High Contrast Media Research Articles

Internally Combined Volume-Surface Integral Equation for a 3-D Electromagnetic Scattering Analysis of High-Contrast Media

An internally combined volume surface integral equation (ICVSIE) for analyzing three-dimensional (3-D) electromagnetic scattering from heterogeneous high-contrast (HC) dielectric objects is proposed. Iteratively solved (generalized) volume integral equations [(g)VIEs] often converge slowly when applied to the analysis of electromagnetic interactions involving dielectric objects with permittivities much higher than those of the surrounding medium [HC breakdown] or discretized using mesh elements much smaller than the wavelength [low-frequency (LF) breakdown]. The proposed ICVSIE hybridizes a VIE and a surface integral equation (SIE) in a manner that eliminates both breakdown phenomena. Just as in previous hybrid VIE–SIE approaches, the ICVSIE invokes Huygens surface equivalence principle to transform the original problem into coupled equivalent exterior and interior scenarios. In the exterior scenario, surface currents radiate in the original surrounding medium; in the interior problem, surface and volume polarization currents radiate in a medium with permittivity closer to that of the object. By judiciously combining integral equations pertinent to both scenarios, the ICVSIE system is assembled. Numerical results show that the proposed ICVSIE is both HC and LF stable when applied to the analysis of 3-D scattering problems.

Uncertainty quantification of two-phase flow problems via measure theory and the generalized multiscale finite element method

The problem of multiphase phase flow in heterogeneous subsurface porous media is one involving many uncertainties. In particular, the permeability of the medium is an important aspect of the model that is inherently uncertain. Properly quantifying these uncertainties is essential in order to make reliable probabilistic-based predictions and future decisions. In this work, a measure-theoretic framework is employed to quantify uncertainties in a two-phase subsurface flow model in high-contrast media. Given uncertain saturation data from observation wells, the stochastic inverse problem is solved numerically in order to obtain a probability measure on the space of unknown permeability parameters characterizing the two-phase flow. As solving the stochastic inverse problem requires a number of forward model solves, we also incorporate the use of a conservative version of the generalized multiscale finite element method for added efficiency. The parameter-space probability measure is used in order to make predictions of saturation values where measurements are not available, and to validate the effectiveness of the proposed approach in the context of fine and coarse model solves. A number of numerical examples are offered to illustrate the measure-theoretic methodology for solving the stochastic inverse problem using both fine and coarse solution schemes.

The effects of contrast media volume on acute kidney injury after transcatheter aortic valve replacement: a systematic review and meta-analysis.

The goal of this systematic review was to assess the effects of contrast media volume on transcatheter aortic valve replacement-related acute kidney injury. A literature search was performed using Medline, EMbase, the Cochrane Database of Systematic Reviews, and clinicaltrials.gov from the inception of these databases through December 2015. Studies that reported relative risk, odds ratio, or hazard ratio comparing the risks of acute kidney injury following transcatheter aortic valve replacement in patients who received high contrast media volume were included. Pooled risk ratio (RR) and 95% confidence intervals (95% CI) were calculated using a random-effect, generic inverse variance method. Four cohort studies composed of 891 patients were included in the analyses to assess the risk of acute kidney injury after transcatheter aortic valve replacement in patients who received high contrast media volume. The pooled RR of acute kidney injury after transcatheter aortic valve replacement in patients who received a large volume of contrast media was 1.41 (95% CI, 0.87 to 2.28) compared with low contrast media volume. The meta-analysis was limited to studies using standard acute kidney injury definitions, and the pooled RR of acute kidney injury in patients who received high contrast media volume is 1.12 (95% CI, 0.78 to 1.62). Our meta-analysis shows no significant association between contrast media volume and risk of acute kidney injury after transcatheter aortic valve replacement.

Reduced-order multiscale modeling of nonlinear p-Laplacian flows in high-contrast media

In this paper, we consider a class of nonlinear flow problems that are modeled through a time-dependent p-Laplacian formulation. Using an approach that combines the Generalized Multiscale Finite Element Method (GMsFEM) and Discrete Empirical Interpolation Method (DEIM), we are able to accurately approximate the nonlinear p-Laplacian solutions at a significantly reduced cost. In particular, GMsFEM allows us to iteratively solve the global problem using a reduced-order model in which a flexible number of multiscale basis functions are used to construct a spectrally enriched coarse-grid solution space. The iterative procedure requires a number of nonlinear functional updates that are simultaneously made more cost efficient through implementation of DEIM. The combined GMsFEM-DEIM approach is shown to be a flexible framework for producing accurate reduced-order descriptions of the nonlinear model for a wide range of parameters and varying levels nonlinearity. A number of numerical examples are presented to illustrate the effectiveness of the proposed methodology.

Super-resolution in high-contrast media

A mathematical theory is developed to explain the super-resolution and super-focusing in high-contrast media. The approach is based on the resonance expansion of the Green function associated with the medium. It is shown that the super-resolution is due to sub-wavelength resonant modes excited in the medium which can propagate into the far-field.

Imaging quality evaluation of low tube voltage coronary CT angiography using low concentration contrast medium.

PurposeTo compare the image quality of prospectively ECG-gated low voltage coronary computed tomography angiography (CTA) with an administration of low concentration contrast medium.Method and MaterialsA total of 101 patients, each with a heart rate below 65 beats per minute (BPM), underwent a prospectively ECG-gated axial scan in CT coronary angiography on a 64-slice CT scanner. All patients were allocated in three groups (group A: n=31, 80kVp, 300 mgI/ml; group B: n=34, 100kVp, 300 mgI/ml; group C: n=36, 120kVp, 370 mgI/ml). The CT attenuation values of aortic root (AR), left main coronary artery (LMA), right main coronary artery (RMA) and chest subcutaneous fat tissue were measured. The contrast-to-noise ratio (CNR) of AR, LMA and RMA were calculated according to the formulas below. The values of computed tomography dose index (CTDI) and dose-length product (DLP) were recorded. Image quality was assessed on a 5-point scale. The results were compared using the one-way ANOVA and rank sum tests.ResultsThe values of CNR and SNR for vessels in group A and group B were not significantly different from group C (each p > 0.05). The effective radiation dose in group A (1.51±0.70 mSv) and group B (2.59±1.24 mSv) were both lower than group C (4.92±2.82 mSv) (each p < 0.05). There was no significant difference among the image quality scores of group A (4.10±0.41), group B (3.90±0.48) and group C (4.04±0.36) (each P > 0.05).ConclusionLow tube voltage coronary CT angiography using low concentration contrast medium does not affect the imaging quality for assessing the coronary arteries compared with high voltage coronary CT angiography using high concentration contrast medium. Meanwhile low concentration contrast medium allowed 47-69% of radiation dose reduction.

Brain Computerized Tomography Scan Findings in a Man After Inadvertent Intrathecal Ionic Contrast Agent Injection

Here is described a serial brain computerized tomography (CT) scan findings in a patient with inadvertent intrathecal injection of Meglumine Diatrizoate 76% for myelography. Intrathecal administration of ionic contrast media may cause severe and lethal neurotoxic reactions owing to their hyperosmolarity. This patient recovered from disease with only minor neurological sequel after 40 days of hospitalization and supportive care. To perform myelography, only non-ionic water soluble contrast agent for intrathecal injection can be used. Here is reported the findings of a serial brain CT scan in a patient with inadvertent intrathecal injection of Meglumine Diatrizoate (an ionic contrast agent not permitted for intrathecal use) for myelography. A 55-year-old man who was a victim of war shell fragment injury and had surgical history of lumbar laminectomy referred with chronic low back pain. He underwent myelography and was unintentionally injected Meglumine Diatrizoate. After a few minutes, he started complaining of severe headache, inability to walk, followed by a decrease in his level of consciousness and myoclonic seizure. His Glascow Coma Scale (GCS) declined to 3 in less than two hours. He was intubated and admitted in the intensive care unit. His initial brain CT scan showed hyperdensity of subarachnoid space consistent with the presence of high density contrast medium as well as diffused white matter hypodensity and increased gray-white matter differentiation compatible with those of cerebral edema Figure 1. After preliminary supportive care a ventricular shunt was introduced to assist faster cerebrospinal fluid turnover. In the second brain CT scan, one day after the event, only the ventriculostomy tube was visible and decrease in brightness of contrast agent in cerebral cortical sulci and decrease in hypodensity of white matter was observed Figure 2. Follow up CT scan after 3 days of continuous medical treatment showed almost complete washout of the contrast medium from subarachnoid space. Although to a lesser extent, white matter hypodensity consistent with cerebral edema still persisted. The patient’s GCS was calculated to be 10 at the time of this follow up CT scan, Figure 3. After 10 days of hospitalization and supportive care, the patient regained full consciousness showing GCS of 15 and only complained of difficulty in walking. CT scan at the time of discharge demonstrated complete resolution of cerebral edema, Figure 4.

Qualitative evaluation of pulmonary CT angiography findings in pregnant and postpartum women with suspected pulmonary thromboembolism.

Background:Considering the importance of using more appropriate imaging technique for accurate diagnosis of pulmonary thromboembolism (PTE) with less side effects, we aimed to evaluate the quality of pulmonary 64-multidetector computed tomographic (MDCT) angiography in pregnant and postpartum women with suspected PTE in Isfahan.Materials and Methods:In this descriptive study, radiological findings of pregnant and postpartum women with suspected PTE who underwent pulmonary 64-MDCT angiography were evaluated. Prevalence of PTE in pregnant and postpartum women, mean of pulmonary arteries density for right and left pulmonary arteries, and their lobar and segmental branches, diagnostic quality of the pulmonary arteries density and their scoring, frequency of diagnostic and nondiagnostic images, mean of radiation dose and mean of bolus time, and the correlation between the quality of the vascular density with the peak density of the pulmonary artery were determined.Results:In this study, 44 pregnant and postpartum women with suspected PTE were selected. The overall prevalence of PTE was 9.1% (4/44). PTE was diagnosed in 1 (3.7%) pregnant and 3 (17.5%) postpartum women (P = 0.14). Mean density of pulmonary trunk was 278.81± 108.16 Hounsfield unit (HU) and 308.41 ± 59.30 HU in pregnant and postpartum women, respectively. Mean of bolus timing, kilovoltage peak (kVp), tube current, and dose length product (DLP) were 12.53 ± 2.36 s, 105.22± 45.71 milliamperage (MA), 382.9 ± 173.5 MA, and 317.98 ± 78.92 mGy/cm, respectively. The rate of nondiagnostic images was 4.5%.Conclusion:Our findings indicated that pulmonary 64-MDCT angiography is an appropriate imaging method for diagnosing PTE in pregnant and postpartum women with suspected PTE. It seems that, using fast CT systems (64-MDCT), in accordance with high flow rate, high contrast medium concentration and low kVp could explain the obtained appropriate quality of images more efficiently than computed tomographic pulmonary angiography (CTPA).

Bayesian compressive sensing for ultrawideband inverse scattering in random media

We develop an ultrawideband inverse scattering technique for reconstructing continuous random media on the basis of Bayesian compressive sensing. In addition to providing maximum a posteriori estimates of the unknown weights, Bayesian inversion provides an estimate of the confidence level of the solution, as well as a systematic approach for optimizing subsequent measurement(s) to maximize information gain. We impose sparsity priors directly on spatial harmonics to exploit the spatial correlation exhibited by continuous media and then efficiently solve for their posterior probability density functions by using a fast relevance vector machine. We linearize the problem by using the first-order Born approximation, which enables us to combine, in a single inversion, measurements from multiple transmitters and ultrawideband frequencies. To extend the method to high-contrast media, we also introduce a Bayesian compressive sensing distorted-Born iterative approach. Further, we illustrate the use of time-reversal strategies to adaptively focus the inversion effort onto subdomains of interest and hence reduce the overall inversion cost. The proposed techniques are applied to a number of scenarios including crosshole and borehole sensing.

Homogenization of diffusion–deformation in dual-porous medium with discontinuity interfaces

Models of homogenized fluid-saturated dual-porous media with weak, or strong discontinuity interfaces (resembling fissures) are derived using the periodic unfolding method. Stress discontinuities at the interfaces are admitted, requesting further restrictions on the applied external forces. The limit models, obtained by a rigorous asymptotic analysis, reflect some non-local effects inherited from the microstructural interactions. In view of obtaining the a priori estimates, standard approaches based on smooth extensions, well fitted for perforated or high-contrast media, cannot be adopted for fissured domains. Therefore, a new approach is developed which enables to control the norm of some off-diagonal terms which in the model equations are generated by the interfaces and are not involved in the energy-related expressions.

ASYMPTOTIC EXPANSIONS FOR HIGH-CONTRAST ELLIPTIC EQUATIONS

In this paper, we present a high-order expansion for elliptic equations in high-contrast media. The background conductivity is taken to be one and we assume the medium contains high (or low) conductivity inclusions. We derive an asymptotic expansion with respect to the contrast and provide a procedure to compute the terms in the expansion. The computation of the expansion does not depend on the contrast which is important for simulations. The latter allows avoiding increased mesh resolution around high conductivity features. This work is partly motivated by our earlier work in [Domain decomposition preconditioners for multiscale flows in high-contrast media, Multiscale Model Simul.8 (2010) 1461–1483] where we design efficient numerical procedures for solving high-contrast problems. These multiscale approaches require local solutions and our proposed high-order expansion can be used to approximate these local solutions inexpensively. In the case of a large-number of inclusions, the proposed analysis can help to design localization techniques for computing the terms in the expansion. In the paper, we present a rigorous analysis of the proposed high-order expansion and estimate the remainder of it. We consider both high- and low-conductivity inclusions.

High-pitch dual-source CT coronary angiography with low volumes of contrast medium

To assess the effect of lower volumes of contrast medium (CM) on image quality in high-pitch dual-source computed tomography coronary angiography (CTCA). One-hundred consecutive patients (body weight 65-85kg, stable heart rate ≤65bpm, cardiac index ≥2.5L/min/m(2)) referred for CTCA were prospectively enrolled. Patients were randomly assigned to one of five groups of different CM volumes (G30, 30mL; G40, 40mL; G50, 50mL; G60, 60mL; G70, 70mL; flow rate 5mL/s each, iodine content 370mg/mL). Attenuation within the proximal and distal coronary artery segments was analysed. Mean attenuation for men and women ranged from 345.0 and 399.1HU in G30 to 478.2 and 571.8HU in G70. Mean attenuation values were higher in groups with higher CM volumes (P < 0.0001) and higher in women than in men (P < 0.0001). The proportions of segments with attenuation of at least 300HU in G30, G40, G50, G60 and G70 were 89%, 95%, 98%, 98% and 99%. CM volume of 30mL in women and 40mL in men proved to be sufficient to guarantee attenuation of at least 300HU. In selected patients high-pitch dual-source CTCA can be performed with CM volumes of 40mL in men or 30mL in women. • High-pitch dual-source coronary angiography is feasible with low contrast media volumes. • Traditional injection rules still apply: higher volumes result in higher enhancement. • The patient's gender is a co-factor determining the level of contrast enhancement. • Volumes can be reduced down to 30-40mL in selected patients.

Asymptotic expansion of steady-state potential in a high contrast medium with a thin resistive layer

We study the steady-state potential in a high contrast medium with a resistive thin layer. We provide asymptotic expansions of the potential at any order, whatever small the layer conductivity is. Approximate transmission conditions at any order and the corresponding variational formulation are given. We prove uniform estimates with respect to the thickness of the layer and with respect to its resistivity. The main insight consists in this expansion at any order and in the corresponding non-standard variational formulation whatever small the layer conductivity is. In particular the limit potential is not continuous across the limit surface (when the membrane thickness shrinks to zero) unlike the soft contrast case and its jump is proportional to the electric flux. Numerical simulations illustrate the theoretical results.

Artifacts caused by insufficient contrast medium filling during C-arm cone-beam CT scans: a phantom study

We investigated artifacts due to late-arriving contrast medium (CM) during C-arm cone-beam computed tomography. We scanned a phantom filled with water or with 100, 50, or 5% v/v concentrations of CM and then virtually produced CM-delayed projection data by partially replacing the projection images. Artifacts as a function of concentration, percentage of filling time, and size and position of the filling area were assessed. In addition, we used an automatic power injector with different injection delays to inject CM during the scans. A decrease in filling times caused by a lag in CM arrival during the scan resulted in a decrease in pixel values, distortion of the filling area, and appearance of streak artifacts. Even a delay of approximately 20% in CM arrival in the total scan time resulted in obvious distortion of the filling area. The distortion and streak artifacts tended to worsen at higher CM concentrations. Use of a minimum CM concentration based on the purpose of the examination and constant filling at the target region are effective for avoiding these artifacts.

Ensemble level multiscale finite element and preconditioner for channelized systems and applications

We consider multiscale flow in porous media. We assume that we can characterize the ensemble of all possible flow scenarios, that is, we can describe all possible permeability configurations needed for the simulations. We construct coarse basis functions that can provide inexpensive coarse approximations that are: (1) adequate for all possible flow scenarios in the given ensemble, (2) robust with respect to the small scales and high variations in each flow scenario. The coarse approximations developed here can be used as a multiscale finite element method, or as the coarse solver in a two-level domain decomposition iterative method. The methods presented here extend, to the ensemble case, some of the results in [J. Galvis, Y. Efendiev, Domain decomposition preconditioners for multiscale flows in high-contrast media: reduced dimension coarse spaces, SIAM Multiscale Model. Simul. 8 (5) (2010) 1621–1644] and [Y. Efendiev, J. Galvis, X.H. Wu, Multiscale finite element methods for high-contrast problems using local spectral basis functions, J. Comput. Phys. 230 (4) (2011) 937–955]. Specifically, ensembles of permeability fields with high-contrast channels and inclusions are considered. Our main objective here is to construct special multiscale basis functions for the whole ensemble of flow scenarios. The coarse basis functions are pre-computed for (selected or constructed) permeability fields with certain topological properties. This procedure is a preprocessing step and it avoids constructing basis functions (or computing upscaling parameters) for each permeability realization. Then, for any permeability, the solution of elliptic equation can be project to the space spanned by these pre-computed basis functions. We apply this coarse multiscale solver to the design of two-level domain decomposition preconditioner. Numerical experiments show that the ensemble level multiscale finite element method converges to the reference solution. Numerical experiments also show that the ensemble level domain decomposition preconditioner condition number is independent of the high-contrast in the coefficient.

HEART's Original [症例] 経皮的冠動脈インターベンション時の造影剤大量使用により全身痙攣をきたした1例

HEART's Original [症例] 経皮的冠動脈インターベンション時の造影剤大量使用により全身痙攣をきたした1例

Numerical analysis of the coupling mechanism in long-range plasmonic couplers at 155 μm

We study the coupling mechanism between a high refractive index contrast waveguide and a plasmonic thin-film waveguide in the IR range. We also propose a novel design of a vertical coupler based on loading the plasmonic waveguide with a high refractive index contrast medium on each side. We achieve a coupling efficiency and an insertion loss of about 95% and 0.2dB, respectively, with a coupling length of only 2.85μm at the working wavelength of 1.55μm.

Diffusion approximation with polarization and resonance effects for the modelling of seismic waves in strongly scattering small-scale media

SUMMARY This paper presents an analytical study of the multiple scattering of seismic waves by a collection of randomly distributed point scatterers. The theory assumes that the energy envelopes are smooth, but does not require perturbations to be small, thereby allowing the modelling of strong, resonant scattering. The correlation tensor of seismic coda waves recorded at a threecomponent sensor is decomposed into a sum of eigenmodes of the elastodynamic multiple scattering(Bethe-Salpeter) equation. Forageneral moment tensor excitation, atotalnumber of four modes is necessary to describe the transport of seismic waves polarization. Their spatiotemporal dependence is given in closed analytical form. Two additional modes transporting exclusively shear polarizations may be excited by antisymmetric moment tensor sources only. The general solution converges towards an equipartition mixture of diffusing P and S waves which allows the retrieval of the local Green’s function from coda waves. The equipartition time is obtained analytically and the impact of absorption on Green’s function reconstruction is discussed. The process of depolarization of multiply scattered waves and the resulting loss of information is illustrated for various seismic sources. It is shown that coda waves may be used to characterize the source mechanism up to lapse times of the order of a few mean free times only. In the case of resonant scatterers, a formula for the diffusivity of seismic waves incorporating the effect of energy entrapment inside the scatterers is obtained. Application of the theory to high-contrast media demonstrates that coda waves are more sensitive to slow ratherthanfastvelocityanomaliesbyseveralordersofmagnitude.Resonantscatteringappears as an attractive physical phenomenon to explain the small values of the diffusion constant of seismic waves reported in volcanic areas.

Speckle generation and analysis of speckle tracking performance in a multi-scatter pressure field

Speckle tracking imaging is used as a method to estimate heart strain. An analysis of accuracy of speckle tracking and its potential to be utilized in quantification of myocardial stress through estimation of heart motion is examined. Multiple scattering effects are modeled using the Kirchoff integral formulation for the pressure field. The method of Pade approximants is used to accelerate convergence and to obtain temporal varying characteristics of the scattered field. Phantoms having varied acoustical contrast media and speckle density are used in this study. The effectiveness of inter-image frame of correlation methods for estimating speckle motion in high contrast media is considered. (NSF Grant 0841392)

Acoustic scattering from weakly coupled porous media

In this work, wave propagation and multiple acoustic scattering from porous media are examined. For a highly porous media, the inertial coupling between solid and fluid phases can be small. In such a case the dilatational and acoustic motions will be weakly coupled. The controlling factor is the ratio of effective inertial mass density of the porous media and the fluid mass density. Using Biot’s analysis as the starting point, the dilatational and acoustic motions, as well as the scattered pressure that ensues are evaluated. The Kirchhoff-Helmholtz integral equation is used to this end. The integral equations are evaluated using a series expansion in terms of derived acoustic contrast properties. Using the local Pade approximant procedure detailed in [The Journal of the Acoustical Society of America, 128(5) EL274-278 (2010)]. Of particular interest, are high contrast media and wavelengths comparable to the average scatterer size.