Magnetic resonance imaging of bovine ovaries in vitro

A sample of 20 bovine ovaries were imaged in vitro using nuclear magnetic resonance (NMR) techniques to determine the visibility of various physiologic structures. In particular, the possibility of using NMR imaging to differentiate atretic follicles from physiologically selected and ovulatory follicles was examined. Five of the 20 ovaries were preserved in formalin, whereas the remaining 15 were preserved in a saline solution and imaged within 18 hours of death. Images weighted by T₁ and T₂ proton spin relaxation rates were obtained along with some three-dimensional (3-D) data sets acquired via a fast imaging with steady-state precession technique. Physiologically different structures were easily identified in the images from their morphology, especially in the 3-D images. Weighting by T₁ and T₂ was able to separate structures in the fresh ovaries in the following manner. Atretic and cohort follicles appear dark in T₁-weighted images and bright in T₂-weighted images. Ovulatory follicles appear bright in both T₁-and T₂-weighted images, whereas prephysiologic selection follicles present an intermediate brightness in T₁-weighted images and appear dark in T₂-weighted images. The corpus luteum appears bright in T₁-weighted images and dark in T₂-weighted images, whereas cysts in the corpus luteum appear dark in T₁-weighted images and bright in T₂-weighted images. The varying brightness of the follicles at different stages of development is hypothesized to be related to different hormone and protein concentrations in the follicular fluid. For example, it is known that physiologically selected preovulatory follicles contain high concentrations of estrogens in a viscous follicular fluid. The increased viscosity may occur only when the follicle fluid contains high concentrations of estrogen and contributes to bright T₁-weighted images. The possibility of using nuclear relaxation-weighted NMR imaging for the study of follicular dynamics and other ovarian biology therefore shows great promise.     

基于随机森林的脑磁共振图像分类

特征选择和分类算法是影响脑磁共振图像分类精度的2个最主要的因素。随机森林算法作为一种优秀组合分类器逐渐成为近年来研究的热点,通过加权脑磁共振3种（T1、T2、PD加权像）图像,采用非统一滑动窗口尺寸提取二维图像的纹理特征、形状特征、HAAR特征、灰度特征以及边缘检测算子、最大类间方差（OTSU）作为随机森林算法的输入特征,从而分类出图像的10类组织。经过对加拿大蒙特利尔神经科学研究院提供的脑仿真核磁共振图像实验,随机森林算法对二维脑MR图像的分类精度可以达到94%以上。     

Magnetic resonance susceptibility contrast induced by capillaries: a numerical comparison of two models

Monte-Carlo computer simulations have proven to be very powerful tools for the analysis of the magnetization decay induced by susceptibility gradients, as well for contrast agent characterization, as for the BOLD effect allowing fMRI. A recent vasculature model containing capillaries and venules uses homogeneous magnetized cylinders as models for the vessels. This modeling is questioned by comparing results obtained from simulation results based on two different models, one using homogeneous cylinders and another taking into account the existence of red blood cells, treated as homogeneous magnetized spheres. The results show the nonequivalence of both models, with the modeling by cylinders systematically overestimating the transverse relaxation rates, and the difference increasing with the adopted value of the diffusion coefficient. The discrepancy is attributed to the dominating role, regarding relaxation, of the local magnetic field in the immediate vicinity of the capillaries, which results in the suggestion of elaborating a “mixed modeling”: the analytical expressions corresponding to the homogeneous cylinder model could be used except when the spin packets are wandering in the immediate vicinity of the capillaries, where accounting for the existence of individual red blood cells (whose motion may be neglected) seems unavoidable.     

双谐振注入混合型有源电力滤波器特性

在分析双谐振注入混合型有源电力滤波器(Double Resonance Injection Hybrid Active Power Filter, DIHAPF)结构和工作原理的基础上,建立了DIHAPF的系统控制模型及包括控制器的电压型逆变器的数学模型。从无功功率静态补偿能力和谐波分压特性两方面分析了DIHAPF注入支路特性,同时研究了DIHAPF的系统稳定性和谐波补偿特性,着重探讨了并联谐振支路谐振频率对系统性能的影响,提出了注入支路谐振频率设计方法。将非线性负载等效为谐波电流源,在PSIM6.0软件平台搭建DIHAPF模型进行仿真验证,并通过6 kV实验样机进行了实验研究。仿真和实验结果证明了理论分析的正确性。     

Magnetic resonance imaging of the female pelvic floor and urethra: body coil versus endovaginal coil

The anatomy of the female pelvic floor and urethra is complex. With the introduction of MRI, the discussion about the normal anatomy of this area has not diminished. The use of a body coil may be contributary to this. In the present study images obtained with an endovaginal coil are compared with those of a quadrature body coil series to study the possible advantages of endovaginal imaging. Axial and radial T₂-w TSE images at a 1.0-T machine were obtained in seven healthy volunteers. The pelvic floor structures as well as the levator ani muscle and the urogenital diaphragm are excellently demonstrated with the endovaginal coil. Also, the urethrovaginal sphincter could be recognized in six volunteers, but only in three with the body coil. In six volunteers a new ligamentous structure, the urethropelvic sling, connecting the urethra to the levator ani muscle and contributing to the supporting mechanism of the urethra is shown with the endovaginal coil. The zonal anatomy of the urethra is excellently shown with the endovaginal coil. The urethral length could only be accurately measured with this coil and ranged from 3.1 to 3.6 cm. Compared with the body coil, endovaginal MRI is excellent in demonstrating the anatomy of the pelvic floor and urethra.     

不平衡磁拉力引起水轮发电机转子的超／次谐波共振

本文利用非线性振动理论的多尺度方法,分析了水轮发电机转子同时存在安装偏心和质量偏心时,不平衡磁拉力引起的超／次谐波共现象,考虑到不平衡磁拉力的非线性后,发现当临界转速接近于２倍或３倍工作转地,会出现超谐波共振,同样,若临界转妆拉于１／２倍工作转速或１／３倍工作转地,会出现次谐波共振,通过模型试验,验证了理论分析的正确性,同时提出了水轮发电机转子设计的新准则。     

金属注射成形烧结软磁合金的研究状况

介绍了国内外采用金属注射成形技术制备的几种烧结软磁合金材料,包括纯铁、软磁不锈钢、铁硅合金、铁钴合金和铁镍合金,并与精密锻造和传统粉末冶金方法进行了对比,分析了采用注射成形技术生产软磁合金材料的优势.     

金属注射成形永磁材料的研究状况

概述了金属注射成形(MIM)技术应用于永磁材料中的特点和优势,分别评述了粘结永磁材料和烧结永磁材料的研究现状,指出了今后的研究方向.     

电压互感器铁心饱和引起铁磁谐振的防治措施

对中性点绝缘的配电系统中电压互感器引起的铁磁谐振现象进行了论述,分析了谐振产生的原因,比较了不同的互感器特性所产生的铁磁谐振的特点与规律,对常用的消除谐振的措施加以对比,提出了防止铁磁谐振的建议.     

Magnetic resonance imaging differentiation of adrenal masses at 1.5 t: t2-weighted images,chemical shift imaging,and gd-dtpa dynamic studies

The purpose of our study was to assess the potential role of spin-echo (SE), chemical shift, and gadolinium-enhanced magnetic resonance imaging (MRI) in the differentiation of adrenal masses.Seventy-two adrenal masses (26 nonhyperfunctioning adenomas, 16 aldosterone-secreting adenomas and 6 other different benign cortical masses, 18 pheochromocytomas, and 6 malignant masses) in 63 patients were evaluated with spin-echo sequences, chemical shift imaging (CSI) and gadolinium diethylenetriamine penta-acetic acid (Gd-DTPA) dynamic studies. Ratios and indices of signal intensity for all examined MRI methods were calculated and examined for significance of difference between different types of adrenal masses.Quantitative magnetic resonance evaluation of adrenal masses showed significant differences (at least<0.01) between nonhyperfunctioning adenomasvs. pheochromocytomas orvs. malignant lesions orvs. aldosterone-secreting adenomas and between pheochromocytomasvs. malignant lesions. The most specific indicators of adrenal mass character proved to be the CSI ratio based on opposed-phase and in-phase two-dimensional fast low-angle shot (FLASH) images, reflecting lipid content in the lesion, and Gd-DTPA dynamic studies ratios reflecting contrast agent inflow and washout in the lesion: Wo_max/last and Dyn_1.2–3.2. There was no overlap of CSI ratio between adenomas and pheochromocytomas. The overlap of ranges of CSI ratio between nonhyperfunctioning adenomas and aldosterone-secreting adenomas was only 18.5%. There was no overlap of Wo_max/last ratio between adenomas and pheochromocytomas, or adenomas and malignant lesions. The overlap of ranges of Dyn_{1.2– 3.2} ratio between pheochromocytomas and malignant lesions was only 17.6%.MRI enables good visualization and specific characterization of adrenal masses. The optimal MRI protocol for the adrenal region is presented.Address for correspondence: Imaging Department, Warsatv Province Hospital, ul. Kondratowicza 8, 03-285 Warsaw, Poland.     

Transient global brain ischemia in the rat: spatial distribution, extension, and evolution of lesions evaluated by magnetic resonance imaging

A newly-developed model of transient global ischemia in the rat was evaluated by magnetic resonance imaging (MRI) in terms of localization of brain lesions, their extent and severity, and temporal evolution. Such a model, consisting of bilateral occlusion of common carotid arteries for 10 minutes and mild hypoxia (15% O₂) for 20 minutes induces delayed neuronal degeneration, necrosis, and gliosis (detected histologically and immunohistochemically). Ischemia was assessed by full suppression of spontaneous electroencephalographic activity. A “hybrid” T₂-/diffusion-weighted MR sequence enhancing more effectively the contrast between injured and intact tissues as compared to T₂-weighted MRI was used at 24, 48, 72, and 96 hours and at 7 days postischemia. Twenty hypoxic-ischemic rats showed a considerable variability in brain damage. In 8, there were no MRI-detectable lesions at any interval. In the other 12 rats, the severity and extension of neuronal damage varied markedly, but the lesions were always localized (monolaterally in 8 and bilaterally in 4 rats) in the occipital, temporal, or parietal cerebral cortex. Mainly, they were of intermediate severity or were severe (as assessed by MRI hyperintensity) and were accompanied by usually less severe lesions in the thalamus and/or caudate putamen. The hippocampus was affected moderately or severely in 4 of 12 rats. In most cases, there was at 48 hours a considerable growth in severity and/or extension of lesions, which usually remained stable at later intervals. In conclusion, MRI allowed us to follow brain lesions during the first week in this relatively simple and noninvasive model of transient global ischemia.     

基于目标场法的磁共振成像小尺寸射频线圈设计

为得到分辨率较高的实蝇磁共振图像(MRI),将目标场法引入小尺寸射频(RF)线圈设计,设计了一种磁场均匀性较好、适合实蝇磁共振成像的小尺寸圆柱体射频线圈。首先采用目标场法求出线圈表面的电流密度分布,利用流函数将电流密度离散为Fourier级数;然后基于Tikhonov正则化的方法,引入最小曲率惩罚函数和最小化电流密度的散度惩罚函数,解决了积分方程的高度病态问题,获得了线圈的绕线形状和感兴趣区域(ROI)内的磁场强度。结果表明,2种惩罚函数对应的惩罚因子λ1和λ2均会对线圈的绕线形状和磁场分布均匀性造成影响;对于不同尺寸的线圈,其合适的λ1和λ2的取值不同。因此,选取λ1和λ2时应综合考虑线圈绕线结构和磁场分布均匀性的要求。     

Measurement of flow in small vessels by magnetic resonance phase mapping techniques: anin vitro andin vivo study

The measurement of blood flow in small arteries is a potential extension of magnetic resonance (MR) angiography. We have compared flow measurements based on MR phase mapping with those obtained by Doppler ultrasound and electromagnetic flowmetry in both phantom and animal models. Correlation between modalities was high forin vitro studies (R ²=0.93–0.98).In vivo, electrocardiogram-gated MR and Doppler ultrasound flow measurements compared to electromagnetic flowmetry showed fair correlation coefficients (R ²=0.73 and 0.66, respectively). However, limits of agreement indicated that in small vessels flow measured by the three modalities could differ by up to ±90 mL/min. For both models, arteries in the range of 3–6.5 mm in diameter produced complementary errors in area and velocity measurements in MR studies. Ungated MR studies showed a reduced agreement (R ²=0.88in vitro, 0.54in vivo), which may in part be due to poor sampling of the velocity pattern. The results show that the high correlation obtainedin vitro cannot be extrapolated to thein vivo situation, where additional physiological and anatomical variables are encountered.     

Influence of cytosolic pH onin vivo assessment of human muscle mitochondrial respiration by phosphorus magnetic resonance spectroscopy

The authors present anin vivo phosphorus magnetic resonance spectroscopy systematic study on the effects of cytosolic pH on skeletal muscle mitochondrial respiration in human calf muscle. In 49 normal subjects, the effect of cytosolic pH on kinetics of phosphocreatine and adenosine diphosphate recovery and on maximum rate of mitochondrial adenosine triphosphate production (Q_max) was evaluation. The results show a strong relationship between the rate of postexercise phosphocreatine recovery and the lowest value of cytosolic pH reached during recovery from exercise (termed minimum pH;r=0.89); in constrast, both adenosine diphosphate recovery halftime andQ _max were independent of cytosolic pH at the end of exercise.     

Is there a correlation in breast carcinomas between tumor size and number of tumor vessels detected by gadolinium-enhanced magnetic resonance mammography?

Tumor vessels are known as a sign of malignancy in breast tumors. Is there a correlation between tumor size and the number of vessels in cases of breast tumor examined by dynamic gadolinium (Gd)-enhanced MR imaging? Eighteen patients (mean age, 46±7 years) underwent dynamic Gd-enhanced MR imaging of the breast by three-dimensional gradient echo sequence using thin-layer technique (2.5 mm) at 1.5T. The dynamic study included one precontrast and four postcontrast sequences (every 90 seconds) in coronal slices. Postprocessing by subtraction method and reconstruction in both transverse and sagittal planes were performed. All carcinomas showed rapid Gd enhancement. Tumor size (0.5 to 31.5 cm³; mean, 6.3±3.7 cm³) and number of vessels (1 to 10; mean, 3±2.1) were detected in summation of all three directions. A significant correlation was found between number of vessels and tumor size (r=0.787,p≤0.01). Breast tumor size significantly correlated with the number of vessels detected by Gd-enhanced MR mammography. The introduced method is a further important step in differentiating a carcinoma from a benign lesion.     

Magnetic resonance imaging and67ga scan versus computed tomography in the staging and in the monitoring of mediastinal malignant lymphoma: a prospective pilot study

Purpose: To assess the potential value of magnetic resonance imaging (MRI) combined with⁶⁷Ga single-photon emission computed tomography (SPECT) versus computed tomography (CT) in the staging and in the monitoring of mediastinal malignant lymphoma. Materials and methods: Twenty-three patients, referred to our institute for the evaluation of lymphoma, underwent CT,⁶⁷Ga scan, and MRI between April 1993 and February 1996 at sequential intervals. The tests studied (MRI,^67Ga, and CT) were performed according to the following schedule: 1) before start of therapy; 2) after four courses of chemotherapy; and 3) 2, 6, 12, and 18 months after the end of treatment. Results: All patients studied at the time of diagnosis had abnormal gallium accumulation in the mediastinum as well as pathologic CT and pathologic signal intensity at MRI. Six months after the end of treatment full consistency was found between the results of MRI and SPECT, whereas during treatment and 2 months after the end of therapy MRI and⁶⁷Ga scan were not in agreement in nine patients. In the 23 patients in follow-up, in CT there were nine false-positive and three false-negative findings; in SPECT three false negatives; in MRI one false positive and one false negative. Conclusion: MRI can give morphologic information similar to CT, even superior due to multiplanarity and with major precision in the distinction between fibrosis and active disease. MRI is thus an alternative to CT. The association with SPECT allows a great diagnostic accuracy in the positive and negative predictive value.     

Water household of the common carp,Cyprinus carpio, when submitted to an osmotic challenge, as determined by diffusion-weighted magnetic resonance imaging at 7 T

In vivo diffusion-weighted magnetic resonance imaging (MRI) was used to determine the effects of an osmotic challenge (1% NaCl) to a freshwater fish, the common carp (Cyprinus carpio). The imaged region covered organs such as the swimbladder, the liver, the kidney, the intestine, the spinal cord, and muscle tissue. A striking difference between salt-treated and control fish was found in the liver. The apparent diffusion coefficient value of livers from control fish was (0.39±0.16) 10⁻⁹ m²/s and of salt-treated fish was (1.23±0.14) 10⁻⁹ m²/s, which points to an increase in extracellular water content. These results were partially confirmed by a decrease in dry/wet weight ratio of the liver tissue. We also found increased levels of stress proteins in liver tissue. TheQ factor of the applied radiofrequency coil dropped dramatically when we performed experiments with salt-exposed fish, indicating an increased conductivity resulting from the increased ion concentration and osmolarity of the fish. The data on plasma osmolarity of salt-exposed fish confirm a significant osmolarity increase upon salt exposure (from 334 to 430 mOsm/kg) and exceeded the osmolarity of the salt water (324 mOsm/kg), indicating that carp tend to cope with an increased salinity by increasing the internal osmolarity (hyperosmotic regulation). These data demonstrate that diffusion-weighted MRI might be a useful and noninvasive tool in the study of osmotic challenges of aquatic organisms.     

Young Investigator Award Presentation at the 13th Annual Meeting of the ESMRMB, September 1996, Prague Quantification of pulmonary water compartments by magnetic resonance

The purpose of this study was to quantify pulmonary water compartments of total, intravascular, and extravascular lung water in excised and perfused sheep lungs with the use of magnetic resonance imaging techniques. Total lung water was measured by proton density maps calculated from multi-spin-echo images. Intravascular lung water was evaluated by magnetic resonance angiography before and after injection of gadolinium diethylenetriamine penta-acetic acid polylysine, a macromolecular paramagnetic contrast agent. Intravascular lung water was calculated from signal intensity histogram changes comparing pre- and postcontrast angiograms. Extravascular water was calculated as the difference between total and intravascular lung water. Quantities of total and intravascular lung water measured by magnetic resonance techniques were compared to reference results obtained from wet/dry weight gravimetry and Evans blue dilution performed after imaging. Magnetic resonance and reference results correlated significantly (total lung water:r=0.93,p<0.001; intravascular lung water:r=0.80,p<0.001; extravascular lung water:r=0.89,p<0.001). Therefore, we conclude that quantitative magnetic resonance techniques are potentially useful for the clinical evaluation of pulmonary water compartments.