JP3159466B2 - MR imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an in-vivo tomography apparatus utilizing a magnetic resonance (hereinafter abbreviated as MR) phenomenon, and is used for medical diagnosis.

[0002]

2. Description of the Related Art A flow velocity measuring method in an MR imaging apparatus includes a method of corresponding to information of intensity, frequency and phase.

[0003] As a method of corresponding to the phase information, there are a spin echo method (hereinafter abbreviated as SE method) and a gradient field method.
There is a de-echo method (hereinafter abbreviated as GFE method) , but the SE method attenuates the signal in a short repetition time (hereinafter abbreviated as TR).
Method is used. However, in the GFE method, since an echo signal is obtained by reversing a magnetic field, a large phase change due to a non-uniform static magnetic field, a chemical shift, or the like occurs. For this purpose, correction data is required. Conventionally, however, correction image data is measured separately from an actual measurement sequence, and correction correction is performed by calculating between images. This method is described in Journal of the Japan Society for Magnetic Resonance Medicine, Vol. 10. S-2 (1990) 27th
It is described on page 8.

[0004]

In the above-mentioned prior art, a plurality of times of photographing are required to obtain image data for correction, so that the measurement time becomes longer and the subject restraining time becomes longer. In addition, since imaging is performed separately, there is a problem in that positional shift between images due to movement of the subject is likely to occur.

SUMMARY OF THE INVENTION It is an object of the present invention to obtain an image showing a flow velocity in which a phase distortion has been corrected by a single photographing, without suppressing the displacement between images due to the movement of the subject without extending the measurement time.

[0006]

In order to achieve the above object, a slice axis phase sensitive sequence for continuously obtaining a plurality of measurement images in a TR and a slice axis phase insensitive sequence for finally obtaining a correction image are provided. The image is taken in a series of one combined sequence.

[0007]

By combining a slice axis phase sensitive sequence for obtaining a measurement image and a slice axis phase insensitive sequence for obtaining a correction image last, the number of measurement images is reduced by one, but only one photographing is required. Therefore, there is no extension of the measurement time.

In addition, the displacement between images due to the movement of the subject can be suppressed because the images are taken simultaneously.

Further, since the correction image is incorporated at the end of a series of sequences, it can be measured at a position where the blood flow velocity is stable during imaging in synchronization with the subject's heartbeat and pulse.
Phase disturbance due to turbulence or the like can be suppressed.

[0010]

The present invention will be described below with reference to examples. FIG.
FIG. 1 is a block diagram showing a configuration of an embodiment of the present invention. 1
Reference numeral 01 denotes a static magnetic field coil for generating a stationary field for determining the resonance frequency of the MR signal and providing a space for injecting the subject, and a gradient magnetic field coil for generating a gradient magnetic field so as to surround the subject in this space. A coil 103 for applying a high-frequency magnetic field (hereinafter abbreviated as an RF pulse) to the object 102 and the subject and receiving the generated MR signal is arranged. Gradient magnetic field coil 1
02 is connected to the gradient magnetic field control unit 104, and the coil 103 is connected to the amplifier and the receiver 106. In order to detect an MR signal from the subject, an RF pulse generating unit 108 and a gradient magnetic field control unit 104 that generate various kinds of nuclides of the subject to resonate by a sequence control unit 107 that controls various pulses and a magnetic field, Controls a receiver 106 that measures after detecting an MR signal generated from a subject, and reconstructs an image with a processing device 109 based on the measurement signal from the receiver 106 and displays the information through a console 110 that exchanges information. It is displayed on the device 111.

A method of implementing the present invention in the above configuration will be described with reference to FIGS.

FIG. 2 shows an overall processing flow of an embodiment of the present invention. Hereinafter, description will be given according to the processing flow of FIG. In this example, a method of photographing using a two-dimensional Fourier transform method will be described.

Processing 201 FIG. 3 shows a slice axis phase insensitive sequence by the GFE method used in this embodiment. This is a sequence in which the phase does not change for spins having a velocity component in which the slice axis flows perpendicular to the slice plane.

The transmitter 102 irradiates an RF pulse 301 including a frequency component capable of selecting a specific slice when the gradient magnetic field 302 is applied. This pulse causes only the nuclear spins in a particular slice to collapse.

In order to generate an echo signal, gradient magnetic fields 303 and 304 for refocus are applied to the slice axis. At this time, in order to prevent a phase change from occurring for the spin having the velocity component X (t) = V · t,

[0016]

(Equation 1)

Γ: resonance frequency G: gradient magnetic field output Here, the output values of the gradient magnetic fields are the same, and t ₁ = t ₃ and t ₂ = 2 · t ₁ .

At the same time, a gradient magnetic field 3 for phase encoding
05 is applied. This is to add position information to the signal, and the intensity is sequentially changed when repeatedly observing. Furthermore, an echo signal is generated by reversing the gradient magnetic field, and the sampling origin of the MR signal is aligned.
A gradient magnetic field 306 is applied t ₅ hours the frequency encoding axis. A gradient magnetic field 307 for observation is applied before t ₆ = t ₅ hours, and an MR signal is observed through the receiver 105. The observed signal is sent to the processing device 106 after being subjected to quadrature detection.

FIGS. 4 (a) and 4 (b) show the gradient axis output of the slice axis of the slice axis phase-sensitive sequence which causes a specific phase change for spins having a velocity component flowing perpendicular to the slice plane. This is an example. The amount of phase change due to the movement is proportional to the difference in the magnitude of the magnetic field changed by the movement in the gradient magnetic field direction and the application time. Therefore, the following relationship is established between the phase and the flow velocity.

[0020]

(Equation 2)

[0021] V: flow rate t _p: application of the gradient magnetic field time t _i: the application interval phase variation Θ of the gradient magnetic field as a fixed parameter, the phase change with respect to the flow rate at the optimal dynamic range if the flow velocity V and the external input parameter is obtained Can be In this case, the sequence can be created by changing one of G, t _p, t _i. (A)
Is an example changing the t _i of to G, and t _p is constant gradient 403. (B) is an example in which the outputs of the gradient magnetic fields 405 and 406 are changed based on the slice axis of FIG. 3 so as to give a phase change, and G ₂ and G ₃ are represented by the following equations.

[0022]

(Equation 3)

[0023] (G '= Θ / 0.36 · γ · V · t p · t i) G: output value G ₂ gradient 303: Output of the gradient 405 value G _3: Output of the gradient 406 Value FIG. 5 shows a series of photographing sequences. In this example, an example of heartbeat synchronous imaging is shown. In the TR, a slice axis phase sensitive sequence 501 for measurement is arranged for each interval I input by the operator, and finally a slice axis phase insensitive sequence 502 for correction is arranged. Sequence 502
Can correct the velocity component of the spin flowing in the direction perpendicular to the slice plane, but cannot correct the spin having an acceleration component such as turbulence. Therefore, it is desirable to avoid such influence. Normally, in the case of imaging synchronized with the heartbeat of the subject, in the time phase near the sequence 502, the blood flow velocity is stable, so that it is less affected by the acceleration component.

FIG. 6 shows a photographing method. The slice plane perpendicular to the blood vessel is photographed in the sequence shown in FIG. in this case,
The blood vessel may be inclined β with respect to the slice plane as shown in FIG. However, in this case, the measured value is only the vertical velocity component with respect to the slice plane.

Processing 202 By taking an image in a series of sequences as described above, in the measurement sequence, a phase change occurs in the blood vessel in accordance with the flow velocity.
In the correction sequence, data in which a phase change does not occur even in the stationary part and the blood vessel is obtained. However, the data obtained by the measurement sequence actually causes the following phase distortion, and it is necessary to correct the phase distortion.

Phase distortion due to gradient magnetic field dynamic characteristics Phase distortion of the detection system Phase distortion due to non-uniform magnetic field Phase distortion due to difference in resonance frequency of water and fat FIG. 7 shows a schematic diagram of the phase distortion correction. (A) is a stationary unit 70
It is assumed that the state of the phase indicated by the dotted line is measured in an example in which the blood vessel 702 exists in 1. The data (b) obtained in the measurement sequence 501 has the same phase distortion as the data (c) obtained in the correction sequence 502 except for the inside of the blood vessel. (D) in which only the inside of the blood vessel has undergone a phase change, in which the typical phase distortion has been eliminated. This correction processing will be described.

FIG. 8 shows a processing flow of the phase distortion correction.

Step 801 The phase correction is performed on the individual data obtained in the step 201. As the processing 801, the processing of step 501 described in Japanese Patent Application Laid-Open No. 61-27643 is used. By this correction, the phase distortion due to the individual data detection system and the phase distortion due to the gradient magnetic field dynamic characteristic can be corrected.

Process 802 The phase of the individual measurement data obtained in the process 801 is corrected with the correction data. The correction is performed on the image as follows. First, a correction angle is obtained by the following equation.

[0030]

(Equation 4)

Here, it is assumed that R1 = PR (x, y) and I1 = PI (x, y).

Α: Correction angle R1: Real part data of the correction data I1: Imaginary part data of the correction data

[0033]

(Equation 5)

Here, it is assumed that R2 = QR (x, y) and I2 = QI (x, y).

R (x, y): real part data of the corrected measurement image I (x, y): imaginary part data of the corrected measurement image R2: real part data of the measurement data I2: measurement The imaginary part data of the data By the above correction, the phase distortion due to the non-uniform magnetic field and the phase distortion due to the difference in the resonance frequency of water and fat can be corrected. However, since this data is not always in a state where the phase of the stationary part is flat (phase 0), it cannot be applied as an image indicating the flow velocity.

Process 803 Further, the data in process 802 is corrected in process 801. However, only the processing after step 603 is performed. As a result, the phase of the stationary portion becomes flat.

Process 804 A phase diagram is reconstructed by the following equation from the measurement image subjected to the phase correction with the correction image.

[0038]

(Equation 6)

Process 805 As described above, a phase diagram showing the phase change of the fluid portion by correcting the overall distortion is displayed on the processing device 111. If the phase of the fluid part in this phase diagram is measured and calculated based on the following equation, it can be converted into a flow velocity.

[0040]

(Equation 7)

V: flow velocity (set flow velocity) Θ ': phase change amount with respect to V Θ: measured phase

[0042]

According to the present invention, a blood flow velocity image in which phase distortion has been corrected can be obtained by a single photographing in a continuous desired time phase synchronized with the heartbeat and the pulse of blood flowing in the direction perpendicular to the cross section.

Further, since the dynamic range can be changed as an external input parameter, it is possible to shoot with optimum sensitivity.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention.

FIG. 2 is a processing flow of the entire present invention.

FIG. 3 is a diagram showing an example of a slice axis phase insensitive sequence for implementing the present invention.

FIG. 4 is a diagram illustrating an example of a slice axis of a slice axis phase insensitive sequence for implementing the present invention.

FIG. 5 is a diagram showing an example of a photographing sequence and a photographing method for implementing the present invention.

FIG. 6 is a diagram showing an example of a blood vessel imaging method for implementing the present invention.

FIG. 7 is a schematic diagram of the phase distortion correction of the present invention.

FIG. 8 is a processing flow of phase distortion correction according to the present invention.

[Explanation of symbols]

301, 401... RF pulse, 302, 303, 30
4,305,306,307,402,403,40
4,405,406 ... Gradient magnetic field.

Continuation of the front page (56) References JP-A-1-232943 (JP, A) JP-A-63-157063 (JP, A) Koichi Sano et al., "Phase distortion correction method in MRI and its application to blood flow imaging Applications, IEICE Transactions (1987), VOL. J70-D, NO. 3, P631-P639 (58) Fields investigated (Int. Cl. ⁷ , DB name) A61B 5/055 JICST file (JOIS)

Claims (2)

(57) [Claims] 1. A magnetic field for generating a static magnetic field, a gradient magnetic field, and a high frequency magnetic field.
Means for detecting magnetic resonance signals from the test object.
Output means and various processes including image reconstruction for the detected signal.
MeansAnd a predetermined sequence of the gradient magnetic field and the high-frequency magnetic field.
Control means for applyingMR imaging device having
In placeThe control means controls a predetermined spin at a predetermined time interval.
The first sequence that produces a phase change with respect to
The second sequence that executes multiple times and does not cause a phase change
Executing after the first sequence, The processing means is configured to store the data obtained in the second sequence.
Data obtained in the first sequence based on the data
An MR imaging apparatus comprising: 2. The MR imaging apparatus according to claim 1, wherein the first sequence and the second sequence are sequences based on a gradient field echo method.