JP2816862B2 - Nuclear magnetic resonance imaging system - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to an MRI multi-echo scan method, and more particularly, to an MRI multi-echo scan method for preventing artifacts occurring in an image after the third echo at the time of tilt or oblique. About the method.

(Prior Art) A nuclear magnetic resonance imaging apparatus includes a magnet unit including a static magnetic field coil that generates a uniform static magnetic field and a gradient magnetic field coil that generates a magnetic field having a linear gradient in each of x, y, and z directions. A transmission / reception unit (including a sequence storage circuit, etc.) for applying an RF pulse to an object placed in a magnetic field formed by the magnet unit and detecting an NMR signal from the object, the transmission / reception unit and the magnet unit And a control image processing unit centered on a computer for controlling the operation of the above and processing the detected data to display an image.

In the above configuration, the sequence storage circuit generates a signal based on the pulse sequence of the multi-echo method shown in FIG. 4 according to a command from the computer. In FIG. 4, RF is a high-frequency rotating magnetic field applied in a direction perpendicular to the static magnetic field. Gs is a gradient magnetic field applied to an axis called a slice axis. The slice gradient applied to the slice axis is for exciting only spins in a specific plane, and the rephase gradient is for removing a phase difference generated at the time of slicing. Gf is a gradient magnetic field applied to an axis called a frequency encoding axis. The read gradient is for observing the spin echo signal, and the dephase gradient is for providing initial phase information in the frequency encode axis direction. Gp is a gradient magnetic field for applying a magnetic field having a different phase to an axis called a phase encoding axis each time. The warp gradient is for providing phase information in the phase encode axis direction. The signal is a first echo, a second echo, a third echo, and a first echo, a second echo, a third echo, and a fourth echo generated at the time TE interval with the fourth inversion pulse. is there. In this way, the multi-echo method applies a large number of inversion pulses during the time TR from the first excitation pulse to the next excitation pulse, and applies a read gradient each time to make an echo appear, thereby laterally relaxing the spin between tissues. Image contrast is provided by utilizing a difference in signal strength based on time. On the other hand, a feature of MRI is that any tomographic plane of the subject can be easily taken. These are called tilt and oblique, and are usually used as routines in MRI.

(Problems to be Solved by the Invention) However, when this multi-echo method is used in combination with a tilt or oblique technique that changes the angle of a tomographic plane to affect the inversion pulse, it is difficult to give an inversion pulse without an angle error. Various artifacts may occur in the image after three echoes, and an image that is clinically satisfactory may not be obtained. The reason why an artifact occurs after the third echo will be described with reference to FIG. Here, it is assumed that the output areas of the first to third read gradients are different from each other.

First echo: occurs at time t1 when the area of the first read gradient following the first inversion pulse becomes equal to the area of the dephase gradient.

Second echo: occurs at time t2 when the area p2 of the first read gradient equals the area p2 of the second read gradient after the echo center time t1 of the first echo.

Third echo: Similarly, it occurs at time t3 when the area p3 of the second read gradient after t2 is equal to the area of the third read gradient.

However, if there is an angle error in the inversion pulse, at time t1 ′,
Pseudo echoes A and B are generated at t2 ', which causes artifacts. The pseudo echo A is a magnetization vector component oriented in the static magnetic field direction by the first inversion pulse and in the anti-static magnetic field direction by the second inversion pulse. The pseudo echo A is added to the area p1 of the dephase gradient after the third inversion pulse. An echo is connected at time t1 'when the areas of the three read gradients become equal.
The pseudo echo B is a magnetization vector component oriented in the 0 ° direction by the first inversion pulse, in the 180 ° direction in the second inversion pulse, and in the 0 ° direction by the third inversion pulse, and is (p1 + p1 + p2) − (2p + p3).
+ P4) = 0, that is, p4 = (p1 + p1 + p2)-(p2 + p3)
At time t2 ', an echo is connected.

As described above, the pseudo echo is generated at the time of tilt or oblique because the output areas of the first to third read gradients are different. The reason for the different read gradient areas is as follows. That is, when there is no tilt or oblique, the read gradient can be an exactly equal output waveform for each echo. On the other hand, with respect to the gradient in the slice axis direction, in order to prevent a reversal image and shading caused by an angle error of the reversal pulse, the fourth
As shown in the figure, a spoiler whose polarity is inverted for each echo is placed before and after the inversion pulse, so that the slice and spoiler gradient outputs when applying each inversion pulse cannot be made equal. Therefore, the output area at each echo is not always constant due to the slew rate characteristic of the gradient and the eddy current induced in the metal component of the magnet due to the switching of the gradient. Therefore, when the gradient in the slice axis direction and the read gradient extend over two or more gradients due to tilt or oblique, the areas of the read gradients in each echo become unequal.

The present invention has been made in view of the above points, and an object of the present invention is to realize an MRI multi-echo scan method in which an artifact does not occur in an image after the third echo in a multi-echo scan method at the time of tilt or oblique. It is in.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a pulse sequence of an MRI multi-echo scan method, in which a slice gradient and a spoiler are applied in a slice axis direction, and a slice at each inversion pulse is applied. With the same output waveform as the gradient, the output area of the spoiler satisfies S2 + S3 = S4 + S5,
Further, by providing a certain time t (t ≧ 0) at which the gradient output value becomes zero between the slice gradient and the spoiler, the gradient output area in the slice axis direction is made equal for each echo, and the output of the read gradient is output. The area is configured to be equal for each echo.

(Function) If the pulse sequence when no tilt or oblique is set as described above, it is suitable when the gradient in the slice axis direction and the gradient in the read axis direction cross over two or more axes due to tilt or oblique. G at the rate
By dividing s, Gf, and Gp, the output area of the read gradient at each echo can be kept equal.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram of a nuclear magnetic resonance imaging diagnostic apparatus embodying the present invention. In the figure, reference numeral 1 denotes a space having a space for inserting a subject therein, and a static magnetic field coil for applying a constant static magnetic field to the subject and a gradient magnetic field for generating a gradient magnetic field surrounding the space. The magnet assembly includes a coil, an RF transmission coil for applying an RF pulse for exciting nuclear spins in the subject, a receiving coil for detecting an NMR signal from the subject, and the like. The static magnetic field coil, the gradient magnetic field coil, the RF transmitting coil, and the receiving coil are connected to a static magnetic field power supply 2, a gradient magnetic field driving circuit 3, an RF power amplifier 4, and a preamplifier 5, respectively. The sequence storage circuit 6 operates the gate modulation circuit 8 according to a command from the computer 7 (at a predetermined timing, the RF of the RF oscillation circuit 9
The output signal is modulated), and an RF pulse signal is applied from the RF power amplifier 4 to the RF transmission coil. In addition, the sequence storage circuit 6 sends the gradient magnetic field drive circuit 3 and the AD by the sequence signal based on FIG.
The converter 11 is also operated. Reference numeral 10 denotes a phase detector for detecting the output of the received signal of the preamplifier 5 using the output of the RF oscillation circuit 9 as a reference signal. This power signal is
The signal is converted into a digital signal by the AD converter 11 and is calculated by the computer 7
Is input to Reference numeral 12 denotes an operation console for inputting instructions for realizing various pulse sequences and various setting values to the computer 7, and 13 denotes a display device for displaying an image reconstructed by the computer 7.

FIG. 2 is a diagram showing a pulse sequence in the multi-echo method when no tilt or oblique is used, which is used in the present embodiment. In the figure, the read gradient has exactly the same output area for each echo. On the other hand, in order to make the gradient in the slice axis direction the same output area for each co, the slice gradient has the same output waveform, the output area of the spoiler satisfies S2 = S3 = S4 = S5, and
A fixed time t between the slice gradient and the spoiler
(T ≧ 0). If re-phasing is required due to slice plane selection, both re-phasing according to the 0th slice gradient and the first slice gradient are adjusted in S1,
After the second slice gradient, as shown in FIG. 3, the adjustment is made based on the slice gradient and the application timing time Δt1, Δt2 of the inversion pulse, or Δt1 and Δt2.

As described above, if the pulse sequence in the multi-echo method when no tilt or oblique is performed is set in this way, the tilt in the slice axis direction and the gradient in the read axis direction become two or more axes due to tilt or oblique. When straddling, Gs, Gf, and Gp are distributed at an appropriate ratio, and the area of the read gradient in each echo can be made equal.

It should be noted that the present invention is not limited to the above embodiment, and various modifications are possible within the scope of the claims. For example, the spoiler may be a rectangular wave instead of a half-wave sin wave. or,
The output area of the spoiler is not S2 = S3 = S4 = S5 but S2 + S3
It only has to satisfy = S4 + S5.

(Effects of the Invention) As described above, according to the present invention, the MRI multi-echo scanning method when there is no tilt or oblique, so that the output area of the read gradient at the time of tilt or oblique is made equal for each echo. Since the pulse sequence is set, it is possible to prevent an artifact that occurs in an image after the third echo at the time of tilt or oblique.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a nuclear magnetic resonance imaging diagnostic apparatus embodying the present invention, FIG. 2 is a diagram showing a pulse sequence showing one embodiment of the present invention, and FIG. 3 is a slice gradient in one embodiment of the present invention. FIG. 4 is a diagram showing a conventional pulse sequence, and FIG. 5 is a diagram for explaining the reason why an artifact occurs. DESCRIPTION OF SYMBOLS 1 ... Magnet assembly, 2 ... Static magnetic field power supply, 3 ... Gradient magnetic field drive circuit, 4 ... RF power amplifier, 5 ... Preamplifier, 6 ... Sequence memory circuit, 7 ... Computer, 8 ... ... Gate modulation circuit, 9 ... RF oscillation circuit, 10 ... Phase detector, 11 ... AD converter, 12 ... Operation console, 13 ... Display device,

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-59-105550 (JP, A) JP-A-63-130056 (JP, A) JP-A-1-107747 (JP, A) JP-A-1- 166751 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) A61B 5/055

Claims (2)

(57) [Claims] An inversion pulse is repeatedly applied to a subject arranged in a static magnetic field, and a slice gradient is generated in a slice axis direction in synchronization with each of the inversion pulses, and a spoiler immediately before and immediately after the slice gradient is provided. By applying a pulse sequence in which a read gradient is applied in synchronization with an echo signal generated in response to the application of each of the inversion pulses, thereby obtaining a tomographic image of the subject. An apparatus, wherein the output waveform of the slice gradient is the same, m is a natural number, S2 is the output area of the spoiler immediately after the slice gradient to be applied simultaneously with the (2m-1) inversion pulse, and S3 is the 2m inversion pulse. The output area of the spoiler immediately before the slice gradient applied simultaneously, S4, the output area of the spoiler immediately after the slice gradient applied simultaneously with the 2m inversion pulse, S5
Is the output area of the spoiler immediately before the slice gradient applied simultaneously with the (2m + 1) inversion pulse, and S2 + S3
= S4 + S5, and by providing a fixed period in which the output value of the gradient magnetic field is zero between the slice gradient and the spoiler, the gradient output area in the slice axis direction is made equal at every interval between the echo signals, A nuclear magnetic resonance imaging diagnostic apparatus comprising: pulse sequence control means for controlling the pulse sequence so that the output area of the read gradient is equal at each interval between the echo signals. 2. The pulse sequence control means comprises: S2 =
The nuclear magnetic resonance imaging apparatus according to claim 1, wherein the pulse sequence is controlled so as to satisfy S3 = S4 = S5.