Magnetic resonance imaging apparatus and imaging method

• US 6,169,398 B1
• Filed: 07/15/1998
• Issued: 01/02/2001
• Est. Priority Date: 07/16/1997
• Status: Expired due to Term

• Current Assignee: Hitachi Medical Corporation (Hitachi, Ltd.)
• Original Assignee: Hitachi Medical Corporation (Hitachi, Ltd.)
• Inventors: Watanabe, Shigeru, Takahashi, Tetsuhiko
• Primary Examiner(s): Oda, Christine K.
• Assistant Examiner(s): Shrivastav, Brij B.
• Application Number:
  US09/115,720
• Time in Patent Office: 902 Days
• Field of Search: 324/309, 324/307
• US Class Current: 324/309
• CPC Class Codes: G01R 33/5615 Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE]
• CPC Class Codes: G01R 33/5618 using both RF and gradient refocusing, e.g. GRASE

Abstract

The MRI apparatus of the present invention performs a pulse sequence for applying successively 180° pulses 102, 103 . . . after application of a 90° pulse 101 at constant intervals and applying readout magnetic fields 111 during the interval between pairs of 180° pulses while the polarity of the magnetic field 111 is inverted several times to collect a plurality of gradient echo signals that are phase encoded differently. Here, the application of the readout gradient magnetic field is controlled such that generation of the gradient echo signals does ot coincide with generation of the spin echo signal. The image reconstruction is performed by using only T2* weighted gradient echo signals to obtain images reflecting difference in magnetic susceptibility and inhomogeneities in local magnetic fields.

Claims

1. A magnetic resonance imaging (MRI) apparatus comprising:

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 a radio frequency magnetic field to cause nuclear magnetic resonance (NMR) in nuclear spins of atoms constituting living body tissue of the object,

a receiving system for detecting echo signals elicited through the NMR,

a signal processing system for performing an image reconstruction operation using the echo signals detected by the receiving system,

control means for controlling the means for generating gradient magnetic fields, the transmission system, the receiving system and the signal processing system in accordance with a predetermined pulse sequence, and

means for displaying the resulting image,

wherein the control means performs a hybrid sequence including application of a radio frequency pulse for exciting the nuclear spins, successive application of a plurality of radio frequency pulses for inverting the nuclear spins at constant intervals and collection of a plurality of gradient echo signals that are phase encoded differently during the interval between pairs of inverting radio frequency pulses, and controls the intervals between the radio frequency pulses and the timing of collection of the gradient echo signals to minimize influence of spin echo signals generated between the inverting radio frequency pulses.

2. A magnetic resonance imaging (MRI) apparatus comprising:

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 a radio frequency magnetic field to cause nuclear magnetic resonance (NMR) in nuclear spins of atoms constituting living body tissue of the object,

a receiving system for detecting echo signals elicited through the NMR,

a signal processing system for performing an image reconstruction operation using the echo signals detected by the receiver means,

control means for controlling the means for generating gradient magnetic fields, the transmission system, the receiving system and the signal processing system in accordance with a predetermined pulse sequence, and

means for displaying the resulting image,

wherein the control means performs a pulse sequence including application of a 90°

pulse as a radio frequency pulse, successive application of 180°

pulses at constant intervals and application of a plurality of readout gradient magnetic fields, the polarity of which is altered several times to generate a plurality of gradient echo signals, and application of phase encoding gradient magnetic fields for encoding the gradient echo signals, and controls the application of the readout gradient magnetic field such that generation of the gradient echo signals does not coincide with generation of the spin echo signal.

3. A magnetic resonance imaging (MRI) apparatus comprising:

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 a radio frequency magnetic field to cause nuclear magnetic resonance (NMR) in nuclear spins of atoms constituting living body tissue of the object,

a receiving system for detecting echo signals elicited through the NMR,

a signal processing system for performing an image reconstruction operation using the echo signals detected by the receiver means,

control means for controlling the means for generating gradient magnetic fields, the transmission system, the receiving system and the signal processing system in accordance with a predetermined pulse sequence, and

means for displaying the resulting image,

wherein the control means performs a pulse sequence including application of a 90°

pulse as a high frequency pulse, successive application of 180°

pulses at constant intervals and application of a plurality of readout gradient magnetic fields, the polarity of which is altered several times to generate a plurality of gradient echo signals, and application of phase encoding gradient magnetic fields for encoding the gradient echo signals, and controls the length of the interval (ETI₂) between pairs of 180°

pulses to be longer than twice of an interval ETI₁/2) between a 90°

pulse and a 180°

pulse so that a spin echo signal is generated at the time when a phase encode value in the high frequency region is applied thereto.

4. An imaging method for obtaining a magnetic resonance image by applying a radio frequency pulse for exciting nuclear spins of atoms constituting an object to be examined, then applying a plurality of high frequency pulses for inverting the nuclear spins at constant intervals, applying readout gradient magnetic fields while the polarity of the readout gradient magnetic fields is altered several times during the adjacent inverting radio frequency pulses to generate a plurality of gradient echo signals, applying a phase encoding gradient magnetic field for encoding the gradient echo signals, and reconstructing a magnetic resonance image based on the gradient echo signals,

wherein the intervals of the radio frequency pulses and the timing of acquisition of the gradient echo signals are controlled to minimize influence of spin echo signals generated between the inverting pulses.

5. The imaging method of claim 4, wherein the radio frequency pulse for exciting the nuclear spins is a 90°

pulse.

6. The imaging method of claim 4, wherein the application of the readout gradient magnetic fields is controlled so that the generation of the spin echo does not coincide with the generation of the gradient echoes.

7. The imaging method of claim 6, wherein the readout gradient magnetic field is applied an even number of times between adjacent inverting radio frequency pulses.

8. The imaging method of claim 4, wherein the interval between the adjacent radio frequency pulses is longer than twice the interval between the exciting radio frequency pulse and the first inverting radio frequency pulse.

9. The imaging method of claim 8, wherein the radio frequency pulse for exciting the nuclear spins is a 90°

pulse.

10. The imaging method of claim 8, wherein the gradient echo signals acquired after an odd-numbered inverting RF pulse are distributed in a k-space in one direction along the ky-axis of the k-space, and the gradient echo signals acquired after an even-numbered inverting RF pulse are distributed in the k-space in an opposite direction along the ky-axis.

11. A magnetic resonance imaging (MRI) apparatus comprising:

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 a radio frequency magnetic field to cause nuclear magnetic resonance (NMR) in nuclear spins of atoms constituting living body tissue of the object,

a receiving system for detecting echo signals elicited through the NMR,

a signal processing system for performing an image reconstruction operation using the echo signals detected by the receiving system,

control means for controlling the means for generating gradient magnetic fields, the transmission system, the receiving system and the signal processing system in accordance with a predetermined pulse sequence, and

means for displaying the resulting image,

wherein the control means performs a first sequence including application of a first radio frequency pulse for exciting the nuclear spins and a second radio frequency pulse(s) for inverting the nuclear spins to generate an echo signal(s), and a second sequence including application of the first radio frequency pulse and gradient magnetic fields whose polarities are alternately inverted while changing the phase encode amount to generate a series of echo signals phase encoded differently, and controls the interval between the first radio frequency pulse and the second radio frequency pulse so that the echo signal in the first sequence is generated before or after generation of the series of echo signals in the second sequence.

12. A magnetic resonance imaging (MRI) apparatus comprising:

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 a radio frequency magnetic field to cause nuclear magnetic resonance (NMR) in nuclear spins of atoms constituting living body tissue of the object,

a receiving system for detecting echo signals elicited through the NMR,

a signal processing system for performing an image reconstruction operation using the echo signals detected by the receiving system,

control means for controlling the means for generating gradient magnetic fields, the transmission system, the receiving system and the signal processing system in accordance with a predetermined pulse sequence, and

means for displaying the resulting image,

wherein the control means performs a first sequence including application of a first radio frequency pulse for exciting the nuclear spins and a second radio frequency pulse(s) for inverting the nuclear spins to generate an echo signal(s), and a second sequence including application of the first radio frequency pulse and gradient magnetic fields whose polarities are alternately inverted while changing the phase encode amount to generate a series of echo signals phase encoded differently, and controls the timing of application of the inverted gradient magnetic fields so that generation of the echo signal in the first sequence substantially overlaps with generation of any one of the echo signals in the second sequence.

13. An imaging method for obtaining a magnetic resonance image by performing a first sequence including application of a first radio frequency pulse for exciting the nuclear spins and a second radio frequency pulse(s) for inverting the nuclear spins to generate an echo signal(s), and a second sequence including application of the first radio frequency pulse and gradient magnetic fields whose polarities are alternately inverted while changing the phase encode amount to generate a series of echo signals phase encoded differently,

wherein the interval between the first radio frequency pulse and the second radio frequency pulse is controlled so that the echo signal in the first is generated before or after generation of the series of echo signals in the second sequence.

14. A magnetic resonance imaging (MRI) apparatus comprising:

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 a radio frequency magnetic field to cause nuclear magnetic resonance (NMR) in nuclear spins of atoms constituting living body tissue of the object,

a receiving system for detecting echo signals elicited through the NMR,

a signal processing system for performing an image reconstruction operation using the echo signals detected by the receiving system,

control means for controlling the means for generating gradient magnetic fields, the transmission system, the receiving system and the signal processing system in accordance with a predetermined pulse sequence, and

means for displaying the resulting image,

wherein the control means controls an interval between a first radio frequency pulse for exciting nuclear spins and a second radio frequency pulse for inverting the nuclear spins so that an echo signal caused by the inverting radio frequency pulse is generated before or after generation of a series of gradient echo signals.

15. A magnetic resonance imaging (MRI) apparatus comprising:

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 a radio frequency magnetic field to cause nuclear magnetic resonance (NMR) in nuclear spins of atoms constituting living body tissue of the object,

a receiving system for detecting echo signals elicited through the NMR,

a signal processing system for performing an image reconstruction operation using the echo signals detected by the receiving system,

control means for controlling the means for generating gradient magnetic fields, the transmission system, the receiving system and the signal processing system in accordance with a predetermined pulse sequence, and

means for displaying the resulting image,

wherein the control means controls gradient magnetic fields to be inverted even times so that generation time of an echo signal caused by a radio frequency pulse for inverting nuclear spins is within generation time of a series of gradient echo signals to restrain the echo signal caused by the inverting radio frequency pulse.

16. A magnetic resonance imaging (MRI) apparatus comprising:

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 a radio frequency magnetic field to cause nuclear magnetic resonance (NMR) in nuclear spins of atoms constituting living body tissue of the object, a receiving system for detecting echo signals elicited through the NMR, a signal processing system for performing an image reconstruction operation using the echo signals detected by the receiver system, control means for controlling the means for generating gradient magnetic fields, the transmission system, the receiving system and the signal processing system in accordance with a predetermined pulse sequence, and means for displaying the resulting image, wherein the control means performs a hybrid sequence including application of a radio frequency pulse for exciting the nuclear spins, successive application of a plurality of radio frequency pulses for inverting the nuclear spins at constant intervals and collection of a plurality of gradient echo signals that are phase encoded differently during the interval between pairs of inverting radio frequency pulses, and controls the intervals between the radio frequency pulses or the timing of collection of the gradient echo signals to minimize influence of spin echo signals generated between the inverting radio frequency pulses.