JPH04352945A - Mr imaging method - Google Patents

Abstract

PURPOSE:To generate N-number of images different in contrast by applying phase rotation to k-order echo to collect data, sequencing the data to create mixed data, and performing Fourier transformation using the mixed data to separate an image by N-cycles. CONSTITUTION:A sequence controller 3 is adapted to operate a magnetic field driving circuit 4 to generate static magnetic field and gradient magnetic field in a magnet assembly 5. A NMR signal obtained by a receive coil of the magnet assembly 5 is input to a phase detector 10 through a head amplifier 9, and an image is reconfigurated according to the data of NMR signals by a computer 2 to be displayed on a display device 12. Concerning each (n) of n=-N/2 to N/2-1(N is a desired number of images), the phase rotation is applied to k-order echo to collect data, and the data is sequenced to create mixed data. Fourier transformation is performed using the mixed data to reconfigurate an image, and the image is separated into N-parts in the warp direction.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to an MR imaging method, and more specifically, to an SSFP sequence (St
eady-State Free Precessio
The present invention relates to an MR imaging method that can simultaneously obtain a plurality of images with different contrasts using a MR imaging method (n sequence).

0002

[Prior art] For example, "JOURNAL OF MAG
NETIC RESONANCE 81, 474-4
83 (1989)” and “MAGNETICRESON”
ANCE IMAGING Vol. 6, pp. 35
3-368 (1988),”
As SSFP sequences, FISP, GRASS, C
Pulse sequences such as E-FAST are known. FIG. 6 illustrates a pulse sequence of the FISP method.

0003

[Problem to be Solved by the Invention] The signal obtained by the SSFP sequence is composed of overlapping echoes of different orders. Conventionally, it was not possible to separate these, so
There was a problem that only one image could be obtained.

[0004] Accordingly, an object of the present invention is to provide an MR imaging method that can simultaneously obtain a plurality of images with different contrasts using an SSFP sequence.

[0005]

[Means for Solving the Problems] The MR imaging method of the present invention is an MR imaging method in which data is collected by an SSFP sequence, where n=-N/2 to N/2- For each n of 1, the phase rotation k・θn (θn = 2πn/N) is expressed as k (k=0,
±1, ±2, ...) to the next echo to collect data Pn, and when the data number in the read direction is r and the data number in the warp direction is m, the data Pn(r,
m)→Mix data Pmix(r, m・N+n+N
/2) rearranging the data to create mix data Pmix from the collected data Pn; reconstructing an image by performing Fourier transform using the mix data Pmix; and reconstructing the reconstructed image. The configuration is characterized in that it has a step of dividing the image into N pieces in the warp direction.

In the above configuration, methods for imparting phase rotation to the k-th echo include a method in which the RF phase is rotated by θn each time with respect to the previous phase, and a method in which the total area of the gradient magnetic field during the time TR is There are ways to insert a spoiler without setting it to 0'' (no rephasing is required).

[0007]

[Operation] In the MR imaging method of the present invention, a phase rotation k·θn is applied to the k-th echo to collect data Pn. The data Pn is obtained by overlapping echoes of different orders, and the data of the echoes of different orders cannot be separated as it is. However, data Pn(r,m)→
Mix data Pmix by rearranging the data so that it becomes mix data Pmix (r, m・N+n+N/2)
, and performs a Fourier transform using the mix data Pmix, it is possible to separate images resulting from the k-th echo in N cycles. That is, N images with different contrasts are obtained.

[0008]Explaining it numerically, it is as follows. S
In the SFP sequence, the signal S(t
)teeth,

[0009]

[Math 1]

[0010]

[0011]

[Math 2]

[0012] Also,

[0013]

[Math 3]

From this, (2) and (3) become (1)
When substituted into

[0015]

[Math 4]

[0016]

(4) represents the FID immediately after RF, and (7
) represents the FID immediately before the RF, (5) (6) (8) (
9) represents higher-order echoes included in them. Since higher-order echoes attenuate by the nth power of (q/2p),
The images of each order of echo will have different contrast and signal intensity.

Next, consider the case where the spin phase is rotated by θ every time TR. At this time, δωTR →
Since it can be expressed as δωTR+θ, (3) becomes

0019
]

[Math 5]

[0020] Therefore, (4) to (9) are

0
021]

[Math 6]

[0022] In other words, the RF given by (11)
Compared to the echo immediately after, the echo of each order is subjected to a phase rotation corresponding to the order. (However, for echoes obtained during RF with phase rotation θ, phase rotation θ is commonly applied to higher-order echoes, so after data collection, phase correction e
It is necessary to multiply by xp(-jθ). The above formula includes this correction. ) The main cause of δω is the inhomogeneity of the static magnetic field, but since the homogeneity of the static magnetic field is usually very good, g(
δω) can be considered to be like a delta function δ(δω). Then, f(t) becomes almost constant, and S(t) includes a very large number of high-order echoes in the time period of interest 0≦t≦TR. High-order echoes are summarized by phase and S(
t) is expressed as

[0023]

[Math 7]

[0024]

[0025] As θn =2πn/N, n=-N/2
When data Pn is collected for each n of ~N/2-1,
As shown in (17), the k-th echo is MOD(2
undergoes a phase rotation of πkn/N, 2π). However, MOD
When (2πkn/N, 2π)>π, -2π+MOD
Since it has the same effect as (2πkn/N, 2π)<0, M
OD(2πin/N, 2π)=-2π+MOD(2πj
The i-th echo and the j-th echo, which are n/N, 2π), are treated as a group with the same phase rotation. Also, similarly,
When MOD(2πkn/N, 2π)<-π, 2π+
Since it has the same effect as MOD(2πkn/N, 2π)>0, MOD(2πin/N, 2π)=2π+MOD(2
The i-th echo and the j-th echo, which are πjn/N, 2π), are treated as a group with the same phase rotation. That is, the echoes are divided into N phase rotation groups as follows.

[0026]

[Math. 8]

If the obtained data Pn are arranged alternately and subjected to Fourier transformation, N images with different contrasts are obtained.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained in more detail below with reference to embodiments shown in the drawings. Note that this invention is not limited to this. FIG. 5 is a block diagram showing an MR apparatus 1 implementing the MR imaging method of the present invention. The computer 2 controls the overall operation based on instructions from the console 13.

The sequence controller 3 operates the magnetic field drive circuit 4 based on the stored sequence,
A static magnetic field and a gradient magnetic field are generated by the static magnetic field coil and the gradient magnetic field coil of the magnet assembly 5. In addition, the modulation circuit 7
, modulates the RF signal generated by the RF oscillation circuit 6 into a predetermined waveform, and applies it from the RF power amplifier 8 to the transmitting coil of the magnet assembly 5.

The NMR signal obtained by the receiving coil of the magnet assembly 5 is input to the phase detector 10 via the preamplifier 9, and further input to the computer 2 via the AD converter 11. The computer 2 reconstructs an image based on the NMR signal data obtained from the AD converter 11 and displays it on the display device 12.

The MR imaging method of the present invention is carried out by procedures stored in the computer 2 and the sequence controller 3. FIG. 1 is a flow diagram of the MR imaging method of the present invention.

In step S1, the desired number of images "N", the number of data "Mr×Mw", and the FOV "Lr×Lw" are determined. Here, Mr is the number of data in the read direction, M
w is the number of data in the warp direction. Further, Lr is the FOV in the read direction, and Lw is the FOV in the warp direction. To give a numerical example, N=2, Mr=Mw=M=64, Lr=L
w=L=30cm.

In step S2, θn is calculated. n
=-N/2 to N/2, and θn =2πn/N. To give a numerical example, when N=2, n=-1, 0, θ-1=±π, θ0=0.

In step S3, one n is extracted. To give a numerical example, take n=-1.

In step S4, data Pn is collected using a pulse sequence of the FISP method. At this time, R
The phase of F is rotated by θn each time with respect to the previous phase. Also, data number m in the warp direction = -Mw/2,...,M
When w/2-1, (Gn)m=(N・m+n+N/2)・ΔGwΔGw
=1/(γ・Tw・Lw・N) γ: Magnetic rotation ratio
Tw: phase encoding gradient (phase encoding time)
Gn) Add m. The number of data Pn is Mr
×Mw. To give a numerical example, N=2, n=-1, M
When r=Mw=M=64, θ-1=π, and the RF phase is inverted every time. Also, m=-32,...,31
It is. Moreover, (G-1)m=(2m)·ΔGw. As a result, 64×64 data is obtained as data P-1.

In step S5, it is checked whether all n have been extracted. If n still remains, the process returns to step S3. Once all n have been extracted, the process proceeds to step R1 in FIG. In the above numerical example, since n=0 remains, the process returns to step S3 and n=0 is extracted. Then, in step S4, θ0 = 0, and the RF phase is not changed. Also, (G0)m=(2m+1)・ΔG
It is w. As a result, data P0 is 64×6
4 data are obtained.

In step R1 shown in FIG. 2, when the data number in the read direction is r and the data number in the warp direction is m, data Pn (r, m) → mix data Pmix
By rearranging the data so that it becomes (r, m・N+n+N/2), mix data P is obtained from the collected data Pn.
Create a mix. The number of data of the mix data Pmix is Mr×N·Mw. To give a numerical example, the above data P-1(r,m) is the mix data Pmix (
r, 2m), and the data P0(r, m) becomes mix data Pmix(r, 2m+1). That is, mix data Pmix is created by arranging data P-1 (r, m) and data P0 (r, m) alternately in the warp direction. Mix data Pmix is 64×128
The number of data will be .

In step R2, two-dimensional digital fast Fourier transform is performed. As a result, FOV “Lr×N・L
An image of “w” is obtained. To give a numerical example, FOV “30
An image of "cm x 60 cm" is obtained.

In step R3, the image obtained in step R2 is separated into N images. At this time, if N is an even number, 1/2 images are taken from both ends in the warp direction and combined into one image. To give a numerical example, FOV “30cm
FOV from the center of the warp direction of the “x60cm” image
Take one image of "30cm x 30cm". Also, take an image of FOV "30cm x 15cm" from the upper end in the warp direction, and take an FOV "30cm x 15cm" image from the lower end.
” and combine them to create an FOV of “30cm x 30cm”
” Take one image.

FIG. 3 is a conceptual diagram showing data or images obtained through the processing of each of the above steps. Step S4
Then, each data (DATA_-N/2
to DATA_N/2-1) is obtained. In step R1, data shown second from the left end in FIG. 3 is obtained. In step R2, the image shown third from the left end in FIG. 3 is obtained. In step R3, each image (IMAGE_-N/2 to IMAGE_N/2
−1) can be obtained.

FIG. 4 is a diagram corresponding to FIG. 3 when N=2. The order of the echo in the image with θ=0 is 0, ±2, ±
4,±6,... Further, the orders of echoes in the image where θ=±π are ±1, ±3, ±5, . . . .

[0042] In the above embodiment, a pulse sequence of the FISP method is used, but in this case, the gradient magnetic fields Gr and Gs between RFs become 0 up to the first moment of their waveforms, so the flow and body This has the effect of reducing motion artifacts caused by motion. Furthermore, in the pulse sequence of the FISP method, images are uneven due to the influence of static magnetic field inhomogeneity, but since images resulting from each echo can be generated separately, images without unevenness can be obtained by superimposing them after phase correction.

[0043]

According to the MR imaging method of the present invention, a plurality of images with different contrasts can be obtained simultaneously by the SSFP sequence. Furthermore, since data is collected by superimposing high-order echoes at the same time, the signal strength is increased, and the S/N ratio is improved compared to the case where data of each echo is collected independently. Furthermore, the load on the gradient amplifier is less than when data of each echo is collected independently.

[Brief explanation of drawings]

FIG. 1 is a flow diagram of the MR imaging method of the present invention.

FIG. 2 is a flow diagram of the MR imaging method of the present invention.

FIG. 3 is a diagram explaining the principle of the MR imaging method of the present invention.

FIG. 4 is a diagram explaining the principle of the MR imaging method of the present invention.

FIG. 5: M carrying out the MR imaging method of the present invention
FIG. 3 is a block diagram of the R device.

FIG. 6 is a pulse sequence diagram of the FISP method.

[Explanation of symbols]

1 MR imaging device 2. Calculator 3 Sequence controller 4 Magnetic field drive circuit 5 Magnet assembly E Echo signal

Claims (2)

[Claims] Claim 1. In an MR imaging method in which data is collected by an SSFP sequence, where the desired number of images is N, a phase rotation k·θn (θn =2πn/N) to k(k
=0, ±1, ±2,…) given to the next echo and data P
When data number n is collected, the data number in the read direction is r, and the data number in the warp direction is m, data Pn (r, m)→
Mix data Pmix (r, m・N+n+N/2)
Rearrange the data so that the mix data Pmix is created from the collected data Pn, reconstruct the image by Fourier transform using the mix data Pmix, and separate the reconstructed image into N pieces in the warp direction. and obtaining N images with different contrasts. 2. The MR imaging method according to claim 1, wherein data Pn is collected using a pulse sequence of the FISP method, and each time the phase of the RF is rotated by θn with respect to the previous phase. Imaging method.