Quiet MRI with novel acoustic noise reduction.

Fast scan techniques, which are used to reduce scanning times, have raised scanning noise levels in magnetic resonance imaging (MRI) systems, resulting in greater patient discomfort and stress. It is well known that this noise is caused by vibration of the gradient coil due to the Lorentz forces generated by the current in the gradient coil, which is placed in a static magnetic field. We have confirmed that MRI noise can be substantially reduced by sealing the gradient coil in a vacuum chamber to block airborne vibration propagation, by supporting the gradient coil independently to block solid vibration propagation and by decreasing the eddy currents induced in RF coils, the RF shield and the static-field-magnet cryostat. Based on these findings, we have developed a silent MRI system in which scanning noise is markedly reduced under a wide range of scanning conditions.     

Characterization of vibration and acoustic noise in a gradient-coil insert

High-speed switching of current in gradient coils within high magnetic field strength magnetic resonance imaging (MRI) scanners results in high acoustic sound pressure levels (SPL) in and around these machines. To characterize the vibration properties as well as the acoustic noise properties of the gradient coil, a finite-element (FE) model was developed using the dimensional design specifications of an available gradient-coil insert and the concentration of the copper windings in the coil. This FE model was then validated using experimentally collected vibration data. A computational acoustic noise model was then developed based on the validated FE model. The validation of the finite-element analysis results was done using experimental modal testing of the same gradient coil in a free-free state (no boundary constraints). Based on the validated FE model, boundary conditions (supports) were added to the model to simulate the operating condition when the gradient-coil insert is in place in an MRI machine. Vibration analysis results from the FE model were again validated through experimental vibration testing with the gradient-coil insert installed in the MRI scanner and excited using swept sinusoidal time waveforms. The simulation results from the computational acoustic noise model were also validated through experimental noise measurement from the gradient-coil insert in the MRI scanner using swept sinusoidal time waveform inputs. Comparisons show that the FE model predicts the vibration properties and the computational acoustic noise model predicts the noise characteristic properties extremely accurately.     

Wavelet-based noise reduction for improved deconvolution of time-series data in dynamic susceptibility-contrast MRI

Dynamic susceptibility-contrast (DSC) MRI requires deconvolution to retrieve the tissue residue function R(t) and the cerebral blood flow (CBF). In this study, deconvolution of time-series data was performed by wavelet-transform-based denoising combined with the Fourier transform (FT). Traditional FT-based deconvolution of noisy data requires frequency-domain filtering, often leading to excessive smoothing of the recovered signal. In the present approach, only a low degree of regularisation was employed while the major noise reduction was accomplished by wavelet transformation of data and Wiener-like filtering in the wavelet space. After inverse wavelet transform, the estimate of CBF·R(t) was obtained. DSC-MRI signal-versus-time curves (signal-to-noise ratios 40 and 100) were simulated, corresponding to CBF values in the range 10–60 ml/(min 100 g). Three shapes of the tissue residue function were investigated. The technique was also applied to six volunteers. Simulations showed CBF estimates with acceptable accuracy and precision, as well as independence of any time shift between the arterial input function and the tissue concentration curve. The grey-matter to white-matter CBF ratio in volunteers was 2.4±0.2. The proposed wavelet/FT deconvolution is robust and can be implemented into existing perfusion software. CBF maps from healthy volunteers showed high quality.     

Acoustic noise reduction in a 4 T MRI scanner

High-field, high-speed magnetic resonance imaging (MRI) can generate high levels of noise. There is ongoing concern in the medical and imaging research communities regarding the detrimental effects of high acoustic levels on auditory function, patient anxiety, verbal communication between patients and health care workers and ultimately MR image quality. In order to effectively suppress the noise levels inside MRI scanners, the sound field needs to be accurately measured and characterized. This paper presents the results of measurements of the sound radiation from a gradient coil cylinder within a 4 T MRI scanner under a variety of conditions. These measurement results show: (1) that noise levels can be significantly reduced through the use of an appropriately designed passive acoustic liner; and (2) the true noise levels that are experienced by patients during echo planar imaging.     

Model gradient coil employing active acoustic control for MRI

Results are presented for a model three-axis gradient coil incorporating active acoustic control which is applied to the switched read gradient during a single-shot rapid echo-planar imaging (EPI) sequence at a field strength of 3.0 T. The total imaging acquisition time was 10.6 ms. Substantial noise reduction is achieved both within the magnet bore and outside the magnet. Typical internal noise reduction over the specimen area is 40 dB_A whereas outside the acoustic chamber the noise level is reduced by 60–67 dB_A. However these results are relative to a control winding which is switched in phase, adding 6 dB_A in its non-optimized mode, which is included in the quoted figures.     

0.5T小型关节MRI仪器中低噪声前置放大器的设计

设计了一个可用于0.5T 小型关节MRI仪器中的低噪声前置放大器,给出了一种低场MRI仪器中LNA制作的可行方案。放大器采用两级级联的形式,第1级采用高电子迁移率晶体管ATF 54143,第2级采用双极型达林顿管ADA 4743。经过测试,所制作放大器的性能指标达到了预定要求,性能良好。放大器的工作频段为18~23 MHz,中心频率处噪声系数NF≈0.6 dB,增益G≈40 dB,带内波动小于1 dB,输入驻波比VSWR≤1.5,输出驻波比VSWR≤1.5。     

真空有载分接开关每相“四个真空灭弧室”安全性分析

对真空有载分接开关中每相用"四个真空灭弧室"的安全性能进行了分析。     

Myocardial viability assessed with tagged MRI

5. Conclusion Tagged MR imaging combined with a low-dose dobutamine stress testing is likely to become a major alternative for the assessment myocardial viability in patients with severe CAD by providing unique quantitative methods, but its future in the clinical arena will be determined by the successful implementation of other MR methods, geared towards direct assessment of the coronary tree and perfusion, combined in a single, comprehensive, and cost-effective examination.     

Indirect current control of active filter for harmonic elimination with novel observer-based noise reduction scheme

Active power filters have been developed recently to solve the harmonics disturbance problems on power system networks. The direct current control scheme is generally used to control the harmonic compensating current directly for current source active power filters. To improve the characteristics of harmonic elimination, the voltage regulation scheme is proposed which regulates the filter capacitor voltage at the input side. Hence the voltage regulation scheme is an indirect current control technique for making sinusoidal line current. To implement this proposed scheme, the electronic differentiator should be used. By using the conventional differentiator, high frequency noise is generated in the controller. This deteriorates both the accuracy and reliability of the controller. To reduce this noise, a novel observer-based noise reduction scheme is proposed for our active filter system. The observer-based algorithm and characteristics of indirect current-controlled active filters are investigated through simulations and experiments.     

水轮机空化声发射信号的联合降噪与特征提取北大核心CSCD

探究水轮机空化诱导的声发射信号随空化状态的变化规律对监测空化具有重要意义。为解决声发射信号携带噪声和特征提取困难的问题,本文建立一种基于自适应迭代滤波分解-奇异谱分析联合降噪和固有时间尺度分解结合特征参数的特征提取方法。首先,采用自适应迭代滤波分解结合相关系数对声发射信号初步降噪,滤除明显噪声分量,重构剩余分量并通过奇异谱分析进一步降噪,将所得信号与趋势分量相加,完成整个降噪过程。然后,对降噪后信号进行固有时间尺度分解,筛选出有效分量,计算其绝对能量和相对能量熵。最后,分析它们随空化系数变化的规律。研究结果表明,绝对能量和相对能量熵随空化系数的变化具有明显的规律性,均能反映水轮机空化的发展状况。     

Adaptive noise reduction using numerically stable fast recursive least squares algorithm

This paper is concerned with adaptive noise reduction based on the fast recursive least squares (FRLS) algorithm. It is well known that the fast recursive least squares (FRLS) algorithm suffers from numerical instability when operating under the effects of finite precision arithmetic. Several numerical solutions of stabilization were proposed in the case of stationary signals. In this work a new version of a numerically stable FRLS algorithm (NS‐FRLS) is proposed. The stability characteristics of this new stabilized algorithm are analysed. The analysis is based on a linear model for the errors in the states of the adaptive filter. Experimental results confirm the merits of adaptive filtering with the NS‐FRLS algorithm over optimum filtering using the solution provided by Wiener–Hopf equations. Copyright © 2006 John Wiley & Sons, Ltd.     

Benign prostate hyperplasia: evaluation of treatment response with DCE MRI

Benign prostate hyperplasia (BPH) is a major disease and its non-surgical therapy a major area of interest. The purpose of this study was to establish perfusion parameters in beagles with BPH using dynamic contrast-enhanced (DCE) MRI and to investigate changes due to the effects of finasteride treatment. Twelve male beagles (mean age 4.4±0.9,years) were divided into a control and treatment group that received a daily dose of 1 mg/kg finasteride. DCE MRI was carried out in a clinical scanner using a 3D spoiled gradient echo sequence prior to and during treatment. 0.2 mmol/kg contrast agent (gadoteridol) was administered with an injection rate of 0.2 ml/s followed by a 15 ml flush of saline. Contrast enhancement was evaluated by pharmacokinetic mapping of a two-compartment model with colour overlay images in addition to regional ROI analysis. Quantitative parameters were defined by the amplitude of contrast enhancement A, the exchange rate k_ep and the time to maximum signal enhancement. Dynamic contrast-enhanced MRI investigations of the prostate revealed two distinct zones, an inner, periurethral zone and an outer, parenchymal zone. The periurethral zone is highly vascularized, whereas the parenchymal zone is moderately vascularized when compared to other parenchymal organs. During treatment, in the parenchymal zone the intensity of enhancement (amplitude A) and the time to maximum signal enhancement increased, while the exchange rate k_ep decreased. Dynamic contrast-enhanced MRI of BPH reveals distinct differences between individual zones within the prostate. Moreover, changes during successful treatment suggest increased blood volume per volume of tissue and decreased vessel leakiness.     

对降低SR电机振动和噪声的两步换相法的实验分析

基于SR电机定子振动特性频域和时域实验结果,对角度位置控制（APC）和电压PWM工况下“两步换相法”的有效性及局限性进行了实验分析。研究表明,SR电机定子振动具有多个主振型,如何从总体最大限度地降低定子振动出发优化两步换相的时间间隔是值得进一步研究的问题;电压PWM方式斩波期间电流关断引起的冲击振动和位置换相引起的冲击振动一样,均可通过两步换相法抑制,但在斩波期间的每一斩波段采用两步换相法有一定的局限,当斩波频率设计值较高或电机固有频率较低时不宜采用,因此应进一步研究电压PWM方式下振动抑制的策略。     

Synthesis and characterization of polyethylenimine-based iron oxide composites as novel contrast agents for MRI

OBJECT: Use of polyethylenimines (PEIs) of different molecular weight and selected carboxylated-PEI derivatives (PEI-COOH) in the synthesis and stabilization of iron oxide nanoparticles, to obtain possible multifunctional contrast agents. MATERIALS AND METHODS: Oxidation of Fe(II) at slightly elevated pH and temperature resulted in the formation of highly soluble and stable nanocomposites of iron oxides and polymer. Composites were characterized and studied by atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffractometry, AC and DC magnetometry, NMR relaxometry and magnetic resonance imaging (MRI). RESULTS: From AFM the dimensions of the aggregates were found to be in the ~150-250 nm size region; the mean diameter of the magnetic core of the compounds named PEI-25, PEI-500 and PEI-COOH60 resulted d approximately 20 +/- 5 nm for PEI-25, d approximately 9.5 +/- 1.0 nm for PEI-500 and d approximately 6.8 +/- 1.0 nm for PEI-COOH60. In PEI-COOH60 TEM and X-ray diffractometry revealed small assemblies of mineral magnetic cores with clear indications that the main constituents are maghemite and/or magnetite as confirmed by AC and DC SQUID magnetometry. For PEI-COOH60, the study of NMR-dispersion profiles revealed r (1) and r (2) relaxivities comparable to superparamagnetic iron-oxide commercial compounds in the whole investigated frequency range 7 < or = nu < or = 212 MHz. CONCLUSION: PEI-25 was studied as possible MRI contrast agent (CA) to map the cerebral blood volume (CBV) and cerebral blood flow (CBF) in an animal model obtaining promising results. The reported compounds may be further functionalized to afford novel multifunctional systems for biomedical applications.     

The noise performance of CMOS Miller operational transconductance amplifiers with embedded current‐buffer frequency compensation

The noise of cascode transistors is usually considered to be negligible. It is demonstrated in this paper that this is not the case for an operational transconductance amplifiers exploiting Miller compensation via an embedded current buffer. Analysis, validated through simulations, shows that the noise power spectral density of the cascode transistor implementing the buffer, can be dominant at high frequencies. Its contribution can be of the same order of magnitude as that of the transistors in the first gain stage. A design equation is also given to limit the increase of this output noise contribution. Copyright © 2016 John Wiley & Sons, Ltd.     

一种含噪声测试的数字化电能表计量性能检测系统

电子式互感器属于电子设备,在A/D采样过程中不可避免的会产生噪声,噪声叠加至采样值中,对电能计量的准确性产生明显影响,有必要对数字化电能表入网运行前展开输入噪声测试,保证数字化电能表入网运行的准确性。因此,研究了一种含噪声测试的数字化电能表计量性能检测系统,检测系统在现有数字化电能表检测项目的基础上,提出了输入噪声测试项目及测试方法,通过研究发现,数字化电能表在噪声条件下误差明显增大及不稳定,当信噪比为30 dB时,数字化电能表误差出现超差现象,建议数字化电能表入网运行前增加输入噪声测试项目。     

Monitoring the survival of islet transplants by MRI using a novel technique for their automated detection and quantification

Object  There is a clinical need to be able to assess graft loss of transplanted pancreatic islets (PI) non-invasively with clear-cut quantification of islet survival. We tracked transplanted PI in diabetic mice during the early post-transplant period by magnetic resonance imaging (MRI) and quantified the islet loss using automatic segmentation technique. Materials and methods  Magnetically labeled islet iso-, allo- and xenografts were injected into the right liver lobes. Animals underwent MRI scanning during 14 days after PI transplantation. MR images were processed using custom-made software, which automatically detects hypointense regions representing PI. It is based on morphological top-hat and bottom-hat transforms. Results  Manually and automatically detected areas, corresponding to PI, differed by 4% in phantoms. Signal loss regions due to PI decreased comparably in all groups during the first week post transplant. Throughout the second week post-transplant, the signal loss area continued in a steep decline in case of allografts and xenografts, whereas the decline in case of isografts slowed down. Conclusion  Automatic segmentation allows for the more reproducible, objective assessment of transplanted PI. Quantification confirms the assumption that a significant number of islets are destroyed in the first week following transplantation irrespective of allografts, xenografts or isografts.     

Interaction of Gd(III) MRI contrast agents with membranes: a review of recent EPR studies

Rational development of new selective paramagnetic contrast agents (PCAs) requires a detailed understanding of their interactions with biological macromolecules. This report shows how some of these interactions can be studied with electron paramagnetic resonance (EPR) through examples of Gd³⁺ complexes interactions with model phospholipid membranes. It is shown that the spin label EPR method can be used to detect: (i) presence and possible location of lipophilic contrast agents in the model membranes, (ii) changes and distortions in membrane organization upon interaction with the PCAs, and (iii) changes in the local polarity of the bilayer and its phase behavior due to addition of Gd³⁺ complexes. This work demonstrates that interaction of Gd³⁺ complexes with phospholipid bilayers can be observed directly from changes in their continuous wave (CW) EPR spectra obtained at frequencies higher than X-band (9.5 GHz), where signals arising from aqueous and lipid-bound Gd³⁺ complexes become resolved. Analysis of frequency dependence of the effectiveg-factors of the EPR signal provides estimates of zero-field splitting (ZFS) parameter for these complexes at physiological conditions and information on how this parameter is affected by interaction with lipids. Multifrequency EPR experiments at high magnetic fields are also useful in providing data on the frequency dispersion of electronic relaxation caused mainly by a modulation of the electron-electron dipolar interaction (ZFS) of these high spin ions.     

Design of high data rate optical transmission systems with self phase modulation and noise

 Self phase modulation (SPM) and amplified spontaneous emission noise (ASE) are the most important degradation effects in optical transmission systems [1–3]. SPM is caused by the Kerr-effect, which describes the signal power dependence of the refractive index. SPM leads to irreversible signal distortions, which can be kept small by an optimised system design. The erbium doped fibre amplifiers (EDFA) create ASE, which accumulates over several transmission spans and degrades the optical signal to noise ratio (OSNR) at the receiver. In this paper, we investigate the signal quality when using non-return-to-zero (NRZ) and return-to-zero (RZ) modulation due to both degradation effects. We further present design rules for the achievable transmission distance in high data rate transmission systems, and give an outlook for the design of future transmission systems. Received: 1 November 2001/Accepted: 31 December 2001