Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-12-17
2001-12-04
Williams, Hezron (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06326786
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for obtaining images by means of magnetic resonance (MR) of an object placed in a static magnetic field comprising the following steps
generation of an excitation RF pulse in a part of the body,
measurement of a plurality of sets of MR signals using a plurality of receiver coils along a trajectory in k-space comprising a first plurality of lines by application of a read gradient and other gradients,
reconstruction of a final image from a combination of the plurality of sets of MR signals measured and distance dependent sensitivities of the plurality of receiver coils. The invention further relates to an MR apparatus arranged for performing such method. In this patent application pixels mean picture elements of a digital image, voxels mean volume-elements of a three-dimensional digital object.
Such a method is known from the article “Simultaneous Acquisition of Spatial Harmonic (SMASH): Fast Imaging with radio frequency Coil Array” by D. K. Sodickson et al, published in Magnetic Resonance in Medicine, vol. 38, page 591-603, 1997. The known method is used, for example, in real time cardiac imaging of human beings. To reduce the acquisition time of MR data in the known method a sub-encoding data acquisition scheme is used wherein the plurality of sets of MR signals are simultaneously measured using the plurality of receiver coils along the trajectory in k-space containing the first plurality of lines using the read gradient and the other gradients. The number of lines corresponds to a reduced number phase-encoding steps in comparison with a number of phase encoding steps in conventional Fourier MR imaging. The receiver coils may be arranged in an array of surface coils. A final MR data set is determined from a specific distance sensitivity function of the set of receiver coils and the sets of MR signals measured. The final MR data set then contains the information of the number of lines of the conventional Fourier MR imaging. The final image is then reconstructed by transforming the final MR data set. The number of sets of MR signals may be equal to the number of receiver coils in the array. Furthermore, the specific distance sensitivity function of the set of receiver coils must have a sinusoidal shape. By the simultaneous measurement of MR data is the acquisition time is reduced. The reduction factor is determined by the number of lines in k-space corresponding to the final image and the number of lines which the sub-encoding data acquisition employs. The reduction in the acquisition time may, for example, enable application in real time cardiac imaging or functional imaging. A disadvantage of the known method is that it may be limited to only a few positions of a field of view and a limited number of orientations of the slice of the object to be image because of the specific sensitivity function of the set of coils. The field of view is defined by a distance in a phase encoding direction covered by the trajectory in the k-space.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide an MR imaging method with an improved degree of freedom in the choice of field of view and orientation of the slice to be imaged. To this end, the method in accordance with the invention is characterised in that for the reconstruction of the final image the method comprises a further step of
reconstruction of receiver coil images from each set of MR signals measured respectively, and in
that the final image is reconstructed from a combination of the receiver coil images and the distance dependent sensitivities of the receiver coils. In t his way the reconstruction of the final image does not depend on a specific sensitivity function of the sets of receiver coils and can be applied for an arbitrary set of receiver coils and the restrictions on the size of the field of view and the orientation of the slice to be image are less severe. Furthermore, non-integer reduction factors can be chosen. This option gives a possibility to shift aliasing artefacts to less important parts of the image. Simultaneous or partly simultaneous measurement of the MR data sets yields a reduction in the acquisition time and a possibility of fast MR imaging compared to conventional MR-imaging.
A particular version of the method according to the invention comprises a step of determining an image vector of the final image from a combination of a generalised inverse of a sensitivity matrix and a receiver coil image vector, wherein an image vector component represents a value of a tissue contrast function at a position of a volume-element selected from a first plurality of equidistant volume-elements in a first plurality of adjacent fields of views, an element S(i,j) of the sensitivity matrix represents a sensitivity at the position of the selected volume-element with respect to a receiver coil selected from the first plurality of receiver coils and a receiver coil image vector component represents a pixel value of a receiver coil image corresponding to the selected receiver coil, a position of the pixel in the receiver coil image corresponding with the position of the selected volume-element in the selected field of view. The generalised inverse of a matrix S is defined as a matrix product (S
H
S)
−1
S
H
, wherein S
H
represents a complex conjugate transpose of the matrix S. The generalised inverse or pseudo inverse is known in applied mathematics for the following property
∥S
(
S
+
a
)−
a
∥=min
x
∥Sx−a∥.
That is, given a vector equation Sx=a, which does not have an exact solution for x due to over determination, the pseudo inverse yields a vector that best fits that equation in the above sense. In the reconstruction method according to the invention use is made of this minimisation property. The pixel wise reconstruction method provides a possibility to include an actual degree of aliasing in the reconstruction method in order to reduce the different types of aliasing. As a result the method according to the invention provides substantially aliasing free final images in case of both non-integer and integer reduction factors.
A further version of the method according to the invention is characterised in that a reduction factor of acquisition of the sets of MR signals amounts to a real value smaller than or equal to the number of receiver coils. The reduction factor is defined as the factor with which the distance between the lines of the trajectory in k-space is increased as compared with the distance between adjacent lines in k-space in conventional Fourier MR imaging. As a result the acquisition time of the sets of MR signals is reduced proportionally. In cardiac imaging, for example, regions of lung tissue of the human being can be often excluded from the image reconstruction due to a negligible signal contribution resulting from the choice of an optimal reduction factor. An optimal choice of the reduction factor can be chosen such that for example a high intensity fold-over of a back chest wall of the human being is directed off the heart in to the lung regions. The optimal choice of the reduction factor may result in different type of aliasing in the final image.
A further version of the method according to the invention is characterised in that the method comprises a step of determining a dimension of the image vector from a number of the first plurality of equidistant voxels in a final field of view of the final image, a distance between subsequent voxels being equal to the field of view. An actual degree of aliasing can thus be determined and taken into account in the reconstruction process of the final image. As a result fold-over artefacts in the final image, are reduced. In case of a non-integer reduction factor the degree of aliasing varies in different pixels of the final image because the actual number of the volume-elements or voxels varies that contributed to a picture element of the single receiver coil images. In general, a pixel value of a single receiver coil
Boesiger Peter
Pruessmann Klaas P.
Scheidegger Markus B.
Weiger Markus
U.S. Philips Corporation
Vargas Dixomara
Vodopia John
Williams Hezron
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