Investigation of high-resolution functional magnetic resonance imaging by means of surface and array radiofrequency coils at 7 T

In this investigation, high-resolution, 1×1×1-mm³ functional magnetic resonance imaging (fMRI) at 7 T is performed using a multichannel array head coil and a surface coil approach. Scan geometry was optimized for each coil separately to exploit the strengths of both coils. Acquisitions with the surface coil focused on partial brain coverage, while whole-brain coverage fMRI experiments were performed with the array head coil. BOLD sensitivity in the occipital lobe was found to be higher with the surface coil than with the head array, suggesting that restriction of signal detection to the area of interest may be beneficial for localized activation studies.     

Low-Field MR Coils: Comparison between Strip and Wire Conductors

This work describes how the cross-sectional shape of radio-frequency coil conductors affects coils performance. This is of particular importance at low Larmor frequencies such as those of low-field magnetic resonance imaging systems where conductor and capacitor losses are the dominant power dissipation mechanisms. We demonstrate that conductors having a circular cross section allow the coil to achieve significantly better performance than the one built using flat strips. The change in coil quality factor due to conductor geometry was verified to be due only to changes in the conductors’ resistance and not their inductance. The results are not limited to low-field proton imaging but they are equally applicable to other situations where the Larmor frequency is in the megahertz range, including nuclear magnetic resonance of other nuclear species at intermediate fields.     

Relative RF coil performance in carotid imaging

PURPOSE: Computer simulations and measurements on human volunteers were used to test the extent to which the quality of carotid imaging might be improved by coil arrays that are not limited by a constraint on the number of RF coil receiver ports. METHODS: Analytic near-field equations for the magnetic and electric fields of a rectangular loop resonator were used to estimate the relative signal-to-noise ratio (rSNR) along the length of a simulated carotid artery as a function of loop size, loop position and vessel depth. The sizes, positions and number of elements in a linear coil array that resulted in the maximum composite SNR along the length of a simulated carotid artery were then estimated. The linear array results were used to predict the total number of elements needed for optimal imaging of the carotid arteries. Also, three normal volunteers were imaged with a variety of RF coils, and the rSNR measurements along the lengths of the carotid artery were evaluated for each coil combination. RESULTS: The analytic simulation and the human volunteer measurements both show that improved SNR (e.g., >300% at the bifurcation) can be obtained with coils tailored to each specific region of the carotid artery in comparison to that obtained with four-element arrays designed and used to image the entire carotid artery. CONCLUSIONS: The resulting number of coil ports, 16 to 24, required for full coverage of the carotid arteries is consistent with the number of channels just becoming available on recently developed clinical scanners.     

Design considerations and coil comparisons for 7 T brain imaging

The development of 300 MHz radio-frequency (RF) head coils analogous to those used at field strengths of 1.5 and 3 T is complicated by increased dissipative losses in conductive tissue, effects arising from the short RF wavelength in biological tissue (about 13 cm at 300 MHz), and the constraints imposed by the use of head gradient sets desirable for mitigating increased static field susceptibility effects. In this study, five RF head coils were constructed and tested on a 7 T scanner including 2 TEM designs, 2 birdcage designs and a local receive-only array. Signal-to-noise ratio, coil reception profiles and interactions between the coil and dielectric head were examined. Particular attention was placed on the coil’s reception in the neck and shoulders, where the head gradient is unable to spatially encode the image. With the use of conductive shields and distributed capacitance, all of the coil designs could be made to image effectively at high field, but each design was found to have subtle differences in field distribution, interaction with the dielectric boundary conditions of the head and fringe fields in the neck and shoulders. In particular, the birdcage and array coils were found to have reducedB ₁ reception field profiles in the neck and shoulders which helped reduce signal detection outside the linear region of the head gradient coil. Although the TEM coils exhibited higher signal detection in the neck and shoulders, all the coils picked up enough signal from these regions to produce artifacts in the brain. These artifacts could be mitigated through use of a conductive shield or by small local dephasing shims sewn into the shoulders of a jacket worn by the subject. Although homogeneous in low-dielectric-constant phantoms, the volume coil’sB ₁ profile was strongly peaked in the center of the head, rendering them spatially complementary to that observed in the surface coil array. The image profile of the surface coil was found to be less dramatically changed from patterns observed at lower field strength. Its dielectric brightening pattern was found to depend on the orientation of the coil with respect to the head.     

Noise properties of a NMR transceiver coil array

The use of multiple radiofrequency (RF) surface coil elements has applications in both fast parallel imaging and conventional imaging techniques. Through implementation of a simple magnetic decoupling network, 50 Omega matching can be achieved in both the transmitter and receiver chains, enabling the use of conventional RF power amplifiers and preamplifiers for transceive applications. Unlike phased array coil arrangements using low impedance preamplifiers for decoupling, the noise correlation between 50 Omega coils decoupled with discrete components has not been characterized. We have measured the dependence of coil quality factor (Q-factor) and noise correlation on coil separation and shown these quantities to be consistent with theoretical arguments, at least at 4 T (170 MHz). Our results suggest that a coil system for transmission and reception of NMR signals with 50 Omega coils can be built to take advantage of all the benefits of conventional array coils and with the added advantages of using conventional amplifiers.     

A four-element phased array coil for high resolution and parallel MR imaging of the knee

A four-element phased array coil for MR imaging of the knee was designed, built and tested for clinical use at 1.5 Tesla. In routine imaging, it provides over twofold increase in signal-to-noise (SNR) compared to two commercially available knee coils, and supports higher spatial image resolution. The phased array knee coil was also tested for its compatibility with parallel MR imaging that reduces imaging time by several folds over conventional MR technique. Results obtained using SiMultaneous Acquisition of Spatial Harmonics (SMASH) technique shows that our phased array knee coil can be used with parallel MR imaging. These improvements may enhance knee diagnosis with higher image quality and reduced scan time.     

Parallel magnetic resonance imaging using coils with localized sensitivities

The purpose of this study was to present clinical examples and illustrate the inefficiencies of a conventional reconstruction using a commercially available phased array coil with localized sensitivities. Five patients were imaged at 1.5 T using a cardiac-synchronized gadolinium-enhanced acquisition and a commercially available four-element phased array coil. Four unique sets of images were reconstructed from the acquired k-space data: (a) sum-of-squares image using four elements of the coil; localized sum-of-squares images from the (b) anterior coils and (c) posterior coils and a (c) local reconstruction. Images were analyzed for artifacts and usable field-of-view. Conventional image reconstruction produced images with fold-over artifacts in all cases spanning a portion of the image (mean 90 mm; range 36-126 mm). The local reconstruction removed fold-over artifacts and resulted in an effective increase in the field-of-view (mean 50%; range 20-70%). Commercially available phased array coils do not always have overlapping sensitivities. Fold-over artifacts can be removed using an alternate reconstruction method. When assessing the advantages of parallel imaging techniques, gains achieved using techniques such as SENSE and SMASH should be gauged against the acquisition time of the localized method rather than the conventional sum-of-squares method.     

结合振动控制的柱面纵向梯度线圈目标场设计方法

在磁共振成像系统的工作过程中,噪声主要是由梯度线圈系统产生的.梯度线圈置于高均匀度超导磁体的室温孔内,并工作于脉冲状态,频繁的开启和关闭会使线圈中电流急剧随时间变化,变化的电流导致线圈受到变化的洛伦兹力作用,从而产生振动,这种高频振动所发出的噪声会对病人产生刺激,严重时甚至会对病人的听觉神经产生损伤.梯度场的场强越强、切换速度越快,所产生的噪声就越大.降低噪声的最根本方法是通过有效的梯度线圈设计,降低洛伦兹力的空间分布.本文针对纵向梯度线圈,在原经典目标场设计方法基础上,加入对振动参量,从而能够有效地降低线圈工作时所产生的噪声.其具体方法是将振动控制函数作为约束条件,通过目标场法建立数学模型,利用MATLAB进行电磁验算.计算结果表明,所提数学模型可有效地降低线圈振动的最大振幅.     

Experimental development of a petal resonator surface coil

A surface coil for MRI was designed and built based on the principles of the petal resonator proposed by Mansfield [J Phys D Appl Phys 21 (1988) 1643]. This resonator coil design was named the petal resonator surface (PERES) coil and is composed of an eight-petal coil array and a central circular coil. A minimum separation of three times the petal coil radius is necessary to significantly decrease the mutual inductance. An analytical function for the PERES Signal-to-noise ratio (SNR) is obtained based on the quasistatic method. Theoretical plots of SNR enhancement yielded 26% and 35% more SNR over the circular coil and phased-array coils. Imaging experiments were first performed using a spectroscopy phantom on a 1.5-T commercial imager. Subsequently, brain images of healthy volunteers were obtained. Clinical MR imager compatibility allows this resonator coil to be used with conventional pulse sequences and imaging protocols. This coil design offers a new alternative to existing surface coils because it significantly increases the SNR.     

Sequential CW-EPR image acquisition with 760-MHz surface coil array

This paper describes the development of a surface coil array that consists of two inductively coupled surface-coil resonators, for use in continuous-wave electron paramagnetic resonance (CW-EPR) imaging at 760 MHz. To make sequential EPR image acquisition possible, we decoupled the surface coils using PIN-diode switches, to enable the shifting of the resonators resonance frequency by more than 200 MHz. To assess the effectiveness of the surface coil array in CW-EPR imaging, two-dimensional images of a solution of nitroxyl radicals were measured with the developed coil array. Compared to equivalent single coil acquired images, we found the visualized area to be extended approximately 2-fold when using the surface coil array. The ability to visualize larger regions of interest through the use of a surface coil array, may offer great potential in future EPR imaging studies.     

Design of a capacitively decoupled transmit/receive NMR phased array for high field microscopy at 14.1T

A design is presented for a "phased array" of four transmit/receive saddle-geometry volume coils for microimaging at 600 MHz within a 45 mm clear-bore vertical magnet. The small size of the coils, approximately 10 mm in length, and high frequency of operation both present considerable challenges for the design of a phased array. The particular design consists of four saddle coils, stacked vertically, in order to produce an array suitable for imaging samples, typical of many microimaging studies, with a large length:diameter ratio. Optimal coil overlap is used to reduce the mutual inductance between adjacent coils, and capacitive networks are used to maximize the isolation between all of the coils. Standard 50 Omega input impedance preamplifiers are used so that the preamplifiers do not have to be integrated directly into the probe. Isolation between coils was better than 20 dB for all coil pairs. An increase in signal-to-noise of 70 +/- 3% was achieved, averaged over the whole array, compared to a single coil of the same dimensions. High resolution phased array images are shown for ex vivo tissue samples.     

A Rat’s Image Using Multichannel Phase Array RF Coil Specifically Designed for Animals: Focused on a Pulse Sequence and Mediator Variable

The study focuses on finding the pulse sequences depicting a rat’s tumor when the size of the field of view was reduced, using coils specifically designed for rats, and obtaining an optimized image of a rat by transforming the parameters, according to each pulse sequence. The manufactured coil is 8-channel phased array coils, and the type is a receive-only coil. The diameter of the coil is 80 mm, and the length is 150 mm. The overlapped distance among each channel was 8 mm, and the lab rats used in the experiment were the commonly used Sprague–Dawley rats. The study used three types of pulse sequences, which are the diffusion weight imaging (DWI), three-dimensional dual echo steady state (3-D DESS), and three-dimensional volumetric interpolated breath-hold (3-D VIBE). Along with the DWI results, pulse sequences of 3-D DESS and 3-D VIBE enabled to distinguish the tumors from that of normal tissues in the brain by optimizing a mediator variable and to illustrate the whole body imaging of a rat.     

4 T Actively detuneable double-tuned 1H/31P head volume coil and four-channel 31P phased array for human brain spectroscopy

Typically 31P in vivo magnetic resonance spectroscopic studies are limited by SNR considerations. Although phased arrays can improve the SNR; to date 31P phased arrays for high-field systems have not been combined with 31P volume transmit coils. Additionally, to provide anatomical reference for the 31P studies, without removal of the coil or patient from the magnet, double-tuning (31P/1H) of the volume coil is required. In this work we describe a series of methods for active detuning and decoupling enabling use of phased arrays with double-tuned volume coils. To demonstrate these principles we have built and characterized an actively detuneable 31P/1H TEM volume transmit/four-channel 31P phased array for 4 T magnetic resonance spectroscopic imaging (MRSI) of the human brain. The coil can be used either in volume-transmit/array-receive mode or in TEM transmit/receive mode with the array detuned. Threefold SNR improvement was obtained at the periphery of the brain using the phased array as compared to the volume coil.     

Optimization of an 8-Channel Loop-Array Coil for a 7 T MRI System with the Guidance of a Co-Simulation Approach

Minimizing coupling between coil elements is technically challenging in designing large-sized, volume-type phased-array coils for human head imaging at ultrahigh fields, e.g., 7 T. As a widely used decoupling method, the capacitive decoupling method has shown excellent performance for loop array. However, building a multi-channel loop array with capacitive decoupling method is laborious that tuning frequency and matching of one coil element will affect adjacent elements and even next adjacent elements. In this study, we made an 8-channel loop-array transmit/receive radio-frequency coil on a 7 T magnetic resonance imaging system with the guidance of frequency domain three-dimensional electromagnetic and radio-frequency circuit co-simulation. The position of decoupling capacitors was investigated and values of all capacitors were predicted from co-simulation. The co-simulation approach cost about 2 days and the error of the predicted and practical capacitance was <5 %. To demonstrate the accuracy of simulation, we evaluated the simulated and measured S-parameter matrixes and B ₁ ⁺ profiles in a birdcage-like excitation mode on a cylindrical water phantom. In addition, B ₁ ⁺ maps and images of human head were shown with the fabricated coil. To demonstrate the parallel imaging performance of this coil array, GRE images using GRAPPA acceleration with the reduction factor R of 1, 2, 3, and 4 were acquired.     

Current CONtrolled Transmit And Receive Coil Elements (C2ONTAR) for Parallel Acquisition and Parallel Excitation Techniques at High-Field MRI

A novel intrinsically decoupled transmit and receive radio-frequency coil element is presented for applications in parallel imaging and parallel excitation techniques in high-field magnetic resonance imaging. Decoupling is achieved by a twofold strategy: during transmission elements are driven by current sources, while during signal reception resonant elements are switched to a high input impedance preamplifier. To avoid B(0) distortions by magnetic impurities or DC currents a resonant transmission line is used to relocate electronic components from the vicinity of the imaged object. The performance of a four-element array for 3 T magnetic resonance tomograph is analyzed by means of simulation, measurements of electromagnetic fields and bench experiments. The feasibility of parallel acquisition and parallel excitation is demonstrated and compared to that of a conventional power source-driven array of equivalent geometry. Due to their intrinsic decoupling the current-controlled elements are ideal basic building blocks for multi-element transmit and receive arrays of flexible geometry.     

An Efficacious Target-Field Approach to Design Shim Coils for Halbach Magnet of Mobile NMR Sensors

The main magnetic fields of mobile nuclear magnetic resonance (NMR) magnets differ from those of conventional NMR and magnetic resonance imaging (MRI) magnets. In the Halbach magnet, the main field B ₀ is perpendicular to the longitudinal axis, the symmetry of the current distribution with respect to the symmetry of the magnetic field differs from that in conventional target-field applications, and the current distribution on the coil surface cannot be expressed in terms of periodic basis functions. To obtain the winding pattern of the coil, an efficacious target-field approach. The surface of a coil is divided into small discrete elements, where each element is represented by a magnetic dipole. From the stream function of the elements, the resultant magnetic field is calculated. The optimization strategy follows an objective function defined by the power dissipation or efficiency of the coil. This leads to the optimum stream function on the coil surface, whose contour lines define the winding patterns of the coil. This paper shows winding patterns designed of shim coils for Halbach magnet and illustrates the craft of a shim coil using flexible printed circuit board. The performance of the coils is verified by simulating the fields they produce over the sensitive volume.     

MRI Visualization of Small Structures Using Improved Surface Coils

In this paper we present the spatial resolution enhancement and noise reduction level achieved with an optimized inductively coupled surface coil specifically designed for our experiments. The technique of designing and implementing customized coils for magnetic resonance imaging of very small structures is described. We have designed a low cost prototype of an inductively coupled circular surface coil, tuned for ¹H magnetic resonance imaging at 200 MHz. The coil is mounted on a customized teflon support. The inductive coupling used in this coil improves the signal-to-noise ratio by reducing various loss mechanisms (specially the dielectric losses). Test images have been acquired to determine the evolution of induced articular lesions in a rabbit animal model, as well as brain tumors in rats. The images show high spatial resolution, excellent B₁ field homogeneity and no “hot spots”. Comparing these images with those acquired with conventional coils, one finds better spatial resolution and signal-to-noise ratio, as well as larger field of view with less intense illumination artifact. The methodology can be used in any application that requires high quality imaging of small structures.     

Design and Demonstration of Four-Channel Received Coil Arrays for Vertical-Field MRI

The layout of radio-frequency received coils is related to signal-to-noise ratio (SNR) in magnetic resonance imaging (MRI). In this paper, different structures of four-channel received coil arrays for vertical-field MRI are constructed and optimized by establishing the relationship between coil geometry and SNR to achieve a high SNR and a uniform SNR distribution in the region of interest (ROI). Then, the SNR distributions of three optimized configurations, including rectangular loops, non-definite shape surface coils, and solenoid loops as the main unit, are simulated and compared. The four-channel coil of solenoid loops as the main unit has been found to have the best performance with the highest mean SNR in the ROI when imaging without acceleration. In addition, g-factor and 2D SENSE SNR in yoz-plane are simply analyzed, which show a sharp decrease in SNR for all the coils. Finally, all the coils are manufactured and operated at a 0.5 T permanent magnet MRI system with phantom and joint imaging experiments. Using pixel-by-pixel manner to evaluate SNR map, the experimental results are consistent with the simulation results, while parallel imaging experiment results show that the major consideration in low field MRI is the improvement of SNR value and uniformity rather than that of the imaging speed. As different constructions of four-channel received coils are investigated, we have found the most effective configuration with high and uniform SNR for vertical-field MRI.     

Fast two-dimensional MR imaging by multiple acquisition with micro B(0) array (MAMBA)

A new method for acquiring MR data in two dimensions is described. This is achieved by combining coils into an array so as to produce a unique local magnetic field within each coil when placed within the B(0) field of a standard MRI scanner. In this way each known location of a coil is associated with a unique resonant frequency. Each coil now represents a location of a pixel in the plane, and after Fourier transformation of the signal the resulting frequency spectrum gives immediately the spin distribution in the plane of the array. In effect, the two-dimensional spatial distribution is frequency encoded without the use of switched gradients or phase encoding. As only static fields are used, this technique offers the potential of fast imaging. Furthermore, signals from different locations would also be inherently time-registered. Initial experiments to demonstrate the principle are described, using a square array of 5 by 5 coils. The currents in the coils were determined by using a genetic algorithm. Echoes from pellet phantoms placed in the array were acquired using standard spin-echo sequences with gradients switched off. The results are promising, with the spectra showing generally good resolution between peaks, enabling localisation in up to half the pixels. Technical difficulties are discussed and possible applications are outlined.     

An optimization method for designing SENSE imaging RF coil arrays

An optimization method in RF coil array design for SENSE imaging is described. Using this method the optimized RF coil geometries can be calculated numerically given the required SENSE imaging performance. Although this method can be applied to optimize the RF coil arrays for both 1D and 2D SENSE imaging, to demonstrate the potential applications of this method, we designed RF coil arrays for 2D SENSE imaging and compared their performance by simulation. An optimized 4-channel receive-only RF coil array designed for 2D SENSE imaging was implemented and tested to demonstrate the feasibility of the proposed technique. Imaging results showed reasonable agreement with the simulations, thus the method can be applied to RF coil array designs for SENSE imaging when optimum imaging performance is desired.