Parallel Excitation Research Articles

Multiple-Input Multiple-Output (MIMO) MRI: Combining Parallel Excitation and Parallel Reception for Enhanced Imaging.

Magnetic resonance imaging (MRI) plays a critical role in visualizing the structure and functions of the human body. In order to accelerate imaging time and improve image quality, radio-frequency (RF) coil receive arrays are commonly employed to acquire the magnetic resonance (MR) signal. Similarly, multiple transmit coils have been shown to accelerate and refine RF excitation. In this work, we investigate the optimization of total imaging time and image accuracy when considering both the transmit and receive coil arrays; we term this strategy multiple-input multiple-output (MIMO) MRI. Our RF pulse design method is modeled by minimizing the excitation errors while simultaneously maximizing the signal-to-noise ratio (SNR) of the reconstructed MR image. It further allows a key tradeoff between the two optimizers. Additionally, multiple acceleration factors, varying numbers of receive coils used, maximum excitation error tolerance, and different excitation patterns are simulated and analyzed in this model. For a given excitation pattern, our method is shown to improve the SNR by 18-130% under certain acceleration schemes, as compared to conventional parallel transmission methods, while simultaneously controlling the excitation error within a desired scope (NRMSE ≤ 0.12).

Analysis of Torque Characteristics of Parallel Hybrid Excitation Machine Drives With Sinusoidal and Rectangular Current Excitations

The parallel hybrid excitation machine (PHEM) consists of axial parallel permanent magnet machine part and doubly salient electromagnetic machine part. The three-phase armature windings of two parts are connected in series. Based on the waveforms of back electromotive force, both sinusoidal and rectangular current excitations can be adopted in PHEM. The torque characteristics of PHEM drives with ideal sinusoidal and ideal rectangular current excitations are analyzed. The average torque versus armature current in brushless ac (BLAC) and brushless dc drives are comparatively analyzed. Furthermore, the torque and torque ripple versus current angle in some typical armature currents are investigated. Moreover, the speed and torque waveforms are given under the vector control and the current pulsewidth modulation control scheme. Finally, the experimental data of the target PHEM agree well with the simulation results. It is shown that in BLAC drives, the PHEM exhibits higher torque density with lower torque ripple.

Absence of Parallel Fibre to Purkinje Cell LTD During Eyeblink Conditioning

Long-term depression (LTD) of parallel fibre/Purkinje cell synapses has been the favoured explanation for cerebellar motor learning such as classical eyeblink conditioning. Previous evidence against this interpretation has been contested. Here we wanted to test whether a classical conditioning protocol causes LTD. We applied a conditioning protocol, using a train of electrical pulses to the parallel fibres as the conditional stimulus. In order to rule out indirect effects caused by antidromic granule cell activation or output from Purkinje cells that might produce changes in Purkinje cell responsiveness, we focused the analysis on the first pulse in the conditional stimulus, that is, before any indirect effects would have time to occur. Purkinje cells learned to respond with a firing pause to the conditional stimulus. Yet, there was no depression of parallel fibre excitation after training.

Parent bending effects on nonadiabatic transition dynamics: Isotopomer-resolved imaging of photodissociation of CF3Br at two source temperatures.

Nonadiabatic transition between electronic states plays a critical role in the photodissociation of the CX3Y (X = H and F; Y = Cl, Br, and I) system, and the transition probability was considered to be closely related to the X-C-Y bending motion. Hereby the effect of F-C-Br bending vibration on the nonadiabatic transition dynamics is studied by time-sliced ion velocity imaging of Br(2P1/2,3/2) isotopomers produced from the photodissociation of title molecules at two source temperatures, 298 K and 473 K, respectively. At the photolysis wavelength 234 nm, the anisotropy parameter (β) of the Br(2P3/2) products decreases from 1.3 at 298 K to 0.9 at 473 K, while the β of Br(2P1/2) remains at almost 2 at two temperatures, indicating the significant effect of bending excitation on the ground channel. Two nonadiabatic dissociation pathways are suggested in the Br(2P3/2) channel. One of them is the parallel excitation from the ground state to the 3 Q 0 state in C3V symmetry, and then transition to the 1 Q 1 state via conical intersection, and the other is the perpendicular excitation to the 3A' state in Cs symmetry and then decomposition along this state in the presence of the avoided crossing between 3A' and 4A' states. Closely related to the F-C-Br bending vibration of CF3Br is the latter transition.

A Review of Virtual Impedance Technology for Grid Connected Inverters

With the development of electric and electronic devices, in order to ensure their reliable work, the demand for the quality of electrical energy is becoming more and more high. However, the harmonic components seriously affect the quality of the power grid, and will cause uneven distribution of Grid inverter, series and parallel resonance, excitation current and fault current. By changing the impedance characteristics of the grid connected inverter, the virtual impedance technology can solve the impact of the above problems. It is easy to realize and is widely used. This paper will describe the application and implementation of virtual impedance technology for grid connected transformer.

Single channel spin‐wave logic cell

Spin-wave (SW) logic cell with a parallel 2-port excitation configuration that operates in XOR and OR logic regimes is described. The peculiarity of the solution is that magnetic field of lateral excitation electrodes couples to magnetic field component of SW resulting from its 2D confinement. The operation principle of the device can be interpreted as single channel multiport interference-based element.

In brief

PAGE 413 A portable in-ear brain-computer interface (BCI) is fabricated for generating and monitoring neural responses in work from Korea. The device is inexpensive, unobtrusive and avoids applying electrodes to the scalp, which can cause discomfort. The BCI is shown to efficiently and accurately record changes in electroencephalogram (EEG) signals in response to hearing different frequencies. PAGE 433 Researchers in China propose a control strategy for railways lines to regulate the energy transferred to traction grids. A single-phase traction rectifier is used to make the train become actively involved in renewable energy generation through regenerative braking. This control scheme can be used for smart grid networks on railway lines. A portable, lightweight and unobtrusive brain-computer interface for biomedical monitoring. PAGE 420 Work from Argentina presents a single instruction, multiple data processor (SIMD) for energy-aware embedded visual processing. The architecture of the chip is designed for dynamic voltage/frequency scaling and achieves good performance with efficient dynamic energy dissipation. The chip can be used in visual processing applications, such as for autonomous vehicle navigation. Controlling energy transfer in railway lines using a single-phase traction rectifier. PAGE 430 A direct laser performance comparison between single- and dual-heatspreader semiconductor disk laser configurations is made in work from the USA. The second heatspreader facilitates heat dissipation and increases the thermal rollover pump power. The design is proposed to be a suitable alternative to diamond-based materials for vertical external-cavity surface-emitting laser applications. A SIMD chip with a designed architecture for visual processing applications. PAGE 418 Researchers in Mexico report a compact spin-wave logic cell based on a single magnonic waveguide as a single channel multiport interference-based element. The spin-wave logic cell is based on a two-port parallel excitation configuration and can be used in functional electronics free of electric charges, while avoiding the Joule heating losses. Dual-heatspreaders in disk lasers are shown to give efficient heat dissipation and increased power to the rollover pump. A spin-wave logic cell for functional electronics free of electronic charges is proposed.

A highly decoupled transmit-receive array design with triangular elements at 7T.

Transmit arrays are essential tools for various RF shimming or parallel excitation techniques at 7T. Here we present an array design with triangular coils to improve diversity in the B1 profiles in the longitudinal (z) direction and allow for next-nearest neighbor decoupling. Two cylindrical 8-channel arrays having the same length and diameter, 1 of triangular coils and the other of rectangular coils, were constructed and compared in phantom imaging experiments using measures of excitation distribution for a variety of RF shim settings and geometry factor maps for different accelerations on different planes. Coupling between elements was -20 dB or better for all triangular coil pairs, but worse than -12 dB for several of the rectangular coil pairs. Both coils could produce adequate shims on a central transverse plane, but the same shim produced worse results off center for the triangular coil array than for the rectangular coil array. Compared to the rectangular coil array, the maximum geometry factor for the triangular coil array was reduced by a factor of 13.1 when using a 2-fold acceleration in the z-direction. An array design with triangular coils provides effective decoupling mechanisms for nearest and next-nearest neighboring elements, as well as diversity in B1 profiles along the z-direction, although this also means that individual slices must be shimmed separately. This design is well suited for parallel transmit applications while also having high receive sensitivity.

Rotation relaxation splitting for optimizing parallel RF excitation pulses with [formula omitted]- and [formula omitted]-relaxations in MRI

Exact solutions of the Bloch equations with T1- and T2-relaxation terms for piecewise constant magnetic fields are numerically challenging. We therefore investigate an approximation for the achieved magnetization in which rotations and relaxations are split into separate operations. We develop an estimate for its accuracy and explicit first and second order derivatives with respect to the complex excitation radio frequency voltages. In practice, the deviation between an exact solution of the Bloch equations and this rotation relaxation splitting approximation seems negligible. Its computation times are similar to exact solutions without relaxation terms. We apply the developed theory to numerically optimize radio frequency excitation waveforms with T1- and T2-relaxations in several examples.

Wide regulated series hybrid excitation alternator

Parallel hybrid excitation alternators (PHEAs) are large in size but offers wide flux regulation capability. Series hybrid excitation alternator has limited flux regulation capability but compact compared to PHEAs. A new rotor excitation topology has been proposed for a series hybrid rotor excitation alternator (SHREA). The proposed machine is not only small in size but also have wide flux regulation capability. The proposed machine can be classified as a series hybrid machine as permanent magnets and field windings are arranged in a single rotor ensuring series addition of permanent magnet flux and field winding flux. In this study, one more rotor configuration for quick maintenance SHREA is also proposed. As the name suggests, for any fault in excitation winding, they can be easily replaced. Two 1.3 kW prototypes with such designs have been validated with finite-element method simulation and hardware testing.

Adaptive integrated parallel reception, excitation, and shimming (iPRES-A) with microelectromechanical systems switches.

Integrated parallel reception, excitation, and shimming coil arrays with N shim loops per radio-frequency (RF) coil element (iPRES(N)) allow an RF current and N direct currents (DC) to flow in each coil element, enabling simultaneous reception/excitation and shimming of highly localized B0 inhomogeneities. The purpose of this work was to reduce the cost and complexity of this design by reducing the number of DC power supplies required by a factor N, while maintaining a high RF and shimming performance. In the proposed design, termed adaptive iPRES(N) (iPRES(N)-A), each coil element only requires one DC power supply, but uses microelectromechanical systems switches to adaptively distribute the DC current into the appropriate shim loops to generate the desired magnetic field for B0 shimming. Proof-of-concept phantom experiments with an iPRES(2)-A coil and simulations in the human abdomen with an 8-channel iPRES(4)-A body coil array were performed to demonstrate the advantages of this innovative design. The iPRES(2)-A coil showed no loss in signal-to-noise ratio and provided a much more effective correction of highly localized B0 inhomogeneities and geometric distortions than an equivalent iPRES(1) coil (88.2% vs. 32.2% lower B0 root-mean-square error). The iPRES(4)-A coil array showed a comparable shimming performance as that of an equivalent iPRES(4) coil array (52.6% vs. 54.2% lower B0 root-mean-square error), while only requiring 8 instead of 32 power supplies. The iPRES(N)-A design retains the ability of the iPRES(N) design to shim highly localized B0 inhomogeneities, while drastically reducing its cost and complexity. Magn Reson Med 80:371-379, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Electromagnetic Torque Performance Analysis of a Parallel Hybrid Excitation Machine With Axial Paralleling of Permanent Magnet Part and Variable Reluctance Part

In this paper, the electromagnetic torque (EM) performance analysis of a parallel hybrid excitation machine (PHEM) is investigated. It is composed of a permanent magnet (PM) part and variable reluctance part. The flux paths of the two parts are independent and the armature windings are connected in series. PHEM is an axial parallel coupling of two different types of brushless machine. The mechanisms of EM torque for the two parts are quite different and many problems of torque coupling still remain unsolved. A 2-D EM-circuit coupling simulation model is built to analyze the torque characteristics and coupling relationships. The simulation results indicate that the PM part plays a main role in the output torque of PHEM. The large inductance ripple of variable reluctance part would cause large torque ripple in PHEM. The target machine is experimentally tested to verify the output torque characteristics. Furthermore, the torque-angle characteristics are studied with different field current and it shows that the best load angle could be around 110° to get maximum output torque and relatively smaller torque ripple.

A Hybrid-Excited Vernier Permanent Magnet Machine Using Homopolar Topology

In this paper, a hybrid-excited vernier permanent magnet (PM) machine is proposed. Two excitation sources, namely, the PM excitation and the electrical excitation are artfully combined together based on the homopolar topology, resulting in a parallel hybrid excitation. The flux regulation capability of the proposed machine is discussed and analyzed. Unlike other hybrid-excited machines, the flux regulation is realized based on the variation of the armature flux linkage rather than directly adjusting the air-gap flux. Performances of the bi-directional flux regulation are evaluated by the 3-D finite-element method. In order to verify the analytical results, the proposed machine is prototyped and tested as a generator. Both no-load and on-load performances are assessed. The experimental results agree well with the analytical ones.

Dipole Approximation to Predict the Resonances of Dimers Composed of Dielectric Resonators for Directional Emission

AbstractIn this paper we develop a fully retarded, dipole approximation model to estimate the effective polarizabilities of a dimer made of dielectric resonators. They are computed from the polarizabilities of the two resonators composing the dimer. We analyze the situation of full cubes as well as split cubes, which have been shown to exhibit overlapping electric and magnetic resonances. We compare the effective dimer polarizabilities to ones retrieved via full‐wave simulations as well as ones computed via a quasi‐static, dipole approximation. We observe good agreement between the fully retarded solution and the full‐wave results, whereas the quasi‐static approximation is less accurate for the problem at hand. The developed model can be used to predict the electric and magnetic resonances of a dimer under parallel or orthogonal (to the dimer axis) excitation. This is particularly helpful when interested in locating frequencies at which the dimer will emit directional radiation.

Injection Locking and Polarization Switching Bistability in a 1550 nm VCSEL Subject to Parallel Optical Injection

We present experimental and theoretical results of bistable polarization switching in a single transverse mode 1550 nm vertical-cavity surface-emitting laser (VCSEL) subject to parallel optical injection. We focus our analysis on the bistability induced by power variation of the master laser found on the recently observed state of simultaneous injection locking of the parallel polarization mode and excitation of the orthogonal polarization mode, IL+PS. Experimental stability maps identifying, in the injected power-frequency detuning plane, where this state is observed, and corresponding bistable regions are reported for several bias currents. We find bistability between the IL+PS state and the single polarization mode injection locked solution. We also find bistability between the IL+PS and different periodic dynamics in the parallel polarization mode. The width of the hysteresis cycle increases when increasing the bias current or when increasing the wavelength of the optical injection beyond the solitary VCSEL wavelength. We theoretically confirm these types of bistability by numerically simulating the spin-flip model and by performing a linear stability analysis for the different stable states. Excellent agreement is found between experimental and theoretical results.

A New High-Temperature Superconducting Vernier Permanent-Magnet Machine for Wind Turbines

A vernier permanent magnet (VPM) machine using high-temperature superconducting (HTS) winding for dual excitation is proposed for wind power generation. The proposed VPM machine adopts the parallel hybrid excitation configuration, the stator consists of an identical tooth pair connected together by the stator yoke and the rotor contains two sets of consequent-pole permanent magnets joined together by the rotor yoke. The armature winding is located at the stator main slots while the field winding using HTS coils is wound on the stator yoke. With the benefit of the HTS field winding, the air-gap flux can be flexibly adjusted by controlling the magnitude of the dc current. Therefore, the machine performance is improved to cope with different operating conditions.

Testing Device for Rheological Properties of Magnetorheological Fluid at High Shear Rate

In order to research the rheological property of MRF at high shear rate, a magnetorheological fluid (MRF) rheological property testing device based on shear mode, is developed. The accurate analytical expressions of MRF rheological parameters in terms of structural dimensions of testing device and shear rate in the parallel plate gap are derived, and the flow state of MRF is considered, which provides the theoretical foundation for testing the rheological property of MRF at high shear rate. The relationship between magnetic indication in the parallel plate gap and excitation electrical current is experimentally obtained. Some elements influencing testing are analyzed and eliminated. The device is used to test the rheological parameters of MRF, and the results show that the yield stress of MRF increases significantly as the magnetic flux density continues to increase, and shear rate has no obvious effect on it. The shear rates in designed device may exceed 3 × 104 s−1, which meets the special demands in MRF testing.

Polarization-dependent plasmon mode mapping of Ag nanowires based on two-photon excitation fluorescence of quantum dots

We show that the plasmon modes of Ag nanowires can be imaged by coating them with a layer of quantum dots (QDs), held off the nanowire surface by a nanoscale dielectric spacer layer. Parallel or perpendicular excitation polarization modulates the intensity maps of two-photon excitation fluorescence (TPEF), which exhibit Fabry–Pérot cavity modes at the excitation or fluorescence wavelength. We attribute this phenomenon to the QDs excited by propagating surface plasmon polaritons or localized surface plasmon modes. The results of the TPEF intensity maps are well explained by theoretical simulations, and the energy transfer process is also discussed.

A Controllable Power Distribution Strategy for Open Winding Hybrid Excitation Generator System

The open winding permanent magnet generator system is suitable for applications such as renewable energy power generation systems with multiple power sources, due to its high power density, relatively few power switches, and high degree of integration. However, for this system, it is difficult to quantitatively control the power distribution between different energy sources. In this paper, an open winding hybrid excitation generator system is presented, and for this system, a controllable power distribution strategy is investigated. In the proposed strategy, the excitation current is regulated according to the error in the current loop of the auxiliary energy source, which can achieve the regulation of the output power of the generator and finally realize the power quantitative control of the auxiliary energy source. Then, the topologies of the hybrid excitation machine are analyzed and chosen with emphasize on the influence of the flux regulation capability on the control capability of system power distribution. The experiments are carried out on a parallel hybrid excitation flux-switching (PHEFS) generator system, and the results verify the correctness and effectiveness of the proposed controllable power distribution strategy and the power distribution capability of the PHEFS generator.

P154 Probing the sensitivity of a novel MR head coil array for TMS/fMRI studies of the motor cortex

Question The typical setup used in online TMS/fMRI experiments presents some limitations, most importantly the poor sensitivity of the MR coil (Fig. 1a) Download : Download high-res image (262KB) Download : Download full-size image . In addition, new imaging techniques like parallel imaging and multiband excitation are not available. To get over these limitations, a dedicated MR coil array (Fig. 1b) for combining TMS with fMRI was developed (Navarro de et al., 2015) . We present results of an online TMS/fMRI experiment over the motor cortex to show the advantages of the new setup. Methods The experiment was performed at 3T (Siemens, Germany). The TMS system included a MagProX100 stimulator and an MRi-B91 MR-compatible TMS coil (MagVenture, Denmark). Six healthy subjects participated in the study. Functional images were acquired using a multiband EPI sequence (TR/TE = 1000/33 ms), 14 slices, 1.5 × 1.5 × 3 mm3. Slices were aligned parallel to the TMS coil and covered primary motor cortex. The stimulation protocol comprised 20 stimulation blocks, with five blocks of each of the following stimulation intensities: 80, 90, 100 and 110% of the individual’s motor threshold distributed randomly over the experiment. A finger tapping task was also conducted. Data analysis was performed using SPM12. The design matrix comprised four task regressors representing the 4 stimulation amplitudes. For each subject, two regions were defined based on fMRI activation maps: (i) the global maximum, and (ii) the maximum within M1. Results Fig. 2a,b Download : Download high-res image (632KB) Download : Download full-size image shows group results as the mean of the BOLD signal intensity change in percent for the two voxels defined above with indicator of statistical significance. In addition, averaged time courses were calculated for global maximum (2c) and the M1 maximum (2d). Conclusions In both regions, a monotonic ascending relationship of signal change amplitude with TMS intensity was observed. While activations at the global maxima were statistically significant for all intensities, results on the M1 maximum are only significant for the 100% and 110% intensities. These results on M1 are consistent with results obtained before (Bestmann et al., 2004) . Contrary to the standard setup, the new one didn’t suffer of any artefacts under the TMS coil, where the global maximal activation was always found. The obtained time courses demonstrated the efficiency of the new setup to acquire the signal with high accuracy.