Radek Kubasek

Determination of pre-emphasis constants for eddy current reduction

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.

Numerical Method of Simulation of Material Influences in Mr Tomography

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.

A Passive Optical Location with Limited Range

We 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.

RL Circuits Modeling With Noisy Parameters

This paper deals with inductor-resistor electrical circuits. The deterministic model of the circuit is replaced by a stochastic model by adding a noise term in both the source and the resistance. The analytic solution of the resulting stochastic differential equations is presented. Numerical simulations are obtained using the stochastic Euler method. Computer programs in C\# are used to generate numerical solutions and their graphical representations. Simulation is verified by measurement of transient event on inductor-resistor electrical circuits.

Automatic adjustment of time-variant thresholds when filtering signals in MR tomography

Removing noise from an FID signal (a signal detected in MR measurement) is of fundamental significance in the analysis of results of NMR spectroscopy and tomography. Optimum solution can be seen in removing noise by means of a digital filter bank that uses half-band mirror frequency filters of the type of low-pass and high-pass filters. A filtering method using digital filters and the approach of automatic threshold adjustment is described in the paper.

Testing the Quality of Magnetic Gradient Fields for Studying Self-Diffusion Processes in Biological Specimens by Magnetic Resonance Methods

Imaging techniques based on the principle of nuclear magnetic resonance (MRI) can be used in the study of molecular transport phenomena in biological systems such as self-diffusion processes. The precision of determining the diffusion constants depends on generating the gradient pulse with high precision. For the purpose of determining the characteristics of time behaviour of gradient pulses a simple measuring method was developed and experimentally tested on a 4.7 T tomograph. The method is based on the principle of measuring the instantaneous frequency of MR signal in the presence of gradient pulse after the excitation of a thin defined layer of the examined specimen placed outside the gradient field centre. Using the above method, errors were found in the amplitude and time integral of generated gradient fields and in determining the diffusion constants for biological tissues

Measurements of Time Characteristics of the Gradient Magnetic Field

Imaging techniques based on the principles of nuclear magnetic resonance (NMR) are modern techniques for the study of chemical, biological and physical properties of substances. The most important are their applications in medical sciences. MR imaging of a specimen weighted with diffusion coefficients requires very accurate data on the time course of the gradient pulse. Diffusion coefficients are determined from the drop of the MR signal measured with and without the application of magnetic field gradients. From the accuracy point of view, the defined course of gradients plays an important role in the computation of coefficients. A minimum rise and fall times, a defined magnitude of the excited gradient of the magnetic field and a symmetry of positive and negative pulses (zero integral of pulses of the same magnitude and opposite polarities) are required. To characterize the time course of gradient pulses or either polarity, simple methods of their measurement has been developed and experimentally tested on a 4.7 T tomograph. The method is based on the principle of instantaneous MR frequency measurements at the presence of a gradient pulse following the excitation of a thin layer situated outside the centre of the gradient field

Pulsed magnetic field fiber optic sensor

In order to the fiber linear birefringence compensation a promising method was chosen for pulsed current sensor design. The method employs orthogonal polarization conjugation by the back direction propagation of the light wave in the fiber. The Jones calculus analysis presents its propriety. An experimental fiber optic current sensor has been designed and realized. The advantage of the proposed method was proved considering to the sensitivity improvement.

The Instruments for Noise Spectroscopy

The article describes basic study of broadband noise signal application in the investigation of materials. The aim is flnd a metrology method utilizable for the research on metamaterials in the frequency range of about 100MHz to 10GHz. The instrumental equipment and other requirements are presented. This research report provides an overview of the current potentialities in the described fleld and summarizes the aspects necessary for noise spectroscopy. In the complex investigation of material structures for the micro-wave application (tensor and composite character), the properties of materials are studied by means of the classic single-frequency methods, which bring about certain di-culties in the process (1). In boundary changes with a size close to the wave-length there can occur wrong information concerning the examined objects (2,3). One of the possible ways of suppressing the negative sources of signals consists in the use of wide- band signals like white noise, and in researching into the problem of absorption in the examined material (4). These methods require a source of noise, a receiving and a transmitting antenna, and A/D conversion featuring a large bandwidth; for our purposes, the bandwidth ranged between 0Hz and 10GHz. Until recently it had not been possible to realize an A/D converter of the described speed, or devices with the above-mentioned bandwidth. Currently, high-end oscilloscopes are available with a sampling frequency of tens of Gsa/s. 2. NOISE SOURCE For UWB systems, several methods of the generation of short pulses with large bandwidth have been developed to date (5). However, these singly-iterative processes are not applicable for noise spectroscopy; in this respect, there is a need of a continuous source of noise signal (ideally of white noise) with the given bandwidth. The type of source referred to is currently being produced by certain manufacturers specialized in this fleld. Importantly, for the noise spectroscopy application we require a comparatively large output power of up to 0dB/mW; the assumed bandwidth char- acteristics range up to 10GHz. Nevertheless, at this point it is appropriate to mention the fact that there occurs the fundamental problem of flnding active devices capable of performing signal ampliflcation at this kind of high frequencies. As a matter of fact, our requirements are thus limited by the current status of technology used in the production of commercially available devices; the highest-ranking solution for the bandwidth of up to 10GHz can be found only up to the maximum of 0dB/mW. Our response to the above-discussed problem consisted in an attempt to produce a noise gener- ator in laboratory conditions as, in principle, this type of generator can be considered as su-cient for testing and basic measurement. In view of the price and availability of noise diodes we decided to apply thermal noise on electrical resistance as the basic source of noise. The speciflc connection is shown in Figure 1. The flrst transistor is in the CC conflguration, where we require mainly a high input impedance of the amplifler. The thermal noise at the input is given by its input parameters. The generator could operate even without a resistor at the transistor input, yet the unconnected input would cause a substantial deterioration of the stability. The second and the third transis- tors form a cascade voltage amplifler in the CE conflguration. The output impedance of the third amplifler is 50› for its matching to coaxial line. Figure 2 shows the realization of the tested noise generator; the BFP620 vf transistors were applied. This type of transistor features the characteristic of ft = 65GHz and the maximum stable ampliflcation of 11dB at the frequency of 6GHz. The overall ampliflcation of the two CE ampliflers in cascade for the output power of 0dB/mW would have to approximate the value of 10000, and there is no hf transistor available for this kind of stable ampliflcation. Therefore, for the stable noise generator we have to accept a lower output power.

Noise generator for noise spectroscopy

The article describes the basic study of broadband noise signal application in the investigation of materials. The aim is to find a metrology method utilizable for the research on metamaterials in the frequency range of about 100 MHz to 10 GHz. The instrumental equipment and other requirements are presented. This article provides an overview of the current potentialities in the described field and summarizes the aspects necessary for noise spectroscopy.