JP6691786B2 - Magnetic resonance imaging equipment - Google Patents

Description

  Embodiments of the present invention relate to magnetic resonance imaging.

  MRI is an imaging method in which nuclear spins of a subject placed in a static magnetic field are magnetically excited by an RF pulse having a Larmor frequency, and an image is reconstructed from MR signals generated by the excitation. The MRI means magnetic resonance imaging, the RF pulse means a radio frequency pulse, and the MR signal means a nuclear magnetic resonance signal. ..

JP, 2014-161499, A JP 2012-40362 A

  An object of the present invention is to provide a magnetic resonance imaging apparatus capable of suppressing deterioration of image quality even when the Larmor frequency changes due to an eddy current magnetic field.

  The magnetic resonance imaging apparatus of one embodiment is estimated from a gradient magnetic field generation circuit that applies the gradient magnetic field pulse according to a pulse sequence including transmission of an RF pulse and application of the gradient magnetic field pulse, and a waveform of the gradient magnetic field pulse. RF transmission in which the output control waveform of the RF pulse is modulated by following the temporal change of the magnetic resonance frequency caused by the temporal change of the eddy current magnetic field, and the modulated RF pulse is transmitted. And a circuit.

The block diagram which shows an example of the whole structure of the MRI apparatus in 1st Embodiment. The schematic diagram which shows an example of the deviation of the Larmor frequency resulting from the time change of each intensity | strength of the 0th-order component of the eddy current magnetic field which a slice selection direction gradient magnetic field pulse produces. The schematic timing diagram which shows an example of the time range of the gradient magnetic field pulse considered in calculation of the intensity | strength of the 0th-order component of an eddy current magnetic field. The schematic diagram which shows an example of a slice profile when the frequency modulation of 1st Embodiment is performed, and when it is not performed. 3 is a flowchart showing an example of the operation flow of the MRI apparatus according to the first embodiment. The flowchart which shows an example of the flow of operation | movement of the MRI apparatus in 2nd Embodiment. The flowchart which shows an example of the flow of operation | movement of the MRI apparatus in 3rd Embodiment. The flowchart which shows an example of the flow of operation | movement of the MRI apparatus in 4th Embodiment.

  Distortion of the gradient magnetic field distribution is known as one of the causes of image quality deterioration of MRI. The distribution of each gradient magnetic field in the slice selection direction, the phase encode direction, and the frequency encode direction is ideally such that the magnetic field strength linearly changes according to the position along the application direction. However, in reality, when a pulse current is supplied to the gradient magnetic field coil, an eddy current is generated, and the magnetic field due to the eddy current is added to the gradient magnetic field to cause distortion of the gradient magnetic field distribution.

  Hereinafter, the magnetic field generated by the eddy current is referred to as "eddy current magnetic field". The eddy current magnetic field is a magnetic field component generated by electromagnetic induction from a metal in the vicinity of the gradient magnetic field coil, mainly due to switching of the gradient magnetic field, and the main cause is the leakage magnetic flux of the gradient magnetic field coil. If the eddy current magnetic field largely deviates from the original magnetic field strength or magnetic field distribution, image quality may be deteriorated.

  In addition, the eddy current magnetic field has components of 0th order to higher order. The primary component of the eddy current magnetic field is, for example, a magnetic field component that increases or decreases substantially in proportion to the distance from the center of the magnetic field. On the other hand, the zero-order component of the eddy current magnetic field can be regarded as a magnetic field component that increases or decreases the static magnetic field strength.

  Here, the Larmor frequency is proportional to the strength of the static magnetic field. Therefore, when the Larmor frequency in the subject in the imaging region changes due to the 0th-order component of the eddy current magnetic field, the relative frequency of the RF pulse with the Larmor frequency as the reference frequency (difference from the reference frequency) changes.

  For example, when the static magnetic field strength of 1.5 Tesla increases to 1.50001 Tesla due to the 0th-order component of the eddy current magnetic field, the Larmor frequency in the subject increases by an amount proportional to 0.00001 Tesla. In that case, in the following embodiments, the phase of the RF pulse current supplied to the RF transmission coil is advanced so that the frequency of the RF pulse increases by the increase in the Larmor frequency.

  That is, in the following embodiments, the output control waveform of the RF pulse is frequency-modulated so as to follow the change in the Larmor frequency estimated from the 0th-order component of the eddy current magnetic field. As a result, the MRI apparatus brings the center frequency of the RF pulse transmitted to the imaging region as close as possible to the actual Larmor frequency in the subject in the imaging region that reflects the 0th-order component of the eddy current magnetic field.

  In the following description, frequency modulation and phase modulation basically have the same meaning. This is because applying the phase modulation so that the phase advances by 360 ° per second is the same as applying the frequency modulation so that the frequency increases by 1 Hz.

  Hereinafter, embodiments of the MRI apparatus and the MRI method will be described with reference to the accompanying drawings. In each drawing, the same elements are designated by the same reference numerals, and overlapping description will be omitted.

<First Embodiment>
FIG. 1 is a block diagram showing an example of the overall configuration of the MRI apparatus 10 of the first embodiment. Note that the block diagram of FIG. 1 is common to the second to fourth embodiments, except that the sequence correction function 66 of FIG. 1 is omitted in the second to fourth embodiments described below. Here, as an example, the constituent elements of the MRI apparatus 10 will be described by dividing them into a bed apparatus 20, a gantry 30, and a control apparatus 40.

First, the bed apparatus 20 includes a support table 21, a tabletop 22, and a tabletop moving mechanism 23 arranged in the support table 21.
The subject P is placed on the top surface of the top plate 22. A plurality of connection ports 25 to which the RF coil device 100 attached to the subject P is connected are arranged on the top surface of the top plate 22.

  The support base 21 supports the top plate 22 so as to be movable in the horizontal direction (Z-axis direction of the device coordinate system). The top moving mechanism 23 adjusts the vertical position of the top plate 22 by adjusting the height of the support base 21 when the top plate 22 is located outside the gantry 30. The top moving mechanism 23 moves the top 22 in the horizontal direction to insert the top 22 into the gantry 30, and after taking an image, the top 22 is taken out of the gantry 30.

  Secondly, the gantry 30 has, for example, a cylindrical shape and is installed in the imaging room. The gantry 30 includes a static magnetic field magnet 31, a shim coil 32, a gradient magnetic field coil 33, and an RF coil 34.

  The static magnetic field magnet 31 is, for example, a superconducting coil and has a cylindrical shape. The static magnetic field magnet 31 forms a static magnetic field in the imaging space by a current supplied from a static magnetic field power supply 42 of the control device 40 described later. The imaging space means, for example, a space in the gantry 30 in which the subject P is placed and a static magnetic field is applied. The static magnetic field magnet 31 may be formed of a permanent magnet without providing the static magnetic field power supply 42.

  The shim coil 32 has, for example, a cylindrical shape and is arranged inside the static magnetic field magnet 31 with the same axis as the static magnetic field magnet 31. The shim coil 32 forms an offset magnetic field that homogenizes the static magnetic field by a current supplied from a shim coil power supply 44 of the control device 40 described later.

  The gradient magnetic field coil 33 is configured, for example, in a cylindrical shape, and is arranged inside the shim coil 32. The gradient magnetic field coil 33 includes an X-axis gradient magnetic field coil 33x, a Y-axis gradient magnetic field coil 33y, and a Z-axis gradient magnetic field coil 33z.

  In this specification, the X-axis, the Y-axis, and the Z-axis are the device coordinate system unless otherwise specified. Here, as an example, the X axis, Y axis, and Z axis of the device coordinate system are defined as follows. First, the vertical direction is the Y-axis direction, and the top plate 22 is arranged so that the normal line direction of its top surface is the Y-axis direction. The horizontal movement direction of the top plate 22 is the Z-axis direction, and the gantry 30 is arranged so that the axial direction thereof is the Z-axis direction. The X-axis direction is a direction orthogonal to the Y-axis direction and the Z-axis direction, and is the width direction of the top plate 22 in the example of FIG.

  The X-axis gradient magnetic field coil 33x forms a gradient magnetic field Gx in the X-axis direction corresponding to a current supplied from an X-axis gradient magnetic field power supply 46x, which will be described later, in the imaging region. Similarly, the Y-axis gradient magnetic field coil 33y forms a gradient magnetic field Gy in the Y-axis direction in the imaging region according to a current supplied from a Y-axis gradient magnetic field power supply 46y described later. Similarly, the Z-axis gradient magnetic field coil 33z forms a gradient magnetic field Gz in the Z-axis direction according to a current supplied from a Z-axis gradient magnetic field power supply 46z, which will be described later, in the imaging region.

  Then, the slice selection direction gradient magnetic field Gss, the phase encoding direction gradient magnetic field Gpe, and the reading direction (frequency encoding direction) gradient magnetic field Gro are arbitrary by combining the gradient magnetic fields Gx, Gy, and Gz in the three axis directions of the apparatus coordinate system. It can be set in the direction of.

  The imaging region is, for example, at least a part of the acquisition range of MR signals used for generating one image or one set of images, and is a region serving as an image. The imaging region is three-dimensionally defined in the device coordinate system as a part of the imaging space, for example. When MR signals are acquired over a wider area than the area to be imaged, for example to prevent aliasing artifacts, the imaging area is part of the MR signal acquisition range. On the other hand, the entire MR signal acquisition range may be an image, and the MR signal acquisition range and the imaging region may match. The “one set of images” is a plurality of images when MR signals of a plurality of images are collectively collected by one pulse sequence, such as multi-slice imaging.

  The RF coil 34 has a cylindrical shape, for example, and is arranged inside the gradient magnetic field coil 33. Here, as an example, the RF coil 34 includes a whole body QD coil (not shown) that also serves to transmit an RF pulse and receive an MR signal. A QD (Quadrature) coil is a quadrature type RF coil device.

  Thirdly, the control device 40 includes a static magnetic field power supply 42, a shim coil power supply 44, a gradient magnetic field power supply 46, a gradient magnetic field pulse waveform generation circuit 47, an RF transmitter 48, an RF receiver 50, and an RF pulse waveform. It has a generation circuit 54, a variable frequency generation circuit 56, a fixed frequency generation circuit 57, a sequence controller 58, a processing circuit 60, an input device 72, a display 74, and a storage circuit 76.

  The gradient magnetic field pulse waveform generation circuit 47, based on the gradient magnetic field pulse waveform of each axis input from the sequence controller 58, a waveform signal for an X-axis gradient magnetic field pulse, a waveform signal for a Y-axis gradient magnetic field pulse, and a Z-axis gradient. Generate a waveform signal for the magnetic field pulse. The gradient magnetic field pulse waveform generation circuit 47 inputs the waveform signal of each of these axes to the gradient magnetic field power supply 46.

  The gradient magnetic field power supply 46 has an X-axis gradient magnetic field power supply 46x, a Y-axis gradient magnetic field power supply 46y, and a Z-axis gradient magnetic field power supply 46z. The X-axis gradient magnetic field power supply 46x, the Y-axis gradient magnetic field power supply 46y, and the Z-axis gradient magnetic field power supply 46z generate the gradient magnetic fields Gx, Gy, and Gz based on the waveform signals of the respective axes input from the gradient magnetic field pulse waveform generation circuit 47. The respective currents for forming are supplied to the X-axis gradient magnetic field coil 33x, the Y-axis gradient magnetic field coil 33y, and the Z-axis gradient magnetic field coil 33z, respectively.

  The fixed frequency generation circuit 57 acquires the center frequency value of the RF pulse calculated by the prescan described later from the sequence controller 58. The fixed frequency generation circuit 57 has, for example, a highly stable crystal oscillator. The fixed frequency generation circuit 57 generates the carrier frequency of the acquired center frequency value by the crystal oscillator, and inputs the generated carrier frequency to the RF pulse waveform generation circuit 54.

  The RF pulse waveform generation circuit 54 has an arithmetic circuit formed on, for example, a semiconductor substrate, and an oscillator (not shown) for a clock signal (hereinafter, referred to as “substrate clock signal”) is also provided on the semiconductor substrate. It is provided. The RF pulse waveform generation circuit 54 generates a digital pulse waveform signal according to the clock signal of the board.

  Further, the RF pulse waveform generation circuit 54 generates an analog pulse waveform signal by performing D / A conversion (Digital to Analogue Conversion) on the digital pulse waveform signal. At this time, the RF pulse waveform generation circuit 54 compresses or expands the analog pulse waveform signal so as to match the bandwidth of the rectangular wave determined by the output control waveform of the RF pulse in the pulse sequence input from the sequence controller 58.

  Then, the RF pulse waveform generation circuit 54 modulates the analog pulse waveform signal to the carrier frequency input from the fixed frequency generation circuit 57, and inputs the modulated pulse waveform signal to the RF transmitter 48.

  The variable frequency generation circuit 56 incorporates a phase synchronization circuit (not shown), a digital direct synthesis oscillator (Direct Digital Synthesizer), a mixer, and the like. The variable frequency generation circuit 56 obtains the modulated pulse waveform signal from the RF pulse waveform generation circuit 54, further performs frequency modulation of Δf on the modulated pulse waveform signal, and outputs the frequency-modulated pulse waveform signal. Input to the RF transmitter 48.

  However, in the first embodiment, the output control waveform of the RF pulse is frequency-modulated by Δf in the processing circuit 60 so as to follow the change in the Larmor frequency estimated from the 0th-order component of the eddy current magnetic field. The pulse sequence after frequency modulation is input to the sequence controller 58. Therefore, in the first embodiment, the variable frequency generation circuit 56 does not execute the frequency modulation of Δf.

  The RF transmitter 48 generates an RF pulse current having a Larmor frequency that causes nuclear magnetic resonance based on the modulated pulse waveform signal input from the RF pulse waveform generation circuit 54 (however, the frequency modulation of Δf is a variable frequency. In a third embodiment to be described later executed by the generation circuit 56, the RF transmitter 48 generates the RF pulse current based on the frequency-modulated pulse waveform signal input from the variable frequency generation circuit 56). The RF transmitter 48 transmits the generated RF pulse current to the RF coil 34. An RF pulse corresponding to this RF current pulse is transmitted from the RF coil 34 to the subject P.

  The whole-body QD coil of the RF coil 34 and the RF coil device 100 attached to the subject P detect and detect the MR signal generated by exciting the nuclear spins in the subject P with the RF pulse. The MR signal is input to the RF receiver 50.

  The RF receiver 50 performs predetermined signal processing on the received MR signal and then performs A / D (analog to digital) conversion to generate raw data that is complex data of the digitized MR signal. .. The RF receiver 50 inputs the raw data of the MR signal to (the image reconstruction function 62 of) the processing circuit 60.

  The sequence controller 58 stores control information necessary for driving the gradient magnetic field power supply 46, the RF transmitter 48, and the RF receiver 50 according to a command from the processing circuit 60. The control information here is, for example, pulse sequence information describing operation control information such as the intensity, application time, and application timing of the pulse current to be applied to the gradient magnetic field power supply 46. The sequence controller 58 drives the gradient magnetic field power supply 46, the RF transmitter 48, and the RF receiver 50 according to the stored predetermined pulse sequence to generate the gradient magnetic fields Gx, Gy, Gz, and RF pulses. The sequence controller 58 may be configured by dedicated hardware, or may include a processor and realize the above functions by software processing by this processor.

The processing circuit 60 may also be configured by dedicated hardware, or may be equipped with a processor and various types can be realized by software processing by this processor. In the following, an example in which the processing circuit 60 realizes various functions by software processing by the processor will be described. Specifically, as shown in FIG. 1, the processing circuit 60 includes a system control function 61, an image reconstruction function 62, an image processing function 64, and a sequence correction function 66, and a program stored in a storage circuit 76. Alternatively, it is realized by executing a program directly stored in the processor of the processing circuit 60.
Here, the processor is, for example, a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), an application specific integrated circuit (ASIC), a programmable logic device (for example, a simple programmable logic device (Simple). This means circuits such as Programmable Logic Device (SPLD), Complex Programmable Logic Device (CPLD), and Field Programmable Gate Array (FPGA).
The number of processors included in the processing circuit 60 and the sequence controller 58 may be one, or may be two or more.

  The system control function 61 performs system control of the entire MRI apparatus 10 in setting the imaging conditions of the main scan, the imaging operation, and the image display after imaging. The imaging condition means, for example, which kind of pulse sequence, under what condition an RF pulse or the like is transmitted, and under what condition an MR signal is collected from the subject P. Examples of imaging conditions include an imaging region as positional information in the imaging space, a flip angle, a repetition time TR (Repetition Time), the number of slices, an imaging region, and a type of pulse sequence such as spin echo method or parallel imaging. Can be mentioned. The imaging part means which part of the subject P, such as the head, chest, abdomen, etc., is to be imaged as an imaging region.

  The “main scan” is a scan for capturing a target diagnostic image such as a T1-weighted image, and does not include a scan for MR signal acquisition for a positioning image and a calibration scan. Scan refers to the operation of collecting MR signals and does not include image reconstruction.

  The calibration scan is performed separately from the main scan in order to determine, for example, undetermined imaging conditions of the main scan and conditions and data used for image reconstruction processing and correction processing after image reconstruction. Is a scan. Examples of the calibration scan include a sequence for calculating the center frequency of the RF pulse in the main scan before the main scan. The pre-scan refers to a calibration scan performed before the main scan.

  Further, the system control function 61 causes the display 74 to display the setting screen information of the imaging condition, sets the imaging condition based on the instruction information from the input device 72, and sets the pulse sequence based on the imaging condition.

  The sequence correction function 66 corrects the pulse sequence (provisionally) set by the system control function 61 and inputs the corrected pulse sequence to the sequence controller 58. The modification here is to calculate the zero-order component of the eddy current magnetic field and modify the output control waveform of the RF pulse so as to follow the change in the Larmor frequency due to the zero-order component of the eddy current magnetic field. It will be described later.

The input device 72 provides a user with a function of setting an imaging condition and an image processing condition.
The image reconstruction function 62 arranges and stores the raw data of the MR signal input from the RF receiver 50 as k-space data according to the number of phase encoding steps and the number of frequency encoding steps. The k-space means frequency space. The image reconstruction function 62 generates image data of the subject P by performing image reconstruction processing including Fourier transform on the k-space data. The image reconstruction function 62 stores the image data immediately after the reconstruction in the storage circuit 76.

The image processing function 64 takes in the image data immediately after reconstruction from the storage circuit 76, performs predetermined image processing on the image data, and stores the image data after the image processing in the storage circuit 76 as display image data.
The storage circuit 76 stores the above-mentioned image data for display by additionally attaching the imaging conditions used for generating the image data for display, information of the subject P (patient information), and the like as supplementary information.

Alternatively, the processing circuit 60, the input device 72, the display 74, and the memory circuit 76 may be configured as one computer and installed in, for example, a control room.
Further, in the above description, the constituent elements of the MRI apparatus 10 are classified into the gantry 30, the bed apparatus 20, and the control apparatus 40, but this is merely an example of interpretation. For example, the top moving mechanism 23 may be regarded as a part of the control device 40.

  Alternatively, the RF receiver 50 may be located within the gantry 30 rather than outside the gantry 30. In this case, an electronic circuit board corresponding to, for example, the RF receiver 50 is arranged in the gantry 30. Then, the MR signal converted from the electromagnetic wave to the analog electric signal by the RF coil device 100 or the like is amplified by the preamplifier in the electronic circuit board, output to the outside of the gantry 30 as a digital signal, and input to the image reconstructing function 62. To be done. At the time of output to the outside of the gantry 30, it is desirable to use an optical communication cable for transmission as an optical digital signal because the influence of external noise is reduced.

  FIG. 2 is a schematic diagram showing an example of the deviation of the Larmor frequency due to the temporal change of each intensity of the zero-order component of the eddy current magnetic field generated by the slice selection direction gradient magnetic field pulse. RF in the upper part of FIG. 2 shows the waveform of the excitation RF pulse as an input signal before being put on the carrier frequency. In the present embodiment, for example, a waveform signal as shown in the upper part of FIG. 2 is added to the carrier frequency in the Larmor frequency band by frequency modulation, and the waveform modulated by the carrier frequency and Δf is output from the RF coil 34 as an RF pulse. Therefore, the upper part of FIG. 2 is different from the temporal change of the intensity of the RF pulse actually output.

  Gss in the middle part of FIG. 2 shows the time change of the intensity of the slice selection direction gradient magnetic field pulse Gss applied simultaneously with the excitation RF pulse, and the bottom part of FIG. 2 shows the eddy current magnetic field generated by the slice selection direction gradient magnetic field pulse Gss. The time change of the deviation of the magnetic resonance frequency (namely, Larmor frequency) resulting from the time change of a 0th-order component is shown. The deviation of the Larmor frequency is the deviation from the original Larmor frequency when there is no eddy current magnetic field. In each of the upper, middle, and lower horizontal axes in FIG. 2, t indicates elapsed time.

  As shown in the middle and lower parts of FIG. 2, when the intensity of the slice selection direction gradient magnetic field pulse Gss increases, the intensity of the 0th order component of the eddy current magnetic field also increases and the Larmor frequency also increases. After that, in the period in which the intensity of the slice selection direction gradient magnetic field pulse Gss is constant, the intensity of the 0th-order component of the eddy current magnetic field is exponentially attenuated according to the time constant, and the Larmor frequency is also reduced.

  However, in the example of FIG. 2, the intensity of the slice selection direction gradient magnetic field pulse Gss decreases before the zero-order component of the eddy current magnetic field becomes zero due to exponential decay. The intensity of the 0th-order component of is also decreased to a negative value, and the Larmor frequency also swings to the negative side of the original frequency. After that, since the intensity of the slice selection direction gradient magnetic field pulse Gss is zero and constant, the 0th-order component of the eddy current magnetic field exponentially recovers according to the time constant, and the Larmor frequency gradually returns to the original frequency.

  Here, when the slice selection direction is, for example, the Z-axis direction, the sequence correction function 66 causes the zero-order eddy current magnetic field having a positive intensity during the period in which the intensity of the Z-axis gradient magnetic field pulse is a constant value (including zero). The time constant τza when the component decays exponentially to zero and the time constant τzb when the zero-order component of the eddy current magnetic field, which has a negative intensity, recovers exponentially to zero are stored.

  Such time constants τza and τzb can be measured at the time of installation and adjustment of the MRI apparatus 10, and can be stored in the sequence correction function 66, the system control function 61, or the like. The time constant τya when the zero-order component of the eddy current magnetic field, which was positive intensity, decays exponentially to zero during the period when the intensity of the Y-axis gradient magnetic field pulse is constant, and the eddy current having negative intensity The time constant τyb when the 0th-order component of the magnetic field exponentially recovers to zero is also stored in the sequence correction function 66 and the system control function 61 by the preliminary measurement as described above.

  The time constant τxa when the zero-order component of the eddy current magnetic field, which was positive intensity, decays exponentially to zero during the period when the intensity of the X-axis gradient magnetic field pulse has a constant value, and the eddy current having negative intensity The time constant τxb when the zero-order component of the magnetic field exponentially recovers to zero is also stored in the sequence correction function 66 and the system control function 61 by the preliminary measurement as in the above.

  The sequence correction function 66 calculates the intensity of the 0th-order component of the eddy current magnetic field generated by each gradient magnetic field pulse as a function of the elapsed time t based on the time changes of the intensity of all the gradient magnetic field pulses and the above time constants. .. Therefore, the sequence correction function 66 calculates the time change (waveform) of the intensity of all the gradient magnetic field pulses Gss, Gpe, and Gro in the slice selection direction, the phase encoding direction, and the reading direction.

  Here, as an example, the waveform of the gradient magnetic field pulse is commanded based on the command value of each supply current (time change thereof) to the X-axis gradient magnetic field coil 33x, the Y-axis gradient magnetic field coil 33y, and the Z-axis gradient magnetic field coil 33z. It is calculated under the assumption that a gradient magnetic field pulse having a waveform according to the value is generated. In the pulse sequence, when the electric power supplied to the gradient magnetic field coil 33 is defined based on the command value of the voltage value (time change of the voltage value) instead of the command value of the current value, the sequence correction function 66 determines that the voltage value The above calculation is executed based on the command value.

  The intensity of the 0th order component of the eddy current magnetic field at a certain time tx within the period of the pulse sequence is the integration of the 0th order component of the eddy current magnetic field generated by all the gradient magnetic field pulses applied from the start time of the pulse sequence to the time tx. It can be calculated as a value (summed value). Since the intensity of the 0th-order component of such an eddy current magnetic field can be calculated by a known technique, detailed description thereof will be omitted.

  Here, in order to calculate the intensity of the 0th order component of the eddy current magnetic field at a certain time tx, it is more accurate to consider all gradient magnetic field pulses applied from the start time of the pulse sequence to the time tx as described above. However, the calculation load is heavy. Further, in consideration of the attenuation due to the time constant, it is considered that the 0th-order component of the eddy current magnetic field generated by other than the gradient magnetic field pulse of the latest predetermined period may be ignored.

  That is, in order to calculate the intensity of the 0th-order component of the eddy current magnetic field at a certain time tx, it is sufficient to consider only all the gradient magnetic field pulses applied within the predetermined period PS ending at the time tx. It can be calculated with various accuracy. This point will be specifically described with reference to FIG.

  FIG. 3 is a schematic timing chart showing an example of the time range of the gradient magnetic field pulse considered in the calculation of the intensity of the 0th order component of the eddy current magnetic field. In FIG. 3, the pulse sequence of the field echo method is shown as an example, but the present embodiment is also applicable to other pulse sequences such as the spin echo method.

  In FIG. 3, each horizontal axis corresponds to the elapsed time t, RF in the upper stage is an RF pulse, Gss therebelow is a gradient magnetic field in the slice selection direction, Gpe therebelow is a gradient magnetic field in the phase encoding direction, and Gro below is read out. Direction (frequency encoding direction) gradient magnetic field, and SIGNAL at the bottom shows the generated MR signal.

Here, as an example, at the beginning of the pulse sequence, the slice selection direction gradient magnetic field pulse Gss is applied together with the transmission of the excitation RF pulse having the flip angle of 90 °.
Next, the phase encoding direction gradient magnetic field pulse Gpe is applied, and the negative polarity readout direction gradient magnetic field pulse Gro is applied.

  Next, the application of the gradient magnetic field pulse Gpe in the phase encoding direction ends, and the polarity of the gradient magnetic field pulse Gro in the reading direction is inverted. The MR signal is detected under the application of the readout direction gradient magnetic field pulse Gro after the polarity reversal.

  Up to this point, the MR signal for one phase encoding step has been acquired. Then, after the repetition time TR has elapsed from the start of the transmission of the excitation RF pulse, the same processing is repeated for the number of phase encodes, and the MR signal for one image is collected.

  Here, on the right side of FIG. 3, attention is paid to time tx during the application of the excitation RF pulse, that is, within the excitation RF pulse. The sequence correction function 66 calculates the intensity of the 0th-order component of the eddy current magnetic field at the time tx in the excitation RF pulse as follows, and executes frequency modulation.

  First, the sequence correction function 66 has a predetermined period so that the intensity of the 0th-order component of the eddy current magnetic field can be calculated with sufficient accuracy based on the conditions of the pulse sequence including the time constants of the 0th-order component of the eddy current magnetic field. Determine PS. For example, as illustrated in FIG. 3, three times or more of the repetition time TR is set as the predetermined period PS.

  Next, the sequence correction function 66 selects all the gradient magnetic field pulses applied within the predetermined period PS that ends at the time tx, based on the conditions of the pulse sequence.

Next, the sequence correction function 66 supplies each current value or each voltage value supplied to the X-axis gradient magnetic field coil 33x, the Y-axis gradient magnetic field coil 33y, and the Z-axis gradient magnetic field coil 33z to generate the selected gradient magnetic field pulse. The command value of (change over time) is obtained from the pulse sequence conditions.
Next, the sequence correction function 66 calculates the temporal change (waveform) of the intensity of each gradient magnetic field pulse based on the acquired current value or command value of each voltage value.

  Next, the sequence correction function 66 calculates the 0th-order component of the eddy current magnetic field generated by each of the gradient magnetic field pulses based on the time change of the intensity of each gradient magnetic field pulse within the predetermined period PS calculated as described above. Calculate and sum as a function of t, respectively. This summed value is the time change of the 0th order component of the eddy current magnetic field in the excitation RF pulse including the time tx. The sequence correction function 66 calculates the time change of the 0th-order component of the eddy current magnetic field in the same manner as above for all the other RF pulses in the pulse sequence.

  Next, the sequence correction function 66 calculates the actual Larmor frequency in the subject P in the RF pulse for each of all the RF pulses included in the pulse sequence. This Larmor frequency is calculated based on the temporal change of the 0th-order component of the eddy current magnetic field calculated for each RF pulse as described above and the magnetic field strength generated by the static magnetic field magnet 31.

  Next, the sequence correction function 66 performs frequency modulation on the output control waveforms of all RF pulses included in the pulse sequence so as to follow the actual Larmor frequency in each RF pulse calculated as described above. By doing so, the output control waveform of each RF pulse is modified. Tracking here means performing frequency modulation so that the frequency in the output RF pulse matches the actual Larmor frequency.

  More specifically, in the case of a quadrature type RF transmission coil such as the whole body QD coil in the RF coil 34, a first axis side and a second axis side orthogonal to each other in the rotating coordinate system are provided. Currents whose phases are different from each other by 90 ° are supplied to generate RF pulses. In the case of the quadrature phase type RF transmission coil, the real part component and the imaginary part component of the RF pulse are supplied to the first current component and the second current component, which are supplied to the first shaft side and the second shaft side which are orthogonal to each other. Corresponding to the second current component. The phase and amplitude of the RF pulse can be calculated from the real part component and the imaginary part component, but the calculation method is publicly known, and therefore the description thereof is omitted here.

  For example, when the actual Larmor frequency reflecting the 0th order component of the eddy current magnetic field is lower than the center frequency determined by the output control waveform of the RF pulse defined by the pulse sequence, the phase of the RF signal in the RF pulse is Frequency modulation is performed on the output control waveform of the RF pulse current so as to be delayed. The RF pulse current after the frequency modulation is supplied to the first axis side and the second axis side of the quadrature phase type RF transmission coil after passing through, for example, a phase divider, and thus The phases of the RF pulse currents supplied to the shaft side and the second shaft side differ from each other by 90 °.

  As a result, the frequency in the RF pulse radiated as an electromagnetic wave from the RF transmission coil substantially matches the actual Larmor frequency in the subject that changes due to the time change of the 0th-order component of the eddy current magnetic field. In this way, the sequence modification function 66 modifies the pulse sequence.

  FIG. 4 shows a conventional slice profile in the case where the frequency modulation of the RF pulse is not executed (FIG. 4B) and a slice profile in the case where the frequency modulation of the RF pulse is executed according to the first embodiment (FIG. 4 ( It is a schematic diagram which shows an example of c)). The slice profile is a one-dimensional diagram showing the intensity of the MR signal generated in response to the application of the RF pulse from the region spatially selected by the gradient magnetic field. FIGS. 4B and 4C show slice profiles in the Z direction as an example. Further, the lower part of each of FIGS. 4B and 4C shows the frequency component of the RF pulse when the frequency modulation is not performed and the frequency component of the RF pulse when the frequency modulation is performed. Here, the shape itself of the frequency component of the RF pulse is assumed to be a rectangle obtained by Fourier transforming the sinc function in any case. However, in FIG. 4B where frequency modulation is not performed, the center frequency f0 of the RF pulse is fixed, and in FIG. 4C where frequency modulation is performed, the center frequency of the RF pulse changes with time. I am supposed to.

On the other hand, in FIGS. 4B and 4C, the slice profile in the Z direction can be expressed by the following (Formula 1) that defines the Larmor frequency f (Z) at the position Z.
f (Z) = f0 + (γ / 2π) · Gz · Z (Equation 1)
Here, f0 is the original Larmor frequency when there is no eddy current magnetic field. Further, γ is a constant called a gyromagnetic ratio, and Gz is the magnitude of the gradient magnetic field in the Z direction. Since the RF pulse has a bandwidth, the slice profile has a thickness determined by this bandwidth and the gradient magnetic field Gz.

When the 0th-order component of the eddy current magnetic field exists and the 0th-order component changes with time, the actual Larmor frequency fluctuates from the original Larmor frequency at each application time of the RF pulse. As shown in a), it also changes with time within each RF pulse. The deviation between the actual Larmor frequency and the original Larmor frequency is represented by Δf (t), and if this deviation is taken into consideration, (Equation 1) becomes the following (Equation 2).
f (Z) = f0 + Δf (t) + (γ / 2π) · Gz · Z (Equation 2)
The graphs shown in the central portions of FIGS. 4B and 4C are graphs corresponding to (Equation 2). If the deviation Δf (t) changes, for example, at times t1, t2, and t3 in the RF pulse, the position of the graph will shift in the Z direction according to the deviation Δf (t).

  Therefore, in the conventional case where frequency modulation is not performed, as shown in FIG. 4B, the center position of the slice profile shifts at times t1, t2, and t3 in the RF pulse. Therefore, the equivalent thickness of the slice profile becomes larger than the original thickness.

  On the other hand, when the frequency modulation according to the present embodiment is executed, the output control waveform of the RF pulse is modified so as to follow the shift of the Larmor frequency due to the 0th-order component of the eddy current magnetic field. The center frequency of the pulse closely matches the actual Larmor frequency. Therefore, as shown in FIG. 4C, the slice profile maintains the original thickness.

  FIG. 5 is a flowchart showing an example of the operation flow of the MRI apparatus 10 of the first embodiment. The operation of the MRI apparatus 10 will be described below in accordance with the step numbers shown in FIG. 5 while appropriately referring to the above-mentioned drawings.

  [Step S1] The system control function 61 (see FIG. 1) sets imaging conditions based on the imaging conditions input to the MRI apparatus 10 via the input device 72. Further, the center frequency value of the RF pulse is calculated by a known prescan, and the center frequency value of the RF pulse is input from the sequence controller 58 to the fixed frequency generation circuit 57. Then, it progresses to step S2.

  [Step S2] The system control function 61 determines the output control waveform and transmission timing of each RF pulse and the pulse sequence including the output control waveform and transmission timing of each gradient magnetic field pulse based on the imaging conditions set in step S1. Provisionally set. Then, it progresses to step S3.

  [Step S3] The sequence correction function 66 acquires from the system control function 61 all the conditions of the pulse sequence provisionally set in step S2. As described above, the sequence correction function 66 determines the predetermined period PS based on the conditions of the pulse sequence so that the intensity of the 0th-order component of the eddy current magnetic field can be calculated with sufficient accuracy (see FIG. 3).

  Next, the sequence correction function 66 calculates the intensity of the 0th-order component of the eddy current magnetic field that temporally changes within the transmission timing and the RF pulse for each RF pulse of the pulse sequence, as described above. The transmission timing and the Larmor frequency in the RF pulse are calculated for each RF pulse based on the calculated intensity of the 0th-order component.

Next, the sequence correction function 66 applies frequency modulation to the output control waveform of each RF pulse so as to follow the transmission timing and the time change of the Larmor frequency in the RF pulse, as described above. Then, the output control waveform of each RF pulse is corrected. In this way, the sequence correction function 66 corrects the pulse sequence provisionally set in step S2.
Then, it progresses to step S4.

  [Step S4] The sequence correction function 66 inputs the pulse sequence corrected in step S3 to the sequence controller 58. Then, it progresses to step S5.

  [Step S5] The sequence controller 58 controls each part of the MRI apparatus 10 in accordance with the pulse sequence input in step S4, and executes the data acquisition of the main scan. Specifically, the subject P is placed on the top plate 22, and a current is supplied from the shim coil power supply 44 to the shim coil 32 to homogenize the static magnetic field formed in the imaging space.

  When the imaging start instruction is input from the input device 72 to the system control function 61, the sequence controller 58 causes the gradient magnetic field pulse waveform generation circuit 47, the gradient magnetic field power supply 46, the RF transmitter 48, and the RF receiver 50 to follow the pulse sequence. , The RF pulse waveform generation circuit 54, the variable frequency generation circuit 56, etc. are driven. As a result, the sequence controller 58 causes the gradient magnetic field to be formed in the imaging region including the imaging region of the subject P, and causes the RF coil 34 (QD coil for the whole body in this example) to transmit the RF pulse to the imaging region.

  Explaining in detail with respect to the RF pulse, the RF pulse waveform generation circuit 54 generates a digital pulse waveform signal based on the clock signal of the substrate as described above, and performs D / A conversion on the digital pulse waveform signal to generate an analog pulse waveform. Generate a signal. At this time, the RF pulse waveform generation circuit 54 compresses or expands the analog pulse waveform signal so as to fit the rectangular wave bandwidth determined by the output control waveform of the RF pulse in the pulse sequence input from the sequence controller 58. .. The RF pulse waveform generation circuit 54 modulates the analog pulse waveform signal to the carrier frequency from the fixed frequency generation circuit 57, and inputs the modulated pulse waveform signal to the RF transmitter 48.

  Here, the rectangular wave determined by the pulse sequence input from the sequence controller 58 to the RF pulse waveform generation circuit 54 is corrected to follow the Larmor frequency in the subject P considering the 0th-order component of the eddy current magnetic field. (Step S3). Therefore, the modulated pulse waveform signal input to the RF transmitter 48 is also corrected to follow the actual Larmor frequency.

  The RF transmitter 48 generates an RF pulse current based on the input pulse waveform signal and transmits it to the whole body QD coil. An RF pulse corresponding to this RF current pulse is transmitted to the subject P from the whole body QD coil.

Therefore, the MR signal generated by the nuclear magnetic resonance in the subject P is detected by the RF coil device 100 (and the QD coil for the whole body) and input to the RF receiver 50. The RF receiver 50 generates the raw data of the MR signal by subjecting the MR signal to the above-described processing, and inputs the raw data of the MR signal to the image reconstruction function 62. The image reconstruction function 62 arranges and stores the raw data of the MR signal as k-space data.
Then, it progresses to step S6.

[Step S6] The image reconstruction function 62 reconstructs image data by performing image reconstruction processing including Fourier transform on the k-space data generated by the main scan, and stores the obtained image data in the storage circuit 76. Save to. The image processing function 64 takes in the reconstructed image data from the storage circuit 76, performs predetermined image processing on the reconstructed image data, generates two-dimensional display image data, and stores the display image data in the storage circuit 76. save. After that, the system control function 61 causes the display 74 to display the image indicated by the display image data.
The above is the description of the operation of the MRI apparatus 10 of the first embodiment.

  Hereinafter, the difference between the first embodiment and the conventional technique will be described. In the related art, in the transmission of the RF pulse, the change in the actual Larmor frequency due to the 0th-order component of the eddy current magnetic field is not taken into consideration. Therefore, in the conventional technique, the shape of the slice profile may be deteriorated (the width of the slice profile may be widened). Further, when a narrow-band frequency-selective RF pulse such as a fat saturation pulse is applied, the Larmor frequency of fat shifts, so that fat to be suppressed is not suppressed and, conversely, should not be suppressed. There was also a problem that the water component was suppressed.

  On the other hand, in the first embodiment, the sequence correction function 66 calculates the 0th-order component of the eddy current magnetic field, and based on the 0th-order component of the eddy current magnetic field, the transmission timing of each RF pulse and the Larmor frequency within the RF pulse. To calculate. Then, the sequence correction function 66 frequency-modulates the output control waveform of each RF pulse so as to follow the transmission timing and the actual Larmor frequency in the RF pulse.

Therefore, the pulse sequence corrected as described above is input to the hardware side (sequence controller 58) of the control device 40 before the main scan. Therefore, the frequency of the RF pulse that is actually output follows the actual Larmor frequency due to the time change of the eddy current magnetic field, and it is possible to obtain the originally intended slice profile. Therefore, in this embodiment, it is possible to realize local excitation matching the imaging conditions. In particular, in the case of an RF pulse having a long transmission period, the improvement effect of the first embodiment is remarkable.
Further, the Larmor frequency of fat or water that changes due to the eddy current magnetic field is also followed inside the RF pulse, so that unnecessary signals such as fat can be suppressed more reliably and the image quality can be improved.

  Furthermore, in the first embodiment, regarding the intensity of the 0th-order component of the eddy current magnetic field calculated for each transmission timing of each RF pulse, only the gradient magnetic field pulse applied within the most recent predetermined period PS is reflected. It The predetermined period PS is determined by the sequence setting function 66 so that the intensity of the 0th-order component of the eddy current magnetic field can be calculated with sufficient accuracy in consideration of the attenuation due to the time constant. Therefore, according to the first embodiment, the calculation load for calculating the intensity of the 0th-order component of the eddy current magnetic field can be minimized.

<Second Embodiment>
In the first embodiment, the pulse sequence (the output control waveform of the RF pulse in the processing circuit 60) is modified, and the modified pulse sequence is input to the hardware side of the control device 40. In the second embodiment, since the RF pulse waveform generation circuit 54 on the hardware side performs frequency modulation on the analog pulse waveform signal, the sequence correction function 66 is omitted.

  Therefore, the apparatus configuration of the MRI apparatus of the second embodiment is the same as that of the MRI apparatus 10 of the first embodiment described in FIG. 1 except that the sequence correction function 66 is omitted. The flow chart will be omitted, and only the differences from the first embodiment will be described with reference to the flowchart (this point, the third embodiment, and the fourth embodiment are also the same).

  FIG. 6 is a flowchart showing an example of the operation flow of the MRI apparatus of the second embodiment. The operation of the MRI apparatus according to the second embodiment will be described below according to the step numbers shown in FIG.

  [Steps S21 and S22] Since the steps are the same as Steps S1 and S2 in FIG. 5 of the first embodiment, duplicate description will be omitted. Then, it progresses to step S23.

  [Step S23] The system control function 61 inputs the pulse sequence set in step S22 to the sequence controller 58. Then, it progresses to step S24.

  [Step S24] Similar to the first embodiment, the static magnetic field formed in the imaging space is made uniform by the shim coil power supply 44 and the shim coil 32.

  Then, when an imaging start instruction is input from the input device 72 to the system control function 61, the sequence controller 58 drives each part of the control device 40 in accordance with the input pulse sequence to collect MR signals as a main scan. To execute. Here, as an example, the main scan is executed by sequentially repeating the following sub steps <1> to <4>.

  <1> The sequence controller 58 commands each current value or each voltage value (time change thereof) supplied to the X-axis gradient magnetic field coil 33x, the Y-axis gradient magnetic field coil 33y, and the Z-axis gradient magnetic field coil 33z according to the pulse sequence. The values are sequentially input to the gradient magnetic field pulse waveform generation circuit 47 and the RF pulse waveform generation circuit 54 in real time, and the output control waveform of the RF pulse is sequentially input to the RF pulse waveform generation circuit 54 in real time. In synchronization with this input, the RF pulse waveform generation circuit 54 causes the vortex according to the command value of the supply current (supply voltage) to the X-axis gradient magnetic field coil 33x, the Y-axis gradient magnetic field coil 33y, and the Z-axis gradient magnetic field coil 33z. The shift amount of the 0th-order component of the current magnetic field and the Larmor frequency is calculated as in the first embodiment.

  <2> The RF pulse waveform generation circuit 54 performs frequency modulation on the analog pulse waveform signal so as to follow the shift amount of the Larmor frequency after generating the analog pulse waveform signal as in the first embodiment. The RF pulse waveform generation circuit 54 further modulates the analog pulse waveform signal frequency-modulated to the carrier frequency from the fixed frequency generation circuit 57 in the same manner as described above, and the modulated pulse waveform signal is supplied to the RF transmitter 48. input.

  <3> The RF transmitter 48 generates an RF pulse current based on the input pulse waveform signal and transmits it to the whole body QD coil. An RF pulse corresponding to this RF current pulse is transmitted to the subject P from the whole body QD coil.

<4> After transmitting the RF pulse, the RF coil device 100 detects the MR signal from the subject P. The detected MR signal is processed in the same manner as in the first embodiment, and finally converted into k-space data by the image reconstruction function 62 and stored.
After the acquisition of MR signals of the main scan is completed by repeating the above <1> to <4>, the process proceeds to step S25.

  [Step S25] Since it is the same as step S6 in FIG. 5 of the first embodiment, redundant description will be omitted. The above is the description of the flowchart in FIG.

  As described above, in the MRI apparatus of the second embodiment, the frequency modulation of the output control waveform of the RF pulse is sequentially executed in real time in the RF pulse waveform generation circuit 54 so as to follow the actual Larmor frequency. It is possible to obtain the same effect as that of the embodiment.

<Third Embodiment>
In the first embodiment, the output control waveform of the RF pulse in the pulse sequence is modified in the processing circuit 60, and in the second embodiment, the output control waveform of the RF pulse in the pulse sequence is the RF pulse waveform generation on the hardware side. The example corrected by the circuit 54 has been described. In the MRI apparatus of the third embodiment, the variable frequency generation circuit 56 frequency-modulates Δf to the center frequency of the RF pulse so as to follow the shift of the Larmor frequency without modifying the output control waveform of the RF pulse in the pulse sequence. Apply.

  FIG. 7 is a flowchart showing an example of the operation flow of the MRI apparatus of the third embodiment. The operation of the MRI apparatus of the third embodiment will be described below according to the step numbers shown in FIG.

  [Steps S31 to S33] Since these steps are the same as steps S21 to S23 of FIG. 6 of the second embodiment, respectively, redundant description will be omitted. Then, it progresses to step S34.

  [Step S34] Similar to the first embodiment, the static magnetic field formed in the imaging space is made uniform by the shim coil power supply 44 and the shim coil 32.

  Then, when an imaging start instruction is input from the input device 72 to the system control function 61, the sequence controller 58 drives each part of the control device 40 in accordance with the input pulse sequence to collect MR signals as a main scan. To execute. Here, as an example, the main scan is executed by sequentially repeating the following sub-steps <1 '> to <4'>.

  The <1 ′> sequence controller 58 changes (changes with time) each current value or each voltage value supplied to the X-axis gradient magnetic field coil 33x, the Y-axis gradient magnetic field coil 33y, and the Z-axis gradient magnetic field coil 33z according to the pulse sequence. The command value is sequentially input to the gradient magnetic field pulse waveform generation circuit 47 and the variable frequency generation circuit 56 in real time, and the output control waveform of the RF pulse is sequentially input to the RF pulse waveform generation circuit 54 in real time. The variable frequency generation circuit 56 synchronizes with the input from the sequence controller 58 to set the command value of the supply current (supply voltage) to the X-axis gradient magnetic field coil 33x, the Y-axis gradient magnetic field coil 33y, and the Z-axis gradient magnetic field coil 33z. Based on this, the 0th order component of the eddy current magnetic field and the shift amount of the Larmor frequency are calculated in the same manner as described above.

  <2 '> Similar to the first embodiment, the RF pulse waveform generation circuit 54 modulates the analog pulse waveform signal to the carrier frequency from the fixed frequency generation circuit 57 after generating the analog pulse waveform signal. The RF pulse waveform generation circuit 54 inputs the modulated pulse waveform signal to the variable frequency generation circuit 56. The variable frequency generation circuit 56 frequency-modulates the pulse waveform signal input from the RF pulse waveform generation circuit 54 so as to follow the shift amount of the Larmor frequency calculated in substep <1 ′>, and after the frequency modulation, The pulse waveform signal is input to the RF transmitter 48.

  <3 '> The RF transmitter 48 generates an RF pulse current in the same manner as described above based on the input pulse waveform signal and transmits it to the whole body QD coil. An RF pulse corresponding to this RF current pulse is transmitted to the subject P from the whole body QD coil.

<4 '> MR signals are detected and stored as k-space data as described above.
After the sub-steps <1 ′> to <4 ′> are repeated, the MR signal acquisition for the main scan is completed, and the process proceeds to step S35.

  [Step S35] Since this step is the same as step S6 in FIG. 5 of the first embodiment, redundant description will be omitted. The above is the description of the flowchart in FIG. 7.

  In the third embodiment, the pulse waveform signal modulated to the carrier frequency from the fixed frequency generation circuit 57 in the RF pulse waveform generation circuit 54 is input to the variable frequency generation circuit 56 so as to follow the shift amount of the Larmor frequency. The signal is further frequency-modulated, and then input to the RF transmitter 48. Therefore, the center frequency of the RF pulse output from the RF coil 34 has a value that follows the actual shift of the Larmor frequency. Therefore, also in the MRI apparatus of the third embodiment, the same effect as that of the first embodiment can be obtained.

<Fourth Embodiment>
The MRI apparatus of the fourth embodiment shifts the carrier frequency, which is the generation source of the RF pulse, on the hardware side so as to follow the shift of the Larmor frequency without modifying the output control waveform of the RF pulse in the pulse sequence. .. Here, as an example, the fixed frequency generation circuit 57 shifts the carrier frequency so as to follow the shift of the Larmor frequency.

  FIG. 8 is a flowchart showing an example of the operation flow of the MRI apparatus of the fourth embodiment. Hereinafter, the operation of the MRI apparatus of the fourth embodiment will be described according to the step numbers shown in FIG.

  [Steps S41 to S43] These steps are the same as steps S21 to S23 of FIG. 6 of the second embodiment, respectively, and thus redundant description will be omitted. Then, it progresses to step S44.

  [Step S44] Similar to the first embodiment, the static magnetic field formed in the imaging space is made uniform by the shim coil power supply 44 and the shim coil 32.

  Then, when an imaging start instruction is input from the input device 72 to the system control function 61, the sequence controller 58 drives each unit of the control device 40 in accordance with the input pulse sequence to collect MR signals as a main scan. To execute. Here, as an example, the main scan is executed by sequentially repeating the following sub-steps <1 ″> to <4 ″>.

  <1 ″> The sequence controller 58 changes the current value or each voltage value (change over time) of each current value or each voltage value supplied to the X-axis gradient magnetic field coil 33x, the Y-axis gradient magnetic field coil 33y, and the Z-axis gradient magnetic field coil 33z according to the pulse sequence. The command value is sequentially input to the gradient magnetic field pulse waveform generation circuit 47 and the fixed frequency generation circuit 57 in real time, and the output control waveform of the RF pulse is sequentially input to the RF pulse waveform generation circuit 54 in real time. The fixed frequency generation circuit 57, based on the command value of the supply current (supply voltage) to the X-axis gradient magnetic field coil 33x, the Y-axis gradient magnetic field coil 33y, and the Z-axis gradient magnetic field coil 33z, the 0th-order component of the eddy current magnetic field and the Larmor. The frequency shift amount is sequentially calculated in real time in the same manner as described above, and the fixed frequency generation circuit 57 follows this shift amount. Generates a carrier frequency shifting the urchin frequencies, the generated carrier frequency to sequentially input in real time RF pulse waveform generation circuit 54.

  <2 ″> The RF pulse waveform generating circuit 54 modulates the analog pulse waveform signal to the carrier frequency from the fixed frequency generating circuit 57 after generating the analog pulse waveform signal as in the first embodiment (here. The frequency of the modulated RF pulse waveform is further shifted in sub-step <1 ″> to follow the actual Larmor frequency). The RF pulse waveform generation circuit 54 inputs the modulated pulse waveform signal to the RF transmitter 48.

  <3 ″> The RF transmitter 48 generates an RF pulse current based on the input pulse waveform signal and transmits it to the whole body QD coil in the same manner as described above. The RF pulse corresponding to this RF current pulse is generated. , Is transmitted to the subject P from the QD coil for the whole body.

<4 ″> MR signals are detected and stored as k-space data as described above.
After the acquisition of MR signals of the main scan is completed by repeating the above substeps <1 ″> to <4 ″>, the process proceeds to step S45.

  [Step S45] Since this step is the same as step S6 in FIG. 5 of the first embodiment, redundant description will be omitted. The above is the description of the flowchart in FIG.

  In the fourth embodiment, the carrier frequency input to the RF pulse waveform generation circuit 54 is shifted so as to follow the shift of the Larmor frequency. Since the RF pulse is generated based on this carrier frequency, the center frequency of the RF pulse output from the RF coil 34 is a modulation that follows the actual shift of the Larmor frequency. Therefore, also in the MRI apparatus of the fourth embodiment, the same effect as that of the first embodiment can be obtained.

  According to each embodiment of the magnetic resonance imaging apparatus described above, it is possible to suppress deterioration of image quality even when the Larmor frequency changes due to the eddy current magnetic field.

<Supplementary information on each embodiment>
[1] In each of the above-described embodiments, an example in which the quadrature QD coil for whole body in the RF coil 34 is used as the RF coil device that transmits the RF pulse has been described. The embodiments of the present invention are not limited to the quadrature method, and the techniques of the above-described embodiments of frequency-modulating the output control waveform of the RF pulse can be applied to the RF coil device that transmits the RF pulse by another method. Is.

[2] Correspondence between claim terms and embodiments will be described. In addition, the correspondence relationship shown below is one interpretation shown for reference and does not limit the present invention.
The gradient magnetic field pulse waveform generation circuit 47, the gradient magnetic field power supply 46, and the gradient magnetic field coil 33 are examples of the gradient magnetic field generation circuit described in the claims.

  In the first embodiment, the sequence correction function 66, the sequence controller 58, the variable frequency generation circuit 56, the RF pulse waveform generation circuit 54, the RF transmitter 48, and the RF coil 34 are examples of the RF transmission circuit according to the claims. Is.

  In the second to fourth embodiments, the sequence controller 58, the variable frequency generation circuit 56, the RF pulse waveform generation circuit 54, the RF transmitter 48, and the RF coil 34 are an example of the RF transmission circuit described in the claims. ..

  The system control function 61 that acquires the imaging condition via the input device 72 and sets the pulse sequence based on the imaging condition is an example of the function implemented by the processing circuit according to the claims.

  [3] Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. The embodiments and their modifications are included in the scope of the invention and the scope thereof, and are included in the invention described in the claims and the scope of equivalents thereof.

10: MRI device,
20: Bed device, 22: Top plate, 30: Gantry,
31: static magnetic field magnet, 32: shim coil unit, 33: gradient magnetic field coil unit,
34: RF coil unit, 40: control device, 60: processing circuit

Claims (6)

A gradient magnetic field generation circuit for applying the gradient magnetic field pulse according to a pulse sequence including transmission of an RF pulse and application of a gradient magnetic field pulse;
Due to temporal changes in the eddy current magnetic fields is estimated from the waveform of the gradient magnetic field pulse, and to follow the temporal changes in the magnetic resonance frequency of the said RF pulse, to the output control waveform of the RF pulse And an RF transmitting circuit that performs modulation and transmits the modulated RF pulse ,
A processing circuit that acquires an imaging condition and sets the pulse sequence based on the imaging condition;
Equipped with
The RF transmission circuit calculates a 0th-order component of the eddy current magnetic field based on the waveform of the gradient magnetic field pulse included in the pulse sequence set by the processing circuit, and based on the 0th-order component of the eddy current magnetic field. A magnetic resonance imaging apparatus that modifies the pulse sequence by performing the modulation. The magnetic resonance imaging apparatus according to claim 1,
The RF transmission circuit such that said center frequency of the RF pulse follows the temporal variation of the Larmor frequency in the subject in the imaging region, the magnetic resonance imaging apparatus for performing the modulation. The magnetic resonance imaging apparatus according to claim 1 or 2 ,
The RF transmission circuit includes a 0th-order component of the eddy current magnetic field generated by all the gradient magnetic field pulses applied within a predetermined period before the application of the RF pulse, for each RF pulse included in the pulse sequence. And a magnetic resonance imaging apparatus that executes the modulation based on the zero-order component of the eddy current magnetic field calculated for each RF pulse. A gradient magnetic field generation circuit for applying the gradient magnetic field pulse according to a pulse sequence including transmission of an RF pulse and application of a gradient magnetic field pulse;
The output control waveform of the RF pulse is made to follow the temporal change in the magnetic resonance frequency within the RF pulse caused by the temporal change of the eddy current magnetic field inferred from the waveform of the gradient magnetic field pulse. An RF transmission circuit that performs modulation and transmits the modulated RF pulse;
Equipped with
The RF transmission circuit includes a 0th-order component of the eddy current magnetic field generated by all the gradient magnetic field pulses applied within a predetermined period before the application of the RF pulse, for each RF pulse included in the pulse sequence. And a magnetic resonance imaging apparatus that executes the modulation based on the zero-order component of the eddy current magnetic field calculated for each RF pulse . The magnetic resonance imaging apparatus according to claim 4 ,
The gradient magnetic field generation circuit based on imaging conditions sets a condition of the pulse sequence including a command value of a supply current or a supply voltage to a gradient magnetic field coil for applying the gradient magnetic field pulse and an output control waveform of the RF pulse. includes a processing circuit that sets Te,
The RF transmission circuit obtains a 0th-order component of the eddy current magnetic field obtained from a command value of a supply current or a supply voltage to the gradient magnetic field coil defined by the pulse sequence, and 0 of the eddy current magnetic field. A magnetic resonance imaging apparatus that sequentially executes, in real time, a process of performing the modulation on the output control waveform of the RF pulse based on the next component and a process of outputting the modulated RF pulse while the pulse sequence is in progress. .. A gradient magnetic field generation circuit for applying the gradient magnetic field pulse according to a pulse sequence including transmission of an RF pulse and application of a gradient magnetic field pulse;
The output control waveform of the RF pulse is modulated by following the temporal change of the magnetic resonance frequency caused by the temporal change of the eddy current magnetic field inferred from the waveform of the gradient magnetic field pulse, An RF transmitter circuit for transmitting the modulated RF pulse;
Equipped with
The RF transmitting circuit includes a time constant of attenuation of the zero-order component of the eddy current field which is stored before execution of the pulse sequence, based on the waveform of the gradient magnetic field pulse, the RF pulse of the output control waveform Modulates,
Magnetic resonance imaging system.

Images