Optimization of Basic Magnetic Field Homogeneity in MR Tomography

Práce se zabývá problematikou měření a mapování magnetického pole v MR tomografu, za účelem nastavování homogenity magnetické indukce. Pozornost je věnována technikám mapování na rotačně symetrickém objemu a způsobům optimalizace magnetických polí s využitím pasivních a aktivních korekčních systémů. Je proveden teoretický rozbor rozkladu magnetických polí s využitím sférických harmonických funkcí a Taylorovy řady. Techniky mapování a aproximace základního magnetického pole jsou ověřeny na 200 MHz tomografu umístěném na pracovišti Ústavu přístrojové techniky Akademie věd České republiky v Brně (ÚPT AV ČR).This thesis is concerned with problems of measuring and mapping of magnetic field in MR tomograph, for purpose of magnetic flux density homogeneity optimization. Attention is paid to mapping techniques on rotary symmetric volume and to ways of magnetic fields optimization with utilization of passive and active correction systems. Theoretical analysis of magnetic field decomposition with utilization of spherical harmonics and numerical decomposition is made. Mapping and approximation techniques of basic magnetic field are verified by experiments in the laboratory at the Institute of Scientific Instruments AS CR in Brno.

Magnetic field decomposition and reverse approximation method

In this work, we developed a method, which is used for approximation of basic magnetic field B0 inside a specific spherical volume. This method uses Legendre multinominals expansion of magnetic flux density field measured at discrete points on spherical volume. Using optimization technique of Least square method we modeled map of magnetic flux density on the desired surface and compared it to measured values of magnetic flux density. The coefficients of spherical function expansion obtained from approximation process can be used for determination of shimming currents which feed correction coils of tomograph. The results of approximated maps are described and compared to measured maps in the experimental part of this article

Arc coupled band: pass filters

A filter design of coupled band – pass filters is very often used in practise because exhibit smaller spread of filter elements then filter ladder configurations. In comparison to cascade synthesis method enable this type of filters to reach smaller filter element sensitivity. However in area of active filter design there are problems with active filter elements (active inductances or FDNR elements) which require relatively high quality factors and consequently relatively high numbers of active elements (OAs) too. In paper are prescribed some class of coupled band – pass filters using lossy active elements with minimised number of active elements (OAs). The possibilities of application and using of developed active band – pass filters are discussed in further details and explained in some examples

Numerical modeling of the inhomogeneity in materials

This article deals with the verification of experimental results obtained by numerical simulation. We solved the effect of changes in the homogeneity of magnetic fields evoked by different samples from conductive and/or magnetic materials and the different types of inhomogeneity in the MR tomograph. Moreover, the paper will describe the suitable magnetic resonance techniques

Numerické modelování nehomogenity v materiálech

Příspěvek se zabývá využitím numerického modelování nehomogenních materiálových vzorků k ověření správnosti výsledků MRI. Pro účely studia deformací MR obrazů magnetického pole v okolí měřených diaparamagnetických vzorků vlivem susceptibility heterogenních materiálů (objektů) vybrána jednoduchá konfigurace vzorku a byly porovnány numericky modelované řezy s experimentálním měřicím průběhem magnetického pole měřeného MR technikou gradientního echa

The reverse reconstruction results obtained from the numerical simulation of MR signals

The paper describes the magnetic resonance imaging method applicable mainly in MRI and MRS in vivo studies. We solved the effect of changes of magnetic fields in MR tomography. This article deals with the reverse reconstruction results obtained from the numerical simulation of MR signals by various techniques, which will be usable for the experimental results verification

Experiments with the Effect of Non-Homogenous Parts into Materials

The article describes possibility of the deformity magnetic field numerical modeling into surroundings of the measured dia – paramagnetic specimens. This deformation study is given for the purposes MR measuring, where MR images are deformed from the point of view of susceptibility heterogeneous materials. For the numerical simulation was chosen elementary configuration. The results was verified by MR experimental measurement. As the MR measurement method was used techniques of the gradient echo