JPH0884719A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE: To obtain a magnetic resonance imaging apparatus which achieves an artifact compensation by detecting the position of a fine RF coil mounted on a subject and motions of the subject. CONSTITUTION: Three markers 17, 18 and 19 comprising a fine RF coil are arranged at a part to be taken and the positions of the markers 17, 18 and 19 are detected to correct a slice surface adjusting a pulse sequence according to the results of the detection. Thus, a control is performed to photograph the same slice constantly to correct a collection data according to the results of the detection thereby preventing the generation of artifact with a proper positioning within a photographing section.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus capable of obtaining an image free from artifacts.

[0002]

2. Description of the Related Art A magnetic resonance imaging apparatus (hereinafter referred to as MR
When a subject such as a human body is imaged by the I device), if the subject moves during execution of the pulse sequence, an artifact called a body movement artifact or motion artifact occurs. Artifacts are phenomena in which images are doubled or blurred.

As a conventional method of suppressing artifacts in an MRI apparatus, there is a method of physically fixing an object on a bed by using a fixture such as a band to prevent body movement itself. However, this method has a drawback in that it cannot be performed depending on the medical condition of the patient and it cannot completely prevent body movement.

It is also considered to detect the body movement as in the EXORCIST method and adjust the parameter of the pulse sequence such as the encoding number according to the detected body movement to suppress the influence of the artifact. However, in the practical EXORCIST method, since body movement is detected by detecting the expansion / contraction motion of the subject in the slice, the imaging target is limited to the flexible part such as the abdomen, It cannot be applied to rigid parts such as parts. Further, there is a drawback that it cannot be dealt with when the patient performs a turning operation such as turning over or a lateral displacement causes the slice plane to be displaced.

[0005]

As described above, the conventional MR is used.
The I-device cannot completely suppress the artifact. Therefore, it is an object of the present invention to provide a magnetic resonance imaging apparatus capable of remarkably preventing the occurrence of artifacts regardless of the state of the imaged region or the object to be imaged.

[0006]

A magnetic resonance imaging apparatus according to the present invention detects a movement of an object to be imaged by detecting the positions of three points of the object to be imaged, and a predetermined cross section of the object to be imaged at all times. And a control unit that adjusts a pulse sequence for magnetic resonance imaging based on a detection result of the detection unit so as to capture an image.

Further, in this magnetic resonance imaging apparatus, the control means is provided with means for adjusting the position of the imaging section by oblique processing of the imaging section and offset processing of the imaging section.

In this magnetic resonance imaging apparatus, the control means is also characterized in that it has means for correcting the collected magnetic resonance signals to adjust the position of the image in the slice plane.

[0009]

According to the magnetic resonance imaging apparatus of the present invention,
By adjusting the pulse sequence according to the movement of the subject and allowing the imaging cross section to follow the movement of the subject, the same cross section can always be photographed, and an image without artifacts can be obtained.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a magnetic resonance imaging apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of this embodiment. Gantry 20
A static magnetic field magnet 1, an X-axis / Y-axis / Z-axis gradient magnetic field coil 2 and a transmission / reception coil 3 are provided therein. Transmit / receive coil 3
Instead of being embedded in the gantry, it may be embedded in the top plate of the bed 13 or directly attached to the subject. Here, an example in the case of photographing the head is shown. Further, separate coils dedicated to transmission and reception may be used instead of the transmission / reception coil.

The static magnetic field magnet 1 as the static magnetic field generator is constructed by using, for example, a superconducting coil or a normal conducting coil. The X-axis / Y-axis / Z-axis gradient magnetic field coil 2 is a coil for generating an X-axis gradient magnetic field Gx, a Y-axis gradient magnetic field Gy, and a Z-axis gradient magnetic field Gz. The transmission / reception coil 3 is used to generate a radio frequency (RF) pulse as a selective excitation pulse for selecting a slice and to detect a magnetic resonance signal (MR signal) generated by magnetic resonance. Sleeper 13
The subject P placed on the top plate is an imageable region in the gantry 20 (a spherical region where an imaging magnetic field is formed, and diagnosis is possible only in this region).
Is inserted into. Markers 17, 18, and 19 made up of minute RF coils for position detection as shown in FIG. 2 are fixed to a region of the subject P to be imaged, here, the head, by a belt 16 or the like. The markers 17, 18, 19 are preferably positioned so as to be the vertices of an equilateral triangle. Although not shown in FIG. 1, the markers 17, 18, and 19 are also connected to the transmitter 5 and the receiver 6 similarly to the transmission / reception coil 3.

Next, the operation of this embodiment will be described. In this embodiment, at least three markers 17, 18,
19 is installed, the marker 17, while the pulse sequence is being executed,
By correcting the pulse sequence while measuring the positions of 18 and 19 so that the position of the imaging cross section (slice) follows the movement of the subject, the same cross section is always photographed regardless of the movement of the subject, and artifacts are suppressed. To do.

First, the principle of the present invention will be described with reference to FIG. The MRI apparatus has an absolute coordinate system having three axes of an X-axis, a Y-axis, and a Z-axis which are orthogonal to each other and whose origin is the center Oe of the gradient magnetic field. In this absolute coordinate system, the basic vectors of the three orthogonal axes are (e1, e2, e3). Typically,
e1 = (1,0,0), e2 = (0,1,0),
e3 = (0,0,1), and the coordinate origin Oe is (0,
0,0).

First, second and third markers 17, 18, 1
The positions of 9 are designated as P1, P2 and P3. 3 markers 1
Let f1, f2 be the basic vectors constituting the plane determined by 7, 18, and 19, the basic vector in the normal direction of this plane be f3, and the origin of this plane be Of. Specifically, the intersection point of the perpendiculars drawn from the first marker P1 to the straight line P2 P3 is the origin Of of this plane, and the unit vector of Of P1 is f1 and the unit vector of Of P2 is f2. In this case, f3 = f1 xf2.

The coordinate system of the imaging section is also defined. It is assumed that the orthogonal vectors constituting the photographing section (plane) are g1 and g2, and the basic vector in the normal direction of the photographing section is g3.
Again, g3 = g1 xg2.

Here, for convenience of explanation, the basic vector g
Let 1 be the basic vector in the read direction, basic vector g2 be the basic vector in the encoding direction, and basic vector g3 be the basic vector in the slice direction. In addition, the image center of the photographing cross section is Og.

Under such a relationship, the basic vector (e1, e2, e3) of the absolute coordinate system of the MRI apparatus and the basic vector (f1, of the coordinate system determined by the marker).
For the relation with f2, f3), use the orthogonal matrix A (1)
It is expressed as shown in the equation.

Similarly, the relationship between the basic vector (f1, f2, f3) of the coordinate system determined by the marker and the basic vector (g1, g2, g3) of the coordinate system of the imaging cross section uses the orthogonal matrix B ( It is expressed as shown in the equation (2).

[0019] Here, the orthogonal matrix A is the markers 17, 18, 19 (subject)
Changes when the positional relationship between the MRI device and the
The cross matrix B is a marker 17, 18 and 19 and an imaging section (slide).
It is constant as long as the positional relationship with the cross section) does not change. You
That is, the orthogonal matrix B does not change even when the subject moves.

Therefore, the cross-section g of the image is taken as A, B, e, f
When expressed using, it becomes as follows. The origin Oe of the absolute coordinate system of the MRI apparatus, the markers 17 and 1
Origin Of of plane determined by 8 and 19 and imaging section
The relation of the image center Og of (slice) is calculated by the equation (4).
Given.

[0021]

[Equation 1]

Therefore, since the orthogonal matrix A and the vector v change according to the movement of the subject, the same cross section can always be photographed by correcting the pulse sequence according to these changes. Further, by shifting the collected data according to these changes, it is possible to perform alignment within the slice plane. This makes it possible to obtain an image without artifacts even when the subject moves.

The pulse sequence for magnetic resonance imaging is, for example, a part of a series of processing (hereinafter referred to as sequence processing) as shown in FIG. 5 (gradient magnetic field strength in the encoding direction in FIG. 5) is changed little by little. It is realized by executing it repeatedly. That is, one sequence unit means a series of processes for collecting echo signals by excitation with one RF pulse. For example, an RF pulse called a 90 ° pulse is applied while applying a gradient magnetic field Gs in the slice direction, then a gradient magnetic field Ge in a predetermined encoding direction is applied, and then a gradient magnetic field Gr in the read direction is applied, and further sliced. A magnetic resonance signal (echo signal) is generated when an RF pulse called a 180 ° pulse is applied while applying a directional gradient magnetic field Gs, and this is a series of processes for detecting an echo signal while applying a predetermined Gr.

The operation of this embodiment based on such a principle will be described with reference to the flow chart shown in FIG. At step # 1, 1 is set to each of the variable i and the variable j. Here, the variables i and j are variables relating to the number of executions of the pulse sequence in one sequence unit.

In step # 2, the first marker 17 and the second marker 17
The positions of the marker 18 and the third marker 19 are detected.
When the position of the marker is detected, the orthogonal matrix A is obtained from the absolute coordinates of the MRI apparatus according to the equation (1).

For measuring the position of each marker, for example, Ma
gnetic Resonance In Medicine, vol. 29, No. 3, 199
3, pp411-415, "Real-time Position Monitoring"
of Invasive Devices Using Magnetic Resonance ", G.
The method described by L. Dumoulin et al. Can be used. Here, signal acquisition is performed four times using a pulse sequence by the Hadamard encoding method as shown in FIG. 6, and X, Y, Z position information can be obtained as follows.

X = −P (ex1) + P (ex2) + P (ex3) −P (ex4) (6) Y = −P (ex1) + P (ex2) −P (ex3) + P (ex4) (7) Z = -P (ex1) -P (ex2) + P (ex3) + P (ex4) (8) Here, P (ex1), P (ex2), P (ex3),
P (ex4) is the marker position determined by the 1st, 2nd, 3rd and 4th signal acquisition, respectively. The polarity of the gradient magnetic field at the time of collecting signals each time is set as follows.

[0028] In step # 3, the degree of movement of the subject is detected from changes in the marker position information, and it is determined whether or not the body movement is within a correctable range. The correction is judged to be impossible in the following cases.

(1) When the object to be imaged is out of the linear control guarantee region of the gradient magnetic field. (2) When the object to be photographed is out of the sensitivity area of the transmission / reception coil 3. (3) The object to be photographed is out of the static magnetic field homogeneity guarantee area.

(4) When the positional relationships among the markers 17, 18 and 19 are greatly different from each other. If it is determined that the correction is impossible, step # 4
Then, the variable i is incremented (+1), and step # 5
Then, it is determined whether the variable i is equal to the upper limit number L. The upper limit number L is the maximum number of times the same sequence unit can be repeatedly executed to detect the marker position. When the variable i reaches the upper limit number L, a predetermined error process is performed, and then the process ends.

If the variable i is not equal to the upper limit number L (less than L), the marker position detecting process of step # 2 is executed again. When it is determined in step # 3 that the correction is possible, the variable i is set to 1 and the variable j is incremented (+1) in step # 6.
To be done. As will be described later, this variable j is a variable relating to the encoding amount.

In step # 7, the pulse sequence is corrected based on the marker position information. The sequence correction includes oblique processing (rotational conversion of gradient magnetic field output) and offset of an imaging section (adding an offset frequency to the carrier wave of the RF pulse).

First, the oblique processing will be described.
The gradient magnetic field pulse sequence (after correction) required by this embodiment is Go (t), and the basic vector (g1,
The gradient magnetic field pulse sequence required to image the plane defined by g2, g3) is defined as Gi (t). As described above, the gradient magnetic field Go (t) is 3 of (X, Y, Z).
It has a component and the three components are (Go1, Go2, Go3). Gi (t) has three components (read, encode, slice), and these three components are (Gi1, Gi2, Gi3).

Since the relationship between the basic vector g of the coordinate axis of the imaging section and the basic vector e of the absolute coordinate system is expressed by the equation (3), the gradient magnetic field Go (t) expressed by the coordinate system and the read, encode, and slice. The relationship with G i (t) expressed by is given by the following equation.

[0035] Here, T represents transposition.

That is, the marker and M
When the relationship with the absolute coordinates of the RI apparatus changes, the orthogonal matrix A changes as described above. Therefore, by correcting the gradient magnetic field using the orthogonal matrix A obtained in step # 2, the subject moves. The same cross section is always taken.

Next, the offset processing of the photographing section will be described. The distance D between the origin Oe of the absolute coordinate system and the shooting section
Is represented by the following equation. D = (v, g3) (10) The offset frequency δf added to the predetermined frequency of the RF pulse is expressed by the following equation.

Δf = γGsD (11) where γ is the gyromagnetic rate and Gs is the gradient magnetic field strength in the slice direction. That is, when the distance D between the origin Oe of the coordinate system of the MRI apparatus and the imaging cross section changes due to the movement of the subject, the frequency offset δf is corrected according to this distance D, so that the subject is always the same even if the subject moves. A cross section is taken.

In step # 8, one sequence unit of the pulse sequence thus corrected is executed to collect data. In step # 9, the collected data is corrected and the position is adjusted within the imaging plane. This correction means performing data shift in the slice plane in the directions of the basic vectors g1 and g2 to perform alignment in the slice plane.

Regarding this data shift correction, for example,
Magnetic Resonance In Medicine, vol. 29, No. 3, 199
3, pp327-334, "Correction of Translational Mo
tion Artifacts in Multi-Slice Spin-Echo Imaging Us
ing Self Calibration ", L.-H. Zang et al. can be used. The outline will be described below.

First, the coordinate origin at the time of photographing the reference section (the section for photographing when the first sequence unit is executed) is set to O.
Let f (s) and Og (s). Since the vector v is expressed as a composite vector of the vector Ofe Of (s) and the vector Of (s) Og (s), the equation (5) is transformed into the following equation.

[0042]

[Equation 2]

[0043] Note that α_l , Β_k Is the expansion coefficient.

The first term on the right side of the equation (12) represents the positional relationship between the absolute coordinate system of the MRI apparatus and the markers 17, 18 and 19, and α (s) is obtained by monitoring the marker position during sequence execution. It is possible. Also,
The second term on the right side represents the positional relationship between the imaging cross section and the markers 17, 18, and 19 and remains unchanged unless the imaging cross section is changed. That is, the value of β is determined when the reference cross section is determined.

During photographing, α is obtained by detecting the positions of the markers 17, 18, and 19, and even if the patient's head moves during photographing and the position changes, the vector v is obtained according to the equation (9). You can

Using the vector v thus obtained,
Positioning of the collected data within the slice plane (data shift in the read direction and the encode direction) is performed. Data shift value (δk
r, δke) is expressed by the following equation.

(Δkr, δke) = ((v, g1), (v, g2)) (13) Based on (δkr, δkez) shown in the equation (13), data shift correction in each imaging cross section is performed as follows. Apply by applying an expression.

Scor (kr, ke) = S (kr, ke) exp [−2πi {δkr (kr / nr) + δke (ke / ne)}] (14) where Scor: Slice in-plane alignment (correction) Post data, S: collected data, kr: coordinates in the read direction, ke: coordinates in the encode direction, nr: number of data points in the read direction, ne: number of data points in the encode direction.

At step # 10, it is judged whether the variable j is equal to the total number M of sequence units to be repeatedly executed, that is, whether all the data necessary for obtaining one image have been collected. If the variable j is not equal to the total number M, then
The marker position detection process of step # 2 is executed again, and if the variable j is equal to the total number M of sequences to be imaged, the process ends.

As described above, according to this embodiment,
By detecting changes in the subject before each sequence unit and correcting the imaging cross-section position according to the amount of fluctuation and correcting the collected data, the same cross-section can always be taken, and There is provided a magnetic resonance imaging apparatus capable of obtaining an image free from artifacts caused by movement of a specimen. Unlike the conventional EXORCIST method, it can be used when the positional relationship between the imaging section and the marker does not change, such as when attaching a marker to the head during imaging of the head.

The present invention is not limited to the above-mentioned embodiments, but can be implemented with various modifications. For example, the above-mentioned imaging is a case of two-dimensional imaging, but it is also applicable to three-dimensional imaging. In that case, the data shift amount δks in the slice direction is (1
5), and data shift correction is performed according to equation (16) δks = (v, g3) (15) Scor (kr, ke, ks) = S (kr, ke, ks) exp [-2πi {δkr ( kr / nr) + δke (ke / ne) + δks (ks / ns)}] (16) Here, ks: coordinates in the slice direction, ns: the number of data points in the slice direction.

Further, in the above-mentioned embodiment, the position of the marker is detected every execution of one sequence unit, but the position of the marker may be detected every execution of a plurality of sequence units in order to shorten the photographing time.

[0053]

As described above in detail, according to the present invention, since the same cross section can be photographed at all times by adjusting the pulse sequence according to the movement of the subject and making the photographing cross section follow the movement, an artifact is produced. It is possible to provide a magnetic resonance imaging apparatus that can obtain a clear image. Further, since an image without artifacts can be obtained, an image with high resolution and high resolution can be obtained by a single inspection, which has the advantage of eliminating the need for re-inspection.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of a magnetic resonance imaging apparatus according to the present invention.

FIG. 2 is a diagram showing a micro RF coil attached to an imaging region for detecting a positional variation of a subject in the present embodiment.

FIG. 3 is a diagram showing the relationship of various coordinate systems for explaining the correction principle of the present embodiment.

FIG. 4 is a flowchart showing the operation of this embodiment.

FIG. 5 is a diagram showing a pulse sequence of the present embodiment.

FIG. 6 is a diagram showing a pulse sequence when detecting the position of a marker.

[Explanation of symbols]

1 ... Static magnetic field magnet, 2 ... X-axis / Y-axis / Z-axis gradient magnetic field coil, 3 ... Transmitting / receiving coil, 4 ... Static magnetic field control device, 5 ... Transmitter, 6 ... Receiver, 7 ... X-axis gradient magnetic field amplifier, 8 ... Y-axis gradient magnetic field amplifier, 9 ... Z-axis gradient magnetic field amplifier, 10 ... Sequencer, 11 ... Computer system, 12 ... Display unit, 1
7, 18, 19 ... Markers.

Claims (3)

[Claims] 1. A means for detecting the movement of an object to be photographed by detecting the positions of three points on the object to be photographed, and a means for constantly photographing a predetermined cross section of the object to be photographed based on the detection result of the detector. And a control means for adjusting a pulse sequence for magnetic resonance imaging. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit includes a unit that adjusts the position of an imaging cross section by an oblique process and an offset process. 3. The magnetic resonance according to claim 1, wherein the control means comprises means for correcting the collected magnetic resonance signal to adjust the position of the image in the slice plane. Imaging equipment.