JP4112998B2 - Magnetic resonance imaging system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention particularly relates to an interventional magnetic resonance imaging apparatus using a catheter tracking technique in which a minute RF coil is inserted into a blood vessel.
[0002]
[Prior art]
The imaging object of the current magnetic resonance imaging apparatus (hereinafter referred to as MRI apparatus) is the main constituent substance of the subject, proton, which is widely used clinically. By imaging the spatial distribution of the proton density and the relaxation phenomenon of the excited nuclear spin, the form or function of the human head, abdomen, limbs, etc. can be photographed two-dimensionally or three-dimensionally.
[0003]
A configuration of a typical MRI apparatus will be described with reference to FIG. The MRI apparatus includes a magnet 902 that generates a static magnetic field around a subject 901, a gradient magnetic field coil 903 that generates a gradient magnetic field in the space, an RF coil 904 that generates a high-frequency magnetic field in this region, and a subject 901 An RF coil 905 for detecting the generated MR signal is provided. The gradient magnetic field coil 903 is configured by gradient magnetic field coils in three directions of X, Y, and Z, and each generates a gradient magnetic field in accordance with a current from the gradient magnetic field power supply 909. The RF coil 904 generates a high-frequency magnetic field in accordance with the signal from the RF transmission unit 910. The NMR signal received by the RF coil 905 is detected by the signal detection unit 906, signal processed by the signal processing unit 907, and converted into an image signal by calculation. The image is displayed on the display unit 908. The gradient magnetic field power source 909, the RF transmission unit 910, and the signal detection unit 906 are controlled by the control unit 911. The control time chart is generally called a pulse sequence. The table 912 is for the subject to lie down. The table 912 is controlled by a control unit, and the position is monitored by an encoder.
[0004]
In recent years, catheters have been used for cardiovascular examinations and treatments. In order to insert a catheter to a desired site in the body, a tracking device that monitors its position is required. Some catheters are provided with a receiving coil sensitive to a magnetic resonance signal for tracking the catheter, and this signal is detected in the presence of a magnetic field gradient. By detecting this signal, the position of the catheter in the subject is determined.
[0005]
[Patent Document 1]
JP-A-7-255691 [0006]
[Problems to be solved by the invention]
However, the length of the vascular catheter is 1 to 1.5 m. For example, the blood vessel is inserted from the femoral artery to the heart, and the tip of the catheter reaches the coronary artery. During this time, there is a desire to always monitor the tip of the catheter with an image. However, the static magnetic field uniform space of the MRI apparatus remains at most in the spherical range with a diameter of 40 cm. Therefore, the surgeon needs to move the patient during insertion of the catheter so that the tip of the catheter is always in an area where the MRI apparatus can take an image. Since this movement is performed during the operation, it is troublesome and reduces the efficiency of the operation. In addition, no suitable method has been proposed so far for control of catheter tracking with table movement.
[0007]
An object of the present invention is to provide an MRI apparatus capable of continuously performing MRI catheter tracking in a wide area at a high speed by moving a table so as to follow the catheter when the catheter is out of the field of view.
[0008]
[Means for Solving the Problems]
In order to solve the above problems, a static magnetic field generating means for generating a static magnetic field in a space in which the subject is arranged, a moving means for moving the table provided with a table on which the subject is placed, and an inclination in the space A gradient magnetic field generating means for generating a magnetic field; a high-frequency magnetic field generating means for causing nuclear magnetic resonance in an atomic nucleus constituting the biological tissue of the subject; and a receiving means for detecting a signal emitted by the nuclear magnetic resonance; In a magnetic resonance imaging apparatus comprising: signal processing means for processing a received signal detected by the receiving means; and display means for displaying an image reconstructed using the received signal.
The receiving means includes at least a catheter coil disposed at a distal end of a catheter to be inserted into the subject, and the signal processing means detects position information of the distal end of the catheter based on a signal from the receiving means. The moving means moves the table based on the position information .
[0009]
[Action]
According to the present invention, the tip of the catheter can always be monitored at the operator's desired timing or fully automatically, and the operator moves the patient during insertion of the catheter and moves the patient to an area where the MRI apparatus can take an image. Work becomes unnecessary. Also, active tracking of the catheter can be used in combination with multiple RF coils.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be specifically described below with reference to the drawings.
FIG. 1 schematically shows how an operator 104 inserts a catheter 102 from a femoral artery of a patient 106 using the MRI apparatus 100 of the present embodiment. The surgeon grasps the position of the catheter while looking at the display unit 101, and inserts the catheter.
[0011]
The MRI apparatus 100 has a static magnetic field strength of 0.3 T, for example, and a magnetic resonance frequency of 12.8 MHz. A high frequency magnetic field corresponding to the nuclear magnetic resonance frequency is received by a chest multiple RF coil composed of four small surface RF coils. The imaging sequence is GrE (Gradient Echo) type multi-shot EPI (Echo Planar Imaging). A signal required for one image of 128 × 128 pixel size is acquired in about 200 ms. The field of view of this image is 30 cm, and the slice thickness is 10 mm. The execution TE is 35 ms, the imaging repetition time TR is 0.2 s, and the number of slices is one. The photographing cross section is a coronal cross section, and the photographing is a fluoroscopic copy performed continuously. Therefore, the total number of shots is 1500, for example, when shooting for 300s continuously. During 300s under MR fluoroscopy, a vascular catheter is inserted from the femoral artery toward the heart. A thin guide wire made of copper wire is inserted into the sheath of the vascular catheter. One end of the guide wire is connected to a tuning / matching circuit and is connected to an RF receiving amplifier. It operates as a non-loop type RF coil and receives MR signals.
[0012]
FIG. 2 is a diagram for explaining an embodiment of the coil signal processing unit of the catheter monitor. As shown in FIG. 2, in this embodiment, a fifth coil, that is, a catheter coil 202 is installed in addition to the four coils for chest imaging. When the received signal from the catheter coil 202 is Fourier-transformed, the catheter coil generally has only local reception sensitivity, so that a signal only in the vicinity of the catheter coil 202 is obtained. By performing the catheter position calculation 203 from the catheter coil image, the position of the tip of the catheter can be determined. In the image processing used as the position calculation method, for example, the position of the pixel having the highest pixel value is regarded as the tip of the vascular catheter. Alternatively, pixels that exceed a certain threshold are extracted, and the tip position is determined from the center of gravity. For the determination by such image processing, since the tip of the catheter is highly sensitive, a catheter coil having a small loop coil at the tip is suitable. The calculated position information is sent to the control unit of the MRI apparatus and used for calculating the table movement amount. The table movement is preferably in the direction orthogonal to the X direction and the Y direction, but is smaller than the Y direction (body axis direction) Y direction.
[0013]
FIG. 3 is a detailed diagram related to signal synthesis of a small RF receiver coil (chest coil) that does not move. In signal acquisition (step 301), AD conversion / detection 20 is performed on the signals of the four small RF receiving coils 205, and four partial images en (i, j) (where n = 1, 2, 3, 4) (ie, A process of creating a result of Fourier transform 209 in FIG. 6 is performed (step 401). Subsequent to signal acquisition (step 301), sensitivity distribution calculation (step 302) of each small RF coil is performed using partial images of four small coils. An example of sensitivity distribution calculation is shown in FIG.
[0014]
That is, the original image en (i, j) is subjected to two-dimensional inverse Fourier transform (step 402), and the measurement K space data pn (i ′, j ′), (n = 1, 2, 3, 4: where i ′ , J ′ represents a data array in the K space), and a two-dimensional low-pass filter in the K space is applied to pn (i ′, j ′) (step 404), and a two-dimensional Fourier transform (step 405). Thus, the sensitivity distribution wn (i, j) (n = 1, 2, 3, 4) (step 407) is obtained. After the sensitivity distribution calculation is completed, signal synthesis is performed based on the sensitivity distribution (step 303).
[0015]
The details of the signal synthesis process are shown in FIG. 5 by blocking the signal acquisition (step 301) and (step sensitivity distribution calculation 302). Signal synthesis (step 303) is an operation for obtaining a composite image s (i, j) (step 304) using en (i, j) and wn (i, j). Indicated. The synthesized image (step 304) obtained as a result is displayed on the display unit 101 of the MRI apparatus. It is also transferred to a computer hard disk and stored as image data.
[0016]
The sensitivity distribution wn (i, j) (n = 1, 2, 3, 4) is stored in a predetermined memory of the signal processing unit, and the signal processing of the second image taken after 0.2 seconds and In the signal processing up to the (n-1) th sheet, after the signal processing (step 301), the sensitivity distribution result wn (i, j) (n = 1, Signal synthesis (step 303) is performed using 2, 3, 4).
[0017]
Then, the result is displayed or transferred (step 304), and this process continues until the table is moved. Only when the table is moved during the n-th shooting in the middle, the position of the RF coil also moves as the table moves, so the sensitivity distribution calculation is performed again. Thereafter, using the updated sensitivity distribution data, processing similar to that for the second to (n-1) th sheets is repeated. Through this process, multiple RF coils are continuously synthesized at high speed during catheter tracking, and appropriate sensitivity distribution calculation is performed even if the position of the small RF receiver coil (chest coil) changes after moving the table. The catheter tracking is performed again without image quality degradation. Although only one slice has been described in FIG. 3, the same processing can be extended for multi-slice imaging.
[0018]
FIG. 7 is a diagram showing an image display example of the present invention. The imaging origin 700 is set substantially equal to the magnetic field center of the MRI apparatus, and the imaging field of view 701 is typically 30 cm × 30 cm. The imaging cross section is coronal (coronal surface) and includes the abdominal aorta, and the vascular catheter 703 advances from the thigh (lower side) toward the heart (upper side). The tip P (x1, y1) 704 of the catheter is in the second visual field 702 near the magnetic field center. The second field of view is in the vicinity of the center of the magnetic field and is a region of, for example, 20 cm × 20 cm with the origin 700 as the center. When the operator sends the catheter from the thigh, the tip of the catheter passes through the abdominal aorta and reaches P ′ (x2, y2) 705 in the direction of the heart. Here, the position of the catheter is omitted in this description because it does not move much in the slice direction (z).
[0019]
In the open type MRI apparatus, the shape of the static magnetic field magnet is devised in order to enhance the accessibility to the subject, and as a result, generally the static magnetic field uniformity around the imaging visual field tends to decrease. Therefore, to image a catheter that has reached P ′ (x2, y2) 705, rather than moving the field of view, the subject is moved more quickly, and P ′ (x2, y2) comes closer to the origin 700. It is important to reset so that. Therefore, in this embodiment, the MRI apparatus extracts the catheter tip from the image information from the signal based on the sensitivity distribution information, and notifies the operator with an alarm when the tip of the catheter goes out of the visual field 702, or It has a mechanism that issues a warning on the image. Or, from the intentional operation of the surgeon (pressing the control button, clicking the mouse, etc.), from the signal based on the sensitivity distribution information, the catheter tip is moved based on the sensitivity distribution information to move the catheter tip. It has a function to move it close to the origin.
[0020]
Specifically, in order to move the tip of the catheter near the origin, the tip of the catheter is detected from the sensitivity distribution information in the first field of view. Then, the control unit sends a control signal in which the origin 700 is set to the distance and direction corresponding to the movement of the catheter tip, and drives the motor based on the control signal so that the catheter tip is in the center of the screen. Let the table move.
[0021]
The image after movement is shown in FIG. When the position of the catheter 102 is at the corner on the image, the table automatically moves and the catheter is at the center of the image (ie, the magnetic field center 105). After the table is stopped, catheter monitoring automatically restarts. Alternatively, when the operator presses the switch 103, the table starts to move slowly, and when the tip of the catheter reaches the magnetic field center 105, the table automatically stops based on the sensitivity distribution information.
[0022]
Here, a warning message that informs the surgeon to move the table is displayed on the display unit 101 with, for example, an alarm lamp lit on the screen. In addition, the frame of the visual field 2 (FIG. 7: 702) is overwritten on the image to inform the operator that the tip of the catheter is out of the center of the visual field. In either case, catheter tracking (fluoroscopy) is automatically restarted after moving the table. If necessary, the fluoroscopy may be continued while the table is moving. However, in this case, the image quality may be deteriorated due to electrical noise or movement of the subject during table movement.
[0023]
In the above embodiment, the catheter is an active catheter, but the present invention can also be applied to a passive catheter. In this case, the catheter does not receive a signal, but a catheter made of a material having a slight magnetic property causes local magnetic field inhomogeneity, resulting in signal loss on the image. Therefore, on the MR image, the distal end portion of the catheter is shown as a darker point than the surroundings. This point is automatically extracted by image recognition, and the relationship with the field of view is recognized as in the above embodiment, and the table is moved as necessary. In addition, after moving the table, the sensitivity distribution calculation of multiple RF coils is performed again, and catheter tracking is automatically restarted. Note that the distal end of the catheter may be input by clicking on the position on the image. In this case, although it is not automatic, image processing is simple.
[0024]
In this embodiment, the multi-shot EPI is taken as an example of the shooting sequence, but another shooting sequence, for example, a normal Gr sequence may be used.
[0025]
【The invention's effect】
In the present invention, when the catheter is out of the field of view, the table can be moved so as to follow the catheter, and MRI catheter tracking can be performed continuously in a wide area at high speed.
[Brief description of the drawings]
FIG. 1 is a diagram of an MRI apparatus showing an outline of the present invention.
FIG. 2 is a diagram showing an overall configuration of the present invention.
FIG. 3 is a schematic diagram of signal processing according to the present invention.
FIG. 4 is a diagram showing sensitivity distribution calculation according to the present invention.
FIG. 5 is a detailed view of signal processing according to the present invention.
FIG. 6 is a schematic diagram of signal processing according to the present invention.
FIG. 7 is an image taken using the present invention.
FIG. 8 is an image taken using the present invention.
FIG. 9 is a diagram of a conventional MRI apparatus.
[Explanation of symbols]
100 gantry, 101 display section, 102 catheter, 104 examiner, 105 magnetic field center, 106 subject

Claims (4)

A static magnetic field generating means for generating a static magnetic field in a space in which the subject is arranged, a moving means for moving the table provided with a table on which the subject is placed, and a gradient magnetic field generating for generating a gradient magnetic field in the space Means, high-frequency magnetic field generating means for causing nuclear magnetic resonance in atomic nuclei constituting the biological tissue of the subject, receiving means for detecting a signal emitted by the nuclear magnetic resonance, and detection by the receiving means In a magnetic resonance imaging apparatus comprising signal processing means for processing a received signal and display means for displaying an image reconstructed using the received signal,
The receiving means includes at least a catheter coil disposed at a distal end of a catheter inserted into the subject,
The signal processing means detects positional information of the tip of the catheter based on a signal from the receiving means,
The magnetic resonance imaging apparatus characterized in that the moving means moves the table based on the position information. The magnetic resonance imaging apparatus according to claim 1.
A magnetic resonance imaging apparatus comprising: means for generating information indicating that the table needs to be moved based on the position information. The magnetic resonance imaging apparatus according to claim 1.
The magnetic resonance imaging apparatus, wherein the signal processing means detects the position information based on a signal from the catheter coil. The magnetic resonance imaging apparatus according to claim 1.
The receiving means further comprises a receiving coil arranged outside the subject,
The magnetic resonance imaging apparatus, wherein the signal processing means detects the position information based on an image reconstructed using a signal from the receiving coil.

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