On the Contribution of Curl‐Free Current Patterns to the Ultimate Intrinsic Signal‐to‐Noise Ratio at Ultra‐High Field Strength

The ultimate intrinsic signal‐to‐noise ratio (SNR) is a coil independent performance measure to compare different receive coil designs. To evaluate this benchmark in a sample, a complete electromagnetic basis set is required. The basis set can be obtained by curl‐free and divergence‐free surface current distributions, which excite linearly independent solutions to Maxwell's equations. In this work, we quantitatively investigate the contribution of curl‐free current patterns to the ultimate intrinsic SNR in a spherical head‐sized model at 9.4 T. Therefore, we compare the ultimate intrinsic SNR obtained with having only curl‐free or divergence‐free current patterns, with the ultimate intrinsic SNR obtained from a combination of curl‐free and divergence‐free current patterns. The influence of parallel imaging is studied for various acceleration factors. Moreover results for different field strengths (1.5 T up to 11.7 T) are presented at specific voxel positions and acceleration factors. The full‐wave electromagnetic problem is analytically solved using dyadic Green's functions. We show, that at ultra‐high field strength (B₀?7T) a combination of curl‐free and divergence‐free current patterns is required to achieve the best possible SNR at any position in a spherical head‐sized model. On 1.5‐ and 3T platforms, divergence‐free current patterns are sufficient to cover more than 90% of the ultimate intrinsic SNR.     

Experimental verification of SNR and parallel imaging improvements using composite arrays

Composite MRI arrays consist of triplets where two orthogonal upright loops are placed over the same imaging area as a standard surface coil. The optimal height of the upright coils is approximately half the width for the 7 cm coils used in this work. Resistive and magnetic coupling is shown to be negligible within each coil triplet. Experimental evaluation of imaging performance was carried out on a Philips 3 T Achieva scanner using an eight‐coil composite array consisting of three surface coils and five upright loops, as well as an array of eight surface coils for comparison. The composite array offers lower overall coupling than the traditional array. The sensitivities of upright coils are complementary to those of the surface coils and therefore provide SNR gains in regions where surface coil sensitivity is low, and additional spatial information for improved parallel imaging performance. Near the surface of the phantom the eight‐channel surface coil array provides higher overall SNR than the composite array, but this advantage disappears beyond a depth of approximately one coil diameter, where it is typically more challenging to improve SNR. Furthermore, parallel imaging performance is better with the composite array compared with the surface coil array, especially at high accelerations and in locations deep in the phantom. Composite arrays offer an attractive means of improving imaging performance and channel density without reducing the size, and therefore the loading regime, of surface coil elements. Additional advantages of composite arrays include minimal SNR loss using root‐sum‐of‐squares combination compared with optimal, and the ability to switch from high to low channel density by merely selecting only the surface elements, unlike surface coil arrays, which require additional hardware. Copyright © 2014 John Wiley & Sons, Ltd.     

Uncompromised MRI of knee cartilage while incorporating sensitive sodium MRI

Sodium imaging is able to assess changes in ion content, linked to glycosaminoglycan content, which is important to guide orthopeadic procedures such as articular cartilage repair. Sodium imaging is ideally performed using double tuned RF coils, to combine high resolution morphological imaging with biochemical information from sodium imaging to assess ion content. The proton image quality of such coils is often harshly degraded, with up to 50% of SNR or severe acceleration loss as compared to single tuned coils. Reasons are that the number of proton receive channels often severely reduced and double tuning will degrade the intrinsic sensitivity of the RF coil on at least one of the nuclei. However, the aim of this work was to implement a double‐tuned sodium/proton knee coil setup without deterioration of the proton signal whilst being able to achieve acquisition of high SNR sodium images. A double‐tuned knee coil was constructed as a shielded birdcage optimized for sodium and compromised for proton. To exclude any compromise, the proton part of the birdcage is used for transmit only and interfaced to RF amplifiers that can fully mitigate the reduced efficiency. In addition, a 15 channel single tuned proton receiver coil was embedded within the double‐resonant birdcage to maintain optimal SNR and acceleration for proton imaging. To validate the efficiency of our coil, the designed coil was compared with the state‐of‐the‐art single‐tuned alternative at 7 T. B1+ corrected SNR maps were used to compare both coils on proton performance and g‐factor maps were used to compare both coils on acceleration possibilities. The newly constructed double‐tuned coil was shown to have comparable proton quality and acceleration possibilities to the single‐tuned alternative while also being able to acquire high SNR sodium images.     

An optimization framework to maximize signal‐to‐noise ratio in simultaneous multi‐slice body imaging

Parallel imaging is essential for the acceleration of abdominal and pelvic 2D multi‐slice imaging, in order to reduce scan time and mitigate motion artifacts. Controlled Aliasing In Parallel Imaging Results IN Higher Acceleration (CAIPIRINHA) accelerated imaging has been shown to increase the signal‐to‐noise ratio (SNR) significantly compared with in‐plane parallel imaging with similar acceleration. We hypothesize that for CAIPIRINHA‐accelerated abdominal imaging the consistency of image quality and SNR is more difficult to achieve due to the subject‐specific coil sensitivity profiles, caused by (1) flexible coil placement; (2) variations in anatomy; and (3) variations in scan coverage along the superior–inferior direction. To test this, a mathematical framework is introduced that calculates the (retained) SNR for in‐plane and simultaneous multi‐slice (SMS)‐accelerated acquisitions. Moreover, this framework was used to optimize the sampling pattern by maximizing the local SNR within a region of interest (ROI) through non‐linear, RF‐induced CAIPIRINHA slice shifts. The framework was evaluated on 14 healthy subjects and the optimized sampling pattern was compared with in‐plane acceleration and CAIPIRINHA acceleration with linear slice shifts, which are primarily used in brain imaging. We demonstrate that the field of view (FOV) in the superior–inferior direction, the coil positioning and the individual anatomy have a large impact on the image SNR (changes up to 50% for varying coil positions and 40% differences between subjects) and image artifacts for simultaneous multi‐slice acceleration. Consequently, sampling patterns have to be optimized for acquisitions employing different FOVs and ideally on an individual basis. Optimization of the sampling pattern, which exploits non‐linear shifts between slices, showed a considerable SNR increase (10–30%) for higher acceleration factors. The framework outlined in this article can be used to optimize sampling patterns for a broad range of accelerated body acquisitions on an individual basis. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

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 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.     

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.     

A radiofrequency coil to facilitate B₁⁺ shimming and parallel imaging acceleration in three dimensions at 7 T

A 15‐channel transmit–receive (transceive) radiofrequency (RF) coil was developed to image the human brain at 7 T. A hybrid decoupling scheme was implemented that used both capacitive decoupling and the partial geometric overlapping of adjacent coil elements. The decoupling scheme allowed coil elements to be arrayed along all three Cartesian axes; this facilitated shimming of the transmit field, B, and parallel imaging acceleration along the longitudinal direction in addition to the standard transverse directions. Each channel was independently controlled during imaging using a 16‐channel console and a 16 × 1‐kW RF amplifier–matrix. The mean isolation between all combinations of coil elements was 18 ± 7 dB. After B shimming, the standard deviation of the transmit field uniformity was 11% in an axial plane and 32% over the entire brain superior to the mid‐cerebellum. Transmit uniformity was sufficient to acquire fast spin echo images of this region of the brain with a single B shim solution. Signal‐to‐noise ratio (SNR) maps showed higher SNR in the periphery vs center of the brain, and higher SNR in the occipital and temporal lobes vs the frontal lobe. Parallel imaging acceleration in a rostral–caudal oblique plane was demonstrated. The implication of the number of channels in a transmit–receive coil was discussed: it was determined that improvements in SNR and B shimming can be expected when using more than 15 independently controlled transmit–receive channels. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Coil combination of multichannel MRSI data at 7 T: MUSICAL

The goal of this study was to evaluate a new method of combining multi‐channel ¹H MRSI data by direct use of a matching imaging scan as a reference, rather than computing sensitivity maps. Seven healthy volunteers were measured on a 7‐T MR scanner using a head coil with a 32‐channel array coil for receive‐only and a volume coil for receive/transmit. The accuracy of prediction of the phase of the ¹H MRSI data with a fast imaging pre‐scan was investigated with the volume coil. The array coil ¹H MRSI data were combined using matching imaging data as coil combination weights. The signal‐to‐noise ratio (SNR), spectral quality, metabolic map quality and Cramér–Rao lower bounds were then compared with the data obtained by two standard methods, i.e. using sensitivity maps and the first free induction decay (FID) data point. Additional noise decorrelation was performed to further optimize the SNR gain. The new combination method improved significantly the SNR (+29%), overall spectral quality and visual appearance of metabolic maps, and lowered the Cramér–Rao lower bounds (?34%), compared with the combination method based on the first FID data point. The results were similar to those obtained by the combination method using sensitivity maps, but the new method increased the SNR slightly (+1.7%), decreased the algorithm complexity, required no reference coil and pre‐phased all spectra correctly prior to spectral processing. Noise decorrelation further increased the SNR by 13%. The proposed method is a fast, robust and simple way to improve the coil combination in ¹H MRSI of the human brain at 7 T, and could be extended to other ¹H MRSI techniques. © 2013 The Authors. NMR in Biomedicine published by John Wiley & Sons, Ltd.     

Whole‐body radiofrequency coil for 31P MRSI at 7 T

Widespread use of ultrahigh‐field ³¹P MRSI in clinical studies is hindered by the limited field of view and non‐uniform radiofrequency (RF) field obtained from surface transceivers. The non‐uniform RF field necessitates the use of high specific absorption rate (SAR)‐demanding adiabatic RF pulses, limiting the signal‐to‐noise ratio (SNR) per unit of time. Here, we demonstrate the feasibility of using a body‐sized volume RF coil at 7 T, which enables uniform excitation and ultrafast power calibration by pick‐up probes. The performance of the body coil is examined by bench tests, and phantom and in vivo measurements in a 7‐T MRI scanner. The accuracy of power calibration with pick‐up probes is analyzed at a clinical 3‐T MR system with a close to identical ¹H body coil integrated at the MR system. Finally, we demonstrate high‐quality three‐dimensional ³¹P MRSI of the human body at 7 T within 5 min of data acquisition that includes RF power calibration. Copyright © 2016 John Wiley & Sons, Ltd.     

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.     

Wireless coils based on resonant and nonresonant coupled‐wire structure for small animal multinuclear imaging

Earlier work on RF metasurfaces for preclinical MRI has targeted applications such as whole‐body imaging and dual‐frequency coils. In these studies, a nonresonant loop was used to induce currents into a metasurface that was operated as a passive inductively powered resonator. However, as we show in this study, the strategy of using a resonant metasurface reduces the impact of the loop on the global performance of the assembled coil. To mitigate this deficiency, we developed a new approach that relies on the combination of a commercial surface coil and a coupled‐wire structure operated away from its resonance. This strategy enables the extension of the sensitive volume of the surface coil while maintaining its local high sensitivity without any hardware modification. A wireless coil based on a two parallel coupled‐wire structure was designed and electromagnetic field simulations were carried out with different levels of matching and coupling between both components of the coil. For experimental characterization, a prototype was built and tested at two frequencies, 300 MHz for ¹H and 282.6 MHz for ¹⁹F at 7 T. Phantom and in vivo MRI experiments were conducted in different configurations to study signal and noise figures of the structure. The results showed that the proposed strategy improves the overall sensitive volume while simultaneously maintaining a high signal‐to‐noise ratio (SNR). Metasurfaces based on coupled wires are therefore shown here as promising and versatile elements in the MRI RF chain, as they allow customized adjustment of the sensitive volume as a function of SNR yield. In addition, they can be easily adapted to different Larmor frequencies without loss of performance.     

Optimizing diffusion MRI acquisition efficiency of rodent brain using simultaneous multislice EPI

Diffusion tensor imaging (DTI) of the brain provides essential information on the white matter integrity and structural connectivity. However, it suffers from a low signal‐to‐noise ratio (SNR) and requires a long scan time to achieve high spatial and/or diffusion resolution and wide brain coverage. With recent advances in parallel and simultaneous multislice (multiband) imaging, the SNR efficiency has been improved by reducing the repetition time (T_R). However, due to the limited number of RF coil channels available on preclinical MRI scanners, simultaneous multislice acquisition has not been practical. In this study, we demonstrate the ability of multiband DTI to acquire high‐resolution data of the mouse brain with 84 slices covering the whole brain in 0.2 mm isotropic resolution without a coil array at 9.4 T. Hadamard‐encoding four‐band pulses were used to acquire four slices simultaneously, with the reduction in the T_R maximizing the SNR efficiency. To overcome shot‐to‐shot phase variations, Hadamard decoding with a self‐calibrated phase was developed. Compared with single‐band DTI acquired with the same scan time, the multiband DTI leads to significantly increased SNR by 40% in the white matter. This SNR gain resulted in reduced variations in fractional anisotropy, mean diffusivity, and eigenvector orientation. Furthermore, the cerebrospinal fluid signal was attenuated, leading to reduced free‐water contamination. Without the need for a high‐density coil array or parallel imaging, this technique enables highly efficient preclinical DTI that will facilitate connectome studies.     

Diffusion tensor imaging using multiple coils for mouse brain connectomics

The correlation between brain connectivity and psychiatric or neurological diseases has intensified efforts to develop brain connectivity mapping techniques on mouse models of human disease. The neural architecture of mouse brain specimens can be shown non‐destructively and three‐dimensionally by diffusion tensor imaging, which enables tractography, the establishment of a connectivity matrix and connectomics. However, experiments on cohorts of animals can be prohibitively long. To improve throughput in a 7‐T preclinical scanner, we present a novel two‐coil system in which each coil is shielded, placed off‐isocenter along the axis of the magnet and connected to a receiver circuit of the scanner. Preservation of the quality factor of each coil is essential to signal‐to‐noise ratio (SNR) performance and throughput, because mouse brain specimen imaging at 7 T takes place in the coil‐dominated noise regime. In that regime, we show a shielding configuration causing no SNR degradation in the two‐coil system. To acquire data from several coils simultaneously, the coils are placed in the magnet bore, around the isocenter, in which gradient field distortions can bias diffusion tensor imaging metrics, affect tractography and contaminate measurements of the connectivity matrix. We quantified the experimental alterations in fractional anisotropy and eigenvector direction occurring in each coil. We showed that, when the coils were placed 12 mm away from the isocenter, measurements of the brain connectivity matrix appeared to be minimally altered by gradient field distortions. Simultaneous measurements on two mouse brain specimens demonstrated a full doubling of the diffusion tensor imaging throughput in practice. Each coil produced images devoid of shading or artifact. To further improve the throughput of mouse brain connectomics, we suggested a future expansion of the system to four coils. To better understand acceptable trade‐offs between imaging throughput and connectivity matrix integrity, studies may seek to clarify how measurement variability, post‐processing techniques and biological variability impact mouse brain connectomics.     

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.     

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.     

磁共振射频发射与接收线圈谐振电路特性仿真分析

目的:使射频线圈的工作频率接近于磁共振成像（MRI）的共振频率,即射频线圈处于谐振状态,从而提高MRI系统射频功率的转化效率、改善MRI图像质量。方法:在介绍MRI射频线圈谐振电路原理的基础上,对射频线圈串联谐振电路和并联谐振电路进行理论推导。采用电子电路设计软件MULTISIM搭建串联谐振电路和并联谐振电路,从射频脉冲作用瞬间、正弦稳态和射频脉冲撤除后3个阶段,分别对其谐振电路波形、幅频特性、相频特性、线圈两端的瞬时压降和稳态电路参数等进行仿真模拟分析。结果:电子线路仿真的结果能够直观地反映串联谐振电路和并联谐振电路的特性。通过电子线路参数的优化调整能使射频线圈处于最佳谐振状态,从而提高射频功率的转化效率和改善MRI图像质量。结论:谐振电路特性的仿真分析为射频线圈的设计提供了技术依据,提高了工作效率,节约了射频线圈谐振电路的设计制造成本。