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 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.     

Transmit-Only/Receive-Only Radiofrequency System for Hyperpolarized 13C MRS Cardiac Metabolism Studies in Pigs

Hyperpolarized ¹³C magnetic resonance spectroscopy in pig models enables metabolic activity mapping, providing a powerful tool for the study of the heart physiology, but requires the development of dedicated radiofrequency coils, capable of providing large field of view with high signal-to-noise ratio (SNR) data. This work describes the simulations and the tests of a transmit-only (TX) volume coil/receive-only (RX) surface coil both designed for hyperpolarized studies of pig heart with a clinical 3T scanner. The coil characterization is performed by developing an SNR model for coil performance in terms of coil resistance, sample-induced resistance and magnetic field pattern. In particular, coil resistances were calculated from Ohm’s law, while magnetic field patterns and sample-induced resistances were calculated using a numerical finite-difference time-domain algorithm. Experimental phantom chemical shift image, showed good agreement with the theoretical SNR-vs-depth profiles and highlighted the advantage of the novel configuration over the single transmit–receive coils throughout the volume of interest for cardiac imaging in pig. Finally, the TX-birdcage/RX-circular configuration was tested by acquiring metabolic maps with hyperpolarized [1-13C] pyruvate injected i.v. in a pig. The results of the phantom and pig experiments show the ability of the coil configuration to image well the metabolites distribution.     

一种用于开放式MRI的射频发射线圈设计

介绍了一种用于开放式MRI系统的射频发射线圈. 此发射线圈为上下2个相同的线圈,分别安装在磁体的2极,两线圈采用非对称的正交方式放置. 线圈为矩形螺线管结构,通过电磁场数值计算的方法对线圈的匝间距进行了优化,使线圈在300 mm的球形区域内达到偏差不超过3 dB的均匀性要求. 根据优化结果制作了一套用于0.23 T开放式MRI系统的发射线圈,并对线圈的均匀性及射频发射的效率进行了测试. 测试结果表明,线圈具有较高的发射效率和较好的均匀性,由此验证了设计方案的可行性.     

Sensitivity enhancement and compensation of RF penetration artifact with planar actively detunable quadrature surface coil

An actively detunable planar quadrature surface coil for human body imaging at 4 T has been constructed and compared with a conventional linear surface coil. The coil could be used as a transmit/receive or a receive-only device in combination with a volume transmit coil. Transmission, reception profiles and the corresponding images acquired with each coil, as well as with both individual modes of the quadrature coil, are presented. Data collected using a tissue equivalent loaded phantom recorded with the linear surface coil demonstrated significant intensity distortions due to RF penetration artifact. The quadrature surface coil, on the other hand, provided compensation of the artifact, separately in its transmission and reception profiles as well as in the resultant images. Substantial sensitivity gain was also observed for the quadrature coil compared to the linear device. Significant advantages of using the quadrature surface coil over the linear device at 4 T have, therefore, been demonstrated.     

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

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 7T MRI scanner. Simulations and measurements of the B(1) field distribution of the transmit coil are described, along with SAR considerations for operation at 7T. Results of imaging the trabecular bone of three volunteers at 1.5T, 3T and 7T are presented, using identical 1.5T and 3T versions of the 7T 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 7T compared to 3T and 1.5T, respectively. The results thus display an approximately linear dependence of SNR with field strength and enable the practical utility of 7T scanners for micro-MRI of trabecular bone.     

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.     

Artifacts caused by transcranial magnetic stimulation coils and EEG electrodes in T(2)*-weighted echo-planar imaging

We investigated the effects of transcranial magnetic stimulation (TMS) coils and electroencephalographic (EEG) electrodes on T(2)*-weighted echo-planar images (EPI) at 2.0 T (gradient-echo EPI, mean TE = 53 ms, 2x2x4 mm(3)). In comparison with anatomic gradient-echo images (3D FLASH, TE = 4 ms, 1x1x1 mm(3)), T(2)*-weighted EPI acquisitions of a water-filled spherical phantom revealed severe signal losses and geometric distortions in the vicinity of TMS coils. Even remote effects were observed for image orientations perpendicular to the coil plane. EEG electrodes and the fixation gel caused milder localized distortions. In humans, complications were avoided by the large distance between the TMS coil and the cortical surface and when using an EPI orientation parallel to the plane of the coil. It is concluded that T(2)*-weighted EPI studies of human brain function may be performed without distortions caused by TMS coils and EEG electrodes.     

Surface normal imaging with a hand-held NMR device

Recently the capabilities of single sided nuclear magnetic resonance (NMR) devices have been extended towards three-dimensional imaging. This paper details the use of a magnetic field sweep coil to obtain spatial resolution in the plane normal to the surface of a hand-held NMR device-the NMR-Mobile Universal Surface Explorer (MOUSE). One-dimensional depth profiles can be recorded by varying the current in the sweep coils. Preliminary results from multi-layer rubber and glass sample phantoms demonstrate a sample penetration depth of 7 mm. Two-dimensional images were acquired via the inclusion of phase encoding coils. Non-destructive cross-sectional images of small rubber phantoms were successfully recorded.     

High-field magnetic resonance imaging using solenoid radiofrequency coils

High-resolution magnetic resonance imaging using dedicated high-field radiofrequency micro-coils at 16.4 T (700 MHz) was investigated. Specific solenoid coils primarily using silver and copper as conductors with enamel and polyurethane coatings were built to establish which coil configuration produces the best image. Image quality was quantified using signal-to-noise ratio and signal variation over regions of interest. Benchmarking was conducted using 5-mm diameter coils, as this size is comparable to an established coil of the same size. Our 1.4-mm-diameter coils were compared directly to each other, from which we deduce performance as a function of conductor material and coating. A variety of materials and conductor coatings allowed us to choose an optimal design, which we used to image a kidney section at 10-micron resolution. We applied zero-fill extrapolation to achieve 5-micron resolution.     

A transmit/receive volume strip array and its mode mixing theory in MRI

MRI is increasingly moving towards higher magnetic field, prompting the need for multi-port transmit/receive RF coil arrays to overcome high-frequency limitations such as penetration depth and dielectric resonance effects. In this work, an arbitrary n-element transmit/receive volume strip array (VSA) and an associated mixing mode theory are described to understand the behavior of a multiple-port cyclic symmetrical VSA in both the physical port space and the complementary mode space; the relations between the two spaces are explicitly formulated. The advantage of mode-space analysis is that an arbitrary n x n impedance matrix which describes any VSA in port space can be diagonalized to a diagonal n x n matrix; thus an analytical solution of Kirchhoff's laws for the VSA becomes manageable when n is large. Based on such an analytical solution, we can (a) generate excitation profile of any desired mixed mode during transmission by manipulating external power sources without the need of physically tuning VSA to the mixed mode; (b) identify the sensitivity profiles of the complementary mode distributions during reception, which were unknown in quadrature and decoupled coils. Many predictions are rigidly verified by extensive test measurements from network analyzer and by MR imaging experiments.     

Practical design of a 4 Tesla double-tuned RF surface coil for interleaved 1H and 23Na MRI of rat brain

MRI is proving to be a very useful tool for sodium quantification in animal models of stroke, ischemia, and cancer. In this work, we present the practical design of a dual-frequency RF surface coil that provides (1)H and (23)Na images of the rat head at 4 T. The dual-frequency RF surface coil comprised of a large loop tuned to the (1)H frequency and a smaller co-planar loop tuned to the (23)Na frequency. The mutual coupling between the two loops was eliminated by the use of a trap circuit inserted in the smaller coil. This independent-loop design was versatile since it enabled a separate optimisation of the sensitivity and RF field distributions of the two coils. To allow for an easy extension of this simple double-tuned coil design to other frequencies (nuclei) and dimensions, we describe in detail the practical aspects of the workbench design and MRI testing using a phantom that mimics in vivo conditions. A comparison between our independent-loop, double-tuned coil and a single-tuned (23)Na coil of equal size obtained with a phantom matching in vivo conditions, showed a reduction of the (23)Na sensitivity (about 28 %) because of signal losses in the trap inductance. Typical congruent (1)H and (23)Na rat brain images showing good SNR ((23)Na: brain 7, ventricular cerebrospinal fluid 11) and spatial resolution ((23)Na: 1.25 x 1.25 x 5mm(3)) are also reported. The in vivo SNR values obtained with this coil were comparable to, if not better than, other contemporary designs in the literature.     

Signal-to-noise ratio comparison of phased-array vs. implantable coil for rat spinal cord MRI

The signal-to-noise ratio (SNR) performance and practicality issues of a four-element phased-array coil and an implantable coil system were compared for rat spinal cord magnetic resonance imaging (MRI) at 7 T. MRI scans of the rat spinal cord at T10 were acquired from eight rats over a 3 week period using both coil systems, with and without laminectomy. The results demonstrate that both the phased array and the implantable coil systems are feasible options for rat spinal cord imaging at 7 T, with both systems providing adequate SNR for 100-mum spatial resolution at reasonable imaging times. The implantable coils provided significantly higher SNR, as compared to the phased array (average SNR gain of 5.3x between the laminectomy groups and 2.5x between the nonlaminectomy groups). The implantable coil system should be used if maximal SNR is critical, whereas the phased array is a good choice for its ease of use and lesser invasiveness.     

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.     

Plate form three-dimensional gradient coils for L-band ESR imaging experiment

A set of plate form three-dimensional magnetic gradient coils was developed and used in electron spin resonance imaging (ESRI) experiment. The coils were processed with whole copper plates instead of wound with copper wires, which made its structure so compact that it was much thinner and smaller comparing to those traditionally used in ESRI. The coil set had a pie-like appearance of which the total thickness was only 14 mm and the outer diameter was 250 mm. The efficiency of the coils could be greater than 10 mT/m/A when distance between the two side-pieces was 63 mm. A maximum gradient strength of more than 200 mT/m could be obtained with driving current of about 20 A in each dimension coil. The spatial linearity was better than 5% in all three dimensions within the available spatial linearity area of larger than a sphere of 40 mm in diameter. The stability of the gradients strength could reach the level of 10(-5). An imaging resolution of better than 1 mm could be achieved with the coil set. Some preliminary practical imaging results show that the developed gradient coil set is suitable for L-band ESRI experiment of biological samples or even in vivo small animals.     

An assessment of the sharpness of carotid artery tissue boundaries with acquisition voxel size and field strength

A measure of the sharpness of vessel wall interfaces in carotid artery MRI may be useful for assessing the conspicuity of the wall's features. An edge detection technique was used to measure the signal intensity gradients in 2D time-of-flight (2D-TOF) and double-inversion recovery black-blood (DIR-BB) carotid artery images of normal subjects that were acquired at 1.5 T with 0.55 x 0.55 x 2.0-mm (0.6 mm3) acquisition voxels and zero filled to reduce the in-plane reconstructed voxel size by one half in each dimension as well as with 0.27 x 0.27 x 2.0-mm (0.15 mm3) acquisition voxels and at 3.0 T with 0.27 x 0.27 x 2.0-mm (0.15 mm3) acquisition voxels using surface coils. The gradient intensities of the lumen-to-background interface varied closely with the contrast-to-noise ratio of the 2D-TOF imaging. For the DIR-BB imaging, in which higher spatial frequency artery structures are visible, the gradient intensities at the interfaces were higher than theoretically predicted at both field strengths with smaller acquisition voxels. The use of acquisition voxels smaller than those previously used at 1.5 T can improve the visualization of carotid artery structures at 1.5 and 3.0 T with surface coil reception.     

Multichannel transceiver dual-tuned RF coil for proton/sodium MR imaging of knee cartilage at 3 T

Sodium magnetic resonance (MR) imaging is a promising technique for detecting changes of proteoglycan (PG) content in cartilage associated with knee osteoarthritis. Despite its potential clinical benefit, sodium MR imaging in vivo is challenging because of intrinsically low sodium concentration and low MR signal sensitivity. Some of the challenges in sodium MR imaging may be eliminated by the use of a high-sensitivity radiofrequency (RF) coil, specifically, a dual-tuned (DT) proton/sodium RF coil which facilitates the co-registration of sodium and proton MR images and the evaluation of both physiochemical and structural properties of knee cartilage. Nevertheless, implementation of a DT proton/sodium RF coil is technically difficult because of the coupling effect between the coil elements (particularly at high field) and the required compact design with improved coil sensitivity. In this study, we applied a multitransceiver RF coil design to develop a DT proton/sodium coil for knee cartilage imaging at 3 T. With the new design, the size of the coil was minimized, and a high signal-to-noise ratio (SNR) was achieved. DT coil exhibited high levels of reflection S11 (～-21 dB) and transmission coefficient S12 (～-19 dB) for both the proton and sodium coils. High SNR (range 27-38) and contrast-to-noise ratio (CNR) (range 15-21) were achieved in sodium MR imaging of knee cartilage in vivo at 3-mm(3) isotropic resolution. This DT coil performance was comparable to that measured using a sodium-only birdcage coil (SNR of 28 and CNR of 20). Clinical evaluation of the DT coil on four normal subjects demonstrated a consistent acquisition of high-resolution proton images and measurement of relative sodium concentrations of knee cartilages without repositioning of the subjects during the same MR scanning session.     

One micrometer resolution NMR microscopy

We obtained a magnetic resonance image of 1 microm resolution and 75 microm(3) voxel volume for a phantom filled with hydrocarbon oil within an hour at 14.1 T. For this work, a specially designed probe with a high sensitivity RF coil and gradient coils generating over 1000 G/cm was built. The optimal pulse sequence was analyzed in consideration of the bandwidth, diffusion coefficients, and T(1) and T(2) relaxations of the medium. The system was applied to the in vivo imaging of a geranium leaf stem to get the images of 2 microm resolution and 200 microm(3) voxel volume.     

Surface coil imaging of rat spine at 7.0 T

An inductively coupled surface coil for imaging the rat spine at 7 T is described. This planar circular probe was made from microwave substrate to limit the size of the coil and to minimize the magnetic susceptibility. The surface coil was used as a single transmit/receive coil and as a receive-only coil with a birdcage body coil for excitation. The signal-to-noise ratio (SNR) of the probe was compared to a 5-cm birdcage coil and exceeded the birdcage coil's SNR by three to six times at superficial structures. The main advantages of the probe are an improved SNR for superficial structures and a simple design and use. Images with 50 × 50 × 500 μm voxels were obtained of the rat spine with excellent anatomical detail.     

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.