MR imaging on ASL technique

• US 6,564,080 B1
• Filed: 03/31/2000
• Issued: 05/13/2003
• Est. Priority Date: 03/31/1999
• Status: Expired due to Term

• Current Assignee: Toshiba Medical Systems Corporation (Canon Inc.), Csb-System Software-Entwicklung & Unternehmensberatung AG
• Original Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Inventors: Kimura, Tokunori
• Primary Examiner(s): Lateef, Marvin M.
• Assistant Examiner(s): Shah, Devaang
• Application Number:
  US09/539,775
• Time in Patent Office: 1,138 Days
• Field of Search: 324/307, 324/309, 128/653.3, 600/407, 600/410
• US Class Current: 600/410
• CPC Class Codes: A61B 5/0263 using NMR
• CPC Class Codes: G01R 33/4835 of multiple slices
• CPC Class Codes: G01R 33/563 of moving material, e.g. flow contrast angiography

Abstract

MR imaging is provided based on the ASL method providing an ASL image. In this imaging, MT effects are steadily canceled to reduce difference errors inherent in excluding stationary tissue from the image and sensitivity is given to only a signal from one-way blood flow. A first RF wave and a first magnetic gradient G_tag both for selective-exciting a tagging slab and a second RF wave and a second magnetic gradient G_cont both for selective-exciting a controlling slab are set, in which offset amounts of exciting central frequencies of both of the first and second RF waves to a central position of an imaging slab are equal to each other and offset positions Offset_tag and Offset_cont of the tagging slab and the controlling slab to the imaging slab are different from each other. Both of the RF pulses are IR pulses. A pulse sequence including the first RF wave and the first magnetic gradient G_tag is performed and a pulse sequence including the second RF wave and the second magnetic gradient G_cont is performed, so that echo signals are acquired for each performance. The echo signals are subject to mutual difference calculation to provide an ASL image.

Claims

1. An MRI system for obtaining an ASL (Arterial Spin Labeling) image of an imaging slab of a subject placed in a static magnetic field by locating a tagging slab and a controlling slab on both sides of the imaging slab, respectively, the system comprising:

storing means for storing information indicative of a first pulse sequence including a first RF pulse and a first magnetic gradient both for selective-exciting the tagging slab and for storing information indicative of a second pulse sequence including a second RF pulse and a second magnetic gradient both for selective-exciting the tagging slab controlling slab, in which offset amounts of exciting central frequencies of both the first and second RF pulses to a central position of the imaging slab are equal to each other and offset positions of both the tagging slab and the controlling slab to the imaging slab are different from each other;

first scanning means for performing toward the subject the first pulse sequence so as to acquire a first MR signal from the imaging slab;

second scanning means for performing toward the subject the second pulse sequence so as to acquire a second MR signal from the imaging slab; and

image producing means for producing the ASL image based on a difference between the first and second MR signals.

2. The system of claim 1, wherein each of the first and second RF pulses consists of a single-frequency continuous RF pulse determined correspondingly to each of the first and second magnetic gradients so as to allow each of the tagging and controlling slabs to be excited at a desired slab position.

3. The system of claim 2 wherein the storing means stores the information concerning the first and second RF pulses and the first and second magnetic gradients determined such that a ratio between thicknesses of the tagging and

controlling slabs and a further ratio between the positional offsets of the tagging and controlling slabs to the imaging slab are equal to each other.

4. The system of claim 3, wherein each of the first and second RF pulses is a pulsed RF pulse having a certain frequency band.

5. The system of claim 1, wherein each of the first and second RF pulses is an IR (inversion recovery) pulse inverting spins of the subject, and each of the first and second scanning means is configured to apply the IR pulse in mutually opposite polarities to the imaging slab during the performance of each of the first and second pulse sequences.

6. The system of claim 5, wherein the imaging slab is located at a head portion of the subject, and the storing means includes means for setting the controlling slab spatially separately from the head portion.

7. The system of claim 1, wherein the image producing means includes mans for extracting from the difference of the MR signals a signal component of not more than a threshold determined as a minimum signal intensity for a blood vessel to be suppressed of the subject.

8. The MRI system of claim 1, wherein the first and second pulse sequences are the same type of pulse sequence enhancing a longitudinal magnetization of spins of the subject.

9. The MRI system of claim 8, wherein each of the first and second RF pulses is an IR (inversion recovery) pulse for inverting the spins, the IR pulse being applied slice-selectively to each of the tagging and controlling slabs.

10. The MRI system of claim 8, wherein each of the first and second pulse sequences includes a pre-saturation pulse for previously saturating the spins of the subject.

11. The MRI system of claim 1, wherein the first and second scanning means are configured to perform the first and second pulse sequences in an interleaved order along which the first and second pulse sequences are repeated by turns.

12. The MRI system of claim 1, wherein each of the first and second scanning means is configured to perform each of the first and second pulse sequences a plurality of times so that the MR signal is acquired the plurality of times from the imaging slab, and

the image producing means includes means for averaging the MR signals acquired over the plurality of times.

13. The MRI system of claim 3, wherein the storing means comprises

providing means for providing, as known amounts, a slab thickness of the imaging slab, a slab thickness of the tagging slab, a distance between the imaging and tagging slabs, and a distance between the imaging and controlling slabs;

and

calculating means for calculating the slab thickness and the positional offset amount of the controlling slab on the basis of the known amounts.

16. The MRI system of claim 1, wherein each of the first and second pulse sequences includes a non-slice selective IR pulse following application of the RF pulse as well as being applied to a region of the subject including the imaging slab, tagging slab, and controlling slab and an imaging pulse train following application of the non-slice selective IR pulse.

17. The MRI system of claim 16, wherein an interval between the application of the non-slice selective IR pulse and the application of the imaging pulse train in each of the first and second pulse sequences is determined so that a spin-lattice relaxation time of stationary tissue contained in the imaging slab becomes an amount to be regarded as being approximately zero on average when the imaging pulse train is applied.

18. The MRI system of claim 16, wherein the non-slice selective IR pulse consists of a plurality of non-slice selective IR pulses.

14. An MRI system comprising:

a first scanning means for applying a first RF wave to a tagging slab to be located in one side of an imaging slab of a subject so as to acquire a first MR signal from the imaging slab;

a second scanning means for applying a second RF wave to a controlling slab to be located, symmetrically to the tagging slab, in the other side of the imaging slab so as to acquire a second MR signal from the imaging slab; and

image data producing means for producing image data on the basis of the first and second MR signals,

wherein the image data producing means comprise

a first and second absolute-value calculating means for calculating absolute values of the first and second MR signals after reconstruction thereof, respectively;

difference means for performing mutual differences between the absolute values of the first and second MR signals; and

extracting means for extracting image data of a desired signal component from differences obtained by the difference means.

15. The MRI system of claim 14, wherein the extracting means is configured to apply to the differences threshold processing with either a threshold determined to suppress the MR signal of a large-diameter blood vessel inflowing from a controlling slab side into the imaging slab and to extract image data indicative of a blood flow inflowing from a tagging slab side into the imaging slab or a further threshold determined to suppress the MR signal of a large-diameter blood vessel inflowing into the imaging slab and to extract image data indicative of perfusion inflowing into the imaging slab.

19. An MR imaging method of obtaining an ASL (Arterial Spin Labeling) image of an imaging slab of an subject placed in a static magnetic field by locating a tagging slab and a controlling slab on both sides of the imaging slab, respectively, the method comprising the steps of:

reading out information indicative of first and second pulse sequences set and stored in memory means, the first pulse sequence including a first RF pulse and a first magnetic gradient both for selective-exciting the tagging slab and the second pulse sequence including a second RF pulse and a second magnetic gradient both for selective-exciting the controlling slab, in which offset amounts of exciting central frequencies of both the first and second RF pulses to a central position of the imaging slab are equal to each other and offset positions of both the tagging slab and the controlling slab to the imaging slab are different from each other;

performing toward the subject both the first and second pulse sequences so as to acquire first and second MR signals from the imaging slab, respectively;

producing the ASL image based on a difference between the first and second MR signals.

20. An MRI system for obtaining an ASL (Arterial Spin Labeling) image of an imaging slab of an subject placed in a static magnetic field by locating a tagging slab and a controlling slab on both sides of the imaging slab, respectively, the system comprising:

storing means for storing information indicative of a first pulse sequence including a first RF pulse and a first magnetic gradient both for selective-exciting the tagging slab and for storing information indicative of a second pulse sequence including a second RF pulse and a second magnetic gradient both for selective-exciting the controlling slab, wherein (i) offset amounts of exciting central frequencies of both the first and second RF pulses to a central position of the imaging slab are equal to each other, (ii) offset positions of both the tagging slab and the controlling slab to the imaging slab are different from each other, and (iii) a ratio between thicknesses of the tagging and controlling slabs and a further ratio between the positional offsets of the tagging and controlling slabs to the imaging slab are equal to each other;

first scanning means for performing toward the subject the first pulse sequence so as to acquire a first MR signal from the imaging slab;

second scanning means for performing toward the subject the second pulse so as to acquire a second MR signal from the imaging slab; and

image producing means for producing the ASL image based on a difference between the first and second MR signals.

21. The system of claim 20, wherein each of the first and second RF pulses is a pulsed RE pulse having a certain frequency band.

22. The MRI system of claim 20, wherein the storing means comprises

providing means for providing, as known amounts, a slab thickness of the imaging slab, a slab thickness of the tagging slab, a distance between the imaging and tagging slabs, and a distance between the imaging and controlling slabs; and

calculating means for calculating the slab thickness and the positional offset amount of the controlling slab on the basis of the known amounts.