Array RF Research Articles

Feasibility study of a new RF coil design for prostate MRI

The combined use of a torso-pelvic RF array coil and endorectal RF coil is the current state-of-the-art in prostate MRI. The endorectal coil provides high detection sensitivity to acquire high-spatial resolution images and spectroscopic data, while the torso-pelvic coil provides large coverage to assess pelvic lymph nodes and pelvic bones for metastatic disease. However, the use of an endorectal coil is an invasive procedure that presents difficulties for both patients and technicians. In this study, we propose a novel non-invasive RF coil design that can provide both image signal to noise ratio and field of view coverage comparable to the combined torso-pelvic and endorectal coil configuration. A prototype coil was constructed and tested using a pelvic phantom. The results demonstrate that this new design is a viable alternative for prostate MRI

Automated tuning of an eight‐channel cardiac transceive array at 7 tesla using piezoelectric actuators

PurposeUltra-high field (UHF) MR scanning in the body requires novel coil designs due to B1 field inhomogeneities. In the transverse electromagnetic field (TEM) design, maximum B1 transmit power can only be achieved if each individual transmit element is tuned and matched for different coil loads, which requires a considerable amount of valuable scanner time.MethodsAn integrated system for autotuning a multichannel parallel transmit (pTx) cardiac TEM array was devised, using piezoelectric actuators, power monitoring equipment and control software. The reproducibility and performance of the system were tested and the power responses of the coil elements were profiled. An automated optimization method was devised and evaluated.ResultsThe time required to tune an eight-element pTx cardiac RF array was reduced from a mean of 30 min to less than 10 min with the use of this system.ConclusionPiezoelectric actuators are an attractive means of tuning RF coil arrays to yield more efficient B1 transmission into the subject. An automated mechanism for tuning these elements provides a practical solution for cardiac imaging at UHF, bringing this technology closer to clinical use. Magn Reson Med 73:2390–2397, 2015. © 2014 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.

Quantitative 23Na MRI of human knee cartilage using dual‐tuned 1H/23Na transceiver array radiofrequency coil at 7 tesla

To develop quantitative dual-tuned (DT) (1) H/(23) Na MRI of human knee cartilage in vivo at 7 Tesla (T). A sensitive (23) Na transceiver array RF coil was developed at 7T. B1 fields generated by the transceiver array coil were characterized and corrected in the (23) Na images. Point spread function (PSF) of the (23) Na images was measured, and the signal decrease due to partial-volume-effect was compensated in [(23) Na] quantification of knee cartilage. SNR and [(23) Na] in anterior femoral cartilage were measured from seven healthy subjects. SNR of (23) Na image with the transceiver array coil was higher than that of birdcage coil. SNR in the cartilage at 2-mm isotropic resolution was 26.80 ± 3.69 (n = 7). B1 transmission and reception fields produced by the DT coil at 7T were similar to each other. Effective full-width-half-maximum of (23) Na image was ∼5 mm at 2-mm resolution. Mean [(23) Na] was 288.13 ± 29.50 mM (n = 7) in the anterior femoral cartilage of normal subjects. We developed a new high-sensitivity (23) Na RF coil for knee MRI at 7T. Our (1) H/(23) Na MRI allowed quantitative measurement of [(23) Na] in knee cartilage by measuring PSF and cartilage thickness from (23) Na and (1) H image, respectively.

Dense Delta-Sigma Phased Arrays

The high cost of high-resolution phase shifters required to maintain precise control over the array beam pattern in traditional phased arrays preclude their use in a variety of emerging millimeter-wave applications. We develop a phased array architecture that obviates the need for such precise phase shifters, based on the use of sub-half-wavelength array element spacing and novel spatial domain delta-sigma processing. We characterize the performance of this architecture in terms of the array signal-to-quantization-noise ratio (SQNR) and the array power transfer efficiency, and demonstrate a tradeoff between these two metrics. As an illustrative design, we show that when constrained to two-bit phase shifters, a four-fold increase in the array density can provide a roughly 6 dB improvement in SQNR over standard design techniques, with an average efficiency loss of less than 1.5 dB with respect to a perfectly tuned ideal array. In our analysis, we account for the effects of mutual coupling, and describe a simple, practical impedance matching network for this architecture. The resulting framework allows a system designer with a given set of circuit, device, and antenna fabrication and integration technologies to choose from a spectrum of tradeoffs between array density and RF component complexity.

A Cost Effective Smart Antenna for Mobile Broadband Systems

A cost effective implementation of a ‘Smart Antenna’ is limited due to the number and high cost of RF array elements, RF distribution as well as phasing elements. Shaped Dielectric Lenses perform the task of collimation, to obtain multiple beams in a fixed set of directions. Lenses are inherently broadband, lower in cost, easy to fabricate, have lower dimensional tolerances, and act as a radome for the electrically small primary radiators that are embedded inside or placed behind the lens.

Precompensation for mutual coupling between array elements in parallel excitation.

Parallel transmission or excitation has been suggested to perform multi-dimensional spatial selective excitation to shorten the pulse width using a coil array and the sensitivity information. The mutual coupling between array elements has been a critical technical issue in RF array designs, which can cause artifacts on the excitation profile, leading to degraded excitation performance and image quality. In this work, a precompensation method is proposed to address the mutual coupling effect in parallel transmission by introducing the mutual coupling coefficient matrix into the RF pulses design procedure of the parallel transmission. 90° RF pulses have been designed using both the original transmit SENSE method and the proposed precompensation method for RF arrays with non-negligible mutual coupling, and their excitation profiles are generated by simulating the Bloch equation. The results show that the mutual coupling effect can be effectively compensated by using the proposed method, yielding enhanced tolerance to insufficient mutual decoupling of RF arrays in parallel excitation, ultimately, providing improved performance and accuracy of parallel excitation.

ICE decoupling technique for RF coil array designs

Parallel magnetic resonance imaging (MRI) requires an array of RF coil elements with different sensitivity distributions and with minimal electromagnetic coupling. The goal of this project was to develop a new method based on induced current compensation or elimination (ICE) for improved coil element decoupling and to investigate its performance in phantom MR images. An electromagnetic decoupling method based on induced current compensation or elimination for nonoverlapping RF coil arrays was developed with the design criteria of high efficiency, easy implementation, and no physical connection to RF array elements. An eigenvalue/eigenvector approach was employed to analyze the decoupling mechanism and condition. A two-channel microstrip array and an eight-channel coil array were built to test the performance of the method. Following workbench tests, MR imaging experiments were performed on a 7T MR scanner. The bench tests showed that both arrays achieved sufficient decoupling with a S21 less than -25 dB among the coil elements at 298 MHz. The MR phantom images demonstrated well-defined sensitivity distributions from each coil element and the unique decoupling capability of the proposed ICE decoupling technique. B1 distributions of the individual elements were also measured and calculated. The theoretical analysis and experiments demonstrated the feasibility of the decoupling method for high field RF coil array designs without overlapping or direct physical connections between coil elements, which provide more flexibility for coil array design and optimization. The method offers a new approach to address the RF array decoupling issue, which is a major challenge in implementing parallel imaging.

Helmholtz-pair transmit coil with integrated receive array for high-resolution MRI of trabecular bone in the distal tibia at 7 T

A Helmholtz-pair local transmit RF coil with an integrated four-element receive array RF coil and foot immobilization platform was designed and constructed for imaging the distal tibia in a whole-body 7 T MRI scanner. Simulations and measurements of the B 1 field distribution of the transmit coil are described, along with SAR considerations for operation at 7 T. Results of imaging the trabecular bone of three volunteers at 1.5 T, 3 T and 7 T are presented, using identical 1.5 T and 3 T versions of the 7 T four-element receive array. The spatially registered images reveal improved visibility for individual trabeculae and show average gains in SNR of 2.8× and 4.9× for imaging at 7 T compared to 3 T and 1.5 T, respectively. The results thus display an approximately linear dependence of SNR with field strength and enable the practical utility of 7 T scanners for micro-MRI of trabecular bone.

Z-axis Interconnections for Next Generation Packaging

In this paper, the use of electrically conducting adhesives (ECA) to form z-axis interconnections for next generation packaging is discussed. In particular, current efforts related to Z-axis interconnections for device level fabrication, integration, and electrical performance are highlighted. A few optimized ECAs were used for hole fill applications to fabricate Z-axis interconnections in laminates. Conductive joints were formed during composite lamination using the ECA. Around 5,000 to 200,000 through holes in the joining cores, formed by laser or mechanical drilling, and having diameters ranging from 50 μm to 750 μm, were filled with an optimized conducting adhesive. The adhesive-filled joining cores/layers were laminated with circuitized subcomposites to produce a composite structure. As a case study, a variety of z-axis interconnect constructions for a flip-chip plastic ball grid array package, rigid-flex, rigid-rigid, package-interposer-package (PIP), RF structures, and PWBs were fabricated and evaluated at both the subcomposite and composite levels to understand structural and electrical integrity. Electrically, S-parameter measurements showed very low loss at multi-gigahertz frequencies. The losses were low enough to support typical SERDES up to 15 Gbps over 750 mm. The present process allows fabrication of z-interconnect conductive joints having diameters in the range of 55 to 500 μm. The processes and materials used to achieve smaller feature dimensions, satisfy stringent registration requirements, and achieve robust electrical interconnections are discussed.

A radiofrequency coil configuration for imaging the human vertebral column at 7 T

We describe the design and testing of a quadrature transmit, eight-channel receive array RF coil configuration for the acquisition of images of the entire human spinal column at 7T. Imaging parameters were selected to enable data acquisition in a clinically relevant scan time. Large field-of-view (FOV) scanning enabled sagittal imaging of the spine in two or three-stations, depending upon the height of the volunteer, with a total scan time of between 10 and 15min. A total of 10 volunteers have been scanned, with results presented for the three subjects spanning the range of heights and weights, namely one female (1.6m, 50kg), one average male (1.8m, 70kg), and one large male (1.9m, 100kg).

DEVELOPMENT OF 61-CHANNEL DIGITAL BEAMFORMING (DBF) TRANSMITTER ARRAY FOR MOBILE SATELLITE COMMUNICATION

An 61-channel digital beamforming transmitter array antenna used in mobile satellite communication is presented in this paper. It can steer 16 beams simultaneously. The interface, architecture and subunits are described in detail. Standard hexagonal array (SHA) and 61-channel RF front ends are designed. Genetic Algorithm is adopted to realize the pattern synthesis with muti- objective optimization. The signal ∞ow and hardware platform of the digital beamforming network are discussed, which can complete the high speed array signal processing with maximum throughput of 34.16Gbps. A novel calibration scheme with high feasibility of project implementation is also proposed. The measurements of array antenna match well with simulation result, which validates the rationality and feasibility of the algorithms.

SU‐GG‐J‐134: MRI‐Guided Adaptive Brachytherapy of Cervical Cancer: Initial Experience with High Resolution Diffusion Imaging

Purpose: MRI has the potential to provide wealthy information related to tumor biology and function, and therefore is the idea imaging modality to provide accurate guidance for radiotherapy. The purpose of this study is to develop and integrate a high resolution diffusion imaging protocol into clinical treatment planning for image‐guided adaptive brachytherapy of cervical cancer. Method and Materials: MR imaging was performed on a clinical 1.5T system using a four‐element phased array RF coil. Patients who underwent routine MRI simulation in conjunction with their weekly brachytherapy treatments were included in the study. Brachytherapy was performed with MRI‐compatible tandem and ovoid applicators, and MRI was performed right after placement of the applicators, but before the treatment. In additional to T2‐weighted TSE sequences for routine treatment planning purposes, high resolution diffusion‐weighted images were also acquired. Diffusion images in the form of apparent diffusion coefficient (ADC) maps were computed, and evaluated along with treatment planning using Brachyvision. The add‐on values of ADC maps for treatment planning were assessed. Results: Diffusion imaging was successfully performed on all the selected patients using DW‐EPI sequence without obvious distortion artifacts from the applicators. Overall image qualities were analyzed, and the diffusion gradient b‐value for optimal tumor delineation was found to be 800 to 1000. When b‐value increased, the signal contrast between the tumor and adjacent normal tissue became stronger, however, image became noisier. With the optimized protocol, ADC maps provided better tumor delineation than T2‐weighted images for treatment plan optimization. Conclusion: High resolution diffusion imaging provided physiological information for tumor and normal tissue, which is less achievable by other imaging modalities. The ADC maps offered excellent image sets for not only depicting the tumor extend for adaptive treatment optimization, but also assessing the overall tissue physiological changes during the treatment course.

Sounding rocket flight experiment for demonstrating “Furoshiki Satellite” for large phased array antenna

University of Tokyo and Kobe University are planning a sounding rocket experiment of large membrane “Furoshiki Satellite” extension and large phased array RF transmission. The paper will describe the concept of “Furoshiki Satellite,” its application to solar power satellite, and the scenario of micro-gravity experiment using a small sounding rocket. University of Tokyo has been proposing the idea of “Furoshiki Satellite,” a large membrane or a net structure, say 1 km × 1 km in size, extended by satellites which hold its corners. The attitude and the shape of the membrane or net structure is controlled by these corner satellites. As one application of Furoshiki Satellite, a large solar power satellite can be configured by several solar cells and RF transmitters placed on several parts of the large net structure. It is difficult to control the position and attitude of the RF transmitters precisely, but using the “retro-directive” method, the tolerance of such position and attitude disturbance will be relaxed by large. This is one of promising systems’ concept of the future large solar power satellite or large antenna, because quite a large area can be obtained without any hard structure, and the weight will not depend very much on the size. To demonstrate the feasibility of the extension of large net structure and phased array performance, micro-gravity experiment is planned using a sounding rocket of JAXA/ISAS, Japan.

Large membrane “Furoshiki Satellite” applied to phased array antenna and its sounding rocket experiment

University of Tokyo and Kobe University are planning a sounding rocket experiment of large membrane “Furoshiki Satellite” extension and large phased array RF transmission. The paper will describe the concept of “Furoshiki Satellite,” its application to phased array antenna, and the scenario of micro gravity experiment using a small sounding rocket. University of Tokyo has been proposing the idea of “Furoshiki Satellite,” a large membrane or a net structure, say 1 km × 1 km in size, extended by satellites which hold its corners. The attitude and the shape of the membrane or net structure is controlled by these corner satellites. As one application of Furoshiki Satellite, a large phased array antenna can be configured by several RF transmitters placed on several parts of the large net structure. It is difficult to control the position and attitude of the RF transmitters precisely, but using the “retro-directive” method, the tolerance of such position and attitude disturbance will be relaxed by large. This is one of promising systems’ concept of the future large solar power satellite or large antenna, because quite a large area can be obtained without any hard structure, and the weight will not depend very much on the size [S. Motohashi, T. Nagamura, Large scaled membrane structure Furoshiki Satellite—its concept and orbital/attitude dynamics, in: Proceedings of 20th International Symposium on Space Technology and Science (ISTS), 1996, p. 96-n-14]. To demonstrate the feasibility of the extension of large net structure and phased array performance, micro-gravity experiment is planned using a sounding rocket of ISAS/JAXA, Japan.

An RF phased array applicator designed for hyperthermia breast cancer treatments

An RF phased array applicator has been constructed for hyperthermia treatments in the intact breast. This RF phased array consists of four antennas mounted on a Lexan water tank, and geometric focusing is employed so that each antenna points in the direction of the intended target. The operating frequency for this phased array is 140 MHz. The RF array has been characterized both by electric field measurements in a water tank and by electric field simulations using the finite-element method. The finite-element simulations are performed with HFSS software, where the mesh defined for finite-element calculations includes the geometry of the tank enclosure and four end-loaded dipole antennas. The material properties of the water tank enclosure and the antennas are also included in each simulation. The results of the finite-element simulations are compared to the measured values for this configuration, and the results, which include the effects of amplitude shading and phase shifting, show that the electric field predicted by finite-element simulations is similar to the measured field. Simulations also show that the contributions from standing waves are significant, which is consistent with measurement results. Simulated electric field and bio-heat transfer results are also computed within a simple 3D breast model. Temperature simulations show that, although peak temperatures are generated outside the simulated tumour target, this RF phased array applicator is an effective device for regional hyperthermia in the intact breast.

Functional MRI using regularized parallel imaging acquisition

Parallel MRI techniques reconstruct full-FOV images from undersampled k-space data by using the uncorrelated information from RF array coil elements. One disadvantage of parallel MRI is that the image signal-to-noise ratio (SNR) is degraded because of the reduced data samples and the spatially correlated nature of multiple RF receivers. Regularization has been proposed to mitigate the SNR loss originating due to the latter reason. Since it is necessary to utilize static prior to regularization, the dynamic contrast-to-noise ratio (CNR) in parallel MRI will be affected. In this paper we investigate the CNR of regularized sensitivity encoding (SENSE) acquisitions. We propose to implement regularized parallel MRI acquisitions in functional MRI (fMRI) experiments by incorporating the prior from combined segmented echo-planar imaging (EPI) acquisition into SENSE reconstructions. We investigated the impact of regularization on the CNR by performing parametric simulations at various BOLD contrasts, acceleration rates, and sizes of the active brain areas. As quantified by receiver operating characteristic (ROC) analysis, the simulations suggest that the detection power of SENSE fMRI can be improved by regularized reconstructions, compared to unregularized reconstructions. Human motor and visual fMRI data acquired at different field strengths and array coils also demonstrate that regularized SENSE improves the detection of functionally active brain regions.

Array Calibration for CDMA Smart Antenna Systems

In this paper, we investigate array calibration algorithms to derive a further improved version for correcting antenna array errors and RF transceiver errors in CDMA smart antenna systems. The structure of a multi-channel RF transceiver with a digital calibration apparatus and its calibration techniques are presented, where we propose a new RF receiver calibration scheme to minimize interference of the calibration signal on the user signals. The calibration signal is injected into a multi-channel receiver through a calibration signal injector whose array response vector is controlled in order to have a low correlation with the antenna response vector of the receive signals. We suggest a model-based antenna array calibration to remove the antenna array errors including mutual coupling errors or to predict the element patterns from the array manifold measured at a small number of angles. Computer simulations and experiment results are shown to verify the calibration algorithms. Keywords: Array calibration, antenna array errors, RF transceiver error, CDMA, smart antennas, multi-channel RF transceiver, model based antenna array calibration.

Digitalization decoupling method and its application to the phased array in MRI *

A general network decoupling method for phased array radio frequency coil used in magnetic resonance imaging is developed. A more flexible digital approach further simplifies the method, which has an even higher signal-to-noise ratio. The performance of the digitalized decoupling method was confirmed by the calculated result by using a high frequency structure simulation software with the method of finite element for a tree-port phased array RF coil. The consistence was well established between the theoretical and simulated results.

Simulation-based investigation of partially parallel imaging with a linear array at high accelerations.

Partially parallel imaging strategies such as SMASH, SENSE, and PILS rely on the sensitivity distribution of phased array RF coils to reduce MRI imaging time. Using an N-element phased array, these techniques allow maximum accelerations, L, such that L < or = N, with acceleration defined as the factor by which scan time is reduced in comparison to traditional, fully gradient encoded acquisitions. As N increases in modern MRI facilities or using special hardware extensions, its role as the primary limitation in partially parallel imaging will be reduced and other limiting factors will become dominant. Two such factors include available SNR and the variation of sensitivity distributions with imaging depth. Simulations have been conducted to evaluate the impact of slice depth and noise on partially parallel reconstructions for the case of a square linear array of overlapped elements that are parallel to the imaging plane. Results indicate that even when sensitivity distributions are exactly known, the linear surface array can only provide high accelerations over a limited imaging depth due to changing suitability of the sensitivity distributions for partially parallel reconstruction. This work emphasizes the importance of simulations for target-based partially parallel array design.

Phased array imaging on a 4.7T/33cm animal research system

Although phased array technology has been standard on clinical magnetic resonance imaging (MRI) systems for many years, it is just now becoming available on small animal MRI systems. Bruker Instruments Ltd. has recently provided multiple rf channels on a 4.7T/33cm Avance, but further development was necessary to complete the phased array implementation. This work discusses the development of this other hardware, specifically the rf array coil, low impedance preamplifiers, excitation coil, and image combination algorithm. Comparison of the array coil to a quadrature coil indicates superior signal-to-noise ratio from the array. In vivo images of a cat spine acquired simultaneously from the individual channels of the array and a sum of squares combination are shown.