NMR-Bildgebung an periodisch bewegten Systemen
The Usage of the magnetic resonance imaging (mri) technique to determine material properties inside a body without destroying the corpus is a major advantage of this method. In this context the aim of mapping local stiffness in a body and representing it in form of an image is commonly refered to as ‘mr-elastography’. This can be accomplished by observing an induced mechanical wave passing through the material. Here it is necessary to measure the local motions spacially resolved and to interpret these displacement data in terms of local stiffness utilizing a model. The local motion itself can be regarded as a periodic and cyclical one whose average displacement is zero. Using appropriate oscillating magnetic field gradients {G (t)} in the mri experiment allows to utilize this periodicity to gain in sensitivity for small amplitudes of local periodic motions. To point out the possible fields of application for these oscillating gradients in mr-elastography and to verify these experimentally is the topic of this thesis. Starting from the time-dependant functions of local motion {xi(r,t)} and experimental magnetic field gradient {G(t)} the general equations of interaction are deduced. The interpretation of the amplitude of cyclic motion {xi(r,t)} and the gradient amplitude – in form of the linear dependant q-space coordinate {q_os} – as a Fourier pair facilitates new representations of the experiment and implies interesting applications. Among these are the determination of the amplitude distribution within a given voxel (i.e. the sample volume which is represented as a single point in the final image) and the correlation of motions along the orthogonal space coordinates (i.e. x,y,z). The feasibility of this concept is demonstrated by the experimental results received on a gel phantom. Besides the motion amplitude (<50 micrometer), wave length and spatial damping of the wave are determinable out of the experimental results. The nature of the propagating wave, consisting of longitudinal and transversal components, can be revealed when correlating the motions along and perpendicular to the propagation direction.
info:eu-repo/classification/ddc/004, Informatik, Gel, Periodische Bewegung, NMR-Bildgebung
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