Overhauser DNP and EPR in a Mobile Setup: Influence of Magnetic Field Inhomogeneity

Power-dependent Overhauser dynamic nuclear polarization (DNP) enhancements and continuous-wave electron paramagnetic resonance (EPR) spectra of nitroxide radicals were measured in the magnetic field of a mobile Halbach-array permanent magnet and compared with results from a commercially available electromagnet. DNP saturation factors for varying microwave power were obtained from both measurement series and used to investigate how the increased magnetic field inhomogeneity present in the Halbach magnet affects the saturation efficiency. An EPR detection system was designed to allow continuous-wave EPR measurements at microwave power up to 20?W. Our results show that despite the lower magnetic field homogeneity, a Halbach-array magnet can be used for EPR and DNP-enhanced nuclear magnetic resonance of high quality providing almost the same performance as a more homogeneous electromagnet.     

L波段三维ESR成像系统的研制(Ⅰ)——L波段ESR成像的磁场及三维梯度磁场系统

研究并实现了L波段电子自旋共振三维成像（3D-EPRI）专用的三维梯度磁场系统，主磁场及扫描磁场系统以及相应的驱动控制系统. 梯度场线圈采用在铜板上用电切割方法加工的 平板式线圈，避免了用铜导线绕制线圈体积较大的缺点，从而缩小了主磁场的体积和极间距 . 梯度场强度在三维方向上均达到200 mT/m，驱动电流为20 A. 三维空 间线性度均优于5％；线性区域大于直径42 mm的球形空间. 两磁极间距离为63 mm，可以容纳通常体积的L波段谐振腔. 主磁场和扫描场线圈固定在同一轭铁架上. 它们可分别产生1.6～ 96 mT和0.2～16 mT的线性变化磁场. 5组磁场线圈（包括主磁场， 扫描磁场和三维梯度磁场）分别由5台独立的恒流驱动电源控制驱动. 电源通过数据接口由计算机控制. 初步成像实 验证明本工作所建立的磁场和梯度磁场系统可以用于EPRI实验.     

EPR在体测量专用调制磁场驱动装置

利用电子顺磁共振（electron paramagnetic resonance,EPR）在体测量人牙齿可以实现无损伤地快速评估人体辐射剂量,具有实际应用价值.本文针对EPR在体测量牙齿剂量的应用特点,研制了专用调制磁场驱动装置,包括功率放大器、调制磁场激励线圈、调制频率设定模块、感应型调制幅度显示模块等.功率放大器采用脉冲功率放大方式取代传统的线性放大方式,用多N-MOSFET管H桥电路,功率容量大、效率高、结构简单,且调制频率设定自如.实验结果表明：（1）此装置可在大于9 cm磁极间距的中心样品位置产生调制幅度为0~0.9 mT的调制磁场,调制频率为10~100 kHz;（2）用该装置与EPR在体测量谱仪配合使用,可以明显观测到1,1-二苯基-2-三硝基苯肼（1,1-diphenyl-2-picrylhydrazyl,DPPH）样品谱线调制增宽过程以及辐射诱发的整体牙齿中的自由基信号,验证了该装置的高调制效率和实用性.     

Measurement of specific absorption rate for clinical EPR at 1200 MHz

The specific absorption rate (SAR) has been measured experimentally at 1200 MHz under the conditions and in the configuration used for in vivo electron paramagnetic resonance (EPR) spectroscopy of human subjects. The measurement of SAR was based on an analysis of the time dependence of the temperature of a model tissue (isolated bovine skeletal muscle) placed adjacent to the surface loop of the EPR resonator. The measured SAR in the tissue at the position that should have the greatest density of microwaves, directly under the wire of the loop, was 3.7±1.2 W/kg at 100 mW incident radio-frequency power to a surface loop resonator with an efficiency of about 0.1 mT/W^1/2. This is substantially below the recommended limit in the relevant regulation for extremities. The method of measurement of SAR used in this study can be adapted for other resonators and other types of clinical applications of EPR spectroscopy and imaging.     

Use of the Frank sequence in pulsed EPR

The Frank polyphase sequence has been applied to pulsed EPR of triarylmethyl radicals at 25 6 MHz (9.1 mT magnetic field), using 256 phase pulses. In EPR, as in NMR, use of a Frank sequence of phase steps permits pulsed FID signal acquisition with very low power microwave/RF pulses (ca. 1.5 mW in the application reported here) relative to standard pulsed EPR. A 0.2 mM aqueous solution of a triarylmethyl radical was studied using a 16 mm diameter cross-loop resonator to isolate the EPR signal detection system from the incident pulses.     

Micromirror with large-tilting angle using Fe-based metallic glass

For enhancing the micromirror properties like tilting angle and stability during actuation, Fe-based metallic glass (MG) was applied for torsion bar material. A micromirror with mirror-plate diameter of 900?μm and torsion bar dimensions length 250?μm, width 30?μm and thickness 2.5?μm was chosen for the tilting angle tests, which were performed by permanent magnets and electromagnet setup. An extremely large tilting angle of over -270° was obtained from an activation test by permanent magnet that has approximately 0.2?T of magnetic strength. A large mechanical tilting angle of over -70° was obtained by applying approximately 1.1?mT to the mirror when 93?mAwas applied to solenoid setup. The large-tilting angle of the micromirror is due to the torsion bar, which was fabricated with Fe-based MG thin film that has large elastic strain limit, fracture toughness, and excellent magnetic property.     

Design and development of the 6–18 MeV electron beam system for medical and other applications

A system for the electron and photon therapy has been designed and developed at SAMEER, IITB, Mumbai. All the components of the system such as the 270° beam bending electromagnet, trim coils, magnet chamber, electron scattering foil, slits, applicators, etc., were designed and fabricated indigenously. The electrons of 6, 8, 9, 12, 15 and 18?MeV energies were provided by a linear accelerator, indigenously designed and made at SAMEER, IITB campus, Mumbai. The electron beam from the LINAC enters the magnet chamber horizontally, and after deflection and focusing in the 270° bending magnet, comes out of the exit port, and travels a straight path vertically down. After passing through the beryllium and tantalum scattering foils, the electron beam gets scattered and turns into a solid cone shape such that the diameter increases with the travel distance. The simulation results indicate that at the exit port of the 270° beam bending magnet, the electron beam has a divergence angle of ≤ 3?mrad and diameter ～2–3?mm, and remains constant over 6–18?MeV. Normally, 6–18?MeV electrons are used for the electron therapy of skin and malignant cancer near the skin surface. On a plane at a distance of 100?cm from the scattering foils, the size of the electron beam could be varied from 10?cm?×?10?cm to 25?cm?×?25?cm using suitable applicators and slits. Different types of applicators were therefore designed and fabricated to provide required beam profile and dose of electrons to a patient. The 6?MeV cyclic electron accelerator called Race-Track Microtron of S. P. Pune University, Pune, was extensively used for studying the performances of the scattering foils, electron beam uniformity and radiation dose measurement. Different types of thermoluminescent dosimetry dosimeters were developed to measure dose in the range of 1–10kGy.     

Biochemical EPR imaging of skin

EPR Imaging (EPRI) of spin labels is a powerful method for investigating skin and can give information about biochemical processes which are involved in numerous skin diseases. Furthermore it enables the non invasive investigation of the liberation, penetration and distribution of spin labelled drugs. The basis of these measurements is spectral spatial EPR imaging employing modulated field gradients and simultaneous field scans (MOSS). A skin region (?=6 mm) was treated with a 10 μl spin label solution (1 mM). EPR spectra of 128 points were recorded in 128 spatial planes resulting in a 128×128 image matrix. A spatial resolution of better than 10 μm can be obtained for a spectral line width of 0.1 mT and a gradient of 4 Tm^?1.In vivo imaging on mammalian skin can be performed by employing surface coils at S-band frequencies, 3 GHz.     

Single loop multi-gap resonator for whole body EPR imaging of mice at 1.2 GHz

For whole body EPR imaging of small animals, typically low frequencies of 250-750 MHz have been used due to the microwave losses at higher frequencies and the challenges in designing suitable resonators to accommodate these large lossy samples. However, low microwave frequency limits the obtainable sensitivity. L-band frequencies can provide higher sensitivity, and have been commonly used for localized in vivo EPR spectroscopy. Therefore, it would be highly desirable to develop an L-band microwave resonator suitable for in vivo whole body EPR imaging of small animals such as living mice. A 1.2 GHz 16-gap resonator with inner diameter of 42 mm and 48 mm length was designed and constructed for whole body EPR imaging of small animals. The resonator has good field homogeneity and stability to animal-induced motional noise. Resonator stability was achieved with electrical and mechanical design utilizing a fixed position double coupling loop of novel geometry, thus minimizing the number of moving parts. Using this resonator, high quality EPR images of lossy phantoms and living mice were obtained. This design provides good sensitivity, ease of sample access, excellent stability and uniform B(1) field homogeneity for in vivo whole body EPR imaging of mice at 1.2 GHz.     

EPR Spectroscopy Using Magnetic Field Gradient Modulated by a Triangular Wave

Magnetic field gradient modulation is one of the techniques to obtain an electron paramagnetic resonance (EPR) spectrum in a selected region of the sample. In this study, the magnetic field gradient modulation using a triangular wave was performed to overcome a problem during the sine wave modulation. Plastic materials were used for the bobbins and cases of the electromagnet to reduce the eddy current loss and drive the gradient coils in three-dimensional directions at a frequency of over 160 Hz. While the EPR signal splitting in a nonselected region, which is a problem in spectral analysis, was observed during the simulation and the actual measurement with the sine wave gradient modulation, the EPR signal broadening without splitting was observed in those with the triangular wave modulation. Thus, it is postulated that the triangular wave is more suitable than the sine one for the field gradient modulation. The spatial resolution was determined to be about 4 or 2 mm at the field gradient of 1 or 2 mT/cm, respectively. The separation of the EPR spectra of two types of radicals was also made by the triangular wave gradient modulation. Authors' address: Hidekatsu Yokoyama, Department of Pharmaceutical Sciences, International University of Health and Welfare, 2600-1, Kitakanemaru, Ohtawara 324-8501, Japan     

上海深紫外自由电子激光波荡器的端部磁结构设计

 针对固定间隙的上海深紫外自由电子激光（SDUV-FEL）混合型波荡器的端部,用Radia程序进行了模拟计算。在端部不加任何电磁线圈补偿的情况下,通过减小端部磁铁、磁极的体积和变动端部磁极位置的方法对波荡器磁场进行了优化,优化以后波荡器横向磁场的边缘场强度降到5×10^-4 T（距离端部磁块边缘10 mm处）,边缘场波形没有了明显突起,优化后的横向磁场的一次积分曲线和二次积分曲线都有很大改善,端口处的一次积分值、二次积分值接近于零。     

L波段三维ESR成像系统的研制(Ⅳ)--系统组成与各部分性能指标

报道了自行研制的L波段三维电子自旋共振成像（3D-ESRI）系统的整机结构及各部分性能指标. 该系统主要由L波段ESR谱仪、三维梯度磁场装置、数据处理及图像重建软件组成. 系统的微波频率为1.05 GHz;最大微波功率500 mW. 采用3-环2-缝再进入式谐振腔,无载Q值>1 000;最大测量体积为φ 20 mm, 高30 mm柱状水溶液样品. 接收系统采用100 kHz锁相放大电路,最大增益可达1×10⁶;时间常数0.02 ms～1 s;磁场调制幅度>0.5 mT. 最大梯度磁场2 mT/cm;三维梯度线性度均优于5 ％;稳定度可达10^-5;主磁场可在1.6～96 mT范围内任意点选择扫场起始点;在0.2～16 mT范围选择磁场扫描宽度. 数据系统为12位A/D实现数据采集,三路8位D/A控制梯度磁场. 采用滤波反投影法实现图像重建, 成像功能包括：二维、三维自旋浓度成像;等浓度线2D图像显示;3D立体和断层图像显示等. 对水溶液和固体模型样品进行ESR成像的结果表明：本系统可以开展较大体积生物样品的ESRI研究.     

Coarse and Fine Control of the EPR Frequency of a Loop-Gap Resonator Using a Single-Turn Coil with a Varactor Diode Attached

Coarse control and fine control of the resonant frequency of a loop-gap resonator (LGR) operating at an electron paramagnetic resonance (EPR) frequency of ca. 650 MHz were achieved using a single-turn coil with a varactor diode attached (a frequency shift coil). When the distance between the LGR and the frequency shift coil was changed from 15 to 10 mm under the condition of constant voltage to the varactor diode (0 V), a shift of the resonant frequency of the LGR of ca. 20 MHz was observed (coarse frequency control). When the voltage applied to the varactor diode was changed from 0 to 15 V at the same distance between the LGR and the frequency shift coil (10 mm), a shift of the resonant frequency of the LGR of ca. 200 kHz was observed (fine frequency control). There were no significant changes in sensitivity of EPR measurements of a phantom (comprised of agar with a nitroxide radical and physiological saline solution) without and with the frequency shift coil. The EPR sensitivity did not change discernibly when the resonant frequency was shifted by the frequency shift coil. Furthermore, radio-frequency phase adjustment for homodyne detection could be performed by using the frequency shift coil without applying frequency modulation to the carrier wave.     

A bridged loop-gap S-band surface resonator for topical EPR spectroscopy.

The design and structure of a bridged loop-gap surface resonator developed for topical EPR spectroscopy and imaging of the distribution and metabolism of spin labels in in vivo skin is reported. The resonator is a one-loop, one-gap bridged structure. A pivoting single loop-coupling coil was used to couple the microwave power to the loop-gap resonant structure. A symmetric coupling circuit was used to achieve better shielding and minimize radiation. The frequency of the resonator can be easily adjusted by trimming the area of the capacitive foil bridge, which overlaps the gap in the cylindrical loop. The working frequency set was 2.2 GHz and the unloaded Q was 720. The B1 field of this resonator was measured and spatially mapped by three-dimensional EPR imaging. The resonator is well suited to topical measurements of large biological subjects and is readily applicable for in vivo measurements of free radicals in human skin.     

ADS注入器Ⅱ超导螺线管的初步测试

介绍了加速器驱动次临界系统（ADS）注入器Ⅱ束流聚焦单元超导螺线管的初步测试。 该测试在高度1 600 mm、 直径510 mm的垂直杜瓦中, 利用型号为cryogenic sms的超导电源对磁体进行了励磁, 并利用霍尔探头测试了其轴向磁场分布情况和漏场情况, 其中心磁场励磁后可达8.20 T, 漏场分布和计算值的相对误差小于10%。 同时, 对励磁过程中骨架的应力应变状态做了测试。 对测试结果的分析表明, 骨架结构设计合理, 应变状态变化平稳, 磁体低温稳定性能良好。The preliminary test of the superconducting solenoid for ADS injectorⅡhas been carried out in order to measure its performances. A vertical dewar with height 1 600 mm and outer diameter 510 mm was employed for this test. The magnet was energized by a cryogenic sms superconducting magnet current source. The axial magnetic distribution and leakage field were also tested through hall probes. The center field can be excited up to 8.20 T and the relative diviation between the measured value and the calculated value of the stray and leakage field is less than 10%. The measurements of the strain and stress status of the magnet skeleton show that the distortion is small and the mechanical performance is robust.     

Plate form three-dimensional gradient coils for L-band ESR imaging experiment

A set of plate form three-dimensional magnetic gradient coils was developed and used in electron spin resonance imaging (ESRI) experiment. The coils were processed with whole copper plates instead of wound with copper wires, which made its structure so compact that it was much thinner and smaller comparing to those traditionally used in ESRI. The coil set had a pie-like appearance of which the total thickness was only 14 mm and the outer diameter was 250 mm. The efficiency of the coils could be greater than 10 mT/m/A when distance between the two side-pieces was 63 mm. A maximum gradient strength of more than 200 mT/m could be obtained with driving current of about 20 A in each dimension coil. The spatial linearity was better than 5% in all three dimensions within the available spatial linearity area of larger than a sphere of 40 mm in diameter. The stability of the gradients strength could reach the level of 10(-5). An imaging resolution of better than 1 mm could be achieved with the coil set. Some preliminary practical imaging results show that the developed gradient coil set is suitable for L-band ESRI experiment of biological samples or even in vivo small animals.     

Field-cycled PEDRI imaging of free radicals with detection at 450 mT

This paper describes the design, construction and use of a field-cycled proton-electron double-resonance imaging (FC-PEDRI) system for the detection and imaging of free radicals. The unique feature of this imager is its use of a 450-mT detection magnetic field in order to achieve good image quality and sensitivity. The detection magnetic field is provided by a superconducting magnet, giving high stability and homogeneity. Field cycling is implemented by switching on and off the current in an internal, coaxial, resistive secondary magnet that partially cancels the superconducting magnet's field at the sample; the secondary magnet is actively shielded to avoid eddy currents. EPR irradiation takes place at approximately 5 mT, following which the field is switched to 450 mT in 40 ms for NMR signal detection. Full details of the imager's subsystems are given, and experiments to image the distribution of stable free radical contrast agents in phantoms and in anesthetized rats are described.     

A Pulse EPR 25 mT Magnetometer with 10 ppm Resolution

A magnetometer designed for permanent magnet manufacturing and operated around 25 mT with 10 ppm absolute accuracy is described. The magnetometer uses pulse electron paramagnetic resonance methodology. The use of a pulsed broadband acquisition allowed reliable measurements in the presence of the magnetic field gradient and in relatively inhomogeneous magnetic fields of un-shimmed magnets.     

Energy efficient iron based electronic field cycling magnet

A new concept for an energy efficient electromagnet for fast field cycling NMR applications as well as its construction and first test results are presented. The magnet, which provides a rectangular sample space of 17 × 25 mm, has an iron yoke and pole pieces optimised with respect to the B₀ homogeneity. The maximum field is 0.66 T at a current of 320 A; its field inhomogeneity for a cylindrical sample (length 7 mm, diameter 6 mm) is about 50 ppm. The power dissipation during polarisation at 0.55 T is as low as 1.4 kW. The magnet is powered by a commercially available power supply and can be rapidly switched with a slew rate of 0.55 T/ms. The system has shown a stability of 50 ppm/h.     

Optimisation of an EPR dosimetry system for robust and high precision dosimetry

Clinical applications of electron paramagnetic resonance (EPR) dosimetry systems demand high accuracy causing time consuming analysis. The need for high spatial resolution dose measurements in regions with steep dose gradients demands small sized dosimeters. An optimization of the analysis was therefore needed to limit the time consumption. The aim of this work was to introduce a new smaller lithium formate dosimeter model (diameter reduced from standard diameter 4.5 mm to 3 mm and height from 4.8 mm to 3 mm). To compensate for reduced homogeneity in a batch of the smaller dosimeters, a method for individual sensitivity correction suitable for EPR dosimetry was tested. Sensitivity and repeatability was also tested for a standard EPR resonator and a super high Q (SHQE) one. The aim was also to optimize the performance of the dosimetry system for better efficiency regarding measurement time and precision. A systematic investigation of the relationship between measurement uncertainty and number of readouts per dosimeter was performed. The conclusions drawn from this work were that it is possible to decrease the dosimeter size with maintained measurement precision by using the SHQE resonator and introducing individual calibration factors for dosimeter batches. It was also shown that it is possible reduce the number of readouts per dosimeter without significantly decreasing the accuracy in measurements.