A vision of 14 T MR for fundamental and clinical science

Objective

We outline our vision for a 14 Tesla MR system. This comprises a novel whole-body magnet design utilizing high temperature superconductor; a console and associated electronic equipment; an optimized radiofrequency coil setup for proton measurement in the brain, which also has a local shim capability; and a high-performance gradient set.

Research fields

The 14 Tesla system can be considered a ‘mesocope’: a device capable of measuring on biologically relevant scales. In neuroscience the increased spatial resolution will anatomically resolve all layers of the cortex, cerebellum, subcortical structures, and inner nuclei. Spectroscopic imaging will simultaneously measure excitatory and inhibitory activity, characterizing the excitation/inhibition balance of neural circuits. In medical research (including brain disorders) we will visualize fine-grained patterns of structural abnormalities and relate these changes to functional and molecular changes. The significantly increased spectral resolution will make it possible to detect (dynamic changes in) individual metabolites associated with pathological pathways including molecular interactions and dynamic disease processes.

Conclusions

The 14 Tesla system will offer new perspectives in neuroscience and fundamental research. We anticipate that this initiative will usher in a new era of ultra-high-field MR.

     

A practical approach to the order reduction of a power system model for small signal stability analysis

A linearized power system model suitable for small signal stability analysis or automatic voltage regulators (AVR) and power system stabilizers (PSS) synthesis is derived in the paper. It is derived using the 5th order synchronous machine model with damping winding, the second order IEEE Type 1 AVR and the third order PSS. A new method of model construction and decoupling is given. Using a singular perturbation method the power system model is separated into decoupled lower order slow and fast subsystems. The new proposed model was tested on a real power system in Serbia.     

Optimized in-phase and opposed-phase MR imaging for accurate detection of small fat or water fractions: theoretical considerations and experimental application in emulsions

Object  

To optimize strategies and measurement parameters for quantification of small fat and water fractions (<10%) in mixtures of both components by 4-point in-phase and opposed-phase gradient-echo imaging and to compare theoretical results with in-vitro experiments using emulsions.     

Quantitative T<Subscript>1</Subscript> and T<Subscript>2</Subscript> MRI signal characteristics in the human brain: different patterns of MR contrasts in normal ageing

Objective

The objective of this study was to examine age-dependent changes in both T₁-weighted and T₂-weighted image contrasts and spin-echo T₂ relaxation time in the human brain during healthy ageing.

Methods

A total of 37 participants between the ages of 49 and 87 years old were scanned with a 3 Tesla system, using T₁-weighted, T₂ weighted and quantitative spin-echo T₂ imaging. Contrast between image intensities and T₂ values was calculated for various regions, including between individual hippocampal subfields.

Results

The T₁ contrast-to-noise (CNR) and gray:white signal intensity ratio (GWR) did not change in the hippocampus, but it declined in the cingulate cortex with age. In contrast, T₂ CNR and GWR declined in both brain regions. T₂ relaxation time was almost constant in gray matter and most (but not all) hippocampal subfields, but increased substantially in white matter, pointing to an age effect on water relaxation in white matter.

Conclusions

Changes in T₁ and T₂ MR characteristics influence the appearance of brain images in later life and should be considered in image analyses of aged subjects. It is speculated that alterations at the cell biology level, with concomitant alterations to the local magnetic environment, reduce dephasing and subsequently prolong spin-echo T₂ through reduced diffusion effects in later life.

     

Hybrid linearization of a power system with FACTS devices for a small signal stability study: Concept and application

This paper deals with a hybrid linearization method of a power system with flexible AC transmission system (FACTS) devices. The linearized power system model provides useful information, which is necessary for the small signal stability study and the controller design. Although the algebraic linearization method provides an accurate and parameterized linear system, it may incur computation burdens for the large power system especially with multiple FACTS devices. The numerical identification of linearized system by utilizing input–output numerical data is highly versatile. However, quite amount of valuable information may be not fully utilized. In this paper, a hybrid method in which the algebraic and the numerical linearization technique are combined is presented. While both the power system and the FACTS devices are separately linearized by using algebraic method, the interaction terms between the power system and FACTS devices are numerically identified by adopting the quadratic optimization technique. The proposed hybrid linearization technique is tested on the WSCC system in which one thyristor-controlled series capacitor is installed. Both eigenvalue analysis and time-domain simulation results verify that the proposed method can effectively identify the linearized model of the power system with FACTS devices.     

Comparison of convergence and stability properties for the state and output solutions of neural networks

A typical neuron cell is characterized by the state variable and the neuron output, which is obtained by passing the state through a nonlinear active device implementing the neuron activation. The paper introduces a wide class of neural networks for which the state solutions and the output solutions enjoy the same convergence and stability properties. The class, which includes as a special case the standard cellular neural networks, is characterized by piecewise‐linear Lipschitz continuous neuron activations, Lipschitz continuous (possibly) high‐order interconnections between neurons and asymptotically stable isolated neuron cells. The paper also shows that if we relax any of the assumptions on the smoothness of the neuron activations or interconnecting structure, or on the stability of the isolated neuron cells, then the equivalence between the convergence properties of the state solutions and the output solutions is in general no longer guaranteed. To this end, three relevant classes of neural networks in the literature are considered, where each class violates one of the assumptions made in the paper, and it is shown that the state solutions of the networks enjoy stronger convergence properties with respect to the output solutions or viceversa. Copyright © 2010 John Wiley & Sons, Ltd.     

Multiple-quantum-well Mach-Zehnder modulators: comparison of calculated and measured results for modulator properties and system performance

A measurement-based model of a /spl pi/ phase-shift multiple-quantum-well Mach-Zehnder modulator with asymmetric Y-branch waveguides is presented. Using this model, calculated and measured results for the transmission contours, /spl alpha/-parameter, intensity modulation, and time-resolved chirp are compared. The suitability of the model for system performance evaluation is demonstrated by examining the implications of the modulator chirp on 10-Gb/s transmission over nondispersion shifted fiber, and the combined effect of the modulator chirp and group delay ripple of a dispersion compensating fiber Bragg grating on 10-Gb/s dispersion compensated transmission. Calculated and measured results for the system performance are compared.     

Study on the stability of charge-ordered state and rectifying properties of heteroepitaxial structure for manganites

We report the study on the stability of charge-ordered (CO) state and rectifying properties of heteroepitaxial structure for manganites R _1-x Ca _x MnO₃ (R = La, Pr, and Sm) by measuring magnetoresistance, magnetization, specific heat, I?V characteristics, and X-ray diffraction. For the La_1-x Ca _x MnO₃ system, near the Ca²⁺concentration of x?=?0.75 the CO state is mostly stabilized and insensitive to external magnetic fields, which is supported by the charge transport, magnetic, and specific heat properties. By analyzing powder X-ray diffraction patterns, we found that the Jahn–Teller (JT) distortion in the CO state is the largest at x?=?0.75, which confirms the close relationship between the JT lattice distortion and the stability of the CO state for La_1-x Ca _x MnO₃. The stability of the CO state is also related to the A-site average ionic radius, 〈r _A〉. With decreasing the radius of the doped rare-earth elements (La, Pr, and Sm), the most stable CO state for the each doped system shifts gradually toward lower Ca²⁺concentration. In addition, we have fabricated simple p–n junctions using a heteroepitaxial structure of La_1-x Ca _x MnO₃/Nb-doped SrTiO₃. These junctions show good rectifying properties for temperature from 85 to 305 K. The photovoltage as a function of light wavelength at room temperature has also been studied and the maximum photovoltage can reach 180 mV. The good rectifying behaviors and strong photovoltaic effect make these simple p–n junctions promising for applications.     

A simple apparatus for safety assessment of magnetically induced torque on active implantable medical devices (AIMDs) under 1.5 T and 3.0 T MRI

Magnetic Resonance Materials in Physics, Biology and Medicine - Our goal is to design and validate a simple apparatus for the safety assessments of magnetically induced torques by four active...     

A parallel prefiltering approach for the identification of a biased sinusoidal signal: Theory and experiments

The problem of estimating the amplitude, frequency, and phase of an unknown sinusoidal signal from a noisy‐biased measurement is addressed in this paper by a family of parallel prefiltering schemes. The proposed methodology consists in using a pair of linear filters of specified order to generate a suitable number of auxiliary signals that are used to estimate—in an adaptive way—the frequency, the amplitude, and the phase of the sinusoid. Increasing the order of the prefilters improves the noise immunity of the estimator, at the cost of an increase of the computational complexity. Among the whole family of estimators realizable by varying the order of the filters, the simple parallel prefilters of orders 2 + 2 and 3 + 3 are discussed in detail, being the most attractive from the implementability point of view. The behavior of the two algorithms with respect to bounded external disturbances is characterized by input‐to‐state stability arguments. Finally, the effectiveness of the proposed technique is shown both by comparative numerical simulations and by a real experiment addressing the estimation of the frequency of the electrical mains from a noisy voltage measurement. Copyright © 2015 John Wiley & Sons, Ltd.     

Optimization method of P+ ω PSS parameters for stability and robustness enhancement in a multimachine power system

Recent steady increase of electric power demand causes power sources to be large and far from cities. Wide area power interchanges lead to larger and more complex power systems. This makes the network susceptible to poor damping power swing oscillations of relatively low frequencies which influence the whole system. This paper describes a newly developed generator's double input signal PSS (P+ ω input PSS) design method. Several power system conditions (power flow and/or power patterns) are considered to satisfy a well‐stabilized power system for each system condition. Major features of this method are: (1) Weighting factor for eigenvalue sensitivity of the oscillation mode is considered when the parameters of the PSS are updated during the optimization process. (2) The new method provides good results for the generator's local and interarea oscillation modes under peak and off‐peak power flow conditions. © 2000 Scripta Technica, Electr Eng Jpn, 131(1): 19–31, 2000     

A comparison of numerical methods for the full‐wave analysis of flange mounted rectangular apertures

The analysis and design of small‐ and medium‐sized aperture antenna arrays is an important task in the development of many microwave systems, and is computationally heavy due to the typical requirements for efficiency and accuracy. Several approaches have already been proposed in the literature, but they have not been compared so far in terms of their computational efficiency. In this paper, we compare different methods, either based on rectangular waveguide modes to expand the field over the aperture, or based on Gegenbauer's polynomials for singular fields representation. Their numerical properties are studied, and their performance compared. It is concluded that similar accuracies are achieved, whilst the approach using the spectral‐domain rectangular waveguide mode formulation is the more robust, and should be considered for the development of general purpose software packages. Copyright © 2000 John Wiley & Sons, Ltd.     

Dynamic stability study of an isolated wind-diesel hybrid power system with wind power generation using IG,PMIG and PMSG: A comparison

This paper presents a comparative study of reactive power control for isolated wind-diesel hybrid power system in three different cases with wind power generation by induction generator (IG), permanent-magnet induction generator (PMIG) and permanent-magnet synchronous generator. The synchronous generator (SG) is used with diesel engine set. A mathematical model of the system based on small signal analysis, is developed considering reactive power flow balance equations. The variable reactive power needed by the system is provided by a static synchronous compensator (STATCOM) when wind power generation is done by IG and PMIG. When permanent-magnet synchronous generator (PMSG) is used for wind power generation, the variable reactive power demand is fulfilled by a voltage source converter (VSC) which is on the load side. A new mathematical approximation model for VSC connected with PMSG is proposed such that the voltage source converter fulfills the increased reactive power requirement of load and also increases its active power equal to the increased input wind power. Proportional and integral (PI) gains of the STATCOM and VSC controllers are optimized using integral square error criterion (ISE). The dynamic responses of the system for small (1%) step increase in load reactive power with and without 1% step increase in input wind power are shown. The paper also shows the dynamic responses of the system for random step change in load reactive power plus random step change in input wind power. The MATLAB/SIMULINK environment is used for simulation.     

Exact and explicit expressions for the small‐signal dynamics and frequency response of switching converters

In this paper, exact and explicit expressions for the dynamics and small‐signal responses of piecewise linear switching converters are derived. The results are very useful for the exact simulation and analysis of converter circuits. These expressions can also be used to develop simplified (approximate) models of converters for practical design purposes. An example is given to show that the well‐known state‐space averaging model is in fact the first‐order approximation of our exact model. Copyright © 2004 John Wiley & Sons, Ltd.     

3D magnetic resonance fingerprinting on a low-field 50 mT point-of-care system prototype: evaluation of muscle and lipid relaxation time mapping and comparison with standard techniques

Objective

To implement magnetic resonance fingerprinting (MRF) on a permanent magnet 50 mT low-field system deployable as a future point-of-care (POC) unit and explore the quality of the parameter maps.

Materials and methods

3D MRF was implemented on a custom-built Halbach array using a slab-selective spoiled steady-state free precession sequence with 3D Cartesian readout. Undersampled scans were acquired with different MRF flip angle patterns and reconstructed using matrix completion and matched to the simulated dictionary, taking excitation profile and coil ringing into account. MRF relaxation times were compared to that of inversion recovery (IR) and multi-echo spin echo (MESE) experiments in phantom and in vivo. Furthermore, B₀ inhomogeneities were encoded in the MRF sequence using an alternating TE pattern, and the estimated map was used to correct for image distortions in the MRF images using a model-based reconstruction.

Results

Phantom relaxation times measured with an optimized MRF sequence for low field were in better agreement with reference techniques than for a standard MRF sequence. In vivo muscle relaxation times measured with MRF were longer than those obtained with an IR sequence (T₁: 182 ± 21.5 vs 168 ± 9.89 ms) and with an MESE sequence (T₂: 69.8 ± 19.7 vs 46.1 ± 9.65 ms). In vivo lipid MRF relaxation times were also longer compared with IR (T₁: 165 ± 15.1 ms vs 127 ± 8.28 ms) and with MESE (T₂: 160 ± 15.0 ms vs 124 ± 4.27 ms). Integrated ΔB₀ estimation and correction resulted in parameter maps with reduced distortions.

Discussion

It is possible to measure volumetric relaxation times with MRF at 2.5 × 2.5 × 3.0 mm³ resolution in a 13 min scan time on a 50 mT permanent magnet system. The measured MRF relaxation times are longer compared to those measured with reference techniques, especially for T₂. This discrepancy can potentially be addressed by hardware, reconstruction and sequence design, but long-term reproducibility needs to be further improved.

     

A new methodology for the identification of PD in electrical apparatus: properties and applications

Applications of a new methodology, aimed at the identification of defects occurring in insulation systems of HV apparatus and based on partial discharge (PD) measurements, are presented in this paper. This methodology relies upon the digital acquisition of a large amount of PD pulses and separates the acquired pulses into homogeneous subclasses. Signal processing tools recognize the presence of noise among the different classes. Identification of basic PD source typologies (i.e., internal, corona and surface discharges) is then achieved, resorting to fuzzy algorithms. The proposed procedure is applied to measurements performed on different HV apparatus, such as cables, transformers and rotating machines. The purpose of this paper is to show that the identification process is robust, regarding the measuring circuit, and flexible, so that it can constitute an advanced tool for condition based maintenance, guiding maintenance experts in making decisions on the condition of the insulation system under test.     

Noise covariance matrices in state‐space models: A survey and comparison of estimation methods—Part I

This paper deals with the estimation of the noise covariance matrices of systems described by state‐space models. Stress is laid on the systematic survey and classification of both the recursive and batch processing methods proposed in the literature with a special focus on the correlation methods. Besides the correlation methods, representatives of other groups are introduced also with respect to their basic idea, estimate properties, assumptions and possible extensions, and user‐defined parameters. Common and dual properties of the methods are highlighted, and a simulation comparison using exemplary MATLAB implementations of the methods is provided.     

A comparison of the wound rotor and nested‐loop rotor brushless doubly fed generator

Both the wound rotor and the nested‐loop rotor can realize the coupling with the power winding (PW) and control winding. And the two types of rotors in a brushless doubly fed generator (BDFG) can realize variable‐speed, constant‐frequency generation. In this paper, the two types BDFG are compared through finite element analysis and experiments. From the finite element simulation results, it can be seen that the air gap flux density harmonic contents, PW line voltage harmonic contents, and the core loss of the nested‐loop rotor BDFG are higher than those of the wound‐rotor BDFG. In the experiments, the efficiency of the wound rotor BDFG is slightly lower than that of the nested‐loop rotor BDFG when the rotor speed is below the natural synchronous speed and is much higher than that of the nested‐loop rotor BDFG when the rotor speed is above the natural synchronous speed. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.