高场和超高场MR下人体内B1场均匀性及SAR随场强变化规律的研究

高场和超高场磁共振成像(MRI)具有高的信噪比,可获得高分辨率的图像,成为磁共振的发展趋势.在磁共振成像系统中,射频线圈产生的射频磁场(B1场)均匀性将直接影响图像质量,且人体组织对射频能量的过多吸收会给受检者带来安全问题,因此研究高场和超高场MR下人体组织B1场均匀性和特定能量吸收率(SAR)随场强变化的关系意义重大.为了准确分析不同场强下人体组织B1场均匀性和SAR值分布,采用时域有限差分法,以真实女性盆腔电磁模型作为磁共振系统中TEN体线圈的负载,在磁通密度为1.5、3、7T下分别对盆腔内部B1场和SAR进行数值计算,比较分析不同场强下人体内部B1场分布均匀性和SAR的变化规律.数值计算结果表明:磁通密度由1.5T分别增加到3和7T时,B1场的均匀性分别降低11.54％和49.97％,盆腔各组织SAR的平均值最大提高4.7和24.14倍,局部最大值最大提高5.5和22倍;而且随着场强增高,人体内深层组织吸收的射频能量显著增加,7T时皮肤、肌肉和组织液的SAR已经超出了安全阈值.可见,B1场均匀性和SAR值随场强增高变化显著,需要根据实际需要对TEM线圈进行优化,以提高成像效果并确保受检者安全.     

混合MOM/FDTD电磁分析方法及其在MR射频线圈设计中的应用

研究的目的在于增强磁共振射频线圈仿真的真实性,提高射频线圈设计水平,从而提高线圈的成像质量。充分利用矩量法（MOM）在计算复杂结构线圈上电流分布的优势,以及时域有限差分（FDTD）方法在仿真人体模型等复杂的不均匀电磁介质中的优势,通过惠更斯等效面将MOM和FDTD有机地结合到一起,形成混合MOM/FDTD射频线圈设计方法,并提出一种新的真实人体电磁模型建立方法,融入到混合方法中,充分考虑线圈与真实人体组织之间复杂的电磁相互作用。由该方法设计的原型线圈扫描图像的信噪比达到193.4dB,相比先前未考虑人体与线圈之间作用的方法,所以得到的图像信噪比提高了38.7 dB,从而验证了仿真设计的正确性。     

基于中国可视人数据集的时域有限差分法人体头部电磁仿真模型构建

目的:为掌握人体组织电磁特性的变化,实现对真实人体结构电磁辐射特性的模拟和仿真计算。方法:在中国数字可视化人体数据集基础上,提出建立真实人体三维电磁模型的方法并利用CST STUDIO SUITE对其进行电磁仿真计算。结果:构建了适用于时域有限差分法计算的网格精度为1 mm3的三维人体头部电磁模型,满足频率为30 GHz以下的电磁仿真计算在数值色散空间离散间隔上的要求。结论:该模型具有很好的可视性和可操作性,对构建数字电磁人以及优化生物电磁检测技术具有重要的理论价值和实践指导意义。     

基于目标场法和CST仿真的低场磁共振射频线圈设计

根据临床对人体肺部磁共振成像的需要,设计一种用于肺部0.06 T低场磁共振成像的3He射频线圈。通过预先设定感兴趣区域（ROI）及其内部的目标磁场,运用目标场法逆向求解ROI内产生目标磁场所需的源电流密度分布,流函数获取射频线圈理想绕组。然后利用CST软件建模简化线圈绕组,场路协同仿真研究负载射频线圈在不同品质因数（Q值）下的射频场B1和特定吸收率（SAR）分布等性能指标。仿真实验结果表明,通过改变射频电路的电容值可调整线圈的Q值,线圈Q值越大,单位功率信号激励下产生的B1场强越高,SAR值越高,传输效率越低,而B1场均匀性不受影响,一直保持在近90%。通过射频线圈仿真,验证了目标场法联合CST仿真设计方法的可行性,可推广至高频射频线圈设计。     

MR射频能量特定吸收率在女性盆腔不同组织中分布规律的研究

目的：磁共振（MR）扫描中,特定吸收率（specific absorption rate,SAR）是衡量被检者安全的最重要指标。为了确保被检者的安全,需了解不同人体组织在MR射频作用下SAR分布规律。方法：采集女性志愿者原始X射线断层成像数据,重建女性盆腔数值断层图像;根据断层图像灰度值分割出皮肤、脂肪、肌肉、骨骼、子宫、子宫腔等不同人体组织原始蒙罩,对分割的断层图像进行网格优化和三角面片优化后,结合不同组织电磁参数,建立女性盆腔电磁数值模型。依据NEMA MS-10标准中的局部SAR计算原则,运用时域有限差分（FDTD）法计算64 MHz下盆腔局部不同组织SAR分布。结果：64 MHz下,女性盆腔电磁模型分割出的六种不同组织中,对SARavg值而言,皮肤处最大,子宫处最小;对SAR1g值而言,皮肤处最大,骨骼处最小。结论：MR扫描下不同人体组织内的SAR值分布各不相同。不同组织内SAR值分布与组织的电磁特性和几何形状,以及组织和射频场源距离等因素相关。研究MR扫描下不同人体组织中SAR值的分布规律,掌握影响SAR值分布的因素,对保证被检者的安全具有重要参考价值。     

永磁体磁共振电特性成像方法研究

磁共振电特性成像,是利用磁共振原始数据对被测生物组织的电特性进行重建的一种成像方法。利用有限元仿真软件,建立0.4 T主磁场下的射频线圈模型,测量仿真中射频场的分布情况。通过射频场空载和负载下射频场的均匀性,证明电特性的不同影响射频场的分布。用0.4 T永磁体磁共振设备对仿体和人体头部进行实验,并根据仿真数据和实验数据对电特性进行重建。结果表明,低场磁共振电特性在有无负载下,感兴趣区域的磁通密度均匀性相差将近4倍,磁通密度模值明显增加,这为磁共振电特性低场下发展提供一定的参考。     

磁共振六通道线圈阵列的数字化仿真与设计

近年来,多通道线圈阵列被广泛应用于磁共振成像,以提高图像的质量。针对局部感兴趣区域内的射频场优化,提出一种由不同尺寸单元构成的六通道线圈阵列,可优化盆腔组织中局部感兴趣区域内的射频场。使用宽度为10和20 cm的两种不同尺寸的线圈单元来构建六通道线圈阵列模型,并对其采用几何重叠法和电容网络法进行去耦,以及运用时域有限差分(FDTD)方法进行仿真和计算,分析和评估其在感兴趣区域内产生的射频场。仿真结果表明,在加载盆腔组织椭圆柱电磁模型情况下,提出的线圈阵列的去耦效果S₁₂和S₁₃分别为-27.19和-33.46 dB,在感兴趣区域内产生的射频场B⁺₁强度平均值,比由宽度为15 cm的相同单元构成的常规线圈阵列高出约5.21%。由不同尺寸单元构成的六通道线圈阵列能够优化感兴趣区域内的射频场,为磁共振线圈设计提供新的思路和方法。     

微型诊疗系统无线供能技术及其生物电磁效应研究

目的 针对人体肠道内微型诊疗系统能量供应问题,研究了一种基于电磁耦合的无线能量传输系统,并对其产生的生物电磁效应进行分析.方法 围绕在人体腹部的发射线圈产生的交变磁场,体内微小线圈产生感应电动势为肠内诊疗系统提供电能.为了研究电磁场在人体组织产生的生物效应,本文首先采用图像分割技术对人体横断面图像进行组织识别,建立了包含56种组织的高精度三维真实人体数字化模型,然后通过数值计算的方法研究了能量传输系统的生物电磁效应.结果 获得电流密度、比吸收率、磁场强度在人体的分布,以及不同频率下全身平均比吸收率.结论 生物电磁效应的研究为无线供能技术在人体肠道诊疗系统的应用奠定了生物安全性基础.     

矩量法及其在磁共振射频线圈中的应用

目的:基于矩量法,实现对具体射频线圈的电磁仿真分析。方法:对不同射频线圈进行建模,基于具体的线圈模型,选取合适的基函数对未知函数进行展开,并代入电磁场下的相应算子方程,从而实现运用矩量法对射频线圈的电磁仿真分析。本文首先通过推导给出矩量法的电磁场计算公式,再将其进行必要的化简,最后通过Matlab工具实现对具体线圈模型的仿真计算。结果:通过对头部线圈,头颈联合线圈以及术中多通道射频接收线圈的仿真,得到电磁场分布结果。结论:与基于比奥-萨法尔定律传统方法计算磁场分布相比,矩量法能够更方便地分析复杂拓扑结构的射频线圈的磁场分布,应用范围更广。同时,该方法比FDTD等方法需要的计算资源少很多,更适合工程设计人员使用。     

基于Computer Simulation Technology的磁共振射频线圈模拟与设计

目的:基于CST(Computer Simulation Technology)软件设计磁共振射频线圈,利用软件功能计算S参数、驻波比,求解每个网格的电场和磁场的方向、大小等,辅助完成实际线圈设计。方法:以单圈线圈为例,设计3层仿真模型:铜箔线圈层、环氧玻璃布层压板层、水模层。正确输入每一层的材料参数,并加入电容、端口等集总参数元件。设计合适的网格划分要求,插入场监视器。结果:数据结果包括一维结果和二、三维结果,其中一维结果包括S参数,驻波比,不连续端口的电流、电压、阻抗,集总参数元件的电流、电压、阻抗等,二、三维结果包括电场分布、磁场分布、表面电流等。从S11参数看出此线圈模型可在21.86 MHz处实现调谐和匹配,实际中可用作0.5 T磁共振成像设备的线圈。电磁场分布结果可以看出电容附近电场强度较高,磁场强度在线圈附近z方向强度较高。结论:磁共振射频线圈的设计可以利用CST软件辅助完成,通过调整线圈的形状、铜箔宽度等建立模型,根据S参数查看调谐匹配、耦合结果,通过场监视器的设置,查看二、三维电磁场分布。由结果再修改模型,最终完成设计。     

7 T body MRI: B1 shimming with simultaneous SAR reduction

The high frequency of the radiofrequency (RF) fields used in high field magnetic resonance imaging (MRI) results in electromagnetic field variations that can cause local regions to have a large specific absorption rate (SAR) and/or a low excitation. In this study, we evaluated the use of a B1 shimming technique which can simultaneously improve the B+1 homogeneity and reduce the SAR for whole body imaging at 7 T. Optimizations for four individual anatomies showed a reduction up to 74% of the peak SAR values with respect to a quadrature excitation and a simultaneous improvement of the B+1 homogeneity varying between 39 and 75% for different optimization parameters. The average SAR was reduced with approximately 50% for all optimizations. The optimized phase and amplitude settings from an elliptical phantom model were applied to four realistic human anatomy models to evaluate whether a generic application without prior knowledge of the detailed human anatomy is possible. This resulted in an average improvement of the B+1 homogeneity of 37% and an average reduction of the maximum and average SAR of 50 and 55%, respectively. It can be concluded that this generic method can be used as a simple method to improve the prospects of 7 T body imaging.     

Electric fields induced in the human body by time-varying magnetic field gradients in MRI: numerical calculations and correlation analysis

The spatial distributions of the electric fields induced in the human body by switched magnetic field gradients in MRI have been calculated numerically using the commercial software package, MAFIA, and the three-dimensional, HUGO body model that comprises 31 different tissue types. The variation of |J|, |E| and |B| resulting from exposure of the body model to magnetic fields generated by typical whole-body x-, y- and z-gradient coils has been analysed for three different body positions (head-, heart- and hips-centred). The magnetic field varied at 1 kHz, so as to produce a rate of change of gradient of 100 T m(-1) s(-1) at the centre of each coil. A highly heterogeneous pattern of induced electric field and current density was found to result from the smoothly varying magnetic field in all cases, with the largest induced electric fields resulting from application of the y-gradient, in agreement with previous studies. By applying simple statistical analysis to electromagnetic quantities within axial planes of the body model, it is shown that the induced electric field is strongly correlated to the local value of resistivity, and the induced current density exhibits even stronger correlation with the local conductivity. The local values of the switched magnetic field are however shown to bear little relation to the local values of the induced electric field or current density.     

The use of MR B+1 imaging for validation of FDTD electromagnetic simulations of human anatomies

In this study, MR B(+)(1) imaging is employed to experimentally verify the validity of FDTD simulations of electromagnetic field patterns in human anatomies. Measurements and FDTD simulations of the B(+)(1) field induced by a 3 T MR body coil in a human corpse were performed. It was found that MR B(+)(1) imaging is a sensitive method to measure the radiofrequency (RF) magnetic field inside a human anatomy with a precision of approximately 3.5%. A good correlation was found between the B(+)(1) measurements and FDTD simulations. The measured B(+)(1) pattern for a human pelvis consisted of a global, diagonal modulation pattern plus local B(+)(1) heterogeneties. It is believed that these local B(+)(1) field variations are the result of peaks in the induced electric currents, which could not be resolved by the FDTD simulations on a 5 mm(3) simulation grid. The findings from this study demonstrate that B(+)(1) imaging is a valuable experimental technique to gain more knowledge about the dielectric interaction of RF fields with the human anatomy.     

The modelling of biological systems in three dimensions using the time domain finite-difference method: II. The application and experimental evaluation of the method in hyperthermia applicator design

The three-dimensional time domain finite-difference (TDFD) method was used to design a 433 MHz circular waveguide applicator for electromagnetic hyperthermia. This applicator has a water-filled lens at one end, providing surface cooling as well as some shaping of the emergent electromagnetic field. An experimental system for scanning the EM field in three dimensions is described. The experimental data were compared with those calculated from the TDFD program and good agreement was found. A realistic three-dimensional model based on an ultrasound scan of a surface tumour was built. Both the EM field and the specific absorption rate (SAR) within the model when irradiated by the applicator were calculated.     

Numerical optimization of a three-channel radiofrequency coil for open, vertical-field, MR-guided, focused ultrasound surgery using the hybrid method of moment/finite difference time domain method

The numerical optimization of a three-channel radiofrequency (RF) coil with a physical aperture for the open, vertical-field, MR-guided, focused ultrasound surgery (MRgFUS) system using the hybrid method of moment (MoM)/finite difference time domain (FDTD) method is reported. The numerical simulation of the current density distribution on an RF coil with a complicated irregular structure was performed using MoM. The electromagnetic field simulation containing the full coil-tissue interactions within the region of interest was accomplished using the FDTD method. Huygens' equivalent box with six surfaces smoothly connected the MoM and FDTD method. An electromagnetic model of the human pelvic region was reconstructed and loaded in the FDTD zone to optimize the three-channel RF coil and compensate for the lower sensitivity at the vertical field. In addition, the numerical MoM was used to model the resonance, decoupling and impedance matching of the RF coil in compliance with engineering practices. A prototype RF coil was constructed to verify the simulation results. The results demonstrate that the signal-to-noise ratio and the homogeneity of the B(1) field were both greatly improved compared with previously published results.     

Magnetic Field Visualization in Applications to Pulsed Electromagnetic Field Stimulation of Tissues

Electromagnetic field visualization is important in multidisciplinary research on the molecular basis of therapeutic effects of pulsed electromagnetic fields (PEMF). We have compared classic PEMF representations by two-dimensional field lines and field magnitude contour plots with a field representation using three-dimensional field isosurfaces. Field simulations were performed for a clinically approved Spinal-Stim® Lite system (Orthofix Inc., McKinney, TX). The relatively simple coil system geometry and the predominantly dielectric properties of the surrounding medium (air and human connective tissue) allowed us to develop analytical expressions for the field. The field model was validated by comparison with experimentally measured field values, and with values calculated using a commercial finite-element analysis software package. Two-dimensional field representations by field lines and field contour plots were less intuitive than three-dimensional field isosurface representations to members of the group without an engineering background. Field isosurfaces, represented as three-dimensional solids, allowed for direct visualization of PEMF targeting of individual organs (lumbar spine), the extent of the therapeutic field value, and the directional field characteristics. The dynamic characteristic of the field was well illustrated by a sequence of field isosurfaces corresponding to the evolution with time of the electric current waveform (sawtooth) powering the coils. The isosurface representation of the field can be extended to any three-dimensional coil system geometry using plotting capabilities of current computer algebra software packages. © 2003 Biomedical Engineering Society. PAC2003: 8750Mn, 8754-n, 8762+n     

基于fluent离心式血泵内流场仿真与结构优化

提高溶血性能,降低溶血率作为血泵性能优化的一个重要指标,对血泵的结构优化具有重要的指导意义。本文基于一款离心式血泵通过使用计算流体力学（CFD）技术,采用非结构化网格、N-S方程和标准K-ε湍流模型在fluent中模拟分析出不同工况下血泵流场内部的剪切力场、压力场等重要参数并根据叶轮流场数据分析,提出了4种不同的结构优化方案;并基于三维快速溶血预估模型计算出不同流量、不同叶轮结构下血泵的溶血性能。仿真结果显示:当叶片与叶轮径向夹角为45°,流量达到5 L/min、转速为2 100 r/min时,扬程为115 mmHg,溶血率达到0.022 1 g/100 L,优化后模型较原模型溶血率提升40.9%,满足人体泵血生理需求。实验结果显示:选用优化后结构进行实验分析,得到扬程的实验数据与仿真数据相互验证,进一步证实了该仿真结果的准确性。     

The modelling of biological systems in three dimensions using the time domain finite-difference method: I. The implementation of the model

A computer method has been developed which uses the time domain finite-difference (TDFD) algorithm to calculate the deposition of the electromagnetic (EM) field in three-dimensional biological models. This, the first of two papers, describes the algorithm and the computer programs developed. The method is demonstrated by calculating the penetration of the EM field from a rectangular waveguide radiating into a homogeneous model, the calculation being carried out in two dimensions for simplicity in this paper.