Tuners – Tuner unit with electromagnetic operator – Predetermined center frequency selector
Reexamination Certificate
2002-04-19
2004-03-23
Gutierrez, Diego (Department: 2859)
Tuners
Tuner unit with electromagnetic operator
Predetermined center frequency selector
Reexamination Certificate
active
06710686
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for the fast acquisition of a magnetic resonance image, wherein magnetic resonance signals from an imaging region are acted on by magnetic gradient fields that define a spatial frequency space so that the spatial frequency space is occupied with magnetic resonance signals having trajectories that are radially directed and proceed through a projection center.
2. Description of the Prior Art
A method of the above type is described in the book by Heinz Morneburg, “Bildgebende Systems für die medizinische Diagnostik,” Publicis MCD Verlag, 3
rd
edition, 1995, pages 176-183.
The localization of the magnetic resonance signals from a subject to be imaged that is necessary for the imaging ensues by superimposition of a uniform magnetic basic field with a magnetic field gradient. When a two-dimensional examination subject is assumed wherein the magnetization aligned by the basic magnetic field was tilted out of the equilibrium position by means of a radio-frequency pulse, then the Larmor frequency is constant in stripes perpendicular to the gradient direction when a gradient field is applied. In other words, the signal amplitude of the magnetic resonance signal as a function of the frequency respectively corresponds to the sum of all spins in this stripe, i.e. represents the projection of the transverse magnetization onto the field gradient. The signal that is detected by the measurement, however, is the time signal that is registered by means of suitable antennas. During further processing, an alternating voltage similar to the Larmor frequency is admixed with the received magnetic resonance signal, so that the signal of the difference frequency (phase-sensitive rectification) directly indicates the magnetic moment of the precessing nuclear magnetization. With the introduction of a “normalized time” or spatial frequency k established by k=&ggr;∫G·dt, where &ggr; is the gyromagnetic ratio, G is the magnetic field gradient and t is the time during which the magnetic field gradient is superimposed on the magnetic basic field, the measured signal and the projection prove to be a Fourier transform pair. A set of magnetic resonance signals that are generated in successively stepped gradients is, accordingly, the two-dimensional Fourier transform of the magnetization. The Fourier space, spatial frequency domain or k-space inverse to the spatial or image domain wherein the test subject is located is thus scanned with a raster of polar coordinates. The graphic presentation of the magnetization, however, ensues in Cartesian coordinates. The magnetic resonance image then can be generated with two different methods. In the first method, the received magnetic resonance signals are re-interpolated onto a Cartesian grid and are subjected to a two-dimensional Fourier transformation. In a second method, the received magnetic resonance signals are subjected to a one-dimensional Fourier transformation, as a result of which projections are calculated and the image is then reconstructed by means of a filtered back-projection.
For imaging three-dimensional subjects, the above considerations for two-dimensional imaging are expanded by one dimension. When it is assumed in the two-dimensional case that N projections are registered, N
2
projections must be registered in the three-dimensional case given the same resolution in the third dimension. If it is desired to avoid the lengthening on the measurement time resulting therefrom and to graphically present only one slice, this slice is cut (defined) from the spatial test subject by means of “selective excitation”, this then being two-dimensionally measured.
The radial scanning of the spatial frequency domain mentioned above is of interest for magnetic resonance imaging because it enables shorter repetition times in the image sequence compared to the occupation of the spatial frequency domain in Cartesian coordinates and is less susceptible to motion artifacts. The shortening of the repetition time arises from the absence of the additional phase coding steps required given Cartesian scanning directions.
Various other methods are known in general for reducing the exposure times in magnetic resonance imaging that are based on parallel and simultaneous data pickup of the magnetic resonance signals with a number of antennas of an antenna array (PPA, or partial parallel acquisition, methods). The number of lines in the spatial frequency domain to be acquired thus can be reduced in conformity with the number of antennas utilized.
One such method is described in the article by Daniel K. Sodicksen, Warren J. Manning, “Simultaneous Acquisition of Spatial Harmonics (SMASH): Fast Imaging with Radiofrequency Coil Arrays”, which appeared in Magnetic Resonance in Medicine, Volume 38, 1997, pages 591 through 603. The location coding or conditioning of the spins therein ensues with phase coding gradients and frequency coding gradients. The spatial frequency space is thus scanned on a Cartesian grid. The magnetic resonance signals, however, are conditioned such that scanning in phase coding direction is only incomplete. The excited magnetic resonance signals, however, are simultaneously received with an antenna array composed of a number of antennas. Because the individual k-space rows are multiplied by weighting factors that are determined from the sensitivity profiles of the antennas (which are assumed to be known), the missing k-space rows can be synthesized, so that k-space is completed in the phase coding direction. A gain in measuring time thus is achieved that corresponds to the phase coding steps that are saved.
The article by Klaas P. Pruessmann, Markus Weiger, Markus B. Scheidecker and Peter Boesiger, “Sense: Sensitivity Encoding for Fast MRI”, which appeared in Magnetic Resonance in Medicine, Volume 42, 1999, pages 952 through 962, describes a PPA method wherein the signals received from the individual antennas, that are incomplete in the phase coding direction, are first subjected to a Fourier transformation. The partial images that are generated in this way, however, exhibit convolutions in the phase coding direction due to the under-scanning. The individual images are then superimposed form a convolution-free overall image using the known sensitivity profiles.
The article by Mark A. Griswold, Peter M. Jakob, Mathias Nittka, James W. Goldfarb and Axel Haase, “Partially Parallel Imaging With Localized Sensitivities (PILS)”, which appeared in Magnetic Resonance in Medicine, Volume 44, 2000, pages 602-609, describes a further PPA method. It is thereby assumed that the individual antennas in the antenna array have only a limited sensitivity profile. It is then adequate for the antennas to be characterized by only two parameters: the position of the center of the sensitivity zone of the corresponding antenna in the overall field of view and the width of the sensitivity zone around this center.
SUMMARY OF THE INVENTION
An object of the present invention is based on the object of specifying a method for fast acquisition of a magnetic resonance image wherein the measuring time that has already been inherently shortened is reduced further given radial scanning of the spatial frequency space.
This object is achieved in a method in accordance with the invention wherein the imaging zone is subdivided into sub-regions; with an antenna of an antenna array allocated to each sub-region. Each antenna has a known position relative to the projection center; and the antennas simultaneously receive the magnetic resonance signals and respectively form reception signals from the received magnetic resonance signals according to their sensitivity. The number of radially directed trajectories is of such a size so that only the sub-zones—regardless of their spatial position in the imaging zone—can be unambiguously reconstructed; from the respectively received signals. The reception signals, taking the positions of the individual receiving antenna into consideration, are transforme
Mertelmeier Thomas
Oppelt Arnulf
Schiff & Hardin & Waite
Vargas Dixomara
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