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  波谱学杂志   2021, Vol. 38 Issue (1): 118-139.  DOI: 10.11938/cjmr20202826
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引用本文 [复制中英文]

马聪伟, 杨鸿毅, 钟凯. 高场磁共振成像1H/31P双调谐射频线圈研究进展[J]. 波谱学杂志, 2021, 38(1): 118-139. DOI: 10.11938/cjmr20202826.
[复制中文]
MA Cong-wei, YANG Hong-yi, ZHONG Kai. Research Progresses of High-Field MRI 1H/31P Dual-Tuned Radio Frequency Coil[J]. Chinese Journal of Magnetic Resonance, 2021, 38(1): 118-139. DOI: 10.11938/cjmr20202826.
[复制英文]

基金项目

国家重点研发计划(2018YFE0205700);中科院合肥大科学中心"高端用户培育基金"资助项目(2019HSC-UE006)

通讯联系人

钟凯, Tel: 0551-65595192, E-mail: kzhong@hmfl.ac.cn

文章历史

收稿日期:2020-04-13
在线发表日期:2020-05-28
高场磁共振成像1H/31P双调谐射频线圈研究进展
马聪伟 1,2, 杨鸿毅 1,2, 钟凯 1,2     
1. 中国科学院 强磁场科学中心, 安徽 合肥 230031;
2. 中国科学技术大学, 安徽 合肥 230026
摘要: 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.
关键词: 高场磁共振成像    射频线圈    1H    31P    双调谐    
Research Progresses of High-Field MRI 1H/31P Dual-Tuned Radio Frequency Coil
MA Cong-wei 1,2, YANG Hong-yi 1,2, ZHONG Kai 1,2     
1. High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, China;
2. University of Science and Technology of China, Hefei 230026, China
Abstract: 1H nucleus is the most commonly used nucleus for magnetic resonance imaging (MRI) due to its high natural abundance in organisms and high MR sensitivity. However, other nuclei are distinctive in physiological processes, such as 31P that is important in the energy metabolisms. The development of high-field MRI allows for MRI research based on 31P nucleus, while high-quality 1H/31P dual-tuned radio frequency (RF) coil is essential for such applications. This article summarized the researches and applications of various 1H/31P dual-tuned RF coils, and presented in vivo 1H MRI and 31P MR spectra of mouse brain at 9.4 T. Potential applications of 1H/31P dual-tuned RF coil for high field were discussed.
Key words: high-field magnetic resonance imaging    radio frequency coil    1H    31P    dual-tuned    
引言

磁共振成像(magnetic resonance imaging,MRI)是一种成熟的非侵入性成像技术.由于MRI没有组织穿透深度限制[1],也没有计算机断层扫描(computed tomography,CT)或X射线中产生的电离辐射[2],因此,其在临床医学和科研中发挥着越来越重要的作用[3].磁场强度是实现MRI的一个物理基础,磁场强度越高,成像分辨率越高.从20世纪80年代初的1.5 T和2.0 T MRI系统、80年代末的4.0 T[4]、90年代末的第一台人体8.0 T[5]系统,逐渐发展到本世纪的9.4 T和15.2 T等系统,可见高场强是MRI系统发展的一个必然趋势.

MRI的另一个物理基础是磁场中有可产生磁共振现象的原子核.基于质子的MRI是当前最为成熟的成像技术,因为观察的需要,基于非质子核(X核),例如31P、23Na或19F等的MRI也具有很高的研究和应用价值[2].磷是机体极为重要的元素之一,在细胞的核糖核酸和脱氧核糖核酸中均有分布,是生物体遗传代谢、生长发育、能量供应等方面不可或缺的元素.但与质子相比,31P核具有较低的天然丰度和MR敏感性,在低场条件下,其MRI信噪比(signal to noise ratio,SNR)低,以致应用困难.随着磁场强度的提高,成像的灵敏度、分辨率都有很大程度的提升,使得31P MRI研究得以逐步开展.

除了足够的磁场强度,实现31P MRI的另一个必要硬件条件是具备相应的磁共振射频(radio frequency,RF)线圈.RF线圈在MRI系统中具有两方面的作用:一是产生拉莫尔频率下的RF磁场B1,以激发处于静磁场B0中的原子核,使磁矩偏转;二是接收磁共振原子弛豫过程中产生的自由感应衰减(free induction decay,FID)信号.RF线圈可直接影响MRI的信号灵敏度、图像SNR以及均匀度等.高场条件下共振频率相对较高,会使RF线圈的性能发生改变,设计研发时要加以考虑.此外,一般不用单一的31P线圈成像,需用1H和31P结合(1H/31P)的双调谐RF线圈.其中,1H MRI可用于定位、匀场及相应的对比分析.但1H核和31P核的拉莫尔频率不同,因此兼容性成为实现1H/31P双调谐RF线圈的必要条件之一.

本文主要对MRI系统的基本组成、RF线圈的主要类型、高场下RF线圈的设计挑战、双调谐RF线圈的实现方法,以及1H/31P双调谐RF线圈的发展与应用进行综述.

1 MRI及MRI系统

MRI作为非侵入性了解人体结构的有力工具,为生物医学的发展做出了巨大贡献.最早是在1938年,Rabi在真空中用分子束测量和描述了原子核的核磁共振(nuclear magnetic resonance,NMR)现象[6].1945年,Bloch和Purcell在凝聚态物质中分别证明这一现象,并于1952年同时获得诺贝尔物理学奖[7].1950年Hahn的自旋回波法[8]和1966年Ernst的傅里叶变换磁共振波谱(magnetic resonance spectroscopy,MRS)[9]是MRI史上最重要的贡献之二.Damadian在1971年发现癌组织较正常组织具有更长的弛豫时间,这个发现推动了NMR在生物医学方向的应用[10].1973年,Lauterbur[11]和Mansfield[12]独立描述了在梯度磁场的作用下,利用空间编码技术可使MR信号成像.随着一系列新的技术、方法和硬件的发展,MRI已经转变为一种卓越的医学诊断工具.

MRI系统主要包括磁体、匀场模块、梯度系统、RF系统以及计算机系统等.磁体可在其中心产生稳定且空间分布均匀的静磁场B0B0场强的提升可以改善图像SNR、对比度以及分辨率等.匀场模块是用来消除磁场的低阶不均匀量,对磁场进行矫正以防止磁场不均匀引起的信号畸变失真[13].梯度系统包括梯度脉冲产生器、梯度脉冲放大器以及梯度线圈.梯度脉冲发生器产生梯度电流,经梯度脉冲放大器放大后驱动梯度线圈产生梯度磁场.梯度线圈在空间三个正交方向形成梯度磁场,可提供图像重建所需的空间编码信息.RF系统包括RF脉冲功率放大器、RF收发(transmit/receive,T/R)开关、低噪声前置放大器、混频器以及RF线圈.RF线圈是信号链路中最前端的传感器,直接作用于磁共振的研究对象.其中RF发射线圈用来产生作用于进动原子核的B1磁场;接收线圈用于接收MR信号.计算机系统负责协调控制MRI设备每个部分的运行和图像重建与处理等[14].图 1是根据文献[14]绘制的MRI系统的主要设备示意图.

图 1 磁共振成像系统的主要设备示意图. 主要包括磁体、匀场模块、梯度系统、射频系统以及计算机系统等. 其中箭头表示信号在设备链路中的传输方向. 上方和下方分别是射频系统的发射和接收设备链路. 其中射频线圈位于射频系统的前端. 中间的设备链路是梯度系统. 计算机系统连接着其它各系统并利用各种磁共振序列加以控制(根据文献[14]绘制) Fig. 1 Block diagram of the main equipment of an MRI system. The MRI system includes the magnet, the shimming module, the gradient system, the RF system and the computer system etc. The arrows indicate the directions of the signal flow in the device links. The upper and the lower links are the transmitting and receiving channels of the RF system respectively. RF coil is at the front end of the RF system. The device link in the middle is the gradient system. The computer system is connected to all the systems and controls them with various MR sequences (Reproduced from Ref. [14])
2 RF线圈

RF线圈是MRI系统的关键部件,用于产生与主磁场B0垂直的激励磁场B1和接收MR信号.当MRI系统工作时,发射机发送RF脉冲,并施加于RF线圈,以产生使原子净磁化强度偏离主磁场方向的B1磁场.RF功率越强,偏离角度即翻转角越大.磁化强度在恢复到原方向的过程即弛豫过程中,会对外激发出电磁波,接收线圈通过电磁感应产生感应电压即MR信号,接收机对MR模拟信号进行放大、混频等处理,最终通过模数转换进入计算机.在RF线圈的研发过程中主要关注线圈的调谐匹配性能、解耦性能、品质因数以及均匀度等问题.同时,因线圈具有多种类型,在设计线圈的过程中要根据实验对象选择合适的线圈构型.尤其在高场下,随着拉莫尔频率的增加,RF线圈的研发有许多新的问题需要考虑.

2.1 RF线圈的研发要点

RF线圈理论上都是由电容和电感组成的谐振电路构成,需要调谐,即在成像过程中,无论是发射脉冲进入发射线圈还是接收线圈输出MR信号,这些线圈都必须工作在其谐振频率即拉莫尔频率下.线圈若失谐,即谐振频率点不在拉莫尔频率,线圈的灵敏度会下降,MR信号的SNR随之下降.一般情况下,线圈的失谐主要是由样品和磁体环境造成的.加载样品使线圈内介电常数增大,即等效电容增大,则谐振频率减小.线圈进入磁体环境会使等效电感变小,谐振频率增大.调整谐振频率(即调谐)意味着调整电容或者电感,以使线圈电路的共振频率与组织中自旋核的磁共振频率匹配.用电感调谐时,可以采用额外铜片,通过移动铜片和线圈之间的距离完成,也可以通过移动线圈屏蔽和线圈的重叠面积实现[15].实际应用中最多最方便的方式是采用电容调谐,通过在回路中串联或者并联可调电容,进而改变可调电容的容值来实现谐振频点移动.

RF系统的另一个问题就是阻抗匹配问题.线圈处于发射状态时,线圈阻抗和传输线特征阻抗匹配才能确保最大的RF功率传递到核自旋.线圈处于接收状态时,阻抗匹配才能保证接收到的微弱MR信号最大化地传输.若阻抗不匹配,则发射脉冲在线圈处会发生反射,降低发射效率;同时接收到的MR信号会存在大量的损失,降低成像SNR.

接收线圈接收到的MR信号是微伏(μV)量级的弱信号,尤其是来自X核的MR信号,经不起外来干扰或长电缆的衰减[14].因此,研发RF线圈的一条原则是尽量降低噪声影响或提高信号强度,一般会采用一个处于RF线圈接收链路前端的低噪声前置放大器对接收到的FID信号进行放大,再传输至接收机进行后续处理.

在设计收发一体式RF线圈的过程中,为了解决发射通道与接收通道之间的耦合、串扰等问题,需要设计收发开关.收发开关[16]的作用是在MRI系统处于发射状态时,接通发射机与RF线圈,将RF功率信号传输至RF线圈,从而激发核磁矩,同时保证RF大功率信号不会进入接收链路而损伤低噪声前置放大器.在MRI系统处于接收状态时,收发开关必须保证接收的MR信号经接收链路传输到接收机,同时隔离发射链路的串扰.

在为多核MRI实验设计多调谐RF线圈时,必须考虑多个通道之间的解耦以防止因耦合导致的频率分裂等问题.一般可采用PIN二极管、电容及电感组成的失谐电路或高阻抗电路等方法来实现良好的解耦.失谐电路的作用是在其中的一个通道工作时,其他通道处于失谐状态;高阻抗电路的作用是在其中一个通道谐振时,阻止其他通道耦合到该通道的信号影响该通道的谐振;还有一种解耦合的办法是采用具有固有几何解耦结构的线圈来设计多通道线圈.

2.2 RF线圈的构型分类

RF线圈按其构型可分为体线圈、表面线圈和阵列线圈.线圈研发时,一般可使用大容量的体线圈作为发射线圈,确保激发产生均匀的射频磁场B1;而使用小的表面线圈作为接收线圈,能够在感兴趣区域(region of interest,ROI)中提供出色的灵敏度.阵列线圈可以综合体线圈和表面线圈的特点,作为发射线圈、接收线圈或者收发线圈使用.

体线圈一般可以将测试对象完全包含,主要作为发射线圈或者收发线圈使用,其类型主要有螺线管线圈、鞍形线圈、亥姆霍兹线圈、鸟笼线圈、横向电磁(transverse electromagnetic,TEM)体线圈等.鸟笼线圈是较常用也较成熟的一种体线圈,在线圈范围内能够产生高度均匀的B1[17].自1946年Pound和Purcell在哈佛大学建立第一个同轴谐振腔线圈[18]以来,越来越多的研究人员利用传输线原理设计并制作TEM体线圈[19-21].目前,TEM体线圈已经是高场领域应用较多的线圈[22],其结构中的传输线可以是同轴线、带状线或微带线、波导等.

表面线圈这个概念由Suryan在1951年首次提出[23].自从Ackerman等[24]在1980年设计出一种简单而高效的表面线圈用于标绘活体组织内的代谢情况图以来,小型RF表面接收线圈已被证明具有很高的灵敏度,并且易于构造.不同类型的RF表面线圈有不同的构型和RF磁场分布特点.最简单且使用最多的表面线圈是由导电材料(或集总元件电路)制成的单一圆形或矩形环(circular-loop,CL)[24],该类型线圈已成功用于许多MRI的研究中,如图 2(a)所示,其产生的B1磁场在线圈中心处垂直于线圈表面[25],且在线圈导体附近较强,中心较均匀.另一种类型的表面线圈是横向磁场表面线圈,常用的横向磁场表面线圈有“蝶形线圈”和“FO8(figure-of-eight)线圈”[26, 27].蝶形线圈和FO8线圈中心处B1磁场方向是平行于其线圈平面的,其中蝶形线圈在线圈中心交叉处产生的B1磁场最大,如图 2(b)所示;FO8线圈产生的B1磁场在平行的导体附近有较大值,且沿着导体方向呈较均匀的带状分布[28],如图 2(c)所示.本文作者采用数值方法模拟仿真了400 MHz(9.4 T高场下的拉莫尔频率)下三种表面线圈空载时的RF磁场分布,三种线圈尺寸接近,并对每个线圈的集总端口施加峰值为1 V的电场激励,获得的B1+(B1可分解为两个旋向相反的圆形极化分量,其中与核磁矩进动旋转方向一致的圆形极化分量是磁共振的有效分量,称为B1+)幅值分布图如图 2所示.

图 2 数值模拟400 MHz下三种表面线圈的射频磁场B1+的幅值分布. (a)单环形(CL)线圈B1+的幅值分布,CL线圈呈单一圆环或矩形环,B1磁场在线圈导体附近较强,中心较均匀;(b)蝶形线圈B1+的幅值分布,线圈由相连的两个环构成,在线圈中心两导体按一定角度交叠,但不短路,其B1磁场的最大值是在线圈中心交叉处;(c) FO8线圈B1+的幅值分布,该线圈由相连的两个半环构成,中心的两条导体是平行且间隔的,其B1磁场在平行的导体附近沿着导体方向呈带状分布 Fig. 2 Numerical simulation of the B1+ amplitude distribution of the three surface coils at 400 MHz. (a) The B1+ amplitude distribution of CL coil. CL coil is a single circular or rectangular ring, the intensity of B1 is relatively strong near the conductor of the coil and uniform in the center; (b) The B1+ amplitude distribution of butterfly coil. Butterfly coil is composed of two rings, which cross in the center at a certain angle but isolated to each other. The maximum B1 intensity is at the intersection of the center of the coil; (c) The B1+ amplitude distribution of FO8 coil. FO8 coil consists of two half-rings with two conductors in the center being parallel and spaced out, and the B1 is banded along the conductor direction near the parallel conductor

偶极子天线是另一种表面线圈结构,在通信领域,偶极子天线已经比较常用,但在MR领域,偶极子天线的应用还不算成熟[29, 30].2009年,Raaijmakers等[31]提出辐射天线可以作为MRI系统的RF线圈使用.波长线的尺寸是导致电偶极子线圈在低频场使用率低的主要原因,但高场时波长线较短,电偶极子线圈尺寸较合适.Raaijmakers等[30]又于2016年证明,7.0 T下偶极子天线可以产生对称且均匀的RF磁场,与环形线圈相比,偶极子天线具有更高的接收灵敏度和发射效率.Lattanzi等[32]在2018年证明,即使偶极子天线典型的辐射方向图在自由空间是全方向的,但其在介质负载附近是有效的磁流变线圈;且与标准环形表面线圈相比,偶极子天线可以产生无旋度和无发散电流,而环路线圈只限于产生无发散电流模式,这也是偶极子线圈在超高场强下的优势所在,可以使超高场下样品成像的SNR接近固有SNR[32].MRI中使用的偶极子有多种类型:偶极子天线[33]、折叠式偶极子天线[34]、圆形偶极子天线[35]、分级偶极子天线[36, 37]、以及线圈和偶极子天线的组合[38].本文作者采用数值方法模拟仿真了400 MHz下偶极子天线的RF磁场,设置偶极子天线(图 3中白色细长圆柱体)的每根天线臂长为1/4波长线,其半径为1/20臂长,其间隙为1%臂长.在偶极子天线中间的集总端口施加峰值为1 V的电场激励,吸收边界层设置在球体的外层为完全匹配吸收边界层.图 3是上述偶极子的B1+幅值分布仿真,结果表明偶极子两端之外区域B1+比中心区域弱;但在中心处xy面内,幅值呈中心对称分布.

图 3 数值模拟400 MHz下偶极子天线的射频磁场B1+的幅值分布. 左侧为yz面的磁场分布,右侧为xy面的磁场分布. 该模型由两个圆柱形导电材料臂(图中白色细长圆柱体)和一个施加在两臂之间峰值为1 V的电压源组成. 天线工作频率400 MHz处的自由空间波长为0.749 m,每个天线臂长0.187 m,并与z轴平行. 天线周围是以完全匹配层(PML)为边界的自由空间域,PML起到吸收外辐射的作用 Fig. 3 Numerical simulation of the B1+ amplitude around the dipole antenna at 400 MHz. Distributions of magnetic field B1+ are depicted in the yz plane (left) and xy plane (right). The model is composed of a dipole with two cylindrical conductive material arms (the white slender cylinder in the left figure) and a voltage source with amplitude of 1 V. Each arm is 0.187 m long as a quarter of the wavelength and parallel to the z-axis. The antenna is in a free space domain and bounded with a perfectly matched layer (PML), which absorbs the radiation without reflection

多个表面线圈的组合称为阵列线圈(图 4为八通道阵列接收线圈示意图).该线圈同时采用前置放大器(Preamp)解耦以及最邻近解耦两种方法来解决通道之间的耦合问题.如果每个通道SNR通过平方和方法组合,则可以在更大的ROI上获得高SNR[29, 39].1990年,Roemer等[40]在一篇开创性工作的论文中首次描述阵列线圈,并用于脊柱MRI实验,其成像优势明显.阵列线圈由小的表面线圈组成,线圈与样品之间紧密耦合,增加了信号,提高了单个元件的局部灵敏度,从而提高了成像SNR.阵列线圈中的每一个通道可以独立获取MR信号,即允许多通道并行成像,从而加快了扫描、减少了图像伪影以及失真等.除了提高SNR、减少扫描时间外,阵列线圈还扩大了成像视野(field of view,FOV).由于阵列线圈具有同时工作的表面线圈元件,因此必须尽量减少元件之间的串扰,否则会影响成像SNR.减少相邻线圈元件之间耦合的有效方法之一是通过阵列元件的部分重叠来实现[41],而减小不相邻线圈元件之间的耦合可以通过低输入阻抗的前置放大器加以解决[42].阵列线圈因其每个通道独立成像而具有上述多个优点,但也由于其多个通道需要多个耦合器、移相器、放大器等硬件电路,导致其电路相对复杂,不易制备.CL、FO8、蝶形线圈和偶极子天线线圈都可以作为阵列线圈的阵列元件,或者作为双调谐RF线圈的基础.同时,阵列线圈的高灵敏度以及高成像速度使其成为高场MRI的RF线圈的一个发展趋势.

图 4 八通道阵列接收线圈示意图,包括线圈及接收通道. 线圈1~8(Coils 1~8)为8个独立的表面线圈,每个线圈有各自独立的接收通道,包括前置放大器(Preamp)和接收机(Receiver)等设备,8个通道的信号由数字多路复用器(Digital MUX)输入计算机进行后处理. 该阵列线圈同时采用前置放大器解耦以及最邻近解耦两种方法来解决通道之间的耦合问题[39] Fig. 4 Schematic of an 8-channel Rx array coil, which includes coils and receiving channels. Coils 1~8 are eight independent surface coils, and each coil has its own independent receiving channel including preamplifier, receiver and other devices. The signals of eight channels are processed by digital MUX and computer. Here two methods are involved to decouple channels. One is application of preamplifiers and the other is overlapping of the adjacent coils[39]
2.3 高场MRI系统RF线圈设计的机遇及主要挑战

高场MRI系统发展的主要动力是提高灵敏度,进而提高成像的空间或时间分辨率.灵敏度是检测到微弱MR信号的能力度量,通常使用SNR进行量化.常用的衡量SNR的方式是特定区域内信号的平均强度和背景噪声的标准差之比.文献[43]给出RF线圈信噪比SNR的一种表达式如下:

$ SNR = K\eta {M_0}{\left( {\frac{{{\mu _0}Q{\omega _0}{V_c}}}{{4Fk{T_c}\Delta f}}} \right)^{\frac{1}{2}}} $ (1)

其中K是取决于接收线圈几何结构的约为1的数值因子,η为填充因数,${M_0}$是与场强${B_0}$呈正比的磁化强度,${\mu _0}$为自由空间的磁导率,Q是线圈的品质因数,${\omega _0}$是拉莫尔角频率,${V_c}$是线圈的体积容量,F是前置放大器的噪声系数,k是玻尔兹曼常数,${T_c}$是线圈与样品的相对温度,$\Delta f$是接收线圈的带宽.

(1) 式中的${M_0}$${\omega _0}$均与场强${B_0}$呈正比.随着${B_0}$增强、${M_0}$增加,MR信号随之增强,图像的SNR也随之提高[14],这是高场MRI的主要优势之一.从线圈优化的角度来讲,若要提高SNR,就需要减小有效温度${T_c}$和噪声系数F,增加线圈填充因数η和品质因数Q.可以采用超导低温线圈减小${T_c}$,采用低噪声的元器件或者材料减小F.而增大η,可以从线圈构型、线圈尺寸,以及线圈和样品的贴合度入手,一般情况下表面线圈可以与样品紧密贴合从而增大η.Q与频率密切相关,高场环境下,拉莫尔频率增大,为保证高场下成像的SNR,就必须保证高场下Q值较高.Q值定义为存储能量和消耗能量的比值[29],消耗能量越小即损耗越小、存储能量越大时,Q值就越大.然而实际上,RF线圈的损耗会随着${B_0}$的增加而增加.RF线圈损耗分为对线圈电路的电阻损耗和对线圈磁场的辐射损耗.高场下,线圈电路由于拉莫频率高、趋肤深度降低,电感器或线圈导电金属的欧姆损耗增加;同时由于电容器的电容值偏小,线圈电路的介电损耗增加.而且高场环境下,当线圈的物理尺寸和波长尺寸相当时,电流在线圈中的分布呈现出波动性,这样不均匀的电流分布会引起磁场均匀度和η的下降、辐射损耗的增加,同时大尺寸线圈在高场下的辐射损耗也会增加.所以,在提高${B_0}$以改善SNR的同时,线圈损耗也随之增加,RF磁场的均匀度也随之下降.为了保证高场下RF线圈Q值的最大化,就需要最小化线圈损耗,这也对高场下RF线圈的制作提出了新的挑战.对于RF线圈的欧姆损耗,可以采用分布式电路或增加线圈金属的表面积来减少,同时也可以通过减小线圈尺寸来减少[44];对于RF线圈的辐射损耗,可以通过减小线圈尺寸、使用高Q值的元器件、采用有效的屏蔽结构以及依据传输线理论制成的微带TEM线圈等方法来减少[45].

3 1H/31P双调谐RF线圈

MRS在化学中的应用由来已久,随着应用研究的深入和高场MRI系统的飞速发展,非质子MRS和MRI已逐渐成熟,由理论应用于实践.质子解耦31P MRS在缺血性心脏病[46]和各种脑疾病[47]的诊断中具有一定的价值.而开展31P MRI/MRS的研究,需要设计专用的1H/31P双调谐RF线圈.1H/31P双调谐RF线圈具有两个谐振频率,分别是质子和31P核的拉莫尔频率,该线圈可以在不移动实验对象和线圈的情况下,利用质子成像进行定位,并对31P核进行MRI/MRS实验.

3.1 双调谐RF线圈的分类

在没有发展出双调谐RF线圈时,一些研究人员首先用质子线圈获得质子的MR信号;再利用质子MR信号的快速自动线性匀场算法确定匀场,并允许该匀场值应用于X核成像;然后在不移动测试对象的情况下切换为X核线圈[48].但是更换线圈时会不可避免地引起磁场变化.双调谐RF线圈的出现,让研究者不需要移动线圈,并改善了${B_0}$匀场的流程和质量.双调谐RF线圈的实现方法有很多种,大致可以通过单线圈、双线圈、Four-ring鸟笼线圈、阵列线圈等方法来实现.表 1列举了近些年常用的双调谐RF线圈.

表 1 双调谐射频线圈的分类及相关原理 Table 1 Classification and related principles of dual-tuned RF coils
3.2 1H/31P双调谐RF线圈的发展及应用

1H/31P双调谐RF线圈的发展经历了单线圈双调谐[49, 75-76]、各种鸟笼线圈改进的双调谐线圈[72, 73, 77]、具有几何解耦结构的双调谐表面线圈[78-83],以及阵列线圈[84-87]等方式,也经历了低场到高场的场强变化.

3.2.1 单线圈1H/31P双调谐RF线圈

1985年,Schnall等[49]首次提出了1H/31P双调谐RF线圈,该线圈采用单个线圈并添加集总元件实现双谐振的方法,使用多级电路,将并联电感器和电容器组成的陷波电路串入线圈[50].此时,串入的并联电路不是用来抑制特定频率的电流,而是使整个线圈在两个不同频率下都产生谐振.文中利用所制备的线圈,在2.0 T下测试的离体狗脑的1H MRS包含了乳酸(Lac)、N-乙酰天门冬氨酸(N-Ac-Asp,或NAA)、脂质(Lip),31P MRS包含了磷酸单脂(PME)、磷酸二酯(PDE)、无机磷(Pi)、磷酸肌酸(PCr)、三磷酸腺苷(γ-ATP、α-ATP、β-ATP)等代谢物,如图 5(a)所示.

图 5 (a) 单线圈双调谐射频线圈电路图以及使用该线圈在2.0 T下获得的狗脑的31P和1H MRS. 该线圈在由L1、C1、C3组成的谐振电路的基础上增加了L2C2并联电路,使得电路中出现两个谐振点[49, 76];(b)双调谐线圈电路图以及利用该线圈在1.5 T下获得的志愿者小腿的横轴质子成像和31P MRS. 该线圈电路中采用串联电感和并联电容等效模拟的1/4波长线,使得线圈可以在两个频率下工作且耦合较小[75];(c)具有平衡匹配电路的线圈电路图以及5.6 T下大鼠脑的1H和31P MRS. 该线圈电路在Schnall等[49]设计的单线圈双调谐射频线圈电路中增加了可调电容C4,与C3一起使得该电路具有平衡匹配的作用[76] Fig. 5 (a) The circuit of a double-tuned single RF coil, and the 31P, 1H MRS of a dog brain using the coil at 2.0 T. The coil is composed of a normal resonant circuit with L1, C1, C3 and an additional circuit of L2C2, which leads to two resonance frequencies[49, 76]; (b) The circuit diagram of double-tuned coil that designed by Leach, and the 1H MRI, 31P MRS of the calf of a volunteer at 1.5 T. In the circuit, the quarter-wavelength line was replaced by series inductances and parallel capacitances, so that the coil can work at two frequencies and the isolation is better[75]; (c) The coil diagram with balanced matching circuit, and 1H, 31P MRS of a anesthetized rat brain at 5.6 T. The adjustable capacitor C4 is added to the circuit designed by Schnall etc.[49], which together with C3 enables the circuit to achieve balance matching[76]

1986年,Leach等[75]采用串联电感和并联电容等效模拟的1/4波长线,调谐至高频时开路、低频时短路,确保非谐振的电路处于高阻抗状态,类似trap的作用.该线圈可以先从ROI获得1H MRI对成像区域定位,再采集31P MRS.文中利用所制备的线圈在1.5 T下测试的志愿者小腿的1H MRI和31P MRS如图 5(b)所示.

1987年,Chang等[76]对Schnall等[49]设计的单线圈双调谐RF线圈进行了改进,设计并添加了平衡匹配电路,可以更好地调谐和匹配两个频率,使得线圈加载时降低介电损耗,消除原结构带来的灵敏度损失,电路结构如图 5(c)所示.使用双调谐射频线圈能够同时监测1H核和31P核,以便更好地关联乳酸水平、pH值和高能磷酸盐的变化.文中采用麻醉的活大鼠,去除头皮,将线圈直接放在裸露的头骨上进行测试.测得的5.6 T下大鼠脑的1H MRS和31P MRS如图 5(c)所示, 1H MRS中除检测到N-乙酰天门冬氨酸外,还检测到了谷氨酸(Glu)、肌酸(Cr)/磷酸肌酸、胆碱(Cho).

早期的单线圈1H/31P双调谐表面RF线圈元器件少,也易于实现,但仅限于31P MRS,基本不能直接实现31P MRI.由图 5(a)5(c)可以发现,由于线圈的改进以及场强的提高,5.6 T下得到的大鼠脑31P MRS比2.0 T下得到的狗脑31P MRS结果更好.由图 5(b)也可以发现,1.5 T低场下的1H MRI不够理想.为了得到更好的1H MRI、31P MRS和31P MRI,就需要设计更高场强下灵敏度更高的1H/31P双调谐RF线圈.结合表 1,可以发现这些单线圈1H/31P双调线圈由于电感器的存在,在高场下有阻抗损耗增加现象,但它们为1H/31P双调谐RF线圈的设计与发展奠定了基础.

3.2.2 鸟笼1H/31P双调谐RF线圈

鸟笼线圈是一种著名的线圈结构,能够产生均匀度良好的B1场,而且鸟笼线圈的正交驱动还可以提高SNR、降低比吸收率(specific absorption rate,SAR,即射频功率沉积).利用鸟笼线圈实现双调谐的方法有:trap电路改进的鸟笼线圈[43, 52, 53]、同轴鸟笼线圈[57]以及Four-ring鸟笼线圈[71, 72-74].由表 1可知,trap电路改进的鸟笼线圈与同轴鸟笼线圈结构不适用于高场MRI系统,它们会造成高场磁共振图像SNR的损失.而Four-ring鸟笼结构双调谐RF线圈自1991年提出以来,在双调谐RF线圈中的应用已经越来越成熟.

Four-ring鸟笼线圈可以看作是三个鸟笼线圈融合在一起,形成一个圆柱体.它的构造涉及三个单独鸟笼结构的组装和调整,内部采用低通(low-pass,LP)鸟笼线圈结构,与两端的外部结构共用其环路,且其长度约为线圈总长度的一半,而外部两端是两个相同的低通或者高通(high-pass,HP)鸟笼结构.Four-ring鸟笼线圈内部结构针对低频原子核进行调整.而外部两个鸟笼通过内部环路耦合,使得鸟笼线圈的基本模式(即k=1模式)分裂为两个新模式,此两种模式具有同向和反向旋转电流分布,并且同向旋转模式以更高的频率出现,用来针对质子频率调谐.由于所有k=2和更高阶模式沿线圈的中心轴都没有射频场分布,而且k=1时的反向旋转模式在线圈中心处的场为0,因此仅可以检测到k=1模式的同向旋转模式.这样便可以得到Four-ring鸟笼双调谐RF线圈.

Four-ring鸟笼1H/31P双调谐RF线圈由Murphy-Boesch等[72]提出,他们描述了Four-ring鸟笼双调谐线圈的两种配置,低通-高通(LP-HP)和低通-低通(LP-LP)结构,如图 6(a)6(b)所示.两种结构的线圈都可以在1.5 T时以双正交模式工作,频率间的调谐相互作用大大减小、调谐速度更快,且质子和31P的灵敏度以及发射效率均优于体积相近的单调谐线圈.图 6(c)是利用LP-LP鸟笼线圈,对志愿者头部进行MR扫描,获得的耦合(1)和质子解耦(2)非局部31P MRS,包含了磷酸单脂、磷酸二酯、磷脂(PE)、磷脂酰胆碱(PC)、无机磷、甘油磷酰乙醇胺(GPE)、甘油磷脂酰胆碱(GPC)、磷酸肌酸、三磷酸核苷(α-NTP、β-NTP、γ-NTP)等代谢物;图 6(d)为利用LP-LP鸟笼线圈对志愿者小腿进行MR扫描,获得的耦合(1)和解耦(2)状态下的31P MRS.

图 6 (a) 低通-高通Four-ring鸟笼线圈,内部结构是低通鸟笼线圈,外部结构是高通鸟笼线圈;(b)低通-低通Four-ring鸟笼线圈,内部结构是低通鸟笼线圈,外部结构也是低通鸟笼线圈;(c)利用低通-低通线圈,对志愿者头部进行MR扫描,在1.5 T下获得的耦合(1)和质子解耦(2)31P MRS;(d)利用低通-低通线圈,对志愿者小腿进行MR扫描,在1.5 T下获得的耦合(1)和解耦(2)31P MRS(根据文献[72]修改) Fig. 6 (a) The configuration of a LP-HP Four-ring birdcage coil. The inner structure is a low-pass birdcage coil, the outer is high-pass; (b) The configuration of a LP-LP Four-ring birdcage coil. Both the inner and outer structures are low-pass birdcage coils; (c) The coupling (1) and proton- decoupled (2) 31P MRS of a volunteer's brain obtained with the LP-LP coil at 1.5 T; (d) The coupling (1) and decoupled (2) 31P MRS of the volunteer's leg obtained with the LP-LP coil at 1.5 T (Reproduced from Ref. [72])

Duan等[73]在2009年利用内部开发的FDTD工具优化了LP-LP Four-ring鸟笼线圈结构的直径、高度尺寸以及电容值等,制作了3.0 T下的1H/31P双调谐RF线圈(图 7).该线圈具有16条腿,使用有机玻璃成型器和厚度为30 µm、宽度为8 mm的背胶铜带构造,外部鸟笼调谐至1H频率127.72 MHz,内部鸟笼调谐至31P频率51.70 MHz.通过测量梯度回波90˚翻转角的RF功率,来对比双调谐线圈和单调谐线圈,并评估实际线圈的效率.结果显示Four-ring鸟笼线圈90˚翻转角所需的RF功率仅比单调谐鸟笼高出8.6%,即8.6%的额外损耗,但其效率却达到单调谐的90%以上.

图 7 低通-低通Four-ring鸟笼1H/31P双调谐射频线圈,其外部和内部均为16条腿的低通鸟笼线圈[73] Fig. 7 A LP-LP 1H/31P double-tuned RF coil, the outer and inner parts are low-pass birdcage coils with 16-rungs[73]
3.2.3 双线圈1H/31P双调谐RF线圈

双调谐RF线圈可以用2.2节中所描述的表面线圈作为基础,这些表面线圈在1H/31P双调谐RF线圈中的应用较多.由表 1可知,利用trap制作的双调谐表面RF线圈会引入一定程度的功率损耗;而具有固有几何解耦结构的双调谐表面RF线圈,因其两个通道的感应耦合小、线圈灵敏度高,在1H/31P双调谐RF线圈的发展过程中应用较多.

肿瘤诊断时,某些磷脂共振幅度的变化可能比肿瘤体积的变化更加敏感.2001年,Klomp等[78]制作了用于1.5 T头颈部肿瘤检测的1H/31P双调谐RF线圈,该线圈采用非平面的1H蝶形线圈与31P环形线圈构成,如图 8(a)所示,两个线圈产生互相垂直的RF磁场,使得两线圈在拉莫尔频率下的电磁耦合最小.文中利用所制作的表面线圈得到1.5 T下人体浅表部分质子解耦的31P MRS,图 8(b)展示了患者下咽肿瘤和弥漫性大B细胞非霍奇金淋巴瘤的31P MRS.

图 8 (a) 具有几何解耦结构的双线圈1H/31P双调谐射频线圈结构.该线圈由直径为50 mm的圆环线圈(图中黑色部分)调谐至1.5 T下31P的拉莫尔频率和非平面的蝶形线圈(图中灰色部分)调谐至1.5 T下1H核的拉莫尔频率构成. ATC capacitors表示ATC电容;(b)两名患者的头颈部31P MRS,左边为肿瘤区域波谱, 右边为肌肉区域波谱,(1)为下咽肿瘤,(2)为弥漫性大B细胞非霍奇金淋巴瘤.用来采集31P MRS的序列参数为:重复时间为1 800 ms,体素大小为25 mm×25 mm×25 mm.对比肌肉组织区域与肿瘤区域的31P MRS,显然肿瘤区域显示相对较高的磷酸单脂(PME)和较低的磷酸肌酸(PCr)信号(根据文献[78]修改) Fig. 8 (a) Structure of a double-coil and double-tuned 1H/31P RF coil which consists of two geometric decoupling coils. One is a 50 mm diameter CL coil (black part in the figure) tuned to the Lamor frequency of 31P and the other is a non-planar butterfly coil (gray part in the figure) to 1H at 1.5 T; (b) 31P MRS of the head and neck region in two patients, with tumor areas on the left and muscle areas on the right. Spectra were obtained from (1) a hypopharyngeal tumor, and (2) a diffuse large B cell non-Hodgkin lymphoma. 31P MRS setting: TR= 1 800 ms, Voxel Size=25 mm×25 mm×25 mm. Comparing 31P MRS in muscle tissue areas and tumor areas, it is obvious that the tumor areas show relatively high PME and low PCr (Reproduced from Ref. [78])

2010年,Alfonsetti等[68]也采用这种几何解耦的双调谐RF线圈结构,设计了由CL和FO8线圈制成的平面表面1H/31P双调谐RF线圈,图 9(a)所示为该线圈的正面和背面示意图.文中对比了CL-CL、FO8-FO8、CL-FO8几种组合的线圈的隔离参数S12,结果表明CL-FO8组合的线圈之间解耦最好.文中利用平面CL-FO8组合的1H/31P线圈在1.5 T下获得了0.1 mol/L NaH2PO4样品的冠状和矢状面的1H MRI[图 9(b)]和31P MRS[图 9(c)].

图 9 (a) 1.5 T下CL-FO8双调谐射频表面线圈的正面及背面.该线圈由外部的CL线圈调谐至63.87 MHz和内部的FO8线圈调谐至25.85 MHz组成,两个通道之间具有很好的隔离.线圈由宽4 mm、厚100 μm的铜箔带及电容器实现;(b) 1.5 T下0.1 mol/L NaH2PO4的冠状面和矢状面的1H SPGR成像.NaH2PO4溶液盛放在半径为6 cm、高为21 cm的圆柱体内. 1H成像参数:成像视野为24 cm,翻转角为10˚,层厚为5 mm,脉冲重复激发次数为2;(c) 1.5 T下0.1 mol/L NaH2PO431P MRS.序列参数:重复时间为5 000 ms,脉冲重复激发次数为64(根据Ref. [68]修改) Fig. 9 (a) The front and back of a CL-FO8 double-tuned RF surface coil at 1.5 T. The coil is composed of an external CL coil tuned to 63.87 MHz and an internal FO8 coil to 25.85 MHz, the two channels are well isolated. The coil is made of copper strips with width of 4 mm, thickness of 100 μm and capacitors; (b) 1H SPGR images of 0.1 mol/L NaH2PO4 in the coronal and sagittal planes at 1.5 T. The solution of 0.1 mol/L NaH2PO4 was filled in a tube with a radius of 6 cm and a height of 21 cm. Imaging parameters: FOV=24 cm, Flip Angle=10˚, Slice Thickness=5 mm, NEX=2; (c) 31P MRS of 0.1 mol/L NaH2PO4 at 1.5 T. Sequence parameters: TR=5 000 ms, NEX=64 (Reproduced from Ref. [68])

2015年,Retico等[80]将微带技术引入1H/31P双调谐RF线圈的研究中,设计了一种微带线表面线圈用于7.0 T下1H MRI和31P MRS.文中利用一个中央微带和两个横向微带构成1H/31P双调谐RF线圈,如图 10(a)所示,分别调谐至1H核和31P核的共振频率.在7.0 T下获得0.1 mol/L KH2PO4样品的轴向面和冠状面的1H MRI和31P MRS,如图 10(b)所示;文中也测试了新鲜牛肉样品,获得了其轴向面的1H MRI和31P MRS,如图 10(c)所示,解剖结构清晰可见,最大强度接近中心微带位置.

图 10 (a) 微带线收发一体表面线圈结构.该线圈由印制在FR4PCB上的一个宽为10 mm的中央微带及两个宽为5 mm的横向微带构成,总尺寸为190 mm×100 mm×63 mm,且通过连接在微带两端的片式电容器进行调谐;(b) 7.0 T下,利用微带线圈获得的KH2PO4的轴向面(左上)和冠状面(右上)的1H MRI,以及31P MRS(下). 1H MRI使用SPGR序列,其参数设置为:重复时间为11 ms,回波时间为6 ms,脉冲重复激发次数为8,图像大小为256×256,成像视野为240 mm×240 mm(轴向面),成像视野为200 mm×200 mm(冠状面).获得31P MRS所采用的参数为:重复时间为2 000 ms,翻转角为90˚;(c) 7.0 T下,新鲜牛肉的轴向面1H MRI以及31P MRS.1H MRI使用SPGR序列,重复时间为11 ms,回波时间为6 ms,脉冲重复激发次数为8,图像大小为512×512,分辨率为230 μm,成像视野为120 mm×120 mm.31P MRS采用参数为:重复时间为2 000 ms,翻转角为90˚[80] Fig. 10 (a) A 7 T T/R surface coil of microstrip line. The coil consists of a central copper microstrip (10 mm wide) and two transverse copper microstrips (5 mm wide) on FR4 PCB with a total size of 190 mm × 100 mm × 63 mm. And the coil is tuned by chip capacitors connected to both ends of the microstrips. (b) 1H MRI in axial planes (upper left), coronal planes (upper right) and 31P MRS (lower) of a KH2PO4 phantom obtained by the coil. 1H image was obtained by SPGR sequence with the following parameters: TR=11 ms, TE=6 ms, NEX=8, Image Size=256×256, FOV=240 mm×240 mm (axial plane), FOV=200 mm×200 mm (coronal plane). 31P MRS: TR=2 000 ms, Flip Angle=90˚. (c) 1H MRI and 31P MRS of a piece of fresh beef obtained by the coil. SPGR 1H MRI: TR=11 ms, TE=6 ms, NEX=8, Image Size=512×512, Resolution=230 μm, FOV=120 mm×120 mm. 31P MRS: TR=2 000 ms, Flip Angle=90˚[80]

2018年,Hong等[81]将调谐至1H核频率的一对偶极子天线与一个四通道31P环形阵列线圈整合,设计了一种1H/31P双调谐RF线圈,且偶极子与环形线圈之间具有强几何解耦.线圈的仿真模型如图 11(a)所示,其中装有直径为14 cm的球形体模.由于123 MHz(3 T时1H核的拉莫尔频率)下的偶极子天线的物理长度过长,偶极子天线在负载附近采用弯曲结构,其电路结构如图 11(b)所示. 环形线圈相邻回路通过重叠和电容解耦方式解耦,而对侧回路通过变压器解耦,如图 11(c)所示.文中在3.0 T下进行MR测量实验,其中体模中每升含有2 g KH2PO4、1.25 g NiSO4·6 H2O和2.6 g NaCl. 如图 11(d)所示,文中展示了在几乎相同线宽(1.95 ppm对1.86 ppm)下有、无偶极子时的31P MRS,显然两者之间具有5%差异的SNR.如图 11(e)所示,给出了在相同线宽(2.34 ppm)下用四通道环形线圈和双调谐鸟笼线圈获得的31P MRS,SNR分别为309和268,显然四通道阵列线圈较双调谐鸟笼线圈具有更高的波谱SNR.该文最后也表明正交弯曲偶极子和环形阵列的组合是一种很有前途的双调谐RF线圈设计方案.

图 11 (a) 正交弯曲偶极子天线和四通道31P线圈的仿真模型. 图中已标出了环形线圈和偶极子天线,蓝色部分是球形体模(直径14 cm,导电率σ = 0.5 S/m,介电常数εr = 80);(b)弯曲偶极子天线示意图.其长为60 cm,弯曲成半径为13 cm的圆顶,用四个电感器将其调谐至123 MHz;(c)四通道31P阵列的电路图.四通道阵列线圈通过向顶部逐渐变细进行修改,形成圆顶形状;(d)在有或没有偶极子天线的情况下,用四通道线圈获得31P MRS;(e)用四通道线圈和双调谐鸟笼线圈获得的31P MRS(根据文献[81]修改) Fig. 11 (a) Simulation model of quadrature bent dipole antenna and four-loop 31P coil with phantom. The loop and dipole have been marked in the figure, and the blue part is the spherical phantom (14 cm diameter, conductivity σ= 0.5 S/m, permittivity εr= 80); (b) Schematic of bent dipole antenna. The dipoles were 60 cm long and bent onto a dome of 13 cm radius, which was tuned down to 123 MHz using four inductors; (c) The 2D representation of the four-loop 31P array. The four-loop array is 27 cm in diameter and 20 cm in length, and the shape of the loop coils was modified by tapering towards the top, resulting in the formation of a rounded dome shape; (d) The 31P MRS acquired using four-loop coil with and without dipole antenna presence; (e) The 31P MRS acquired using four-loop coil and dual-tuned birdcage coil (Reproduced from Ref. [81])

Du等[82]也采用偶极子和环线圈结合的方式于2018年提出了一种用于超高磁场7.0 T MRI的1H/31P双调谐多通道RF线圈.其中,采用四通道偶极子阵列作为1H MRI线圈,用四通道环形阵列作为31P MRI线圈.图 12(a)所示为该线圈的仿真模型,其建立在一个外径为190 mm、长度为500 mm的圆柱体上.该双调谐线圈其中一组通道的电路结构如图 12(b)所示,环形线圈尺寸为138×111 mm2,铜皮宽度为6 mm,偶极子单元尺寸为260×3.17×1.41 mm3,偶极子线圈放置在环形线圈上方5 mm的中心位置,以避免两个元件接触,并且可以最小化两种线圈之间的耦合.文中采用数值模拟方法对线圈的性能进行了评估,并在7.0 T MRI系统上进行了测试,其中体模采用直径为123 mm、长度为232 mm的圆柱体结构,由3.6 g/L NaCl,3.75 g/L NiSO4·6H2O和1.65 g/L CH6O6P2组成.采用2D梯度回波脉冲序列进行1H MRI,参数设置为:重复时间TR=100 ms,回波时间TE=4 ms,接收带宽Bandwidth=260 Hz,翻转角Flip Angle=45˚,FOV=150×150 mm2,层厚Slice Thickness=5 mm,矩阵大小Matrix Size=128×128.如图 12(c)所示为横截面(左)、矢状面(中)和冠状面(右)的1H MRI.采用化学位移成像序列获取31P MRS,参数设置:TR=1 000 ms,TE=2.3 ms,Bandwidth=10 kHz,Flip Angle=90˚,FOV= 250×250 mm2,Slice Thickness=80 mm,空间分辨率Spatial Resolution=3×3×8 cm3,平均次数Average=64,矢量大小Vector Size=1 024,Matrix Size=8×8,插值分辨率Interpolation Resolution=16×16.图 12(d)所示,具有网格覆盖的1H磁共振图像用作31P MRS的定位(左),其提供了每个体素中的MRS数据.图 12(d)中也描绘了中心体素中31P MRS的放大结果(右).该双调谐线圈的设计在7.0 T MRI系统中的成功应用再一次证明偶极子与环形线圈的组合在高场下具有很好的应用前景.

图 12 (a) 线圈的仿真模型.蓝色部分是盐水模型.盐水模型是直径为150 mm、长度为200 mm的圆柱体结构(电导率σ = 0.6 S/m,介电常数εr= 78).(b)双调谐线圈阵列单元的电路结构. 通过优化得到线圈集总元件的数值,红色三角形代表激励端口. (c)横截面(左)、矢状面(中)和冠状面(右)的1H MRI.(d) 7 T MRI系统下的31P MRS分析.具有网格覆盖的1H MR图像用作31P MRS的定位(左),其提供了网格中每个体素的MRS数据,其峰值对应于31P化学位移.右图为中心区域31P MRS的放大结果(根据文献[82]修改) Fig. 12 (a) Simulation model of coil with phantom. The blue section is saline phantom. The phantom is a cylindrical water model with a diameter of 150 mm and a length of 200 mm (conductivity σ = 0.6 S/m, permittivity εr = 78). (b) Circuit structure of double-tuned coil array unit. The values of the lumped elements of coil were achieved by optimization, the red triangle represents the excitation port. (c) 1H MR images in the transverse (left), sagittal (middle), and coronal planes (right). (d) 31P MRS under the 7 T MRI system. 1H MR image with grid overlay was used as the positioning image of the 31P MRS (left). MRS data in each voxel of a grid were presented, and the peaks were corresponding to 31P chemical shift. The enlarged 31P spectrum can be shown in the right (Reproduced from Ref. [82])
3.2.4 阵列1H/31P双调谐RF线圈

阵列线圈由许多小表面阵列元件构成,具有良好的灵敏度和成像视野等特点,是高场MRI的RF线圈的发展趋势之一.Goluch等[84]在2015年设计并制作了用于7.0 T下小腿肌肉研究的1H/31P双调谐收发阵列线圈,该线圈由两个1H通道和三个31P通道组成.仿真结果表明该线圈比同等大小的单线圈或鸟笼线圈具有更高的发射效率和SNR,在7.0 T下提高了单次激发局部区域所得波谱的数据质量.Brown等[85]于2016年研发了一个3.0 T下具有高31P核SNR1H/31P人体下肢RF线圈,该线圈由两个(1H和31P)八通道收发阵列线圈构成,环绕在直径为17 cm的圆柱外壳结构上,且1H线圈位置相对31P偏移22.5˚.该线圈能够对人体下肢进行高时空分辨率的1H和31P磁共振实验,且腓肠肌中31P核的SNR是鸟笼线圈的两倍以上.

多通道阵列线圈结合高效的脉冲序列在超高场下可获得强烈的31P MRI信号,并对几种神经再生条件下的潜在能量代谢损伤提供新的见解,如阿茨海默症和帕金森症,以及精神分裂等精神障碍疾病.Brown等[86]于2015年设计出用于7.0 T脑部成像的1H/31P双调谐阵列线圈.该阵列线圈使用双核嵌套设计策略,由八通道1H阵列线圈和八通道31P阵列线圈组成,图 13(a)所示为其阵列单元电路图.图 13(b)所示为阵列线圈以及接口电路的实物图.文中利用所制线圈获得了覆盖全脑且灵敏度较高的1H MP-RAGE(磁化预处理快速采集梯度回波)图像、31P MRS,以及PCr和γ-ATP在矢状面上的图像如图 13(c)所示.31P MRS、单代谢物(PCr和γ-ATP)图像均从3D-化学位移成像(3D-chemical shift imaging,3D-CSI)数据集中得到.

图 13 (a) 双调谐射频线圈阵列单元原理图.嵌套阵列线圈的一个31P和一个1H线圈用黑色突出显示,相邻元素用浅灰色显示;(b) 1H/31P阵列线圈和接口电路的实物图.接口电路中包括了电缆trap电路、功率分配器、发射/接收开关和前置放大器;(c) 1H/31P数据集:(1)矢状面1H MP-RAGE图像,在额叶、顶叶和枕叶显示良好的灰质/白质对比.(2) 4.66 mL体素的31P-CSI波谱,体素位于(1)中的虚线框.(3) PCr和(4) γ-ATP在矢状面上的波谱图像来源于3D-CSI数据[86] Fig. 13 (a) Schematic diagram of double-tuned coil array unit. For simplicity one 31P and one 1H coil of the nested array coil are highlighted in black while neighboring elements are displayed in light gray; (b) Photograph of the 1H/31P array coil and interface. The interface circuit includes the cable trap circuit, the power dividers, transmit/receive switches, and preamplifiers; (c) 1H/31P data set: (1) 1H MP-RAGE image in the sagittal plane, which shows excellent gray/white matter contrast in the frontal, parietal, and occipital lobes. (2) Typical 31P-CSI spectrum from a 4.66 mL voxel whose location is outlined in (1), spectroscopic images of (3) PCr and (4) γ-ATP in the sagittal plane derived from the 3D-CSI data[86]

Avdievich等[87]于2020年研制出用于9.4 T人脑的1H/31P双调谐阵列线圈.该线圈中的1H线圈以及31P线圈均由环绕头部的8个表面环以及上方位置的两个垂直环构成,并且将31P和1H阵列线圈放置在同一层中,如图 14(a)所示.1H线圈以及31P线圈之间通过重叠来减小耦合,而两个阵列线圈的各个通道之间也可以通过重叠来减小耦合,该双调谐线圈在两个频率下都能获得较高的传输效率.文中在人头部上方位置增加了两个垂直的交叉环改善了脑部覆盖率.获得了横向人脑1H梯度回波图像和图像中2个不同体素位置的31P MRS,以及矢状面和横向面PCr的31P磁共振谱成像(31P MRSI,使用FOV上的5个平均值获取数据),如图 14(b)所示.

图 14 (a) 1H阵列线圈(上)和31P阵列线圈(下左)的电磁仿真模型以及1H/31P阵列线圈的实物图(下右).1H和31P阵列线圈均采用环绕头部的八个表面环以及头顶位置的两个垂直环构成,且1H线圈和31P线圈放置在同一层,两者之间通过重叠来减小耦合.图中,1H和31P回路分别用红色和黄色虚线标记;(b)横向GRE活体1H图像(上左)和图像中显示的2个不同体素位置获得的31P MRS(上右)以及中央矢状面(下左)和横向面(下右)PCr的31P磁共振谱成像(31P MRSI).使用成像视野为240 mm×240 mm×200 mm上的5个平均值获取数据,体素尺寸为12 mm×12 mm×20 mm[87] Fig. 14 (a) Models of 1H array coil (upper), 31P array coil (lower left), and the photo of the 1H/31P array coils (lower right). The 1H and 31P array coils are composed of eight surface coils around the head and two vertical coils on top of the head. The 1H, 31P coils are placed on the same layer, and the coupling between them is reduced by coils overlapping. In the photo, the 1H and 31P coil loops are marked with red and yellow dotted lines respectively. (b) Transversal GRE in vivo 1H image (upper left), 31P MRS of two different voxels (upper right) and PCr 31P MRSI maps in the central sagittal (lower left) and transversal (lower right). Data are acquired using 5 averages over FOV of 240 mm×240 mm×200 mm and the Voxel Size of 12 mm×12 mm×20 mm[87]
3.3 9.4 T下1H/31P双调谐RF线圈测试

作者所在的项目团队设计了一种9.4 T双线圈1H/31P双调谐RF线圈,该线圈采用具有几何解耦结构的双调谐RF线圈制成.其中一个线圈调谐至31P核的共振频率(162 MHz),另一线圈调谐至1H核的共振频率(400 MHz),两个谐振频率下各通道的反射系数S11S22均小于-25 dB,通道之间的传输系数S12S21也均小于-25 dB,说明该线圈具有良好的匹配和解耦性能.

在9.4 T下采用上述1H/31P双调谐RF线圈进行测试,获得了活体小鼠的1H MRI,如图 15(a)所示,参数设置为TR=350 ms,TE=3 ms,Flip Angle=30˚,Slice Thickness=0.5 mm,Image Size=192×192,FOV=14×14 mm2,Resolution=0.073 mm,Average=8,Bandwidth=30 kHz.同时采集了活体小鼠脑的31P MRS,如图 15(b)所示,参数设置为TR=2 000 ms,Flip Angle=60˚,累加次数为64,扫描时间为128 s.

图 15 (a) 小鼠脑1H MRI. 采用具有几何解耦结构的1H/31P双调谐射频线圈在9.4 T下利用T1-FLASH序列获得了1H MRI结果,SNR约为30;(b)小鼠脑31P MRS.同样条件下利用Singlepulse_31P序列获得了小鼠脑的31P MRS信号 Fig. 15 (a) 1H MRI of mouse brain. The results were obtained at 9.4 T by the sequence of T1-FLASH with a 1H/31P double-tuned RF coil, which consists of two geometric decoupling coils. And the SNR is 30; (b) 31P MRS of a mouse brain at 9.4 T. The MRS was acquired with the sequence of Singlepulse_31P

由于线圈灵敏度和场强问题,很少有文献报道得到样品或者组织的31P磁共振图像,大部分文献使用1H/31P双调谐RF线圈得到1H MRI以及31P MRS.上述测试结果表明,9.4 T下,在较低的累加次数(64)和扫描时间(约2 min)下,PCr的SNR已接近20,说明线圈具有非常高的灵敏度.若同时对扫描序列以及扫描参数等进行优化,有望获得更强的信号,使9.4 T高场下31P MRI有实现的可能.

3.4 高场下1H/31P双调谐RF线圈的发展趋势

由上述内容可以发现,Four-ring鸟笼双调谐射频线圈、具有几何解耦结构的双调谐RF线圈以及阵列双调谐RF线圈都适合高场下1H/31P双调谐RF线圈的设计.

2018年,Ha等[74]提出了一种新的折叠式Four-ring鸟笼双调谐RF线圈,应用于9.4 T下的质子与23Na双核信号采集,其中23Na MRI的SNR是单调谐线圈的93%.Four-ring鸟笼1H/23Na双调谐RF线圈在9.4 T超高场下的成功应用,也可以推广到1H/31P双调谐RF线圈的制作,在成像区域可以得到均匀的B1磁场,并且获得高分辨率图像.

高场下为了得到更高SNR31P MRI,可以将具有几何解耦结构的1H/31P双调谐RF线圈中的31P线圈进行优化,利用正交激励使得31P MRI的SNR提高.首先,具有几何解耦结构的双调谐RF线圈,两个通道均只能工作在线性模式下而不能工作在正交模式.但是,如果将两个通道均用做同一原子核的射频激励端口,且相位相差90˚,就可以实现同一原子核的正交激励模式,从而提高成像SNR.综上,9.4 T下,可以对3.3节中设计的1H/31P双调谐RF线圈的31P通道进行优化.具体可以使用PIN二极管让原来的1H线圈在PIN处于不同的状态时分别调谐至1H频率或者31P频率,此时结合原31P线圈便对31P核实现了正交激励模式.

在高场成像方面,由于阵列线圈均由多个小尺寸的表面线圈阵列单元构成,避免了高频下波长效应的局限性,从而具有较高的灵敏度.高场下将高灵敏度几何解耦结构的1H/31P双调谐表面RF线圈作为阵列单元制成收发一体阵列线圈,可以得到高SNR的图像.其中,具有几何解耦结构的表面线圈可以用CL与FO8、蝶形线圈制成,也可以采用CL与微带线、同轴线、波导、带状线等传输线原理制作的TEM[25-27, 88, 89]线圈、单/偶极子天线[33-38, 81, 82]制成,微带线等线圈以及偶极子天线在高场下已被证明具有优越的性能[69, 38].前面部分已对偶极子天线与环线圈结合的1H/31P双调谐阵列RF线圈做了介绍,该类型线圈是高场下很有前途的双调谐RF线圈设计方案.

研究表明,使用均匀体积线圈作为发射线圈,且使用相控阵线圈作为接收线圈时,通常会获得较高SNR[40].Avdievich等[90] 2007年在高场下结合31P相控阵以及1H/31P体积线圈,研制了一种主动失谐的1H/31P双调谐RF线圈,用于4.0 T人脑31P MRS研究.该线圈由收发一体的TEM 1H/31P双调谐体积线圈和四通道的31P阵列接收线圈构成,文中利用所制线圈获得了人脑的1H MRI以及31P MRS.van Uden等[91]于2019年设计并制作了用于测试3.0 T下人脑31P MRS的1H/31P双调谐RF线圈,该线圈由1H/31P双调谐鸟笼线圈和8通道的31P阵列线圈组成,在保证两个频率下均匀发射场的同时提高了31P MRS的SNR.高场下利用体发射线圈产生均匀的RF激发场,采用阵列接收线圈获得高SNR的成像结果,两者结合可以制成收发分离且同时具有高均匀度和高灵敏度特性的1H/31P双调谐RF线圈.

4 结论与展望

高场下可实现高SNR、高分辨率的MRI,使基于31P的磁共振研究有望取得新的成果,其中高性能的1H/31P双调谐RF线圈是其重要前提条件.本文回顾了1H/31P双调谐RF线圈的研发历程和相关应用,包括介绍单线圈、Four-ring鸟笼线圈、含trap电路的线圈、具有几何解耦的线圈以及阵列线圈等构型的技术特点,以及这些线圈由低场到高场的相关应用.其中Four-ring鸟笼线圈等体线圈具有较好的RF均匀度,几何解耦的线圈具有较高的灵敏度,阵列线圈兼具高灵敏度和射频均匀度的特点.

目前大多数的1H/31P双调谐RF线圈都可以实现31P MRS,然而31P MRI研究报道依然很少.已开展的研究涉及人脑、人小腿、人咽喉、狗脑、大鼠脑等,如用31P MRS研究了人体下咽肿瘤细胞、淋巴瘤的变化.本文还展示了利用自行研发的一种9.4 T高场1H/31P双调谐RF线圈在较短时间内获得的较高SNR小鼠脑的31P MRS,证明了高场下用该线圈开展31P的磁共振研究的可行性.随着高场下1H/31P双调谐RF线圈相关技术的发展,将来有望在活体动物获得高质量的31P MRI,并促进相关科研的发展,如研究细胞膜的变化、代谢情况或肿瘤细胞的变化等.

致谢 感谢中国科学院磁共振技术联盟科研仪器设备研制项目(2020gz1003)的支持.

利益冲突  无


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