Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1988-06-01
1990-04-10
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324312, G01R 3320
Patent
active
049163965
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an improved MRI (magnetic resonance imaging) method and, more particularly, to an MRI method by which a scan is made in a short time in collecting data by Fourier transform.
BACKGROUND ART
An MRI (magnetic resonance imaging) instrument comprises a magnet portion, a transmitting-and-receiving portion for applying RF pulses or RF electromagnetic radiation to an object under examination placed in the magnetic field produced by the magnet portion and for detecting the NMR signal from the object, and a control-and-image processing portion which controls the operation of the transmitting-and-receiving portion and the operation of the magnet portion and processes the obtained data to image the object. The magnet portion consists of static magnetic field coils for generating a uniform and static magnetic field H.sub.0 and gradient magnetic field coils for setting up magnetic fields which are parallel with the static magnetic field H.sub.0 and have linear gradients in the directions of the x, y, z axes, respectively.
The control-and-image processing portion of the MRI instrument constructed as described above produces a pulse sequence by saturation recovery or repetitive inversion recovery method. Data is collected by making use of Fourier transform, and an image is reconstructed. At this time, data is collected from a square or rectangular region in a two-dimensional Fourier plane as shown in FIG. 9(a), by scanning it. When Fourier transform is employed, data concerning the nuclear spin density of the object is directly obtained by two-dimensional Fourier transform.
In FIG. 9(a), as the distance from the origin O increases, the spatial frequency increases. Let f be the highest spatial frequency in the horizontal and vertical directions. The highest spatial frequency in the diagonal direction is 2.sup.1/2 .multidot.f. Therefore, the resolution differs from direction to direction. Consequently, the obtained image becomes nonuniform in a peculiar way. The resolution in the diagonal direction is excessively high at the sacrifice of signal to noise ratio. In order to reconstruct an image free of these problems, a circular window function is applied to the data region as shown in FIG. 9(b). Specifically, the data contained in the hatched portion is discarded, and only the data contained in the circle is used.
In the conventional MRI instrument, a circular window function is applied to the data region to reconstruct an image. Therefore, even the data not used for the reconstruction is collected. This results in a useless operation during a scan.
DISCLOSURE OF THE INVENTION
It is an object of the invention to provide an MRI method which permits a scan to be made in a short time by collecting only the data that is used to reconstruct an image.
An MRI method according to the invention comprises the steps of: defining a circular region whose center lies at the point in a two-dimensional Fourier plane at which the spatial frequency is zero, the Fourier plane corresponding to a region containing an image to be reconstructed; observing the spin echo signals emanating from a region inside the circular region by an ordinary Fourier method, said region having a length greater than a predetermined length as measured in the direction in which data obtained by the observation is read out; and observing the spin echo signals emanating from the remaining region inside the circular region while shortening the time for which a reading gradient magnetic field is applied whenever an excitation is effected and, at the same time, effecting an additional phase shift encoding.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an MRI, instrument according to the invention;
FIGS. 2-4 are diagrams for illustrating the operation of the instrument shown in FIG. 1;
FIGS. 5 and 6 are diagrams for illustrating the operation of another instrument according to the invention;
FIGS. 7 and 8 are diagrams for illustrating the operation of a further instrument according to the invention;
REFERENCES:
patent: 4727325 (1988-02-01), Matsui et al.
patent: 4746864 (1988-05-01), Satoh
patent: 4748410 (1988-03-01), Macovski
patent: 4786872 (1988-11-01), Hoshino
Kojima Moonray
Tokar Michael J.
Yokogawa Medical Systems Limited
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