Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Utility Patent
1998-07-15
2001-01-02
Oda, Christine K. (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Utility Patent
active
06169398
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus (abbreviated as “MRI apparatus” hereinafter) and an imaging method utilizing the MRI apparatus. In particular, it relates to an MRI apparatus and an imaging method capable of producing images with high sensitivity to difference in magnetic susceptibility and inhomogeneities in local magnetic fields.
2. Related Art
Known imaging methods utilizing MRI apparatuses include various kinds of sequences for fast imaging based on the spin echo imaging method or the gradient echo imaging method. Among such imaging methods, the fast spin echo method (FSE method) is an imaging method utilizing the multiple-echo method in which multiple echoes are generated by repeatedly applying 180° pulses to inverse magnetization generated by excitation with 90° pulses. The FSE method encompasses a method where each echo signal is differently phase-encoded to obtain one image fast, and a method in which differently phase-encoded echo signals are divided into several sequence arrays to produce an image having a quality substantially the same as that obtainable in the ordinary spin echo method.
The echo planar imaging method (EPI method) is a method for acquiring a plurality of echoes with one excitation pulse by rapidly inverting read-out gradient magnetic fields without utilizing inversion by radio frequency (RF) pulses. This method enables very fast imaging, and is extremely sensitive to inhomogeneities of static magnetic fields.
Further, there are also known a hybrid sequence, which utilizes both inversion by RF pulses and inversion of gradient magnetic fields (Japanese Patent Publication No. Hei 6-46985).
In the hybrid sequence, as shown in
FIG. 8
, a 90° pulse RF is applied concurrently with gradient magnetic fields for slice selection Gs, and then several 180° RF pulses are applied at constant intervals (echo time interval ETI). Between these 180° pulses, several read-out gradient fields Gf are applied while the magnetic field polarity is inverted alternately. This generates several gradient echo signals GE
1
-GE
4
. When the same period of time as the interval between the 90° pulse and the first 180° pulse (ETI/2) passes after the application of the 180° pulse, a spin echo signal SE is generated. That is, the spin echo signal SE is generated at the center of the several generated gradient echo signals. For example, after the first 180° pulse, the echo signals are generated in such an order as GE
1
, GE
2
, SE, GE
3
and GE
4
.
In the method for scanning k-space, as shown in the same figure, the phase encoding is controlled so that the spin echo signals SE fall in the low frequency region of the k-space (near the center of the k-space), and so that the T2* weighted gradient echo signals GE
1
-GE
4
before and after SE, which reflect inhomogeneities in local magnetic fields or difference in magnetic susceptibility, fall in the radio frequency region. This enables fast image acquisition with a contrast similar to that of conventional spin echo images.
While the FSE method and the hybrid sequence can thus provide images that enjoy the advantages of spin echo images, their sensitivity to difference in magnetic susceptibility and inhomogeneities of local magnetic fields is low. They are, therefore, difficult to utilize in the BOLD method, a promising technology for examining diseases producing difference of magnetic susceptibility, such as hematoma, and measurement of brain functions.
On the other hand, the EPI method exhibits high sensitivity to difference in magnetic susceptibility and inhomogeneities of local magnetic fields, and realizes extremely short measurement time. It is, therefore, an effective imaging method for the examination of the aforesaid diseases and brain function measurement. However, it requires a strong power source capable of enabling acquisition of the signals before they are attenuated by transverse relaxation due to T2*. This make the apparatuses for the method too expensive for use as a standard apparatus for commercial use.
The object of the present invention is to provide an MRI apparatus capable of fast imaging which exhibits high sensitivity to difference in magnetic susceptibility and inhomogeneities in local magnetic fields as a standard commercial apparatus, and to provide a novel imaging method.
SUMMARY OF THE INVENTION
The MRI apparatus of the present invention essentially consists of means for generating a static magnetic field in a space where an object to be examined is placed, means for generating gradient magnetic fields in the space, a transmission system for emitting radio frequency (RF) pulses to cause nuclear magnetic resonance in nuclear spins of atoms constituting living body tissues of the object, a receiving system for detecting echo signals elicited through the nuclear magnetic resonance, a signal processing system for performing image reconstruction operation using the echo signals detected by the receiving system, control means for controlling the means for generating the gradient magnetic fields, the transmission system, the receiving system and the signal processing system in accordance with a given pulse sequence, and means for displaying the resulting image,
wherein the control means executes, as the pulse sequence, a hybrid pulse sequence for applying RF pulses for exciting the nuclear spins, then applying a RF pulse for inverting the nuclear spins several times at constant intervals, and acquiring a plurality of gradient echo signals that are phase-encoded in different magnitudes between adjacent inverting RF pulses. In this sequence, the intervals of the RF pulses and the timing of the acquisition of the gradient echo signals are controlled to minimize the influence of spin echoes observed between the inverted pulses.
The RF pulse for excitation is typically, but not limited to, a RF pulse having a flip angle of 90° (90° pulse).
In the MRI apparatus of the present invention, the influence of spin echoes is minimized and the reconstruction is performed by using the T2* weighted gradient echo signals, and, therefore, the MRI apparatus can provide images reflecting difference in magnetic susceptibility and inhomogeneities in local magnetic fields.
In accordance with one embodiment of the apparatus of the present invention, the control means controls the timing of the generation of the gradient echo signals so as not to coincide with the timing of the generation of the spin echoes, and images are reconstructed by using only the gradient echo signals without generating spin echoes. Such timing of the generation of the gradient echo signals can be realized by controlling the timing of the gradient magnetic field application in the read-out direction.
In another embodiment of the MRI apparatus of the present invention, the control means executes the hybrid sequence so that the interval (interpulse time) of the inverting RF pulses is longer than twice of the interval between the RF pulse for excitation and the first inverting RF pulse and that the spin echo signals are generated at time points to phase-encode them in a high frequency region and to phase-encode the other gradient echo signals in a lower frequency region.
In this embodiment, since the generated spin echo signals are distributed (arranged) in a high frequency region of the k-space, and the gradient echo signals not containing spin-echo are distributed in a lower frequency region of the k-space, the T2* weighted gradient echo signals become predominant in determining the contrast, as in the first embodiment, and images reflecting difference in magnetic susceptibility and inhomogeneities in local magnetic fields can be provided.
Also in this case, the RF pulse for excitation is typically, but not limited to, a RF pulse having a flip angle of 90° (90° pulse).
The imaging method of the present invention is an imaging method comprising the steps of applying a RF pulse for exciting nuclear spins of an object, then applying RF pulses for inverting the nuclear spins seve
Takahashi Tetsuhiko
Watanabe Shigeru
Hitachi Medical Corporation
Oda Christine K.
Pennie & Edmonds LLP
Shrivastav Brij B.
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