Ultra-high Spatial Resolution Research Articles

Apertureless Near‐Field Vibrational Imaging of Block‐Copolymer Nanostructures with Ultrahigh Spatial Resolution

Nanodomains formed by microphase separation in thin films of the diblock copolymers poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) and poly(styrene-b-ethyleneoxide) (PS-b-PEO) were imaged by means of infrared scattering-type near-field microscopy. When probing at 3.39 mum (2950 cm(-1)), contrast is obtained due to spectral differences between the C--H stretching vibrational resonances of the respective polymer constituents. An all-optical spatial resolution better than 10 nm was achieved, which corresponds to a sensitivity of just several thousand C--H groups facilitated by the local-field enhancement at the sharp metallic probe tips. The results demonstrate that infrared spectroscopy with access to intramolecular dimensions is within reach.

Femtosecond laser photoemission microscopy of capillary nanotips with ultrahigh spatial resolution

A nanolocalized electron beam emitted from silicon nanotubes has been used to image the aperture of a quartz capillary with a channel diameter of 100 nm. An analogous nickel-coated capillary was used as a tip for imaging deposited organic nanostructures by means of femtosecond laser photoelectron projection microscopy (LPPM). Organic nanocomplexes deposited onto the tip surface change the dependence of the photoelectron response signal on the energy density of probing femtosecond laser pulses. An analysis of the LPPM images of capillary nanotips shows that the spatial resolution achieved in these experiments is on a level of 5 nm.

Fiber optic confocal laser Doppler velocimeter using an all-fiber laser source for high resolution measurements

We demonstrate and analyze a novel fiber optic confocal laser Doppler velocimeter using an ultra-narrow linewidth all-fiber laser source centered at around 1550 nm (eye-safe region). The narrow spectral linewidth of the fiber laser (<10 kHz) is used to achieve an extremely high velocity resolution (~0.0075 m/s), which is an order of magnitude better as compared to the commonly used semiconductor diode lasers or He-Ne lasers based systems. The directional optical circulator based design used in our system is much simpler to implement and is power conserving compared to the conventional Michelson interferometer based designs. We perform Gaussian beam propagation analysis by using the ABCD law to study the performance of the confocal design. The analysis is in good accord with our experimental results. The confocal design is capable of providing ultrahigh spatial resolution (~5microm, in both lateral and longitudinal directions) for high-precision velocity distribution measurement applications.

Different physiological MRI noise between cortical layers.

Significantly higher temporal fluctuations of the blood oxygenation level-dependent (BOLD) signal in the living rat group compared to that in the dead rat group were observed in the cortex, suggesting the existence of physiological information in the signal fluctuations. A similar analysis shows significantly different fluctuations between visual cortical layers. The highest fluctuations were observed in layers 4 and 5 and the lowest in layer 1. Given the consistency with published electrophysiology studies anticipating high spontaneous activity in the deeper layers (particularly layer 4), and low activity in superficial layers, we hypothesize that the BOLD signal temporal fluctuations may reflect cortical neuronal activity. Temporal fluctuations in ultrahigh spatial resolution data of the rat brain were measured in two ways. In the first, analyses were performed according to known layer widths, and in the second equal lines of 117 micro along the cortex were selected. The second approach yielded temporal fluctuations along the cortex that resemble known neuronal density distributions including the intralayer structure, particularly within layer 5.

Inter-frame filtering for list-mode EM reconstruction in high-resolution 4-D PET

Dynamic (4-D) PET imaging is a routinely used technique applied in clinical studies, and is now also finding application for high resolution animal PET systems. The usual method of implementing the image reconstruction involves separating the acquired data out into lower count discrete time frames, resulting in increased image noise. This paper presents two further developments for one-pass 4-D list-mode expectation-maximization (EM) reconstruction, which has already demonstrated enhanced reconstruction quality for image sequences compared to conventional strategies, as well as significantly reducing reconstruction time. The technique, which is an extension of one-pass 3-D list-mode EM, filters a previously reconstructed frame, which is then used as a starting image for the following frame, thus including prior object information. A new approach to post-reconstruction smoothing is also proposed, which in some cases is able to reduce noise without loss of resolution. Using measured quad-HIDAC data from a moving point source and a specially designed liquid flow experiment, and a rat scan, the two approaches are compared to the uniform image initialization method in terms of spatial and temporal resolution, noise, and accuracy of time-activity curves. Preliminary results, using resolution recovery, indicate the potential of achieving ultra-high spatial resolution with good temporal resolution, arising from improving the use of otherwise wasted prior information.

Noise reduction in MR angiography with nonlinear anisotropic filtering.

To evaluate three-dimensional nonlinear anisotropic filtering in suppressing image noise in high spatial resolution magnetic resonance angiograms (MRA) acquired with hybrid undersampled projection reconstruction and phase contrast vastly undersampled isotropic projection reconstruction (PC-VIPR). Three-dimensional nonlinear anisotropic filtering was quantitatively analyzed and evaluated through the measurement of contrast to noise ratio (CNR) in PC-VIPR images and contrast enhanced peripheral MRA images. To filter MRA images with ultra-high spatial resolution and poor CNR, a spatial frequency dependent nonlinear anisotropic filtering algorithm was proposed that uses two-step processing to filter the whole spatial frequency data. Three-dimensional nonlinear anisotropic filtering was shown to be effective in suppressing noise and improving CNR in MRA with isotropic spatial resolution. Higher CNR was achieved using spatial frequency dependent nonlinear anisotropic filtering. A typical CNR gain of between 50-100% was shown in our studies. Three-dimensional nonlinear anisotropic filtering significantly improved CNR in MRA images with isotropic spatial resolution. Spatial frequency dependent nonlinear anisotropic filtering further improved CNR for MRA images with ultra-high spatial resolution and low CNR.

Imaging detector designs based on flat panel PMT

Over the last 10 years, because of close proximity positioning, small field of view imagers have demonstrated superior spatial resolution and sensitivity at multiple orientations compared to the conventional nuclear camera. Design favoring high spatial resolution at a reasonable cost is the main goal and to this aim a number of photodetector designs have been proposed. The recent availability of Hamamatsu Flat Panel photomultiplier (PMT) makes it a promising and realistic candidate by significant improving compactness, packing fraction of PSPMT designs and lowering costs. In this paper we analyze the spatial resolution potential of a small gamma imager based on Hamamatsu H8500 PMT coupled to different scintillation material arrays like CsI(Tl) and NaI(Tl), with crystal pixel size ranging between 0.2 and 1.8mm. The imaging performances and limitations for application in scintimammography, linphoscintigraphy and small animal imaging are critically explored. The results show very good position linearity, spatial resolution and gain. Furthermore the array structure of anodes favours the exploiting of new methods of charge analysis for a precise calculation of gamma ray interaction position. Unfortunately the light sampling is still limited by too big anode size that impairs photodetector performances for ultra high spatial resolution applications.

Using Synchrotron Transmission FTIR Microspectroscopy as a Rapid, Direct, and Nondestructive Analytical Technique To Reveal Molecular Microstructural−Chemical Features within Tissue in Grain Barley

The objective of this study was to use synchrotron transmission FTIR microspectroscopy as a rapid, direct, and nondestructive analytical technique to reveal molecular microstructural-chemical features within tissue in grain barley. The results showed that synchrotron transmission FTIR microspectroscopy could provide spectral, chemical, and functional group characteristics of grain barley tissue at ultrahigh spatial resolutions. The spatially localized structural-chemical distributions of biological components (lignin, cellulose, protein, lipid, and carbohydrates) and biological component ratios could be imaged. Such information on molecular microstructural-chemical features within the tissue can be used for plant breeding programs for selecting superior varieties of barley for special purposes and for prediction of grain barley quality and nutritive value for humans and animals.

In vitro evaluation of normal and abnormal lungs with ultra-high-resolution CT.

Synchrotron radiation microtomography is a new technique with high directionality of a synchrotron radiation beam and a high-resolution detector array. In this study, we estimated the visualization of the peripheral human lung structure with an ultra-high-resolution computed tomography (CT) system, the synchrotron radiation CT. The synchrotron radiation CT system uses the bending magnet beamline emitted from the 8.0 GeV electron storage ring. Six lung specimens were obtained at autopsy: 3 normal, 1 cellular alveolitis, 1 diffuse alveolar damage, and 1 pulmonary hemorrhage. Each specimen was cut down to a cylindrical sample with 6-mm diameter and 15- to 25-mm height. The synchrotron radiation CT images of these lung samples were obtained using the ultra-high-resolution image detector system with a charge coupled device (CCD) array detector (1024 x 1024 pixels with 12 x 12 microm2 pixel size). After that, the sample specimens were sliced to 200 microm (micrometer) thickness, and were observed with a stereomicroscope and by contact radiography. Finally, approximately 10-microm thick microscopic images were obtained and compared with the synchrotron radiation CT images. The synchrotron radiation CT could depict the peripheral lung including peripheral airways, airspaces, and alveolar walls individually. Each finding in the 3 disease processes seen on the synchrotron radiation CT images correlated well with the microscopic findings. The synchrotron radiation CT allows microtomographic imaging of human lung specimens with ultra high-spatial resolution (12 microm) on a level approaching that of conventional histopathology, without sectioning.

Nanotomography based on double asymmetrical Bragg diffraction

Nondestructive investigations at ultrahigh spatial resolution can be carried out with synchrotron-based x-ray computer tomography. The performance of presently used detectors is restricted by scintillation properties, optical light transfer and charge coupled device granularity to a limit of 1 μm spatial resolution and efficiency of a few percent. A recently developed detector, called Bragg magnifier, exploits double asymmetrical Bragg diffraction to efficiently produce hard x-ray images with magnification factors up to 100×100 and pixel size of less than 200×200 nm2. Combining it with tomography, we obtained full volumetric information of a sample with spatial resolution well below 1 μm in less than 40 min.

Ultra-high resolution index of refraction profiles of femtosecond laser modified silica structures

The combination of selective chemical etching and atomic force microscopy has been used for the first time to make ultra-high spatial resolution (20 nm) index of refraction profiles of femtosecond laser modified structures in silica glass.

Ultra-high temporal and spatial resolutions using femtosecond and attosecond pulses

We show that ultra-high temporal Δt and spatial Δx optical resolutions can be provided simultaneously only by using a pulse with an ultra-short central wavelength of the wavepacket or/and decreasing the distance between the light source and sample. Such behaviour is the result of the existence of a cut-off frequency ωco in the Fourier time expansion of an electromagnetic wavepacket when we consider its transmission through an aperture. The phenomena could be important for achievement of femtosecond or attosecond temporal resolutions together with confinement of light to nanometre-scale transverse dimensions in near-field scanning optical microscopy and spectroscopy.

Very Long Baseline Array Observations of the Zeeman Effect in H[TINF]2[/TINF]O Masers in W3 IRS 5

We present Very Long Baseline Array (VLBA) observations of the Zeeman effect in H2O masers in W3 IRS 5. These observations have yielded the first ultrahigh spatial resolution maps of the magnetic field in a star-forming region measured via the Zeeman effect—masers for which we can independently measure the magnetic field are separated by ≤25 mas (60 AU). This is the first VLBA detection of the Zeeman effect in H2O masers in a star-forming region. These observations also remove the confusion in observed magnetic fields from closely spaced masers. We shall explicitly demonstrate that observations at lower resolution can yield lower magnetic fields than with the VLBA, even though there are no field reversals within the larger beam.

Application of the Phase-Retrieval X-Ray Diffractometry to an Ultra-High Spatial Resolution Mapping of SiGe Films near the Absorption Edge of Ge

A recently developed new experimental analytical X-ray diffraction method for the direct non-destructive characterization of single-crystal alloys is applied to map the complex structure-factor of SiGe layers with an ultra-high spatial resolution of 5.8 Å. The technique is based on analytical measurements of X-ray phase and amplitude changes in a narrow polychromatic region of synchrotron radiation near the absorption edge of the alloy impurity. These atomic spatial resolution studies have allowed observation and preliminary analysis of surface and interface nanoscale sublayers, where the crystal structure-factor may noticeably differ from the bulk material.

Near-field scanner for moving molecules.

We have fabricated using electron beam nanolithography a fixed slit near-field optical scanning device which uses near-field fluorimetry to achieve 200 nm spatial resolution of objects moving over the slits. We explore the basic physics of operating narrow slits in the waveguide cutoff mode and present data from the passage of extended double-stranded DNA molecules passing over the slits as a first example of how this device can be used to do ultrahigh spatial resolution mapping of long polymers.

A study of sclerochronology by laser ablation ICP-MS

Sclerochronology is to shells what dendrochronology is to trees, i.e., growth structures within some shells (in this case the bivalve mollusc Arctica islandica L., the Ocean Quahoc) resemble, in form and content, those growth structures in trees, which grow with an annual periodicity. By utilising laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), allied with stable isotope analysis to sample the growth structures within shells, environmental conditions (e.g., ambient temperature, salinity, seasonality and productivity) prevalent at the time of shell genesis may be ascertained. A limiting factor in the retrieval of any data from biogenic carbonates such as these is the size of the repeated growth structures from which to extract information. In the case of Arctica, these can be as low as tens of µm. Micro-drilling techniques, such as those commonly used in isotope analyses, are constrained by the size of the drill bit used to collect the sample. LA-ICP-MS, with its ultra-high spatial resolution (10–20 µm ablation pits) and precision, delivers highly constrained analysis allowing accurate multi-element sampling of seasonal growth bands within shells. It may be feasible to use the elements strontium, magnesium and barium as records of inter-annual environmental conditions, thus removing the need to carry out stable isotope analyses in some instances, particularly where the size of the material to be sampled demands the high resolution of LA-ICP-MS.

Ultrahigh-spatial-resolution photoelectron projection microscopy using femtosecond lasers

Ultrahigh spatial resolution of two-photon photoelectron images (as high as 3 nm, which is the best value that has been achieved to date in photoelectron microscopy with spatial resolution) is obtained when silicon nanotips are irradiated by the second harmonic of a pulsed femtosecond Ti: sapphire laser. In addition, the absolute value of the two-photon external photoeffect coefficient is measured.

Advanced one-dimensional x-ray microscope for the Omega Laser Facility

An analytical study of the on-axis aperture defect (spherical aberration) convolved with diffraction, for a simple spheric-based Kirkpatrick–Baez (KB) grazing incidence x-ray microscope, indicates the optimum performance is achieved by using a short object plane to mirror distances, steep grazing angles, and high x-ray energies. Other aberrations occur for off-axis field points, but if the spherical aberration can be reduced significantly the image improvement of an extended object should be dramatic. In the quest for ultrahigh x-ray spatial resolution, the Los Alamos one-dimensional (1D) KB (at the time of writing, under fabrication) fully exploits these facts in a 40 mm working distance, steep grazing angle multilayer mirror, 4.316 keV design. A rigorous analysis of Zernike surface deformations, and all other fabrication/assembly errors indicates negligible image degradation of the predicted very high resolution performance; a similar study of existing KBs closely models the 3–5 μm resolutions over the relevant field sizes. In addition, a novel analytical and ray trace optimization technique allows larger working distances at even higher resolutions. Due to instrument length constraints, however, this approach was not incorporated into the 4.316 keV 1DKB; although, in the future such designs could be easily accommodated on The National Ignition Facility.

Development of constrained DET for measurements of dissolved iron in surface sediments at sub-mm resolution

The technique of diffusive equilibration in thin-films (DET) was used to measure pore-water concentrations of dissolved Fe at high (1 mm) and ultra-high (400 μm) spatial resolution in the surface sediments (0–12 mm) and immediate overlying water of Esthwaite Water, UK. DET measurements were made using an unconstrained DET probe and a new constrained DET probe with 200-μm wide compartments in a 400-μm thick ceramic sheet. Good agreement between profiles obtained by conventional and constrained DET measurements confirmed the performance of the new procedure, which in principle can be used to measure most major components of pore waters. Single point maxima observed with conventional DET were revealed as systematic peaks with constrained DET. All Fe pore-water profiles showed tightly defined maxima in the surface sediments. Double maxima observed for both spring and winter were probably due to tightly defined zones of bacterial oxidation of Fe(II) although sampling artifacts could not be completely ruled out. Differences in ultra-high resolution vertical concentration profiles measured only 2 mm apart provided evidence of small scale horizontal heterogeneity. Such observations suggest that fluxes through the sediment water interface calculated from concentration profiles should be treated as spatially specific values. Average fluxes can only be obtained from several replicate measurements.

A triple-head SPECT system with parallel-hole collimators of different acceptance angles

We proposed to use three different parallel-hole collimators for a triple-head SPECT system. One of the collimators had a small collimator acceptance angle to provide ultra-high spatial resolution and the other two had larger collimator acceptance angles to achieve high counts. A new 2D reconstruction algorithm that combined the data acquired from different collimators was derived to take advantages of both high resolution and high sensitivity. The algorithm was evaluated using a computer-simulated matrix of spherical sources. For noise-free data, the accuracy (mainly determined by spatial resolution) obtained from combination of the collimator acceptance angles of 1.35, 4.05 and 6.75° (or 1.35, 5.40 and 9.45°) was slightly inferior to that obtained from three same LEHR collimators (with a 2.70° collimator acceptance angle). This is because the modulation transfer function (MTF) resulting from three different collimators decreases more quickly at low frequencies but becomes comparable at high frequencies as compared with the MTF of the 2.70° collimator. With noisy data, however, the image quality obtained with three different collimators was better than that resulting from any combinations of three same collimators. The improvement was only achieved by using the derived algorithm, while the conventional FBP algorithm did not improve image quality even with the same collimator configuration.