JPH03215244A - Three-dimensional mr imaging method - Google Patents

Abstract

PURPOSE:To obtain an effective image without increasing a scanning time by magnifying second direction objective FOV and matrix and applying the second warp gradient to store the same and extracting only objective Fourier data to perform two-dimensional image reconstituting operation. CONSTITUTION:A calculator 2 calculated second warp direction magnified FOV and matrix from second warp direction objective FOV and matrix and calculates a warp step based thereon to apply the second warp gradient. The number changing the second warp gradient is limited only to the second warp direction objective matrix and scanning is performed centering around a low frequency part. The calculator 2 stores the data obtained by scanning in the region corresponding to the second warp direction objective matrix in the second warp direction magnified matrix and performs Fourier transform with respect to the second warp direction magnified matrix to take out only objective Fourier data and throws away aliasing data and performs 2D fourier transform to form an image. Since scanning is performed the number of times corresponding to the number of objective matrice, a scanning time is not increased.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a three-dimensional MR imaging method, and more specifically, a three-dimensional MR imaging method that can remove aliasing images and efficiently obtain effective images. Regarding the method.

[Prior 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 second warp direction (hereinafter referred to as objective FOV) is LP
Since it is not possible to limit the passband and stopband using F, etc., it is possible to use the RF sensitivity distribution, for example, as shown in Figure 10, or to limit the band by selective RF excitation, as shown in Figure 11. It is used and prescribed.

[Problems to be Solved by the Invention] However, when using the RF sensitivity distribution, since the RF sensitivity distribution is generally gentle, as shown by diagonal lines in FIG. There is a problem that both ends are crushed by aliasing images.

On the other hand, when using RF selective excitation, it is difficult to ideally cut out a rectangle, so as shown in Figure 11,
There is also the problem that signals outside the target FOV enter, and both ends are crushed by aliasing images.

Therefore, it is conceivable to consider in advance the portions that will be crushed in the aliased image and set an FOV larger than the target FOV so that even if both ends are crushed, a good image can be obtained within the range of the target FOV.

However, in this case, scanning time increases and unnecessary data is obtained, resulting in a problem of decreased efficiency.

Therefore, an object of the present invention is to provide a three-dimensional MR imaging method that can efficiently obtain effective images from which aliasing images are removed without increasing scan time.

[Means for Solving the Problem] The three-dimensional MR imaging method of the present invention is a three-dimensional MR imaging method in which scanning is performed by applying a first warp gradient to the warp axis and applying a second warp gradient to the slice axis. , a second warp direction objective FOV and a second warp direction expansion FO larger than the second warp direction objective matrix.
an enlargement step of setting V and a second warp direction enlargement matrix, a second warp direction enlargement warp step is set based on the second warp direction enlargement FOV and the second warp direction enlargement matrix, and in the second warp direction enlargement warp step, and an enlarged second warp gradient application step of applying a second warp gradient centered on a low frequency at the warp number of the second warp direction objective matrix, and the obtained data is applied to the second warp gradient in the second warp direction enlarged matrix. a data storing step of storing the data in an area corresponding to the warp direction objective matrix and storing O in other areas;
a Fourier transform step of performing Fourier transform on the data stored in the warp direction enlargement matrix to obtain Fourier data; a purpose Fourier data extraction step of extracting only the purpose Fourier data corresponding to the second warp direction purpose matrix; and a purpose of the extraction. A structural feature of this method is that it includes an image generation step of performing a two-dimensional image reconstruction operation on Fourier data and generating an image.

[Operation] In the three-dimensional MR imaging method of the present invention, in the enlargement step, an enlarged FOV larger than the target FOV is set in consideration of the portion that will become an aliased image.

However, the step of applying the enlarged second warp gradient scans only the number of target matrices in the second warp direction in the enlarged matrix.

Then, in the data storage step, the data obtained by the scan is stored in an area corresponding to the target matrix in the enlarged matrix, and 0 is stored in other areas.

Using this data, the Fourier transform step performs Fourier transform, but the Fourier data of the number of enlarged matrices obtained is data separated according to each position,
Objective Fourier data and objective FOV corresponding to objective FOV
Can be separated from external data.

The target Fourier data extraction step extracts only the target Fourier data, and the image generation step generates an image based on the target Fourier data.

Therefore, for the target FOV, an effective image without aliasing can be obtained.

On the other hand, since scanning is performed only for the number of target matrices, an increase in scanning time can also be avoided.

[Embodiments] The present invention will be explained in more detail below based on embodiments shown in the drawings. Note that this invention is not limited to this.

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

The computer 2 controls the overall operation based on instructions from the console 13.

The sequence storage circuit 3 operates the gradient magnetic field drive circuit 4 based on the stored sequence, and generates a static magnetic field by the static magnetic field coil and the gradient magnetic field coil of the magnet assembly 5.
Generate a gradient magnetic field. It also controls the gate modulation circuit 7 and modulates the RF signal generated by the RF oscillation circuit 6 to
F is applied from the power amplifier 8 to the transmitting 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 inputted to the computer 2 via the AD converter 11.

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

FIG. 5 shows the basic sequence of the three-dimensional MR imaging method.

After applying the 90° pulse, a first warp gradient is applied to the warp axis, a second warp gradient is applied to the slice axis, and a defase gradient is applied to the lead axis.

Next, a 180° pulse is applied, and then a lead gradient is applied to the lead axis to obtain an echo signal. This is repeated at a period of T while changing the magnitude of the first warp gradient and the second warp gradient.
Repeat with R to obtain all three-dimensional data.

Although the above basic sequence is conventionally known, as will be understood from the following explanation, in the present invention, the method of applying the second warp gradient is different from the conventional method.

That is, FIG. 1 is a flowchart showing the operation of the main part of the three-dimensional MR imaging method of the present invention.

In step S1, the user specifies the desired second warp direction FOV and second warp direction matrix via the console l3. Hereinafter, these will be referred to as the second warp direction objective FOV and the second warp direction objective matrix.

In step S2, the computer 2 calculates a second warp direction enlarged FOV and a second warp direction enlarged matrix from the second warp direction objective FOV and the second warp direction objective matrix. This enlarges the second warp direction objective FOV and the second warp direction objective matrix by about 1.1 to 1.5 times.

FIG. 3 is a slice shape conceptual diagram showing the relationship between the second warp direction objective FOV and the second warp direction enlarged FOV,
FIG. 4 is a conceptual diagram showing the relationship between the second warp direction objective matrix and the second warp direction expansion matrix.

In step S3, the warp step GWS2 is performed based on the second warp direction enlargement FOV and the second warp direction enlargement matrix.
Calculate. The calculation formula is GWS2 = 1 / (-
r-TSW-FOV2)...■(
However, γ is the nuclear gyromagnetic ratio, TSW is the warp time, and FOV2 is the expanded FOV in the second warp direction.

Step S4 represents application of the second warp gradient in the sequence shown in FIG.

The warp step of the second warp gradient applied here is the GWS2. Further, the number of changes in the second warp gradient is only for the second warp direction objective matrix. Then, scanning is performed centering on the low frequency part (scanning is performed centering on the middle warp (0 warp) in the second warp direction).

The computer 2 stores the data obtained by this scan in the area corresponding to the second warp direction objective matrix in the second warp direction enlargement matrix, as shown in FIG. 6 (d in the figure indicates this data). ). Furthermore, calculator 2
stores O in areas other than the area where data is stored.

In step S5, Fourier transform is performed on the second warp direction enlargement matrix shown in FIG. 6 stored in step S4. As a result, Fourier data as shown in FIG. 7 is obtained. By appropriately setting the enlarged FOV in the second warp direction, valid Fourier data (target Fourier data) equal to the number of target matrices in the second warp direction can be obtained. The rest becomes aliasing data.

In step S6, only the target Fourier data shown in FIG. 7 is extracted and the aliasing data is discarded.

In step S7, 2D Fourier transform is performed based on the extracted target Fourier data to generate an image. This image is for the second warp direction objective FOV and does not include an aliased image.

In step S8, this image is displayed on the display device 12.

As you can understand from the above explanation, the actual scan is
Since scanning is performed only as many times as there are objective matrices in the second warp direction, there is no increase in scanning time, and the resulting images do not include aliasing images.

Next, FIG. 8 shows a sequence in which the selective excitation method is applied to the basic sequence of the three-dimensional imaging method shown in FIG.

That is, 90° pulse and 180. ``By applying a slice gradient at the same time as applying a pulse to the slice axis,
Data is obtained by selectively exciting only a predetermined portion (first slab) in the second warp direction. The refase gradient is used to restore the phase of spins disturbed by the slice gradient. Following the scanning of this first slab, scanning is performed in the same sequence as above using 90'' pulses and 180'' pulses having frequencies different from the 90° pulses and 180° pulses. As a result, data on a portion different from the above (second slab) can be obtained.

Thereafter, the sequence is repeated for necessary parts (slabs) in the same manner, and one repetition period TR is completed. Repeat this as many times as necessary.

The sequence shown in Fig. 8 is shown in Fig. 9 (a), (b), and (c).
As shown in , the target field of view is divided into multiple slabs and only the necessary slabs are scanned.
Since the image is distorted due to aliasing at both ends of each slab, there is a problem in that gaps are created even in continuous slabs.

However, by applying this invention, this problem can be solved.

That is, the size of each slab is enlarged, and only the matrix corresponding to the original slab is scanned, data other than the one obtained by scanning is filled with 0 and Fourier transformed.
If you extract only the target Fourier data and perform 2DF processing,
It becomes possible to obtain images of each slab without aliasing images. Therefore, a truly continuous slab without gaps is also possible.

As another embodiment, D transform such as Charp-Z-transform may be used instead of Fourier transform in step S5.
One example is one using an algorithm equivalent to FT.

[Effects of the Invention] According to the three-dimensional MR imaging method of the present invention, an image free of aliasing can be efficiently obtained without extending the scan time.

[Brief explanation of drawings]

FIG. 1 is a flowchart of essential parts of an embodiment of the three-dimensional MR imaging method of the present invention, FIG. 2 is a block diagram of an MRI system implementing the three-dimensional MR imaging method of the present invention, and FIG. 3 is a second warp direction. A conceptual diagram showing the relationship between the objective FOV and the second warp direction enlarged FOV, FIG. 4 is a conceptual diagram showing the relationship between the second warp direction matrix and the second warp direction enlargement matrix, and FIG. 5 shows the basics of the three-dimensional MR imaging method. Sequence diagram showing the sequence, Figure 6 is the second
To the warp direction expansion matrix. A conceptual diagram showing the state in which data obtained by scanning and 0 are stored. Figure 7 is a conceptual diagram of Fourier data obtained by performing Fourier transformation using the matrix shown in Figure 6. Figure 8 is a conceptual diagram of the Fourier data obtained by performing Fourier transformation using the matrix shown in Figure 6. Figure 8 is a conceptual diagram showing the state in which the data obtained by scanning and 0 are stored. Figure 9 is an explanatory diagram of the principle of the sequence shown in Figure 8, Figure 10 is a conceptual diagram of field of view restriction using RF sensitivity distribution, Figure 11 is selective excitation by RF. FIG. 2 is a conceptual diagram of visual field restriction using . (Explanation of symbols) 1... MRI system 2... Computer.

Claims (1)

[Claims] 1. In a three-dimensional MR imaging method in which scanning is performed by applying a first warp gradient to a warp axis and a second warp gradient to a slice axis, a second warp direction objective FOV and a second warp an enlargement step of setting a second warp direction enlargement FOV and a second warp direction enlargement matrix larger than the direction objective matrix; and a second warp direction enlargement warp step based on the second warp direction enlargement FOV and the second warp direction enlargement matrix. and an enlarged second warp gradient application step of applying a second warp gradient mainly at low frequencies at the warp number of the second warp direction objective matrix in the second warp direction enlargement warp step, and the obtained data is a data storage step of storing data in an area corresponding to the second warp direction objective matrix in a second warp direction expansion matrix, and storing 0 in other areas; and the data stored in the second warp direction expansion matrix. a Fourier transform step of performing Fourier transform to obtain Fourier data; a target Fourier data extraction step of extracting only target Fourier data corresponding to the second warp direction target matrix; and two-dimensional image reconstruction of the extracted target Fourier data. A three-dimensional MR imaging method comprising: an image generation step of performing calculations and generating an image.