Current Resolution Limit Research Articles

Optical fiber interferometer array for scanless Fourier-transform spectroscopy

We report a spatial heterodyne Fourier-transform spectrometer implemented with an array of optical fiber interferometers. This configuration generates a wavelength-dependent stationary interferogram from which the input spectrum is retrieved in a single shot without scanning elements. Furthermore, fabrication and experimental deviations from the ideal behavior of the device are corrected by spectral inversion algorithms. The spectral resolution of our system can be readily scaled up by incorporating longer optical fiber delays, providing a pathway toward surpassing current spectroscopy resolution limits.

Noise characteristics of InAs/GaSb superlattice infrared photodiodes

AbstractA sophisticated noise measurement setup employing a switching unit to measure statistically relevant numbers of InAs/GaSb superlattice photodiodes has been developed. The noise current resolution limit of 20 fA Hz‐1/2 enables the characterization of focal plane array‐sized InAs/GaSb superlattice homojunction photodiodes for the long‐wavelength infrared at 77 K. To resolve midwavelength infrared photodiodes a junction area of about (400 µm)2 is required. Without switching unit and by using a dedicated low noise amplifier, noise currents down to 2 fA Hz‐1/2 can be achieved, allowing the noise characterization of mid‐wavelength photodiodes with smaller junction areas. With this setup long‐wavelength and mid‐wavelength InAs/GaSb superlattice photodiodes, with generation‐recombination limited dark current behavior, are investigated at 77 K. The measured diode noise follows the theoretically predicted shot noise level within the white noise part of the spectra. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-Resolution Fluorescence Diffuse Optical Tomography Developed with Nonlinear Upconverting Nanoparticles

Fluorescence diffuse optical tomography (FDOT) is an emerging biomedical imaging technique that can be used to localize and quantify deeply situated fluorescent molecules within tissues. However, the potential of this technique is currently limited by its poor spatial resolution. In this work, we demonstrate that the current resolution limit of FDOT can be breached by exploiting the nonlinear power-dependent optical emission property of upconverting nanoparticles doped with rare-earth elements. The rare-earth-doped core-shell nanoparticles, NaYF(4):Yb(3+)/Tm(3+)@NaYF(4) of hexagonal phase, are synthesized through a stoichiometric method, and optical characterization shows that the upconverting emission of the nanoparticles in tissues depends quadratically on the power of excitation. In addition, quantum-yield measurements of the emission from the synthesized nanoparticles are performed over a large range of excitation intensities, for both core and core-shell particles. The measurements show that the quantum yield of the 800 nm emission band of core-shell upconverting nanoparticles is 3.5% under an excitation intensity of 78 W/cm(2). The FDOT reconstruction experiments are carried out in a controlled environment using liquid tissue phantoms. The experiments show that the spatial resolution of the FDOT reconstruction images can be significantly improved by the use of the synthesized upconverting nanoparticles and break the current spatial resolution limits of FDOT images obtained from using conventional linear fluorophores as contrast agents.

THE HIGH-ENERGY, ARCMINUTE-SCALE GALACTIC CENTER GAMMA-RAY SOURCE

Employing data collected during the first 25 months' observations by the Fermi-LAT, we describe and subsequently seek to model the very high energy (>300 MeV) emission from the central few parsecs of our Galaxy. We analyze the morphological, spectral and temporal characteristics of the central source, 1FGL J1745.6-2900. Remarkably, the data show a clear, statistically significant signal at energies above 10 GeV, where the Fermi-LAT has an excellent angular resolution comparable to the angular resolution of HESS at TeV energies, which makes meaningful the joint analysis of the Fermi and HESS data. Our analysis does not show statistically significant variability of 1FGL J1745.6-2900. Using the combination of Fermi data on 1FGL J1745.6-2900 and HESS data on the coincident, TeV source HESS J1745-290, we show that the spectrum of the central gamma-ray source is inflected with a relatively steep spectral region matching between the flatter spectrum found at both low and high energies. We seek to model the gamma-ray production in the inner 10 pc of the Galaxy and examine, in particular, cosmic ray (CR) proton propagation scenarios that reproduce the observed spectrum of the central source. We show that a model that instantiates a transition from diffusive propagation of the CR protons at low energy to almost rectilinear propagation at high energies (given a reasonable energy-dependence of the assumed diffusion coefficient) can well explain the spectral phenomenology. In general, however, we find considerable degeneracy between different parameter choices which will only be broken with the addition of morphological information that gamma-ray telescopes cannot deliver given current angular resolution limits.We argue that a future analysis done in combination with higher-resolution radio continuum data holds out the promise of breaking this degeneracy.

Neutron computed tomography of rat lungs

Using conventional methods, three-dimensional imaging of the lung is challenging because of the low contrast between air and tissue and the large differences in dimensions between various pulmonary structures. The small distal airway structures and the high air-to-tissue ratio of lung tissue require an imaging technique which reliably discriminates between air and water. The objective of this study was to assess whether neutron computed tomography would satisfy such a requirement. This method utilizes the unique characteristic of neutrons of directly interacting with the atomic nucleus rather than being scattered by the atomic shell. Neutron computed tomography was tested in rats and allowed differentiation of larger lung structures (e.g., lobes) and distal airways. Airways could be identified reliably down to the sixth bronchial generation, in some cases even down to the tenth generation. The lung could be stabilized for sufficiently long exposure times to achieve an image resolution of 50–60 µm, which is the current physical resolution limit of the neutron computed tomography facility. Neutron computed tomography allowed excellent lung imaging without the need for additional tissue preparation or contrast media. The enhanced structural resolution obtained by applying this new research technique may improve understanding of lung physiology and respiratory therapy.

Super‐resolution MRI using microscopic spatial modulation of magnetization

A new super-resolution method is presented to overcome limitations of spatial resolution in MRI. In contrast to previous attempts that are based on simple field of view shifts, the new method additionally modulates the longitudinal magnetization within the imaging plane for each shift, allowing the acquisition of new and independent k-space data. With this approach, resolution improvements in up to three dimensions are possible, and the total acquisition time linearly scales with the improvement factor for each dimension. First super-resolution experiments in a geometric phantom and in brain tissue of two healthy volunteers clearly demonstrate the feasibility and advantages of this new method, which has the capability to extend current resolution limits in MRI.

A study of virtual lithography process for polymer directed self-assembly

For the feature size scaling down to tens of nanometers, the top-down approaches are getting more severe because the extremely ultra-violet (EUV) technique, the high-index fluid-based immersion ArF lithography, and the double patterning technology (DPT) under development may be cover one or two generations. An alternative technology to extend lithography patterning beyond current resolution limits is to combine the top-down lithography and bottom-up assembly. In this paper, an directed self-assembly lithography process of “bottom-up” block copolymer self-assembly, is modeled and simulated in molecular-scale. Impacts of block polymer components on pattern formation are analyzed and discussed.

Funktionelle Magnetresonanztomographie bei ultrahohen Feldern

Functional magnetic resonance imaging (fMRI) is currently the primary method for non-invasive functional localization in the brain. With the emergence of MR systems with field strengths of 4 Tesla and above, neuronal activation may be studied with unprecedented accuracy. In this article we present different approaches to use the improved sensitivity and specificity for expanding current fMRT resolution limits in space and time based on several 7 Tesla studies. In addition to the challenges that arise with ultra-high magnetic fields possible solutions will be discussed.

21 Femur Ultrasound (FemUS) Improves Fracture Discrimination by Evaluating Direct and Guided Waves Through Trabecular and Cortical Parts of the Femur

Recent progress in adaptive beamforming techniques for medical ultrasound has shown that current resolution limits can be surpassed. One method of obtaining improved lateral resolution is the Minimum Variance (MV) beamformer. The frequency domain implementation of this method effectively divides the broadband ultrasound signals into sub-bands (MVS) to conform with the narrow-band assumption of the original MV theory. This approach is investigated here using experimental Synthetic Aperture (SA) data from wire and cyst phantoms. A 7 MHz linear array transducer is used with the SARUS experimental ultrasound scanner for the data acquisition. The lateral resolution and the contrast obtained, are evaluated and compared with those from the conventional Delay-and-Sum (DAS) beamformer and the MV temporal implementation (MVT). From the wire phantom the Full-Width-at-Half-Maximum (FWHM) measured at a depth of 52 mm, is 16.7 μm (0.08λ) for both MV methods, while the corresponding values for the DAS case are at least 24 times higher. The measured Peak-Side-lobe-Level (PSL) may reach −41 dB using the MVS approach, while the values from the DAS and MVT beamforming are above −24 dB and −33 dB, respectively. From the cyst phantom, the power ratio (PR), the contrast-to-noise ratio (CNR), and the speckle signal-to-noise ratio (sSNR) measured at a depth of 30 mm are at best similar for MVS and DAS, with values ranging between −29 dB and −30 dB, 1.94 and 2.05, and 2.16 and 2.27 respectively. In conclusion the MVS beamformer is not suitable for imaging continuous targets, and significant resolution gains were obtained only for isolated targets.

The second solar spectrum and the hidden magnetism

AbstractApplications of the Hanle effect have revealed the existence of vast amounts of “hidden“ magnetic flux in the solar photosphere, which remains invisible to the Zeeman effect due to cancellations inside each spatial resolution element of the opposite-polarity contributions from this small-scale, tangled field. The Hanle effect is a coherency phenomenon that represents the magnetic modification of the linearly polarized spectrum of the Sun that is formed by coherent scattering processes. This so-called “Second Solar Spectrum” is as richly structured as the ordinary intensity spectrum, but the spectral structures look completely different and have different physical origins. One of the new diagnostic uses of this novel spectrum is to explore the magnetic field in previously inaccessible parameter domains. The earlier view that most of the magnetic flux in the photosphere is in the form of intermittent kG flux tubes with tiny filling factors has thereby been shattered. The whole photospheric volume instead appears to be seething with intermediately strong fields, of order 100G, of significance for the overall energy balance of the solar atmosphere. According to the new paradigm the field behaves like a fractal with a high degree of self-similarity between the different scales. The magnetic structuring is expected to continue down to the 10m scale, 4 orders of magnitude below the current spatial resolution limit.

3D Fiber‐Deposited Electrospun Integrated Scaffolds Enhance Cartilage Tissue Formation

AbstractDespite the periodical and completely interconnected pore network that characterizes rapid prototyped scaffolds, cell seeding efficiency remains still a critical factor for optimal tissue regeneration. This can be mainly attributed to the current resolution limits in pore size. We present here novel three‐dimensional (3D) scaffolds fabricated by combining 3D fiber deposition (3DF) and electrospinning (ESP). Scaffolds consisted of integrated 3DF periodical macrofiber and random ESP microfiber networks (3DFESP). The 3DF scaffold provides structural integrity and mechanical properties, while the ESP network works as a “sieving” and cell entrapment system and offers?at the same time?cues at the extracellular matrix (ECM) scale. Primary bovine articular chondrocytes were isolated, seeded, and cultured for four weeks on 3DF and 3DFESP scaffolds to evaluate the influence of the integrated ESP network on cell entrapment and on cartilage tissue formation. 3DFESP scaffolds enhanced cell entrapment as compared to 3DF scaffolds. This was accompanied by a higher amount of ECM (expressed in terms of sulphated glycosaminoglycans or GAG) and a significantly higher GAG/DNA ratio after 28 days. SEM analysis revealed rounded cell morphology on 3DFESP scaffolds. Spread morphology was observed on 3DF scaffolds, suggesting a direct influence of fiber dimensions on cell differentiation. Furthermore, the ESP surface topology also influenced cell morphology. Thus, the integration of 3DF and ESP techniques provide a new set of “smart” scaffolds for tissue engineering applications.

Dynamic Basis for One-Dimensional DNA Scanning by the Mismatch Repair Complex Msh2-Msh6

The ability of proteins to locate specific sites or structures among a vast excess of nonspecific DNA is a fundamental theme in biology. Yet the basic principles that govern these mechanisms remain poorly understood. For example, mismatch repair proteins must scan millions of base pairs to find rare biosynthetic errors, and they then must probe the surrounding region to identify the strand discrimination signals necessary to distinguish the parental and daughter strands. To determine how these proteins might function we used single-molecule optical microscopy to answer the following question: how does the mismatch repair complex Msh2-Msh6 interrogate undamaged DNA? Here we show that Msh2-Msh6 slides along DNA via one-dimensional diffusion. These findings indicate that interactions between Msh2-Msh6 and DNA are dominated by lateral movement of the protein along the helical axis and have implications for how MutS family members travel along DNA at different stages of the repair reaction.

Low-Invasive Diagnosis of Metallic Prosthesis Osseointegration by Electrical Impedance Spectroscopy

In this paper, a digital-measurement method for low-invasive clinical diagnosis of metallic prosthesis osseointegration is proposed. Electrical impedance spectroscopy is exploited to characterize the quality of the tissue at the interface between the bone and the prosthesis. The method overcomes current resolution limits of biological electrical-impedance analysis by means of several polarization levels. Electrical modeling through an equivalent circuit is used to define a quantitative index of osseointegration. In vitro experimental results of the proposed method validation show its sensitivity and its effectiveness in low-depth prostheses, such as in dentistry applications

Fabrication of ultrathin magnetic structures by nanostencil lithography in dynamic mode

The fabrication of magnetic elements containing constrictions is demonstrated using nanostencil lithography in dynamic mode, i.e., by a continuous translation of a shadow mask with respect to the sample. The authors quantify the current resolution limits of this technique, demonstrating edge profile widths of 120nm and thickness variations of 10%, and discuss prospects and challenges of dynamic nanostencil lithography.

X-ray tests of a microchannel plate detector and amorphous silicon pixel array readout for neutron radiography

High-performance large area imaging detectors for fast neutrons in the 5–14MeV energy range do not exist at present. The aim of this project is to combine microchannel plates or MCPs (or similar electron multiplication structures) traditionally used in image intensifiers and X-ray detectors with amorphous silicon (a-Si) pixel arrays to produce a composite converter and intensifier position sensitive imaging system. This detector will provide an order of magnitude improvement in image resolution when compared with current millimetre resolution limits obtained using phosphor or scintillator-based hydrogen rich converters. In this study we present the results of the initial experimental evaluation of the prototype system. This study was carried out using a medical X-ray source for the proof of concept tests, the next phase will involve neutron imaging tests. The hybrid detector described in this study is a unique development and paves the way for large area position sensitive detectors consisting of MCP or microsphere plate detectors and a-Si or polysilicon pixel arrays. Applications include neutron and X-ray imaging for terrestrial applications. The technology could be extended to space instrumentation for X-ray astronomy.

Design of a prototype microchannel plate detector with cooled amorphous silicon array readout for neutron radiography

High-performance large-area imaging detectors for fast neutrons in the 5–14 MeV energy range do not exist at present. The aim of this project is to combine microchannel plates or MCPs (or similar electron multiplication structures) traditionally used in image intensifiers and X-ray detectors with amorphous silicon pixel arrays to produce a composite converter and intensifier position-sensitive imaging system. This detector will provide an order of magnitude improvement in image resolution when compared with current millimetre resolution limits obtained using phosphor- or scintillator-based hydrogen-rich converters. In this study the detection of fast neutrons is based on neutron capture in silicon rather than proton recoil in hydrogen-rich converters. This will reduce the effect that light spreading has on image resolution when using conventional phosphor-based converters. The threshold in the silicon capture cross-section will reduce the effect of neutron scatter on the detectability of small features in fast neutron radiographs. In this study we highlight the prototype detector design, present its main advantages and the current status of the detector build phase.

From micro to nano: recent advances in high-resolution microscopy

Improving the spatial resolution of optical microscopes is important for a vast number of applications in the life sciences. Optical microscopy allows intact samples and living cells to be studied in their natural environment, tasks that are not possible with other microscopy methods (e.g. electron microscopy). Major advances in the past two decades have significantly improved microscope resolution. By using interference and structured light methods microscope resolution has been improved to approximately 100 nm, and with non-linear methods a ten times improvement has been demonstrated to a current resolution limit of approximately 30 nm. These methods bring together old theoretical concepts such as interference with novel non-linear methods that improve spatial resolution beyond the limits that were previously assumed to be unreachable.

Resolution in quartz crystal oscillator circuits for high sensitivity microbalance sensors in damping media

The use of quartz crystal oscillators as high sensitivity microbalance sensors is limited by the frequency noise present in the circuit. To characterise the behaviour of the sensors, it is not enough to determine their experimental sensitivity, but rather it is essential to study the frequency fluctuations in order to establish the sensor resolution. This is fundamental in the case of oscillators for damping media, because the level of noise rises due to the strong decline of the quality factor of the resonator.In this paper, a comparative study of noise and resolution is presented with respect to the frequency and the quality factor. The study has been made using four oscillators designed to be used in quartz crystal microbalance sensors in damping media. The four circuits have been designed at increasing frequencies in order to improve the sensitivity. Also, the current resolution limit of a microbalance oscillator is determined using an AT resonator, since this does not depend on the frequency. However, when working in liquid, the damping of the resonator makes the resolution diminish due to a worsening the quality factor. The relationship between the resolution limit and the frequency and characteristics of the liquid medium has been determined. The resolution worsens when the density and viscosity of the liquid is increased. However, in this case, the increase in frequency implies a small increase in the resolution. Therefore, when working below the maximum quality factor, for similar values, the resolution can be improved by elevating the work frequency.

Spot sizes on Sun-like stars

The total area coverage by starspots is of interest for a variety of reasons, but direct techniques only provide estimates of this important quantity. Sunspot areas exhibit a lognormal size distribution irrespective of the phase of the activity cycle, implying that most sunspots are small. Here we explore the consequences if starspot areas were similarly distributed. The solar data allow for an increase in the fraction of larger sunspots with increasing activity. Taking this difference between the size distribution at sunspot maximum and minimum, we extrapolate to higher activity levels, assuming different dependences of the parameters of the lognormal distribution on total spot coverage. We find that, even for very heavily spotted (hypothetical) stars, a large fraction of the spots are smaller than the current resolution limit of Doppler images and hence might be missed on traditional Doppler maps.

The role of preoperative ultrasound scan in detecting lymph node metastasis before sentinel node biopsy in melanoma patients

To evaluate the efficacy of preoperative ultrasound (US) scanning in identifying lymph node metastasis before sentinel node biopsy (SNB), we conducted a prospective study on 125 patients with primary cutaneous melanoma (CM). We prospectively enrolled 125 patients with >1 mm thick CM and candidate for SNB. Preoperatively, patients underwent US scanning of regional lymphatic basins and FNA of suspected lymph nodes (LN). All patients underwent lymphatic mapping and SNB. Combined with fine-needle aspirate (FNA) of suspect LN, US scan allowed the correct preoperative detection of 12 out of 31 histologically positive lymphatic basins, specificity and sensitivity being 100 and 39%, respectively. The false negative rate (61%) was mainly linked to tumor deposits less than 2 mm in diameter, which can be considered the current spatial resolution limit of this technique. Preoperative US scan could reduce the number of SNB, thus avoiding the stress of this surgical procedure in approximately 10% of patients and reducing health care costs. As a non-invasive and relatively inexpensive technique, lymph node US scan can be part of the preoperative staging process of patients' candidate for SNB in order to avoid unnecessary surgical procedures.