Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-11-13
2003-11-25
Lefkowitz, Edward (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06653834
ABSTRACT:
The invention relates to a magnetic resonance imaging method in which the k-space is sampled by the acquisition of magnetic resonance signals.
The invention also relates to a magnetic resonance imaging system that is arranged to carry out such a magnetic resonance imaging method.
A magnetic resonance imaging method and a magnetic resonance imaging system of this kind are known from U.S. Pat. No. 5,713,358.
According to the known magnetic resonance imaging method the k-space is subdivided into central regions and peripheral regions. The central regions are sampled at a higher rate, that is, more often per unit of time, than the peripheral regions. The magnetic resonance image is reconstructed from the most recently acquired magnetic resonance signals from the central regions, supplemented by magnetic resonance signals from peripheral regions that have been acquired somewhat longer ago. The known magnetic resonance imaging method succeeds in faithfully reproducing fast changes of the coarse structures in the magnetic resonance image. For fine structures only slow changes can be faithfully reproduced, because the magnetic resonance signals from the peripheral regions of the k-space, that is, magnetic resonance signals having comparatively large values of the wave number and hence short wavelengths, are refreshed at a low frequency only.
It is an object of the invention to provide a magnetic resonance imaging method that enables the formation of magnetic resonance images that are even more faithful renditions of the object to be imaged.
This object is achieved by means of a magnetic resonance imaging method in accordance with the invention wherein
a selected region is adjusted on the basis of a priori known information concerning a part of an object to be imaged,
the selected region of the k-space is sampled, and
the magnetic resonance image is reconstructed from the magnetic resonance signals that have wave vectors in the sampled selected region in the k-space.
The invention is based on the recognition of the fact that a relationship exists between properties, such as the three-dimensional shape, the dimensions and the temporal contrast variations of or in the part of the object to be imaged and the shape of the region in the k-space with the wave vectors of the magnetic resonance signals of a considerable signal level that are generated by RF excitation in the object to be examined. This is because the magnetic resonance signals represent the Fourier transform of the part of the object to be imaged. The term k-space denotes the reciprocal space of the geometrical space in which the object to be imaged is situated; the k-space is notably the reciprocal space of the geometrical range of the magnetic resonance signals of the receiving antennas, that is, the so-called field of view. Positions in the k-space represent the wave vectors of the magnetic resonance signals. A part of the object that completely and uniformly fills the field of view of the magnetic resonance imaging system generates magnetic resonance signals having wave vectors that are concentrated in a small region at the center of the k-space. A part of the object that is much smaller than the field of view generates magnetic resonance signals that have wave vectors in an extensive region in the k-space. A part of the object that has an elongate three-dimensional shape gives rise to magnetic resonance signals that are concentrated in a region which is oriented in a direction in the k-space that corresponds to the direction that extends transversely of the elongate three-dimensional shape in the geometrical space. The magnetic resonance imaging method in accordance with the invention takes into account the three-dimensional shape of the part of the object to be examined. Consequently, the magnetic resonance imaging method succeeds in accurately sampling, in as far as necessary, the relevant positions in the k-space while avoiding the unnecessary sampling of positions in the k-space. In this context relevant positions in the k-space are to be understood to mean vectorial k values with magnetic resonance signals that are necessary to reconstruct a magnetic resonance image that has a high diagnostic quality and in which small details of low contrast are suitably visibly reproduced. The magnetic resonance imaging method in accordance with the invention thus succeeds in forming magnetic resonance images at a high rate and with a high diagnostic quality.
Preferably, magnetic resonance images of a high diagnostic quality are formed of a patient to be examined.
Furthermore, the scanning or sampling of the selected region constitutes effective filtering in respect of the wave number (or the wavelength) of the magnetic resonance signals; this is because the magnetic resonance signals acquired in the selected region of the k-space constitute a component with a given range of wave numbers of the totality of generated magnetic resonance signals as determined by the choice of the selected region. Effective filtering is thus adjusted by adjustment of the selected region. This adjustable effective filtering enables given structures in the magnetic resonance image to be intensified or suppressed at option.
The invention is particularly suitable for use in conjunction with a three-dimensional magnetic resonance imaging method such as, for example, 3D FFE (Fast Field Echo). During such an FFE sequence magnetic resonance signals are generated by exciting spins in the object to be examined, for example a patient to be examined, by means of an RF pulse which rotates the spins through a given flip angle relative to the steady magnetic field. Successive gradient echoes are then generated by application of a read-out gradient in the read-out direction and by application of mutually perpendicular phase encoding gradients in the direction perpendicular to the read-out gradient. The individual plane in the k-space in which the central region is selected preferably extends perpendicularly to read-out direction. For example, the read-out direction in the k-space is the k
x
direction and the individual plane containing the selected region is situated in the (k
y
,k
z
) plane. The selected region in the k-space is sampled at a comparatively low rate, that is, relative to the sampling rate in the read-out direction, that is, transversely of the plane in which the selected region is situated. At individual points in the selected region (in the (k
y
,k
z
) plane) each time a large number of points is sampled on a line or line segment in the read-out direction.
These and other aspects of the invention will be elaborated on the basis of the following embodiments which are defined in the dependent claims.
Preferably, the magnetic resonance imaging method in accordance with the invention takes into account the shape and/or the orientation of the part of the object to be imaged in order to select the region to be sampled in the k-space. The selected region to be sampled in the k-space can thus be accurately made to correspond to the Fourier transform of the part of the object to be examined that is to be imaged. Consequently, exactly the magnetic resonance signals that are necessary for the reconstruction of the magnetic resonance image of the part to be imaged are acquired. It has been found that mainly the shape and the orientation of the selected region in the k-space have a strong effect on the adjustment of the effective filter. The intensification of desired parts in the magnetic resonance image and the suppression of undesirable parts in the magnetic resonance image can be suitably controlled on the basis of the shape and the orientation of the selected region in the k-space. This adjustment of the effective filtering is based on the recognition of the fact that usually a difference exists between the shape and orientations of respective parts of the object whose image is either desired or not desired.
Preferably, in accordance with the invention different sampling densities of magnetic resonance signals are used in different directions in the selecte
Beck Gabriele Marianne
Harvey Paul Royston
Hoogeveen Romhild Martijn
Van Den Brink Johan Samuel
Koninklijke Philips Electronics , N.V.
Shrivastav Brij B.
Vodopia John
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