvements in lateral resolution in scanning electron microscopy, recent developments of interest here concern extension of the incident beam energy, E(0), over two decades, from approximately 20 keV to approximately 0.1-0.5 keV and the possibility of changing the take-off emission, alpha, of detected secondary electrons. These two degrees of freedom for image acquisition permit a series of images of the same field of view of a specimen to be obtained, each image of the series differing from the others in some aspect. The origins of these differences are explored in detail and they are tentatively interpreted in terms of the change in the secondary electron emission yield delta vs. E(0), delta = f(E(0)), and also of the change in delta vs. alpha, partial differentialdelta/ partial differentialalpha. Various origins for the chemical contrast and topographic contrast have been identified. Illustrated by correlating a secondary electron image and a backscattered electron image, use of the scatter diagram technique facilitates image comparison. The difference between the lateral resolution and the size of the minimum detectable detail is outlined to avoid possible errors in nanometrology. Some aspects related to charging are also considered and possible causes of contrast reversal are suggested. Finally, the suggested strategy consists of the acquisition of various images of a given specimen by changing one parameter: primary beam energy and take-off angle for conductive specimens; working distance or beam intensity for high-resolution experiments; scanning frequency for insulating specimens.

SABCMS, a new approach to higher lateral resolution of laser probe measurement system

In order to achieve a high lateral resolution required for ultraprecision measurement of microstructural workpieces, shaped annular beam confocal measurement system (SABCMS), a new approach is proposed for ultraprecision measurement of 3D microstructures. SABCMS uses a binary optical element based on the diffraction principle to shape a Gauss laser beam into an annular beam, and optimizes the normalized inner radius of annular beam aperture ɛ to decrease the main lobe of airy spot, to increase the lateral resolution and measurement range. In comparison with an annular beam shaping technique used in a conventional annular lens confocal measurement system, the annular beam shaping technique used in SABCMS reduces the energy loss, and eliminates the effect of the inner ring edges diffraction of annular lens on the measurement system and improves the defocused property. Experimental comparison and analysis indicate that, as ɛ increases, both the lateral resolution and measurement range of SABCMS increase, and the spatial resolution improves, too, while the axial resolution of SABCMS reduces; the lateral resolution of SABCMS is better than 0.2 μm when wavelength of incidence laser beam λ = 632.8 nm, numerical aperture of measuring lens NA = 0.85, and ɛ = 0.5. It is therefore concluded that SABCMS is a new approach to further improvement of lateral resolution in laser probe measurement systems.

Restauration d'images ultrasonores en contrôle non destructif

The evolution of manufacturing techniques and the required quality show the importance of noninvasive diagnostics in industrial processes. Ultrasounds are widely used for this purpose, thanks to their innocuous character and their intrinsic qualities. In later years, the amelioration of ultrasonic techniques has required a number of improvements, including image processing. The present works focuses on the improvement of lateral resolution to optimize the localization and the size of the defects. To this end, we have developped a 3-D model that describes the propagation of ultrasonic waves and their interaction with a point defect. Reconstructing the image requires a 2-D deconvolution through an inverse filter. Tests made on Bscam images show clearly the efficiency of this technique to improve the image resolution.[Journal translation]

High-resolution wide-field surface plasmon microscopy.

This paper describes the application of a Köhler illuminated high-resolution wide-field microscope using surface plasmons to provide the image contrast. The response of the microscope to a grating structure in both the Fourier and the image planes is presented to demonstrate image formation by surface waves. The effect of spatial filtering in the back focal (Fourier) plane to enhance image contrast is described. We also discuss how the surface wave contrast mechanism affects the imaging performance of the microscope and discuss factors that can be expected to lead to even greater improvements in lateral resolution and sensitivity.

Influence of the elliptical illumination on acquisition and correction of coherent aberrations in high-resolution electron holography

In high-resolution off-axis electron holography, the interpretable lateral resolution is extended up to the information limit of the electron microscope by means of a correcting phase plate in Fourier space. A plane illuminating electron wave is generally assumed. However, in order to improve spatial coherence, which is essential for holography, the object under investigation is illuminated with an elliptically shaped electron source. This special illumination imposes a variation of beam directions over the field of view. Therefore, due to the interaction of beam tilt and coherent wave aberration, the effective aberrations vary over the field of view yielding a loss of isoplanicity. Consequently, in the past the aberrations were only corrected successfully for a small part of the field of view. However, a thorough analysis of the holographic imaging process shows that the imaging artifacts introduced by the elliptical illumination can be corrected under reconstruction by means of a phase curvature, which models the illuminating wave front. Applied in real space, this phase curvature is seamlessly incorporated into the correction process for coherent wave aberration resulting in an improvement of interpretable lateral resolution up to the information limit for the whole field of view.

Objective measurements of image quality

Tissue harmonic imaging (THI) and compound imaging have been reported clinically to improve contrast resolution, tissue differentiation and overall image quality. However, there have been limited studies to date to quantify objectively the improvements in image quality achieved with these new imaging techniques. The aim of this study was to quantify differences in image quality that exist between conventional B-mode imaging, harmonic imaging, compound imaging and harmonic compound imaging. An ATL HDI 5000 scanner with three probes (C5-2, L7-4 and L12-5) was tested with two different types of test object, the Gammex-RMI model 404 GS LE and the Gammex-RMI 403 GS LE. The measurement limitations associated with subjective analysis methods were not present in this study because an automated image analysis program was used to determine the image quality parameters. Therefore, subtle differences between the four imaging modes could be detected. Significant improvements in lateral resolution and slice thickness as a function of depth were found with THI. Contrast resolution and anechoic target detection improved with compound imaging, and harmonic compound imaging improved lateral resolution, slice thickness as a function of depth and contrast resolution. (E-mail: jacinta.browne@northglasgow.scot.nhs.uk)

Physics of tissue harmonic imaging

Tissue harmonic imaging is used on almost all modern diagnostic ultrasound equipment. The clinical success of this modality involves a combination of fortuitous factors. The improvement in lateral resolution obtained by transmitting on a fundamental frequency and receiving on a harmonic is well known. Other advantages include a deeper focal range with less absorption than would have been obtained by transmitting and receiving at twice the fundamental frequency. Natural apodization considerably reduces the 3-D volume of interaction with tissue. Simulations indicate that harmonic imaging is more robust in the presence of aberration. Analysis shows that the improvements over what would have been achieved by transmitting at twice the frequency are for shallower focal depths. Reasons for the noticeably enhanced contrast of harmonic imaging are discussed.

Atomic force microscopy of nickel dot arrays with tuning fork and nanotube probe

The advantages of tuning fork scanning force microscopy are combined with the unique properties of carbon nanotubes to improve the spatial resolution of atomic force microscopy (AFM) images of nickel dot arrays. These arrays have high relief features that prevent the resolution of the actual dot size and shape with a regular cantilever tip. The modification of two key parts of a commercial AFM, the force sensor, and the probe allowed the acquisition of true data on feature size and shape. AFM images of Ni dot arrays were taken using conventional optical detection of a commercial cantilever, tuning fork with attached commercial cantilever tip, and tuning fork with attached multiwalled carbon nanotube. The resolutions of the AFM images were compared, and it was shown that probing with a carbon nanotube provided a 30% improvement of lateral resolution compared to a conventional AFM tip.

XPEEM WITH ENERGY-FILTERING: ADVANTAGES AND FIRST RESULTS FROM THE SMART PROJECT

The second development step of the SMART project, i.e. an energy-filtered but not yet corrected photoelectron emission microscope, operates at the undulator U49/1-PGM beamline at BESSY II. It already demonstrates the variety of methods of the final version: microscopy, spectroscopy and electron diffraction. Some recent experimental results are reported for these three operation modes. In addition, the theoretical improvement of lateral resolution and transmission of PEEMs in general by using an energy filter is discussed for systems without and with aberration correction.

Apertureless scanning near-field magneto-optical microscopy of magnetic multilayers.

We imaged magnetic domains in Pt/Co/Pt multilayers using an apertureless scanning near-field optical microscope operating in reflection mode. As the magneto-optical effects are weak for this kind of structure, a polarization modulation technique with a photoelastic modulator was used to reveal the contrast between magnetic domains. In the case of a Pt/Co/Pt trilayer structure, a strong improvement in lateral resolution is observed compared with far-field magneto-optical images and good sensitivity is achieved. In the case of a Pt/[Co/Pt]Pt multilayer structure, stripe domains of 200 nm width could be resolved, in good agreement with images obtained by magnetic force microscopy on the same structure.

Fabrication and characterization of advanced probes for magnetic force microscopy

Advanced probes for magnetic force microscopy have been produced by e −-beam lithography and Ar +-ion etching. The probes consist of individual CoCrPt, Fe, or Ni particles with a typical size of 100×100×100 nm 3, located at the tip apex of commercial cantilevers. They were compared to conventional thin film probes by performing magnetic force imaging on hard disc test patterns and soft garnet films. The advanced probes yield a considerable improvement in lateral resolution due to the three-dimensional confinement of the magnetically active tip volume. For soft magnetic samples the improvements are even more striking since the low stray field of the new probe allows nondestructive imaging at much smaller probe–sample spacing than achievable by conventional probes.

Imaging properties of high aperture multiphoton fluorescence scanning optical microscopes

A theory for multiphoton fluorescence imaging in high aperture scanning optical microscopes employing finite sized detectors is presented. The effect of polarisation of the fluorescent emission on the imaging properties of such microscopes is investigated. The lateral and axial resolutions are calculated for one‐, two‐ and three‐photon excitation of p‐quaterphenyl for high and low aperture optical systems. Significant improvement in lateral resolution is found to be achieved by employing a confocal pinhole. This improvement increases with the order of the multiphoton process. Simultaneously, it is found that, when the size of the pinhole is reduced to achieve the best possible resolution, the signal‐to‐noise ratio is not degraded by more than 30%. The degree of optical sectioning achieved is found to improve dramatically with the use of confocal detection. For two‐ and three‐photon excitation axial full width half‐maximum improvement of 30% is predicted.

Single-walled carbon nanotube probes for high-resolution nanostructure imaging

Single-walled carbon nanotube (SWNT) tips have been used to image nanostructures with high resolution. Studies of gold nanocrystal standards showed that SWNT tips provide a significant improvement in lateral resolution with respect to multi-walled nanotube tips and microfabricated Si tips. The nanotube tips were also used to resolve substructure within SWNTs deposited on surfaces. These results suggest that observed 1.5 nm high structures can correspond to several SWNTs aligned in parallel. In addition, SWNT tips exhibited superior resolution compared to conventional tips when imaging biological nanostructures, such as double-stranded DNA. The potential and future challenges of SWNT tips are discussed.

Three‐dimensional seismic refraction tomography: A comparison of two methods applied to data from the Faeroe Basin

This paper presents a comparison of two tomographic methods for determining three‐dimensional (3‐D) velocity structure from first‐arrival travel time data. The first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays. The second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors. Both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration. Travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing. Both inverse methods are applied to 3‐D ocean bottom seismometer (OBS) data collected in 1993 over the Faeroe Basin, consisting of 53,479 travel times recorded at 29 OBSs. This is one of the most densely spaced, large‐scale, 3‐D seismic refraction experiments to date. Different starting models and values for the free parameters of each tomographic method are tested. A new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion, but the latter method provides a simpler model for little additional computational expense when applied to the Faeroe Basin data. Bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints. An inversion of synthetic data corresponding to 100% data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3‐D refraction experiments to provide well‐resolved, long‐wavelength velocity models. The similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion.

Resolution Issues in Confocal Magnetooptic Scanning Laser Microscopy

The influence of confocal pinhole size on the lateral imaging resolution in magnetooptic scanning laser microscopes (MO-SLM) is investigated both theoretically and experimentally. Various scanning microscope configurations are studied theoretically using a scalar diffraction, transfer function model. A surprising theoretical prediction is that for the commonly used differential detector magnetooptic (MO) microscope, no improvement in lateral resolution is achieved by the use of a confocal imaging arrangement. A series of experimental tests measuring the MO step response for a range of confocal pinhole sizes has been carried out and the results appear to verify the theoretical predictions.

Carbon Nanotube Tips: High-Resolution Probes for Imaging Biological Systems

Herein, we report the first atomic force microscopy (AFM) imaging studies with carbon nanotube tips that address their potential to improve lateral resolution and to probe biological systems. Multiwall carbon nanotubes (MWNTs) and single-wall carbon nanotubes (SWNTs) have been attached to the ends of single-crystal silicon cantilever tip assemblies and used to image amyloid â(1-40) derived protofibrils and fibrils by tapping mode AFM. Image analysis shows that the average resolution obtained with the nanotube tips is significantly better than that obtained with our best silicon tips and provides new insight into the structure and assembly mechanism of amyloid fibrils. The potential for imaging with molecular resolution using these tips is discussed. The feature resolution obtained by AFM is determined in large part by the size and shape of the probe tip used for imaging.1,2 Commercially available probes consist of microfabricated pyramids of Si or Si3N4 that have end radii of curvature as small as 10 nm but are often much larger.2,3 These tips place significant constraints on potential lateral resolution, and furthermore, the pyramidal shape restricts the ability of these tips to access narrow and deep features. Small cone angle carbon protrusions with radii of 10-30 nm deposited on the ends of conventional tips significantly improve depth but not lateral resolution.4 Recently, a potential breakthrough in probe technology was achieved by attaching MWNTs to the ends of Si tips.5 In this work, the cylindrical geometry of the MWNT tips was exploited to image a deep grating with excellent fidelity, although potential improvements in lateral resolution were not investigated. A typical scanning electron microscopy (SEM) image of a MWNT tip attached to a conventional single-crystal silicon cantilever tip assembly is shown in Figure 1.5,6 This MWNT tip extends ca. 1.8 μm from the end of the pyramidal Si tip to which it is attached. The diameter of the major portion of the nanotube tip, 100 nm, is much larger than a single MWNT and corresponds to a bundle of tightly packed tubes.7 The MWNT bundles attached to the Si probes have diameters of 75 ( 27 nm; attached SWNT bundles8 typically are smaller with diameters of 45 ( 8 nm. The high aspect ratio nanotube tips have obvious advantages for probing deep crevices and steep features.5 Additionally, it is known that nanotubes elastically buckle above a critical force.5,9 Buckling limits the maximum force applied to a sample,10 which can prevent damage to delicate organic and biological samples, and at the same time makes the tips very robust.9 To explore the resolution of these tips and their applicability to high-resolution imaging of biological samples, we have investigated amyloid-â 1-40 (Aâ40) fibrils produced in vitro. Aâ fibrils are the primary constituent of the insoluble core of amyloid plaques that are characteristic of Alzheimer’s disease.11 Previous AFM studies have shown that Aâ initially forms protofibril structures12,13 and subsequently larger fibrils;12-14 branching of fibrils has also been observed.13,14 A typical image15 of an Aâ fibril obtained with a MWNT tip (Figure 2) shows higher resolution than observed in previous AFM studies of the fibrils. Key features include (1) a fibril Y-branch (left arrow) with the

Scanning electrode quartz crystal analysis Application to metal coatings

Scanning electrode quartz crystal analysis (SEQCA) has been applied to the monitoring of quartz surfaces that are partially coated with gold. The lateral resolution for gold coatings is better than that for insulating coatings. The mass sensitivity in the centre region is better than that in the edge region. On the other hand, the lateral resolution is independent of the area and location of the gold coating. It may be possible to distinguish a conducting coating from an insulating one. Gold coatings can be located in NaCl solutions if the concentration is ⩽10 mM. The possibility of further improvement in lateral resolution is discussed.

Experimental study of amplitude and phase detection limits in electron holography

The well-known advantages of electron holography are the availability of amplitude and phase of the electron wave and the resulting improvement of lateral resolution by correction of the coherent aberrations. The noise properties of the holographic method are very important: what are the smallest amplitude and phase differences detectable well above noise level in the reconstructed wave? Results of statistical evaluations collected from experimental holograms are compared with the existing theory.