Electricity: measuring and testing – Particle precession resonance
Reexamination Certificate
2001-03-07
2002-09-17
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
C324S307000
Reexamination Certificate
active
06452387
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to magnetic resonance imaging (MRI), and more particularly, the invention relates to steady-state magnetic resonance imaging sequences and a method of establishing nuclear magnetization in a steady-state position to thereby accelerate MRI signal acquisition.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Magnetic resonance (MR) imaging is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During a MR experiment, each nuclear spin responds to four different effects—precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MR experiments, a combination of these effects occurs periodically.
Refocused steady-state free precession (SSFP) sequences have recently gained popularity in magnetic resonance imaging, due to improved gradient hardware. One drawback with such sequences is the slow and often oscillatory signal progression as a steady-state is established, as shown in
FIG. 1
a
. Imaging during this transient period can result in image artifacts. Alternatively, waiting for a steady-state reduces the scan-time efficiency of the method. An objective of the present invention is to “catalyze ” or speed up the evolution of a steady-state as in
FIG. 1
b
. An ideal catalyzing sequence would achieve the steady-state much more quickly and with no oscillation.
The steady-state nuclear magnetization in steady-state sequences is a non-trivial function of many parameters. The present invention is directed to generating a sequence that catalyzes the steady-state magnetization based on the steady-state and the transient response in MRI sequences.
SUMMARY OF THE INVENTION
In accordance with the invention, a steady-state condition for tipped nuclear spins is accelerated or catalyzed by first determining magnetization magnitude of the steady-state and then scaling magnetization along one axis (Mz) to at least approximate the determined magnetization magnitude. Then the scaled magnetization is rotated to substantially coincide with a real-valued eigenvector extension of the tipped steady-state magnetization. Any error vector will then decay to the steady-state condition with reduced oscillation.
In one embodiment, the magnetic resonance imaging utilizes steady-state free precession (SSFP). The scaling and rotating steps are followed by the steps of applying read-out magnetic gradients and detecting magnetic resonance signals from the tipped nuclear spins. The magnetization magnitude is determined by eigenvector analysis, and the eigenvector extension is a real-valued eigenvector determined in the analysis.
REFERENCES:
“Characterization and Reduction of the Transient Response in Steady-State MR Imaging” by Brian A. Hargreaves, Shreyas S. Vasanawala, John M. Pauly, and Dwight G. Nishimura Magnetic Resonance in Medicine vol. 46 pp. 149-158 Jul. 2001.*
Deimling et al., “Magnetization Prepared True FISP Imaging, ”Proceedings of the 2nd Annual Meeting of SMR, San Francisco, 1994, p. 495.
Nishimura et al., “Analysis and Reduction of the Transient Response in SSFP Imaging,” Proceedings of the 8th Annual Meeting of ISMRM, Denver, Apr. 1, 2000, p. 301.
Pauly et al., “Parameter Relations for the Shinnar-Le Roux Selective Excitation Pulse Design Algorithm,” IEEE Trans Med Imaging, vol. 10, No. 1, 1991, pp. 53-54.
Vasanawala et al., “Fluctuating Equilibrium MRI, ” Magnetic Resonance in Medicine, 42:876, 1999, 2 pp.
Vasanawala et al., “Linear Combination Steady-State Free Precession MRI,” Magnetic Resonance in Medicine, 43:82, Jan. 2000, 2 pp.
Hargreaves Brain A.
Nishimura Dwight G.
Pauly John M.
Vasanawala Shreyas
Board of Trustees of the Leland Stanford Junior University
Fetzner Tiffany A.
Lefkowitz Edward
Woodward Henry K.
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