Numerical methods and software tools for simulation,design, and resonant mode analysis of radio frequency birdcage coils used in MRI

Design of magnetic resonance imaging (MRI) radiofrequency (RF) coils using lumped circuit modeling based techniques begins to fail at high frequencies, and therefore more accurate models based on the electromagnetic field calculations must be used. Field calculations are also necessary to understand the interactions between the RF field and the subject inside the coil. Furthermore, observing the resonance behavior of the coil and the fields at the resonance frequencies have importance for design and analysis. In this study, finite element method (FEM) based methods have been proposed for accurate time‐harmonic electromagnetic simulations, estimation of the tuning capacitors on the rungs or end rings, and the resonant mode analysis of the birdcage coils. Capacitance estimation was achieved by maximizing the magnitude of the port impedance at the desired frequency while simultaneously minimizing the variance of RF magnetic field in the region of interest. In order for the proposed methods to be conveniently applicable, two software tools, resonant mode and frequency domain analyzer (RM‐FDA) and Optimum Capacitance Finder (OptiCF), were developed. Simulation results for the validation and verification of the software tools are provided for different cases including human head simulations. Additionally, two handmade birdcage coils (low‐pass and high‐pass) were built and resonance mode measurements were made. Results of the software tools are compared with the measurement results as well as with the results of the lumped circuit modeling based method. It has been shown that the proposed software tools can be used for accurate simulation and design of birdcage coils. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 13–32, 2015     

实时MRI引导放射治疗中边缘磁场对电子枪束流特性的影响

目的 观察实时MRI引导放射治疗（MRIgRT）系统中MR边缘磁场对电子枪束流特性的影响。方法 以电磁场仿真软件模拟平行配置MRIgRT系统中1.5T MR不同边缘磁场下Varian 600C电子枪和Litton L2087电子枪的束流特性。结果 2款电子枪在磁场中的整体表现一致。随磁感应强度增大,Litton L2087和Varian 600C电子枪电流分别缓慢增加至无磁场时的2.72%和1.93%。电子枪阳极出口处电流在经历初始稳定水平后开始减少,达到最小值后电流缓慢增加,电流损失最大分别可达80.25%和73.48%;高场强处,束流半径出现周期性变化;束流均方根发射度存在波动。结论 平行配置的MRIgRT系统中,边缘磁场会对电子枪束流轨迹产生影响,导致大量电流损失,均方根发射度变化巨大。     

Changes in abdominal wall thickness during laparoscopy: implications for the use of magnetic assisted surgery

Abstract

Introduction: Recently, magnetic solutions have been proposed to minimize surgical invasiveness. These are comprised of deployable instruments containing magnets which are inserted into the abdominal cavity through a single access point. The manipulation of the internal elements occurs via magnets held on the external surface of the abdominal wall. This technology relies on the magnetic force between the magnets, which is inversely related to the abdominal wall thickness (AWT). The aim of this study was to establish the expected change in AWT from before and after initiation of pneumoperitoneum.

Material and methods: Patients scheduled for laparoscopic procedures were assessed by ultrasound for AWT immediately before and during laparoscopy. Change of AWT during laparoscopy was calculated. Statistical analysis was performed using Student’s t-test.

Results: Thirty-two patients undergoing various laparoscopic procedures were included. Twenty patients were male (62.5%) and ten were morbidly obese (31%). Mean age was 51?years (range 18–76) and average BMI was 28.1?kg/m² (range 19.0–41.0). AWT decreased on average by 15.6% once pneumoperitoneum was initiated in both obese and non-obese patients (p?=?.01).

Conclusion: Our data suggest that following preoperative assessment of AWT with abdominal wall ultrasound, more patients than expected might be candidates for the use of trans-abdominal magnetic devices.     

In vivo MRI assessment of a novel GdIII‐based contrast agent designed for high magnetic field applications

Gd₃L is a trinuclear Gd³⁺ complex of intermediate size, designed for contrast agent applications in high field magnetic resonance imaging (H₁₂L is based on a trimethylbenzene core bearing three methylene‐diethylenetriamine‐ N,N,N″,N″‐tetraacetate moieties). Thanks to its appropriate size, the presence of two inner sphere water molecules and a fast water exchange, Gd₃L has remarkable proton relaxivities at high magnetic field (r₁ = 10.2 vs 3.0 mM ^?1 s^?1 for GdDOTA at 9.4 T, 37°C, in H₂O). Here we report an in vivo MRI feasibility study, complemented with dynamic γ scintigraphic imaging and biodistribution experiments using the ¹⁵³Sm‐enriched analog. MRI experiments were performed at 9.4 T in mice with Gd₃L and the commercial contrast agent gadolinium(III)‐1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetate (GdDOTA). Gd₃L was well tolerated by the animals at the dose of 8 µmol Gd kg^?1 body weight. Dynamic contrast enhanced (DCE) images showed considerably higher signal enhancement in the kidney medulla and cortex after Gd₃L injection than after GdDOTA injection at an identical dose. The relaxation rates, ΔR₁, were calculated from the IR TrueFISP data. During the excretory phase, the ΔR₁ for various tissues was similar for Gd₃L and GdDOTA, when the latter was injected at a three‐fold higher dose (24 vs 8 µmol Gd kg^?1 body weight). These results point to an approximately three times higher in vivo relaxivity (per Gd) for Gd₃L relative to GdDOTA, thus the ratio of the relaxivities of the two compounds determined in vitro is retained under in vivo conditions. They also indicate that the two inner sphere water molecules per Gd in Gd₃L are not substantially replaced by endogenous anions or other donor groups under physiological conditions. Gd₃L has a pharmacokinetics typical of small, hydrophilic complexes, involving fast renal clearance and no retention in the blood pool. The dynamic γ scintigraphic studies and the biodistribution experiments performed in Wistar rats with ¹⁵³Sm‐enriched ^*Sm₃L are also indicative of a fast elimination via the kidneys. Copyright © 2008 John Wiley & Sons, Ltd.     

HR-VW-MRI与数字减影血管造影检查在评估脑动脉狭窄病变中的价值比较

目的探讨高分辨率血管壁磁共振成像（HR-VW-MRI）评估脑血管狭窄与数字减影血管造影（DSA）金标准对照的临床诊断价值。方法收集本院2016年1月~2019年12月入院25例近期发生缺血性脑病患者,行CT血管造影、磁共振血管成像检查证实存在脑血管狭窄,在1周内行DSA检查及HR-VW-MRI,分别测量、计算狭窄段脑动脉DSA与HR-VW-MRI狭窄率,对责任区血管进行高分辨率磁共振血管壁重建、T1WI增强,根据线性回归分析,Pearson相关性分析评价HR-VW-MRI测量脑血管狭窄率与DSA狭窄率的相关性,并探寻脑动脉狭窄原因。结果25例患者共有20例完成HR-VW-MRI和DSA数据测量。HRVW-MRI狭窄率为62.66±13.79,DSA狭窄率为63.78±13.10,Pearson相关性分析显示两组检查方式呈正相关（P < 0.05）,线性回归分析显示两组方式检查相关性良好（R²=0.880,P < 0.05）,HR-VW-MRI发现脑动脉狭窄段责任斑块。结论HR-VW-MRI检查在测量脑血管狭窄方面与DSA具有良好的相关性,因具有无创性,良好的组织结构分辨率,能通过三维角度更好的显示脑血管壁结构及管腔大小,分析病因,做出诊断,在替代DSA检查方面具有一定的优势。     

低场磁共振水成像结合常规磁共振成像在胆道及泌尿系统疾病诊断中的应用

目的评价低场磁共振水成像结合常规磁共振成像（MRI）在胆道及泌尿系统疾病诊断中的临床价值。方法回顾性分析31例临床怀疑患有胆道、泌尿系统疾病患者的低场（0．2T）磁共振胰胆管成像（MRCP）、磁共振尿路成像（MRU）及常规MRI图像。结果低场MRCP、MRU结合常规MRI对胆道、泌尿系统性疾病的定位诊断符合率为100％,定性诊断总符合率为96．5％。结论低场磁共振水成像结合常规MRI在胆道及泌尿系统疾病诊断中的诊断是非常有效和准确的方法。     

Evaluation of a Subject specific dual-transmit approach for improving B1 field homogeneity in cardiovascular magnetic resonance at 3T

Background

Radiofrequency (RF) shading artifacts degrade image quality while performing cardiovascular magnetic resonance (CMR) at higher field strengths. In this article, we sought to evaluate the effect of local RF (B₁ field) shimming by using a dual-source–transmit RF system for cardiac cine imaging and to systematically evaluate the effect of subject body type on the B₁ field with and without local RF shimming.

Methods

We obtained cardiac images from 37 subjects (including 11 patients) by using dual-transmit 3T CMR. B₁ maps with and without subject-specific local RF shimming (exploiting the independent control of transmit amplitude and phase of the 2 RF transmitters) were obtained. Metrics quantifying B₁ field homogeneity were calculated and compared with subject body habitus.

Results

Local RF shimming across the region encompassed by the heart increased the mean flip angle (μ) in that area (88.5 ± 15.2% vs. 81.2 ± 13.3%; P = 0.0014), reduced the B₁ field variation by 42.2 ± 13%, and significantly improved the percentage of voxels closer to μ (39% and 82% more voxels were closer to ± 10% and ± 5% of μ, respectively) when compared with no RF shimming. B₁ homogeneity was independent of subject body type (body surface area [BSA], body mass index [BMI] or anterior-posterior/right-left patient width ratio [AP/RL]). Subject specific RF (B₁) shimming with a dual-transmit system improved local RF homogeneity across all body types.

Conclusion

With or without RF shimming, cardiac B1 field homogeneity does not depend on body type, as characterized by BMI, BSA, and AP/RL. For all body types studied, cardiac B₁ field homogeneity was significantly improved by performing local RF shimming with 2 independent RF-transmit channels. This finding indicates the need for subject-specific RF shimming.