In vivo 1H magnetic resonance imaging and spectroscopy of the rat spinal cord using an inductively-coupled chronically implanted RF coil.

An inductively coupled, chronically implanted short-solenoid coil was used to obtain in vivo localized 1H NMR spectra and diffusion-weighted images from a rat spinal cord. A 5 x 8 mm two-turn elliptically shaped solenoid coil was implanted in rats at the site of a T-12 vertebral-level laminectomy. Excitation was achieved solely by a 3 x 3 cm external surface coil, and signal detection was achieved by inductively coupling the external coil to the implanted coil. The image signal-to-noise ratio (SNR) obtained with the inductively-coupled implanted coil was compared with that obtained using a linear or a quadrature external surface coil. The implanted coil provided a gain by over a factor of 3 in SNR. The implanted coil was used to measure localized 1H spectra in vivo at the T13/L1 spinal-cord level within a 1.85 x 1.85 x 4.82 mm (16.5 microL) volume. With 256 averages, a approximately 3-s repetition delay and respiratory gating, a high-quality spectrum was acquired in 13 min. In addition, water translational diffusion was measured in three orthogonal directions using a stimulated-echo imaging sequence, with a short echo time (TE), to produce a quantitative map of diffusion in a rat spinal cord in vivo.     

A novel multi-segment surface coil for neuro-functional magnetic resonance imaging

The construction of two novel mult-segment (MS) resonators are described. The signal-to-noise ratio (SNR) and B₁ homogeneity of the coils are compared with that of a surface coil and a standard quadrature head coil. The images obtained with the MS designs revealed a surface coil-like fall-off in signal with depth. The SNR offered by MS coils was found to be better than the head coil at depths less than approximately 6 cm. Bilateral motor cortex activation on normal subjects performing finger tapping tasks is demonstrated.     

Three‐element phased‐array coil for imaging of rat spinal cord at 7T

To overcome some of the limitations of an implantable coil, including its invasive nature and limited spatial coverage, a three‐element phased‐array coil is described for high‐resolution magnetic resonance imaging (MRI) of rat spinal cord. This coil allows imaging both thoracic and cervical segments of rat spinal cord. In the current design, coupling between the nearest neighbors was minimized by overlapping the coil elements. A simple capacitive network was used for decoupling the next neighbor elements. The dimensions of individual coils in the array were determined based on the signal‐to‐noise ratio (SNR) measurements performed on a phantom with three different surface coils. SNR measurements on a phantom demonstrated higher SNR for the phased array coil relative to two different volume coils. In vivo images acquired on rat spinal cord with our coil demonstrated excellent gray and white matter contrast. To evaluate the performance of the phased array coil under parallel imaging, g‐factor maps were obtained for acceleration factors of 2 and 3. These simulations indicate that parallel imaging with an acceleration factor of 2 would be possible without significant image reconstruction–related noise amplifications. Magn Reson Med 60:1498–1505, 2008. © 2008 Wiley‐Liss, Inc.     

The NMR phased array

We describe methods for simultaneously acquiring and subsequently combining data from a multitude of closely positioned NMR receiving coils. The approach is conceptually similar to phased array radar and ultrasound and hence we call our techniques the "NMR phased array." The NMR phased array offers the signal-to-noise ratio (SNR) and resolution of a small surface coil over fields-of-view (FOV) normally associated with body imaging with no increase in imaging time. The NMR phased array can be applied to both imaging and spectroscopy for all pulse sequences. The problematic interactions among nearby surface coils is eliminated (a) by overlapping adjacent coils to give zero mutual inductance, hence zero interaction, and (b) by attaching low input impedance preamplifiers to all coils, thus eliminating interference among next nearest and more distant neighbors. We derive an algorithm for combining the data from the phased array elements to yield an image with optimum SNR. Other techniques which are easier to implement at the cost of lower SNR are explored. Phased array imaging is demonstrated with high resolution (512 x 512, 48-cm FOV, and 32-cm FOV) spin-echo images of the thoracic and lumbar spine. Data were acquired from four-element linear spine arrays, the first made of 12-cm square coils and the second made of 8-cm square coils. When compared with images from a single 15 x 30-cm rectangular coil and identical imaging parameters, the phased array yields a 2X and 3X higher SNR at the depth of the spine (approximately 7 cm).     

对比正交鸟笼线圈与CTL脊柱线圈颅脑MR图像质量

目的 探讨正交鸟笼线圈与CTL脊柱线圈在颅脑扫描中的应用价值.方法 采用MRI所配备的正交鸟笼头线圈和CTL脊柱线圈对标准ACR水模行轴位T1FLAIR序列扫描,对每层图像的信噪比(SNR)及均匀度百分比(PIU)进行测量计算,并对其结果进行配对t检验.随机抽取使用头颅正交鸟笼线圈和CTL脊柱线圈进行颅脑检查的100例...     

Quadrature coil design for high-resolution carotid artery imaging scores better than a dual phased-array coil design with the same volume coverage

PURPOSE: To evaluate the ability of a custom-built coil design to provide improved signal-to-noise ratio (SNR) and less signal drop with increasing depth at the carotid artery. MATERIALS AND METHODS: Phased-array surface coils can provide a high SNR to image the carotid vessel wall. However, given the required field-of-view (FOV) and penetration depth, these coils show either a fast signal drop with increasing depth or a moderate SNR at increased coil size. A quadrature surface coil (a butterfly coil in conjunction with a linear single-loop coil) was compared with a phased-array coil in phantom and human studies. RESULTS: The phantom studies showed that the quadrature coil has better SNR over the required FOV than a standard phased-array coil (26% at 3 cm depth). CONCLUSION: The quadrature coil enables better image quality to be achieved.     

Novel RF coil geometry for lower extremity imaging.

A receive-only phased-array coil was designed to image the lower extremities. The array consists of four volume coils placed on two cylindrical formers. The coil array has the ability to image both legs simultaneously over a 40 cm longitudinal field of view (FOV). Experiments using phantoms show an increase in signal-to-noise ratio (SNR) in regions of interest through the center of the coil by an average factor of 2.8 over the body coil and 1.5 over the GE 4-channel torso array. In vivo data acquired from 10 subjects show that the X array provided similar SNR improvement in spin-echo images and more vascular details in angiographic images compared to the torso array.     

Simple, low-cost multipurpose RF coil for MR microscopy at 9.4 T.

An inductively coupled RF coil is introduced for high-resolution MR studies at 9.4 T. The coil offers the flexibility of use as an implantable coil for local imaging inside the body or as a surface coil for imaging below complex surfaces. Successful operation of this coil at strong magnetic field requires special design considerations. In this note, implementation issues of the coil are discussed in detail and practical solutions to overcome some of these difficulties are offered. Imaging performance of the coil and its versatility are demonstrated with images acquired from rat spinal cord when the coil is implanted, and mouse spine and brain when the coil is placed on the surface.     

32‐Channel RF coil optimized for brain and cervical spinal cord at 3 T

Diffusion and functional magnetic resonance imaging of the spinal cord remain challenging due to the small cross‐sectional size of the cord and susceptibility‐related distortions. Although partially addressable through parallel imaging, few highly parallel array coils have been implemented for the cervical cord. Here, we developed a 32‐channel coil that fully covers the brain and c‐spine and characterized its performance in comparison with a commercially available head/neck/spine array. Image and temporal signal‐to‐noise ratio were, respectively, increased by 2× and 1.8× in the cervical cord. Averaged g‐factors at 4× acceleration were lowered by 22% in the brain and by 39% in the spinal cord, enabling 1‐mm isotropic R = 4 multi‐echo magnetization prepared gradient echo of the full brain and c‐spine in 3:20 min. Diffusion imaging of the cord at 0.6 × 0.6 × 5 mm³ resolution and tractography of the full brain and c‐spine at 1.7‐mm isotropic resolution were feasible without noticeable distortion. Improvements of this nature potentially enhance numerous basic and clinical research studies focused on spinal and supraspinal regions. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

A 16-element phased-array head coil

Volume-array coils offer increased signal-to-noise ratio (SNR) over standard volume coils near the array elements while preserving the SNR at the center of the volume. As the number of array elements is increased, the SNR advantage as well as the complexity of actually constructing the array increases also. In this study, a 16-channel receive-only array for imaging of the brain is demonstrated and compared to a circularly polarized (CP) head coil of similar shape and diameter. The array was formed from a 2 × 8 grid of square elements placed on a cylindrical form. Mutual coupling was minimized by a combination of overlapping element placement and current-reducing matching networks. Simultaneous data acquisition from the 16 individual elements was performed using a four-channel receiver system with each channel time domain multiplexed by a factor of 4. Theoretical and experimental comparisons between the array and a standard CP head coil show that the array offers an increase in SNR of nearly a factor of 3 near its surface while maintaining a comparable SNR to that of the CP head coil in the center of the region of interest.     

Coil selection for magnetic resonance imaging of the cervical and thoracic spine using a vertical magnetic field

In order to optimize the coil selection for cervical and thoracic spine imaging the signal characteristics of two different solenoidal surface coils (15 cm and 30 cm diameter, respectively) as well as the head coil and body coil were determined using a 0.3 T MR scanner with a vertical magnetic field. Signal-to-noise ratio curves were obtained for each coil using tube phantoms and a human-like phantom. The findings were compared with images obtained in two healthy volunteers. The head coil was found to be superior for imaging of the cranio-cervical junction while the 15 cm surface coil gave better results in the remaining part of the cervical spine and the upper thoracic spine. The body coil was superior for imaging of the thoracic region at the level of the shoulders (T4-T6) but the 30 cm surface coil was better for the more caudal part of the thoracic spine. Combined phantom and in vivo studies are also recommended for evaluation of future, improved coils.     

An improved quadrature or phased-array coil for MR cardiac imaging

A tailored receive-only coil for cardiac imaging has been designed. The coil consists of two overlapping coil elements and can be used either as a quadrature surface coil or as a phased-array coil. Through phantom experiments and images of the heart, the authors have shown that the improved cardiac coil provided a signal-to-noise ratio 1.6 times higher than a conventional quadrature spine coil, 1.4 times higher than that of a single coil (having the same shape and total dimension), and three times higher than the body coil at the depth of the posterior wall of the heart. The authors have also shown that the cardiac coil improved image quality everywhere in the heart. This coil will enhance routine clinical cardiac studies as well as other examinations such as myocardial perfusion, wall motion, and coronary artery imaging.     

Eight-channel phased array coil and detunable TEM volume coil for 7 T brain imaging.

An eight-channel receive-only brain coil and table-top detunable volume transmit coil were developed and tested at 7 T for human imaging. Optimization of this device required attention to sources of interaction between the array elements, between the transmit and receive coils and minimization of common mode currents on the coaxial cables. Circular receive coils (85 mm dia.) were designed on a flexible former to fit tightly around the head and within a 270-mm diameter TEM transmit volume coil. In the near cortex, the array provided a fivefold increase in SNR compared to a TEM transmit-receive coil, a gain larger than that seen in comparable coils at 3 T. The higher SNR gain is likely due to strong dielectric effects, which cause the volume coil to perform poorly in the cortex compared to centrally. The sensitivity and coverage of the array is demonstrated with high-resolution images of the brain cortex.     

Detunable transverse electromagnetic (TEM) volume coil for high-field NMR.

Most high-field MRI systems do not have the actively detuned body coils that are integral to clinical systems operating at 1.5T and lower field strengths. Therefore, many clinical applications requiring homogeneous volume excitation in combination with local surface coil reception are not easily implemented at high fields. To solve this problem for neuroimaging applications, actively detunable transverse electromagnetic (TEM) head coils were developed to be used with receive-only surface coils for signal-to-noise ratio (SNR) gains and improved spatial coverage from homogeneously excited regions. These SNR and field of view (FOV) gains were achieved by application of a detunable TEM volume coil to human brain imaging at 4T.     

Intensity correction of phased-array surface coil images

Phased-array coils distribute the high signal-to-noise ratio (SNR) performance of their small component surface coils over the larger area covered by the entire array. The inhomogeneous sensitivity profiles of the component surface coils result in images with very high signal near the phased-array and decreased signal far from the array. This paper presents a postprocessing algorithm for correcting these coil-related intensity variations. The algorithm's performance was evaluated by correcting images of volunteers acquired with several different receive-only phased-array surface coils.     

串联线圈大鼠MR成像的初步应用研究

目的 探讨串联线圈进行大鼠3.0 T MR扫描的效果.方法 应用研制的串联线圈与随机所配的3个线圈,分别对自制的水溶液模型运用相同的快速恢复快速自旋回波序列(FRFSE-XL)进行扫描,选择4组图像中同一位置的层面,采用单幅图像测量信噪比(SNR).12只SD大鼠平均分为3个模型组和1个正常组.运用FRFSE-XL序列、快速扰相梯度回波(FSPGR)和多体素波谱(Probe-SI)序列,分别对大鼠脑损伤、脑氢质子MR波谱(1H-MRS)、脊髓损伤(SCI)和大鼠腹部进行了初步应用研究.结果 串联线圈的SNR(39.7)比随机所配的3个线圈中SNR(6.41)最好的膝关节线圈高出6倍以上.大鼠颅脑损伤的T2WI和T1WI能清楚分辨灰质和白质,很好地显示脑室的结构和血肿的位置及大小.注射6-羟基多巴胺(6-OHDA)2周后,大鼠脑部1H-MRS能显示氮-乙酰天冬氨酸/肌酸(NAA/Cr)的比值减小(注射6-OHDA前为1.240,注射2周后为0.781).大鼠胸髓T2WI可清楚显示卵圆形的脊髓,显示"H"形脊髓白质;大鼠SCI模型T2WI清楚显示大鼠胸部脊髓损伤的部位和程度.大鼠腹部扫描,FSPGR序列T1WI时间为8 s,有利于克服呼吸等运动的影响,清楚显示大鼠腹部的结构.结论 应用3.0 T MR结合串联线圈为活体大鼠模型的MR检查提供了很好的方法.     

Proton spectroscopic imaging of the human brain using phased array detectors

Two and four-coil phased array detectors have been developed to increase the sensitivity of proton spectroscopic imaging of the human brain. These include a quadrature figure-8 coil for the study of the vertex, several arrays of 2-4 small overlapping (6-8 cm diameter) circular coils and a combination figure-8 coil plus circular coil. These were constructed in our laboratory and tested to assess their utility for brain spectroscopy. Methods for optimally combining the data from the independent receivers based on the analytical coil maps or measured signal to noise ratios (SNRs) of the data were investigated. High spatial resolution (0.2-0.4 cm³ voxel size) two- or three-dimensional chemical shift images of normal brain were obtained in 17-minute acquisitions. These spatial resolutions are comparable to those previously obtained with conventional small surface coils, but the specialized detectors allow this sensitivity to be achieved for a larger region or for previously inaccessible areas such as the top of the head. The coverage and SNR increases demonstrated are similar to those obtained in magnetic resonance phased array imaging.     

Hybrid phased array for improved internal auditory canal imaging at 3.0-T MR

PURPOSE: To develop and evaluate a hybrid phased array for internal auditory canal (IAC) imaging at 3.0 T. MATERIALS AND METHODS: A hybrid phased array was designed and built as two circular surface receive-only coils combined with a volume transmit-receive birdcage head coil for simultaneous image acquisition. Phantom and volunteer images were obtained to assess the coil performance. RESULTS: The phantom data show that significant signal-to-noise ratio (SNR) improvement was achieved in the region corresponding to the inner ear, i.e., by a factor of 2.5 compared to the standard head coil data. Volunteer IAC image quality was deemed superior as compared to images acquired at 3.0 T using a standard head coil. CONCLUSION: This hybrid array combined with three-dimensional fast spin-echo (FSE) acquisition resulted in improved high spatial resolution IAC imaging.     

Simultaneous acquisition of spatial harmonics (SMASH): Fast imaging with radiofrequency coil arrays

To determine the optimum configuration of a phased array MR coil system for human cardiac applications, the sensitivity of 10 flexible array designs operating under ideal conditions was calculated at 13 points circling the myocardium of a model torso whose geometry was determined from healthy volunteers. The array geometries that were evaluated included continuous strips of 2,4,6, and 10 circular coils of diameter equal to half the torso thickness wrapped laterally around the torso, 2 pairs of coils located on the left side of the chest and back, clusters of 3 coils in 2 orientations, clusters of 4 and 6 coils, and a hybrid cross of 6 coils. The 4-, 6-, and 10-coil strip arrays out-performed the other designs for a given number of coils, yielding average theoretical sensitivity improvements of 45%, 53%, and 55% relative to a single flexible coil positioned at the point closest to the anterior myocardium, compared with about 30% for 4- and 6-coil clusters and the 2-pair geometry (P < 0.02). A flexible 4-coil strip array was constructed for a clinical 1.5 T scanner with 15-cm diameter circular surface coils on flexible circuit board. The signal-to-noise ratio (SNR) of this coil at the 13 cardiac locations was measured in 15 normal volunteers and compared with the SNR measured in images acquired with standard commercial MR coils: a body coil, a flexible torso array, a general purpose flexible coil, and, in 4 subjects, a dual array coil. In the prone orientation, the average myocardial SNR improvement of the 4-coil strip array was 650% relative to the whole body coil, compared with 310–340% for the other commercial coils (P < 0.00005). The twofold advantage over the commercial coils persisted in supine studies (P < 0.00005, n = 5). Thus, flexible circumferential phased arrays of strips of surface coils of diameter comparable with the depth of the heart generally out-perform many other standard geometries for a given number of coils, and can yield dramatically improved SNR over coils available for general use in the torso.     

Signal-to-noise ratio comparison between surface coils and implanted coils

The signal-to-noise ratio (SNR) from implanted coils is widely known to be superior to that from surface coils. This article addresses the quantitative aspects of this improvement by explicitly evaluating the magnetic vector potential in a conducting medium of finite extent for both implanted and surface coils. The predictions of the model are tested with actual image data from spin warp experiments on gelatin phantoms. The authors derived a simplified expression that yields the gain in SNR of an implanted coil relative to that of a surface coil and is valid in many practical situations.