JP5733490B2 - Magnetic resonance imaging apparatus and program - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus, an imaging method, and a program that scan an imaging region including a predetermined tissue and a background tissue and obtain image data of the predetermined tissue.

  As one of the MRA (Magnetic Resonance Angiography) methods, a scan without contrast medium and a scan with contrast medium are executed, and an image obtained by a scan without contrast medium and a scan with contrast medium are obtained. A method is known in which a blood vessel is imaged by obtaining a difference from the obtained image (see, for example, Patent Document 1).

JP 2006-122301 A

  In the method of Patent Document 1, when a positional shift occurs between an image obtained by a scan without a contrast medium and an image obtained by a scan with a contrast medium, a background such as an organ or a vein is formed together with an artery. Tissue outlines may also be drawn. Therefore, it is desired that the outline of the background tissue is not drawn as much as possible.

The invention of the first aspect is a magnetic resonance imaging apparatus that scans an imaging region including a predetermined tissue and a background tissue and obtains image data of the predetermined tissue,
Scan means for executing a first scan that scans the imaging region, and a second scan that scans the imaging region so that the predetermined tissue is emphasized rather than the first scan;
Find the difference between the absolute value of the k-space data of the imaging region obtained by the first scan and the absolute value of the k-space data of the imaging region obtained by the second scan; Image data calculating means for calculating image data of the predetermined tissue based on phase information of k-space data obtained by the second scan;
Is a magnetic resonance imaging apparatus.

The invention of the second aspect is an imaging method for imaging the predetermined tissue based on a magnetic resonance signal obtained by scanning an imaging region including a predetermined tissue and a background tissue,
The imaging region is scanned so that the absolute value of the k-space data of the imaging region obtained by the first scan that scans the imaging region and the predetermined tissue is emphasized more than the first scan. Obtain the difference between the absolute value of the k-space data of the imaging region obtained by the second scan, and based on the difference and the phase information of the k-space data obtained by the second scan, An imaging method comprising: an image data calculating step for calculating image data of the predetermined tissue.

The invention of the third aspect is a program of a magnetic resonance imaging apparatus that scans an imaging region including a predetermined tissue and a background tissue and obtains image data of the predetermined tissue,
The imaging region is scanned so that the absolute value of the k-space data of the imaging region obtained by the first scan that scans the imaging region and the predetermined tissue is emphasized more than the first scan. Obtain the difference between the absolute value of the k-space data of the imaging region obtained by the second scan, and based on the difference and the phase information of the k-space data obtained by the second scan, It is a program for causing a computer to execute image data calculation processing for calculating image data of the predetermined tissue.

  By calculating the image data using the difference between the absolute values of the k-space data and the phase information of the k-space data, between the imaging region in the first scan and the imaging region in the second scan. Even if a positional shift occurs, image data in which the contour of the background tissue is sufficiently reduced can be obtained.

1 is a schematic diagram of a magnetic resonance imaging apparatus according to an embodiment of the present invention. It is explanatory drawing of the scan performed in this embodiment. 2 is a diagram showing a processing flow of the MRI apparatus 100. FIG. It is explanatory drawing of simulation conditions. It is explanatory drawing of a simulation result. It is explanatory drawing of the conditions of a phantom experiment. It is explanatory drawing of an experimental result.

  Hereinafter, embodiments for carrying out the invention will be described in detail with reference to the drawings, but the present invention is not limited to the following embodiments.

FIG. 1 is a schematic view of a magnetic resonance imaging apparatus according to an embodiment of the present invention.
A magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus, MRI: Magnetic Resonance Imaging) 100 includes a magnetic field generator 2, a table 3, a cradle 4, a contrast medium injector 5, a receiving coil 6, and the like. Yes.

  The magnetic field generator 2 includes a bore 21 in which the subject 14 is accommodated, a superconducting coil 22, a gradient coil 23, and a transmission coil 24. The superconducting coil 22 applies a static magnetic field B0, the gradient coil 23 applies a gradient magnetic field, and the transmission coil 24 transmits an RF pulse. In place of the superconducting coil 22, a permanent magnet may be used.

  The cradle 4 is configured to be movable from the table 3 to the bore 21. The subject 14 is transported to the bore 21 by the cradle 4.

The contrast agent injection device 5 injects a contrast agent into the subject 14.
The receiving coil 6 is attached to the abdomen of the subject 14 and receives a magnetic resonance signal.

  The MRI apparatus 100 further includes a sequencer 7, a transmitter 8, a gradient magnetic field power supply 9, a receiver 10, a central processing device 11, an input device 12, and a display device 13.

  Under the control of the central processing unit 11, the sequencer 7 sends information for executing the pulse sequence to the transmitter 8 and the gradient magnetic field power supply 9. Specifically, under the control of the central processing unit 11, the sequencer 7 sends RF pulse information (center frequency, bandwidth, etc.) to the transmitter 8, and gradient magnetic field information (gradient magnetic field strength, etc.). The gradient magnetic field power supply 9 is sent.

  The transmitter 8 outputs a drive signal for driving the RF coil 24 based on the information sent from the sequencer 7.

  The gradient magnetic field power supply 9 outputs a drive signal for driving the gradient coil 23 based on the information sent from the sequencer 7.

  The receiver 10 processes the magnetic resonance signal received by the receiving coil 6 and transmits it to the central processing unit 11.

  The central processing unit 11 implements various operations of the MRI apparatus 100 such as transmitting necessary information to the sequencer 7 and the display device 13 and reconstructing an image based on a signal received from the receiver 10. The operation of each unit of the MRI apparatus 100 is controlled. The central processing unit 11 is configured by, for example, a computer. In addition, the central processing unit 11 has an image data calculation unit 111. The image data calculation unit 111 calculates image data according to the equation (7) described later. The central processing unit 11 is an example of the image data calculation unit 111, and functions as this unit by executing a predetermined program.

  The input device 12 inputs various commands to the central processing unit 11 according to the operation of the operator 15. The display device 13 displays various information.

  The MRI apparatus 100 is configured as described above. In the present embodiment, the MRI apparatus 100 is used to perform a scan for obtaining an artery image. Next, a scan executed to obtain an artery image in the present embodiment will be described.

FIG. 2 is an explanatory diagram of the scan executed in the present embodiment.
In this embodiment, two scans are performed. The first scan is a scan SC _non without contrast medium, and the second scan is a scan SC _ca with contrast medium. In the scan SC _ca with the contrast agent, the contrast of the artery is enhanced compared to the scan SC _non without the contrast agent. Therefore, by obtaining the difference between the data obtained by the scan SC _non without the contrast agent and the data obtained by the scan SC _ca with the contrast agent, an image in which the artery is depicted and the background tissue is suppressed is obtained. Is possible.

In the conventional method, the imaging region is displaced between the end of the first scan (the scan SC _non without the contrast agent) and the start of the second scan (the scan SC _{ca with the} contrast agent). In this case, there is a problem that the outline of the background tissue remains without being removed. However, in the method of this embodiment, even if the imaging region is displaced between the end of the first scan and the start of the second scan, the outline of the background tissue is hardly left. There is an effect that can be done. The reason why this effect can be obtained will be described below.

First, it is assumed that the imaging region of the subject is located at the reference position. The k-space data S ₁ (k _x , k _y ) at this time can be expressed by the following equation (1).
ρ (x, y): image data at coordinates (x, y)

Further, assume that the imaging region of the subject is displaced from the reference position by Δx in the x direction and by Δy in the y direction. The k-space data S ₂ (k _x , k _y ) when the imaging region is displaced by Δx and Δy can be expressed by the following equation (2).

Comparing equation (1) and equation (2), it can be seen that the displacement of the position in the real space corresponds to the change of the phase in the k space. Therefore, if the absolute value is taken for each of the k-space data S ₁ (k _x , k _y ) and S ₂ (k _x , k _y ) and the difference between them is taken, the signal becomes zero. That is, the following relationship holds.
| S ₂ (k _x , k _y ) | − | S ₁ (k _x , k _y ) | = 0 (3)

In the present embodiment, the background tissue is removed by paying attention to the fact that even if the imaging region is displaced by Δx and Δy, if the difference between the absolute values of the k space is taken, the signal becomes zero. However, in the present embodiment, the artery must be depicted while removing the background tissue. Therefore, in order to realize background tissue removal and arterial rendering, k-space data S ₁ (k _x , k _y ) is collected by scan SC _non without contrast agent, while k-space data Consider that S ₂ (k _x , k _y ) is collected by the contrast-enhanced scan SC _ca so that the arterial contrast is enhanced over the contrast-less scan SC _non . In this case, the k-space data S ₁ (k _x , k _y ) obtained by the scan SC _non- contrast agent is considered to have a sufficiently small arterial signal, and therefore expressed by the following equation (4): Can do.
ρ ₁ (x, y): Background tissue signal

Also, k-space data S ₂ (k _x , k _y ) obtained by the scan SC _ca with the contrast agent can be expressed by the following equation (5).
ρ ₀ (x, y): Arterial signal ρ ₁ (x, y): Background tissue signal

Here, assuming that the k-space data when the background tissue is not drawn and only the artery is drawn is S ₀ (k _x , k _y ), S ₀ (k _x , k _y ) can be expressed by the following formula ( 6).

Next, a difference ΔS between the absolute value of S ₂ (k _x , k _y ) and the absolute value of S ₁ (k _x , k _y ) is obtained, and the phase of the difference ΔS and S ₂ (k _x , k _y ) is obtained. Consider image data obtained by multiplying P and ΔS · P obtained by the multiplication and inverse Fourier transform. Assuming that the image data at this time is ρ ′ (x, y), ρ ′ (x, y) can be expressed by the following equation (7).

In formula (7), for convenience of explanation, S ₁ (k _x , k _y ) is simply represented by “S ₁ ”, and S ₂ (k _x , k _y ) is represented simply by “S ₂ ”.

In Expression (7), | S ₂ | − | S ₁ | represents the difference ΔS between the absolute value of S ₂ (k _x , k _y ) and the absolute value of S ₁ (k _x , k _y ). , S ₂ / | S ₂ | represents the phase P of S ₂ (k _x , k _y ). Equation (7) can be further modified as follows.

Here, when S ₀ / S ₂ << 1 is set and the first order of S ₀ / S ₂ is taken into consideration, Equation (8) can be modified as follows.

Referring to Equation (9), in addition to the first term arterial signal ρ ₀ (x, y), the second term remains as an extra signal separate from the arterial signal ρ ₀ (x, y). . However, it is considered that the second term can be removed by executing MIP, for example. Therefore, as shown in Expression (7), a difference ΔS between the absolute value of S ₂ (k _x , k _y ) and the absolute value of S ₁ (k _x , k _y ) is obtained, and the difference ΔS and S ₂ ( It can be seen that by multiplying by the phase P of k _x , k _y ) and performing inverse Fourier transform, image data in which the artery is emphasized and drawn and the background tissue is sufficiently reduced can be obtained.

Next, a process flow of the MRI apparatus 100 when an image of the artery of the subject is taken by sequentially executing the scan SC _non without the contrast agent and the scan SC _ca with the contrast agent (see FIG. 2) will be described. .

FIG. 3 is a diagram showing a processing flow of the MRI apparatus 100.
In step ST1, a scan SC _non without contrast medium is executed (see FIG. 2). Data S ₁ (k _x , k _y ) in the k space of the imaging region can be collected by the scan SC _non without contrast medium. If the imaging region is displaced by breathing or heartbeat during the scan SC _non , the imaging region may be scanned using the respiratory synchronization method or the electrocardiogram synchronization method. After collecting k-space data S ₁ (k _x , k _y ) of the imaging region, the process proceeds to step ST2.

In step ST2, a scan SC _ca with contrast medium is executed (see FIG. 2). Data S ₂ (k _x , k _y ) in the k space of the imaging region can be collected by the scan SC _{ca with the} contrast agent. When the imaging region is displaced by breathing or heartbeat during the scan SC _ca , the imaging region may be scanned using the respiratory synchronization method or the electrocardiogram synchronization method. In the scan SC _ca with the contrast agent, the contrast of the artery is enhanced compared to the scan SC _non without the contrast agent. After collecting k-space data S ₂ (k _x , k _y ) of the imaging region, the process proceeds to step ST3.

  In step ST3, the image data calculation unit 11 (see FIG. 1) calculates the image data using Expression (7), and the flow ends. By calculating the image data using Equation (7), it is possible to obtain the image data of the artery with sufficiently reduced background tissue.

  In order to verify that image data of an artery with sufficiently reduced background tissue can be obtained, a simulation was performed. Hereinafter, simulation results will be described with reference to FIG. 4 and FIG.

FIG. 4 is an explanatory diagram of simulation conditions.
FIG. 4A is a diagram schematically showing the background tissue B, and FIG. 4B is a diagram schematically showing the artery V and the background tissue B. The background tissue B is simulated by a circle, and the artery V is simulated by a vertical bar. It is assumed that the background tissue B in FIG. 4B is displaced by Δx in the x direction and Δy in the y direction with respect to the background tissue B in FIG. 4A.

  4A1 to 4A4 respectively show the absolute value image, the phase image, the real part image of the MR signal, and the imaginary part image of the MR signal in the background tissue B of FIG. 4A. . 4 (b1) to 4 (b4) are respectively an absolute value image, a phase image, an image of the real part of the MR signal, and an imaginary part of the MR signal of the artery V and the background tissue B of FIG. 4 (b). An image is shown.

  FIG. 5 is an explanatory diagram of a simulation result of an image obtained by the difference between the background tissue B in FIG. 4A and the artery V and the background tissue B in FIG.

FIG. 5A shows the difference S ₂ − between the k-space data S ₂ of the artery V and the background tissue B in FIG. 4B and the k-space data S ₁ of the background tissue B in FIG. seeking S _1, a differential image obtained by inverse Fourier transform of the difference S ₂ -S ₁ data of the k-space.
FIG. 5B is a difference image obtained by the difference between the absolute value image of the background tissue B shown in FIG. 4A1 and the absolute value image of the artery V and the background tissue B shown in FIG. is there.
FIG. 5C is a difference image obtained using Expression (7) of the present embodiment.

  In the images of FIGS. 5A and 5B, the outline of the circle simulating the background tissue B remains, but the image of FIG. 5C obtained by using Expression (7) of the present embodiment. It can be seen that the signal of the background tissue B is almost completely removed. Therefore, according to this embodiment, it can be seen that image data of an artery with sufficiently reduced background tissue can be obtained.

  Furthermore, a phantom experiment was conducted in order to verify the influence of the second term of Equation (7) on the image quality. Hereinafter, the results of the phantom experiment will be described with reference to FIGS.

FIG. 6 is an explanatory diagram of conditions for the phantom experiment.
6A is a diagram illustrating a phantom that simulates the background tissue B, and FIG. 6B is a diagram illustrating a phantom that simulates the artery V and the background tissue B. FIG. The background tissue B is represented by a circle having a groove at the center, and the artery V is represented by a horizontal bar. It is assumed that the background tissue B in FIG. 6B is displaced by Δx in the x direction and Δy in the y direction with respect to the background tissue B in FIG. 6A.
FIGS. 6A1 to 6A4 respectively show the absolute value image, phase image, k-space absolute value, and k-space phase of the phantom of FIG. 6A. FIGS. 6B1 to 6B4 respectively show the absolute value image, phase image, k-space absolute value, and k-space phase of the phantom of FIG. 6B.

FIG. 7 is a diagram illustrating an experimental result of an image obtained by a difference between the phantom that simulates the background tissue B in FIG. 6A and the phantom that simulates the artery V and the background tissue B in FIG. .
FIG. 7A shows the phantom k-space data S ₂ simulating the artery V and the background tissue B in FIG. 6B and the phantom k-space data simulating the background tissue B in FIG. 6A. calculates the difference _S 2 -S ₁ and S _1, a differential image obtained by inverse Fourier transform of the difference _S 2 -S ₁ data of the k-space.
FIG. 7B shows the difference between the absolute value image of the phantom that simulates the background tissue B in FIG. 6A1 and the absolute value image of the phantom that simulates the artery V and the background tissue B in FIG. It is the obtained difference image.
FIG. 7C is a difference image obtained using Expression (7) of the present embodiment.

  In the image of FIG. 7A, the signal of the background tissue B remains without being removed, and in the image of FIG. 7B, the outline of the background tissue B remains. On the other hand, in the image of FIG. 7C obtained by the method of the present embodiment, it can be seen that the background tissue B is sufficiently reduced and the horizontal bar simulating the artery V is drawn with emphasis. In the image of FIG. 7C, a ghost due to the influence of the second term of the formula (7) appears. However, the ghost of the degree shown in FIG. If it is done, it is considered that it can be removed sufficiently.

  In this embodiment, the contrast medium is used in the two scans. However, if the second scan can emphasize the artery more than the first scan, the contrast medium is not always necessary. Alternatively, the non-contrast MRA may be used for imaging. In this embodiment, the artery is emphasized in the second scan, but the artery may be emphasized in the first scan.

In the present embodiment, artery image data is calculated using Expression (7). However, the arterial image data can be calculated by using the difference | S ₂ | − | S ₁ | of the absolute value of the k-space data and the phase S ₂ / | S ₂ | of the k-space data. In this case, an expression different from the expression (7) may be used.

  In this embodiment, the image data of the artery is obtained. However, the present invention can also be applied when obtaining image data of another tissue other than an artery, such as a vein.

DESCRIPTION OF SYMBOLS 100 MRI apparatus 2 Magnetic field generator 3 Table 4 Cradle 5 Contrast agent injection apparatus 6 Reception coil 7 Sequencer 8 Transmitter 9 Gradient magnetic field power supply 10 Receiver 11 Central processing unit 12 Input apparatus 13 Display apparatus 14 Subject 21 Bore 22 Superconducting coil 23 Gradient coil 24 Transmitting coil 111 Image data creation means

Claims (6)

A magnetic resonance imaging apparatus that scans an imaging region including a predetermined tissue and a background tissue and obtains image data of the predetermined tissue,
Scan means for executing a first scan that scans the imaging region, and a second scan that scans the imaging region so that the predetermined tissue is emphasized rather than the first scan;
Find the difference between the absolute value of the k-space data of the imaging region obtained by the first scan and the absolute value of the k-space data of the imaging region obtained by the second scan; Image data calculating means for calculating image data of the predetermined tissue based on phase information of k-space data obtained by the second scan;
A magnetic resonance imaging apparatus.   The magnetic resonance imaging apparatus according to claim 1, wherein the first scan is a scan without a contrast medium, and the second scan is a scan with a contrast medium.   The magnetic resonance imaging apparatus according to claim 1, wherein the first scan and the second scan are scans without a contrast medium. The magnetic resonance imaging apparatus according to claim 1, wherein the image data calculation unit calculates image data of the predetermined tissue based on the following expression.
  The magnetic resonance imaging apparatus according to claim 1, wherein the predetermined tissue includes an artery. A program of a magnetic resonance imaging apparatus that scans an imaging region including a predetermined tissue and a background tissue and obtains image data of the predetermined tissue,
The imaging region is scanned so that the absolute value of the k-space data of the imaging region obtained by the first scan that scans the imaging region and the predetermined tissue is emphasized more than the first scan. Obtain the difference between the absolute value of the k-space data of the imaging region obtained by the second scan, and based on the difference and the phase information of the k-space data obtained by the second scan, Image data calculation processing for calculating image data of the predetermined tissue;
A program to make a computer execute .