Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-02-04
2002-12-03
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S309000
Reexamination Certificate
active
06489770
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus. More particularly, it relates to a magnetic resonance imaging apparatus that is suitable for an imaging performed by a multi-gradient echo type pulse sequence that aims at shortening the measurement time (i.e., for example, an echo planar spectroscopic imaging that allows space distribution information about the chemical shift to be measured at high speed).
The magnetic resonance imaging apparatus is an apparatus that performs the imaging in the following way: An object placed within a static magnetic field is irradiated with a radio frequency pulse with a specific frequency corresponding to a specific substance included in the object, thereby giving rise to a magnetic resonance phenomenon of the specified substance. Then, a magnetic resonance signal occurring from the object is utilized so as to image the chemical and physical information on the substance.
In particular, a method that has been known as a magnetic resonance spectroscopic imaging (MRSI) is as follows: A difference in the magnetic resonance frequency (hereinafter, referred to as a chemical shift), which is attributed to a dissimilarity in the chemical bond of a variety of molecules, is measured, thereby obtaining a space distribution image for each molecule (hereinafter, referred to as a chemical shift image).
Here, the magnitude of the chemical shift attributed to the dissimilarity in the chemical bond of the molecules is, usually, of the order of ppm (i.e., 1/1-millionth), which is exceedingly small. On account of this, in the MRSI, it becomes important to adjust the homogeneity of the magnetic field that exerts influences on the magnetic resonance frequency.
In general, one of the factors that exert considerable influences upon the homogeneity of the magnetic field is the existence of an object. This condition requires that the homogeneity of the magnetic field be enhanced in a state where the object is placed within the magnetic field. As a method of adjusting the homogeneity of the magnetic field, there has been proposed the following method (for example, in literatures such as “Journal of Magnetic Resonance”, Vol. 77, pp. 40-52 (1988)): Two kinds of values, i.e., the respective offset values of gradient magnetic fields in mutually different three directions and amounts of the electric currents to be flowed through the respective shimming coils, are varied so as to superimpose, on the static magnetic field, the magnetic fields that the respective gradient magnetic field generating coils generate and magnetic fields that the respective shimming coils generate, thereby adjusting the degree of the magnetic field homogeneity.
In this method, with a phantom employed as the subject to be measured, a reference image is scanned in advance that indicates current-magnetic field distribution characteristics of the respective gradient magnetic field generating coils and the respective shimming coils. Next, the object is inserted into the magnetic field, and a magnetic field distribution image within the object is measured as a target image of a subject to be shimmed. Then, the current-magnetic field distribution characteristics of the respective gradient magnetic field generating coils and the respective shimming coils are obtained from the reference image. Moreover, combinations of the distribution characteristics (values of the currents to be flowed through the respective coils) are calculated so as to uniform the magnetic field distribution within the object, then adjusting of the magnetic field is carried out on the basis of the combinations calculated.
As the reference image and the target image used here, a phase distribution image obtained by MRI (magnetic resonance imaging) is generally employed. In order to measure this phase distribution image, an imaging sequence called a spin echo pulse sequence is usually used. In particular, a pulse sequence is used in which the spin echo time and the gradient echo time are shifted by an amount of &Dgr;t.
SUMMARY OF THE INVENTION
In the conventional technique described in the above-mentioned literature, however, there exists a problem that no consideration has been given to the adjustment of the homogeneity of the magnetic field in the case where the multi-gradient echo type pulse sequence is used for the purpose of shortening the measurement time.
Namely, in the multi-gradient echo type pulse sequence, gradient polarity of a high-intensity gradient magnetic field is inverted continuously in a short period of time so as to measure echo train signals including a plurality of echo signals. This causes an eddy current of large amplitude to occur on such places as the inner surface of a bore within a magnet or the surface of the object. Accordingly, there exists a serious problem that a magnetic field induced by the eddy current makes the magnetic field distribution inhomogeneous at the detection time of the echo signals. However, in the spin echo pulse sequence in the conventional technique with which the reference image is measured, one echo signal is measured with a single excitation, and it is not repeated to invert the polarity of high-intensity gradient magnetic field in a short period of time. On account of this, there appears no influence of the eddy current upon the reference image data obtained. This results in a problem that it is impossible to perform an adjustment in which the eddy current has been taken into consideration.
Incidentally, there are many cases where, in order to cancel out the influences of the eddy current, a gradient coil equipped with an active shield is used. It is difficult, however, to completely cancel out the eddy current occurring on the inner surface of the magnet bore, and thus the active shield exhibits no effect of eliminating the eddy current on the object surface. Consequently, it has been required to execute the adjustment of the degree of the magnetic field homogeneity in which the eddy current has been taken into consideration.
It is an object of the present invention to improve the adjustment of the homogeneity of the magnetic field in the magnetic resonance imaging apparatus that uses the multi-gradient echo type imaging pulse sequence.
A representative configuration (a first configuration) of a representative magnetic resonance imaging apparatus according to the present invention includes a static magnetic field generating unit for generating a static magnetic field to be applied to an object, a shimming magnetic field generating unit (shimming coil) for generating a N-channel (N≧3) shimming magnetic field that adjusts the degree of the static magnetic field homogeneity, a gradient magnetic field generating unit (gradient magnetic field generating coil) for generating first, second and third directions of gradient magnetic fields to be applied to the object, a radio frequency pulse generating unit (radio frequency pulse generating coil) for generating a radio frequency pulse to be applied to the object, a signal detecting unit for detecting a magnetic resonance signal generated from the object, a computing unit for performing computation in accordance with the magnetic resonance signal detected by the signal detecting unit, a pulse sequence controlling unit for controlling operations of the shimming magnetic field generating unit, the gradient magnetic field generating unit, the radio frequency pulse generating unit and the signal detecting unit, and a static magnetic field homogeneity degree-adjusting unit for controlling the shimming magnetic field generating unit and/or the gradient magnetic field generating unit so as to adjust the degree of the static magnetic field homogeneity.
Moreover, the pulse sequence controlling unit executes a static magnetic field-adjusting pulse sequence control that causes the magnetic resonance signal to be detected while applying at least one direction of gradient magnetic field from among the gradient magnetic fields with gradient polarity of the one direction of gradient magnetic field be
Arana Louis
Hitachi Medical Corporation
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