Abstract-Generally all Magnetic Resonance Imaging (MRI) techniques are affected by magnetic and electric properties of measured materials, resulting in errors in MR image. Using numerical simulation we can solve the effect of changes in homogeneity of static and RF magnetic fields caused by specimen made from conductive and/or magnetic material in MR tomograph. This paper deals with numerical simulation of material susceptibility influence to magnetic field.

Magnetic field gradients play a fundamental role in fast magnetic resonance imaging (MRI) methods and spectroscopy imaging. Precise information on gradient waveform shape and rise times is often very useful in experimental MRI. We present a simple method for the measurement of the gradient time course and static magnetic field changes. The method does not require any specialized hardware and can be used with a standard volume coil and a special phantom filled with deionized or doped water. The method is based on an analysis of the instantaneous frequency variation of an MR signal in the time domain, acquired from a mechanically selected thin slice of phantom located at the gradient isocentre. The measurement and approximation of the course of time gradients and of the changes in the static magnetic field result in pre-emphasis constants that substitute the constants determined manually in a time-consuming manner. The described method facilitates the determination of pre-emphasis constants for the compensation of eddy current effects in MR systems. Our results show that residual gradients in the region of interest within 0.5 ms after the gradient is switched off are smaller than 5 µT m−1 (for an applied test gradient of 72 mT m−1) and the shift of the static field is smaller than 0.2 µT. This accuracy is very important for the development of MR spectroscopic imaging technologies using samples with very short relaxation times, and for EPI pulse sequences.

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SummaryWe know active and passive methods of a location. This article deals only with a passive location of dynamic targets. The passive optics location is suitable just for tracking of targets with mean velocity which is limited by the hardware basis. The aim of this work is to recognize plasma, particles etc. It is possible to propose such kind of evaluation methods which improve the capture probability markedly. Suggested method is dealing with the short-distance evaluation of targets. We suppose the application of three independent principles how to recognize an object in a scanned picture. These principles use similar stochastic functions in order to evaluate an object location by means of simple mathematical operations. Methods are based on direct evaluation of picture sequence by the help of the histogram and frequency spectrum. We find out the probability of unidentified moving object in pictures. If the probability reaches a setting value we will get a signal. The processing of dynamic pictures and their filtration are a significant part of work. Static objects, background (trees, buildings) must be filtered off before. This filtration is being also done by means of the probability function. Probability distribution of an object position is gained from a sequence of more pictures.

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