Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/4818—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space
  • G01R33/482—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space using a Cartesian trajectory
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/4828—Resolving the MR signals of different chemical species, e.g. water-fat imaging

Definitions

• the invention relates to an imaging method for the investigation of substances in which a precession of at least some nuclear spins with an additional one by indirect nuclear spin-nuclear spin interaction
• the invention relates in particular to a method for determining the spin-spin relaxation time T 2 .
• Nuclear spins are excited by nuclear magnetic resonance tomography.
• the excited nuclear spins relax in equilibrium states. This requires an energy transfer.
• spherically symmetrical magnetic cores which do not have an electrical quadrupole nuclear moment, only an interaction with time-varying magnetic fields ⁇ B (t) is possible. They have vertical components.
• This relaxation process takes place on a microscopic scale (1 - 10 A) and is described by the longitudinal or spin-lattice relaxation time Ti.
• the last name refers to the idea that the molecular movements realize a thermal "phonon bath", the so-called lattice. The closer the
• Typical values for Ti in liquids are between 10 ⁇ 4 s and 10 s. Due to the much slower movement of the atoms in the crystal lattice, Ti is around two in solids Orders of magnitude (10 ⁇ 2 to 10 3 ) longer.
• This relaxation of the transverse magnetization is characterized by a transverse relaxation time T 2 , which is also known as spin-spin relaxation time.
• T 2 transverse relaxation time
• T 2 is almost independent of the strength of the main magnetic field B 0 .
• relaxation of the transverse magnetization is always associated with the relaxation.
• T 2 can therefore never be larger than Ti.
• T 2 is approximately one order of magnitude below Ti, whereas in solids the "exposure time" x is significantly longer, so that here T 2 «is usually Ti.
• Nuclear magnetic resonance imaging is used, among other things, to obtain spectroscopic information or image information about a substance.
• a combination of nuclear magnetic resonance imaging with Techniques of magnetic resonance imaging (MRI) provide a spatial picture of the chemical composition of the substance.
• Magnetic resonance imaging is, on the one hand, a sophisticated imaging method that is in clinical use worldwide. On the other hand, magnetic resonance imaging is also a very important examination tool for industry and research outside of the medical field. Applications are, for example, investigations of food, quality controls, preclinical investigations of medicines in the pharmaceutical industry or the investigation of geological structures, such as pore sizes in rock samples for petroleum exploration.
• Larmor frequency depends on the strength of the magnetic field and on the magnetic properties of the substance, in particular on the gyromagnetic constant ⁇ of the core.
• the gyromagnetic constant ⁇ is a characteristic quantity for each atom type.
• a substance to be examined, or a person to be examined, is used in the
• the uniform magnetic field is also referred to as the polarization field B 0 and the axis of the uniform magnetic field as the z-axis.
• the individual magnetic moments of the spins in the tissue precess with their characteristic Larmor frequency around the axis of the uniform magnetic field.
• a net magnetization M z is generated in the direction of the polarization field, the randomly oriented magnetic components canceling each other in the plane perpendicular to this (xy plane).
• an excitation field Bi is additionally generated.
• the excitation field Bi is polarized in the xy plane and has a frequency that is as close as possible to the Larmor frequency.
• the net magnetic moment M z can be tilted into the xy plane in such a way that a transverse magnetic magnetization M t arises.
• the transverse component of the magnetization rotates in the xy plane with the Larmor frequency.
• Magnetic resonance spectroscopy enables the measurement of the spatial density distribution of certain chemical components in a material, especially in biological tissue.
• EPSI Echo Planar Spectroscopic Imaging
• MRI magnetic resonance imaging
• MRS magnetic resonance spectroscopy
• NMR imaging methods are used to select layers or volumes which, with the appropriate irradiation of high-frequency pulses and the application of magnetic gradient fields, deliver a measurement signal which is digitized and stored in a one- or multi-dimensional field in a measuring computer.
• the desired image information is obtained (reconstructed) from the recorded raw data by means of a one-dimensional or multi-dimensional Fourier transformation.
• a reconstructed slice image consists of pixels, a volume data set consists of voxels.
• a pixel picture element
• a voxel volume pixel
• the dimensions of a pixel are on the order of 1mm 2 , those of a voxel of 1mm 3 .
• the geometries and dimensions can be variable.
• the lipids cover a fairly broad frequency range, which coincides with that of most metabolites.
• the suppression of also known as lipid suppression is
• Signals from substances that are located outside the brain, but within the layer to be examined, are required because the signals caused thereby can be much larger than signals in brain regions to be examined.
• lipids in the human head are predominantly in the periphery of the skull, one way of suppressing lipids is to not even stimulate the nuclear spins in the periphery.
• a spatially localized spectrum is achieved by signal suppression in regions outside a volume to be examined. Such techniques are referred to as single voxel techniques.
• STEAM single voxel technique
• the known single voxel techniques have the disadvantage that the spatial distribution of chemical substances can only be examined to a limited extent.
• a further disadvantage of the known methods is a limitation of the signal suppression outside of a target volume by imperfections of the slice selection, whereby a low lipid suppression is achieved and / or only a selection of rectangular target volumes is possible.
• the BASING method includes a frequency-selective rephasing pulse in connection with immediately before and after switching gradient pulses with opposite signs, which leads to dephasing.
• fMRI Functional magnetic resonance imaging
• DOH deoxyhemoglobin
• Oxyhemoglobin has a magnetic susceptibility that is essentially that of
• Brain activity is made possible by applying an investigation using functional NMR methods that measure the NMR signal with a time delay (echo time). This is also known as susceptibility-sensitive measurement.
• the biological mechanism of action is in the
• Literature known as the BOLD effect (Blood Oxygen Level Dependent Effect) and leads to up to approx. 5% fluctuations in the image brightness in activated brain regions in susceptibility-sensitive magnetic resonance measurements with a field strength of a static, for example 1.5 Tesla strong magnetic field.
• DOH contrast agent
• other contrast agents can also occur which cause a change in the susceptibility. Suppression of lipid signals is also advantageous here. Frequency-selective lipid presaturation is preferably used.
• the invention has for its object to improve the resolution of the recorded images and to reduce the influence of interference signals.
• this object is achieved in that, after the excitation pulse and a recording of at least one high-resolution image, at least one
• Rephasing pulse which rephases the transverse magnetization, is applied and that further images are then recorded with a lower resolution. It is advantageous to carry out the method in such a way that the high-resolution image is generated before the excitation pulse.
• the high-resolution image prefferably be generated after the excitation pulse.
• the resolution of the further images is varied over time.
• the excitation pulse prefferably be a 90 ° pulse.
• the rephasing pulse is a 180 ° pulse.
• the high-resolution image is taken as a reference image (REF ⁇ hires>) and that the other images have a lower resolution (KEY ⁇ low-res>) and that the resolution of the further images by a link to the reference image (REF ⁇ hi-res>) is improved.
• the high-resolution image in a k-space corresponds to a larger matrix size than the other, lower-resolution images.
• phase correction is carried out on at least some of the lower-resolution images.
• phase correction is carried out in such a way that discontinuities in marginal areas of measurement time intervals are corrected.
• the lower-resolution images are transformed into reconstructed images after the phase correction.
• the reconstructed images are combined with one another in such a way that at least one graphical representation of a relaxation time T 2 * is created.
• the reconstructed images are linked to one another in such a way that a signal-to-noise Ratio is improved.
• a suitable method for obtaining images is a Fourier transformation.
• a fast Fourier transformation (FFT) is suitable for increasing the speed.
• the imaging method is preferably a spectroscopic echo planar imaging method, in particular a repeated three-dimensional (x, y, t) echo planar imaging method, which consists of a repeated application of a two-dimensional echo planar image coding.
• Spatial coding takes place in the shortest possible time, which is repeated several times during a signal drop and is preferably 20 to 100 ms. The repetition of the echo planar coding several times during a signal drop shows a course of the signal drop in the sequence of reconstructed individual images.
• the echo planar imaging according to the invention is very fast. It is therefore particularly suitable for capturing functional images of the entire brain, which would otherwise require much longer acquisition times. With a field strength of, for example, 1.5 T, the time required to record one layer is approximately 100 ms, which, with reasonable coverage of the entire brain in 32 layers, for example, requires a total recording time of approximately 4 seconds.
• the hemodynamic response function (Hae odynamic Response
• Curve should, however, be recorded in a time grid that is sufficient to make a good data adjustment.
• the keyhole (“keyhole”) imaging method provides for separating a signal in the reciprocal k-space into two different areas: firstly in a central area with low spatial frequencies, which is responsible for the contrast in the generating image, and secondly in the outer Regions of the k-space that have a high spatial frequency and contain the essential information about the spatial resolution In the case of several measurements in succession in which contrast changes are examined, it is expedient to base the examination only on the central region of the k-space lay.
• FIG. 1 shows Fig. 1 in four fields - a, b, c and d k-spaces and associated locations and
• T 2 shows a time course of a relaxation time T 2 with an excitation sequence and measurement time windows for its detection.
• partial image a shows a k-space which can be converted into a real space by means of a Fourier transformation, which is shown in partial image b.
• sub-picture c only 16 central lines of k-space are recorded.
• the low-resolution image shown in partial image d is produced by a Fourier transformation.
• a high-resolution reference image (REF ⁇ hi-res>) is first recorded for the keyhole method. This image is obtained by evaluating the data of an entire k-space. Keyhole images (KEY ⁇ low-res>) are then recorded.
• the high-resolution images can be represented as follows:
• the dynamic images are generated through central areas of the individual recorded images, peripheral areas of the reference image leading to a spatial resolution.
• FIG. 1 A sequence diagram is shown in FIG. 1
• echo planar imaging takes place after an excitation pulse, preferably a 90 ° pulse. However, it is also possible for the echo planar imaging to take place before the excitation pulse.
• the relaxation time T 2 * is recorded.
• the first picture is taken with a high-resolution image, which takes a longer time.
• preferred time acquisition windows are marked separately.
• a high-resolution HI-RES EPI image is taken immediately after the excitation pulse.
• the keyhole images are reconstructed to produce fully resolved images by means of a phase correction.
• phase correction methods are suitable for this.
• RF Radio Frequency
• This modulated MR signal is now scanned for a sufficiently long time, ie approximately until Mj «is completely dephased, and in sufficiently short time intervals.
• Steps 2 and 3 are repeated as often as the sectional image should have halftone dots, ie (N ⁇ * N x ) times. With each repetition, the gradient strength G or the duration of the application is varied, as is necessary for correct spatial coding.
• the steps mentioned are sufficient for the implementation.
• the second and fourth steps can be omitted.
• the result is spatially resolved frequency spectra from which the relative concentration of individual chemical components can be calculated. These can be differentiated because the effective magnetic field at the location of a nucleus and thus its precession frequency depend on its mother molecule, which shields the external magnetic field to a greater or lesser extent.
• protons as resonant nuclei for the investigation of biological tissue.
• the very strong signals of water and lipids with concentrations in the double-digit molar range are to be suppressed in order to avoid the interesting ones
• the phase coding can be partially connected to the reading of the MR signal.
• the advantage lies in a factor of N x shortened measuring time.
• the measurement data are reinterpreted in a suitable manner, preferably as (k x , k ⁇ ) layers at different times t. Formally, this is done by rearranging the measurement data.
• the data can then be processed using the usual methods of conventional spectroscopic imaging.
• the coordinates (k x , k ⁇ ) are only shown as examples. The person skilled in the art can select suitable (k x , k ⁇ ) for each examination.

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• Physics & Mathematics (AREA)
• High Energy & Nuclear Physics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)

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• 1999
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