Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-03-13
2004-02-03
Gutierrez, Diego (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S310000, C324S307000
Reexamination Certificate
active
06686739
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for operating a magnetic resonance device in which a magnetic resonance signal is recorded for a span of time, and in which, in order to produce a magnetic resonance spectrum, the magnetic resonance signal in the time domain is subjected to a Fourier transformation.
2. Description of the Prior Art
Magnetic resonance spectroscopy has been used for more than four decades in basic research in physics, chemistry, and biochemistry, for example as an analytical technique or for the structural clarification of complex molecules. As is the case for magnetic resonance tomography, magnetic resonance spectroscopy is based on the principle of magnetic nuclear spin resonance. However, the primary goal of spectroscopy is not imaging, but rather an analysis of a material. Resonant frequencies of isotopes that have a magnetic moment, for example
1
H,
13
C, or
31
P, are dependent on the chemical structure of molecules in which the above-named isotopes are bonded. A determination of the resonant frequencies therefore makes it possible to differentiate between different materials. The signal intensity at the various resonant frequencies provides information concerning a concentration of the corresponding molecules.
If a molecule is brought into a basic magnetic field of a magnetic resonance device, as takes place in spectroscopy, electrons of the molecule shield the basic magnetic field for atomic nuclei of the molecule. Due to this effect, the local magnetic field at the locus of an atomic nucleus changes by a few millionths of the external basic magnetic field. The accompanying variation of the resonant frequency of this atomic nucleus is called chemical displacement. In this way, molecules can be identified on the basis of their chemical displacement. Since, from the point of view of measurement technology, frequency differences can be acquired more easily and more precisely than can absolute frequencies, the chemical displacement is indicated in ppm relative to a reference signal, for example the operating frequency of the magnetic resonance device.
A resonance line of an atomic nucleus can be split into a number of lines if additional atomic nuclei having a magnetic moment are located in the environment of the atomic nucleus under observation. The cause for this is due to the phenomenon known as spin-spin coupling between the atomic nuclei. The magnetic flux density of the basic magnetic field that is experienced by an atomic nucleus depends not only on the electron shell surrounding this atomic nucleus, but also on the orientation of the magnetic fields of the neighboring atoms. Because the resolution capacity of the magnetic resonance device is often too low, the spin-spin coupling is thereby often not visible in the spectra.
Clinical magnetic resonance spectroscopy is understood to refer to magnetic resonance spectroscopy with the use of clinical magnetic resonance tomography devices. The methods used in localized magnetic resonance spectroscopy are distinguished from those used in magnetic resonance imaging essentially by virtue of the fact that in spectroscopy, in addition to the tomographic locus resolution, chemical displacement is resolved.
Currently, in clinical applications two types of localization methods for magnetic resonance spectroscopy are dominant. A first type includes individual volume techniques based on echo methods, in which a spectrum of a target volume selected beforehand on the basis of proton images is recorded. A second type includes spectroscopic imaging methods (Chemical Shift Imaging, CSI) that simultaneously enable the recording of spectra of a multiplicity of spatially contiguous target volumes.
The individual volume techniques standardly used today are based on acquisition of a stimulated echo or of a secondary spin echo. In both cases, a locus resolution takes place through successive selective excitations of three orthogonal layers. The target volume is defined by the slice volume of these three layers. Only the magnetization of the target volume experiences all three selective high-frequency pulses and thus contributes to the stimulated, or secondary spin echo. The spectra of the target volume is obtained through one-dimensional Fourier transformation of a time signal corresponding to the stimulated echo, or to the secondary spin echo.
Spectroscopic imaging methods are used both in clinical phosphorus spectroscopy and in proton spectroscopy. A 3D CSI pulse sequence includes for example the following steps: After a slice-layer-selective 90° high-frequency pulse, for a defined period of time a combination of magnetic phase coding gradients of the three spatial directions is activated, and subsequently the magnetic resonance signal is read out in the absence of all gradients. This procedure is repeated as often as necessary with different combinations of phase coding gradients until the desired locus resolution has been achieved. A four-dimensional Fourier transformation of the magnetic resonance signals supplies the desired spatial distribution of the resonance lines. If the above-described non-selective high-frequency pulse is replaced by a slice-selective excitation, consisting of a frequency-selective high-frequency pulse and a corresponding magnetic gradient, one phase coding direction can be omitted, and given a 2D CSI pulse sequence of this sort the measurement time is reduced in relation to the 3D CSI pulse sequence.
In clinical proton spectroscopy, the intensive water signals are often suppressed using so-called water suppression methods. A method for water suppression is, for example, the CHESS technique, in which the nuclear spins of the water molecules are first selectively excited by narrowband 90° high-frequency pulses, and their cross-magnetization is subsequently de-phased through the switching of magnetic field gradients. For an immediately subsequent spectroscopic imaging method, in the ideal case no detectable magnetization of the water molecules is therefore available. In methods using water suppression, however, metabolites are at least partially also saturated, so that, disadvantageously, these metabolites contribute only slightly, or not at all, to a magnetic resonance signal, and appear only weakly, or not at all, in the associated spectrum.
In the above-described individual volume techniques and spectroscopic imaging methods, magnetic resonance signals of a particular chronological length, for example a free induction decay or a second half of a spin echo, are subjected to a Fourier transformation, as a part of a very comprehensive procedure—from the point of view of magnetic resonance physics—for the production of a spectrum. Among other things, this results in the resonance lines, in particular the water resonance lines, of a spectrum experiencing an undesired line broadening, so that metabolite resonance lines adjacent to the water resonance line are covered due to the broadening, and, in addition, a precise frequency determination in particular of metabolite resonance lines, and thereby the identification thereof as particular molecules, is made more difficult.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of the type described above for the operation of a magnetic resonance device that reduces the cited disadvantages of the prior art.
The above object is achieved in accordance with the principles of the present invention in a method for operating a magnetic resonance device, wherein a magnetic resonance signal is recorded for a time span and wherein, in order to produce a magnetic resonance spectrum, the magnetic resonance signal in the time domain is subjected to a Fourier transformation, and wherein, before the Fourier transformation, the magnetic resonance signal is weighted with a bell-shaped window function.
Due to the fact that before the Fourier transformation the magnetic resonance signal is weighted with a bell-shaped window function, a line broadening of resonance lines is prevented.
Fetzner Tiffany A.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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