JP3748714B2 - Magnetic resonance imaging device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging method and apparatus, and more particularly, to a magnetic resonance imaging method and apparatus for labeling arterial spin and imaging a multi-slice perfusion image.
[0002]
[Prior art]
One type of functional imaging using magnetic resonance, that is, fMRI (functional magnetic resonance imaging), is imaging of a perfusion image by an ASL (arterial spin labeling) method. In this method, arterial blood is labeled by spin reversal or the like, and the distribution of labeled blood flowing into the imaging region is imaged. The ASL method is used, for example, for the purpose of imaging intracerebral blood flow (CBF (Cerebral Blood Flow)).
[0003]
Blood labeling is performed on the upstream side of the blood flow flowing into the imaging region, and the imaging region is scanned at a predetermined time from the labeling to collect the labeled magnetic resonance signals. In that case, in order to increase the efficiency of imaging, so-called multi-slice scanning is performed in which a plurality of slices are scanned during 1TR (repetition time).
[0004]
[Problems to be solved by the invention]
When the ASL method is executed by multi-slice scanning, the arrival time of the labeled spin to the imaging region differs depending on the position of each slice. Therefore, the spin relaxation states are different, and the signal intensity of the magnetic resonance signal is different for each slice. . For this reason, the brightness of the perfusion image is different for each slice, and there is a problem that it is difficult to perform a correct diagnosis.
[0005]
The present invention has been made to solve the above problems, and an object thereof is to realize a magnetic resonance imaging method and apparatus for obtaining a multi-slice perfusion image with uniform brightness.
[0006]
[Means for Solving the Problems]
(1) A first invention for solving the above-mentioned problem is a magnetic resonance imaging method for imaging a multi-slice perfusion image by labeling arterial blood spins, and changing the scan order for a plurality of slices. A magnetic resonance imaging method, wherein imaging is repeated a plurality of times, and the plurality of captured images are added for each same slice.
[0007]
(2) A second invention for solving the above-described problem is a magnetic resonance imaging apparatus for imaging a multi-slice perfusion image by labeling arterial blood spins, and changing the scanning order for a plurality of slices. An magnetic resonance imaging apparatus comprising: an imaging unit that repeats imaging a plurality of times; and an adding unit that adds the plurality of captured images for each same slice.
[0008]
(Function)
In the present invention, for a plurality of slices, imaging is repeated a plurality of times while changing the scanning order, and images obtained by imaging a plurality of slices per slice are added for each same slice, and the brightness of the image is made uniform.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the embodiment. FIG. 1 shows a block diagram of the magnetic resonance imaging apparatus. This apparatus is an example of an embodiment of the present invention. An example of an embodiment relating to the apparatus of the present invention is shown by the configuration of the apparatus. An example of an embodiment related to the method of the present invention is shown by the operation of the apparatus.
[0010]
The configuration of this apparatus will be described. As shown in FIG. 1, in the present apparatus, a substantially cylindrical static magnetic field generator 2 forms a uniform static magnetic field (main magnetic field) in its internal space. Inside the static magnetic field generating unit 2, a gradient coil (coil) part 4 and a head coil (head coil) part 6 having a substantially cylindrical shape are arranged sharing a central axis. The head of the subject 8 is inserted into a substantially columnar space formed inside the head coil unit 6. The subject 8 is carried into and out of the magnetic field space by a transfer means (not shown).
[0011]
A gradient driving unit 10 is connected to the gradient coil unit 4. The gradient drive unit 10 gives a drive signal to the gradient coil unit 4 to generate a gradient magnetic field. Three types of gradient magnetic fields are generated: a slice gradient magnetic field, a readout (read out) gradient magnetic field, and a phase encoding (phase encode) gradient magnetic field.
[0012]
A transmission unit 12 is connected to the head coil unit 6. The transmission unit 12 applies a drive signal (RF (radio frequency) signal) to the head coil unit 6 to generate an RF magnetic field, thereby exciting the spin in the body of the subject 8.
[0013]
A magnetic resonance signal generated by the excited spin is detected by the head coil unit 6. A receiving unit 14 is connected to the head coil unit 6. The receiving unit 14 is configured to receive a signal detected by the head coil unit 6.
[0014]
An analog-to-digital converter 16 is connected to the receiver 14. The analog / digital converter 16 converts the output signal of the receiver 14 into a digital signal.
[0015]
A computer 18 receives a digital signal from the analog / digital converter 16 and stores it in a memory (not shown). A data space is formed in the memory. This data space constitutes a two-dimensional Fourier space. The computer 18 reconstructs an image of the subject 8 by performing two-dimensional inverse Fourier transform on the data in the two-dimensional Fourier space.
[0016]
A controller 20 is connected to the computer 18. The control unit 20 is connected to the gradient driving unit 10, the transmission unit 12, the reception unit 14, and the analog / digital conversion unit 16. The control unit 20 controls the gradient driving unit 10, the transmission unit 12, the reception unit 14, and the analog / digital conversion unit 16 based on a command given from the computer 18.
[0017]
The static magnetic field generation unit 2, the gradient coil unit 4, the head coil unit 6, the gradient drive unit 10, the transmission unit 12, the reception unit 14, the analog / digital conversion unit 16, the computer 18 and the control unit 20 are the imaging means in the present invention. It is an example of an embodiment. The computer 18 is an example of an embodiment of the adding means in the present invention.
[0018]
A display unit 22 and an operation unit 24 are connected to the computer 18. The display unit 22 displays various information including a reconstructed image output from the computer 18. The operation unit 24 is operated by an operator and inputs various commands and information to the computer 18.
[0019]
The operation of this apparatus will be described. Imaging is performed under the control of the control unit 20. As one specific example of magnetic resonance imaging, a case where a perfusion image related to CBF is captured by the ASL method will be described. For this imaging, for example, a pulse sequence as shown in FIG. 2 is used.
[0020]
FIG. 2 is a time chart of a pulse sequence for collecting magnetic resonance signals. The pulse sequence proceeds from left to right along the time axis t. The execution of this pulse sequence is controlled by the control unit 20.
[0021]
First, as shown in (1) of FIG. 2, RF excitation is performed by an inversion pulse. RF excitation is performed by the head coil unit 6 driven by the transmission unit 12. At this time, the slice gradient magnetic field Gs is applied as shown in (2). The application of the slice gradient magnetic field Gs is performed by the gradient coil unit 4 driven by the gradient drive unit 10. Thereby, the spin of a predetermined part of the head of the subject 8 is inverted (selectively inverted).
[0022]
The selective inversion of spin is performed as shown in FIG. That is, as shown in the figure, when imaging within the slab from the distance z2 to z3 in the body axis (z-axis) direction of the subject 8 with, for example, five slices a to e, one mode Then, as indicated by the solid line, the spin of the slab from the distance z1 to z2 is reversed. The number of slices is not limited to five and may be set arbitrarily.
[0023]
Here, blood macroscopically flows from left to right along the z-axis. Accordingly, the spin of the slab upstream of the blood flow is reversed as viewed from the imaging target region. Note that the slab thickness for reversing the spin is made larger than the slab thickness of the imaging target region. Such selective inversion is called a tag.
[0024]
In other modes, the selective inversion of the spin is performed on the slab from the distance z3 to the distance z4 downstream of the imaging target region as shown by the broken line by changing the slice gradient magnetic field Gs. Such selective inversion is referred to as control. Tags and controls are alternated.
[0025]
As will be described later, the perfusion image is obtained as an image of a difference between an image captured in the tag state (tag image) and an image captured in the control state (control image). A technique for capturing a perfusion image using a combination of a tag and a control as described above is called EPISTAR (echo planar imaging with signaling targeting RF).
[0026]
Note that the technique for capturing a perfusion image is not limited to EPISTAR. For example, PICORE (proximal inversion with a control for resonance effects), FAIR (flow alternative inversion), and other techniques, such as the use of FIR (flow alternated inversion) and other techniques, are used. Good.
[0027]
In PICORE, as shown in FIG. 4, tags similar to those of EPISTAR are performed, and control is performed by so-called off-resonance RF excitation in which the frequency of the inversion pulse is shifted from the magnetic resonance frequency of the spin. Due to the off-resonance, no spin inversion occurs in the entire excitation range in the control state. In FAIR, as shown in FIG. 5, the spin in the imaging range is inverted by RF excitation selected by the tag, and the non-selection inversion is performed by the control.
[0028]
In the above technique, on the upstream side of the imaging region, spin inversion and non-inversion (EPISTAR, PICORE) or non-inversion and inversion (FAIR) are performed by tags and controls, respectively. Non-inversion (EPISTAR, PICORE) or spin inversion (FAIR) is performed.
[0029]
Hereinafter, an example of EPISTAR will be described, but PICORE, FAIR, and other techniques are based on this. Assuming that the tag image is captured first, spin inversion is performed on the upstream side of the imaging region. After the inversion of the spin, as shown in (2) of FIG. 2, the spin is dephased with the slice gradient magnetic field Gs whose amplitude is changed.
[0030]
After a time TI from the spin inversion, the slice a is scanned by EPI (echo planar imaging). The time TI is appropriately set in the range of 600 to 2000 ms, for example.
[0031]
In EPI, as shown in (1) and (2) of FIG. 2, for example, RF excitation for slice a is performed by a 90-degree pulse and a slice gradient magnetic field Gs. After the spin excitation, spin rephasing is performed by the slice gradient magnetic field Gs.
[0032]
During the time TI, the blood whose spin is inverted upstream flows into the slice a. The spin in the inflowing blood is in the middle of longitudinal relaxation from reversal, and the macroscopic longitudinal magnetization intensity is different from the surrounding tissue where the position is fixed. Such a spin is RF-excited by a 90-degree pulse together with the spins of surrounding tissues, and the direction of the spin is tilted by 90 degrees.
[0033]
Next, after a predetermined time TE, as shown in (1) and (2), the slice a is selectively excited with the 180-degree pulse and the slice gradient magnetic field Gs, and the spin direction is rotated by 180 degrees.
[0034]
Subsequently, a read gradient magnetic field Gr as shown in (3) first dephases the spin, and then periodically reverses the direction of the magnetic field, so that a plurality (for example, 256) is obtained as shown in (1). ) Magnetic resonance signals, that is, spin echoes, are generated sequentially. At this time, in synchronization with it, a large phase encoding is first applied to the spin by the phase encoding gradient magnetic field Gp as shown in (4), and then the phase encoding is changed little by little by a blip pulse. A different phase encoding is applied to each of the plurality of spin echoes obtained sequentially.
[0035]
FIG. 2 shows an example of EPI that uses spin echo, but is not limited thereto. EPI that uses gradient echo, SE (spin echo) method, GRE (gradient echo) method, Of course, any imaging method such as the Spiral method may be used. Hereinafter, an example of EPI using spin echo will be described.
[0036]
The spin echo is received by the head coil unit 6. The received signal is input to the computer 18 via the receiver 14 and the analog / digital converter 16. The computer 18 stores the input signal as measurement data in a memory. As a result, spin echo data for one screen relating to the slice a is collected in the memory. The memory space that collects the spin echo data constitutes a two-dimensional Fourier space.
[0037]
Subsequently, spin echo data regarding slices b, c, d, and e is sequentially collected by EPI in the same manner as described above. Thereby, as conceptually shown in FIG. 6, after the inversion (tag) of the spin on the upstream side of the imaging region by the inversion pulse, the spin echo data for one screen regarding the slices a to e is sequentially collected. The spin echo data of each slice is stored in a different area of the memory.
[0038]
Next, imaging regarding slices a to e is performed in the control state. A pulse sequence according to the pulse sequence shown in FIG. 2 is also used for this imaging, except that the spin inversion by the inversion pulse is performed on the downstream side of the imaging region as shown in FIG. Thereby, as conceptually shown in FIG. 6, after the inversion (control) of the spin on the downstream side of the imaging region by the inversion pulse, the spin echo data for one screen regarding the slices a to e is sequentially collected. The spin echo data of each slice is stored separately from the data collected in the tag state in different areas of the memory.
[0039]
Using the echo data collected in the memory, the tag image and the control image are reconstructed for each slice. Image reconstruction is performed by two-dimensional inverse Fourier transform. The tag image and the control image obtain a difference between the same slices.
[0040]
The tag image includes an image of blood that has flowed in a state in which spin is reversed on the upstream side and in the middle of longitudinal relaxation. The control image includes an image of blood that has flowed without spin inversion. That is, in the tag image and the control image, the macroscopic magnetization states of the spins of blood flowing from the upstream are different from each other. On the other hand, for tissues other than blood, since the positions are fixed, there is no difference in magnetization state between the tag and the control, so the same image is included. Therefore, by taking the difference between the two images, the same image disappears, and an image of blood whose magnetization states are different from each other, that is, an image of blood that has flowed in with the spin reversed on the upstream side is obtained. This is nothing but a perfusion image.
[0041]
In this way, in the perfusion image using the difference in magnetization state, the image signal intensity is weak. Therefore, imaging is performed a plurality of times (for example, 100 times) for each slice, and the obtained perfusion images are fully added. The image signal intensity is increased and the S / N (signal-to-noise ratio) is improved.
[0042]
In this apparatus, the EPI order is sequentially changed when performing imaging for each slice a plurality of times. An example is conceptually shown in FIG. As shown in the figure, in the first imaging, after TI from the tag (T), data is collected by EPI in the order of slices a, b, c, d, and e, and then from the control (C). After TI, data collection by EPI is performed in the order of slices a, b, c, d, and e.
[0043]
In the second imaging, data collection by EPI is performed in the order of slices b, c, d, e, and a after the TI from the tag (T), and then the slices b and c after the TI from the control (C). , D, e, and a, respectively, in order of data collection.
[0044]
In the third imaging (not shown), data collection by EPI is performed in the order of slices c, d, e, a, and b after TI from the tag (T), and then, after TI from control (C), slice c , D, e, a, and b, respectively, in order of data collection.
[0045]
Thereafter, similarly, the scan for each slice is performed while sequentially changing the order of performing EPI. Thus, for example, as shown in FIG. 8, the slice a is scanned first in the first shooting, scanned fifth in the second shooting, and scanned fourth in the third shooting. In the second shooting, the third scan is performed, and in the fifth shooting, the second data collection is performed. From the sixth time onward, the above operations are repeated. Slices b, c, d, and e are scanned according to the above, starting from the order 2, 3, 4, and 5, respectively.
[0046]
And the perfusion image regarding slice a adds all the difference images of the tag image imaged multiple times as mentioned above, and a control image. The same applies to slices b, c, d, and e. Note that the change of the data collection order is not necessarily performed regularly as described above, and may be performed irregularly. However, it is a condition to maintain order equality for all slices.
[0047]
In this case, as shown in FIG. 9, the time from when the tag or control is applied until the scan is performed differs as t1 to t5 depending on the order, but the scan order is changed for each slice. Since it is changed every time, a perfusion image based on the magnetic resonance signal after time t1 from the reversal of spin or a perfusion image based on the magnetic resonance signal after time t5 is obtained equally for every slice.
[0048]
For this reason, if such images are fully added for each slice, the image signal intensity becomes uniform in the perfusion image of any slice. That is, a perfusion image with uniform brightness can be obtained for all slices.
[0049]
The above is an example of a magnetic resonance imaging apparatus using a horizontal magnetic field. The magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus using a so-called vertical magnetic field in which the direction of the static magnetic field is perpendicular to the body axis of the subject. Needless to say.
[0050]
【The invention's effect】
As described above in detail, according to the present invention, a magnetic resonance imaging method and apparatus for obtaining a multi-slice perfusion image with uniform brightness can be realized.
[Brief description of the drawings]
FIG. 1 is a block diagram of an exemplary apparatus according to an embodiment of the present invention.
FIG. 2 is a time chart illustrating an example of a pulse sequence used in imaging by an apparatus according to an embodiment of the present invention.
FIG. 3 is a diagram showing an example of a tag and control profile by an example apparatus according to an embodiment of the present invention.
FIG. 4 is a diagram showing an example of a tag and control profile by the apparatus according to the embodiment of the present invention.
FIG. 5 is a diagram showing an example of a tag and control profile by an example device according to an embodiment of the present invention;
FIG. 6 is a time chart of an example of multi-slice scanning by an example of an embodiment of the present invention.
FIG. 7 is a time chart of an example of a plurality of times of imaging performed by the apparatus of the example of the embodiment of the present invention.
FIG. 8 is a diagram illustrating an example of the order of multi-slice scans in a plurality of imaging operations performed by the apparatus according to the exemplary embodiment of the present invention.
FIG. 9 is a time chart of multi-slice scan execution by an example apparatus according to an embodiment of the present invention;
[Explanation of symbols]
2 Static Magnetic Field Generation Unit 4 Gradient Coil Unit 6 Head Coil Unit 8 Subject 10 Gradient Driving Unit 12 Transmitting Unit 14 Receiving Unit 16 Analog / Digital Conversion Unit 18 Computer 20 Control Unit 22 Display Unit 24 Operation Unit

Claims (2)

A magnetic resonance imaging device for labeling arterial blood spins and imaging multi-slice perfusion images,
An imaging unit that repeats imaging a plurality of times while changing the order so that all the scans in each of a plurality of slices of three or more slices that are scanned within 1 TR time are performed;
A magnetic resonance imaging apparatus comprising: an adding unit that adds the plurality of images of the same slice in a plurality of images of the plurality of slices obtained by the imaging. The magnetic resonance imaging apparatus according to claim 1,
The method of imaging the perfusion image is a method by EPISTAR (echo planar imaging with signal targeting using alternating RF), PICORE (proximal inversion with a control for off resonance effects) or FAIR (flow alternated inversion recovery). A magnetic resonance imaging apparatus.

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