JP3181285B2 - MRI data collection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to 3DF in MRI (magnetic resonance imaging).
The present invention relates to a data collection method that does not cause artifacts in an image near the center of a slice even when data is collected by T (three-dimensional Fourier transform) and image reconstruction is performed.

(Prior Art) When data is collected and reconstructed by the 2DFT (two-dimensional Fourier transform) method of MRI, an RF pulse sequence in the SE (spin echo) method shown in FIG.
Immediately after the ゜ pulse and the 180 ゜ pulse, inflow due to the FID (free induction decay) signal occurs. This flow of the FID signal has a shape as schematically shown in FIG. 8A (R in the drawing, E indicates the encoding direction) in the raw data. When an image is reconstructed by performing a Fourier transform (FT) in the presence of the inflow of the FID signal, an artifact extending in the encoding direction is generated in the middle of the image as shown in FIG. 8 (B). Since the region of interest (ROI) in an image is usually near the center of the image, the presence of such artifacts will result in a ROI
The image inside is very difficult to see.

Thus, for example, in U.S. Pat.
Artifacts in the 2DFT method are removed using a so-called "hair clipper method". In this hair clipper method, the direction in which the FID signal is generated is reversed for each encoding, as shown in FIG. 9 (A). As a result, as shown in FIG. 9 (B), the inflow artifact moves to both ends of the image along the direction of encoding.

(Problems to be Solved by the Invention) However, in the 3DFT method including the slice direction in addition to the read direction and the encode direction, the n-plane slice obtained by the above-described clipper method has a vertical direction (with respect to the slice plane). When the Fourier transform is performed vertically, the artifacts have the same frequency components as shown in FIG. 10 (S represents the slice direction), so that n (n Gathers, for example, 256) artifacts. As a result, the artifact extends along the encoding direction to the vicinity of the center of the image, which makes it difficult to diagnose an object in the ROI located near the center of the image.

For reference, FIG. 11 shows the state of occurrence of artifacts when no hair clipper method was performed. Without any clipper technique, the artifacts, including the lead, encode, and slice directions, will appear exactly in the center of the image, making it difficult to read objects in the ROI.

The present invention has been made in view of the above circumstances, even in the case of MRI in which data is collected by 3DFT and image reconstruction is performed,
An object of the present invention is to provide an MRI data collection method capable of avoiding artifacts that hinder diagnosis on an image.

[Configuration of the invention]

(Means for Solving the Problems) In order to solve the above problems, the present invention provides a three-dimensional Fourier transform using encoding performed by applying a gradient magnetic field for slicing and a gradient magnetic field for phase encoding performed by a magnetic resonance imaging apparatus. In a data acquisition method of MRI (magnetic resonance imaging) based on the method, the magnetic resonance imaging apparatus changes the generation direction of the echo of the magnetization spin in the real space for each encoding by applying the gradient magnetic field for phase encoding, When the distance between the gradient magnetic field center of the slice gradient magnetic field and the center of the selective excitation slab in the slice direction is within a predetermined range, the generation direction of the echo in the real space is changed for each encode by applying the slice gradient magnetic field. To collect the echo data.

Preferably, by the magnetic resonance imaging apparatus,
The distance between the center of the gradient magnetic field of the slice gradient magnetic field and the center of the selective excitation slab in the slice direction is d, the thickness of the selective excitation slab in the slice direction is W, and a = d / W and a = n + δ (n Is an integer of 0, ± 1, ± 2, ..., δ is 0 ≦
When a parameter a that satisfies δ ≦ 1) is obtained, the generation direction of the echo in the real space is changed for each encoding by applying the slice gradient magnetic field according to the range of the real number δ. Collect echo data.

(Operation) According to the method of the present invention, the distance between the center of the magnetic field of the gradient magnetic field for slicing and the center of the slab (selective excitation portion in the slice direction) is within a predetermined range when acquiring data by the three-dimensional Fourier transform method in MRI. At this time, so-called clipper processing is performed in which the direction in which the echo is generated is changed for each encoding of the slice gradient magnetic field. For this reason, when an image is reconstructed from echo data, noise that causes an artifact moves to both ends (four corners) of the slice plane, and the artifact does not occur at the center of the image.

(Example) Hereinafter, an example of the present invention will be described with reference to FIGS. 1 (A) to 6 (B). First, the hair clipper method performed in the present invention will be described in detail using a 2DFT method as an example.

FIGS. 1 (A) and (B) show the n-th SE method, respectively.
In the first (1 ≦ n ≦ 256) encoding, the spin is defeated by 90 ° at the first averaging (collecting the first signal for one encoding; the same applies to the second and subsequent averaging). FIG. 8 is a graph showing the motion of magnetization when the magnetic recording medium is tilted by 180 ° and after that.
FIGS. 1 (C) and 1 (D) show the case where the spin was defeated by 90 ° at the second averaging and the case where
It is a graph which shows the movement of magnetization at the time of 0 degree fall. In each figure, the hatched arrow indicates the main echo, the black arrow indicates the FID signal, and the direction of the arrow indicates the direction in which the signal is generated.

That is, in FIG. 1 (B), the echo is emitted in the negative direction and the FID signal is emitted in the positive direction with respect to the amplitude of the frequency (this state is referred to as “state”), and in FIG. 1 (D). , Echo and FID signals are emitted in the positive direction (this state is referred to as "state").

When these signals are represented on a graph with respect to time and frequency, they are as shown in FIGS. 2 (A) to 2 (C). FIG. 2 (A) is a signal diagram corresponding to the state shown in FIG. 2 (B).
FIG. 2 (C) is a signal diagram corresponding to
It is a signal diagram when sampling (signal collection) is performed in the state of + (-).

That is, in the states shown in FIGS. 2A and 2B, the frequency component which becomes an artifact appears as the original low frequency, but as shown in FIG. When sampling is performed in reverse for each encoding, a signal diagram obtained by extracting only the FID signal at this time is as shown in FIG. 3 (A). An apparatus for detecting the FID signal recognizes the FID signal as if it were a very high frequency signal as shown in FIG. 3 (B).

The artifact component mistaken for this high frequency is 2DFT
Is performed, the image moves to both ends along the encoding direction on the image. As a result, an image as shown in FIG. 9B is obtained. In this way, the "hair clipper method" recognizes an artifact that is a low-frequency component as if it were a high-frequency component by changing the spin defeating direction for each encoding when viewed in the encoding direction. It is a technique to make it.

As described above, in the case of 2 averaging or more, the direction in which the spin is defeated is changed according to the number of averaging, but when only 1 averaging is performed, the odd-numbered and even-numbered encodings are performed. The directions of the spins are changed with each other (for example, the odd-numbered state is in the positive state, and the even-numbered state is in the negative state).

The method of the present invention applies this principle to the 3DFT method. The MR imaging based on the 3DTF method is performed by a conventionally known magnetic resonance imaging apparatus.

For simplicity, consider the case of one averaging.
Table 1 shows that the above-described designation of the spin direction applied to the hair clipper method in the 2DFT method is directly applied to the 3DFT method including the slice direction.

In the table, と corresponds to the states shown in FIGS. 1B and 1D, respectively.

However, in this method, as shown in FIG. 10, artifacts are gathered near the center of the slice of the image.

Therefore, in the present invention, the hair clipper process is also performed in the slice direction in the 3DFT method in a manner uniquely shown in Table 2.

In the table, the meaning is the same as in Table 1.

By applying the hair clipper treatment unique to the present invention shown in Table 2,
The artifact moves to the four corners in the slice direction of the image and becomes as shown in FIG. Therefore, according to the method of the present invention, the diagnosis of the ROI in the center of the image is not hindered by the artifact.

Hereinafter, more detailed embodiments of the method of the present invention implemented based on the above-described principle will be described.

FIG. 5 is a graph showing the positional relationship of each image in the slice direction. When the hatched area in the figure is excited, the center of the slab is shifted from the center of the gradient magnetic field to the slab thickness (the thickness of one slice). X equal to the number of slices). Therefore, in this state, when the hair clipper processing shown in Table 2 is not performed, as shown in the figure, the artifact appears at the pitch of the slab thickness W in the slice direction from both ends of the slab along the encode direction passing through the center of the gradient magnetic field. . That is, when viewed from the center of the slab, the artifacts are located at the four corners of the image, and the image is easy to see.

On the other hand, when the hair clipper processing shown in Table 2 is performed, artifacts are generated at locations separated by W / 2 on both sides in the slice direction from points corresponding to both ends of an image in the encoding direction passing through the center of the gradient magnetic field. Therefore, at this time, artifacts occur at both ends of the image just along the encoding direction from the center of the slab, that is, at the center of the slice of the image, and the image is more difficult to see than in the case of FIG. .

Therefore, in the present embodiment, the case where the clipper processing unique to the present invention is performed or not is determined based on the distance between the center of the slab and the center of the gradient magnetic field so that the artifact moves to the peripheral edge of the image as much as possible.

That is, when the distance between the center of the slab and the center of the gradient magnetic field is d and the thickness of the slab is W, a parameter of a = d / W is introduced. This parameter a is a = n + δ (n is 0, ± 1,
An integer of ± 2,..., Δ is also expressed as a real number satisfying 0 ≦ δ ≦ 1).

In this embodiment, (1) 0 ≦ δ ≦ 1/4 or 3/4 <
When δ <1, clipper processing in the slice direction is performed,
(2) When 1/4 <δ ≦ 3/4, the hair clipper process in the slice direction is not performed.

6 (A) and 6 (B) are graphs showing the positional relationship between the center of the gradient magnetic field and the slab. In both figures, the thickness (ST) of one slice is 2 mm and the number of slices (NS) is 32. (W = ST × NS = 2 × 32 = 64 (mm)). In FIG. 6 (A), the center of the slab is +20 mm away from the center of the gradient magnetic field in the slice direction, and in FIG. 6 (B), the center of the slab is −52 mm away from the center of the gradient magnetic field in the slice direction. Set at a point and excite.

In FIG. 6A, a = d / W = 20/64 = 0.3125. Therefore, when a = n + δ, n = 0, δ =
0.3125, which is 1/4 <δ ≦ 3/4, which corresponds to the above-described case (2). Therefore, in FIG. 6 (A), the clipper processing in the slice direction is not performed, and the artifact occurs at the intersection of the encode direction axis passing through the center of the gradient magnetic field and both ends of the slab.

On the other hand, in FIG. 6 (B), a = d / W = −52 / 64 = −
0.8125. Therefore, at a = n + δ, n =
−1, δ = 0.1875, and 0 ≦ δ ≦ 1/4 and the above (1)
Corresponds to the case of Therefore, in FIG. 6 (B), clipper processing in the slice direction is performed, and the artifact is located at a point -W / 2 (= −32 mm) away in the slice direction from the intersection of the encode direction axis passing through the gradient magnetic field center and both ends of the slab. Occurs.

Therefore, in both FIGS. 6 (A) and 6 (B), the artifact does not fall within the range of 1/2 slab thickness and 1/2 slab length from the center of the slab, and there is no hindrance to the diagnosis of ROI within this range. Absent.

〔The invention's effect〕

As described above, in MR imaging in which the method of the present invention is performed, noise processing is performed so that artifacts move to both ends (four corners) in the slice direction on an image obtained by the 3DFT method, and an artifact is present at the center of the image. Since it does not exist, diagnosis in the ROI at the center of the image can be performed more accurately.

[Brief description of the drawings]

FIGS. 1A and 1B are graphs showing the motion of magnetization when the spin is tilted 90 ° and 180 ° at the first averaging in the encoding by the SE method, respectively. (C) and (D) are graphs showing magnetization motions when the spin is tilted 90 ° and 180 ° at the second averaging, respectively. FIGS. 2 (A) and (B) are graphs respectively. FIGS. 1B and 1D are signal diagrams corresponding to the states of magnetization, and FIG. 2C is a state in which the magnetization state of FIG. 3 (A) is a signal diagram of the FID shown in FIG. 2 (C), and FIG. 3 (B) is a signal detected by the apparatus. Signal diagram,
FIG. 4 is an image diagram in which artifacts are moved to the four corners by the method of the present invention, FIG. 5 is a graph diagram showing a positional relationship of each image in a slice direction, and FIGS. 6 (A) and (B) are slab centers, respectively. FIG. 7 is a graph showing a positional relationship between slabs having different positions and the center of the gradient magnetic field, FIG. 7 is a graph showing a sequence of RF pulses in the SE method, FIG. 8 (A) is a schematic diagram of FID on raw data, FIG. FIG. 8 (B) is an image in which an artifact has occurred at the center, FIG. 9 (A) is a graph showing the direction in which the FID emerges in relation to time and phase, and FIG. 9 (B) is an image in which the artifact has both ends. Image view moved to the 10th
FIG. 11 is an image diagram in which artifacts are gathered at the center in the slice direction, and FIG. 11 is an image diagram when the hair clipper method is not performed at all in the 3DFT method.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-63-11141 (JP, A) JP-A-62-224336 (JP, A) JP-A-59-63551 (JP, A) JP-A-2- 95346 (JP, A) US Patent 4,618,182 (US, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) A61B 5/055 JICST file (JOIS)

Claims (2)

(57) [Claims] An MRI (magnetic resonance imaging) data collection method based on a three-dimensional Fourier transform method using encoding by applying a gradient magnetic field for slicing and a gradient magnetic field for phase encoding performed by a magnetic resonance imaging apparatus, The magnetic resonance imaging apparatus changes the generation direction of the echo of the magnetization spin in the real space for each encode by applying the gradient magnetic field for phase encoding, and selectively excites the gradient magnetic field center and the slice direction of the gradient magnetic field for slice. When the distance from the center of the slab is within a predetermined range, the data of the echo is collected by changing the generation direction of the echo in the real space for each encoding by applying the gradient magnetic field for slicing. How to collect MRI data. 2. The MRI data acquisition method according to claim 1, wherein the distance between a gradient magnetic field center of the slice gradient magnetic field and a center of the selective excitation slab in the slice direction is d by the magnetic resonance imaging apparatus. Assuming that the thickness of the selective excitation slab in the slice direction is W, a = d / W and a = n + δ
(N is an integer of 0, ± 1, ± 2,..., Δ is a real number satisfying 0 ≦ δ ≦ 1). When a parameter a that satisfies the parameter a is obtained, an echo is generated in the real space according to the range of the real number δ. The echo data is collected by changing the direction for each encode by applying the slice gradient magnetic field.
MRI data collection method.