Magnetic resonance imaging apparatus and method configured for susceptibility-emphasized imaging with improved signal-to-noise ratio
View Patent ↗A susceptibility-emphasized image having a preferable signal-to-noise ratio can be obtained by a method including measuring a plurality of echo signals using the echo planar method, dividing the plurality of echo signals into a first echo signal group and a second echo signal group, acquiring an image data from the first echo signal group while acquiring a mask data from the second echo signal group, and obtaining the susceptibility-emphasized image through use of the image data and the mask data.
1. A magnetic resonance imaging apparatus comprising:
a static magnetic field generating unit that generates a static magnetic field in an imaging space in which an object to be examined is disposed;
a gradient magnetic field generating unit that generates gradient magnetic fields in the imaging space;
a transmission unit that irradiates the object with an RF pulse;
a reception unit that receives and detects a nuclear magnetic resonance signal generated from the object;
a measurement controller configured to control the operation of the gradient magnetic field generating unit, the transmission unit and the reception unit to obtain measurement of a plurality of echo signals from the object, based on a predetermined pulse sequence which causes the gradient magnetic fields to include a phase blip gradient magnetic field and a frequency encode gradient magnetic field with an inverted polar character; and
an arithmetic processor configured to divide the plurality of echo signals into a first echo signal group and a second echo signal group, acquire image data from the first echo signal group and mask data from the second echo signal group, and obtain a susceptibility-emphasized image through use of the image data and the mask data.
2. The magnetic resonance imaging apparatus according to claim 1 , wherein the arithmetic processor acquires mask image data by Fourier transforming the second echo signal group and acquires filtered mask data by low-pass filtering and Fourier transforming the second echo signal group, so as to acquire the mask data from the difference between the phase of the mask image data and the phase of the filtered mask data.
3. The magnetic resonance imaging apparatus according to claim 1 , wherein the arithmetic processor sets the echo signals measured in the anterior side of the pulse sequence as a first echo signal group and the echo signals measured in the posterior side of the pulse sequence as a second echo signal group.
4. The magnetic resonance imaging apparatus according to claim 3 , wherein the measurement controller applies a re-phasing gradient magnetic field between the measurement of the first echo signal group and the measurement of the second echo signal group.
5. The magnetic resonance imaging apparatus according to claim 3 , wherein the arithmetic processor shares a part of the plurality of echo signals between the first echo signal group and the second echo signal group.
6. The magnetic resonance imaging apparatus according to claim 5 , wherein the measurement controller makes the polar characteristic of the phase blip gradient magnetic field to be applied upon measurement of the first echo signal group and the polar characteristic of the phase blip gradient magnetic field to be applied upon measurement of the second echo signal group different.
7. The magnetic resonance imaging apparatus according to claim 3 , wherein the measurement controller increases intensity of the frequency encode gradient magnetic field to be applied and reduces the application time upon measurement of the second echo signal group compared to the frequency encode gradient magnetic field to be applied upon measurement of the first echo signal group, and broadens reception frequency bandwidth of a sampling window upon measurement of the second echo signal group compared to the reception frequency bandwidth of a sampling window upon measurement of the first echo signal group.
8. The magnetic resonance imaging apparatus according to claim 1 , wherein the arithmetic processor, from among the plurality of echo signals, sets the echo signals measured in the odd-numbered order as the first echo signal group and sets the echo signals measured in the even-numbered order as the second echo signal group.
9. The magnetic resonance imaging apparatus according to claim 8 , wherein the measurement controller applies the phase blip gradient magnetic field with respect to every two echo signals.
10. The magnetic resonance imaging apparatus according to claim 1 , wherein the measurement controller makes the number of echoes in the first echo signal group and the number of echoes in the second echo signal group different.
11. The agnetic resonance imaging apparatus according to claim 10 , wherein:
the measurement controller measures only the echo signals corresponding to a high-spatial frequency region in the K-space with respect to the second echo signal group; and
the arithmetic processor uses the data of the echo signals corresponding to the same low-spatial frequency region in the first echo signal group as the data of the echo signal corresponding to the low-spatial frequency region in the K-space with respect to the second echo signal group.
12. A susceptibility-emphasized imaging method performed by a magnetic resonance imaging apparatus, the method comprising:
a measurement step that measures a plurality of echo signals from an object to be examined based on a predetermined pulse sequence which causes a phase blip gradient magnetic field and a frequency encode gradient magnetic field with an inverted polar character, to be applied; and
an arithmetic processing step that divides the plurality of echo signals into a first echo signal group and a second echo signal group, acquires image data from the first echo signal group and mask data from the second echo signal group, and obtains a susceptibility-emphasized image through use of the image data and the mask data.
13. The susceptibility-emphasized imaging method according to claim 12 , wherein the arithmetic processing step, from among the plurality of echo signals, sets the echo signal measured in the anterior side of the pulse sequence as a first echo signal group, and sets the echo signals measured in the posterior side as a second echo signal group.
14. The susceptibility-emphasized imaging method according to claim 12 , wherein the arithmetic processing step, from among the plurality of echo signals, sets the echo signals measured in the odd-numbered order as the first echo signal group, and sets the echo signals measured in the even-numbered order as the second echo signal group.
15. The susceptibility-emphasized imaging method according to claim 12 , wherein the measurement step makes the number of echo signals in the first echo signal group and the number of echo signals in the second echo signal group different.