JP3342721B2 - MRI equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MRI apparatus,
More specifically, the present invention relates to an improvement in an MRI apparatus that performs MRI imaging using a plurality of echoes.

[0002]

2. Description of the Related Art FIG. 6 is a diagram illustrating an example of an MRI imaging method using a plurality of echoes. The k-space ksp is 400
(= N) views. Each view is +200
It is associated with a phase from * gws to -199 * gws (gws is a gradient amount in one warp step). The k-space ksp is divided into 5 (= S) blocks b1, b2, b3, b4 of 80 (= M) continuous views.
b5, and block numbers 1 to 5 are assigned to them (note that depending on N and S, the number of views of some blocks may be slightly different from the number of views of other blocks). Also, view numbers # 1 to # 80 are added to the views of each block in the order of larger phases on the positive side.

[0003] The first multiple echoes from the first to the fifth order are divided into blocks b1, b2, b3, b
A large phase gradient is applied to the most positive side of the blocks b4 and b5, and data of the # 1 view of each block b1, b2, b3, b4 and b5 is collected. For the second multiple echoes of the first to fifth orders, the respective blocks b1, b2, b3, b4
A second large phase gradient is applied to the positive side in b5, and data of the # 2 view of each block b1, b2, b3, b4, b5 is collected. Generally, for the k-th multiple echoes of the first to fifth orders, each block b1, b2, b
A large phase gradient is given to the k-th positive side in 3, b4, b5, and #k of each block b1, b2, b3, b4, b5
Collect data for the view. This is repeated until the 80th time, and data of N (400 = 5 × 80) views in k-space ksp is collected. Then, an image is reconstructed based on the N-view data.

FIG. 7 is a pulse sequence diagram of the RARE method for collecting a plurality of first to fifth echoes E1 to E5 of the first time (echoes E4 and E5 are not shown).
From the first-order echo E1, data corresponding to the view # 1 having the largest phase (phase gradient = + 200 * gws) in the block b1 is obtained. From the secondary echo E2, the largest phase in the block b2 (phase gradient = + 120 *
gws). Data corresponding to view # 1 is obtained. 3
From the next echo E3, data corresponding to the view # 1 having the largest phase (phase gradient = + 40 * gws) in the block b3 is obtained. Although not shown, data corresponding to the view # 1 having the largest phase (phase gradient = −40 * gws) in the block b4 is obtained from the fourth-order echo E4. From the fifth-order echo E5, data corresponding to the view # 1 having the largest phase (phase gradient = -120 * gws) in the block b5 is obtained.

[0005] As shown in FIG. 6, a DC block including a DC view is a block b3. Therefore, FIG.
As shown in the figure, the third-order echo E in charge of the block b3
The time 3 · Te up to 3 is the DC echo time TE. Note that Te is an echo interval. Since the DC block predominantly determines the contrast of the image, in other words, the DC echo time TE determines the contrast of the image.

[0006]

In the conventional MRI apparatus, the block number j of the DC block is j = (S + 1) / 2 (1). Therefore, the DC echo time TE is as follows: TE = Te · j = Te · (S + 1) / 2 (2) The echo interval Te is a minimum value Te_m due to the limitation of the hardware of the MRI apparatus (for example, the capability of the gradient power supply).
have in. Therefore, the expression (2) is as follows: TE ≧ Te_min · (S + 1) / 2 (3) The expression (3) indicates that the DC echo time TE is restricted by the number S of blocks.

That is, in the conventional MRI apparatus, if the number of blocks S is increased in order to shorten the data collection time, DC
The echo time TE increases, and the small DC echo time T
There is a problem that an image with a contrast of E cannot be obtained. Conversely, if an image with a contrast of a small DC echo time TE is to be obtained, the number of blocks S cannot be increased and the data collection time cannot be reduced. Was. Therefore, an object of the present invention is to provide an MRI apparatus which is improved so that the DC echo time TE is not restricted by the number S of blocks, and which can obtain a contrast image with a small DC echo time TE even if the number S of blocks is increased. To provide.

[0008]

An MRI apparatus according to the present invention divides an N-view constituting a k-space into S blocks each including M views as one block, and forms a plurality of echoes from the first order to the S-order. MR to share and collect data
In the I device, a DC echo time designating means for directly or indirectly designating a DC echo time TE up to an echo sharing a DC block including a DC view, and a DC block so as to satisfy the DC echo time TE In addition to determining the order i of the echo to be shared, block assignment echo determining means for determining the order of the echo sharing the blocks other than the DC block according to the order i is provided. In the above configuration,
To directly specify the DC echo time TE means to explicitly specify the DC echo time. Also, DC
Specifying the echo time TE indirectly means that by specifying a parameter other than the DC echo time,
It means that the echo time is determined.

[0009]

In the conventional MRI apparatus, the echo corresponding to the DC block is fixed at the center of the first to S-order echoes. In the MRI apparatus of the present invention, the first to S-order echoes are used. Is not fixed to the center echo, and the DC block is assigned to the echo of order i that satisfies the designated DC echo time TE, and the blocks other than the DC block are assigned to the echoes of order other than the order i. Therefore, the DC echo time TE is no longer limited by the number S of blocks, and even if the number S of blocks is increased, an image having a small DC echo time TE can be obtained.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on an embodiment shown in the drawings. It should be noted that the present invention is not limited by this. FIG. 1 shows an MRI apparatus 1 according to one embodiment of the present invention.
It is a block diagram of. The computer 2 controls the overall operation based on instructions from the console 13. The sequence controller 3 operates the gradient magnetic field drive circuit 4 based on the stored sequence to generate a gradient magnetic field with the gradient magnetic field coil of the magnet assembly 5. Further, it controls the gate modulation circuit 7, modulates the RF pulse generated by the RF oscillation circuit 6 into a predetermined waveform, and applies the RF pulse from the RF power amplifier 8 to the transmission coil of the magnet assembly 5.

The NMR signal obtained by the receiving coil of the magnet assembly 5 is input to a phase detector 10 via a preamplifier 9 and further input to a computer 2 via an AD converter 11. The computer 2 reconstructs an image based on the NMR signal data obtained from the AD converter 11 and displays the image on the display device 12. FIG. 2 is an explanatory diagram of an MRI imaging method using a plurality of echoes performed by the MRI apparatus 1. The k-space ksp is composed of 400 (= N) views. Each view is associated with a phase from + 200 * gws to -199 * gws (gws is the amount of gradient in one warp step). The k-space ksp is divided into 5 (=
S) divided into blocks b1, b2, b3, b4, b5, and assigned block numbers 1 to 5 (note that N
And S, the number of views of some blocks may be slightly different from the number of views of other blocks.) Also, view numbers # 1 to # 80 are added to the views of each block in the order of larger phases on the positive side.

Then, for the first plurality of echoes from the first to the fourth order, blocks b2, b3, b4, b5
A large phase gradient is given to the most positive side of
Data of the # 1 views # 2, b3, b4 and b5 are collected. For the block b1, no echo is made to correspond, and no data is collected. The blocks b2, b3, b4, b
5, the second largest positive phase gradient is applied to the positive side, and data of the # 2 view of each block b2, b3, b4, b5 is collected. For the block b1, no echo is made to correspond, and no data is collected. Generally, for the k-th multiple echoes of the first to fourth orders, the block b2
A large phase gradient is given to the k-th positive side in b3, b4, and b5, and data of the #k view of each block b2, b3, b4, and b5 is collected. For the block b1, no echo is made to correspond, and no data is collected. This is the eighth
Repeat till the 0th time, blocks b2 and b in k-space ksp
Data of a total of 320 = 4 × 80 views of 3, b4 and b5 are collected. Then, in order to correct that data on the block b1 is not collected, a complex conjugate using the Fourier transform characteristic or a reconstruction method using a Hilbert transform process is executed to reconstruct an image.

FIG. 3 is a pulse sequence diagram of the RARE method for collecting a plurality of first to third echoes of the first time. From the primary echo E1, the view # of the largest phase (phase gradient = + 120 * gws) in the block b2
Data corresponding to 1 is obtained. From the secondary echo E2, the largest phase in the block b3 (phase gradient = + 4
0 * gws), data corresponding to view # 1 is obtained. From the third-order echo E3, data corresponding to the view # 1 having the largest phase (phase gradient = −40 * gws) in the block b4 is obtained. Although not shown, data corresponding to view # 1 having the largest phase (phase gradient = −120 * gws) in block b5 is obtained from the fourth-order echo E4. The difference between the pulse sequence A of the RARE method in FIG. 3 and the pulse sequence B of the RARE method in FIG. 7 is a gradient applied to the warp axis.

Now, as shown in FIG. 2, the DC block including the DC view is a block b3. Therefore, FIG.
As shown in the figure, the secondary echo E in charge of the block b3
The time 2 · Te up to 2 is the DC echo time TE. Note that Te is an echo interval. In the above example, the block b3 is assigned to the secondary echo E2. However, if the block b3 is assigned to the primary echo E1, the DC echo time TE = Te. Thus, the echo time TE can be reduced regardless of the number S of blocks. Therefore, even if the number S of blocks is increased, an image having a small DC echo time TE can be obtained.

FIGS. 4 and 5 show the above MRI apparatus 1.
It is a flowchart of operation | movement at the time of implementing the MRI imaging method using multiple echoes. After setting the subject on the magnet assembly 5, when the user gives an instruction to use the console 13 to execute the MRI imaging method using a plurality of echoes, the computer 2 executes the following processing.

In step S1, a desired number S of blocks for dividing the k-space and a desired DC echo time TE are designated. For example, S = 5, TE = 50 ms. In step S2, the block number j of the original DC block is calculated. Since this block number j is the center from 1 to S, j = (S + 1) / 2. For example, if S = 5, j = (5 + 1) / 2 = 3
Becomes In step S3, j is applied to the block number i of the actual DC block. That is, i = j. For example, i = j = 3.

In step S4, an echo interval Te is calculated. The block number i of the actual DC block corresponds to the i-th echo, and the echo time up to the i-th echo is the DC echo time TE specified in step S1.
Therefore, the echo interval Te is Te = TE / i. For example, if TE = 50 ms and i = 3, Te =
50/3 = 16.7 ms. In step S5, it is checked whether the echo time Te is longer than the minimum value Te_min determined by the hardware limitation of the MRI apparatus 1. If Te ≧ Te_min is not satisfied, execution is not possible, and the process proceeds to step S6. If Te ≧ Te_min, the process can be performed, and the process proceeds to step S7. For example, Te = 16.7m
If s, Te_min = 20 ms, Te ≧ Te_min is not satisfied, so that execution is impossible, and the process proceeds to step S6. Step S
At 6, the block number i of the actual DC block is incremented by one, and the process returns to step S4. For example, i = 3-
1 = 2, and the process returns to step S4. In step S4, the echo interval Te is determined by Te = TE / i. For example, if TE = 50 ms and i = 2, Te = 5
0/2 = 25 ms. In step S5, it is checked whether the echo time Te is longer than the minimum value Te_min. For example, if Te = 25 ms and Te_min = 20 ms, Te
≧ Te_min, so that the operation can be performed.
Proceed to.

In step S7, the block number (j-
i) Only blocks from +1 to block number S are assigned to echoes of order 1 to order S- (ji). For example, if j = 3 and i = 2, only blocks b2, b3, b4 and b5 from block number 2 to block number 5 are assigned to the echoes of degree 1 to degree 4. In step S8, a scan is executed to collect data. For example, blocks b2 to b5 from block number 2 to block number 5
When only b3, b4, and b5 are assigned to echoes of order 1 to order 4, data of the solid line portion of the k-space ksp in FIG. 2 is obtained. Data of the broken line portion of the k-space ksp in FIG. 2 cannot be obtained. That is, the data becomes asymmetric.

Proceeding to FIG. 5, in step R1, it is checked whether the data is asymmetric. If it is asymmetric, the data in k-space is not ready, so the process proceeds to step R2. If the data is not asymmetric, k-space data is available, so step R
Proceed to 3. In step R2, a complex conjugate using Fourier transform characteristics, a Hilbert transform process, or the like is used to correct that data for blocks from block number 1 to block number (ji) has not been collected. An image is obtained by executing the reconstruction method. For example, j
= 3, i = 2, no data has been collected for the block of block number 1, and in order to correct this, a complex conjugate using the Fourier transform characteristic or a reconstruction using the Hilbert transform process, etc. Perform the method to obtain an image. In step R3, an image is obtained by executing a normal reconstruction method.

As described above, in the MRI apparatus 1, when performing MRI imaging using a plurality of echoes, the DC echo time TE is no longer limited by the number of blocks S, and even if the number of blocks S is increased, a small DC echo time An image with TE contrast can be obtained. Since the order of the echo to be used is reduced, the repetition time TR can be shortened, and the number of slices can be shortened, whereby the effect of shortening the data collection time can be obtained.

The MRI apparatus 1 not only eliminates the problem caused by the minimum echo interval Te_min determined by hardware limitations but also obtains two images from one echo train by the 2-contrast method. Problems caused by the limitation of the echo interval can also be solved. In other words, if the user sets the desired Te_min,
This means that it is possible to increase the echo interval Te without changing the DC echo time TE. However, in this case, sampling becomes easy, and an effect of improving the SN ratio is obtained.

Further, in the above-described embodiment, the order of the echo corresponding to the DC block is advanced to reduce the DC echo time TE without reducing the echo interval Te. As an example, the DC echo time T is increased without increasing the echo interval Te.
In order to increase E, there is a method in which the order of the echo corresponding to the DC block is moved backward.

[0023]

According to the MRI apparatus of the present invention, when performing MRI imaging using a plurality of echoes, the DC echo time TE can be reduced without reducing the echo interval Te. Conversely, the DC echo time TE can be increased without increasing the echo interval Te.
For this reason, an image having a wide range of contrast from an image having a small DC echo time TE to an image having a large DC echo time TE can be suitably obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram of an embodiment of an MRI apparatus of the present invention.

FIG. 2 is an explanatory diagram of MRI imaging using a plurality of echoes performed by the apparatus of FIG. 1;

FIG. 3 is a view showing an example of a pulse sequence used for MRI imaging using a plurality of echoes.

FIG. 4 shows an MR using a plurality of echoes in the MRI apparatus of FIG.
It is a flowchart of the data collection operation | movement at the time of implementing I imaging method.

FIG. 5 shows an MR using a plurality of echoes in the MRI apparatus of FIG.
It is a flowchart of the reconstruction operation | movement at the time of implementing I imaging method.

FIG. 6 is an explanatory diagram of conventional MRI imaging using multiple echoes.

FIG. 7 is an exemplary diagram of a pulse sequence used in MRI imaging using a plurality of echoes in the related art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 MRI apparatus 2 Computer b1-b5 Block E1-E3 Echo ksp k space

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-245123 (JP, A) JP-A-5-317289 (JP, A) JP-A-6-205758 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) A61B 5/055 JICST file (JOIS)

Claims (2)

(57) [Claims] 1. An MRI apparatus which divides N views constituting a k-space into S blocks each including M views as one block, and assigns each of the divided blocks to a plurality of echoes from a first order to an S order to collect data. Which directly or indirectly specifies the DC echo time TE up to the echo sharing the DC block including the DC view
C echo time designating means, and the order of the echoes sharing the DC block so as to satisfy the DC echo time TE, including the case where the result of the echo sharing with respect to the block becomes asymmetric in the k space. i, and according to its order i, D
An MRI apparatus comprising: a block sharing echo determination unit that determines an order of an echo sharing a block other than the C block. 2. An MRI apparatus which divides N views constituting a k-space into S blocks each including M views as one block, and assigns each of the divided echoes to a plurality of echoes from a first order to an S order to collect data. Which directly or indirectly specifies the DC echo time TE up to the echo sharing the DC block including the DC view
C echo time designating means, and determines the order i of the echo sharing the DC block so as to satisfy the DC echo time TE, and determines the order of the echo sharing the blocks other than the DC block according to the order i. The N views constituting the k-space are divided into S blocks from the high-frequency side, where each successive M views are one block, and block numbers 1 to S are sequentially assigned to each block. And when the minimum echo interval possible in the MRI apparatus is Te_min and j = (S + 1) / 2, the block-sharing echo determination means determines that i ≦ j and TE /
The order i of the echo sharing the DC block is determined so as to satisfy i ≧ Te_min, and the block number (j
-I) Only blocks from +1 to block number S are shared by echoes of order 1 to order S- (ji), and data on blocks from block number 1 to block number (ji) An MRI apparatus further comprising reconstruction means for executing a complex conjugate using a characteristic of a Fourier transform or a reconstruction method using a Hilbert transform process in order to correct that no data is collected.