MAGNETIC RESONANCE IMAGING APPARATUS AND FLUID-ENHANCED IMAGE ACQUISITION METHOD

• US 20130293231A1
• Filed: 01/11/2012
• Published: 11/07/2013
• Est. Priority Date: 01/17/2011
• Status: Active Grant

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi Medical Corporation (Hitachi, Ltd.)
• Inventors: Hirai, Kosuke
• Granted Patent: US 9,435,871 B2
• Time in Patent Office: Days
• US Class Current: 324/309
• CPC Class Codes: A61B 5/055 involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
• CPC Class Codes: G01R 33/4828 Resolving the MR signals of different chemical species, e.g. water-fat imaging
• CPC Class Codes: G01R 33/4836 using an RF pulse being spatially selective in more than one spatial dimension, e.g. a 2D pencil-beam excitation pulse
• CPC Class Codes: G01R 33/5602 by filtering or weighting based on different relaxation times within the sample, e.g. T1 weighting using an inversion pulse
• CPC Class Codes: G01R 33/5613 Generating steady state signals, e.g. low flip angle sequences [FLASH]
• CPC Class Codes: G01R 33/5635 Angiography, e.g. contrast-enhanced angiography [CE-MRA] or time-of-flight angiography [TOF-MRA]

Abstract

In order to acquire an image with enhanced contrast between a fluid portion and a stationary portion without extending the aging time even when an IR pulse is used as an RF pre-pulse, the RF pre-pulse is applied to a region upstream of an imaging region so as to excite longitudinal magnetization of the fluid portion in a negative direction, an echo signal is measured from the imaging region, and an image with enhanced contrast of the fluid portion with respect to the stationary portion is acquired on the basis of phase information of an image reconstructed by using the echo signal.

Claims

1. A magnetic resonance imaging apparatus comprising:

a measurement control unit configured to control measurement of an echo signal from an imaging region of an object to be examined including a fluid portion, on the basis of a predetermined pulse sequence; and

an arithmetic processing unit configured to obtain an image with enhanced contrast between the fluid portion and a stationary portion using the echo signal,

wherein;

the pulse sequence is formed by an RF pre-pulse portion provided with an RF pre-pulse which performs labeling on the fluid portion by exciting the longitudinal magnetization of the fluid portion in a negative direction and a measurement sequence portion configured to measure an echo signal from the imaging region into which the labeled fluid portion is flown; and

the arithmetic processing unit obtains a fluid-enhanced image with enhanced contrast between the fluid portion and the stationary portion based on phase information of the image.

2. The magnetic resonance imaging apparatus according to claim 1, wherein the measurement control unit performs labeling on the fluid portion by applying the RF pre-pulse to a region upstream from the imaging region, and executes the measurement sequence before the longitudinal magnetization of the labeled fluid portion is recovered to null or above.

3. The magnetic resonance imaging apparatus according to claim 2, wherein:

the RF pre-pulse includes an IR pulse which excites the longitudinal magnetization at α

(90<

α

<

270)-degrees; and

the measurement sequence portion includes a transverse magnetization generating RF pulse which generates transverse magnetization by exciting the longitudinal magnetization at β

(0<

β

≦

90)-degrees.

4. The magnetic resonance imaging apparatus according to claim 3, wherein the measurement control unit sets a waiting time (TI) between the IR pulse in the RF pre-pulse portion and the transverse magnetization generating RF pulse in the measurement sequence portion, as the time for maintaining the state that the longitudinal magnetization of the labeled fluid portion by the IR pulse is in a negative direction.

5. The magnetic resonance imaging apparatus according to claim 4, wherein the measurement control unit controls measurement of the echo signal by differentiating the phase of the transverse magnetization between the labeled fluid portion and the stationary portion by π

, right after the application se magnetization generating RF pulse.

6. The magnetic resonance imaging apparatus according to claim 1, wherein the arithmetic processing unit, on the basis of the phase image reconstructed using the echo signal measured by the measurement sequence portion, determines the weighting coefficient for each pixel in the image, thereby obtaining the fluid-enhanced image using the weighting coefficient.

7. The magnetic resonance imaging apparatus according to claim 6, wherein the arithmetic processing unit, on the basis of a phase difference image between the phase image which is reconstructed using the echo signal measured only by the measurement sequence portion and the phase image which is reconstructed using the echo signal measured in the RF pre-pulse portion and the measurement sequence portion, determines the weighting coefficient for each of the pixel, creates a mask image representing the distribution of the weighting coefficient, and multiplies the mask image by the absolute value image of the reconstructed image for each pixel, thereby obtaining the fluid-enhanced image.

8. The magnetic resonance imaging apparatus according to claim 7, wherein the arithmetic processing unit acquires the weighting coefficient in the phase difference image, by converting the phase of the fluid portion into the value of [0˜

1] and converting the phase of the other stationary portion into [1].

9. The magnetic resonance imaging apparatus according to claim 2, wherein:

the fluid portion has a first fluid portion and a second fluid portion having different flow directions;

the RF pre-pulse portion includes a first RF pre-pulse and a second RF pre-pulse having different flip angles;

the measurement control unit performs labeling on the first fluid portion by applying the first RF pre-pulse on a first region upstream of the first fluid portion, performs labeling on the second fluid portion by applying the second pre-pulse to a second region upstream of the second fluid portion, and sets the region between the first region and the second region as the imaging region; and

the arithmetic processing unit obtains the fluid-enhanced image by differentiating the contrast between the first fluid portion and the second fluid portion.

10. The magnetic resonance imaging apparatus according to claim 9, wherein:

the first RF pre-pulse is an IR pulse with flip angle of φ

₁(90<

φ

₁<

270)-degrees;

the second RF pre-pulse is an IR pulse with flip angle φ

₂(0<

φ

₂≦

90)-degree;

the measurement control unit executes the measurement sequence portion before the longitudinal magnetization of the first fluid portion recovers to null or above; and

the arithmetic processing unit enhances the contrast of the first fluid portion on the basis of the phase information, and enhances the contrast of the second fluid portion on the basis of the signal intensity difference between the fluid portion and the stationary portion.

11. The magnetic resonance imaging apparatus according to claim 2, wherein:

the RF pre-pulse is for executing 2-dimensional spatially-selective excitation; and

the measurement control unit applies the RF pulse for executing 2-dimensional spatially-selective excitation on the region which intersects with at least a part of a fluid region upstream of the imaging region, for labeling at least a part of the fluid portion.

12. The magnetic resonance imaging apparatus according to claim 11, wherein:

the fluid portion has a first fluid portion and a second fluid portion;

the RF pre-pulse includes a first RF pre-pulse and a second RF pre-pulse having different flip angles for executing the 2-dimensional spatially-selective excitation; and

the measurement control unit applies the first RF pre-pulse to a region which intersects with at least a part of the first fluid portion upstream of the imaging region, and applies the second RF pre-pulse to a region which intersects with at least a part of the second fluid portion upstream of the imaging region.

13. The magnetic resonance imaging apparatus according to claim 2, wherein:

the RF pre-pulse portion has a first RF pre-pulse and a second RF pre-pulse; and

the measurement control unit performs labeling on the fluid portion by applying the first RF pre-pulse to a first region upstream of the fluid portion, applies the second RF pre-pulse to a second region which is adjacent to downstream of the first region, and sets at least a part of the second region as the imaging region.

14. The magnetic resonance imaging apparatus according to claim 13, wherein the measurement control unit applies the second RF pre-pulse at the timing that the longitudinal magnetization of the fluid portion flipped in a negative direction by the first RF pre-pulse recovers by T1 to null, and controls measurement of the echo signal from the imaging region by the measurement sequence portion within the time that the longitudinal magnetization of the stationary portion in the second region flipped in a negative direction by the second RE pre-pulse maintains the negative direction state.

15. A fluid-enhanced image acquisition method including:

a measurement step of controlling measurement of an echo signal from an imaging region of an object to be examined including a fluid portion, on the basis of a predetermined pulse sequence; and

an arithmetic processing step of obtaining an image with enhanced contrast between the fluid portion and a stationary portion using the echo signal,

wherein;

the pulse sequence has an RF pre-pulse portion comprising an RF pre-pulse for labeling the fluid portion by exciting the longitudinal magnetization of the fluid portion in a negative direction and a measurement sequence portion which measures an echo signal from the imaging region into which the labeled fluid portion is flown; and

the arithmetic processing step obtains an image with enhanced contrast between the fluid portion and the stationary portion based on phase information of the image.

16. The fluid-enhanced image acquisition method according to claim 15, wherein the measurement step applies the RF pre-pulse to a region upstream from the imaging region, and executes the measurement sequence portion before the longitudinal magnetization of the fluid portion recovers to null.

17. The fluid-enhanced image acquisition method according to claim 16, wherein:

the fluid portion includes a first fluid portion and a second fluid portion having different directions;

the RF pre-pulse portion has a first RF pre-pulse and a second RF pre-pulse having different flip angles;

the measurement step performs labeling on the first fluid portion by applying the first RF pre-pulse on a first region upstream of the first fluid portion, performs labeling on the second fluid portion by applying the second RF pre-pulse on a second region upstream of the second fluid portion, and sets a region between the first region and the second region as the imaging region; and

the arithmetic processing step obtains the fluid-enhanced image by differentiating the contrast between the first fluid portion and the second fluid portion.

18. The fluid-enhanced image acquisition method according to claim 16, wherein:

the RF pre-pulse is an RF pulse for executing 2-dimensional spatially-selective excitation; and

the measurement step applies the RF pulse for executing 2-dimensional spatially-selective excitation to a region which intersects with at least a part of the fluid portion upstream of the imaging region, for labeling at least a part of the fluid portion.

19. The fluid-enhanced image acquisition method according to claim 15, wherein:

the RF pre-pulse portion has a first RF pre-pulse and a second RF pre-pulse; and

the measurement step labels the fluid portion by applying the first RF pre-pulse on a first region upstream of the fluid portion, applies the second RF pre-pulse to a second region adjacent to the side downstream of the first region, and sets at least a part of the second region as the imaging region.