Zdenek Dokoupil

Magnetic field mapping around metal implants using an asymmetric spin-echo MRI sequence

Magnetic susceptibility of materials that are used for body implants causes much distortion in MR images. This paper deals with mapping the magnetic field induction in the vicinity of these materials. A modified spin-echo method is used, in which via inserting a time interval of a defined length the range of phase modulation is reduced below the value 2π. Even with a large B0 inhomogeneity it is not necessary to unwrap the phase jumps. The method is suitable for studying the effects of body-implant materials on MR images and for measuring their magnetic susceptibility.

Magnetic susceptibility measurement using 2D magnetic resonance imaging

The authors describe a nuclear magnetic resonance (NMR) method for measuring the magnetic susceptibility of bar-shaped samples that have an arbitrary cross-section and do not produce an MR signal. The method is based on the measurement of the 2D map of the reaction field in the vicinity of a non-ferromagnetic sample and on the calculation of magnetic susceptibility from a known reaction field. The verification of the technique was realized via modelling the measured configuration in the ANSYS program by means of the finite element method and through an experimental measurement of MR-compatible and MR-incompatible materials carried out on a tomograph. A great advantage of the proposed susceptibility evaluation method consists in the use of only standard commercially used devices without the need of any special sequences. The method is suitable for bar-shaped samples having an arbitrary cross-section; moreover, conditions are given for the selection of the cross-section/length ratio of a sample to be measured.

Mapping of magnetic field around small coils using the magnetic resonance method

The paper deals with measuring magnetic induction by imaging techniques based on magnetic resonance (MR). It describes experimental results of mapping the magnetic induction of helical and toroidal coils performed by modified MR imaging techniques. The results are compared with the theoretical calculation and with the measurement carried out using an ordinary magnetometer. The conclusions derived from the measurement will be used in the design of a new type of coil with complex electronic control of the magnetic field.

Diffusion MRI: mitigation of magnetic field inhomogeneities

Abstract The article reports on certain artifacts that emerge during the in vitro diffusion-weighted imaging of physical samples. In this context, the authors analyze the influence of magnetic field inhomogeneity, temperature, or eddy currents and consider artifact mitigation procedures. A technique reducing the examined spurious effects was designed, experimentally verified, and denominated as the three measurement method. The technique proved to be useful mainly for the evaluation of a DWI image measured with a diffusion gradient in the z axis, where the relative measurement error decreased to 3.38 % (during measurement using two images, the relative error was greater than 19 %). For small errors within the measurement of diffusion constants of a deionized water sample (< 5 %) it was necessary to select a b-factor value larger than 200·106 s.m-2. Temperature stabilization with accuracy better than 0.1 °C during the entire measuring process is a necessary prerequisite for the measurement of biological or material samples with relative accuracy lower than 1 %.

Correction of artifacts in diffusion-weighted MR images

The authors describe the techniques for measuring diffusion coefficients and creating DWI and DTI images. In order to increase the accuracy of diffusion coefficient measurement, it is necessary to eliminate the influence of artifacts such as the influence of magnetic field inhomogeneity, eddy currents, or temperature. For these artifact sources, the article presents experimental detection of the magnitude of errors, which is realized using the three measurements method, and proposes correction possibilities. The results are compared with diffusion measurements at different sizes of 6-factors.

Noise Generator with DSP96002

When measuring the transfer and properties of many electronic circuits it is necessary to use a measuring signal with noise of defined properties. In the paper, the design and software implementation are described of a generator of pseudorandom binary noise signal of adjustable length and with repetition of pseudorandom binary signal sequence. The parameters of noise outputs are stable, well defined and variable over a wide range. The noise generator described in this paper is a very simplified and software-emulated version of the HP 722A noise generator produced by Hewlett-Packard (1971).

Comparing Saddle, Slotted-tube and Parallel-plate Coils for Magnetic Resonance Imaging

Abstract The paper is concerned with a comparison of the properties of RF coils of three configurations for MRI measurements on small animals. In comparison with the classical saddle coil the proposed modification of slotted-tube coil exhibits identical homogeneity of B1 field in a larger space. The parallel-plate coil has a satisfactory homogeneity of B1 field over the whole internal space. The signal-to-noise ratio measured for all three coils is roughly the same and is given by the magnitude of RF pre-amplifier noise. As the slotted-tube and parallel-plate coils have a lower inductance compared with the saddle coil, they can be tuned to resonance on the 200 MHz frequency even at larger dimensions. The results show that the parallel-plate coil has very good properties for the measurement of small animals.

MRI-Based Visualization of the Relaxation Times of Early Somatic Embryos

Abstract The large set of scientific activities supported by MRI includes, among others, the research of water and mineral compounds transported within a plant, the investigation of cellular processes, and the examination of the growth and development of plants. MRI is a method of major importance for the measurement of early somatic embryos (ESE) during cultivation, and in this respect it offers several significant benefits discussed within this paper. We present the following procedures: non-destructive measurement of the volume and content of water during cultivation; exact three-dimensional differentiation between the ESEs and the medium; investigation of the influence of ions and the change of relaxation times during cultivation; and multiparametric segmentation of MR images to differentiate between embryogenic and non-embryogenic cells. An interesting technique consists in two-parameter imaging of the relaxation times of the callus; this method is characterized by tissue changes during cultivation at a microscopic level, which can be monitored non-destructively.

Data Processing in Studying the Growth of Early Spruce Embryos, Using MR Imaging Techniques

The technique of MRI (Magnetic Resonance Imaging) can be used to determine the number of hydrogen nuclei in biological tissues or when monitoring the growth of tissue culture. To assess the growth rate, the increase in the content of proton nuclei, and the shape of clusters of early growing somatic embryos, examinations were conducted using the MR techniques [1]. The measurements were part of research aimed at verifying the hypothesis of increased water content during the growth of tissue culture contaminated with cadmium. The culture to be measured was placed in the working space of a tomograph and a suitable direction of the slice plane was chosen, in which the MR image was then measured. The intensity of an image weighted by spin density is proportionate to the number of proton nuclei in the chosen slice, and the MR image is the mapping of their distribution in the cluster of growing tissue culture under examination [2]. The same technique was used to characterize the growth of early spruce embryos contaminated with lead and zinc. In the course of long-term measurements it is not possible to guarantee the stability of MR system and therefore the results were first normalized. The intensity integral was calculated from the MR images, which characterizes the number of protons of the growing cluster. The changes in the integral during the growth were established. The growth of tissue culture was evaluated from the size of clusters in diffusion images, which characterizes the change in the volume of clusters. The techniques for processing the data measured are given in the present paper.

Use of Magnetic Resonance to Determine the Circumference of Radial Slices of Norway Spruce

In the study of the growth of softwoods an interesting parameter is the rate of the growth of their needles. This parameter is significant in the evaluation of how economically trees use water, in particular the consumption of water in their growth in thickness and height. This is important in the study of the assimilation characteristics of plants and in the determination of physiological parameters of woods. The aim of the present study is to calculate, as accurately as possible, the total surface of Norway spruce needles on the basis of the circumference and shape of their radial slices.