|
|
Some comments on frequency selective excitation in newly proposed MRSI sequences
Starčuk jr., Zenon ; Horký, Jaroslav ; Starčuk, Zenon ; Mlynárik, V. ; Gruber, S. ; Moser, E.
In many pathologies it is desirable to compare the metabolism inside a lesion, near the lesion, and in healthy tissue. The diagnostic value of MRS is enhanced by spectroscopic imaging (MRSI), permitting the simultaneous acquisition of spatially resolved spectra. In this way, assessing metabolic information about different brain regions within a single examination is possible. As with all in vivo spectroscopy, low signal-to-noise ratio (SNR) is the fundamental limiting factor in MRSI due to very low concentrations of metabolites of interest. Proton spectroscopic imaging, however, offers additional technical challenges as compared to single-voxel techniques, especially if acquisition of short echo time (TE<30 ms) is required. Critical to the success of proton MRSI studies of the human brain is the elimination of the very intense water and lipid signals arising from outside the volume of interest (VOI). Water suppression in MRSI can be very problematic in MRSI, in which the achievable degree of water suppression is limited by B.sub.0./sub. and B.sub.1./sub. inhomogeneities and water T.sub.1./sub. variations, invariably present throughout the larger VOIs. Suppression of liquid signals (from bone marrow and subcutaneous fat) is often severely complicated due to the fact that their relaxation behavior differs substantially from that of water. One of the basic approaches used to reduce the undesired contamination if the MRSI signals of interest consists in the use of the STEAM or PRESS selective excitation of the VOI. Both STEAM and PRESS techniques suffer from some drawbacks. STEAM reduces the sensitivity of measurement, PRESS has higher RF power requirements.
|
|
|
High resolution multivoxel spectroscopy of human brain at 3 Tesla
Mlynárik, V. ; Gruber, S. ; Starčuk, Zenon ; Starčuk jr., Zenon ; Horký, Jaroslav ; Moser, E.
Localisation in in vivo NMR spectroscopy can be achieved using different concepts. Single voxel localized spectroscopy exploits three slice selective pulses defining a cube (or a parallelepiped) in their intersection. By proper combination of spoiling gradients the excited magnetization outside the cube is dephased and does not contribute to the NMR signal. Another way of localisation is combination of slice selection with phase encoding of the NMR signal in the other two dimensions by means of gradient magnetic fields. The latter method is referred to as spectroscopic imaging and provides an array of spectra corresponding to individual voxels defined by the phase encoding. Further subdivision of the voxels is possible by Hadamard encoding of the excitation pulses.
|
|
|
Temperature stabilization of semiconductor lasers for direct measurement of index of refraction of air
Matoušek, Vít
Laser interferometers are even more precise distance measurement devices with resolution in nanometer or sub-nanometer region. If interferometric measurements are carried out under atmospheric conditions (usual situation in industry), they measure optical path length of an unknown distance instead of its true geometrical value. It is caused by an index of refraction of air that introduces a multiplicative constant to measured results. If we want to obtain correct values, the knowledge of the index of refraction is necessary. Generally, the index of refraction can be measured by two ways: indirectly or directly. The first of them is based on parametric analysis of atmospheric properties as: relative humidity, pressure, temperature, concentration of CO.sub.2./sub. etc. Values of these parameters are processed then by Edlen formulas with 10.sup.-7./sup. order [1]. The direct methods are more precise then Edlen formulas (more than 10.sup.-7./sup.) but their practical implementation is more difficult. Devices that directly measure the index of refraction are called refractometers.
|
|
|
Temperature stabilization of semiconductor lasers for direct measurement of index of refraction of air
Matoušek, Vít
Laser interferometers are even more precise distance measurement devices with resolution in nanometer or sub-nanometer region. If interferometric measurements are carried out under atmospheric conditions (usual situation in industry), they measure optical path length of an unknown distance instead of its true geometrical value. It is caused by an index of refraction of air that introduces a multiplicative constant to measured results. If we want to obtain correct values, the knowledge of the index of refraction is necessary. Generally, the index of refraction can be measured by two ways: indirectly or directly. The first of them is based on parametric analysis of atmospheric properties as: relative humidity, pressure, temperature, concentration of CO.sub.2./sub. etc. Values of these parameters are processed then by Edlén formulas with 10.sup.-7./sup. order [1]. The direct methods are more precise then Edlén formulas (more than 10.sup.-7./sup.) but their practical implementation is more difficult. Devices that directly measure the index of refraction are called refractometers.
|
|
|
Workplace for stabilization of laser frequency
Buchta, Zdeněk
This paper describes a new appliance for demonstration of frequency stabilization of a laser diode. That is an original solution which is appointed for hands-on teaching students of optoelectronic courses. The arrangement consists of a laser diode driver, temperature controller and driver for the photodetector.
|
|
|
Semiconductor lasers with high coherence and their application, activities of the Coherence Optics Department in ISI, ASCR
Lazar, Josef
Pracoviště oddělení Koherenční optiky svými aktivitami navazuje na tradici výzkumu a vývoje v oblasti laserů s vysokou koherencí a jejich aplikaci především pro metrologické účely. Tým Ing. Františka Petrů, DrSc. vyvinul první plynový laser v tehdejší ČSSR a následně dovedl technologii He-Ne laserů k vysoké dokonalosti. Ve svém úsilí se soustředil na frekvenčně stabilizované lasery pro laserovou interferometrii a následně v souvislosti s definicí jednotky délky prostřednictvím rychlosti šíření světla ve vakuu též na konstrukci laseru - primárního etalonu délky, resp. optické frekvence.295 The workplace of the Coherence Optics Department continues in the tradition of the research and development in the area of lasers with high coherence and their application especially for metrological purposes. The team of Ing. František Petrů, DrSc, developed first gaseous laser in the former CSSR and then led the technology of He-Ne lasers to highest perfection. It focused its striving to the frequency-stabilised lasers for laser interferometry and then, in correspondence with the definition of the unit of length by means of light propagation in vacuum, also to the construction of a laser - primary length, i.e. optical frequency etalon.
|
|
|
Semiconductor lasers for metrology and telecommunications
Růžička, Bohdan
The subject of my work is realisation of a semiconductor source of coherent radiation in the infra-red area with relative frequency stability at least of 10.sup.-8./sup., which could become the basis of a future etalon of wavelengths for optical communication in the Czech Republic. I am a member of a research team which solves these problems at the workplace of the Institute of Scientific Instruments of the Academy of Sciences of the Czech Republic.
|
|
| |
|
| |
|
|
3D Construction View of Differential Pumping Chamber with Detector Board of Environmental Scanning Electron Microscope
Neděla, Vilém
This paper deals with construction of the differential pumping chamber with a detector board for adaptation of the scanning electron microscope (SEM) to the environmental scanning electron microscope (ESEM). ESEM allows work at higher pressure in some parts of the microscope. It focuses especially on the 3D construction view of the differential pumping chamber together with the detector board of the differential pumping chamber and it comments reasons of necessity of the assembly of these parts into the environmental scanning electron microscope.
|
guest ::
