首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到10条相似文献,搜索用时 234 毫秒
1.
A large water resonance is usually present in MRS signals. Time-domain quantitation methods require a good suppression of these components in order to obtain accurate parameter estimates. In this paper, we analyze one of the most successful methods for solvent suppression, the maximum-phase finite impulse response filter (MP-FIR), and identify its drawbacks. A new filtering method is proposed to overcome the limitation of MP-FIR.  相似文献   

2.
The location and masking of the brain and surrounding cerebrospinal fluid (CSF) in two-dimensional (2D) dual-echo fast spin-echo (FSE) magnetic resonance (MR) images of the head is achieved by an automated procedure with a voxel-based computational algorithm. Linear scale-space features are derived from the short-echo, proton-density (PD)-weighted images. The second-order Gaussian derivative (the Laplacian) operator is applied at three different spatial scales as a measure of image convexity/concavity with a first-order Gaussian derivative measure (the squared gradient) at a single scale used to circumscribe cortical regions. A mask obtained from the long-echo, T2-weighted image is used to remove extracerebral components of the visual system. A three-dimensional (3D) connectivity analysis then identifies the largest connected volume as the brain. Five dual-echo fast spin-echo images acquired by repeated scanning of the same normal volunteer were used to verify reproducibility; and coronal and axial acquisitions from another normal volunteer to demonstrate the method's robustness to data collected with non-cubic voxels. Images acquired from five individuals with Alzheimer's disease are also presented to show that the algorithm can be used in cases of non-normative anatomy. Validity is affirmed by demonstrating that cerebral volumes estimated by this method for all 12 images are highly correlated (R = 0.98) with estimates obtained by an expert human operator.  相似文献   

3.
While the inherent low sensitivity of in vivo MR spectroscopy motivated a trend towards higher magnetic fields, B(0), it has since become apparent that this increase does not seem to translate into the anticipated improvement in spectral resolution. This is attributed to the decrease of the transverse relaxation time, T(2)*, in vivo due to macro- and mesoscopic tissue susceptibility. Using spectral contrast-to-noise ratio (SCNR) arguments, we show that if in biological systems the linewidth (on the frequency scale) increases linearly with the field, the spectral resolution (in parts per million) improves approximately as the fifth-root of B(0) for chemically shifted lines and decreases as about B(0)(4/5) (in hertz) for a structure of J-coupled multiplets. It is also shown that for any given B(0) there is a unique voxel size that is optimal in spectral resolution, linking the spectral and spatial resolutions. Since in practical applications the spatial resolution may be dictated by the target anatomy, nomograms to determine the B(0) required to achieve the desired spectral resolution at that voxel size are presented. More generally, the scaling of the nomograms to determine the achievable spectral and spatial resolutions at any given field is described.  相似文献   

4.
卡尔曼滤波法用于ICP-AES分析时,在白色噪音情况下能给出准确的分析结果,当试样的某些组分未知时,测量噪音将不是白色的,分析结果将不准确。本文提出了一种加权增量卡尔曼滤波法来解决这个问题。  相似文献   

5.
We discuss the spectral lineshapes of reflectance and modulated reflectance (MR) measurements on optoelectronic device structures such as epi-layers, quantum wells (QWs), vertical-cavity surface emitting-lasers (VCSELs) and resonant-cavity light-emitting diodes (RCLEDs). We consider the various methods for the extraction of built-in electric fields and band-gap energies from Franz-Keldysh oscillations (FKO), using the example of a tensilely strained InGaAs QW system, whose InGaAsP barriers yield strong FKO. We describe how critical point transition energies can be easily obtained by eye from Kramers-Kronig (KK) transforms of low field or QW modulation spectra, using the example of the modulated transmittance spectra of dilute-nitrogen InGaAsN p-i-n structures. We also discuss how the ordinary reflectivity spectrum, usually acquired at the same time as the MR signal, may also be exploited to extract layer thicknesses and compositions, and information about the active QW absorption spectrum in VCSEL and RCLED structures.  相似文献   

6.
The sensitivity of spectroscopic methods based on nuclear magnetic resonance (MR) is limited, in particular by the magnitude of noise in the signal being measured. In MR tomography and, above all, in localized spectroscopy and spectroscopic MR imaging, this problem becomes even more pronounced. When gradient magnetic fields are used, it cannot be fully ruled out that there will be a change in the basic magnetic field due to the eddy currents in conducting materials in the neighborhood of the sample being measured. This results in a local change in instantaneous frequency of the resonance of nuclei and in a distortion of spectral lines or MR image. For methods that eliminate this distortion and for an accurate calculation of the constants of (in particular long) preemphasis filters, techniques have been developed and experimentally tested that are based on measuring the instantaneous frequency of the signal detected with a very low signal-to-noise ratio. Adaptive filtering methods and filtering based on filter banks have been developed to reduce the level of noise. Results of these two types of filtering are described in the paper. The filtering techniques developed can be used also in other applications and thus contribute to increasing quality of methods for examining the properties of biological and chemical substances. Authors' address: Eva Gescheidtova, Department of Theoretical and Experimental Electrical Engineering, Faculty of Electrical Engineering and Communication, Brno University of Technology, Kolejni 2906/4, Brno 61200, Czech Republic  相似文献   

7.
In the field of optical pattern recognition, there are a large number of filters that possess identification capabilities for certain kinds of input distortions. The resolution of the spectral components plays an important role in the overall processing time of a correlator that may employ such filters. In this paper, two different techniques are discussed: low pass phase filtering and high pass phase filtering. This is achieved by binarizing the amplitude information which then allows certain frequencies to pass and others to be blocked. Their response to different kinds of input distortions is presented and the effect of phase quantisation is considered in detail.  相似文献   

8.
在介绍肿瘤样品代谢物的核磁共振波谱技术的研究方法的基础上,从离体组织和活体组织两个方面综述核磁共振波谱(NMR)在诊断肿瘤方面的应用进展,分析了它在肿瘤诊疗中的应用前景.在离体组织方面,人们通过1H和31P-NMR谱观测病人的体液样品、培养的细胞、切除的组织等来研究脂质、磷脂等代谢物的分布,观测肿瘤与对照组织之间的差别.其中利用组织提取物的方法能够得到分辨率较高的图谱,非常适合应用于肿瘤诊断和治疗方法的研究.高分辨魔角旋转(HR-MAS)的方法在肿瘤诊断研究方面展现出新的生命力.利用高分辨魔角旋转技术可以直接得到组织细胞中很多分子水平的代谢物结构和组成信息,因此它在癌症的早期诊断中具有很好的前景.在活体核磁共振波谱诊断肿瘤方面,主要应用1H和31P核磁共振波谱,结合MRI为非侵人性肿瘤分析提供了一种临床可用的方法.MRI与MRS技术的结合将使核磁共振波谱在医学领域有更大的应用空间.  相似文献   

9.
Cross-polarized magic-angle-spinning NMR (CPMAS-NMR) techniques are assumed to be only semi-quantitative in the assessment of carbon distribution in humic substances or natural organic matter, due to a number of interferences such as spinning side bands (SSB) in spectra, paramagnetic species in samples, and low or remote protonation of aromatic carbons. Fast rotor spin rates or direct polarization NMR techniques are normally applied to improve quantitative signal detectability. Variable contact time pulse sequences were used here to obtain CPMAS-NMR spectra of organic compounds of known structure and different humic substances. Integration of spectral areas, previously subtracted of SSB, and relative stoichiometric factors were used for mathematical elaboration to calculate the elemental content in samples. These values did not significantly differ from those obtained by direct determination of elemental content with quantitative elemental analysis. Our results showed that the carbon observed CPMAS-NMR provides a quantitative representation of the whole carbon content in humic substances.  相似文献   

10.
PCA和SPA的近红外光谱识别白菜种子品种研究   总被引:2,自引:0,他引:2  
为了实现对不同品种白菜种子的快速无损鉴别,应用近红外光谱技术获取白菜种子的光谱反射率,首先采用变量标准化校正和多元散射校正对原始光谱进行预处理;其次,采用主成分分析法(PCA)对光谱数据进行聚类分析,从定性分析的角度得到三种不同白菜种子的特征差异,并采用连续投影算法(SPA)选取特征波长;最后,分别基于全波段光谱、PCA分析得到的前3个主成分变量以及SPA算法选取的特征波长,建立了最小二乘支持向量机(LS-SVM)和偏最小二乘判别(PLS-DA)模型进行白菜种子不同品种的鉴别。从主成分PC1、PC2得分图中可以看出,主成分1和2对不同种类白菜种子具有很好的聚类作用。基于特征波长建立的PLS-DA和LS-SVM模型的判别结果优于基于主成分变量建立的模型,其中基于特征波长建立的LS-SVM模型识别效果最优,建模集和预测集的品种识别率均达到100%。结果表明,通过SPA算法选取的6个特征波长变量能够很好的反映光谱信息,提出的SPA算法结合LS-SVM预测模型能获得满意的分类结果,为白菜种子品种的识别提供了一种新方法。  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号