JPH07241280A - Data collecting method and imaging method of mri device and mri device - Google Patents

Abstract

PURPOSE:To decrease the artifacts and ringing artifacts by phase distortions even if phase correction is not executed by setting the number of sampling before an echo center in time at a specific value and sampling echo signals, thereby collecting data. CONSTITUTION:Data are collected by sampling the echo signals at the sampling point of N-pieces of the number (m) of sampling before the echo center in time and the number (p) of sampling after the echo center in time at the time of collecting the data with an MRI device 100. At this time, m<p, NX20%<=mNX35%, m+p=N are set. A real number component Re and a virtual number component Im are obtd. by subjecting the prescribed data to two-dimensional Fourier transform and an image of the absolute value Z=(Re<2>+ Im<2>)<1/2> is formed in image reconstitution with a calculator 7. As a result, the artifacts by the phase distortion are decreased without making phase correction. In addition, dealing with the short echo time is made possible without losing the advantage of an asymmetrical echo method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention is applied to MRI (Magnetic R
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a data acquisition method, an imaging method, and an MRI apparatus in an esonance imaging apparatus, and more specifically, to a data acquisition method, an imaging method, and an MRI apparatus in an MRI apparatus that can reduce image reconstruction time.

[0002]

2. Description of the Related Art FIG. 8 is an explanatory view of a data acquisition method by a normal full echo method in an MRI apparatus. That is, when the number of data in the frequency axis direction of the k-space is N, the number of samplings N / time preceding the echo center (the time position after the echo time TE from the excitation pulse in the pulse sequence SG for proton excitation) 2 and the echo signal is sampled at N sampling points of the sampling number N / 2, which is later than the echo center in time, and sampling data is collected. Therefore, as shown in FIG. 9, the collected sampling data fills the k-space S. In image reconstruction, two-dimensional Fourier transform is performed on the data in the k-space S to obtain a real number component Re and an imaginary number component Im, and an image having an absolute value Z = √ {Re ² + Im ² } is generated.

FIG. 10 is an explanatory view of a conventional data collection method by the asymmetric echo method in the MRI apparatus. That is, the number of samplings m which is before the echo center in time
(Normally, m is about N × 10%.) And (m + N /) of the sampling number N / 2 after the echo center in time.
2) Echo signals are sampled at the sampling points to collect sampling data. Therefore, as shown in FIG. 11, the collected sampling data does not fill the k-space, resulting in a missing portion E. In the image reconstruction, first, the central axis Gy in the frequency axis direction is assumed to be the k-space S corresponding to the echo center, and the central axis Gy is used.
Phase correction is performed using sampling data in the range (m + m) that is symmetrical with respect to. This phase correction is performed due to inhomogeneity of the main magnetic field, local inhomogeneity due to the difference in magnetic susceptibility in the living body,
This is because the sampling data includes the phase distortion caused by the phase shift due to the blood flow, and the phase distortion is removed. Then, the data H of half the k-space (FIG. 11)
Then, a two-dimensional Fourier transform is performed to obtain a real number component Re and an imaginary number component Im, and an image of the real number component Re is generated. Usually, since the desired image cannot be obtained by using the imaginary number component Im, the imaginary number component Im is not used.

This special image reconstruction mathematically corresponds to the case where t0 = 0 in (Expression 1) described later, the second term becomes zero, and the desired image whose intensity is 1/2 that of the real number component is obtained. Is obtained.

[0005]

In the above-mentioned full echo method, no phase correction is required, but since the sampling number N / 2 of data is collected before the echo center, the echo time TE cannot be shortened. Therefore, when it is desired to shorten the echo time TE, the asymmetric echo method is used. However, in the above conventional asymmetric echo method,
Since the phase correction is necessary, there is a problem that the image reconstruction time becomes long. If you do not correct the phase,
The phase distortion causes artifacts in the image. In addition, ringing artifacts will occur unless a special image reconstruction as in the conventional example is used. Therefore, an object of the present invention is to provide a data acquisition method, an imaging method, and an MRI apparatus in an MRI apparatus, which is an improvement of the asymmetric echo method so that the above-mentioned phase correction is unnecessary and the image reconstruction time can be shortened. is there.

[0006]

According to a first aspect of the present invention, in an MRI apparatus, the sampling number before the echo center is m, and the sampling number after the echo center is p, k. -Where N is the total number of data in the frequency axis direction of the space, m <p and N × 20% ≦ m
MRI characterized in that the echo signal is sampled and data is collected with ≦ N × 35% and m + p ≦ N.
A data collection method in an apparatus is provided.

According to a second aspect of the present invention, in the MRI apparatus, the number of samplings before the echo center in time is m, the number of samplings after the echo center in time is p, and the number of samplings after the echo center is in the frequency axis direction of the k-space. , Where N is the total number of data in N, and N × 20% ≦ m ≦ N × 35%,
In addition, the echo signal is sampled with m + p = N, sampling data is collected, two-dimensional Fourier transform is applied to the sampling data, the real number component Re and the imaginary number component Im are obtained, and the absolute value Z = √ {Re ² + Im ² } is provided to provide an imaging method in an MRI apparatus.

According to a third aspect of the present invention, in the MRI apparatus, the number of samplings before the echo center in time is m, the number of samplings after the echo center in time is p, and the number of samplings after the echo center is in the frequency axis direction of the k-space. , Where N is the total number of data in N, and N × 20% ≦ m ≦ N × 35%,
Also, the echo signal is sampled by collecting m + p <N, sampling data is collected, 0 is substituted in the missing portion of the sampling data in the k-space whose center in the frequency axis direction corresponds to the echo center, and the k-space 2D Fourier transform is applied to the data of
And the imaginary number component Im, the absolute value Z = √ {Re ² + I
There is provided an imaging method in an MRI apparatus characterized by generating an image of m ² }.

In a fourth aspect, the present invention is an MRI apparatus for collecting data at an asymmetrical sampling point in which the sampling number m before the echo center is smaller than the sampling number p after the echo center in time. , K-
When the total number of data in the space frequency axis direction is N,
There is provided an MRI apparatus comprising a data collection means for sampling an echo signal and collecting data by setting N × 20% ≦ m ≦ N × 35% and m + p ≦ N.

In a fifth aspect, the present invention is an MRI apparatus for acquiring data at an asymmetric sampling point in which the sampling number m before the echo center in time is smaller than the sampling number p after the echo center in time. , K-
When the total number of data in the space frequency axis direction is N,
N × 20% ≦ m ≦ N × 35%, and m + p = N, data collection means for sampling an echo signal and collecting data by sampling, and two-dimensional Fourier transform for the sampling data, and a real number component Re and An MRI apparatus comprising: an image reconstructing unit that obtains an imaginary number component Im and generates an image having an absolute value Z = √ {Re ² + Im ² }.

In a sixth aspect, the present invention is an MRI apparatus for acquiring data at an asymmetric sampling point in which the sampling number m before the echo center is smaller than the sampling number p after the echo center in time. , K-
When the total number of data in the space frequency axis direction is N,
In N × 20% ≦ m ≦ N × 35%, and m + p <N, data collecting means for sampling the echo signal and collecting the sampling data, and in the k-space whose center in the frequency axis direction corresponds to the echo center "0" is substituted for the missing portion of the sampling data, the two-dimensional Fourier transform is applied to the data in the k-space to obtain the real number component Re and the imaginary number component Im, and the absolute value Z = √ {Re ² + Im ² }
And an image reconstructing means for generating an image of the above.

[0012]

The asymmetric echo signal x '(t) shown in FIG.
It corresponds to sampling data by the asymmetric echo method.
The center of echo is set to time t = 0, the sampling start time in the asymmetric echo method is set to -t0, and h (t) = 1 t ≧ -t0 h (t) = 0 t <-t0 ) Is assumed, the asymmetric echo signal x ′ (t) has a window function h (t) as the symmetric echo signal x
Since it is equivalent to the product of (t) multiplied, it can be expressed as x ′ (t) = x (t) · h (t). Therefore, the Fourier transform F [x ′ (t)] of the asymmetric echo signal x ′ (t) is F [x ′ (t)] = F [x (t) · h (t)] = F [x (t) )] * F [h (t)] (a). Here, if t0 = 0, then F [h (t)] = δ (f) / 2−i / (2πf) (where δ (f) is a delta function), so F [h (t + t0) ] = Exp {i · 2π · t0} · (δ (f) / 2−i / (2πf)) (b) By substituting the expression (b) into the expression (a) and rearranging, F [x ′ (t)] = I (f) F [x (t)] = X (f) ) Is obtained.

[0013]

[Equation 1]

The real number component of (Equation 1) is a symmetric echo signal x
It is the sum of the half power image of (t), the sinc function of the frequency component determined by t0, and the convolution of the symmetrical echo signal. This means that ringing artifacts cannot be ignored if t0 is small, but
A large 0 indicates that ringing artifacts can be ignored. If t0 is small as in the conventional case and the special image reconstruction as in the conventional example is not used, the ringing artifact becomes a problem.

In the data collecting method, the imaging method and the MRI apparatus in the MRI apparatus of the present invention, the sampling number m before the echo center in time is k.
Since the space is 20% or more of the total number N of data in the frequency axis direction, the above-mentioned t0 is larger than that of the conventional asymmetric echo method (usually about 10%). Therefore,
Although no phase correction is performed, the artifacts due to the phase distortion can be reduced. Also, ringing artifacts are so small that they can be ignored without performing special image reconstruction as in the conventional example. Moreover, since the sampling number m temporally before the echo center is set to 35% or less of the total number N of data in the frequency axis direction of the k-space, the advantage of the asymmetric echo method is not lost and a short echo time TE can be supported. . Since the phase correction is not performed, the image reconstruction time can be shortened.

[0016]

The present invention will be described in more detail with reference to the embodiments shown in the drawings. The present invention is not limited to this. FIG. 1 is a block diagram of an embodiment of the MRI apparatus of the present invention. In this MRI apparatus 100, the magnet assembly 1 has a space portion (hole) for inserting a subject therein, and a static magnetic field for applying a constant static magnetic field to the subject so as to surround this space portion. A coil, a gradient magnetic field coil for generating a gradient magnetic field (the gradient coil includes coils for reading, phase encoding, and slice selection axes), and an RF for exciting spins of nuclei in a subject. A transmitter coil that gives a pulse and an NMR (Nuclear Magnetic Resonance) from the subject.
A receiving coil or the like for detecting the signal is arranged.
The static magnetic field coil, the gradient magnetic field coil, the transmitting coil and the receiving coil are connected to the main magnetic field power source 2, the gradient magnetic field driving circuit 3, the RF power amplifier 4 and the preamplifier 5, respectively.

The sequence storage circuit 8 operates the gradient magnetic field drive circuit 3 in accordance with a command from the computer 7 on the basis of a pulse sequence such as a spin echo method or a gradient echo method, so that the gradient magnetic field coil of the magnet assembly 1 receives a gradient. While generating a magnetic field, the gate modulation circuit 9 is operated to modulate the high frequency output signal of the RF oscillation circuit 10 into a pulsed signal having a predetermined timing and a predetermined envelope, which is applied as an RF pulse to the RF power amplifier 4 to generate an RF signal. Power amplifier 4
After the power is amplified by (1), it is applied to the transmission coil of the magnet assembly 1 to selectively excite the target slice of the subject.

The preamplifier 5 includes a magnet assembly 1
The NMR signal from the subject detected by the receiving coil is amplified and input to the phase detector 12. The phase detector 12 is
The output of the RF oscillation circuit 10 is used as a reference signal, and the preamplifier 5
The NMR signal from is phase-detected and given to the A / D converter 11. The A / D converter 11 converts the analog signal after phase detection into a digital signal and inputs it to the computer 7.
The computer 7 performs an image reconstruction operation on the digital signal from the A / D converter 11 to generate an image of a target slice (proton density image). This image is displayed on the display device 6. In addition, the computer 7 is a console 13
Responsible for overall control such as receiving information input from.

FIG. 2 is an explanatory diagram of a first example of a data collection method by the asymmetric echo method in the MRI apparatus 100. That is, the sampling number m temporally before the echo center (m is N × 25% in the first example) and the sampling number p temporally after the echo center (first number)
In the example, p is N × 75%. ), The echo signal is sampled at N sampling points to collect sampling data. Therefore, as shown in FIG. 3, the collected sampling data fills the k-space. In image reconstruction, two-dimensional Fourier transform is performed on the data in the k-space S to obtain a real number component Re and an imaginary number component Im, and an image having an absolute value Z = √ {Re ² + Im ² } is generated.
In the above-described data acquisition method and imaging method of the first example, the sampling number m temporally before the echo center is set to 25% of the total number N of data in the frequency axis direction of the k-space, so that t0 is large and the phase is large. Although not corrected, the artifacts due to phase distortion can be reduced. Also, ringing artifacts are so small that they can be ignored without performing special image reconstruction as in the conventional example. Moreover, the advantage of the asymmetrical echo method is not lost, and a short echo time TE can be dealt with. Further, since the phase correction is not performed, the image reconstruction time can be shortened. Further, since the high frequency component (Q portion shown in FIG. 3) which has not been collected conventionally is included, the image resolution can be improved.

FIG. 4 is an explanatory diagram of a second example of the data collection method by the asymmetric echo method in the MRI apparatus 100. That is, the sampling number m temporally before the echo center (m is N × 35% in the second example) and the sampling number p temporally after the echo center (second)
In the example, p is N × 50%. ), The echo signal is sampled at (m + p) sampling points to collect sampling data. Therefore, as shown in FIG. 5, the collected sampling data does not fill the k-space S, resulting in a missing portion F1. In image reconstruction,
First, assuming that the center axis Gy in the frequency axis direction is the k-space S corresponding to the center of the echo, "0" is substituted for the missing portion F1 of the sampling data. And that k-
Two-dimensional Fourier transform is applied to the data of the space S to obtain the real number component Re and the imaginary number component Im, and the absolute value Z
An image of = √ {Re ² + Im ² } is generated.

In the above-described data acquisition method and imaging method of the second example, since the sampling number m temporally before the echo center is set to 35% of the total number N of data in the frequency axis direction of k-space, t0 is Large, no phase correction is performed, but ringing artifacts can be reduced. Also,
A short echo time TE can be supported without losing the advantage of the asymmetric echo method. Further, since the phase correction is not performed, the image reconstruction time can be shortened.

FIG. 6 is an explanatory diagram of a third example of the data collection method by the asymmetric echo method in the MRI apparatus 100. That is, the sampling number m temporally before the echo center (m is N × 20% in the third example) and the sampling number p temporally after the echo center (third example).
In the example, p is N × 30%. ), The echo signal is sampled at (m + p) sampling points to collect sampling data. Therefore, as shown in FIG. 7, the collected sampling data does not completely fill the k-space S and produces missing portions F1 and F2. In image reconstruction, first, assuming that the central axis Gy in the frequency axis direction corresponds to the echo center, a k-space S is assumed, and "0" is substituted for the missing portions F1 and F2 of the sampling data, and k
Two-dimensional Fourier transform is performed on the data in the space S to obtain the real number component Re and the imaginary number component Im, and the absolute value Z
An image of = √ {Re ² + Im ² } is generated.

In the above-described data acquisition method and imaging method of the third example, since the sampling number m temporally before the echo center is set to 20% of the total number N of data in the frequency axis direction of k-space, t0 is Although it is large and phase correction is not performed, it is possible to reduce artifacts and ringing artifacts due to phase distortion. Moreover, the advantage of the asymmetrical echo method is not lost, and a short echo time TE can be dealt with. Also,
Since no phase correction is performed, the image reconstruction time can be shortened.

[0024]

According to the data collecting method, the imaging method, and the MRI apparatus in the MRI apparatus of the present invention, the sampling number m before the echo center in time is k.
-Because 20% or more and 35% or less of the total number N of data in the frequency axis direction of the space, it is possible to reduce the artifacts and ringing artifacts due to the phase distortion without performing the phase correction. Also, without losing the advantages of the asymmetric echo method,
It can cope with a short echo time TE. Since the phase correction is not performed, the image reconstruction time can be shortened. It is especially useful in time-of-flight angiography.

[Brief description of 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 a first example of a data collection method by the asymmetric echo method in the MRI apparatus of FIG.

FIG. 3 is an explanatory diagram of k-space according to the data collection method of FIG.

4 is an explanatory diagram of a second example of a data collection method by the asymmetric echo method in the MRI apparatus of FIG.

5 is an explanatory diagram of k-space according to the data collection method of FIG.

6 is an explanatory diagram of a third example of a data collection method by the asymmetric echo method in the MRI apparatus of FIG.

7 is an explanatory diagram of k-space according to the data collection method of FIG.

FIG. 8 is an explanatory diagram of a data collection method by the full echo method.

9 is an explanatory diagram of k-space according to the data collection method of FIG.

FIG. 10 is an explanatory diagram of a data collection method by a conventional asymmetric echo method.

11 is an explanatory diagram of k-space according to the data collection method of FIG.

FIG. 12 is an explanatory diagram of an asymmetric echo signal, a symmetric echo signal, and a window function.

[Explanation of symbols]

100 MR device 1 Magnet assembly 7 Computer 8 Sequence controller 9 Gate modulation circuit 10 RF oscillation circuit 13 Operator's console N k-Number of all data in the frequency axis direction of k-space m Number of samplings temporally before echo center p Time from echo center Sampling number after TE TE Echo time SG Pulse sequence for proton excitation

Claims (6)

[Claims] 1. In the MRI apparatus, the number of samplings before the echo center in time is m, the number of samplings after the echo center in time is p, and the total number of data in the frequency axis direction of the k-space is N. When, m <p, N × 20% ≦ m ≦ N × 35%, and m
A data collection method in an MRI apparatus, characterized in that an echo signal is sampled as + p ≦ N to collect data. 2. In the MRI apparatus, the number of samples before the echo center in time is m, the number of samples after the echo center in time is p, and the total number of data in the frequency axis direction of k-space is N. When, m <p, N × 20% ≦ m ≦ N × 35%, and m
+ P = N, the echo signal is sampled, sampling data is collected, and 2 is applied to the sampling data.
A method for imaging in an MRI apparatus, characterized in that a ^two -dimensional Fourier transform is performed to obtain a real number component Re and an imaginary number component Im to generate an image having an absolute value Z = √ {Re ² + Im ² }. 3. In the MRI apparatus, the number of samplings before the echo center in time is m, the number of samplings after the echo center in time is p, and the total number of data in the frequency axis direction of k-space is N. When, m <p, N × 20% ≦ m ≦ N × 35%, and m
The echo signal is sampled by collecting + p <N, sampling data is collected, "0" is substituted for the missing portion of the sampling data in the k-space whose center in the frequency axis direction corresponds to the echo center, and the k-space 2D Fourier transform is performed on the data of to obtain the real number component Re and the imaginary number component Im, and the absolute value Z = √ {Re ² + Im ² }
Imaging method in an MRI apparatus, which is characterized by generating an image. 4. A sampling number m preceding the echo center in time is a sampling number p following the echo center in time.
In an MRI apparatus that collects data at less asymmetrical sampling points, when N is the total number of data in the frequency axis direction of k-space, N × 20% ≦ m ≦ N × 35% and m + p ≦ N An MRI apparatus comprising a data collecting means for sampling an echo signal and collecting data. 5. A sampling number m preceding the echo center in time is a sampling number p following the echo center in time.
In an MRI apparatus that acquires data at a smaller number of asymmetrical sampling points, when N is the total number of data in the frequency axis direction of k-space, N × 20% ≦ m ≦ N × 35%, and m + p = N A data collecting means for sampling an echo signal and collecting data by sampling, and a two-dimensional Fourier transform on the sampling data to obtain a real number component Re and an imaginary number component Im to obtain an absolute value Z = √ {Re ² + Im ² }. An MRI apparatus comprising: an image reconstructing unit that generates an image. 6. A sampling number m preceding the echo center in time is a sampling number p following the echo center in time.
In an MRI apparatus that acquires data at less asymmetrical sampling points, when N is the total number of data in the frequency axis direction of k-space, N × 20% ≦ m ≦ N × 35% and m + p <N Data collecting means for sampling an echo signal and collecting data by sampling, and "0" is assigned to the missing portion of the sampling data in the k-space whose center in the frequency axis direction corresponds to the echo center, and the k-space 2D Fourier transform is applied to the data of to obtain the real number component Re and the imaginary number component Im, and the absolute value Z = √ {Re ² + Im ² }
And an image reconstructing means for generating an image of the MRI apparatus.