Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-06-30
2001-06-26
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06252401
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for reconstructing a planar tomogram of an examination subject from magnetic resonance signals in an inhomogeneous magnetic field with a magnetic resonance device that has a known inhomogeneous main magnetic field and/or at least one known non-linear gradient magnetic field.
2. Description of the Prior Art
Imaging by means of magnetic resonance techniques makes use of the frequency dependency of the magnetic resonance signal on the magnetic field strength for purposes of spatial resolution. Common methods for reconstructing tomograms presume a homogenous main magnetic field and strictly linear gradient magnetic fields. Inhomogeneities of the main magnetic field cause distortions or deformations in the frequency encoding direction. Given non-linearities of the gradient fields, the distortions are present not only in the tomographic image plane, but also perpendicular thereto, given slice excitations with a selection gradient. The distortions or deformations relate to the geometric position of the reconstructed spin density in the examination subject and likewise to the reconstructed image intensity.
Heretofore, distortions have been corrected in the image plane, exclusively. PCT Application WO 95/30908 teaches a method wherein a generalized Fresnel transformation is performed in the readout direction (GFT reconstruction). The GFT transformation takes into account a known location dependency of the main magnetic field in the readout direction.
The method described in German OS 195 40 837 uses two auxiliary data records which describe a shifting of the measured location relative to an actual location of a signal origin. From one of the auxiliary data records, a corrected auxiliary data record is created. In an image data record, a location correction then occurs in a first coordinate direction using the corrected auxiliary data record. A first intensity correction also occurs. Subsequently, a location correction in the second coordinate direction occurs, along with a second intensity correction. Alternatively, the location correction can occur by a Fresnel transformation of the raw data record including the corrected auxiliary data.
The article “Simulation of the influence of magnetic field inhomogeneity and distortion correction in MR imaging” (J. Weiss, L. Budinsky,
Magnetic Resonance Imaging,
8, 1990: 483-489) teaches the correction of image distortions by a postprocessing of a conventionally acquired image. The information about magnetic field inhomogeneities which is required for this is obtained from the phase of separately registered spin-echo images. Deformations in the slice direction, i.e. slice curvatures, cannot be corrected with this method.
The slice curvature problem with respect to the main magnetic field non-homogeneity conventionally has been avoided by a 3D imaging wherein a spatial resolution occurs in the slice direction by means of an additional phase encoding. The phase encoding is relatively insensitive to main magnetic field inhomogeneities relating to deformations. The longer measuring time compared to multi-slice pickups and the resulting higher susceptibility to movement artefacts are disadvantageous, however. There are consequently fundamental restrictions in the application of certain techniques which are based on a rapid imaging, such as contrast agent examination or dynamic studies. Deformations relating to non-linear gradient fields heretofore have remained uncorrected.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for reconstructing a planar tomogram from magnetic resonance signals in inhomogeneous magnetic fields, with which method deformations in the direction of the main magnetic field also can be corrected.
The object is achieved in a method in accordance with the invention having the following steps. Original image elements are generated by means of multi-slice excitation, these original image elements being generated from magnetic resonance signals of original volume elements that are arranged in the examination subject in several curved layers that are disposed adjacent to one another succession. The spatial position of the original image elements is determined by means of the known magnetic fields. Image elements of a planar tomogram are generated, these image elements being representative of the volume elements of a planar slice that is situated within the curved slice, from the original image elements which are arranged in an environment of the respective image elements.
The term “original volume element” means the measured, i.e. distorted, volume element (voxel). “Volume element “means the real, undistorted voxel, which agrees with the measured one given ideal magnetic fields.
First, several neighboring slices which are deformed by non-ideal magnetic fields are measured. From this, at least one planar slice which is free of deformations is reconstructed. Altogether, the multi-slice excitation covers a 3D volume which, in contrast to the ideal case wherein the main magnetic field is homogeneous and the gradient magnetic fields are strictly linear, does not form a cuboid, but instead forms a region bounded by curvilinear surfaces. An advantage of the inventive method is a complete correction of deformations due to basic field inhomogeneities and gradient field non-linearities in all three spatial directions. This produces a functionality as in the ideal case. The position of the image elements indicates the actual anatomical position. Thus, for example, distance measurements in the entire image region are anatomically accurate. The slices of 3D-corrected measurements can then be graphically positioned onto 2D-corrected or 3D-corrected reference images with straight auxiliary lines, without further difficulty.
In one embodiment the image elements have respective intensity values, and the original image elements have respective original intensity values, and the intensity values are generated from surrounding original intensity values which are weighted with allocated weight factors. The preparation of the planar tomograms from the measured 3D volumes which are deformed due to non-ideal magnetic fields occurs using reconstruction methods which are derived from the known multiplanar reconstruction methods. The required precise spatial position of the original volume elements is obtained from coefficient tables of an allocated series expansion of the magnetic field, as are already used in the deformation correction in an image plane. Given main magnetic field inhomogeneities and gradient field non-linearities, the position of an image element is shifted. Together with the shift in the tomographical plane itself, which already can be calculated with the algorithms of the 2D deformation correction, the spatial position can be determined for each image element in a slice and for all neighboring slices using the coefficient tables. The spatial position of the original image elements enter into the method for multiplanar reconstruction as input values, in order to reconstruct a planar slice therefrom.
In another embodiment the weighting factors are determined by corresponding volume contents of the original volume element and the volume element. Given sharp deviations of the magnetic fields from the ideal distribution, the different volume contents of the original volume elements and the volume elements are taken into consideration in the methods for multiplanar reconstruction.
In a further embodiment, the volume contents are taken into account in the weighting factors by determining the weighting factors as a ratio of the volume contents of the respective volume elements relative to the volume contents of respective original volume elements.
In another embodiment, the weight factors are respectively formed by the volume contents of an overlap volume of the volume element and the original volume element relative to the original volume element. All original image elements that surround the image element to be reco
Faber Roland
Werthner Harald
Fetzner Tiffany A.
Oda Christine
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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