JP2004329669A - MRI apparatus and MRI imaging method - Google Patents

Abstract

Provided is an MRI apparatus and an MRI imaging method capable of stably generating an imaging trigger pulse even when noise due to a gradient magnetic field or the like is mixed in an electrocardiographic waveform of an electrocardiographic synchronization method.
In an MRI imaging by an electrocardiographic synchronization method, when noise (magnetic flux noise) caused by a change in magnetic flux of a magnetic field formed by a gradient magnetic field coil and the like is superimposed on an electrocardiographic waveform from a subject, a control unit is provided. The sequence control circuit 24 calculates a section in which magnetic flux noise occurs based on pulse sequence information used for MRI imaging, and supplies an imaging section signal generated from the calculation result to the electrocardiogram unit 7. Then, the electrocardiographic unit 7 generates an imaging trigger pulse from the electrocardiographic waveform excluding the magnetic flux noise by controlling its own gate function using the imaging section signal, and the control unit 4 generates the imaging trigger pulse. Based on the pulse, MRI imaging is performed by an electrocardiographic synchronization method.
[Selection diagram] Fig. 1

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an MRI apparatus, and more particularly, to an MRI apparatus and an MRI imaging method having an MRI imaging function using an ECG gating method.
[0002]
[Prior art]
In magnetic resonance imaging (MRI), nuclear spins of a subject tissue placed in a static magnetic field are excited by a high-frequency signal having the Larmor frequency, and an image is reconstructed from a magnetic resonance signal generated by the excitation. This is a diagnostic imaging method.
[0003]
An MRI apparatus is an image diagnostic apparatus using a magnetic resonance signal detected from a living body, and can obtain not only anatomical diagnostic information but also many diagnostic information such as biochemical information and functional diagnostic information. Has become indispensable in the field of diagnostic imaging. In particular, heart-enhanced MRI has made great progress in recent years, and has been developed to a level widely used in clinical settings in the diagnosis of myocardial ischemia and myocardial infarction.
[0004]
In recent years, it has become possible to analyze the functions of the brain and heart by developing a high-speed imaging method of MRI. In particular, when imaging a moving organ such as the heart, it is necessary to reduce the imaging time. However, according to the EPI (Echo-planar-imaging) method, which is one of the high-speed imaging methods, one is required. The imaging time of a single image has become 100 msec or less, and it has become possible to obtain an MRI image of a heart at a desired time phase. In this cardiac MRI imaging, a method of simultaneously acquiring an electrocardiographic waveform (ECG) of a subject during the imaging, and performing MRI imaging of a predetermined slice cross section after a predetermined delay time from the generation timing of, for example, an R wave of the electrocardiographic waveform. (For example, see Patent Document 1).
[0005]
When an electrocardiographic waveform is collected from a subject during MRI imaging, the amplitude of the obtained electrocardiographic waveform depends on the position and physique of the heart of the subject, as well as the mounting position and mounting state of the electrodes. For this reason, in order to stably generate a trigger pulse (hereinafter, referred to as an imaging trigger pulse) used for imaging from the electrocardiogram waveform, a trigger level proportional to the size of the electrocardiogram waveform is set. A method of generating an imaging trigger pulse by comparing the magnitudes of electrocardiographic waveforms has been adopted.
[0006]
FIG. 9 shows an electrocardiogram waveform in conventional cardiac MRI imaging and a time chart of MRI imaging in each phase. FIG. 9A shows an electrocardiogram waveform (solid line) and an imaging from the electrocardiogram waveform. A trigger level (broken line) used to generate a trigger pulse is shown superimposed. FIG. 9B shows the generation timing of the imaging trigger pulses F1 and F2 generated by comparing the above-mentioned trigger level with the electrocardiogram waveforms E1 and E2, and FIG. 9C shows each time phase with respect to the imaging trigger pulses F1 and F2. The MRI imaging section (T1 to T4) is shown.
[0007]
For example, when performing MRI imaging in the section T02 based on the R wave (R2) of the electrocardiographic waveform E2, the trigger level is set based on the value of the R wave (R1) which is the maximum value of the electrocardiographic waveform E1 in the section T01. Then, this trigger level is compared with the electrocardiographic waveform E2. Then, when the amplitude of the electrocardiographic waveform E2 exceeds the trigger level, an imaging trigger pulse F2 is generated, and MRI imaging in the section T02 (for example, MRI imaging of the time phases T1 to T4 by the EPI method) is performed according to the imaging trigger pulse F2. Done. The trigger level in this case has, for example, an attenuation characteristic having a predetermined time constant (for example, 1 second) with respect to the maximum value R1 of the electrocardiographic waveform E1, as shown in FIG. 9A.
[0008]
[Patent Document 1]
JP-A-8-182661 (pages 3-4, FIGS. 11-12)
[0009]
[Problems to be solved by the invention]
In MRI imaging, in order to detect an MR signal from the subject, it is necessary to irradiate the subject with an RF magnetic field for excitation or a gradient magnetic field for giving positional information to the MR signal based on an imaging trigger pulse. is there. When MRI imaging and acquisition of an electrocardiographic waveform are performed simultaneously, a cable connecting a plurality of ECG sensors mounted on the body surface of the subject and an electrocardiograph is arranged on the body surface of the subject, and this cable and the ECG A closed loop is formed by the capacitive coupling between the sensors. For example, when the excitation RF magnetic field and the gradient magnetic field are switched at a high speed as in the case of EPI imaging, the magnetic flux crossing the closed loop greatly changes, so that an induced current is generated in the cable and the electromagnetic wave is generated. There is a problem that it is mixed as noise into the shape.
[0010]
When noise caused by a change in magnetic flux (hereinafter referred to as magnetic flux noise) mixed by such a mechanism has a magnitude that cannot be ignored with respect to the electrocardiographic waveform, the imaging trigger level is changed by the magnetic flux noise instead of the R wave. Is formed. For this reason, it becomes impossible to detect the R wave of the electrocardiographic waveform immediately after the occurrence of magnetic flux noise, and it becomes difficult not only to perform MRI imaging at a desired time phase but also to perform MRI imaging using a contrast agent. In some cases, the contrast agent may flow out of the imaging target tissue before the imaging, and a desired imaging effect may not be obtained.
[0011]
The present invention has been made in view of the above problems, and an object of the present invention is to provide an MRI that can stably perform MRI imaging by an electrocardiographic synchronization method even when magnetic flux noise is mixed in an electrocardiographic waveform. An apparatus and an MRI imaging method are provided.
[0012]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, an MRI apparatus according to the present invention according to claim 1 is an MRI apparatus that acquires an MR signal in synchronization with a biological signal obtained from a subject, and an MRI apparatus obtained from the subject. A waveform generating means for deleting or reducing a signal in a predetermined section of the first waveform signal to generate a second waveform signal; and collecting an MR signal in a predetermined time phase of the subject based on the second waveform signal. Image data generating means for performing reconstruction processing on the collected MR signals to generate MRI image data.
[0013]
An MRI apparatus according to a second aspect of the present invention is an MRI apparatus that collects an MR signal in synchronization with a biological signal obtained from a subject, and obtains a first waveform signal obtained from the subject. In addition, a waveform generation means for deleting or reducing a signal in a predetermined section of the first waveform signal to generate a second waveform signal, and selecting either the first waveform signal or the second waveform signal A MR unit for acquiring MR signals in a predetermined time phase of the subject based on the waveform signal selected by the waveform selecting unit and performing a reconstruction process on the collected MR signals to generate MRI image data The waveform selecting means selects the second waveform signal when the noise mixed in the first waveform signal is large.
[0014]
On the other hand, an MRI imaging method of the present invention according to claim 13 is an MRI imaging method for acquiring an MR signal in synchronization with a biological signal obtained from a subject, wherein the first waveform signal obtained from the subject is Generating a second waveform signal by deleting or reducing the signal in a predetermined section of; and collecting an MR signal in a predetermined time phase of the subject based on the second waveform signal; Performing a reconstruction process on the MR signal to generate MRI image data.
[0015]
Therefore, according to the present invention, in the MRI imaging by the electrocardiogram synchronous method, even when the magnetic flux noise by the MRI apparatus is mixed in the electrocardiogram waveform, the imaging trigger pulse can be obtained stably, MRI image data can be generated with high accuracy.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
A feature of the embodiment of the present invention described below is that, in an acquisition section (imaging section) of an MR signal, a magnetic flux noise mixed in an electrocardiographic waveform is eliminated and a stable imaging trigger pulse is generated.
[0017]
(Structure of the device)
Embodiments of the present invention will be described with reference to FIGS. 1 and 2 with respect to a configuration of an MRI apparatus in which an MR signal is acquired using the EPI method. FIG. 1 is a block diagram showing the overall configuration of the MRI apparatus according to the present embodiment, and FIG. 2 is a functional block diagram of an electrocardiographic unit which is an important element of the present embodiment.
[0018]
The MRI apparatus 100 includes a static magnetic field generating unit 1 and a gradient magnetic field generating unit 2 for generating a magnetic field, a transmitting and receiving unit 3 for transmitting and receiving an RF pulse signal, a control unit 4 for controlling the entire system, image reconstruction and image A high-speed operation / storage unit 5 for performing storage is provided. Further, the MRI apparatus 100 displays a bed 8 on which the subject 11 is placed, an electrocardiographic unit 7 and an ECG sensor 9 for collecting electrocardiographic waveforms, an input section 22 for inputting various data and commands, and displays MRI image data. The display unit 21 is provided.
[0019]
The static magnetic field generation unit 1 includes, for example, a main magnet 13 that is a superconducting magnet and a static magnetic field power supply 26 that supplies a current to the main magnet 13, and forms a strong static magnetic field around the subject 11. Further, the gradient magnetic field generation unit 2 includes a gradient magnetic field coil 14 in the X, Y, and Z axis directions orthogonal to each other, and a gradient magnetic field power supply 25 for supplying a current to these gradient magnetic field coils 14.
[0020]
The gradient magnetic field control signal is supplied from the control unit 4 to the gradient magnetic field power supply 25, and the space in which the subject 11 is placed is encoded. That is, by controlling the pulse current supplied from the gradient magnetic field power supply 25 to the X, Y, and Z axis gradient magnetic field coils 14 based on the gradient magnetic field control signal, the gradient magnetic fields in the X, Y, and Z axis directions are synthesized. The slice selection gradient magnetic field Gs, the phase encoding gradient magnetic field Ge, and the readout (frequency encoding) gradient magnetic field Gr, which are orthogonal to each other, can be set in arbitrary directions. Then, the gradient magnetic field in each direction is superimposed on the static magnetic field by the main magnet 13 and applied to the subject 11.
[0021]
The transmitting / receiving unit 3 includes an irradiation coil 15 for irradiating the subject 11 with an RF pulse, a receiving coil 16 for detecting an MR signal, and a transmitter 17 and a receiver 18 connected to these coils. However, the irradiation coil 15 and the reception coil 16 may be provided separately. The transmitter 17 has the same frequency as the magnetic resonance frequency determined by the static magnetic field strength of the main magnet 13, drives the irradiation coil 15 with an RF pulse current modulated by a selective excitation waveform, and sends an RF pulse to the subject 11. Irradiate. On the other hand, the receiver 18 performs A / D conversion after performing signal processing such as intermediate frequency conversion, phase detection, and filtering on a signal received as an MR signal by the reception coil 16.
[0022]
Next, the control unit 4 includes a main control circuit 23 and a sequence control circuit 24. The main control circuit 23 includes a CPU and a storage circuit (not shown), and has a function of controlling the entire apparatus. The storage circuit of the main control circuit 23 has a storage function of temporarily storing a shooting start instruction signal input from the input unit 22, a shooting method, information on a pulse sequence, image display format information, and the like. On the other hand, the CPU of the main control circuit 23 sends pulse sequence information (for example, the magnitude of the pulse current applied to the gradient magnetic field coil 14 and the irradiation coil 15) to the sequence control circuit 24 based on the information input from the input unit 22. Information about the application time and application timing).
[0023]
The sequence control circuit 24 of the control unit 4 includes a CPU and a storage circuit (not shown). The pulse sequence information transmitted from the main control circuit 23 is temporarily stored in the storage circuit, and then the pulse sequence information is stored in accordance with the pulse sequence information. The gradient magnetic field power supply 25, the transmitter 17, and the receiver 18 are controlled. Particularly, when a pulse current is supplied from the gradient magnetic field power supply 25 to the gradient magnetic field coil 14 to collect an MR signal, an imaging section signal indicating a signal collection section (hereinafter referred to as an imaging section) is generated, and It is supplied to the power unit 7. Further, the timing of the applied magnetic field to the gradient magnetic field power supply 25 and the transmitter 17 is controlled in accordance with the imaging trigger pulse supplied from the electrocardiogram unit 7.
[0024]
On the other hand, the high-speed operation / storage section 5 includes a high-speed operation circuit 19 and a storage circuit 20. Then, the high-speed operation circuit 19 performs reconstruction by two-dimensional Fourier transform on the MR signal transmitted from the receiver 18 via the sequence control circuit 24, and converts real-space image data (MRI image data). Generate.
[0025]
The storage circuit 20 includes an MR signal storage circuit 28 that stores MR signals, and an image data storage circuit 29 that stores MRI image data. The MR signal storage circuit 28 stores the MR signal that has been subjected to the intermediate frequency conversion, the phase detection, and the A / D conversion by the receiver 18. Further, the image data storage circuit 29 stores reconstructed image data generated by performing a two-dimensional Fourier transform by the high-speed operation circuit 19 on the MR signal once stored in the MR signal storage circuit 28, that is, MRI image data. Is saved.
[0026]
The couch 8 has a structure in which the subject 11 can be moved to an arbitrary position in the body axis direction in order to set a desired imaging position, and can be inserted into the opening of the main magnet 13.
[0027]
Next, the input unit 22 includes various input devices such as a switch, a keyboard, and a mouse, and a display panel on a console, and allows the operator to perform imaging such as a patient ID, an imaging start instruction signal, an imaging method, a pulse sequence, and the like. Information about conditions or display methods, instruction signals for moving the mechanism, and the like are input. Then, the input information is temporarily stored in the storage circuit of the main control circuit 23.
[0028]
The display unit 21 includes a display image data storage circuit (not shown), a conversion circuit, and a monitor. The MRI image data stored in the image data storage circuit 29 of the high-speed operation / storage unit 5 transmits the main control circuit 23 to the main control circuit 23. The display image data is supplied to the display image data storage circuit. The MRI image data and the accompanying information such as various characters and numerals input from the input unit 22 are combined in the display image data storage circuit, and the conversion circuit performs D / A conversion and television format conversion. After that, it is displayed on a monitor such as a CRT or a liquid crystal.
[0029]
On the other hand, the electrocardiographic unit 7 is for generating a trigger pulse for MRI imaging based on the output of the ECG sensor 9 mounted on the body surface of the subject 11, and as shown in FIG. Is an electrocardiograph 71 that converts an electrocardiographic waveform detected by the ECG sensor 9 into a digital signal, a gate circuit 72 that blocks a signal for a predetermined period to eliminate magnetic flux noise mixed into the electrocardiographic waveform, A trigger level setting circuit 73 for setting a trigger level based on the peak value (R wave) of the radio wave form, and a comparison circuit 74 for comparing the set trigger level with the electrocardiographic waveform to generate a trigger pulse for imaging. Have.
[0030]
Next, an outline of the EPI method used in the present embodiment will be described with reference to FIG. FIG. 3A shows an electrocardiographic waveform output from the electrocardiograph 71 of the electrocardiographic unit 7. FIG. 3B shows an acquisition period of an MR signal necessary for generating MRI image data in the time phases T1 to T4. That is, FIG. 3B collects MR signals of a desired slice cross section in the time phase T1 based on the R wave of the electrocardiographic waveform shown in FIG. 3A, and then, in the time phase T2 to the time phase T4. 5 shows a case in which MR signals of the slice section are sequentially acquired. In collecting the MR signals of each time phase, for example, 256 types of encoded data are sequentially collected.
[0031]
FIG. 3C shows a pulse sequence when an MR signal of a desired slice section is acquired by using the EPI method. FIG. 3C-1 shows an irradiation timing of the RF pulse and detection of the MR signal. FIG. 3 (c-2) shows the slice selection gradient magnetic field Gs, FIG. 3 (c-3) shows the read (frequency encoding) gradient magnetic field Gr, and FIG. 3 (c-4) shows the phase encode gradient magnetic field Ge. Each is shown.
[0032]
For example, after a time T1 from the R wave, the slice slice is selected by the slice selection gradient magnetic field Gs (c-2 in FIG. 3), and the selected slice slice is irradiated with an RF pulse (c- in FIG. 3). 1). When the irradiation of the RF pulse is completed, the positional information is added by the application of the gradient magnetic fields Gr and Ge in the readout direction and the phase encode direction with respect to the slice cross section (c-3 in FIG. 3 and c-4 in FIG. 3). The first MR signal is received from the slice section by the receiving coil 16 of the transmitting / receiving section 3 (c-1 in FIG. 3).
[0033]
Next, the polarity of the gradient magnetic field Gr in the readout direction is alternately reversed (c-3 in FIG. 3), and the gradient magnetic field in the phase encode direction is also sequentially changed (c-4 in FIG. 3), whereby the phase encode in the slice cross section is obtained. The MR signals in the directions are sequentially acquired (c-1 in FIG. 3), and the MR signals necessary for reconstructing the MRI image data of the time phase T1 are acquired. Similarly, acquisition of MR signals for generating MRI image data of the time phases T2 to T4 is subsequently performed. In the EPI method, the polarity switching of the gradient magnetic field Gr is required several tens to several hundred times in a period (for example, 100 msec) during which one MRI image data is generated. Noise is easily generated.
[0034]
(Image data generation procedure and basic operation of the device)
An MRI image data generation procedure and a basic operation of the apparatus according to the present embodiment based on the above-described imaging method will be described with reference to FIGS. FIG. 6 is a flowchart showing a procedure for generating MRI image data.
[0035]
Prior to MRI imaging, the operator wears the ECG sensor 9 on a predetermined part of the body surface of the subject 11. FIG. 4 shows a state in which the subject 11 is arranged at the imaging position of the MRI image, and for example, two ECG sensors 9-1 and 9-2 are mounted on the body surface of the subject 11. The ECG sensors 9-1 and 9-2 are connected to the electrocardiograph 71 of the electrocardiogram unit 7 via cables 10-1 and 10-2. The electrocardiographic waveform of the subject 11 detected by the ECG sensors 9-1 and 9-2 is converted into a digital signal by the electrocardiograph 71. On the other hand, the main control circuit 23 reads the electrocardiographic waveform converted into a digital signal from the electrocardiograph 71, displays the electrocardiogram on the display unit 21, and automatically measures the R-wave interval from the displayed electrocardiographic waveform (FIG. 6). Step S1).
[0036]
Next, the operator inputs an MRI imaging mode from the input unit 22 (step S1). When this command is supplied to the main control circuit 23, the main control circuit 23 switches the display unit 21 to a data input screen for MRI imaging. The operator uses an input device such as a mouse or a keyboard provided in the input unit 22 to determine the number of MRI images to be captured in one heartbeat based on the already measured R-wave interval of the electrocardiographic waveform, and The time phase of the MRI image (that is, the time from the R wave of the electrocardiographic waveform to the irradiation of the RF pulse) is set, and imaging conditions such as an imaging method, a display method, or a pulse sequence necessary for MRI imaging are input (FIG. 6) Step S2).
[0037]
In general, when the EPI method is applied, for example, data of one MRI image can be collected in about 100 msec, and in consideration of a margin of about 20%, the number of images that can be collected in one heartbeat in a healthy person is considered. Is about 4 to 5 sheets. Therefore, an example in which four MRI images of the time phases T1 to T4 are acquired during one heartbeat by the pulse sequence of the EPI method will be described below as an example.
[0038]
Upon receiving the data via the input unit 22, the main control circuit 23 enables four MRI images of the time phases T1 to T4 based on the imaging method (EPI method) which has been set. The information is converted into pulse sequence information (for example, information on the intensity, application time, application timing, and the like of the pulse current applied to the gradient coil 14 and the irradiation coil 15). Further, the photographing section Tx is calculated based on the application time and the number of phases, and the calculation result and the pulse sequence information are supplied to the sequence control circuit 24. Then, the sequence control circuit 24 sends a control signal for the EPI method based on the ECG synchronization to the gradient magnetic field power supply 25, the transmitter 17, the receiver 18, and the ECG unit 7 according to the information.
[0039]
When the setting of the imaging conditions is completed, the operator injects an MR contrast agent (for example, Gd-DTPA (gadolinium diethylenamine pentaacetic acid)) into the subject 11 through the heart catheter as necessary, and then performs imaging. A start command signal is input from the input unit 22 (Step S3 in FIG. 6).
[0040]
Next, a method of generating an imaging trigger pulse for setting a time phase of an image in the first MRI imaging performed according to the imaging start command signal will be described with reference to FIG. The section T00 in FIG. 5 is a section for generating an imaging trigger pulse for the first MRI imaging, the section T01 is the first MRI imaging section for the time phases T1 to T4, and the section T02 is the second MRI imaging section. An MRI imaging section is shown.
[0041]
5A is an MRI imaging section of each time phase, and FIG. 5B is a digitized electrocardiographic waveform output from the electrocardiograph 71 of the electrocardiographic unit 7 shown in FIG. FIG. 5C shows an imaging section signal sent from the sequence control circuit 24 of the control unit 4 to the gate circuit 72. FIG. 5D shows an electrocardiogram waveform (solid line) output from the gate circuit 72 and a trigger level (dashed line) output from the trigger level setting circuit 73. FIG. The photographing trigger pulse output from the circuit 74 is shown.
[0042]
In a section T00 immediately after the operator inputs the MRI imaging start command, an ECG waveform E0 (FIG. 5 (d)) detected by the ECG sensor 9, converted into a digital signal by the electrocardiograph 71 and passed through the gate circuit 72 ) Is supplied to the trigger level setting circuit 73. In the trigger level setting circuit 73, the magnitude AE0 of the peak value (R wave) of the electrocardiographic waveform is detected, and a trigger level having an attenuation characteristic of a predetermined time constant (for example, 1 sec) based on AE0 (FIG. 5) The waveform (d) is generated and supplied to the first input terminal of the comparison circuit 74. On the other hand, an electrocardiogram waveform (solid line in FIG. 5D) from the electrocardiograph 71 is supplied to a second input terminal of the comparison circuit 74, and the electrocardiogram waveform is compared with the trigger level. When the radio wave waveform becomes larger than the trigger level, the photographing trigger pulse F1 is output from the comparison circuit 74 to the main control circuit 23 of the control unit 4 (step S4 in FIG. 6).
[0043]
Next, a procedure of MRI imaging performed based on the above-described imaging trigger pulse will be described with reference to FIGS. 1, 3, and 5. FIG. The sequence control circuit 24 of the control unit 4 shown in FIG. 1 measures a predetermined delay time T1 with a timer configured by software based on an imaging trigger pulse F1 generated at the timing of the R wave of the electrocardiographic waveform E1, and performs imaging. After a time T1 from the trigger pulse, a slice section for MRI imaging is set (c-2 in FIG. 3). Further, the slice section is irradiated with an RF pulse. That is, the sequence control circuit 24 sends a control signal to the gradient magnetic field power supply 25, and after a time T1, the gradient magnetic field power supply 25 supplies the three gradient magnetic field coils 14 based on the control signal from the sequence control circuit 24. Set the pulse current. Further, the pulse current is supplied to the gradient magnetic field coils 14 in the X, Y, and Z directions to form a slice selection gradient magnetic field Gs for selecting a slice cross section. On the other hand, the pulse sequence control circuit 24 supplies a control signal to the transmitter 17 to receive the MR signal in the slice section, and sets the frequency and phase of the RF pulse to be supplied to the irradiation coil 15, An RF pulse current is supplied to the irradiation coil 15.
[0044]
The RF pulse current has the same frequency as the magnetic resonance frequency determined by the magnetic field strength of the region of interest and is modulated by the selective excitation waveform. The irradiation coil 15 irradiates the region of interest of the subject 11 with an RF pulse by supplying the RF pulse current from the transmitter 17.
[0045]
When the irradiation of the RF pulse is completed, the sequence control circuit 24 sends a control signal to the gradient magnetic field power supply 25 again to add position information to the MR signal received from the slice section, and the gradient magnetic field power supply 25 Supplies a pulse current to the gradient magnetic field coil 14 in the X, Y, and Z directions to form a first gradient magnetic field Gr1 in the reading direction and a first gradient magnetic field Ge1 in the phase encoding direction with respect to the slice cross section. Then, the MR signal is received by the receiving coil 16 in a state where the MR signal is phase-modulated by the mutually perpendicular gradient magnetic fields Gr1 and Ge1.
[0046]
The receiver 18 performs A / D conversion on the MR signal supplied from the reception coil 16 after performing signal processing such as intermediate frequency conversion, phase detection, and filtering, and sends the signal to the sequence control circuit 24. Then, the sequence control circuit 24 stores this MR signal in the MR signal storage circuit 28 as first encoded data.
[0047]
Next, under the control of the sequence control circuit 24, the gradient of the gradient magnetic field in the phase encoding direction and the second gradient magnetic field Gr2 in the reading direction in which the polarity of the first gradient magnetic field Gr1 in the reading direction is reversed. In order to change the predetermined amount ΔGe, the second gradient magnetic field Ge2 is formed by the gradient magnetic field coils 14 in the X, Y, and Z directions, and the MR signal obtained by the same procedure as in the case of the first encode data is converted to the second encode data. The data is stored in the MR signal storage circuit 28 of the storage circuit 20 as data.
[0048]
Hereinafter, similarly, the third reading direction gradient magnetic field Gr3 to the Nth reading direction gradient magnetic field Gr3 in which the gradient magnetic field is alternately reversed, and the third phase encoding direction gradient magnetic fields Ge3 to Ge in which the gradient magnetic field is sequentially increased and decreased. MR signals obtained by applying the gradient magnetic field GeN in the N-th phase encoding direction are sequentially stored in the MR signal storage circuit 28 as third to N-th encoding data.
[0049]
Next, the sequence control circuit 24 sends a control signal for capturing an MRI image of the time phase T2 to the gradient magnetic field power supply 25 after a time T2 from the imaging trigger pulse F1, and the gradient magnetic field power supply 25 A pulse current is supplied to the magnetic field coil 14 to form a slice selection gradient magnetic field Gs for selecting a desired slice section (in this case, the same slice section as the time phase T1). On the other hand, the pulse sequence control circuit 24 controls the transmitter 17 to supply an RF pulse current to the irradiation coil 15, and the irradiation coil 15 irradiates the region of interest of the subject 11 with an RF pulse.
[0050]
Next, similarly to the case of the time phase T1, the gradient magnetic field coils 14 in the X, Y, and Z directions use the first gradient magnetic field Gr1 to the Nth gradient magnetic field GrN in the readout direction with respect to the slice cross section, and the gradient magnetic field in the phase encode direction. The first to Nth MR signals are received by the receiving coil 16 while sequentially applying the first to Nth gradient magnetic fields Ge1 to GeN. The MR signal received by the receiving coil 16 is subjected to intermediate frequency conversion, phase detection, and filtering in the receiver 18 and then A / D converted. The first to Nth encoded data are stored in the T2 area. Hereinafter, similarly, the MR signals in the time phase T3 and the time phase T4 are collected, and the first encoded data to the N-th encoded data of each time phase in the time phase T3 area and the time phase T4 area of the MR signal storage circuit 28. (Step S5 in FIG. 6).
[0051]
In this manner, the N-th first encoded data to the N-th encoded data arranged in each of the four regions (that is, the T1 region to the T4 region) of the MR signal storage circuit 28 are operated at high speed. The high-speed operation circuit 19 of the unit 5 performs image reconstruction by two-dimensional inverse Fourier transform, and performs high-speed operation and storage of the four MRI image data of the time phases T1 to T4 in the generated section T01. 29 (step S6 in FIG. 6).
[0052]
Further, the main control circuit 23 selects all of the four MRI image data or desired MRI image data from the MRI image data according to the display format of the display method previously input from the input unit 22. Then, it is displayed on the monitor of the display unit 21 together with the accompanying information (step S7 in FIG. 6).
[0053]
In parallel with the generation and display of the MRI image data of the time phases T1 to T4 in the section T01, the electrocardiogram unit 7 in FIG. 2 is output from the electrocardiograph 71 in the section T01 shown in FIG. An imaging trigger pulse in the section T02 is generated based on the electrocardiographic waveform. In this case, noise (magnetic flux noise) resulting from high-speed switching of the gradient magnetic fields Gr1 to GrN in the readout direction and the gradient magnetic fields Ge1 to GeN in the phase encoding direction is superimposed on the electrocardiographic waveform. For example, in FIG. 4 already shown, the ECG sensors 9-1 and 9-2 and the cables 10-1 and 10-2 attached to the body surface of the subject 11, and further, the ECG sensor 9-1 and the ECG sensor 9-2. A closed loop is formed by the capacitive coupling formed between them. The magnetic flux density of the gradient magnetic field crossing the closed loop changes with time, so that an induced current (magnetic flux noise) is generated. This magnetic flux noise is superimposed on the electrocardiographic waveform from the ECG sensor 9 and is recorded on the electrocardiograph 71. And converted into a digital signal.
[0054]
In order to generate an imaging trigger pulse from the electrocardiographic waveform (FIG. 5B) of the electrocardiograph 71 into which the magnetic flux noise is mixed in this manner, in the present embodiment, as shown in FIG. Is supplied to the first input terminal of the gate circuit 72. On the other hand, the sequence control circuit 24 of the control unit 4 calculates a photographing section Tx in the T01 section from the pulse sequence information set in advance, and outputs a photographing section signal (FIG. 5C) indicating the photographing section Tx to the gate circuit. 72 to a second input terminal. Then, the gate circuit 72 eliminates the above-described magnetic flux noise by blocking the signal in the imaging section Tx in the electrocardiographic waveform of the electrocardiograph 71 (step S8 in FIG. 6) (solid line in FIG. 5D).
[0055]
The electrocardiogram waveform from which the magnetic flux noise has been eliminated by the gate circuit 72 is supplied to a trigger level setting circuit 73. In the trigger level setting circuit 73, the magnitude AE1 of the peak value (R-wave) of the electrocardiographic waveform is detected, and a trigger level having a predetermined time constant (for example, 1 sec) attenuation characteristic based on the AE1 (see FIG. 5 (d) is generated and supplied to the first input terminal of the comparison circuit 74 (step S9 in FIG. 6). On the other hand, an electrocardiogram waveform (solid line in FIG. 5D) from the electrocardiograph 71 is supplied to a second input terminal of the comparison circuit 74, and the electrocardiogram waveform is compared with the trigger level to obtain an electrocardiogram. When the shape becomes larger than the trigger level, the photographing trigger pulse F2 is output from the comparison circuit 74 to the sequence control circuit 24 (step S10 in FIG. 6).
[0056]
The sequence control circuit 24 that has received the imaging trigger pulse F2 sets the times T1 to T4 based on the imaging trigger pulse F2, and collects the MR signals of the time phases T1 to T4 in the same procedure as in the section T01. Perform by law. Then, the obtained MR signals are sequentially stored in the MR signal storage circuit 28 of the storage circuit 20 as first to N-th encoded data in each time phase. Next, the high-speed operation circuit 19 generates MRI image data by performing image reconstruction on the stored N encoded data of each time phase, stores the MRI image data in the image data storage circuit 29, and stores the desired MRI image. The data is displayed on the display unit 21 via the main control circuit 23 (steps S5 to S7 in FIG. 6).
[0057]
According to the above-described embodiment of the present invention, when generating MRI image data by the electrocardiographic synchronization method, when the magnetic flux noise caused by the magnetic flux change of the gradient magnetic field of the MRI apparatus 100 is superimposed on the electrocardiographic waveform, Also, the imaging trigger pulse based on the R wave of the electrocardiographic waveform can be stably generated. Therefore, it is possible to reliably collect the MR signal of each time phase synchronized with the R wave of the electrocardiographic waveform, and it is possible to accurately generate MRI image data at a desired time phase. This is effective in the case where large magnetic flux noise occurs, such as in MRI imaging using a pulse sequence of the method.
[0058]
Further, the imaging efficiency is improved for the above reason, and the burden on the subject 11 can be reduced. Also, in the case of MRI imaging using a contrast agent, the timing at which the contrast agent exists in the tissue to be imaged can be accurately grasped.
[0059]
(First Modification)
Next, a first modification of the present embodiment will be described with reference to FIG. The gate circuit 72 of FIG. 2 in the above embodiment eliminates the magnetic flux noise by interrupting the electrocardiographic waveform output from the electrocardiograph 71 during the MRI imaging section Tx. The feature of the example is that the above-described setting of the electrocardiographic waveform can be selected based on an imaging method used for MRI imaging or a pulse sequence.
[0060]
As described above, the EPI method, which is one of the high-speed imaging methods, requires a high-speed switching of the gradient magnetic field when collecting an MR signal. Therefore, other imaging methods (for example, the SE (Spin-echo) method or the FE method) (Field-echo) method), a large magnetic flux noise is generated. The application of the above-described embodiment to an imaging method that generates a large magnetic flux noise such as the EPI method is extremely effective, but the magnetic flux noise is less than the amplitude of the electrocardiographic waveform as in the SE method or the FE method. If it is smaller, the setting for blocking the electrocardiographic waveform in the imaging section Tx may not be necessary in some cases.
[0061]
In particular, when the above-described embodiment is applied to a subject 11 having a so-called arrhythmia in which the R-wave interval is indefinite, there is a possibility that the R-wave of the electrocardiographic waveform may be blocked, and the imaging efficiency is reduced. there is a possibility. FIG. 7 shows a case where the above embodiment is applied to an electrocardiographic waveform obtained from an arrhythmic subject 11. FIG. 7 (a) shows an electrocardiographic waveform section, and FIG. 7 (b). 7C shows an electrocardiographic waveform output from the electrocardiograph 71, and FIG. 7C shows an imaging section signal sent to the gate circuit 72. FIG. 7D shows an electrocardiographic waveform (solid line) output from the gate circuit 72 and an output signal (dashed line) of the trigger level setting circuit 73, and FIG. 5 shows a shooting trigger pulse to be performed. As shown in FIG. 7, for example, when the R wave interval T01 is significantly narrower than other R wave intervals T02 and T03, the R wave in the section T02 is included in the imaging section of the electrocardiographic waveform in the section T01. It becomes impossible to generate the photographing trigger pulse F2 in the section T02.
[0062]
In the first modification, if the operator selects an imaging method such as the EPI method in setting the imaging conditions (step S2 in FIG. 6), this selection information is transmitted via the main control circuit 23 of the control unit 4 to the user. The sequence control circuit 24 that has received the signal selects transmission / non-transmission of the imaging section signal to the gate circuit 72 based on the selection information of the imaging method. For example, in the case of an imaging method in which magnetic flux noise is large as in EPI imaging, as described in the above embodiment, the gate circuit 72 determines the electrocardiographic waveform output from the electrocardiograph 71 in the imaging section Tx. When the magnetic flux noise is sufficiently smaller than the amplitude of the electrocardiographic waveform in the SE method or the FE method, the electrocardiographic waveform is passed as it is.
[0063]
According to this modification, it is possible to select a method of generating an imaging trigger pulse that is particularly suitable for the subject 11 having an arrhythmia, and as a result, it is possible to improve imaging efficiency.
[0064]
Note that as a result of measuring the R-wave interval in step S1 of FIG. 6 showing the procedure of the above embodiment, if a remarkable arrhythmia is detected in the electrocardiographic waveform of the subject 11, the first modified example It is preferable to apply the imaging trigger pulse generation method shown in FIG.
[0065]
(Second Modification)
Next, a second modification of the present embodiment will be described with reference to FIG. The feature of the second modification is that it is determined whether or not the magnetic flux noise mixed into the electrocardiographic waveform is allowable, and the transmission / non-transmission of the imaging section signal to the gate circuit 72 is performed manually or automatically based on the determination result. Is to make a choice.
[0066]
For example, the input unit 22 is newly provided with a selection device for selecting transmission / non-transmission of the imaging section signal, and the operator displays the selection device in, for example, the measurement of the R wave interval of the electrocardiographic waveform in step S1 in FIG. The electrocardiographic waveform displayed on the unit 21 is observed, and transmission / non-transmission of an imaging section signal is selected using the selection device based on the magnitude of magnetic flux noise superimposed on the electrocardiographic waveform. If the magnitude of the magnetic flux noise cannot be tolerated, the imaging section signal is supplied to the gate circuit 72 to cut off the electrocardiographic waveform during the imaging section Tx, thereby eliminating the magnetic flux noise.
[0067]
Alternatively, it may be automatically measured that the generation of the imaging trigger pulse is difficult due to the magnetic flux noise, and the cutoff of the imaging section Tx with respect to the electrocardiographic waveform may be automatically set based on the measurement result. For example, FIG. 8 shows an electrocardiogram waveform (solid line in FIG. 8A) and a trigger level (dashed line in FIG. 8A) when an imaging section signal is not supplied to the gate circuit 72, and an imaging trigger pulse (FIG. 8). (B)), and when the magnetic flux noise is large, the magnetic flux generates an imaging trigger pulse, so that the imaging trigger pulse is generated with respect to the normal R wave generation frequency (once / sec). The frequency of occurrence is significantly higher. Therefore, by observing the frequency of occurrence, it is possible to automatically determine whether or not the magnitude of the magnetic flux noise is allowable.
[0068]
In this case, the storage circuit of the main control circuit 23 stores in advance the allowable value of the occurrence frequency of the photographing trigger pulse. Then, at the beginning of imaging, the main control circuit 23 controls the sequence control circuit 24 to perform MRI imaging without supplying an imaging section signal to the gate circuit 72, and is output from the comparison circuit 74 of the electrocardiographic unit 7. The shooting trigger pulse is received via the sequence control circuit 24 and counted. When the counted value exceeds the occurrence frequency allowable value, an instruction to supply a radiographing interval signal is sent to the sequence control circuit 24, and the sequence control circuit 24 sends a radiographing interval signal to the gate circuit 72 of the electrocardiographic unit 7. Is supplied, the electrocardiographic waveform output from the electrocardiograph 71 is cut off by the gate circuit 72 during the imaging section Tx, and magnetic flux noise is eliminated.
[0069]
According to the second modification, even in an imaging method in which the magnitude relationship between the magnetic flux noise and the electrocardiographic waveform is not uniquely determined, an imaging trigger pulse can be generated with high accuracy, and therefore, an efficient electrocardiographic synchronization method can be used. MRI imaging can be performed.
[0070]
In the second modification, the counting result and an alarm signal based on the counting result are displayed on a monitor of the display unit 21 or a display panel of the input unit 22 so that the operator can display the displayed information. The transmission setting of the photographing section signal may be performed from the input unit 22 based on this.
[0071]
As described above, the specific embodiments of the present invention have been described. However, the present invention is not limited to the above embodiments, and can be modified and implemented. For example, in the above embodiment, the case where the magnetic flux noise superimposed on the electrocardiographic waveform in the imaging section is eliminated by the gate function has been described. However, the method may be performed by another method. The amplitude may be attenuated so as to be sufficiently small with respect to the amplitude. In the above description, the EPI method has been described as an example of the MRI imaging method, but the present invention is not limited to this method.
[0072]
Further, although the generation of the imaging trigger pulse in the above description was performed in the electrocardiogram unit 7, the electrocardiographic waveform output from the electrocardiograph 71 is directly supplied to the control unit 4, and the generation of the imaging trigger pulse is performed by software processing. And the method is not limited to the method shown in FIG. On the other hand, the number of time phases set in one heartbeat may be other than 4 time phases, and the MRI image data of each time phase may be obtained in different slice cross sections. Further, the above-described embodiment can be applied to MRI imaging without administration of a contrast agent.
[0073]
【The invention's effect】
As described above, according to the present invention, in the MRI imaging by the ECG gating method, even when the magnetic flux noise by the MRI apparatus is mixed in the electrocardiogram waveform, the imaging trigger pulse can be stably obtained, An MRI apparatus and an MRI imaging method capable of accurately generating MRI image data can be provided.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an overall configuration of an MRI apparatus according to an embodiment of the present invention.
FIG. 2 is a block diagram showing a configuration of an electrocardiographic unit in the embodiment.
FIG. 3 is a diagram showing a method of collecting an MRI signal using the EPI method in the embodiment.
FIG. 4 is a diagram showing a mounting method of the ECG sensor and generation of magnetic flux noise in the embodiment.
FIG. 5 is a view showing a method for generating a shooting trigger pulse in the embodiment.
FIG. 6 is a flowchart showing an MRI image data generation procedure in the embodiment.
FIG. 7 is an exemplary view for explaining a case where a first modification of the embodiment is effective;
FIG. 8 is a view for explaining a second modification of the embodiment.
FIG. 9 is a diagram showing a conventional imaging trigger pulse generation method.
[Explanation of symbols]
1. Static magnetic field generator
2. Gradient magnetic field generator
3 ... Transceiver
4 ... Control unit
5 High-speed operation / storage unit
7: ECG unit
8 ... sleeper
9… ECG sensor
13: Main magnet
14 ... Gradient field coil
15 ... irradiation coil
16 ... Reception coil
17 ... Transmitter
18 ... Receiver
19 High-speed arithmetic circuit
20 ... Memory circuit
21 Display unit
22 Input unit
23… Main control circuit
24: Sequence control circuit
25 ... Gradient magnetic field power supply
26: Static magnetic field power supply
28 ... MR signal storage circuit
29 ... Image data storage circuit
100 ... MRI system

Claims (13)

An MRI apparatus for acquiring an MR signal in synchronization with a biological signal obtained from a subject,
Waveform generation means for generating a second waveform signal by deleting or reducing a signal in a predetermined section of the first waveform signal obtained from the subject;
Image data generating means for collecting MR signals of the subject in a predetermined time phase based on the second waveform signal, and performing MRI image data by performing reconstruction processing on the collected MR signals; An MRI apparatus characterized by the above-mentioned. An MRI apparatus for acquiring an MR signal in synchronization with a biological signal obtained from a subject,
Waveform generating means for obtaining a first waveform signal obtained from the subject, and deleting or reducing a signal in a predetermined section of the first waveform signal to generate a second waveform signal;
Waveform selecting means for selecting either the first waveform signal or the second waveform signal;
Image data generating means for collecting MR signals in a predetermined time phase of the subject based on the waveform signal selected by the waveform selecting means, and performing reconstruction processing on the collected MR signals to generate MRI image data; Prepare,
The MRI apparatus, wherein the waveform selecting means selects the second waveform signal when noise mixed into the first waveform signal is large. 3. The MRI apparatus according to claim 1, wherein the waveform generating unit removes or reduces magnetic flux noise during MRI imaging mixed in the predetermined section of the first waveform signal. 4. The MRI apparatus according to claim 3, wherein the magnetic flux noise includes noise mixed into the first waveform signal due to an RF pulse or a gradient magnetic field applied to the subject. The MRI apparatus according to claim 1, wherein the waveform generation unit has a gate function of blocking passage of the first waveform signal in the predetermined section. The waveform generating means may delete or reduce a signal in a predetermined section of the first waveform signal and generate a second waveform signal based on a shooting section set based on the selected shooting method. The MRI apparatus according to claim 1 or 2, wherein Trigger level setting means for setting a trigger level based on the magnitude of a specific signal in the second waveform signal, further comprising a photographing trigger generated by comparing the specific signal in the second waveform signal with the trigger level The MRI apparatus according to claim 1, wherein an MR signal of the subject in a predetermined time phase is collected based on a pulse. 2. The trigger level setting means according to claim 1, wherein a trigger level having an attenuation characteristic of a predetermined time constant is generated based on a value of a specific signal in the first waveform signal or the second waveform signal. MRI equipment. The apparatus further includes a waveform generation control unit, and when the selected imaging method belongs to a preset low magnetic flux noise imaging method, the waveform generation control unit controls the waveform generation unit to control the first waveform. The MRI apparatus according to claim 1, wherein a signal is set to the second waveform signal. 3. The MRI apparatus according to claim 2, further comprising a waveform signal display unit, wherein the waveform selection unit is controlled based on the first waveform signal displayed on the waveform signal display unit. The apparatus further includes a trigger pulse counting unit that counts the shooting trigger pulse, wherein the waveform selecting unit is configured to, when the frequency of occurrence of the shooting trigger pulse counted by the trigger pulse counting unit is larger than a predetermined value, 3. The MRI apparatus according to claim 2, wherein the second waveform signal is selected instead of the second waveform signal. The apparatus further comprises warning display means, wherein the warning display means displays a warning signal when the frequency of occurrence of the photographing trigger pulse counted by the trigger pulse counting means is greater than a predetermined value. Item 13. An MRI apparatus according to Item 11. An MRI imaging method for acquiring an MR signal in synchronization with a biological signal obtained from a subject,
Generating a second waveform signal by deleting or reducing a signal in a predetermined section of the first waveform signal obtained from the subject;
Collecting an MR signal in a predetermined time phase of the subject based on the second waveform signal;
Performing a reconstruction process on the acquired MR signals to generate MRI image data.

Images