JPH0549619A - Three-dimensional mr imaging method - Google Patents

Abstract

PURPOSE:To efficiently obtain an image whose picture quality is satisfactory within necessary range by obtaining an image having uniform density in a target FOV, and also, constituting and processing slab data so that aliasing image is not included. CONSTITUTION:A target slab, target slab thickness, a depth FOV corresponding to the target slab thickness, and a depth direction matrix are designated (S1). Such pseudo target slab thickness as the target slab thickness is set in the roughly flat part of an excitation shape (S2), and a depth direction pseudo-target FOV being larger than a depth direction target FOV is set (S3). Also, a depth direction pseudo-target matrix whose number of matrixes is larger than that of a depth direction target matrix is set (S4), and the scan of only the number of depth direction target matrixes in the depth direction pseudo-target matrix is executed (S5). Data obtained by the scan is subjected to Fourier- transformation (S6), only target Fourier data is fetched (S7), and based thereon, 2D Fourier-transformation is executed (S8), and an image is generated (S9).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a three-dimensional MR imaging method, and more particularly to a three-dimensional MR imaging method using a selective excitation method.

[0002]

2. Description of the Related Art In an MRI system, a three-dimensional MR imaging method is known in which a first warp gradient is applied to a warp axis and a second warp gradient is applied to a slice axis.
In this three-dimensional MR imaging method, the field of view in the depth direction (hereinafter, this is referred to as a target FOV) cannot limit the pass band and stop band by the LPF or the like, and therefore, as shown in FIG. It is specified using the band limitation by excitation.

[0003]

In the conventional selective excitation of the RF signal, it is difficult to make the excitation shape ideally rectangular. Therefore, as shown in FIG. 12, the thickness of the half width of the excitation shape becomes the target FOV. It defines the thickness of the slab (hereinafter referred to as the target slab) that is excited to match (hereinafter referred to as the target slab thickness). However, since the excitation intensity is reduced at both ends of the target slab, the slice in that portion has a low intensity image, and the image obtained by the reformation has a problem that it becomes a striped image due to intensity unevenness. Also,
There is a problem that a signal outside the target FOV enters and both ends are crushed by an aliasing image as shown by the diagonal lines in FIG.

Accordingly, the present invention is to provide a three-dimensional MR imaging method capable of obtaining an image of uniform density for a target FOV and preventing the image from containing an aliasing image.

[0005]

According to a first aspect, the present invention applies an RF signal and a slice gradient to selectively excite a slab, and a first warp gradient to a warp axis. , A three-dimensional MR imaging method in which a second warp gradient is applied to a slice axis to obtain data on the slab, and a pseudo-objective slab thickness is set so that the target slab thickness falls within a substantially flat portion of the excitation shape. A pseudo that sets a slab step, a depth-direction pseudo-objective FOV that is larger than the depth-direction objective FOV corresponding to the pseudo-objective slab thickness, and a depth-direction pseudo-objective matrix in which the number of matrices is larger than the depth-direction objective matrix. Objective step and the pseudo objective slab thickness, depth direction pseudo objective FO
V, a slice gradient, a depth-direction pseudo-objective warp step, and the like are set based on the depth-direction pseudo-objective matrix, and the low-frequency is centered at the depth-direction pseudo-objective warp step and the number of warps in the depth-direction objective matrix. A second warp gradient applying step of applying a second warp gradient to the area, storing the obtained data in an area corresponding to the depth direction objective matrix in the depth direction pseudo objective matrix, and storing the obtained data in other areas. Is a data storage step of storing 0, a Fourier transform step of performing Fourier transform on the data stored in the depth direction pseudo-objective matrix to obtain Fourier data, and an objective corresponding to the depth direction objective matrix from the Fourier data. About the target Fourier data extraction step for extracting only Fourier data and the extracted target Fourier data Perform dimensional image reconstruction operation, it provides a three-dimensional MR imaging method characterized in construction in that it has an image generation step of generating an image.

In a second aspect, the present invention applies an RF signal and a slice gradient to selectively excite a slab, while applying a first warp gradient to a warp axis and a second warp gradient. In a three-dimensional MR imaging method for applying data to a slice axis to obtain data on the slab, a pseudo-target slab step for setting a pseudo-target slab thickness such that the target slab thickness falls within a substantially flat portion of the excitation shape, and the pseudo-target slab step. A pseudo-objective step of setting a depth-direction pseudo-objective FOV larger than the depth-direction object FOV corresponding to the target slab thickness, and a depth-direction pseudo-objective matrix having a larger number of matrices than the depth-direction object matrix; Target slab thickness, depth direction pseudo-objective FOV, depth direction pseudo-objective matrix based slice gradient, depth direction pseudo-objective warp step, etc. Pseudo-objective warp step setting step, which sets the depth-direction objective matrix, target average count, and pseudo-objective average count setting step that sets the pseudo-target average count based on the depth-direction pseudo-target matrix A second warp gradient applying step of applying a second warp gradient by deciding a warp number that increases the number of averaging one time centering on a low frequency, and performing a Fourier transform on the obtained data of the depth direction pseudo objective matrix. Fourier transform step for obtaining Fourier data, target Fourier data extraction step for extracting only target Fourier data corresponding to the depth direction target matrix from the Fourier data, and two-dimensional image reconstruction operation for the extracted target Fourier data And an image generating step for generating an image, and Providing three dimensional MR imaging how.

[0007]

In the three-dimensional MR imaging method of the present invention,
In the pseudo-purpose slab step, the pseudo-purpose slab thickness is set so that the target slab thickness fits in the almost flat part of the excitation shape so that the target slab thickness can be uniformly excited.

In the pseudo-objective step, the depth-direction pseudo-objective F larger than the depth-direction objective FOV is taken into consideration in consideration of a portion which becomes an aliasing image in the image of the pseudo-purpose slab thickness.
Set the OV. Further, a depth-direction pseudo-purpose matrix having a larger number of matrices than the depth-direction target matrix is set so that the slice thickness per slice in the depth direction is not changed.

The second warp gradient applying step scans only the number of depth direction target matrices in the depth direction pseudo target matrix.

Then, in the data storing step, the data obtained by the scan is stored in an area corresponding to the depth direction objective matrix in the depth direction pseudo objective matrix, and 0 is stored in the other portions.

The Fourier transform step carries out the Fourier transform using this data. The obtained Fourier data of the number of pseudo objective matrices in the depth direction is the data separated according to each position, and is the target slab thickness. It is possible to separate the corresponding target Fourier data from other data.

The target Fourier data extraction step extracts only the target Fourier data, so that the image generation step generates an image.

Therefore, an image with almost uniform excitation intensity can be obtained for the target FOV, and the image does not include the aliasing image.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in more detail based on the embodiments shown in the drawings. However, this does not limit the present invention.

FIG. 2 is a block diagram of an MRI system 1 for implementing the three-dimensional MR imaging method of the present invention.

The computer 2 controls the entire operation based on an instruction from the console 13.

The sequence storage circuit 3 operates the gradient magnetic field drive circuit 4 based on the stored sequence,
A static magnetic field coil and a gradient magnetic field coil of the magnet assembly 5 generate a static magnetic field and a gradient magnetic field. Further, the gate modulation circuit 7 is controlled to modulate the RF signal generated by the RF oscillation circuit 6, and the modulated RF signal is applied from the RF power amplifier 8 to the transmission coil of the magnet assembly 5.

The echo signal obtained by the receiving coil of the magnet assembly 5 is input to the phase detector 10 via the preamplifier 9 and further to the computer 2 via the AD converter 11.

The computer 2 calculates image data based on the echo signal data obtained from the AD converter 11 and displays the image on the display device 12.

FIG. 5 shows a three-dimensional MR according to an embodiment of the present invention.
It is an illustration figure of the sequence which concerns on an imaging method.

Simultaneously with applying the 90 ° pulse and the 180 ° pulse, the slice gradient is applied to selectively excite only the first slab, and the first warp gradient is applied to the warp axis, and the second warp gradient is applied. Apply to the slice axis and obtain the data for the first slab. The rephase gradient is
This is to restore the spin phase disturbed by the slice gradient. Following the scan for the first slab, a scan is performed in the same sequence as described above using 90 ° pulses and 180 ° pulses having different frequencies from the 90 ° pulses and 180 ° pulses. As a result, the data of the second slab is obtained. In the same manner, the sequence is repeated for necessary slabs, and one repetition cycle TR is completed. This is repeated as many times as necessary.

The operation of the main part of the three-dimensional MR imaging method according to the embodiment of the present invention will be described below with reference to the flow chart shown in FIG. In step S1,
The user, via the console 13, the target slab, its slab thickness t (hereinafter referred to as the target slab thickness t), and the depth direction FOV corresponding to the target slab thickness t (hereinafter referred to as the depth direction target FOV). ) And a depth direction matrix (hereinafter, depth direction target matrix).

In step S2, the computer 2 calculates the pseudo target slab thickness T from the target slab thickness t. As shown in FIG. 3, the pseudo-target slab thickness T is set so that the target slab thickness t is approximately 1.5 to 1.6 times the target slab thickness t so that the target slab thickness t fits in a substantially flat portion of the pump shape. It was expanded to.

In step S3, the pseudo purpose slab thickness T
The depth-direction pseudo-objective FOV and the depth-direction pseudo-objective matrix are calculated in correspondence with. The depth direction pseudo objective FOV is obtained by expanding the depth direction objective FOV by about 2.0 times. Also, the depth direction pseudo-objective matrix is 1 in the depth direction.
The number of matrices is increased to about 2.0 times the depth direction target matrix so that the slice thickness per sheet is not changed.

Depth-direction objective FOV and depth-direction pseudo-objective FO
The relationship of V is shown in FIG. FIG. 4 shows the relationship between the depth direction objective matrix and the depth direction pseudo objective matrix.

In step S4, based on the pseudo-target slab thickness T, the depth-direction pseudo-objective FOV, and the depth-direction pseudo-objective matrix, the slice gradient and the depth-direction pseudo-objective warp step GW.
Set S2, etc. This depth direction pseudo-purpose warp step
GWS2 is set according to the following equation. GWS2 = 1 / (γ ・ TSW ・ FOV2) where γ is the nuclear gyromagnetic ratio, TSW is the warp time, and FOV2 is the depth direction pseudo objective FOV.

Step S5 represents applying the second warp gradient in the sequence shown in FIG. The warp step of the second warp gradient to be printed here is
It is WS2. The number of changes in the second warp slope is
Depth direction target matrix only. Then, scanning is performed centering on the low frequency part (scanning is centered on the warp (0 warp) in the middle in the depth direction).

The computer 2 stores the data obtained by this scan in the area corresponding to the depth direction objective matrix in the depth direction pseudo objective matrix, as shown in FIG. 6 (d in this figure is Represents data.). Further, the computer 2 stores 0 in the area other than the area in which the data is stored.

In step S6, the discrete Fourier transform (DFT) is performed on the depth direction pseudo objective matrix shown in FIG. 6 stored in step S5. As a result, Fourier data as shown in FIG. 7 is obtained. In the figure, the shaded area is the target Fourier data corresponding to the depth direction target matrix. Others are data and aliasing data corresponding to the inclined portion of the excitation shape in the pseudo-purpose slab thickness T.

In step S7, only the target Fourier data as shown in FIG. 7 is taken out and other data is discarded.
In step S8, 2D Fourier transform is performed based on the extracted target Fourier data to generate an image. This image is an image with substantially uniform excitation intensity for the target slab (target slab thickness t) and does not include an aliasing image. Step S9
Then, this image is displayed on the display device 12.

As described above, according to this three-dimensional MR imaging method, it is possible to obtain an image of the target FOV with a substantially uniform excitation intensity and prevent the image from including an aliasing image.

In the continuous slab, it is possible to deal with the fact that the pseudo-purpose slab thickness calculated in step 2 overlaps with the adjacent pseudo-purpose slab thickness by performing scanning twice for every one-purpose slab. Further, instead of the discrete Fourier transform in step S5, an algorithm equivalent to the discrete Fourier transform such as Charp-Z-transform may be used.

Next, a three-dimensional MR according to another embodiment of the present invention.
The operation of the main part relating to the imaging method will be described with reference to the flowcharts shown in FIGS.

In step S21, the user designates the target slab, the target slab thickness t, the depth direction target FOV, and the depth direction target matrix m (even number of m ≧ 4) via the operation console 13. Further, the average number n of times of repeating views performed for the same slice of the slab (hereinafter referred to as the target average number n) is designated.

In step S22, the computer 2 calculates the pseudo target slab thickness T from the target slab thickness t. As shown in FIG. 3, the pseudo-target slab thickness T is 1.5 to 1.6 times the target slab thickness t so that the target slab thickness t fits in a substantially flat portion of the pump shape. It is an enlarged version.

In step S23, the depth-direction pseudo-objective FOV and the depth-direction pseudo-objective matrix M are calculated in correspondence with the pseudo-objective slab thickness T. The depth direction pseudo objective FOV is obtained by expanding the depth direction objective FOV by about 2.0 times. Further, the depth direction pseudo-purpose matrix M has the number of matrices increased to about 2.0 times the depth direction target matrix m so as not to change the slice thickness per slice in the depth direction. Depth direction pseudo-objective matrix M
Is calculated as the following equation, where c is a multiple of the depth direction target matrix m. M = INT (m × c / 2 + 0.999) × 2 (1) However, INT means integerization, and adding 0.999 to integerization means rounding up. Further, the reason why the number is multiplied by 1/2 and then converted into an integer and then multiplied by 2 is to obtain an even number of matrices within the range multiplied by c.

The target FOV in the depth direction (where FO is
Shown as V1. ), Depth-direction objective matrix m, depth-direction pseudo-objective FOV (indicated by FOV2 in the formula), and depth-direction pseudo-objective matrix M have a relationship of FOV1 × M = FOV2 × m (2) It seems that.

At step S24, the pseudo-purpose slab thickness T,
Based on depth direction pseudo-objective FOV and depth direction pseudo-objective matrix M, slice gradient, depth direction pseudo-objective warp step
Set GWS2 etc. This depth direction pseudo-objective warp step GWS2 is set as in the following equation. GWS2 = 1 / (γ ・ TSW ・ FOV2) (3) where γ is the nuclear magnetic rotation ratio and TSW is the warp time.

In step S25, the pseudo target average number of times corresponding to the depth direction pseudo target matrix M is made equal to the scan time calculated from the depth direction target matrix m specified by the user and the target average number of times n. Determine N.
That is, N = n × m / M (4)

Further, the average number of times Na of views repeated for all matrix elements of the depth-direction pseudo-objective matrix M from this pseudo-objective average number of times N, and the depth-direction pseudo-objective to be performed by one view more than the average number of times Na. The partial matrix Mb in the matrix M is calculated by Na = INT (N) ... (5) Mb = n × m−Na × M (6)

In step S26, the warp number corresponding to this partial matrix Mb is determined centered on the low frequency part (centered on the warp in the depth direction (0 warp)). Then, as shown in the sequence of FIG. 5, the second warp gradient is applied. Here, the warping step of the second warp gradient to be applied is GWS2. The number of changes in the second warp gradient is the depth-direction pseudo-objective matrix M for the average number Na, and is only the warp number corresponding to the partial matrix Mb except the average number Na (that is, N).
a × M + 1 × Mb. ).

A comparison between the scanning method thus performed and the conventional method is illustrated in FIG. Conventional example (b) is
The depth direction target matrix m = 4, and the target average number of times n = 3. In the present embodiment (a), M = INT (4 × 2/2 + 0.999) × 2 = 8 from the equation (1) with c = 2. Further, from the formulas (4), (5), and (6), N = 3 × 4/8 = 1.5 Na = INT (1.5) = 1 Nb = 3 × 4-1 × 8 = 4 Is.

In step S27, the computer 2 performs a discrete Fourier transform (DFT) on the data of the depth direction pseudo objective matrix M obtained by this scan. This allows
Fourier data is obtained as shown in FIG. In the figure, the shaded area is the target Fourier data corresponding to the depth direction target matrix m. Others are data and aliasing data corresponding to the inclined portion of the excitation shape in the pseudo-purpose slab thickness T.

In step S28, only the target Fourier data as shown in FIG. 11 is taken out and the other data is discarded. In step S29, 2D Fourier transform is performed based on the extracted target Fourier data to generate an image. This image is an image with substantially uniform excitation intensity for the target slab (slab thickness t) and does not include an aliasing image. Step S
At 30, the image is displayed on the display device 12.

As described above, according to this three-dimensional MR imaging method, it is possible to obtain an image of the target FOV with a substantially uniform excitation intensity, and it is possible not to include an aliasing image in the image.

[0046]

According to the three-dimensional MR imaging method of the present invention, it is possible to obtain an image of a target FOV with a substantially uniform excitation intensity and prevent the image from including an aliasing image. I can. Therefore, it becomes possible to efficiently obtain an image with good image quality in the required range.

[Brief description of drawings]

FIG. 1 is a main part flowchart of one embodiment of a three-dimensional MR imaging method of the present invention.

FIG. 2 is a block diagram of an MRI system that implements the three-dimensional MR imaging method of the present invention.

FIG. 3 is a conceptual diagram showing a relationship among a target slab thickness, a pseudo target slab thickness, a depth direction target FOV, and a depth direction pseudo target FOV.

FIG. 4 is a conceptual diagram showing a relationship between a depth direction objective matrix and a depth direction pseudo objective matrix.

FIG. 5 is an exemplary diagram of a sequence according to the three-dimensional MR imaging method of the present invention.

FIG. 6 is a conceptual diagram showing a state in which data obtained by scanning and 0 are stored in a depth direction pseudo objective matrix.

7 is a conceptual diagram of Fourier data obtained by performing a Fourier transform with the matrix shown in FIG.

FIG. 8 is a main part flowchart of another embodiment of the three-dimensional MR imaging method of the present invention.

FIG. 9 is a main part flowchart of another embodiment of the three-dimensional MR imaging method of the present invention.

FIG. 10 is a conceptual diagram showing a relationship between a depth direction pseudo objective matrix and a pseudo objective average number of times.

11 is a conceptual diagram of Fourier data obtained by performing a Fourier transform using the matrix shown in FIG.

FIG. 12 is a conceptual diagram of field limitation using selective excitation by an RF signal.

[Explanation of symbols]

 1 ... MRI system 2 ... Computer M ... Depth direction objective matrix m ... Depth direction objective matrix T ... Pseudo objective slab thickness t ... Objective slab thickness

Claims (2)

[Claims] 1. A slab is selectively excited by applying an RF signal and a slice gradient, a first warp gradient is applied to a warp axis, and a second warp gradient is applied to a slice axis. In the three-dimensional MR imaging method for obtaining the data of the above, a pseudo-target slab step for setting a pseudo-target slab thickness such that the target slab thickness falls within a substantially flat portion of the excitation shape, and a depth corresponding to the pseudo-target slab thickness. Depth-direction pseudo-objective FOV larger than the depth-direction objective FOV and a depth-direction pseudo-objective matrix in which the number of depth-direction pseudo-objectives is larger than that in the depth-direction objective matrix; Set the slice gradient, depth direction pseudo-purpose warp step, etc. based on the target FOV and depth direction pseudo-purpose matrix A second warp gradient applying step of applying a second warp gradient centering on a low frequency at a warp step and a warp number of the depth direction target matrix; A data storage step of storing the data in the area corresponding to the depth direction objective matrix and storing 0 in the other area, and performing Fourier transform on the data stored in the depth direction pseudo objective matrix to obtain Fourier data. A transformation step, a target Fourier data extraction step of extracting only the target Fourier data corresponding to the depth direction target matrix from the Fourier data, and a two-dimensional image reconstruction operation for the extracted target Fourier data to generate an image And a three-dimensional MR image having an image generation step. Easing method. 2. Applying an RF signal and slice gradient
Is applied to selectively excite the slab, and
Apply the second slope to the warp axis and slide the second warp slope.
3rd order applied to the s-axis to obtain data about the slab
In the original MR imaging method, the target slab thickness fits in the almost flat part of the excitation shape
Pseudo-objective slab step to set such pseudo-objective slab thickness
And the target FOV in the depth direction corresponding to the pseudo target slab thickness.
Larger depth-direction pseudo objective FOV and depth-direction objective matrix
Depth-direction pseudo-objective matrix with more matrix than cus
And a pseudo-purpose step for setting a cus, and the pseudo-purpose slab thickness, depth direction pseudo-purpose FOV, depth direction
Slice gradient, depth direction pseudo-objective based on pseudo-objective matrix
Pseudo-purpose warp step setting to set warp steps etc.
Constant step, depth direction objective matrix, average number of objectives, depth direction pseudo
Pseudo-objective flats that set the average number of pseudo-objectives based on the statistical matrix
Increase the average count once, centered on low frequencies, based on the uniform count setting step and the pseudo target average count.
Determine the warp number to take and apply the second warp gradient
The second warp gradient applying step and the obtained data of the depth direction pseudo objective matrix are
Fourier transform to obtain Fourier data Fourier
A transformation step and from the Fourier data to the depth direction target matrix
Corresponding objective Fourier extracting only Fourier data
D) Data extraction step and two-dimensional image reconstruction for the extracted target Fourier data
Image generation step of performing calculation and generating an image
A three-dimensional MR imaging method characterized by having
Law.