一种可拆装式的脑外科手术用磁共振成像线圈

在临床磁共振成像(MRI)应用中,射频线圈的设计是非常关键的,针对不同的应用目的,合适的线圈能获得质量更好的图像. 有的应用需要线圈提供均匀性较好的射频场,而有的应用则需要线圈在特定区域内提供高的信噪比(SNR). 但是线圈很难同时得到好的射频场（B₁场）、空间均匀性和高的SNR,需要根据实际应用情况进行折衷设计. 针对MRI在脑外科手术中的应用特点,设计并制作了一种新颖的、适用于脑外科手术的MRI接收和发射共用射频线圈. 该线圈采用可分拆式结构,在脑外科手术支架上可以进行反复组装和拆卸,减少了MRI对医生手术的影响. 仿真结果和人体成像实验表明,该线圈能产生均匀的射频场、有较高的SNR和较大的成像范围,满足脑外科手术的需要.     

增强低场下脑功能磁共振成像显著性的研究

实现了基于低场0.35 T磁共振成像系统的大脑功能磁共振成像（functional Magnetic Resonance Imaging,fMRI）的研究. 基于质子密度加权的快速自旋回波(Turbo Spin Echo,TSE)图像,重点研究增强低场fMRI显著性的方法,目的在于提高低场fMRI的可用性. 结果表明：健康受试者在执行手动任务期间,大脑运动区的信号强度变化可以由基于血管外质子信号增强 (Signal Enhancement by Extravascular water Protons,EEP)的对比机制探测. 优化TSE序列参数能提高图像SNR和扫描速度,并在统计分析中增加外在屏蔽图像,可以有效地提高低场下fMRI研究结果的显著性.     

低场核磁共振短死时间射频线圈与射频开关的设计

本文以核磁共振（NMR）射频线圈振铃信号产生原理为对象进行分析研究,提出了一种适用于低场环境下由环状间隙腔线圈与螺线管线圈构成的收发分离式短死时间射频线圈设计方案,采用优化调谐匹配网络提高发射效率;根据射频线圈方案需求设计了快速切换的射频开关及驱动．在此基础上依据仿真结果制作了短死时间射频线圈,并应用于自主研制的低场9.51 MHz便携式NMR谱仪系统,进行NMR实验,结果显示可将收发切换时间缩短至10 μs以内,验证了该设计方案的可行性．     

NQR炸药探测系统中射频线圈的设计研究

基于核四极矩共振(Nuclear Quadrupole Resonance, NQR)炸药探测原理,对探测系统中拾取信号的关键部件射频线圈进行了优化设计,并确定了小型螺线管型线圈的直径、长度和匝数. 试验测试表明,此线圈射频场均匀性好、信噪比大、灵敏度高,可以快速准确探测到NQR信号,验证了线圈设计理论的有效性和制作方法的可行性. 该设计方法对提高隐藏炸药探测的准确率以及不同试验条件下射频线圈的设计具有重要意义.     

低场MRI系统中2通道脊椎线圈的研制

实现了用于低场磁共振成像系统的２通道脊椎线圈,它的基本结构是两个8字型的表面线圈．另外,讨论了射频线圈的去耦问题,最后在永磁0.3T磁共振成像系统上进行了实验．     

高场磁共振成像1H/31P双调谐射频线圈研究进展

在众多可产生磁共振现象的原子核中,¹H核凭借其在生物体中含量高、磁共振信号强的优势,成为磁共振成像的主要研究对象．但其它杂核在生命科学相关研究中同样具有不可替代的独特性,如³¹P核广泛参与了生物体内的能量代谢过程,是非质子成像研究领域的重要内容．MRI向更高场强的发展使得杂核成像逐渐普及,其核心部件是高质量的¹H/³¹P双调谐射频线圈．本文总结了与¹H/³¹P双调谐射频线圈相关的研究与应用,展示了9.4 T下小鼠脑的质子磁共振成像及磁共振磷谱,并讨论了高场¹H/³¹P双调谐射频线圈的潜在应用价值．     

1.5 T下高介电材料几何结构对发射场影响的仿真研究

近年来研究发现,在高场及超高场磁共振成像（MRI）中,高介电材料在提高磁共振射频线圈性能,以及增强图像信噪比方面具有极大的应用潜力.当前高介电材料研究主要集中于其对磁共振图像信噪比的改善,但对于高介电材料几何结构,以及其对发射场分布均匀度影响的研究不多.本研究利用电磁仿真的方法定量分析了1.5 T下,高介电材料几何结构对水模感兴趣区内发射效率均值和发射场$ {B}_{\text{1}}^{\text{+}} $均匀度的影响.结果表明,高介电材料的几何结构对$ {B}_{\text{1}}^{\text{+}} $均匀度会产生较大影响;比较了不同几何结构的高介电衬垫之后发现,加入四等分圆筒状高介电衬垫后,感兴趣区内发射效率提升最高,同时$ {B}_{\text{1}}^{\text{+}} $均匀度也保持良好.该结果对高介电材料应用于MRI具有重要的参考价值.     

利用主动式回馈场的磁共振成像研究

磁共振成像在现今临床医学诊断上占有相当重要的地位.如何增强成像对比度是磁共振成像研究领域中极为重要的课题.传统成像中利用磁共振性质的差异性来获得成像对比度,然而当这些性质的差异性小时,即使经过时间演化,还是难以得到可视的对比度.为了解决此问题,辐射阻尼是其中一个可能的方案.之前的研究显示,辐射阻尼对极小频率差亦能够在短时间的演化下得到相当强的对比度.然而辐射阻尼的产生需要高敏感度的射频线圈等限制.该研究将介绍如何利用外加的线路,模拟辐射阻尼现象,进而控制自旋动态,以达到增强成像对比度并进一步设计新的磁共振成像方式.     

三维数值模拟射频热等离子体的物理场分布

射频热等离子体的产生包含了丰富、复杂的物理场分布,正确认识这些物理场分布对射频热等离子体在工业领域的应用有指导作用.本文建立了三维射频热等离子体的热-电-磁-流动强耦合数学物理模型,考虑了感应线圈的真实螺线管结构,开发了用于计算三维射频热等离子体复杂电磁场的C++程序代码,计算了射频热等离子体重要物理场,如温度场、流场和电磁场的分布情况.结果表明,射频热等离子体各物理场分布具有三维非对称效应,感应线圈结构对温度场和流场的空间分布有重要影响.研究结果对优化、控制射频热等离子体的实际应用过程有重要的指导意义.     

低场磁共振系统的低噪声前置放大器研究设计

噪声前置放大器是磁共振射频接收子系统中的重要组件,它的性能优劣直接决定了最终磁共振图像的好坏.目前市场上的低噪声前置放大器大多基于中高场磁共振系统开发,而针对低场磁共振系统的很少;另外,商用低场磁共振系统的低噪声前置放大器价格相对较贵,并且多采用两级放大结构,结构复杂、调试难度大、成本相对较高.在此背景下,针对0.5 T低场磁共振设备利用Keysight公司的先进设计系统（ADS）软件对低噪声前置放大器进行研究设计,采用一级放大结构,探索电路设计与布局对放大器性能的影响.实测结果表明自主设计的低噪声前置放大器在21 MHz共振频率附近噪声系数为0.5 dB左右,增益达到了30 dB,能够满足低场磁共振应用的要求.     

NMR strong off-resonance irradiation without sample overheating

A prototype NMR probe for long RF pulse has been constructed. Its main elements are two coils mounted in the concentric position. The first bigger coil is wound around a glass dewar tube and the second smaller coil is placed inside the dewar. These two coils are thermally isolated by the dewar. A long and strong RF pulse is applied to the bigger coil. The smaller detection coil inside the dewar contains a sample and to this coil a short RF pulse is applied. The two coils are independently tuned and electrically isolated. During the operation of the strong RF pulse the smaller coil has a high resistance to ground (very low Q factor) and does not absorb energy from the bigger coil. During the operation of the short on-resonance RF pulse the bigger coil is detuned to a higher frequency, but the resonance circuit with the small coil is in the electrical resonance. The NMR probe may be used in off-resonance experiments in which long and strong RF pulses are applied to the bigger coil and thereby the problem of the sample overheating is avoided.     

Towards a complete coil array

A complete RF coil system, as has been previously defined, is capable of generating any steady-state RF field, at the MR frequency, that is compatible with Maxwell's equations. A coil system is complete if it is capable of generating all basis vector fields in the multipole expansion of the electromagnetic fields. A complete coil system has the potential to reach the ultimate intrinsic signal-to-noise as an MRI receiver coil. It also offers maximum flexibility in tailoring the spatial RF field distribution as an excitation coil. Here, computer simulations have been performed on array coils employing composite coil elements, assuming the current loops are small and can be approximated by magnetic dipoles. We demonstrate that a coil array can be configured to approximate a truncated complete array coil and to generate the basis magnetic vector fields up to certain orders in the multipole expansion of the electromagnetic fields.     

An open volume, high isolation, radio frequency surface coil system for pulsed magnetic resonance

We present an open volume, high isolation, RF system suitable for pulsed NMR and EPR spectrometers with reduced dead time. It comprises a set of three RF surface coils disposed with mutually parallel RF fields and a double-channel receiver (RX). Theoretical and experimental results obtained with a prototype operating at about 100 MHz are reported. Each surface RF coil (diameter 5.5 cm) was tuned to f₀ = 100.00 ± 0.01 MHz when isolated. Because of the mutual coupling and the geometry of the RF coils, only two resonances at f₁ = 97.94 MHz and f₂ = 101.85 MHz were observed. We show they are associated with two different RF field spatial distributions. In continuous mode (CW) operation the isolation between the TX coil and one of the RX coils (single-channel) was about −10 dB. By setting the double-channel RF assembly in subtraction mode the isolation values at f₁ or f₂ could be optimised to about −75 dB. Following a TX RF pulse (5 μs duration) an exponential decay with time constant of about 600 ns was observed. The isolation with single-channel RX coil was about −11 dB and it increased to about −47 dB with the double-channel RX in subtraction mode. Similar results were obtained with the RF pulse frequency selected to f₂ and also with shorter (500 ns) RF pulses. The above geometrical parameters and operating frequency of the RF assembly were selected as a model for potential applications in solid state NMR and in free radical EPR spectroscopy and imaging.     

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.     

Effects of coil dimensions and field polarization on RF heating inside a head phantom

Deterioration of radiofrequency (RF) inhomogeneity with increasing static magnetic field in magnetic resonance imaging (MRI) is one of the fundamental challenges preventing their clinical rendition and posing safety hazards. Variation in RF coil designs could help redistribute RF energy absorption over the imaged object. This work is intended to determine experimentally the difference in RF heating produced within a human head phantom by in situ measurement of RF inhomogeneity as a function of coil design utilized at 8 T. The heating patterns of 1/4 wavelength (long) and 1/8 wavelength 11-cm (short) transverse electromagnetic (TEM) coils loaded with a homogeneous human head phantom at 340 MHz were evaluated. In addition, different transmit/receive (T/R) configurations were used in search for the possibility of "hot-spot" formation. Fluoroptic thermometry was used to measure temperatures in multiple positions in a head phantom made of ground turkey breast for RF powers corresponding to a specific absorption rate (SAR) of 4.0 W/kg for 10 min. Numerical simulations were performed to study the general RF power deposition patterns in phantoms at 340 MHz including the effects of field polarization. The temperature increases varied from 0 to 0.8 degrees C for the long RF coil, while the short RF coil produced a maximum temperature change of 0.5 degrees C. Similar to ultra high-field electromagnetic simulations, these measurements revealed low peripheral and high deep-tissue heating at 8 T. The findings indicated that the largest temperature changes for both cases were less than 1 degrees C. While these results showed an increase in localized heating due to RF pulses at 8 T, they highlight that RF inhomogeneity could be redistributed using different RF coil designs through which the hot spots could be made cooler.     

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.     

A microstrip transmission line volume coil for human head MR imaging at 4T

A high-frequency RF volume coil based on the use of microstrip transmission line (MTL) has been developed for in vivo 1H MR applications on the human head at 4T. This coil is characterized by major advantages: (i) completely distributed coil circuit, (ii) high-quality factor (Q), (iii) simple coil structure, and (iv) better sensitivity and less signal-intensity variation in the MR image of the human head compared with an RF shielded birdcage coil of similar coil size. The proposed MTL volume coil does not require additional RF shielding for preventing Q degradation from radiation losses due to the unique MTL structure; thus, it provides a maximal useable space inside the volume coil when compared with most volume coils available at high fields with the same overall coil size. The intrinsic B(1) distribution of the MTL volume coil effectively compensates for the dielectric resonance effect at 4T and improves the signal homogeneity in human head MR images in the transaxial planes. The results of this study demonstrate that the MTL volume coil design provides an efficient and simple solution to RF volume coil design for human MR studies at high fields.     

Flat RF coils in static field gradient nuclear magnetic resonance

The use of flat RF coils allows considerable gains in the sensitivity of static field gradient (SFG) nuclear magnetic resonance (NMR) experiments. In this article, this effect is studied theoretically as well as experimentally. Additionally, the flat coil geometry has been studied theoretically depending on magnetic field gradient, pulse sequence and amplifier power. Moreover, detecting the signal directly from the free induction decay (FID) turned out to be quite attractive for STRAFI-like microimaging experiments, especially when using flat coils. In addition to wound rectangular flat coils also spiral flat coils have been developed which can be manufactured by photolithography from printed circuit boards.     

高温超导磁共振成像射频接收线圈的研究

为了提高低场磁共振成像系统的信噪比,提出了具有失谐电路的Bi2223带高温超导射频接收线圈.该线圈采用了电耦合方式传输超导谐振回路的磁共振信号,这种方式有利于进一步制成正交结构或相阵结构的超导接收线圈.为了防止趋肤效应降低超导接收线圈的性能,采用化学腐蚀的方法先将超导带的包套去掉,然后再制成超导主谐振电感.采用一种双探测线圈法对高温超导接收线圈和相同结构的常规铜线圈的Q值进行了测量,结果表明超导接收线圈比常规铜线圈的Q值约高一倍.     

Multiple Parallel Round Leg Design for Quadrature Birdcage Coil in Ultrahigh-Field MRI

Copper foil has been widely employed in conventional radio frequency (RF) birdcage coils for magnetic resonance imaging (MRI). However, for ultrahigh-field (UHF) MRI, current density distribution on the copper foil is concentrated on the surface and the edge due to proximity effect. This increases the effective resistance and distorts the circumferential sinusoidal current distribution on the birdcage coils, resulting in low signal-to-noise ratio (SNR) and inhomogeneous distribution of RF magnetic (B₁) field. In this context, multiple parallel round wires were proposed as legs of a birdcage coil to optimize current density distribution and to improve the SNR and the B₁ field homogeneity. The design was compared with three conventional birdcage coils with different width flat strip surface legs for a 9.4 T (T) MRI system, e.g., narrow-leg birdcage coil (NL), medium-leg birdcage coil (ML), broad-leg birdcage coil (BL) and the multiple parallel round wire-leg birdcage coil (WL). Studies were carried out in in vitro saline phantom as well as in vivo mouse brain. WL showed higher coil quality factor Q and more homogeneous B₁ field distribution compared to the other three conventional birdcage coils. Furthermore, WL showed 12, 10 and 13% SNR increase, respectively, compared to NL, ML and BL. It was proposed that conductor’s shape optimization could be an effective approach to improve RF coil performance for UHF MRI.