Combination of surface and ‘vertical’ loop elements improves receive performance of a human head transceiver array at 9.4 T

Ultra‐high‐field (UHF, ≥7 T) human magnetic resonance imaging (MRI) provides undisputed advantages over low‐field MRI (≤3 T), but its development remains challenging because of numerous technical issues, including the low efficiency of transmit (Tx) radiofrequency (RF) coils caused by the increase in tissue power deposition with frequency. Tight‐fit human head transceiver (TxRx) arrays improve Tx efficiency in comparison with Tx‐only arrays, which are larger in order to fit multi‐channel receive (Rx)‐only arrays inside. A drawback of the TxRx design is that the number of elements in an array is limited by the number of available high‐power RF Tx channels (commonly 8 or 16), which is not sufficient for optimal Rx performance. In this work, as a proof of concept, we developed a method for increasing the number of Rx elements in a human head TxRx surface loop array without the need to move the loops away from a sample, which compromises the array Tx performance. We designed and constructed a prototype 16‐channel tight‐fit array, which consists of eight TxRx surface loops placed on a cylindrical holder circumscribing a head, and eight Rx‐only vertical loops positioned along the central axis (parallel to the magnetic field B₀) of each TxRx loop, perpendicular to its surface. We demonstrated both experimentally and numerically that the addition of the vertical loops has no measurable effect on the Tx efficiency of the array. An increase in the maximum local specific absorption rate (SAR), evaluated using two human head voxel models (Duke and Ella), measured 3.4% or less. At the same time, the 16‐element array provided 30% improvement of central signal‐to‐noise ratio (SNR) in vivo relative to a surface loop eight‐element array. The novel array design also demonstrated an improvement in the parallel Rx performance in the transversal plane. Thus, using this method, both the Rx and Tx performance of the human head array can be optimized simultaneously.     

A 22‐channel receive array with Helmholtz transmit coil for anesthetized macaque MRI at 3 T

The macaque monkey is an important model for cognitive and sensory neuroscience that has been used extensively in behavioral, electrophysiological, molecular and, more recently, neuroimaging studies. However, macaque MRI has unique technical differences relative to human MRI, such as the geometry of highly parallel receive arrays, which must be addressed to optimize imaging performance. A 22‐channel receive coil array was constructed specifically for rapid high‐resolution anesthetized macaque monkey MRI at 3 T. A local Helmholtz transmit coil was used for excitation. Signal‐to‐noise ratios (SNRs) and noise amplification for parallel imaging were compared with those of single‐ and four‐channel receive coils routinely used for macaque MRI. The 22‐channel coil yielded significant improvements in SNR throughout the brain. Using this coil, the SNR in peripheral brain was 2.4 and 1.7 times greater than that obtained with single‐ or four‐channel coils, respectively. In the central brain, the SNR gain was 1.5 times that of both the single‐ and four‐channel coils. Finally, the performance of the array for functional, anatomical and diffusion‐weighted imaging was evaluated. For all three modalities, the use of the 22‐channel array allowed for high‐resolution and accelerated image acquisition. Copyright © 2013 John Wiley & Sons, Ltd.     

Performance evaluation of a 32‐element head array with respect to the ultimate intrinsic SNR

The quality of an RF detector coil design is commonly judged on how it compares with other coil configurations. The aim of this article is to develop a tool for evaluating the absolute performance of RF coil arrays. An algorithm to calculate the ultimate intrinsic signal‐to‐noise ratio (SNR) was implemented for a spherical geometry. The same imaging tasks modeled in the calculations were reproduced experimentally using a 32‐element head array. Coil performance maps were then generated based on the ratio of experimentally measured SNR to the ultimate intrinsic SNR, for different acceleration factors associated with different degrees of parallel imaging. The relative performance in all cases was highest near the center of the samples (where the absolute SNR was lowest). The highest performance was found in the unaccelerated case and a maximum of 85% was observed with a phantom whose electrical properties are consistent with values in the human brain. The performance remained almost constant for 2‐fold acceleration, but deteriorated at higher acceleration factors, suggesting that larger arrays are needed for effective highly‐accelerated parallel imaging. The method proposed here can serve as a tool for the evaluation of coil designs, as well as a tool to guide the development of original designs which may begin to approach the optimal performance. Copyright © 2009 John Wiley & Sons, Ltd.     

Resonant inductive decoupling (RID) for transceiver arrays to compensate for both reactive and resistive components of the mutual impedance

Transceiver surface coil arrays improve transmit performance (B₁/√kW) and B₁ homogeneity for head imaging up to 9.4 T. To further improve reception performance and parallel imaging, the number of array elements must be increased with a corresponding decrease in their size. With a large number of small interacting antennas, decoupling is one of the most challenging aspects in the design and construction of transceiver arrays. Previously described decoupling techniques using geometric overlap, inductive or capacitive decoupling have focused on the elimination of the reactance of the mutual impedance only, which can limit the obtainable decoupling to –10 dB as a result of residual mutual resistance. A novel resonant inductive decoupling (RID) method, which allows compensation for both reactive and resistive components of the mutual impedance between the adjacent surface coils, has been developed and verified experimentally. This method provides an easy way to adjust the decoupling remotely by changing the resonance frequency of the RID circuit through the adjustment of a variable capacitor. As an example, a single‐row (1 × 16) 7‐T transceiver head array of n = 16 small overlapped surface coils using RID decoupling between adjacent coils was built. In combination with overlapped coils, the RID technique achieved better than –24 dB of decoupling for all adjacent coils. Copyright © 2013 John Wiley & Sons, Ltd.     

An orthogonal-based decoupling method for MRI phased array coil design

A new 2 T 3‐element orthogonal knee coil array based on the three‐dimensional orthogonality principle was designed, constructed and used in a series of pilot magnetic resonance imaging (MRI) studies on a standardized phantom, and human and pig knees. The coil elements within this new coil array are positioned orthogonal to one another allowing problematic mutual coupling effects to be minimized without the use of any passive mutual decoupling schemes. The proposed method is appropriate for the design of transmit, receive and/or transceive radiofrequency (RF) coil arrays for applications in animal/human MRI and spectroscopic studies. Experimental results demonstrated that the 3‐element orthogonal knee coil array could be angled arbitrarily, including at 90°, relative to the main static magnetic field (B₀) whilst maintaining normal operation with minimal loss of efficiency and functionality. Initial trials with a pig knee specimen further showed that the greatest signal intensity in the patellar ligament (parallel collagen fibres) was observed when the orthogonal knee coil array and the pig knee specimen were angled at ~55° to B₀, which may have potential uses in magic angle MR applications. Copyright © 2011 John Wiley & Sons, Ltd.     

Radial magnetic resonance imaging (MRI) using a rotating radiofrequency (RF) coil at 9.4 T

The rotating radiofrequency coil (RRFC) has been developed recently as an alternative approach to multi‐channel phased‐array coils. The single‐element RRFC avoids inter‐channel coupling and allows a larger coil element with better B₁ field penetration when compared with an array counterpart. However, dedicated image reconstruction algorithms require accurate estimation of temporally varying coil sensitivities to remove artefacts caused by coil rotation. Various methods have been developed to estimate unknown sensitivity profiles from a few experimentally measured sensitivity maps, but these methods become problematic when the RRFC is used as a transceiver coil. In this work, a novel and practical radial encoding method is introduced for the RRFC to facilitate image reconstruction without the measurement or estimation of rotation‐dependent sensitivity profiles. Theoretical analyses suggest that the rotation‐dependent sensitivities of the RRFC can be used to create a uniform profile with careful choice of sampling positions and imaging parameters. To test this new imaging method, dedicated electronics were designed and built to control the RRFC speed and hence positions in synchrony with imaging parameters. High‐quality phantom and animal images acquired on a 9.4 T pre‐clinical scanner demonstrate the feasibility and potential of this new RRFC method.     

Decoupling of a double‐row 16‐element tight‐fit transceiver phased array for human whole‐brain imaging at 9.4 T

One of the major challenges in constructing multi‐channel and multi‐row transmit (Tx) or transceiver (TxRx) arrays is the decoupling of the array's loop elements. Overlapping of the surface loops allows the decoupling of adjacent elements and also helps to improve the radiofrequency field profile by increasing the penetration depth and eliminating voids between the loops. This also simplifies the design by reducing the number of decoupling circuits. At the same time, overlapping may compromise decoupling by generating high resistive (electric) coupling near the overlap, which cannot be compensated for by common decoupling techniques. Previously, based on analytical modeling, we demonstrated that electric coupling has strong frequency and loading dependence, and, at 9.4 T, both the magnetic and electric coupling between two heavily loaded loops can be compensated at the same time simply by overlapping the loops. As a result, excellent decoupling was obtained between adjacent loops of an eight‐loop single‐row (1 × 8) human head tight‐fit TxRx array. In this work, we designed and constructed a 9.4‐T (400‐MHz) 16‐loop double‐row (2 × 8) overlapped TxRx head array based on the results of the analytical and numerical electromagnetic modeling. We demonstrated that, simply by the optimal overlap of array loops, a very good decoupling can be obtained without additional decoupling strategies. The constructed TxRx array provides whole‐brain coverage and approximately 1.5 times greater Tx efficiency relative to a transmit‐only/receive‐only (ToRo) array, which consists of a larger Tx‐only array and a nested tight‐fit 31‐loop receive (Rx)‐only array. At the same time, the ToRo array provides greater peripheral signal‐to‐noise ratio (SNR) and better Rx parallel performance in the head–feet direction. Overall, our work provides a recipe for a simple, robust and very Tx‐efficient design suitable for parallel transmission and whole‐brain imaging at ultra‐high fields.     

Comparison of RF body coils for MRI at 3 T: a simulation study using parallel transmission on various anatomical targets

The performance of multichannel transmit coil layouts and parallel transmission (pTx) RF pulse design was evaluated with respect to transmit B₁ (B₁ ⁺) homogeneity and specific absorption rate (SAR) at 3 T for a whole body coil. Five specific coils were modeled and compared: a 32‐rung birdcage body coil (driven either in a fixed quadrature mode or a two‐channel transmit mode), two single‐ring stripline arrays (with either 8 or 16 elements), and two multi‐ring stripline arrays (with two or three identical rings, stacked in the z axis and each comprising eight azimuthally distributed elements). Three anatomical targets were considered, each defined by a 3D volume representative of a meaningful region of interest (ROI) in routine clinical applications. For a given anatomical target, global or local SAR controlled pTx pulses were designed to homogenize RF excitation within the ROI. At the B₁ ⁺ homogeneity achieved by the quadrature driven birdcage design, pTx pulses with multichannel transmit coils achieved up to about eightfold reduction in local and global SAR. When used for imaging head and cervical spine or imaging thoracic spine, the double‐ring array outperformed all coils, including the single‐ring arrays. While the advantage of the double‐ring array became much less pronounced for pelvic imaging, with a substantially larger ROI, the pTx approach still provided significant gains over the quadrature birdcage coil. For all design scenarios, using the three‐ring array did not necessarily improve the RF performance. Our results suggest that pTx pulses with multichannel transmit coils can reduce local and global SAR substantially for body coils while attaining improved B₁ ⁺ homogeneity, particularly for a “z‐stacked” double‐ring design with coil elements arranged on two transaxial rings. Copyright © 2015 John Wiley & Sons, Ltd.     

Cardiac proton spectroscopy using large coil arrays

Large coil arrays are widely used in clinical routine for cardiovascular imaging providing extended spatial coverage and enabling accelerated acquisition using parallel imaging approaches. This work investigates the use of large coil arrays in single‐voxel cardiac spectroscopy for the detection of myocardial creatine and triglyceride content. For this purpose, a navigator‐gated and cardiac‐triggered point‐resolved spectroscopy sequence was implemented, and data obtained in 11 healthy volunteers using 32‐ and 5‐element coil arrays were compared. For combination of the individual coil element signals, four strategies were evaluated differing in the manner of estimation of the complex coil weights and the amount of additional information required for coil combination. In all volunteers, and with both the 32‐ and 5‐channel coil arrays, triglyceride‐to‐water (0.44 ± 0.19% and 0.45 ± 0.17%) and total creatine‐to‐water (0.05 ± 0.02% and 0.05 ± 0.01%) contents were computed. The values were found to agree well, showing an intraclass correlation coefficient of 0.76 (p < 0.003). The results revealed a gain in signal‐to‐noise ratio of approximately 24% with the 32‐channel coil relative to the 5‐channel array. The findings may foster the integration of cardiac spectroscopy into clinical practice using large coil arrays, provided that appropriate reconstruction algorithms are implemented. Copyright © 2012 John Wiley & Sons, Ltd.     

An embedded four‐channel receive‐only RF coil array for fMRI experiments of the somatosensory pathway in conscious awake marmosets

fMRI has established itself as the main research tool in neuroscience and brain cognitive research. The common marmoset (Callithrix jacchus) is a non‐human primate model of increasing interest in biomedical research. However, commercial MRI coils for marmosets are not generally available. The present work describes the design and construction of a four‐channel receive‐only surface RF coil array with excellent signal‐to‐noise ratio (SNR) specifically optimized for fMRI experiments in awake marmosets in response to somatosensory stimulation. The array was designed as part of a helmet‐based head restraint system used to prevent motion during the scans. High SNR was obtained by building the coil array using a thin and flexible substrate glued to the inner surface of the restraint helmet, so as to minimize the distance between the array elements and the somatosensory cortex. Decoupling between coil elements was achieved by partial geometrical overlapping and by connecting them to home‐built low‐input‐impedance preamplifiers. In vivo images show excellent coverage of the brain cortical surface with high sensitivity near the somatosensory cortex. Embedding the coil elements within the restraint helmet allowed fMRI data in response to somatosensory stimulation to be collected with high sensitivity and reproducibility in conscious, awake marmosets. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

Improved SNR of phased-array PERES coils via simulation study

A computational comparison of signal-to-noise ratio (SNR) was performed between a conventional phased array of two circular-shaped coils and a petal resonator surface array. The quasi-static model and phased-array optimum SNR were combined to derive an SNR formula for each array. Analysis of mutual inductance between coil petals was carried out to compute the optimal coil separation and optimum number of petal coils. Mutual interaction between coil arrays was not included in the model because this does not drastically affect coil performance. Phased arrays of PERES coils show a 114% improvement in SNR over that of the simplest circular configuration.     

A dedicated eight‐channel receive RF coil array for monkey brain MRI at 9.4 T

The neuroimaging of nonhuman primates (NHPs) realised with magnetic resonance imaging (MRI) plays an important role in understanding brain structures and functions, as well as neurodegenerative diseases and pathological disorders. Theoretically, an ultrahigh field MRI (≥7 T) is capable of providing a higher signal‐to‐noise ratio (SNR) for better resolution; however, the lack of appropriate radiofrequency (RF) coils for 9.4 T monkey MRI undermines the benefits provided by a higher field strength. In particular, the standard volume birdcage coil at 9.4 T generates typical destructive interferences in the periphery of the brain, which reduces the SNR in the neuroscience‐focused cortex region. Also, the standard birdcage coil is not capable of performing parallel imaging. Consequently, extended scan durations may cause unnecessary damage due to overlong anaesthesia. In this work, assisted by numerical simulations, an eight‐channel receive RF coil array was specially designed and manufactured for imaging NHPs at 9.4 T. The structure and geometry of the proposed receive array was optimised with numerical simulations, so that the SNR enhancement region was particularly focused on monkey brain. Validated with rhesus monkey and cynomolgus monkey brain images acquired from a 9.4 T MRI scanner, the proposed receive array outperformed standard birdcage coil with higher SNR, mean diffusivity and fractional anisotropy values, as well as providing better capability for parallel imaging.     

An eight‐channel sodium/proton coil for brain MRI at 3 T

The purpose of this work is to illustrate a new coil decoupling strategy and its application to a transmit/receive sodium/proton phased array for magnetic resonance imaging (MRI) of the human brain. We implemented an array of eight triangular coils that encircled the head. The ensemble of coils was arranged to form a modified degenerate mode birdcage whose eight shared rungs were offset from the z‐axis at interleaved angles of ±30°. This key geometric modification resulted in triangular elements whose vertices were shared between next‐nearest neighbors, which provided a convenient location for counter‐wound decoupling inductors, whilst nearest‐neighbor decoupling was addressed with shared capacitors along the rungs. This decoupling strategy alleviated the strong interaction that is characteristic of array coils at low frequency (32.6 MHz in this case) and allowed the coil to operate efficiently in transceive mode. The sodium array provided a 1.6‐fold signal‐to‐noise ratio advantage over a dual‐nuclei birdcage coil in the center of the head and up to 2.3‐fold gain in the periphery. The array enabled sodium MRI of the brain with 5‐mm isotropic resolution in approximately 13 min, thus helping to overcome low sodium MR sensitivity and improving quantification in neurological studies. An eight‐channel proton array was integrated into the sodium array to enable anatomical imaging.     

Analytical modeling provides new insight into complex mutual coupling between surface loops at ultrahigh fields

Ultrahigh‐field (UHF) (≥7 T) transmit (Tx) human head surface loop phased arrays improve both the Tx efficiency (B ₁⁺/√P ) and homogeneity in comparison with single‐channel quadrature Tx volume coils. For multi‐channel arrays, decoupling becomes one of the major problems during the design process. Further insight into the coupling between array elements and its dependence on various factors can facilitate array development. The evaluation of the entire impedance matrix Z for an array loaded with a realistic voxel model or phantom is a time‐consuming procedure when performed using electromagnetic (EM) solvers. This motivates the development of an analytical model, which could provide a quick assessment of the Z‐matrix. In this work, an analytical model based on dyadic Green's functions was developed and validated using an EM solver and bench measurements. The model evaluates the complex coupling, including both the electric (mutual resistance) and magnetic (mutual inductance) coupling. Validation demonstrated that the model does well to describe the coupling at lower fields (≤3 T). At UHFs, the model also performs well for a practical case of low magnetic coupling. Based on the modeling, the geometry of a 400‐MHz, two‐loop transceiver array was optimized, such that, by simply overlapping the loops, both the mutual inductance and the mutual resistance were compensated at the same time. As a result, excellent decoupling (below ?40 dB) was obtained without any additional decoupling circuits. An overlapped array prototype was compared (signal‐to‐noise ratio, Tx efficiency) favorably to a gapped array, a geometry which has been utilized previously in designs of UHF Tx arrays.     

Size‐adaptable 13‐channel receive array for brain MRI in human neonates at 3 T

Neonatal brain injury suffered by preterm infants and newborns with some medical conditions can cause significant neurodevelopmental disabilities. MRI is a preferred method to detect these accidents and perform in vivo evaluation of the brain. However, the commercial availability and optimality of receive coils for the neonatal brain is limited, which in many cases leads to images lacking in quality. As extensively demonstrated, receive arrays closely positioned around the scanned part provide images with high signal‐to‐noise ratios (SNRs). The present work proposes a pneumatic‐based MRI receive array that can physically adapt to infant head dimensions from 27‐week premature to 1.5 months old. Average SNR increases of up to 68% in the head region and 122% in the cortex region, compared with a 32‐channel commercial head coil, were achieved at 3 T. The consistent SNR distribution obtained through the complete coil size range, specifically in the cortex, allows the acquisition of images with similar quality across a range of head dimensions, which is not possible with fixed‐size coils due to the variable coil‐to‐head distance. The risks associated with mechanical pressure on the neonatal head are minimal and the head motion is restricted. The method could be used in coil designs for other age groups, body parts and subjects.     

Inherently decoupled 1H antennas and 31P loops for metabolic imaging of liver metastasis at 7 T

High field ³¹P spectroscopy has thus far been limited to diffuse liver disease. Unlike lower field‐strength scanners, there is no body coil in the bore of the 7 T and despite inadequate penetration depth (<10 cm), surface coils are the current state‐of‐the‐art for acquiring anatomical images to support multinuclear studies. We present a system of proton antennas and phosphorus loops for ³¹P spectroscopy and provide the first ultrahigh‐field phosphorus metabolic imaging of a tumor in the abdomen. Herein we characterize the degree to which antennas are isolated from underlying loops. Next, we evaluate the penetration depth of the two antennas available during multinuclear examinations. Finally, we combine phosphorus spectroscopy (two loops) with parallel transmit imaging (eight antennas) in a patient. The loops and antennas are inherently decoupled (no added circuitry, <0.1% power coupling). The penetration depth of two antennas gives twice that of conventional loops. The liver and full axial slice of the abdomen were imaged with eight transmit/receive antennas using parallel transmit B1‐shimming to overcome image voids. Phosphorus spectroscopy from a liver metastasis resolved individual peaks for phosphocholine and phosphoethenalomine. Proton antennas are inherently decoupled from phosphorus loops. By using two proton antennas it is possible to perform region‐of‐interest image‐based shimming in over 80% of the liver volume, thereby enabling phosphorus spectroscopy of localized disease. Shimming of the full extent of the abdominal cross‐section is feasible using a parallel transmit array of eight antennas. A system architecture capable of supporting eight‐channel parallel transmit and multinuclear spectroscopy is optimal for supporting multiparametric body imaging, including metabolic imaging, for monitoring the response of patients with liver metastases to cancer treatments and for patient risk stratification. In the meantime, the existing infrastructure using two antennas is sufficient for preliminary studies in metabolic imaging of tumors in the liver.     

Noise correlations and SNR in phased‐array MRS

The acquisition of magnetic resonance spectroscopy (MRS) signals by multiple receiver coils can improve the signal‐to‐noise ratio (SNR) or alternatively can reduce the scan time maintaining a reliable SNR. However, using phased array coils in MRS studies requires efficient data processing and data combination techniques in order to exploit the sensitivity improvement of the phased array coil acquisition method. This paper describes a novel method for the combination of MRS signals acquired by phased array coils, even in presence of correlated noise between the acquisition channels. In fact, although it has been shown that electric and magnetic coupling mechanisms produce correlated noise in the coils, previous algorithms developed for MRS data combination have ignored this effect. The proposed approach takes advantage of a noise decorrelation stage to maximize the SNR of the combined spectra. In particular Principal Component Analysis (PCA) was exploited to project the acquired spectra in a subspace where the noise vectors are orthogonal. In this subspace the SNR weighting method will provide the optimal overall SNR. Performance evaluation of the proposed method is carried out on simulated ¹H‐MRS signals and experimental results are obtained on phantom ¹H‐MR spectra using a commercially available 8‐element phased array coil. Noise correlations between elements were generally low due to the optimal coil design, leading to a fair SNR gain (about 0.5%) in the center of the field of view (FOV). A greater SNR improvement was found in the peripheral FOV regions. Copyright © 2009 John Wiley & Sons, Ltd.     

An inverse method to design RF coil arrays optimized for SENSE imaging

A new method to design MRI RF coils that are optimized for SENSE (sensitivity encoding) imaging is introduced. In this approach, the inverse problem was solved where the surface current density distribution on a coil former was calculated to maximize the SNR(sense) within a volume of interest (VOI). For that purpose, an analytic relationship was formulated between the SNR(sense) and surface current density on the coil former. The SNR at pixel rho in a SENSE-MR image, SNR(sense,rho), is inversely proportional to the g-factor: therefore, the g-factor was formulated in terms of the B1 distribution of the coils. Then, by specifying the geometry of the desired coil former and using a finite element mesh (FEM), the surface current distribution was calculated to maximize the SNR(sense), by minimizing (1/SNR(sense)) in the VOI using a least squares procedure. A simple two-coil array was designed and built to test the method and phantom images were collected. The results show that the new coil design method yielded better uniformity and SNR in SENSE images compared to those of standard coils.     

Small‐animal,whole‐body imaging with metamaterial‐inspired RF coil

Particular applications in preclinical magnetic resonance imaging require the entire body of an animal to be imaged with sufficient quality. This is usually performed by combining regions scanned with small coils with high sensitivity or long scans using large coils with low sensitivity. Here, a metamaterial‐inspired design employing a parallel array of wires operating on the principle of eigenmode hybridization was used to produce a small‐animal imaging coil. The coil field distribution responsible for the coil field of view and sensitivity was simulated in an electromagnetic simulation package and the coil geometrical parameters were optimized for whole‐body imaging. A prototype coil was then manufactured and assembled using brass telescopic tubes with copper plates as distributed capacitance. Its field distribution was measured experimentally using the B₁⁺ mapping technique and was found to be in close correspondence with the simulated results. The coil field distribution was found to be suitable for large field of view small‐animal imaging and the coil image quality was compared with a commercially available coil by whole‐body scanning of living mice. Signal‐to‐noise measurements in living mice showed higher values than those of a commercially available coil with large receptive fields, and rivalled the performance of small receptive field and high‐sensitivity coils. The coil was deemed to be suitable for some whole‐body, small‐animal preclinical applications.