High-contrast Structures Research Articles

Dual resonance in a waveguide-coupled ring microresonator

Eigenmodes of an isolated circular ring or disk microresonator are degenerate because of its rotational symmetry. Coupling of the microresonator to a straight bus guide breaks the rotational symmetry of the device. As a result, two slightly different resonant frequencies occur. While this effect is known in microring lasers, it is usually neglected in microring-based filters and add-drop de/multiplexers for telecom applications. Resonant frequency splitting does not follow from the usual ‘optical circuit’ analysis of the microring devices in which the coupling between the bus guide and ring is described by the 2 × 2 coupling matrix. It is shown here that the full 4 × 4 scattering matrix that takes into account also the back-reflections in the coupling region has to be used in order to include this effect into consideration. The magnitude of the resonant frequency splitting due to coupling to the bus guides in small high-contrast structures is estimated by two-dimensional numerical modelling based on bi-directional mode expansion and FDTD algorithms.

Improvements in Skull Base Imaging through High Resolution Flat-Panel Detector-Based Volume-CT (fpVCT)

Introduction: It was shown in numerous experimental studies using small-sized specimens that flat-panel based Volume-CT (fpVCT) allows the imaging of high-contrast structures such as the temporal bone in much higher resolution than current multislice-CT (MSCT). Unfortunately, when larger objects such as whole human skull bases are scanned, the image quality can be expected to be lower due to several effects that include, among others, scatter, beam hardening, and dose desaturation.

Objective assessment of image quality in conventional and digital mammography taking into account dynamic range

The goal of this work is to develop a method to objectively compare the performance of a digital and a screen-film mammography system in terms of image quality. The method takes into account the dynamic range of the image detector, the detection of high and low contrast structures, the visualisation of the images and the observer response. A test object, designed to represent a compressed breast, was constructed from various tissue equivalent materials ranging from purely adipose to purely glandular composition. Different areas within the test object permitted the evaluation of low and high contrast detection, spatial resolution and image noise. All the images (digital and conventional) were captured using a CCD camera to include the visualisation process in the image quality assessment. A mathematical model observer (non-prewhitening matched filter), that calculates the detectability of high and low contrast structures using spatial resolution, noise and contrast, was used to compare the two technologies. Our results show that for a given patient dose, the detection of high and low contrast structures is significantly better for the digital system than for the conventional screen-film system studied. The method of using a test object with a large tissue composition range combined with a camera to compare conventional and digital imaging modalities can be applied to other radiological imaging techniques. In particular it could be used to optimise the process of radiographic reading of soft copy images.

High-resolution ultrasound-modulated optical tomography in biological tissues

We present a novel implementation of high-resolution ultrasound-modulated optical tomography that, based on optical contrast, can image several millimeters deep into soft biological tissues. A long-cavity confocal Fabry-Perot interferometer, which provides a large etendue and a short response time, was used to detect the ultrasound-modulated coherent light that traversed the scattering biological tissue. Using 15-MHz ultrasound, we imaged with high-contrast light-absorbing structures placed >3 mm below the surface of chicken breast tissue. The resolution along the axial and the lateral directions with respect to the ultrasound propagation direction was better than 70 and 120 microm, respectively. The resolution can be scaled down further by use of higher ultrasound frequencies. This technology is complementary to other imaging technologies, such as confocal microscopy and optical-coherence tomography, and has the potential for broad biomedical applications.

Noncontact scanning impedance imaging in an aqueous solution

We present a method for imaging based on noncontact electrical impedance measurements and mechanical scanning. Measurement results are shown for an initial system based on this concept. An impedance probe design is presented, applicable to the test system. Line-scan data plots of high impedance contrast structures show a good fit to a theoretical physical model. Image resolutions on the order of 100μm are indicated for the initial system. Two-dimensional impedance images of biological tissue generated by this technique are shown.

SOI nanophotonic waveguide structures fabricated with deep UV lithography

To reduce the dimensions of photonic integrated circuits, high-contrast wavelength-scale structures are needed. We developed a fabrication process for Silicon-on-insulator nanophotonics, based on standard CMOS processing techniques with deep UV lithography. Measurements using either end-fire incoupling or grating-based fibre couplers show photonic crystal waveguides with moderate propagation losses (7.5 dB/mm) and photonic wires with very low propagation losses (0.24 dB/mm).

Diagnostic possibilities of cone-beam computed tomography in the facial skeleton

We want to demonstrate a retrospective analysis of the patient group who received cone-beam computed tomography (CBCT) imaging in our department focussing on indications extending the established range. For postprocessing purposes eFilm Workstation 1.8.3 (Merge eFilm, Milwaukee, WI, USA) was used. Examinations were either recorded on CD-ROM or documented as printouts. Beside established indications we noticed an increasing number of examinations of traumatologic disorders. New diagnostic possibilities were seen for osteomyelitis and orthodontic problems. Concerning high-contrast structures CBCT proved to be an equivalent alternative to CT leading to reduced radiation exposures of patients. Furthermore, the number of conventional tomographies has markedly decreased. In general, pathologies of the facial skeleton offer a large number of indications for CBCT examinations, but technical limitations have to be kept in mind.

Large Cosmic Variance in the Clustering Properties of Ly Emitters at z 5

We reported in a previous paper the discovery of a large-scale structure of Ly? emitters (LAEs) at z = 4.86 ? 0.03 with a projected size of 20 h Mpc ? 50 h Mpc in narrowband data of a 25' ? 45' area of the Subaru Deep Field (?0 = 0.3, ?0 = 0.7, and H0 = 70 h70 km s-1 Mpc-1). However, the surveyed area, which corresponds to 55 h Mpc ? 100 h Mpc, was not large enough to conclude that what we were seeing was a typical distribution of z 5 LAEs. In this Letter, we report the results of follow-up imaging of the same sky area using a new narrowband filter (NB704, ?c = 7046 ? and FWHM = 100 ?) to detect LAEs at z = 4.79, i.e., LAEs lying closer to us by 39 h Mpc on average than the z = 4.86 objects. We detect 51 LAEs at z = 4.79 ? 0.04 down to NB704 = 25.7, and we find that their sky distribution is quite different from the z = 4.86 LAEs'. The clustering of z = 4.79 LAEs is very weak on any scale, and there is no large-scale high-contrast structure. The shape and the amplitude of the angular correlation function are thus largely different between the two samples. These results demonstrate a large cosmic variance in the clustering properties of LAEs on scales of ~50 h Mpc.

Suppression of intensity transition artifacts in statistical x-ray computer tomography reconstruction through radon inversion initialization.

Statistical reconstruction (SR) methods provide a general and flexible framework for obtaining tomographic images from projections. For several applications SR has been shown to outperform analytical algorithms in terms of resolution-noise trade-off achieved in the reconstructions. A disadvantage of SR is the long computational time required to obtain the reconstructions, in particular when large data sets characteristic for x-ray computer tomography (CT) are involved. As was shown recently, by combining statistical methods with block iterative acceleration schemes [e.g., like in the ordered subsets convex (OSC) algorithm], the reconstruction time for x-ray CT applications can be reduced by about two orders of magnitude. There are, however, some factors lengthening the reconstruction process that hamper both accelerated and standard statistical algorithms to similar degree. In this simulation study based on monoenergetic and scatter-free projection data, we demonstrate that one of these factors is the extremely high number of iterations needed to remove artifacts that can appear around high-contrast structures. We also show (using the OSC method) that these artifacts can be adequately suppressed if statistical reconstruction is initialized with images generated by means of Radon inversion algorithms like filtered back projection (FBP). This allows the reconstruction time to be shortened by even as much as one order of magnitude. Although the initialization of the statistical algorithm with FBP image introduces some additional noise into the first iteration of OSC reconstruction, the resolution-noise trade-off and the contrast-to-noise ratio of final images are not markedly compromised.

Midfacial imaging using digital volume tomography

Digital volume tomography (DVT) has been described to visualize high-contrast structures in a quality comparable to a computed tomography (CT), but requiring less radiation exposure. DVT was applied as a three-dimensional imaging modality using the NewTom 9000. After determining the NewTom 9000's suitability for imaging of fine osseous structures of the facial skeleton using a dried human skull, the clinical application was extended to traumatological cases of the midface, starting with postoperative imaging of orbitozygomatical fractures. Thereafter, preoperative imaging and visualization of bone involvement in tumors were included. This paper summarizes our clinical experiences in cases of orbitozygomatical fractures and tumors involving the midface.

Megavoltage cone-beam computed tomography using a high-efficiency image receptor

Purpose: To develop an image receptor capable of forming high-quality megavoltage CT images using modest radiation doses. Methods and Materials: A flat-panel imaging system consisting of a conventional flat-panel sensor attached to a thick CsI scintillator has been fabricated. The scintillator consists of individual CsI crystals 8 mm thick by 0.38 mm × 0.38-mm pitch. Five sides of each crystal are coated with a reflecting powder/epoxy mixture, and the sixth side is in contact with the flat-panel sensor. A timing interface coordinates acquisition by the imaging system and pulsing of the linear accelerator. With this interface, as little as one accelerator pulse (0.023 cGy at the isocenter) can be used to form projection images. Different CT phantoms irradiated by a 6-MV X-ray beam have been imaged to evaluate the performance of the imaging system. The phantoms have been mounted on a rotating stage and rotated while 360 projection images are acquired in 48 s. These projections have been reconstructed using the Feldkamp cone-beam CT reconstruction algorithm. Results and Discussion: Using an irradiation of 16 cGy (360 projections × 0.046 cGy/projection), the contrast resolution is ∼1% for large objects. High-contrast structures as small as 1.2 mm are clearly visible. The reconstructed CT values are linear ( R 2 = 0.98) for electron densities between 0.001 and 2.16 g/cm 3, and the reconstruction time for a 512 × 512 × 512 data set is 6 min. Images of an anthropomorphic phantom show that soft-tissue structures such as the heart, lung, kidneys, and liver are visible in the reconstructed images (16 cGy, 5-mm-thick slices). Conclusions: The acquisition of megavoltage CT images with soft-tissue contrast is possible with irradiations as small as 16 cGy.

A new computation method for a staggered grid of 3D EM field conservative modeling

A new three-dimensional (3D) MT modeling scheme conserving electric current and magnetic flux is developed. The scheme is based on finite difference (FD) staggered rectangular non-uniform grid formulation for the secondary electric field with continuous components of tangential electric and normal magnetic fields, in contrast to existing FD algorithms with a discontinuous E-field at the face of the cells. The scheme leads to a sparse 13-band complex symmetrical system of linear equations, which is effectively solved by fast and stable conjugate gradient (CG) methods. The preconditioning procedure was used to decrease the condition of a number of an ill-conditioned matrix system by several orders and stably and quickly solves the matrix system. The special module for the correction of divergence-free current J greatly increased the speed of convergence and accuracy, especially at low frequencies and for high-contrast resistivity or conductivity structures. A special procedure was developed to improve the accuracy of tangential magnetic and vertical electrical components at the Earth’s surface and at the interface with a large conductivity contrast. The validity of the new algorithm was demonstrated for difficult models with high-contrast resistivity structures including topography and for COMMEMI project models.

A Fourier transform theory for photon localization and evanescence in photonic bandgap structures

We propose an approach to the problem of describing photon behaviour near the bandgap region of photonic crystal structures using a Fourier transform formalism defined in optical reciprocal-space. A modified Debye-Waller approximation in conjunction with the use of a resonant refractive index provides an analytic solution to the coupled-mode equations describing photon propagation in a periodic high dielectric contrast structure. Overall, this represents a new technique for solving the inverse scattering problem. We use this method to consider photon behaviour at the Brillouin zone edges of a one-dimensional photonic crystal structure: our results showing agreement with conventional theory for the standard assumption of an infinitely long grating structure. However, when a photon is located in the bandgap region of a finite (five periods used as an example) high refractive index contrast grating structure, we find that photon velocity becomes superluminal as the photon wave becomes evanescent. This is in agreement with the basic precepts of the uncertainty principle. Additionally, as the bandgap strength increases, the probability of transmission through the dielectric barrier reduces exponentially, with an associated reduction in tunnelling time.

Homogenization of Elliptic Difference Operators

We develop some aspects of general homogenization theory for second order elliptic difference operators and consider several models of homogenization problems for random discrete elliptic operators with rapidly oscillating coefficients. More precisely, we study the asymptotic behavior of effective coefficients for a family of random difference schemes whose coefficients can be obtained by the discretization of random high-contrast checker-board structures. Then we compare, for various discretization methods, the effective coefficients obtained with the homogenized coefficients for corresponding differential operators.

Diffuse Baryons in Groups and Clusters of Galaxies

To predict the X-ray observables associated to the diffuse baryons in clusters of galaxies, we develop a new physical approach to model such a hot intra-cluster plasma. Such approach is based on punctuated equilibria. and comprises the following blocks: Monte Carlo ``merging histories'' of dark matter potential wells; the central hydrostatic disposition for the ICP, reset to a new equilibrium after each merging episode; conditions of shock, or of closely adiabatic compression at the boundary with the external gas, preheated by stellar energy feedbacks. We predict the L-T relation, consistent with the data as for shape and scatter. This we combine with the mass distribution provided by the hierarchical clustering for different COBE-normalized CDM models, to predict the z-resolved luminosity functions, the source counts, the redshift distributions and contribution of the unresolved groups and clusters to the soft X-ray background. When compared with the recent ROSAT surveys, our results confirm that the critical cosmology with Standard CDM is ruled out by its overproduction of local clusters. On account of underproduction, instead, we rule out open cosmologies except for a narrow range around \Omega_o=0.5; even there, we find the consistency with the full data base to be hardly marginal. For \Omega_o=0.3 in flat geometry, we obtain acceptable fits. For the tilted CDM perturbation spectrum with high baryonic content in the critical universe, we obtain marginal consistency. Finally, we discuss the effective limitations of X-ray clusters and groups as cosmological signposts, and their brighter prospects toward the astrophysics of the ICP and the cosmogony of large, high-contrast structures.

Window width as a dosage-relevant factor in high-contrast structures in CT

To establish the relationship between window width and dose to be applied in low-dose high-resolution (HR) computed tomography (CT). Low-dose HR CT-scans of the petrous bone displayed with different window values were analyzed for identification of osseous details. For two homogeneous phantoms, standard deviation of CT-numbers were measured in order to calculate the fraction of pixels not displayed within the correct grey level. The broadest window used (4000 HU) allowed the best distinction of osseous structures. Standard deviation of CT-numbers in the phantoms varied between 28.5 and 43.2 (mean = 33.6) HU. Thus, only 26.6% of all pixels are displayed in the correct grey level for a 1000 HU-window, but 82.3% for a 4000 HU-window. When using low doses and HR-reconstruction algorithms, large window widths allow for an optimal assessment of high-contrast structures. Under these conditions, even high standard deviations of the CT-numbers will not compromise image evaluation. Due to the restricted number of grey-levels that can be distinguished by the human eye, image noise is compressed into a smaller number of discernable grey-levels. Because of the inverse square root function between dose to be applied and window width used (derived in the article), an even minor enlargement of window permits considerable reductions of patient exposure while the discernable image noise remains constant.

Spiral CT with reduced radiation dosage

To examine the clinical use of a reduced dose in high-resolution spiral-CT (HR-SCT) in combination with a multiplanar volume reconstruction technique (MPVR) to decrease image noise. Facial bones, skull and extremities of 33 patients with different pathological findings were examined with spiral-CT using a slice thickness of 1 mm (pitch 1.7-2.0), a tube amperage of 40 mA and tube voltages of 80 or 120 kV. Additionally, 2-D, 3-D, and MPVR images reconstructions were performed. MPVR takes advantage of displaying planar volumes of variable thickness. Increased image noise was negligible if high contrast structures like bones were examined. However, very small bony structures like the stapes of the middle ear were not seen. Soft tissues could also be imaged by using the MPVR-technique. This technique did reduce image noise while at the same time partial volume averaging was increased. The skin entrance dose per investigation did not exceed 3 mGy, which is based on twelve dose measurements in two patients. It is feasible to perform HR-SCT with secondary reconstructions of facial bones, skull and extremities using reduced skin entrance doses by 10 to 20 fold to below 3 mGy. With the exception of imaging very small structures, sufficient image quality can be accomplished in routine examinations without relevant diagnostic compromise.

The cosmological mass distribution from Cayley trees with disorder

We present a new approach to the statistics of the cosmic density field and to the mass distribution of high-contrast structures, based on the formalism of Cayley trees. Our approach includes in one random process both fluctuations and interactions of the density perturbations. We connect tree-related quantities, like the partition function or its generating function, to the mass distribution. The Press \& Schechter mass function and the Smoluchowski kinetic equation are naturally recovered as two limiting cases corresponding to independent Gaussian fluctuations, and to aggregation of high-contrast condensations, respectively. Numerical realizations of the complete random process on the tree yield an excess of large-mass objects relative to the Press \& Schechter function. When interactions are fully effective, a power-law distribution with logarithmic slope -2 is generated.

Three-dimensional display of the heart, aorta, lungs, and airway using CT.

In previous studies of human anatomy, three-dimensional display of CT data has required laborious manual boundary tracking, except for high-contrast structures such as the spine. Automated boundary tracking techniques have been extended so that they can function well for both high-contrast and soft-tissue interfaces. These methods have been applied to the in vivo study of human lungs, heart, aorta, and larynx in this paper.

Comparison of ground-based and spacecraft observations of the infrared emission from Venus: The nature of thermal contrasts

Comparisons are made between observations of spatial variations in the thermal emission from Venus obtained with ground-based telescopes and those from spacecraft. In particular, we concentrate on measurements of solar-related structure at low and mid-latitudes, limb-darkening, and on the high-contrast polar structure. We conclude that (1) the solar-related emission is predominantly wavenumber 2, although it contains a significant diurnal component; (2) the relative amplitudes of the semidiurnal and diurnal components vary with latitude; (3) thermally excited temperature waves or, alternatively, solar-driven vertical motions of the cloud top are better able to account for the magnitude of the solar-locked emission than brightness temperature contrasts resulting from variations in aerosol microphysical properties; (4) the equatorial limb-darkening shows the top of the main cloud to be diffuse and approximately uniformly mixed with the gas; (5) polar collars are persistent at least for several months but disappear on occasion; and (6) collars have been observed at both poles simultaneously, but whether simultaneous appearance is the exception or the rule is still in question.