ESR谱仪磁场过零扫描

通过附加反向直流磁场,使通用的ESR谱仪的磁场实现过零扫描,这样就能记录下某些材料的样品在零场附近微波信号的变化.本文介绍两种附加反向直流磁场的实验方法,即外加补偿的亥姆霍兹线圈,或直接借用ESR谱仪本身的快扫描线圈,来实现过零扫描.     

NMR detection and one-dimensional imaging using the inhomogeneous magnetic field of a portable single-sided magnet

A portable, nuclear magnetic resonance (NMR) probe is described which utilises the intrinsic inhomogeneity of the field produced by a single-sided magnet to provide spatial encoding of the NMR signal. The probe uses a longitudinally magnetized hollow cylinder, and a figure-8 radiofrequency (RF) surface coil. The system has been used to measure NMR relaxation times and one-dimensional NMR profiles of rubber phantoms.     

Characterization of Filler-rubber Interaction,Filler Network Structure,and Their Effects on Viscoelasticity for Styrene-butadiene Rubber Filled with Different Fillers

For styrene-butadiene rubber (SBR) compounds filled with the same volume fraction of carbon black (CB), precipitated silica and carbon–silica dual phase filler (CSDPF), filler-rubber interactions were investigated thru bound rubber content (BRC) of the compounds and solid-state ¹H low-field nuclear magnetic resonance (NMR) spectroscopy. The results indicated that the BRC of the compound was highly related to the amount of surface area for interaction between filler and rubber, while the solid-state ¹H low-field NMR spectroscopy was an effective method to evaluate the intensity of filler-rubber interaction. The silica-filled compound showed the highest BRC, whereas the CB-filled compound had the strongest filler-rubber interfacial interaction, verified by NMR transverse relaxation. The strain sweep measurements of the compounds were conducted thru a rubber process analyzer; the results showed that the CSDPF-filled compound presented the lowest Payne effect, which is mainly related to the weakened filler network structure in polymer matrix. The temperature sweep measurement, tested by dynamic mechanical thermal analysis, indicated that the glass transition temperature did not change when SBR was filled with different fillers, whereas the storage modulus in rubbery state and the tanδ peak height were greatly affected by the filler network structure of composites.     

Two-dimensional imaging by the continuous-wave EPR

To be able to perform a two-dimensional study of free radical distribution by the continuous-wave electron paramagnetic resonance method in the X-band, special coils producing a magnetic gradient of 8 G/mm have been designed and constructed. The EPR spectra recorded for this gradient were subjected to the procedure of deconvolution in order to elicit information on the concentration of the radical distribution. The data obtained were used as the source file of the program reconstructing the image. The reconstruction was based on the iterative simultaneous algebraic reconstruction technique (Andersen A.H., Kak A.C.: Ultrason. Imag. 6, 81–94, 1984). The quality of the generated images depends on the angle of the sample axis to the gradient direction set by a goniometer and on the deconvolution procedures applied. The first tests on artificially generated phantoms indicated a dependence of the obtained images on the magnetic field gradients applied. The determined spatial distribution of radicals has confirmed their uniform distribution in the sample. The preliminary tests were performed for diphenyl-picrylhydrazyl. Having proved the reliability of the method, analogous measurements were also performed for plyphenylene sulphide PPS-V1 and indicated a homogeneous distribution of radicals in the whole volume of the sample. The images obtained confirmed the uniform distribution of the radicals.     

Chemical exchange magnetic resonance imaging (CHEMI)

Systems investigated with NMR spectroscopy are sometimes heterogeneous with respect to chemical composition, rates of chemical exchange, and other properties influencing magnetic resonance parameters. A method was developed to spatially encode reaction kinetic information and produce NMR images sensitive to chemical exchange. A modified spin-echo pulse sequence was used to allow chemical shift-selective imaging and chemical exchange encoding. 1H and 31P images with microscopic resolution were obtained which yielded chemical exchange as a function of position. Chemical exchange images of the base-catalyzed proton exchange of acetylacetone and of the enzyme-catalyzed 31P transfer between PCr and ATP were obtained at 8.4 T in phantoms at 360 and 146 MHz, respectively. These images demonstrate a means of investigating kinetic heterogeneity and compartmentalization of reactions that are important in the study of both living and non-living systems.     

Direct magnitude and phase imaging of myelin using ultrashort echo time (UTE) pulse sequences: A feasibility study

In this paper, we aimed to investigate the feasibility of direct visualization of myelin, including myelin lipid and myelin basic protein (MBP), using two-dimensional ultrashort echo time (2D UTE) sequences and utilize phase information as a contrast mechanism in phantoms and in volunteers. The standard UTE sequence was used to detect both myelin and long T2 signal. An adiabatic inversion recovery UTE (IR-UTE) sequence was used to selectively detect myelin by suppressing signal from long T2 water protons. Magnitude and phase imaging and T2* were investigated on myelin lipid and MBP in the forms of lyophilized powders as well as paste-like phantoms with the powder mixed with D₂O, and rubber phantoms as well as healthy volunteers. Contrast to noise ratio (CNR) between white and gray matter was measured. Both magnitude and phase images were generated for myelin and rubber phantoms as well white matter in vivo using the IR-UTE sequence. T2* values of ~ 300 μs were comparable for myelin paste phantoms and the short T2* component in white matter of the brain in vivo. Mean CNR between white and gray matter in IR-UTE imaging was increased from − 7.3 for the magnitude images to 57.4 for the phase images. The preliminary results suggest that the IR-UTE sequence allows simultaneous magnitude and phase imaging of myelin in vitro and in vivo.     

L波段三维ESR成像系统的研制（Ⅱ）——L波段三维ESR成像系统

叙述了一个L波段（1.05 GHz）用于ESR和ESR成像的装置，用这套自制装置实现了3D ESR成像. 该装置由L波段ESR谱仪、三组梯度场线圈及控制单元和PC机数据采集系统组成. 样品腔是一个3-环2-缝再进入式谐振腔，可放入直径为20 mm、 长30 mm的H₂O样品，空谐振腔的频率是1.05 GHz. 微波振荡频率用自动频率控制（AFC）的方法自动锁在有载腔的频率上. 梯度场线圈沿X-，Y-和Z-轴产生线性梯度场，在中心40 mm球形范围内梯度场强度为2 mT/cm. 依照Lauterbur's方法进行3D ESR 图像重建. 用该系统检测了样品中TEMPO氮氧自由基的3D空间分布. 得到了TEMPO的2D、3D ESR图像、用像素灰度表示的自旋密度分布图及3D ESR-CT图像.     

任意两共轴圆线圈间的互感系数及磁感线的分布

从一个圆电流的矢势出发,导出了任意两共轴圆线圈间互感系数的函数表达式,根据其函数表达式,由数学软件MatLab绘出了互感系数随两线圈间距离及两线圈半径变化的分布图.然后再从一个圆电流的矢势出发,导出了任意两共轴圆电流线圈磁感线所满足的方程,按照磁感线方程也绘出了其磁感线在空间的分布图.     

Intracranial microvascular imaging at 7 T MRI with transceiver RF coils

Purpose

To investigate intracranial microvascular images with transceiver radio-frequency (RF) coils at ultra-high field 7 T magnetic resonance imaging (MRI).

Materials and methods

We designed several types of RF coils for the study of 7 T magnetic resonance angiography and analyzed quantitatively each coil's performance in terms of the signal-to-noise ratio (SNR) profiles to evaluate the usefulness of RF coils for microvascular imaging applications. We also obtained the microvascular images with different resolutions and parallel imaging technique.

Results

The overlapped 6-channel (ch) transceiver coil exhibited the highest performance for angiographic imaging. Although other multi-channel coils, such as 4- or 8-ch, were also suitable for fast imaging, these coils performed poorly in homogeneity or SNR for angiographic imaging. Furthermore, the 8-ch coil was poor in SNR at the center of the brain, while it had the highest SNR at the periphery.

Conclusion

The present study has demonstrated that the overlapped 6-ch coil with large-size loop coils provided the best performance for microvascular imaging or angiography with the ultra-high-field 7 T MRI, mainly because of its long penetration depth together with high SNR.     

On surface coils and depth pulses

A computer program has been developed which calculates the signal response to a generalised depth pulse both on and off resonance. Results are displayed on a grey scale for any plane which includes the axis of the circular surface coil used to apply the pulse. Use of separate (coaxial) transmitter and receiver coils can be accommodated, and both in-phase and out-of-phase signal components can be displayed. Some methods of improving spatial resolution are demonstrated and the off-resonance performance of several depth pulses is compared. Results of the computer simulations are compared with experimental results on phantoms using a simple surface coil.     

HIRFL-CSR电子冷却装置高精度螺线管线圈制作及磁场测量

 采用特殊工艺制作了HIRFL-CSR电子冷却装置冷却段高精度螺线管线圈,两个产生反向磁场的线圈同轴、平行地放置在特制的测量装置上,高精度霍尔探头位于测量装置中心平面上,探头测量面与测量装置轴线重合,测量单个线圈磁场的横向分量,调节线圈几何轴相对于测量装置轴线的夹角,测得线圈磁轴的偏角小于1×10^-3。     

Active shielding of cylindrical saddle-shaped coils: application to wire-wound RF coils for very low field NMR and MRI

We provide an exact expression for the magnetic field produced by cylindrical saddle-shaped coils and their ideal shield currents in the low-frequency limit. The stream function associated with the shield surface current is also determined. The results of the analysis are useful for the design of actively shielded radio-frequency (RF) coils. Examples pertinent to very low field nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are presented and discussed.     

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.     

Simulation and experimental verification of the diffusion in an anisotropic fiber phantom

Diffusion weighted magnetic resonance imaging enables the visualization of fibrous tissues such as brain white matter. The validation of this non-invasive technique requires phantoms with a well-known structure and diffusion behavior. This paper presents anisotropic diffusion phantoms consisting of parallel fibers. The diffusion properties of the fiber phantoms are measured using diffusion weighted magnetic resonance imaging and bulk NMR measurements. To enable quantitative evaluation of the measurements, the diffusion in the interstitial space between fibers is modeled using Monte Carlo simulations of random walkers. The time-dependent apparent diffusion coefficient and kurtosis, quantifying the deviation from a Gaussian diffusion profile, are simulated in 3D geometries of parallel fibers with varying packing geometries and packing densities. The simulated diffusion coefficients are compared to the theory of diffusion in porous media, showing a good agreement. Based on the correspondence between simulations and experimental measurements, the fiber phantoms are shown to be useful for the quantitative validation of diffusion imaging on clinical MRI-scanners.     

Eddy current simulation in thick cylinders of finite length induced by coils of arbitrary geometry

Eddy currents are inevitably induced when time-varying magnetic field gradients interact with the metallic structures of a magnetic resonance imaging (MRI) scanner. The secondary magnetic field produced by this induced current degrades the spatial and temporal performance of the primary field generated by the gradient coils. Although this undesired effect can be minimized by using actively and/or passively shielded gradient coils and current pre-emphasis techniques, a residual eddy current still remains in the MRI scanner structure. Accurate simulation of these eddy currents is important in the successful design of gradient coils and magnet cryostat vessels. Efficient methods for simulating eddy currents are currently restricted to cylindrical-symmetry. The approach presented in this paper divides thick conducting cylinders into thin layers (thinner than the skin depth) and expresses the current density on each as a Fourier series. The coupling between each mode of the Fourier series with every other is modeled with an inductive network method. In this way, the eddy currents induced in realistic cryostat surfaces by coils of arbitrary geometry can be simulated. The new method was validated by simulating a canonical problem and comparing the results against a commercially available software package. An accurate skin depth of 2.76 mm was calculated in 6 min with the new method. The currents induced by an actively shielded x-gradient coil were simulated assuming a finite length cylindrical cryostat consisting of three different conducting materials. Details of the temporal-spatial induced current diffusion process were simulated through all cryostat layers, which could not be efficiently simulated with any other method. With this data, all quantities that depend on the current density, such as the secondary magnetic field, are simply evaluated.     

Double-quantum-filtered NMR signals in inhomogeneous magnetic fields

The possibility of exciting and detecting proton NMR double-quantum coherences in inhomogeneous static and radiofrequency magnetic fields was investigated. For this purpose specialized pulse sequences which partially refocus the strongly inhomogeneous evolution of the spin system and generate double-quantum buildup and decay curves were implemented on the NMR MOUSE (mobile universal surface explorer). The theoretical justification of the method was developed for the simple two-spin-1/2 system. The performances of the same pulse sequences were also tested on a solid-state high-field NMR spectrometer. It was shown that DQ decay curves have a better signal-to-noise ratio in the initial time regime than DQ buildup curves. The double-quantum buildup and decay curves were recorded for a series of cross-linked natural rubber samples. These curves give access to quantitative values of the ratio of proton total residual dipolar couplings which are in good agreement with those measured in homogeneous fields. A linear dependence of these ratios on the sulfur-accelerator content was found.     

多孔介质体元内随机运动的核磁共振成像研究

多孔介质体元内随机运动的核磁共振成像是一种透视流体微观无规则运动的方法.在多孔介质中,存在两种无规则运动.一种是分于扩散.它是由于分子的布朗运动引起的;另一种是弥散,它是由多孔介质中毛管网络的复杂结构造成的.多孔介质中的液体弥散和分子扩散会在成像体无内产生相位分散.这种相位分散致使回波信号发生衰减.采用适当的梯度脉冲.可以从这种信号衰减中获得视扩散系数(ADC)的分布图像.本文作者在4.7T高场超导核磁共振成像系统上.采用特制的高梯度线圈获得了扩散和机扩散系数分布的图像.在水和丙酮模型中获得了扩散系教图像,其扩散系数值与标准值相符.在人体肾结石中获得了扩散系数分布图像,在不同流速下,获得了多孔介质中的视扩散系数分布图像.并分析了视扩散系数与速度的关系.在多孔介质中定量确定扩散系数和机扩散系数及其空间分布将会极大地提高核磁共振成像技术在渗流力学研究中的应用能力.     

Development of a laser photothermoacoustic frequency-swept system for subsurface imaging: theory and experiment

In conventional biomedical photoacoustic imaging systems, a pulsed laser is used to generate time-of-flight acoustic information of the subsurface features. This paper reports the theoretical and experimental development of a new frequency-domain (FD) photo-thermo-acoustic (PTA) principle featuring frequency sweep (chirp) and heterodyne modulation and lock-in detection of a continuous-wave laser source at 1064 nm wavelength. PTA imaging is a promising new technique which is being developed to detect tumor masses in turbid biological tissue. Owing to the linear relationship between the depth of acoustic signal generation and the delay time of signal arrival to the transducer, information specific to a particular depth can be associated with a particular frequency in the chirp signal. Scanning laser modulation with a linear frequency sweep method preserves the depth-to-delay time linearity and recovers FD-PTA signals from a range of depths. Preliminary results performed on rubber samples and solid tissue phantoms indicate that the FD-PTA technique has the potential to be a reliable tool for biomedical depth-profilometric imaging.     

Segmental anisotropy in strained elastomers detected with a portable NMR scanner

Single-side NMR is particularly suitable for measurements of segmental anisotropy induced in elastomers by uniaxial forces or local strain. Proton transverse nuclear magnetic relaxation was investigated with the NMR-MOUSE by recording the Hahn-echo decay in cross-linked natural rubber bands. This provided information on the dependence of the Hahn-echo decay on the angle between the direction of the uniaxial stretching force and the axis Z defined direction perpendicular to the magnet pole faces of the NMR-scanner. The anisotropy effect on the Hahn-echo decay is correlated with the extension ratio, and it is more evident in the liquid-like regime of the decay. A weaker segmental anisotropy is detected by 1H solid- and Hahn-echo decays recorded by multi-pulse sequences. A qualitative understanding of the angular dependence is obtained by an analytical theory of the Hahn-echo decay adapted to the case of stretched elastomers and to strongly inhomogeneous magnetic fields. Using angular-dependent 1H residual second van Vleck moments and correlation times reported previously [P.T. Callaghan and E.T. Samulski, Macromolecules 30, 113 (1997)] from stretched natural rubber bands the segmental anisotropy measured in inhomogeneous magnetic fields by the Hahn-echo decay was numerically simulated. As an example of a macroscopic distribution of local segmental anisotropy, 1H Hahn-echo decays were measured by the NMR-MOUSE sensor in a stretched cross-linked natural rubber plate with a circular cut in the center.     

The Evaluation of Dielectric Resonators Containing H2O or D2O as RF Coils for High-Field MR Imaging and Spectroscopy

Electromagnetic resonators consisting of low-loss dielectric material and/or metallic boundaries are widely used in microwave technologies. These dielectric resonators usually have high Q factors and well-defined field distributions. Magnetic resonance imaging was shown as a way of visualizing the magnetic field distribution of the resonant modes of these resonators, if the dielectric body contains NMR sensitive nuclei. Dielectric resonators have also been proposed as RF coils for magnetic resonance experiments. The feasibility of this idea in high-field MR is discussed here. Specifically, the dielectric resonances of cylindrical water columns were characterized at 170.7 MHz (4 T¹H Larmor frequency), and evaluated as NMR transmit and receive coils. The dielectric resonance of a cylindrical volume of D₂O was used to image a hand at 170.7 MHz. This study demonstrated that MRI is an effective way of visualizing the magnetic field in dielectric structures such as a water cylinder, and can potentially be generalized to solid-state dielectric devices. The possible applications of dielectric resonators other than simple cylindrical volumes in MRI and MR solution spectroscopy at high field strengths are also discussed.