Transverse Array Research Articles

MRI Coil Design Using Boundary-Element Method With Regularization Technique: A Numerical Calculation Study

A new boundary element method (BEM) based method is described for the design of coils for magnetic resonance imaging (MRI) systems. BEM is an effective approach for solving the electromagnetic forward problem and has been used in the design of MRI gradient coils. However, BEM-based gradient coil design faces an ill-posed mathematical problem, which is conventionally handled by means of a Lagrange multiplication method. This work attempts to improve the BEM method for MRI coil designs by applying the Tikhonov regularization scheme to solve the ill-posed matrix system formulated by the BEM forward model. The objective of the study is not to design some specific gradient or radio-frequency (RF) coils for MRI system, but to discuss the design scheme with practical regularization and constraints. The proposed approach was explained in the design of MRI coils including biplanar transverse gradient coils and RF phased array coils. With the consideration of the practical engineering requirements, physical constraints such as wire intervals are transformed into mathematical constraints and formulated into BEM equations. The examples demonstrate that the proposed method is efficient and flexible for the design of MRI coils with arbitrary geometries and engineering constraints.

Cryofixation rapidly preserves cytoskeletal arrays of leaf epidermal cells revealing microtubule co‐alignments between neighbouring cells and adjacent actin and microtubule bundles in the cortex

Accurate preservation of microtubule and actin microfilament arrays is crucial for investigating their roles in plant cell development. Aldehyde fixatives such as paraformaldehyde or glutaraldehyde preserve cortical microtubule arrays but, unless actin microfilaments are stabilized with drugs such as m-maleimidobenzoyl N-hydroxysuccinimide ester (MBS), ethylene glycol bis[sulfosuccinimidylsuccinate] (sulfo-EGS) or phalloidin, their arrays are often poorly preserved. Cryofixation, used primarily for electron microscopy, preserves actin microfilaments well but is used rarely to fix plant cells for optical microscopy. We developed a novel whole-mount cryofixation method to preserve microtubule and microfilament arrays within Tradescantia virginiana leaf epidermal cells for investigation using confocal microscopy. Cortical microtubule arrays were often oriented in different directions on the internal and external faces of the epidermal cells. A number of arrays were aligned in several directions, parallel to microtubules of neighbouring cells. Actin microfilaments were particularly well preserved possibly due to the speed with which they were immobilized. No transverse cortical microfilament arrays were observed. On occasion, we observed co-aligned microfilament and microtubule bundles lying adjacent to the plasma membrane and positioned side by side suggesting a potential direct interaction between the cytoskeletal filaments at these locations. Cryofixation is therefore a valuable tool to investigate the interactions between cytoskeletal arrays in plant cells using confocal microscopy.

Interfacial pH-gradient induced micro-capillary filling with the aid of transverse electrodes arrays in presence of electrical double layer effects

In the present work, we outline the design and analysis of a micro-capillary filling mechanism through the aid of interfacial pH gradients (and hence interfacial tension gradients) generated by employing arrays of transverse electrodes inducing step changes in voltages, in a natural buffer system that requires low power and no synthetic ampholytes. The capillary transport is modulated by a dynamic and non-trivial coupling between the interfacial tension and viscous resistances, as a consequence of the underlying intermolecular interactions. The competing effects of the driving and the retarding forces effectively determine the displacement, velocity and acceleration characteristics of the capillary front, in a dynamically evolving manner. A comprehensive theoretical model of capillary dynamics is developed here to address these issues in details, thereby revealing the combined influence of the interfacial electrochemistry and the applied transverse voltages, as guided by the pertinent fundamental thermodynamic principles governed by free energy considerations and the physico-chemical phenomena over interfacial scales. Non-trivial implications of the pH-gradient driven micro-capillary transport are aptly emphasized, so as to offer significant physical insights on the adopted strategy as a guiding principle for facilitating capillary filling processes by inducing a modulation in the effective interfacial energy. Particular implications on the capillary filling time are also pinpointed, revealing the effectiveness of the adopted design strategy. Finally, a universal scaling relationship of the capillary filling time as a function of the pertinent operating parameters is derived, so as to provide a generalized guideline for implementing the design scheme. A non-dimensional parameter, depending simultaneously on the inter-electrode pitch and the transverse voltage, is identified, which may be kept to a minimal limit within the other operating constraints of the chosen system, towards minimizing the capillary filling time.

The Structure of GaN-Based Transverse Junction Blue LED Array for Uniform Distribution of Injected Current/Carriers

In this study, we demonstrate a GaN-based transverse junction blue LED array. This device was realized by the regrowth of n-type GaN layers on the sidewall of p-type GaN and undoped multiple quantum wells (MQWs). Due to the transverse flow of injection carriers, problems related to nonuniform current distribution, nonuniform carrier distribution among different MQWs, and bias-dependent shape of the electroluminescence spectra such as that occurring in traditional GaN-based blue LEDs with vertical p-n junctions and large active area (>1 mm2) are all greatly minimized in our structure.

Half-Mode Substrate Integrated Waveguide Transverse Slot Array Antennas

Transverse slot array antennas fed by a half-mode substrate integrated waveguide (HMSIW) are proposed and developed in this paper. The design concept of these new radiating structures is based on the study of the field distribution and phase constant along the HMSIW as well as on the resonant characteristics of a single slot etched on its top conducting wall. Two types of HMSIW-fed slot array antennas, operating, respectively, in X-band and Ka-band, are designed following a procedure similar to the design of slot array antennas fed by a dielectric-filled rectangular waveguide. Compared with slot array antennas fed by a conventional rectangular waveguide, such proposed HMSIW-fed slot array antennas possess the advantages of low profile, compact size, low cost, and easy integration with other microwave and millimeter wave planar circuits. It is worth noting that the width of HMSIW slot array antennas is reduced by nearly half compared to that of slot array antennas fed by a substrate integrated waveguide.

Analysis and Design of a Reflection-Cancelling Transverse Slot-Pair Array with Grating-Lobe Suppressing Baffles

This paper presents the analysis and design of a reflection-cancelling transverse slot-pair array antenna with baffles by using the Spectrum of Two-Dimensional Solutions (S2DS) method. For the transverse slot array, the slot spacings with more than one free-space wavelength cause the grating-lobes. The baffles suppress the grating-lobes effectively. A one-dimensional slot array is extracted from the 2D array with in-phase excitation by assuming periodicity in the transversal direction. The uniform excitation over the finite array is synthesized iteratively to demonstrate the fast and accurate results by S2DS. A unit design model with the baffles is introduced to determine the initial parameters of the slot-pairs, which greatly accelerate the iterations process. Experiments at 25.3 GHz demonstrate the suppression of the grating lobes to the level less than -20.0 dB and also the good uniformity of the aperture field distribution.

High‐field head radiofrequency volume coils using transverse electromagnetic (TEM) and phased array technologies

This article describes technological advances in quadrature transverse electromagnetic (TEM) volume coils and phased arrays reported recently from our laboratory developed for MRI and MRS imaging of the human brain. The first part of this work presents a new method for tuning TEM volume coils based on measurements of the radiofrequency current distribution in the coil elements. This technique facilitates bench adjustment of the coils' homogeneity and is particularly important for tuning double-tuned TEM volume coils. We have also used this method to optimize other TEM configurations such as a quadrature TEM half-volume coil and a split TEM coil. TEM half-volume coils provide greater sensitivity over localized regions than conventional full-volume coils, and the split TEM coil provides greater patient access and ease of use. The second part of this work describes the development of single-tuned and double-tuned transmit TEM volume coils in combination with phased arrays. A variety of different techniques for active detuning of single-tuned and double-tuned TEM volume coils are presented along with the development of phased arrays and transmission line preamplifier decoupling. The final section describes the use of counter rotating current (CRC) surface coils in phased arrays. Because of the intrinsic isolation of CRC coils from transmit volume coils, CRC arrays can be used simultaneously with volume coils for both reception and transmission. Near the center of the human head where both the phased array and the volume coil produce similar sensitivities, simultaneous reception enhances the signal-to-noise ratio. Conversely, simultaneous transmission can be used to boost the transmit field in peripheral brain regions from the volume coil to provide a more homogeneous transmit field.

Stage-specific crosstalk between light, auxin, and ethylene during low-pH-induced root hair formation in lettuce (Lactuca sativa L.) seedlings

In the light, transfer of lettuce seedlings precultured on liquid medium at pH 6.0 to fresh medium at pH 4.0 induces root hair formation. However, no root hairs form in the dark. Here, we investigated how light induces root hair formation. Randomization of the transverse cortical microtubule (CMT) arrays which occurs in root epidermal cells in the light prior to root hair initiation was not observed in the dark. However, addition of indole-3-acetic acid (IAA) or 1-aminocyclopropane-1-carboxylic acid (ACC) induced CMT randomization and root hair formation. In these cases, CMT randomization occurred in almost the same time-dependent manner as under light. However, root hair initiation was delayed for several hours in the dark. These results suggest that light promotes CMT randomization and root hair initiation via auxin and ethylene signaling but light additionally influences root hair initiation independently of these signaling mechanisms. Furthermore, addition of a microtubule-depolymerizing drug in the dark disrupted the transverse CMT arrays and initiated root hair formation; however, root hair elongation was still suppressed. Root hairs elongated when IAA or ACC was applied with the drug. These results suggest that light also promotes root hair elongation via auxin and ethylene signaling.

Numerical analysis of heat transfer in a manifold microchannel heat sink with high efficient copper heat spreader

A manifold microchannel heat sink integrated with a high efficient copper heat spreader is presented. A series analysis of three-dimensional fluid flow and heat transfer performance in this mirochannel heat sink and conventional structure are performed by CFX commercial software package. The temperature difference along the flow direction in the new microchannel heat sink is less than that of the conventional microchannel heat sink due to effect of the transverse channel arrays. The maximum heat flux input of the new microchannel heat sink increases 75% more than the conventional structure with the flow rate of 1 m/s. The new design has better heat transfer characteristics than conventional one for the full range of flow rates considered.

Generation and Weak Beam Control of Two-Dimensional Multicolored Arrays in a Quadratic Nonlinear Medium

We report on the first experimental observation of 2D multicolored transverse arrays in a quadratic nonlinear medium under the pump of two crossly overlapped femtosecond beams. The 2D reproducible patterns are caused by cascaded noncollinear quadratic nonlinear couplings between the input pulses and quadratic spatial solitons originated from spatial breakup of one of the input beams with spatial ellipticity. A seed supercontinuum pulse is then diffracted and amplified with phase preservation, resulting in the formation of up-converted multicolor 2D transverse arrays. By seeding with weak second harmonic pulses, the 2D multicolored transverse patterns are suppressed due to weak beam control of the induced quadratic spatial solitons.

Onset of Vortex Shedding in a Periodic Array of Circular Cylinders

Hopf bifurcation of steady base flow and onset of vortex shedding over a transverse periodic array of circular cylinders is considered. The influence of transverse spacing on critical Reynolds number is investigated by systematically varying the gap between the cylinders from a small value to large separations. The critical Reynolds number behavior for the periodic array of circular cylinders is compared with the corresponding result for a periodic array of long rectangular cylinders considered in [Balanchandar, S., and Parker, S. J., 2002, “Onset of Vortex Shedding in an Inline and Staggered Array of Rectangular Cylinders,” Phys. Fluids, 14, pp. 3714–3732]. The differences between the two cases are interpreted in terms of differences between their wake profiles.

Transverse lightwave circuits in microstructured optical fibers: resonator arrays

Novel type of microstructured optical fiber couplers is introduced where energy transfer is enabled by transverse resonator arrays built into a fiber crossection. Such a design allows unlimited spatial separation between interacting fiber cores which, in turn, eliminates inter-core crosstalk via proximity coupling, thus enabling scalable integration of many fiber cores. Moreover, in the limit of weak inter-resonator coupling, resonator arrays exhibit moderate polarization dependence.

Experimental study of inclined film flow along periodic corrugations: The effect of wall steepness

Gravity-driven film flow along an inclined periodic wall with transverse rectangular corrugations is studied experimentally. The effect of corrugation steepness (=height∕length) is considered in detail and an interesting contrast emerges between properties of the flow that are apparently independent of corrugation steepness and properties that are strongly affected by it. The steady interaction between the wall and the flow leads to a statically deformed free surface, whose amplitude is independent of the corrugation heights tested. Beyond the maximum steady free-surface amplitude, a three-dimensional pattern is established (consisting of transverse arrays of depressions along corrugation valleys), again at conditions independent of corrugation height. On the contrary, steep corrugations expand significantly the stable region of steady flow. Also, fully developed traveling waves (emerging from ambient noise under unsteady conditions) are significantly larger and more regular than under the same conditions along a flat wall. This difference is attributed to the continuous interaction of traveling pulses with the steadily deformed substrate.

A Waveguide Broad-Wall Transverse Slot Linear Array with Reflection-Canceling Inductive Posts and Grating-Lobe Suppressing Parasitic Dipoles

A design of a linearly-polarized non-resonant waveguide broad-wall transverse slot linear array with suppressed grating lobes is presented. Each unit element in the array consists of a transverse slot, an inductive post and a parasitic dipole-pair at a height of half of the free space wavelength. It is designed as an isolated unit without considering mutual coupling by using the Method of Moments (MoM) for radiation suppression in grating beam direction and reflection cancellation at the input. The elements thus designed are used in a travelling wave array environment. It is predicted that the reflection is less than -20dB at 11.95 GHz while the grating lobes are suppressed by more than 15dB. The design and the characteristics of the array are confirmed by measurements.

The focal plane detector of the magnetic spectrometer PRISMA

The characteristics and performance of the focal plane detector system developed for the magnetic spectrometer PRISMA are presented. It is composed of a multi-wire parallel plate avalanche counter (providing X, Y and timing signals) followed by a multi-parametric transverse field ionization chamber array (providing multiple Δ E signals). The detector array has been built to match the very large acceptance of PRISMA and provides optimal performance in the complete identification (nuclear charge, energy and mass) of nuclei produced in heavy-ion binary reactions.

Grating Lobes Suppression in Transverse Slot Linear Array with a Dual Parasitic Beam of Strip Dipoles

A new technique called the Dual Parasitic Beam (DPB) technique is proposed to suppress grating lobes in a rectangular waveguide broad wall transverse slot array. This technique involves an extra layer of parasitic strip dipoles that generate the DPB to suppress the grating lobes without opposing the main beam of the original slot linear array. A full wave EM analysis in Method of Moments (MoM) is conducted to compute the coupling excitation coefficients as well as the far field patterns of the slot and dipole currents. Analysis shows that a suitable dimension and arrangement of dipoles are needed to get a desired level of dipole excitations to meet the grating suppression condition. It is found that the grating lobes can be suppressed as much as 15 dB in the presence of the parasitic dipoles. Experiments are conducted to confirm the computed results.

EUS Staging of Upper GI Malignancies: results of a Prospective, Randomized Trial

Upper GI malignancies can be staged with radial or linear EUS. Electronic 270-degree transverse array radial EUS (R-EUS) provides high quality cross-sectional images, but cannot guide Fine Needle Aspiration (FNA). Linear EUS (L-EUS) provides longitudinal images of malignancies and the ability to guide FNA of abnormal lymph nodes or masses. Methods: In a prospective, randomized trial, patients with a documented upper GI (UGI) malignancy underwent L-EUS immediately followed by R-EUS (63%) or R-EUS immediately followed by L-EUS (37%).

Transverse Slot Antenna Array in the Broad Wall of a Rectangular Waveguide Partially Filled With a Dielectric Slab

A compact and wide-band transverse slot antenna array in the broad wall of a rectangular waveguide partially filled with an H-plane -dielectric slab is presented in this paper. By partially filling an H-plane dielectric slab in the waveguide, the inter-element spacing between slots is reduced to be about 0.8 free-space wavelengths to avoid grating lobes. In addition, the partially filled material provides extra flexibility to adjust the slot impedance as desired within a relatively wide frequency range. A new feeding network is designed for this antenna array, which can prevent the shift in the pointing angle of the array's main radiation beam with the change of frequency. Experimental data for a 2/spl times/4 antenna array operating at X-band show that stable radiation pattern over a wide frequency range can be obtained. The measured gain is 16.9 dB at 10 GHz and the fluctuation is less than 2 dB over a frequency band of 1.5 GHz. The slot array's 10-dB return loss bandwidth is 13% and the cross-polarization level is better than -25 dB.

Secondary walls in hyaline cells of Sphagnum

The cytoskeleton and ultrastructural events associated with cell differentiation and secondary cell wall and pore formation in hyaline cells of Sphagnum are investigated. Microtubules reorient from random arrays in undifferentiated hyaline cells to transverse arrays in elongating cells. Once cells are fully elongated, broad bands of microtubules aggregate into a spiral that predicts the site of secondary cell wall deposition. The secondary wall has a similar fibrillar composition to the primary wall. After the secondary wall is deposited, the thin primary wall covering the pore breaks down, usually by cell-wall degradation at the centre of the pore and around its margin. Finally, the hyaline cell undergoes cytoplasmic degeneration. The orientation of microtubules associated with hyaline-cell formation and secondary cell wall patterning resembles ultrastructural development in tracheary elements of higher plants. The similarities in cytoskeletal arrays during cell differentiation and secondary-wall formation suggest a fundamental pathway of development shared by bryophytes and higher plants.

Secondary Wall Formation in Elaters of Liverworts and the Hornwort Megaceros

Elaters of most liverworts and some hornworts are characterized by prominent spiral secondary walls. Investigation of elater development in two liverworts, Symphyogyna and Radula, and the hornwort Megaceros, showed similarities in the process of secondary wall formation but significant differences in secondary wall structure. Elaters elongate before secondary wall deposition and have a transverse microtubule array indicative of diffuse growth. After elongation, microtubules aggregate into spiral bands that predict the pattern of the future secondary wall: two narrow spirals in Symphyogyna and Radula and a single broad spiral in Megaceros. Secondary walls of liverwort elaters are composed of a single electron‐dense layer, but secondary walls of Megaceros are formed of three distinctive layers: an outer electron dense layer, a central fibrillar layer, and a thin inner layer. Elaters of liverworts and Megaceros stain with TUNEL, which indicates that they may undergo some aspects of programmed cell degeneration after secondary wall development.