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Geometric Distortions (geometric + distortion)
Selected AbstractsEffect of slice angle on inhomogeneity artifact and its correction in slice-selective MR imagingCONCEPTS IN MAGNETIC RESONANCE, Issue 4 2009Kwan-Jin Jung Abstract The inhomogeneity of a local magnetic field causes an image artifact of geometric distortion and intensity abnormality because of the slice offset and readout shift in slice-selective MR imaging. It has been found that this artifact can be corrected by the projection of the slice offset onto the readout axis at a certain oblique slice angle. The slice angle for the artifact correction is determined by the amplitude of slice selection and readout gradients, and is independent of the magnetic field inhomogeneity and the main magnetic field direction. In addition, the existing view-angle tilting technique is found to be valid only for the slice orientation orthogonal to the object axis. The slice angle effect on the inhomogeneity artifact was confirmed experimentally through phantom and volunteer's head imaging for both regular and view-angle tilted spin echo sequences at 3 T. © 2009 Wiley Periodicals, Inc.Concepts Magn Reson Part A 34A: 238,248, 2009. [source] The role of bioinformatics in two-dimensional gel electrophoresisPROTEINS: STRUCTURE, FUNCTION AND BIOINFORMATICS, Issue 8 2003Andrew W. Dowsey Abstract Over the last two decades, two-dimensional electrophoresis (2-DE) gel has established itself as the de facto approach to separating proteins from cell and tissue samples. Due to the sheer volume of data and its experimental geometric and expression uncertainties, quantitative analysis of these data with image processing and modelling has become an actively pursued research topic. The results of these analyses include accurate protein quantification, isoelectric point and relative molecular mass estimation, and the detection of differential expression between samples run on different gels. Systematic errors such as current leakage and regional expression inhomogeneities are corrected for, followed by each protein spot in the gel being segmented and modelled for quantification. To assess differential expression of protein spots in different samples run on a series of two-dimensional gels, a number of image registration techniques for correcting geometric distortion have been proposed. This paper provides a comprehensive review of the computation techniques used in the analysis of 2-DE gels, together with a discussion of current and future trends in large scale analysis. We examine the pitfalls of existing techniques and highlight some of the key areas that need to be developed in the coming years, especially those related to statistical approaches based on multiple gel runs and image mining techniques through the use of parallel processing based on cluster computing and the grid technology. [source] A fast spherical inflation method of the cerebral cortex by deformation of a simplex mesh on the polar coordinatesINTERNATIONAL JOURNAL OF IMAGING SYSTEMS AND TECHNOLOGY, Issue 1 2008Min Jeong Kwon Abstract The convoluted shape of the cerebral cortex makes it difficult to analyze and visualize the neuronal activation area. One way of overcoming this problem is to use a spherical inflation method to draw a cerebral cortex on a spherical surface. The task of mapping the cortical surface on a sphere has several obstacles, namely, the overlap between the polygons of the surface, the heavy computation demand, and the geometric distortions inherent in the process. This article proposes a method of mapping the three-dimensional (3D) cortical surface represented in a simplex mesh to a sphere surface, which does not have any overlap between the polygons and minimizes the geometric distortions as well as the computation time. The proposed method consists of the two steps of preprocessing and refinement. In the preprocessing step, the 3D cortical surface is mapped onto a sphere without any overlap between the polygons by iterative deformation. In the refinement step, the mapped surface is adjusted to minimize its linear distortion. The experimental results show the efficiency and performance of the proposed mapping method. © 2008 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 18, 9,16, 2008 [source] Stimulated echo induced misestimates on diffusion tensor indices and its remedyJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 6 2010Tzu-Chao Chuang PhD Abstract Purpose: To report possible erroneous estimates of diffusion parameters in the twice-refocused spin-echo (TRSE) technique, proposed to eliminate eddy-current-induced geometric distortions in diffusion-weighted echo-planar imaging, when stimulated echo signals are inappropriately included. Materials and Methods: Eleven subjects were included for imaging experiments on two 1.5 Tesla systems using the TRSE sequence. Three versions, two with unbalanced crusher gradients inserted to dephase the stimulated echo from the b = 0 images and one with balanced crusher gradients, were implemented. The apparent diffusion coefficients (ADC) and fractional anisotropy (FA) were derived and compared. Results: The ADCs obtained with unbalanced crusher gradients were closer to values reported in the literature. Stimulated echo led to ADC over-estimations by 34.2%, 50.4%, 54.0%, 51.5%, 24.0%, and 41.9% in the genu of corpus callosum, splenium of corpus callosum, bilateral corona radiata, internal capsule, mediofrontal gyrus, and the cuneus, respectively (P < 0.01), with concomitant reduction in FA in highly anisotropic regions. Over-estimations of diffusion coefficients were found to be roughly equal along all directions. Conclusion: Formation of stimulated echo in the TRSE technique can lead to erroneous estimations of the diffusion parameters, even if no prominent morphological artifacts are seen. J. Magn. Reson. Imaging 2010;31:1522,1529. © 2010 Wiley-Liss, Inc. [source] Specimen-induced distortions in light microscopyJOURNAL OF MICROSCOPY, Issue 1 2007M. SCHWERTNER Summary Specimen-induced aberrations affect the imaging properties in optical 3D microscopy, especially when high numerical aperture lenses are used. Studies on aberrations are often properly concerned with the degradation of image quality such as compromised resolution or reduced signal intensity. Apart from these, aberration effects can also introduce geometric image distortions. The effects, discussed here are particularly strong when thick biological specimens are investigated. Using a high numerical aperture interferometer, we measured wavefront aberrations in transmission mode and quantified geometric distortions associated with specimen-induced aberrations. This assessment for a range of biological specimens allows estimation of the accuracy of spatial measurements. The results show that high-resolution spatial measurements can be significantly compromised by specimen-induced aberrations. [source] Improvements in parallel imaging accelerated functional MRI using multiecho echo-planar imaging,MAGNETIC RESONANCE IN MEDICINE, Issue 4 2010Heiko Schmiedeskamp Abstract Multiecho echo-planar imaging (EPI) was implemented for blood-oxygenation-level-dependent functional MRI at 1.5 T and compared to single-echo EPI with and without parallel imaging acceleration. A time-normalized breath-hold task using a block design functional MRI protocol was carried out in combination with up to four echo trains per excitation and parallel imaging acceleration factors R = 1,3. Experiments were conducted in five human subjects, each scanned in three sessions. Across all reduction factors, both signal-to-fluctuation-noise ratio and the total number of activated voxels were significantly lower using a single-echo EPI pulse sequence compared with the multiecho approach. Signal-to-fluctuation-noise ratio and total number of activated voxels were also considerably reduced for nonaccelerated conventional single-echo EPI when compared to three-echo measurements with R = 2. Parallel imaging accelerated multiecho EPI reduced geometric distortions and signal dropout, while it increased blood-oxygenation-level-dependent signal sensitivity all over the brain, particularly in regions with short underlying T*2. Thus, the presented method showed multiple advantages over conventional single-echo EPI for standard blood-oxygenation-level-dependent functional MRI experiments. Magn Reson Med 63:959,969, 2010. © 2010 Wiley-Liss, Inc. [source] Radial single-shot STEAM MRIMAGNETIC RESONANCE IN MEDICINE, Issue 4 2008Kai Tobias Block Abstract Rapid MR imaging using the stimulated echo acquisition mode (STEAM) technique yields single-shot images without any sensitivity to resonance offset effects. However, the absence of susceptibility-induced signal voids or geometric distortions is at the expense of a somewhat lower signal-to-noise ratio than EPI. As a consequence, the achievable spatial resolution is limited when using conventional Fourier encoding. To overcome the problem, this study combined single-shot STEAM MRI with radial encoding. This approach exploits the efficient undersampling properties of radial trajectories with use of a previously developed iterative image reconstruction method that compensates for the incomplete data by incorporating a priori knowledge. Experimental results for a phantom and human brain in vivo demonstrate that radial single-shot STEAM MRI may exceed the resolution obtainable by a comparable Cartesian acquisition by a factor of four. Magn Reson Med 59:686,691, 2008. © 2008 Wiley-Liss, Inc. [source] | ||||||||||