Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-06-07
2003-02-04
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000, C324S306000, C600S419000
Reexamination Certificate
active
06515477
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance signal collection method and apparatus and a magnetic resonance imaging apparatus, and more particularly to a method and apparatus for successively collecting magnetic resonance signals along a plurality of radial trajectories that pass through the center of a k-space, and a magnetic resonance imaging apparatus that employs such a magnetic resonance signal collection apparatus.
Magnetic resonance imaging by radial scanning involves, as exemplarily shown in
FIG. 1
, collecting a magnetic resonance signal of an imaged object along each of a plurality of radial trajectories passing through the center of a k-space, or a frequency space, performing one-dimensional inverse Fourier transformation on the magnetic resonance signal for each trajectory to generate projections in a plurality of directions of the imaged object, and back-projecting the projections to reconstruct a tomographic image.
To form the radial trajectories, a gradient magnetic field for reading out the magnetic resonance signals is generated by a vector sum of two gradient magnetic field components whose gradient directions are orthogonal to each other. By generating the two gradient magnetic field components using the sine (sin &thgr;) and cosine (cos &thgr;) of the trajectory angle &thgr;, the radial trajectories passing through the center of the k-space are formed.
Since all the trajectories pass through the center of the k-space, the time difference is small among data that relate to generation of a substantial portion of a reconstructed image. Thus, radial scanning is suitable for imaging motion of a moving imaged object with good temporal resolution.
Radial scanning includes sequential scanning and interleaved scanning. Assuming that k-space data are to be collected using eight trajectories, sequential scanning collects the data in the order of a sequence of trajectories
1
through
8
having an angle &thgr; sequentially increasing counterclockwise, as exemplarily shown in
FIG. 2
, to collect the data of all the trajectories during a round.
On the other hand, interleaved scanning collects, for example, the data along odd-numbered trajectories
1
,
3
,
5
and
7
during a first round, and collects the data along even-numbered trajectories
2
,
4
,
6
and
8
during a second round, as shown in
FIGS. 3 and 4
, respectively. The trajectories in the second round are interleaved with those in the first round and all data are completed by the data collection over the two rounds.
Sequential scanning exhibits a large time difference between the first and last trajectories, giving rise to false images such as streaking artifacts in the reconstructed image due to inconsistency between the data of those trajectories in a moving imaged object.
Interleaved scanning has a smaller number of trajectories per round, i.e., a smaller factor value, and therefore the time difference is decreased between the first and last trajectories in each round, reducing false images. However, since there remains a sequential characteristic within each round, scan conditions must be adjusted such as by modifying the factor value according to the motion velocity to effectively reduce the false images.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a magnetic resonance signal collection method and apparatus that suppress false images caused by motion of an imaged object, and a magnetic resonance imaging apparatus that employs such a magnetic resonance signal collection apparatus.
In accordance with a first aspect of the invention, there is provided a magnetic resonance signal collection method for successively collecting magnetic resonance signals along a plurality of radial trajectories passing through the center of a k-space, comprising randomizing the order of selecting the trajectories.
In accordance with a second aspect of the invention, there is provided a magnetic resonance signal collection method for successively collecting magnetic resonance signals along a plurality of radial trajectories passing through the center of a k-space, comprising setting an angular difference between first and next trajectories at 90°, and then defining new trajectories alternately in two adjacent regions formed by dividing the k-space by the first and next trajectories so that the angular difference between adjacent trajectories is a repeatedly bisected angle in sequence.
In accordance with a third aspect of the invention, there is provided a magnetic resonance signal collection apparatus for successively collecting magnetic resonance signals along a plurality of radial trajectories passing through the center of a k-space, comprising view control means for randomizing the order of selecting the trajectories.
In accordance with a fourth aspect of the invention, there is provided a magnetic resonance signal collection apparatus for successively collecting magnetic resonance signals along a plurality of radial trajectories passing through the center of a k-space, comprising view control means for setting an angular difference between first and next trajectories at 90°, and then defining new trajectories alternately in two adjacent regions formed by dividing the k-space by the first and next trajectories so that the angular difference between adjacent trajectories is a repeatedly bisected angle in sequence.
EFFECT
According to the present invention, the order of selecting trajectories is randomized. Alternatively, the angular difference between first and next trajectories is set at 90°, and then new trajectories are defined alternately in two adjacent regions formed by dividing the k-space by the first and next trajectories so that the angular difference between adjacent trajectories is a repeatedly bisected angle in sequence. Thus, the sequential characteristic of trajectories is eliminated, thereby reducing inconsistency in data between the trajectories caused by motion of an imaged object.
Therefore, the present invention can provide a magnetic resonance signal collection method and apparatus that suppress false images caused by motion of an imaged object, and a magnetic resonance imaging apparatus that employs such a magnetic resonance signal collection apparatus.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.
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patent: 6188922 (2001-02-01), Mistretta et al.
patent: 6215306 (2001-04-01), Tsai et al.
patent: 6222365 (2001-04-01), Taniguchi et al.
patent: 6400152 (2002-06-01), Cline et al.
patent: 1079237 (2001-02-01), None
Bushong, Stewart C. “Magnetic Resonance Imaging Physical and Biological Principles” Second Edition 1996 textbook published by Mosby-Year Book, Inc. pp. 203-205.*
Sersa et al., Article “Excitation of Arbitrary Shapes by Gradient Optimized Random Walk in Discrete k-Space” Magnetic Resonance In Medicine, vol. 37, No. 6 pp. 920-931 (Jun.) 1997.*
Mason et al., Article “A Method to Measure Arbitrary k-space Trajectories for Rapid MR Imaging” Magnetic Resonance Medicine vol. 38 pp. 492-496 1997.
Tasaka Nobuyuki
Tsukamoto Tetsuji
Fetzner Tiffany A.
Ge Yokogawa Medical Systems, Limited
Kojima Moonray
Lefkowitz Edward
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