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Dive into the research topics where R. F. Remis is active.

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Featured researches published by R. F. Remis.


Inverse Problems | 2000

On the equivalence of the Newton-Kantorovich and distorted Born methods

R. F. Remis; P.M. van den Berg

We show that the Newton-Kantorovich and distorted Born methods for the computational solution of the nonlinear inverse scattering problem are equivalent. This was already shown for the discrete matrix case. Here we present an analysis based on the analytic representations of the integral operators. We first briefly review both methods and then show that they are equivalent.


IEEE Transactions on Microwave Theory and Techniques | 1997

A modified Lanczos algorithm for the computation of transient electromagnetic wavefields

R. F. Remis; P.M. van den Berg

A new method for computing transient electromagnetic wavefields in inhomogeneous and lossy media is presented. The method utilizes a modified Lanczos scheme, where a so-called reduced model is constructed. A discretization of the time variable is then superfluous. This reduced model represents the transient electromagnetic wavefield on a certain bounded interval in time. Some theoretical aspects of the method are highlighted and numerical results showing the performance of the method for two-dimensional (2-D) configurations are given. Also, comparisons between this Lanczos method and the finite-difference time-domain (FDTD) method are made.


IEEE Transactions on Medical Imaging | 2015

CSI-EPT: A Contrast Source Inversion Approach for Improved MRI-Based Electric Properties Tomography

Edmond Balidemaj; Cornelis A.T. van den Berg; Johan Trinks; Astrid L.H.M.W. van Lier; Aart J. Nederveen; Lukas J.A. Stalpers; Hans Crezee; R. F. Remis

Electric properties tomography (EPT) is an imaging modality to reconstruct the electric conductivity and permittivity inside the human body based on B1+ maps acquired by a magnetic resonance imaging (MRI) system. Current implementations of EPT are based on the local Maxwell equations and assume piecewise constant media. The accuracy of the reconstructed maps may therefore be sensitive to noise and reconstruction errors occur near tissue boundaries. In this paper, we introduce a multiplicative regularized CSI-EPT method (contrast source inversion-electric properties tomography) where the electric tissue properties are retrieved in an iterative fashion based on a contrast source inversion approach. The method takes the integral representations for the electromagnetic field as a starting point and the tissue parameters are obtained by iteratively minimizing an objective function which measures the discrepancy between measured and modeled data and the discrepancy in satisfying a consistency equation known as the object equation. Furthermore, the objective function consists of a multiplicative Total Variation factor for noise suppression during the reconstruction process. Finally, the presented implementation is able to simultaneously include more than one B1+ data set acquired by complementary RF excitation settings. We have performed in vivo simulations using a female pelvis model to compute the B1+ fields. Three different RF excitation settings were used to acquire complementary B1+ fields for an improved overall reconstruction. Numerical results illustrate the improved reconstruction near tissue boundaries and the ability of CSI-EPT to reconstruct small tissue structures.


Inverse Problems | 2004

Electromagnetic inversion using a reduced-order three-dimensional homogeneous model

Neil V. Budko; R. F. Remis

Nonlinearized electromagnetic inversion of a three-dimensional homogeneous model of arbitrary but known support is considered. We propose a reduced-order representation of the cost functional based on the shift invariance property of the Arnoldi decomposition. Numerical experiments demonstrate acceleration up to a factor of 100 with respect to the usual repetitive solution of the forward scattering problem. We also discuss the applicability of a homogeneous model for estimating the effective constitutive parameter of an inhomogeneous target.


Physics in Medicine and Biology | 2016

In vivo electric conductivity of cervical cancer patients based on B₁⁺ maps at 3T MRI.

Edmond Balidemaj; P. De Boer; A.L.H.M.W. Van Lier; R. F. Remis; Lukas J.A. Stalpers; G.H. Westerveld; A J Nederveen; C.A.T. Van den Berg; J. Crezee

The in vivo electric conductivity (σ) values of tissue are essential for accurate electromagnetic simulations and specific absorption rate (SAR) assessment for applications such as thermal dose computations in hyperthermia. Currently used σ-values are mostly based on ex vivo measurements. In this study the conductivity of human muscle, bladder content and cervical tumors is acquired non-invasively in vivo using MRI. The conductivity of 20 cervical cancer patients was measured with the MR-based electric properties tomography method on a standard 3T MRI system. The average in vivo σ-value of muscle is 14% higher than currently used in human simulation models. The σ-value of bladder content is an order of magnitude higher than the value for bladder wall tissue that is used for the complete bladder in many models. Our findings are confirmed by various in vivo animal studies from the literature. In cervical tumors, the observed average conductivity was 13% higher than the literature value reported for cervical tissue. Considerable deviations were found for the electrical conductivity observed in this study and the commonly used values for SAR assessment, emphasizing the importance of acquiring in vivo conductivity for more accurate SAR assessment in various applications.


SIAM Journal on Scientific Computing | 2013

A Krylov Stability-Corrected Coordinate-Stretching Method to Simulate Wave Propagation in Unbounded Domains

Vladimir Druskin; R. F. Remis

The Krylov subspace projection approach is a well-established tool for the reducedorder modeling of dynamical systems in the time domain. In this paper, we address the main issues obstructing the application of this powerful approach to the time-domain solution of exterior wave problems. We use frequency-independent perfectly matched layers to simulate the extension to infinity. Pure imaginary stretching functions based on Zolotarev’s optimal rational approximation of the square root are implemented leading to perfectly matched layers with a controlled accuracy over a complete spectral interval of interest. A new Krylov-based solution method via stabilitycorrected operator exponents is presented which allows us to construct reduced-order models (ROMs) that respect the delicate spectral properties of the original scattering problem. The ROMs are unconditionally stable and are based on a renormalized bi-Lanczos algorithm. We give a theoretical foundation of our method and illustrate its performance through a number of numerical examples in which we simulate two-dimensional electromagnetic wave propagation in unbounded domains, including a photonic waveguide example. The new algorithm outperforms the conventional finitedifference time-domain method for problems on large time intervals.


IEEE Transactions on Magnetics | 2002

Absorbing boundary conditions and perfectly matched layers - an analytic time-domain performance analysis

A.T. de Hoop; P.M. van den Berg; R. F. Remis

The time-domain performance of a number of absorbing boundary conditions invoked on the boundary of a domain of computation, as well as of a perfectly matched layer surrounding such a domain, is carried out for a test configuration for which analytic expressions for the relevant field quantities exist. The test configuration consists of a small loop antenna radiating into a homogeneous, isotropic half-space. On the planar boundary of this halfspace, either an absorbing boundary condition is invoked or a perfectly matched layer is started that is truncated at some finite depth of penetration. For a loop parallel to the boundary, closed-form analytic expressions for all field components of the spuriously reflected field are presented for all truncation conditions involved. A number of important features show up that might be masked in purely numerical implementations of the procedures under consideration.


International Journal of Hyperthermia | 2016

Hyperthermia treatment planning for cervical cancer patients based on electrical conductivity tissue properties acquired in vivo with EPT at 3 T MRI

Edmond Balidemaj; H. P. Kok; G. Schooneveldt; Astrid L.H.M.W. van Lier; R. F. Remis; Lukas J.A. Stalpers; Henrike Westerveld; Aart J. Nederveen; Cornelis A.T. van den Berg; J. Crezee

Abstract Introduction The reliability of hyperthermia treatment planning (HTP) is strongly dependent on the accuracy of the electric properties of each tissue. The values currently used are mostly based on ex vivo measurements. In this study, in vivo conductivity of human muscle, bladder content and cervical tumours, acquired with magnetic resonance-based electric properties tomography (MR-EPT), are exploited to investigate the effect on HTP for cervical cancer patients. Methods Temperature-based optimisation of five different patients was performed using literature-based conductivity values yielding certain antenna settings, which are then used to compute the temperature distribution of the patient models with EPT-based conductivity values. Furthermore, the effects of altered bladder and muscle conductivity were studied separately. Finally, the temperature-based optimisation was performed with patient models based on EPT conductivity values. Results The tumour temperatures for all EPT-based dielectric patient models were lower compared to the optimal tumour temperatures based on literature values. The largest deviation was observed for patient 1 with ΔT90 = −1.37 °C. A negative impact was also observed when the treatment was optimised based on the EPT values. For four patients ΔT90 was less than 0.6 °C; for one patient it was 1.5 °C. Conclusions Electric conductivity values acquired by EPT are higher than commonly used from literature. This difference has a substantial impact on cervical tumour temperatures achieved during hyperthermia. A higher conductivity in the bladder and in the muscle tissue surrounding the tumour leads to higher power dissipation in the bladder and muscle, and therefore to lower tumour temperatures.


Journal of Computational Physics | 2014

An extended Krylov subspace model-order reduction technique to simulate wave propagation in unbounded domains

Vladimir Druskin; R. F. Remis; Mikhail Zaslavsky

Abstract In this paper we present a novel extended Krylov subspace reduced-order modeling technique to efficiently simulate time- and frequency-domain wavefields in open complex structures. To simulate the extension to infinity, we use an optimal complex-scaling method which is equivalent to an optimized perfectly matched layer in which the frequency is fixed. Wavefields propagating in strongly inhomogeneous open domains can now be modeled as a non-entire function of the complex-scaled wave operator. Since this function contains a square root singularity, we apply an extended Krylov subspace technique to construct fast converging reduced-order models. Specifically, we use a modified version of the extended Krylov subspace algorithm as proposed by Jagels and Reichel [14] , since this algorithm allows us to balance the computational costs associated with computing powers of the wave operator and its inverse. Numerical experiments from electromagnetics and acoustics illustrate the performance of the method.


international conference on acoustics, speech, and signal processing | 2012

On preconditioned conjugate gradient method for time-varying OFDM channel equalization

Zijian Tang; R. F. Remis; Magnus Lundberg Nordenvaad

We consider using the conjugate gradient (CG) algorithm to equalize a time-varying channel in an orthogonal frequency division multiplexing (OFDM) system. Preconditioning technique to accelerate the convergence of the CG algorithm is discussed, where we show that when the Doppler spread becomes higher, the commonly used diagonal preconditioner, despite its simpleness, can perform even worse than without preconditioner. In such a case, a preconditioner with a more complex structure is proposed.

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P.M. van den Berg

Delft University of Technology

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Neil V. Budko

Delft University of Technology

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Andrew G. Webb

Leiden University Medical Center

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Wyger M. Brink

Leiden University Medical Center

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J. T. Zimmerling

Delft University of Technology

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