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| 3. |
- Ericsson, A, et al.
(författare)
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Measurements of magnetic field variations in the human brain using a 3D-FT multiple gradient echo technique.
- 1995
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Ingår i: Magnetic Resonance in Medicine. - 0740-3194 .- 1522-2594. ; 33:2, s. 171-7
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Tidskriftsartikel (refereegranskat)abstract
- A magnetic resonance 3DFT multiple gradient-echo technique was used for measurements of the proton spectrum for each voxel in the measured slice. Water, fat, magnetic field and T2 distributions in the head of a normal volunteer and a patient with intracerebral hematoma were computed. Magnetic field variations caused by the head were calculated after correction for the static magnetic field inhomogeneity. Large local magnetic field variations up to 3 ppm were found in the human brain near interfaces between air or bone and brain tissues and 0.5 ppm between hematoma and brain tissue. Information about magnetic field variations could be useful for shimming procedures in vivo and for correcting artifacts in imaging and spectroscopy.
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| 4. |
- Görke, U, et al.
(författare)
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Visualization of anisotropic pulsations in extraembryonic compartments of incubated quail eggs by NMR microimaging.
- 1996
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Ingår i: Magnetic Resonance Imaging. - 0730-725X .- 1873-5894. ; 14:9, s. 1079-84
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Tidskriftsartikel (refereegranskat)abstract
- Multi-plane tagging time-of-flight and bipolar gradient phase-encoding and amplitude-weighting NMR microimaging techniques have been employed to study transport in extraembryonic compartments of fertilized quail eggs during the first seven days of incubation. After the fourth day coherent and incoherent motions became visible in certain regions, in particular in the upper part of the albumen. Coherent motions as visualized by deformations of multiplane tagging grids were specified by local velocities less than 1 mm/s at most. On the other hand, incoherent motions led to much more pronounced phenomena. The multiplane tagging grid completely faded in a region comprising the upper third of the albumen after 4.5 incubation days. Images weighted by signal attenuation owing to incoherent displacements indicate motions in the same region. The fact that a reproducible localization of the motions is only possible when signals from different transients are averaged indicates that the incoherence at least partly is temporal in nature. The signal intensity of a volume selected in the incoherent-motion region consequently fluctuates with time. The Fourier analysis of these fluctuations revealed a distinct pulsation with a frequency of 0.4 Hz.
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| 5. |
- Müller, H P, et al.
(författare)
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Six-dimensional spin density/velocity NMR microscopy of percolation clusters.
- 1996
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Ingår i: Magnetic Resonance Imaging. - 0730-725X .- 1873-5894. ; 14:7-8, s. 955-8
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Tidskriftsartikel (refereegranskat)abstract
- Using computer-simulated random-site percolation networks as templates, three-dimensional percolation cluster objects were fabricated. The pore space was filled with water and experimentally investigated with the aid of NMR microimaging. A pulse sequence for six-dimensional spin density/velocity NMR imaging was employed for the combined record of the three-dimensional spin-density distribution and the three-dimensional velocity vector field of water percolating through the pore space. An evaluation procedure for the NMR image data was established that reliably renders the characteristic parameters (fractal dimensionality, fractal dimensionality of the backbone, correlation length).
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| 7. |
- Smedby, Örjan, et al.
(författare)
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Timing adjustment in gadolinium MR angiography by raw data recombination. Technical note.
- 1999
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Ingår i: Acta Radiologica. - 0284-1851 .- 1600-0455. ; 40:4, s. 462-4
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Tidskriftsartikel (refereegranskat)abstract
- To solve the problem of injection timing in gadolinium MR angiography, a simple procedure is proposed which allows the acquisition interval to be chosen after injection. Starting simultaneously with the injection, several consecutive acquisitions are made, after which raw data acquired in a contiguous interval with a variable starting time are recombined to one data set, which is then used for delayed image reconstruction.
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| 9. |
- Weis, J, et al.
(författare)
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1H-spectroscopic imaging with read gradient during acquisition in inhomogeneous fields : analysis, measurement strategy, and data processing.
- 1997
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Ingår i: Magnetic Resonance Materials in Physics, Biology and Medicine. - 0968-5243 .- 1352-8661. ; 5:3, s. 201-12
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Tidskriftsartikel (refereegranskat)abstract
- The proton magnetic resonance spectroscopic imaging techniques that use read gradient during acquisition produce proton spectra with high spatial and moderately high spectroscopic resolution in a reasonable time for in vivo applications. These techniques suffer mainly from the spatial and spectral distortions caused by the convolution of spectral/spatial information (chemical-shift artifacts) and from the spectral shifts caused by static magnetic field inhomogeneities. The investigators analyze the chemical-shift artifacts in the presence of nonnegligible static magnetic field inhomogeneities and propose a postdetection processing scheme to correct for such effects. Spectral artifacts caused by chemical shifts, spectral line overlapping, streak broadening, and magnetic field inhomogeneities are discussed. The postdetection data processing scheme is demonstrated on measurements of a phantom as well as a human leg.
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| 10. |
- Weis, J, et al.
(författare)
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Characterization of human head vasculature byprelocation parameters
- 1999
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Ingår i: Magnetic Resonance Imaging. - 0730-725X .- 1873-5894. ; 17:3, s. 411-415
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Tidskriftsartikel (refereegranskat)abstract
- A data reduction procedure, originally proposed for characterization of fractals and random percolation clusters, has been used to evaluate the vascular system of the human head. The motivation behind this study arose from the wish to study empirically transport properties of vascular systems and to find a suitable formalism for their description. MR angiographic data acquired by a standard 3D inflow method were used. The evaluated parameters refer to the backbone fractal dimensionality and the correlation length. The fractal dimensionality of the backbone was found to be 1.71 for the human head vasculature. This value fits the theoretical range of random percolation networks. It is concluded that concepts of percolation theory might have some value for characterizing the structure and transport properties of the vascular system.
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