一种矢量原子磁力仪中旋转磁场的设计及实验验证

目前国内外报道了几种矢量原子磁力仪的技术方案,其中基于磁场旋转调制法的矢量原子磁力仪可实现对矢量磁场的连续测量。本文对磁场旋转调制法矢量原子磁力仪的工作原理进行了阐述,对“旋转磁场”的设计、产生以及标定方法进行了介绍,在此基础上基于抽运-检测型原子磁力仪对旋转磁场和矢量磁场的叠加磁场进行了实验测量,验证了实测磁场的平均值、峰峰值与理论计算结果的符合情况,分析了实测磁场的平均值与理论平均值的偏差起源,并通过实验验证了矢量原子磁力仪的连续测量能力。本文研究内容为连续测量型矢量原子磁力仪的研制奠定了技术基础。     

利用质子信号模拟器校准质子进动式磁强计

介绍利用四象限模拟乘法器等构成质子进动信号模拟器来校准质子磁强计。此方法能以物理常数γ＇ｐ和频率计直接校准质子磁强计。校准的不确定度小于 １．４×１０～(－７)。可以取代使用替代法在弱磁场标准装置上校准质子磁强计。     

自由弛豫信号截取时长对NMOR铷原子磁力仪性能的影响

基于非线性磁光旋转(nonlinear magneto-optical rotation, NMOR)效应的铷原子磁力仪通过特定时长的自由弛豫正弦振荡信号可获得拉莫尔进动频率和外磁场值,因此自由弛豫信号的截取时长能够影响原子磁力仪的性能指标。重点研究了在500 nT背景磁场条件下自由弛豫信号截取时长对原子磁力仪灵敏度和磁场测量值的影响。测量结果表明,用噪声功率谱方法标定的灵敏度指标随着自由信号截取时长的增加逐渐减小并趋于稳定,约0.2 pT/Hz^1/2;交变磁场实验表明该磁力仪所得的磁场值是特定截取时长内磁场波动的平均。因此本文NMOR原子磁力仪所得的高灵敏度指标一定程度上是通过数学方法平均了自由弛豫信号截取时长内的磁场值波动。该方法对其他原子磁力仪或实验装置灵敏度的标定和磁场测量值的分析具有一定的借鉴意义。     

Improvement of magnetometer calibration using Levenberg–Marquardt algorithm

The algorithm, the model, and the measurement strategy are important for the calibration of three‐axis magnetometers. A new calibration model with clear physical meaning is proposed, and scale factors, offsets, and nonorthogonal angles are directly illustrated. One limitation of traditional iteration calibration algorithms is the influence of the initial parameters. In this paper, the Levenberg–Marquardt algorithm is proposed to solve the troublesome procedure of initial parameter selection, so that it can improve the calibration performance of three‐axis magnetometers. The validity of this method is proved by simulation, and the estimated parameters are found to be close to the prearranged parameters. The experimental system mainly consists of a three‐axis fluxgate magnetometer (DM‐050), two‐dimensional nonmagnetic rotation equipment, and a proton magnetometer. The magnetic field intensity is obtained by the proton magnetometer. Experimental results show that the root‐mean‐square error is reduced from 84.177 to 4.076 nT. In addition, the influence of the initial parameters and measurement strategy is analyzed. Experimental results show that the Levenberg–Marquardt algorithm is not sensitive to the initial parameters and the 36 static measurements strategy is more reliable than the rotation measurement strategy. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Magnetic field gradient system for nuclear magnetic resonance microimaging

In this study we present an orthogonal magnetic field gradient system for nuclear magnetic resonance (NMR) microimaging applications. The construction details are given for a prototype assembly for proton microscopy inside a 50-mm vertical bore magnet, which is designed to fit into a commercial 300-MHz NMR probe. This system has been used to acquire images of the human spinal cordin vitro. Its performance has been evaluated and compared to that predicted by computer simulation.     

运用谐振触发方式的RSD脉冲电源

本文提出了一种运用谐振原理的可重复RSD脉冲电源。分析电路的工作特性并对其重要元件磁开关进行了重点设计。给出了实验结果，最后提出了几种改进电路参数的方法。     

Breathhold imaging of the upper abdomen using a circular polarized-array coil: comparison with standard body coil imaging

Current studies emphasize the use of array coils to decrease noise and increase the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). We applied Tl-weighted and T2-weighted standard nonbreathhold spin echo (SE) sequences and Tl-weighted FLASH, TurboFLASH, T2-weighted spin-echo time (TSE), and heavily T2-weighted half Fourier acquisition single-shot TSE (HASTE) sequences during breathhold for abdominal imaging in 15 normal volunteers. The breathhold scans were performed using both a standard coil and a circular polarized array coil. We analyzed the signal intensity (SI), SNR, and CNR of abdominal organs in all sequences. SNRs increased in all cases by an overall factor of 3 due to an 8% increase in overall Sis and a 50% decrease in noise when applying the array coil. Although the array-coil FLASH sequence performed at least as well as the respective SE sequence, the SNRs of the array-coil TurboFLASH, TSE breathhold, and HASTE sequences were generally lower. We conclude that array-coil imaging significantly improves fast imaging of the abdomen.     

Three-dimensional magnetic field analysis of a system containing coil currents and iron elements using the virtual current method

This paper proposes a method of analyzing the three-dimensional magnetic field of a system containing coil currents and magnetic substances using virtual currents. Various methods have been presented for analysis of a three-dimensional static field in an open boundary space; however, in a large-scale problem, the computational size easily exceeds the acceptable limit of computing time and machine capacity. The basic idea of the proposed method is to divide the whole region into interior and exterior regions with an artificial boundary and to substitute virtual currents for source currents and magnetic substances within the boundary. The use of virtual currents reduces the number of elements to be analyzed and shortens computing time while maintaining accuracy of analysis. In this paper, a method of determining virtual current is proposed and principal parameters of virtual currents that affect accuracy of analysis are examined. The conditions under which the virtual current method can be applied are examined through analysis of errors introduced by the virtual current method. This method has been applied to analyze magnetic shielding of a magnet for magnetic resonance imaging. This example proves that the proposed method is effective in reducing the scale of analysis and computing time.     

Double-tuned four-ring birdcage resonators forin vivo 31P-nuclear magnetic resonance spectroscopy at 11.75 T

A high signal-to-noise ratio (SNR) in³¹P-nuclear magnetic resonance (³¹P-NMR) spectroscopy can be obtained only with goodB ₀-field homogeneity and optimal coil sensitivity. This demands double-tumed coils with a highly sensitive³¹P channel and an additional¹H channel for¹H-magnetic resonance imaging, shimming,¹H decoupling, and nuclear Overhauser enhancement (NOE). For studies on an 11.75 T magnet, we built coils derived from the four-ring birdcage design originally described by Murphy-Boesch. A comparison with conventional, single-tuned coils shows that, in spite of double tuning, there is no significant loss in³¹P sensitivity while the¹H channel provides the requested performance. The coil design offers the advantage of circular polarization on both channels. Supported by the Deutsche Forschungsgemeinschaft (Ki 433/2-2 and Graduiertenkolleg Ha 1232/8-2).     

磁共振成像仪通信及脉冲生成模块的设计

针对小型化核磁共振成像仪主控板的功能需要提出新的设计方式,采用 FPGA开发板作为主控板,以Nios II嵌入式软核作为协处理器,通过在其上移植μC／OS‐Ⅱ操作系统及 lw IP协议栈,实现与计算机的网络通信;使用Verilog硬件描述语言编写脉冲序列生成模块,实现对指令的解析以及执行,形成用户要求的脉冲序列。最终测试结果表明,主控板的通信速率以及通过示波器采集得到的脉冲序列均满足成像仪系统要求。而且这种软硬结合的设计方式与以往全软件的设计相比,使成像仪精度更高,性能更加稳定。     

Measurements of left ventricular dimensions using real-time acquisition in cardiac magnetic resonance imaging: comparison with conventional gradient echo imaging

This study investigates the use of real-time acquisition in cardiac magnetic resonance imaging (MRI) for measurements of left ventricular dimensions in comparison with conventional gradient echo acquisition. Thirty-one subjects with a variety of left ventricular morphologies to represent a typical clinical population were studied. Short-axis data sets of the left ventricle (LV) were acquired using a conventional turbo-gradient echo and an ultrafast hybrid gradient echo/echo planar sequence with acquisition in real-time. End-diastolic volume (EDV), end-systolic volume (ESV), ejection fraction (EF) and left ventricular mass (LV mass) were measured. The agreement between the two acquisitions and interobserver, intraobserver and interstudy variabilities were determined. The bias between the two methods was 5.86 ml for EDV, 0.23 ml for ESV and 0.94% for EF. LV mass measurements were significantly lower with the real-time method (mean bias 14.38 g). This is likely to be the result of lower spatial resolution and chemical shift artefacts with the real-time method. Interobserver, intraobserver and interstudy variabilities were low for all parameters. In conclusion, real time acquisition in MRI can provide accurate and reproducible measurements of LV dimensions in subjects with normal as well as abnormal LV morphologies, but LV mass measurements were lower than with conventional gradient echo imaging. Presented in abstract form at the International Society of Magnetic Resonance in Medicine meeting in Denver, Colorado in April 2000.     

The NMR sensitivity achievable with a slotted-tube resonator

The signal-to-noise ratio achievable with the slotted-tube resonator (STR), a fundamental type of high radiofrequency coil for nuclear magnetic resonance (NMR) experiments, was formulated in an equation. This equation is based on formulae presented by Hoult and Richards [J Magn Reson 24, 71 (1976)] and Hoult and Lauterbur [J Magn Reson 34, 425 (1979)]. The equation assumes that the sample is positioned within the homogeneous region of magnetic fluxB ₁ generated by the STR, and involves no unknown factors. The NMR sensitivity of an experiment with an STR can therefore be predicted, and the equation is applicable to any nuclear species, static field strength, and dimensions of the sample and STR.     

通过人工磁异常环对航空磁测系统的标定

航空磁测系统具有广泛的用途,但是得到的数据通常包含多种来源的噪声,降低了系统对磁目标的探测能力。实验搭建了一个人工的大面积磁异常环电流线圈,并使用直升机机载光泵磁力仪对线圈在不同高度产生的磁场信号进行测量。通过对得到的信号进行去噪处理,能够标定航磁系统,进而用于航磁测量或者磁目标探测。实验和数据处理结果表明,这套系统能够在较远的距离探测到nT级的磁异常目标。     

开关磁阻电机磁通波形与铁耗计算

对开关磁阻电机的磁通波形进行了分析。得到了电机各部分平均磁通的数学表达式；然后对平均磁密波形进行傅里叶级数分解，把非正弦磁密波形分解和转化为一系列不同频率的正弦磁密波形。对于磁滞损耗，采用考虑了小滞环的磁滞损耗计算数学模型进行计算。对于涡流损耗，在采用传统正弦磁通波形计算公式的基础上，加上涡流损耗修正因子计算。最后对一台6／4结构的开关磁阻电机进行了铁耗计算，同时将计算结果与采用有限元法的结果进行了比较。验证了该方法的正确性。     

Clinical experience of osteoarticular MRI using a dedicated system

Magnetic resonance imaging (MRI) has a significative impact on diagnosis of musculoskeletal diseases. At present, joint diseases are evaluated with total-body systems, this fact representing an obstacle to MR diffusion in the osteoarticular field. The last technological advances have allowed the development of a cost-effective, compact and easy-to-install MR system. The system is constituted by a 0.2-T permanent unit, weighing 800 kg. The unit is used only for limb examination. To verify the diagnostic accuracy of the new system a study based on 1902 lower limb examinations was carried out between October 1992 and February 1994. Of these patients, 301 underwent surgery during which the MR findings were verified. Quite satisfying overall results were obtained, particularly in case of knee trauma, comparable to those provided by total body units with higher magnetic field. It must be noted, however, that in 3% of the investigated knee diseases, the examinations could not be performed due to technical limitations related to the magnet size. The authors believe that the limited field of view (11 cm) does not allows accurate staging of the malignant lesions concerning soft tissue and bone, which require a wider loco-regional staging. They also believe that the particular structure of the magnet allows for a comfortable management of pediatric, elderly, and acute patients.     

开关磁阻电机磁通波形与铁耗计算

对开关磁阻电机的磁通波形进行了分析,得到了电机各部分平均磁通的数学表达式;然后对平均磁密波形进行傅里叶级数分解,把非正弦磁密波形分解和转化为一系列不同频率的正弦磁密波形。对于磁滞损耗,采用考虑了小滞环的磁滞损耗计算数学模型进行计算。对于涡流损耗,在采用传统正弦磁通波形计算公式的基础上,加上涡流损耗修正因子计算。最后对一台6/4结构的开关磁阻电机进行了铁耗计算,同时将计算结果与采用有限元法的结果进行了比较,验证了该方法的正确性。     

12/8极双通道开关磁阻电机非线性数学模型与有限元分析

与传统的12/8极开关磁阻电机不同,双通道开关磁阻电机线圈之间采用一对极绕组反向串联的结构形式,由两套独立的功率电路驱动形成新型的双通道控制系统,该系统能够实现容错控制以提高可靠性.本文在分析双通道开关磁阻电机原理和结构的基础上,建立了电机每个通道绕组的磁路等效数学模型,在一个相周期内对磁路各部分磁导进行了详细分解计算,确定了以磁导表示的磁通矩阵,推导出绕组自感及各相之间互感的计算方法.该模型考虑了电机轭部磁路和材料非线性磁饱和的影响,提高了模型的实用性.有限元分析结果验证了该数学模型的正确性.     

复合绝缘子硅橡胶材料老化性能分析

分析了青海电网部分长时间服役的复合绝缘子硅橡胶材料的老化现象,研究了绝缘子用硅橡胶材料在高原地区的应用状况及其老化规律,采用核磁共振方法对硅橡胶材料进行了检测.结果表明,该检测方法是可行的,可快速、准确地监测电网运行中复合绝缘子的老化程度.     

Characterization of pheochromocytomas using quantitati analysis of the parameter T2 of the mass (T2-QMRI)

The usefullness of MR imaging in the characterization of pheochromocytomas was evaluated in 9 patients with 13 pheochromocytomas and 1 paraganglioma. In two patients the tumors were multiple and in one, extra-adrenal. Two patients conformed to MEN2 syndrome. Adrenal masses were characterized using several parameters: (a) visual inspection of the signal intensities in T1-, T2-, and Gd-DTPA-enhanced T1-weighted images; (b) observed signal intensity and observed signal intensity ratios (adrenal mass/liver, adrenal mass/retroperitoneal fat and adrenal mass/ subcutaneous fat) in the T2-weighted images; (c) calculated T2 relaxation times and calculated T2 relaxation time ratios (adrenal mass/liver, adrenal mass/retroperitoneal fat and adrenal mass/subcutaneous fat) of the adrenal masses.All pheochromocytomas had a T2 relaxation time greater than 82 ms with a maximum value of 134.3 ms. These values were calculated using T2 quantitative analysis methods based on in-house designed mathematical routines (T2-QMRI). The signal intensities of the tumor on T2-weighted spin-echo images were extremely high.As a conclusion of this preliminary report it is postulated that T2 quantitative MRI examination (T2-QMRI) is an accurate method for the diagnosis and characterization of pheochromocytomas because it gives positive diagnostic results whether the pheochromocytoma is secreting or nonfunctioning. It can also detect extra-adrenal pheochromocytomas (paragangliomas). Comparative studies with other adrenal tumors is essential to assess accuracy of this method for characterizing these tumors.Address for correspondence: Areteion University Hospital, Radiology Department, VasSofias 76 st, 11528 Athens, Greece. Additional reprints of this chapter may be obtained from the Reprints Department, Chapman & Hall, One Venn Plaza, New York, NY 10119.     

Left ventricular radial tagging acquisition using gradient-recalled-echo techniques: sequence optimization

Myocardial tagging with magnetic resonance (MR) imaging offers unique possibilities for noninvasive left ventricular (LV) strain analysis. True three-dimensional strain analysis can be achieved with tags implemented in cardiac short axis and long axis images. Spin-echo (SE) techniques have been used for these studies. However, this approach is time-consuming: images at different phases of the cardiac cycle have to be obtained in successive measurements and hence the total number of measurements equals the number of time frames. Moreover, the images are often degraded by flow and motion artifacts. The purpose of this study was to optimize a faster and more robust MR tagging sequence for use on a clinical whole-body 1 T MR system with optimal persistence of the tags during the entire cardiac cycle. The tagging pulses were implemented in gradient-recalled-echo (GRE) sequences and compared to SE-based acquisitions. The effects of the use of flow-compensating gradients, the excitation angles, and the angles of the saturation pulses have been studied with MR signal simulations and in comparative measurements in volunteers. GRE acquisitions with flow-compensating gradients are robust techniques for myocardial tagging acquisitions. Use of optimized flip angles and saturation pulses can significantly improve delineation of the tag and can be used up to at least 700 ms after the R-wave. Therefore, LV tagging with GRE acquisitions using optimized MR parameters is a robust and promising technique.