JP6678214B2 - Magnetic resonance imaging equipment - Google Patents

Description

  Embodiments of the present invention relate to magnetic resonance imaging.

  MRI is an imaging method in which a nuclear spin of a subject placed in a static magnetic field is magnetically excited by an RF pulse having a Larmor frequency, and an image is reconstructed from an MR signal generated by the excitation. In addition, the above-mentioned MRI means magnetic resonance imaging (Magnetic Resonance Imaging), RF pulse means high frequency pulse (radio frequency pulse), MR signal means nuclear magnetic resonance signal (nuclear magnetic resonance signal). .

  In MRI, to obtain spatial position information, gradient magnetic fields orthogonal to each other are applied while being superimposed on a static magnetic field. Therefore, the gradient magnetic field generation system of the MRI apparatus includes a gradient magnetic field coil that applies spatial position information to the MR signal by applying a gradient magnetic field in the imaging space where the subject is placed.

  The distortion of the gradient magnetic field distribution is known as one of the causes of image quality deterioration of MRI. Ideally, each gradient magnetic field strength in the slice selection direction, the phase encoding direction, and the frequency encoding direction changes linearly according to, for example, a position along the application direction. However, actually, when a pulse current is supplied to the gradient coil, an eddy current is generated, and a magnetic field due to the eddy current is applied to the gradient magnetic field, thereby distorting the gradient magnetic field distribution.

  The distortion of the gradient magnetic field distribution generally increases as the distance from the center of the magnetic field increases. Therefore, when the off-center is set as the imaging region, the image quality is deteriorated due to the distortion of the gradient magnetic field distribution.

In this specification, the term “off-center” means a region away from the center of the magnetic field to a degree outside the DSV.
The DSV (Diameter Spherical Volume) is a spherical region centered on the magnetic field center, which is the geometric center of the static magnetic field magnet, and indicates a region having a predetermined magnetic field uniformity.

JP 2012-40362 A

  An object of the present invention is to reduce the occurrence of an artifact artifact.

  In one embodiment, the MRI apparatus has an imaging condition setting unit.

  The imaging condition setting unit, when the imaging region is inside a region having a predetermined magnetic field uniformity, the amplitude of the first gradient magnetic field pulse in the slice selection direction applied simultaneously with the excitation RF pulse, and the refocus RF pulse The pulse sequence conditions are changed so as to be separated from the amplitude of the second gradient magnetic field pulse in the slice selection direction applied at the same time.

FIG. 1 is a block diagram showing an example of the overall configuration of an MRI apparatus according to the embodiment. FIG. 2 is a schematic plan view showing an example of a configuration of an arm RF coil device as an example of a wearable RF coil device that detects an MR signal. FIG. 4 is a schematic timing chart showing an example of a pulse sequence of the SE method. FIG. 4 is a schematic timing chart showing an example of a pulse sequence of the FSE method in the same notation as FIG. 3. FIG. 9 is a schematic timing chart showing an example of a method of increasing the amplitude of the slice selection direction gradient magnetic field pulse Gssr to equalize the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr when the respective amplitudes are different. FIG. 10 is a schematic timing chart showing an example of a method of reducing the amplitude of the slice selection direction gradient magnetic field pulse Gssr to equalize the amplitudes of the slice selection direction gradient magnetic field pulses Gssr and Gssr when the respective amplitudes are different. FIG. 6 is a schematic diagram conceptually showing the effect of equalizing the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr as shown in FIG. 5 when the Z-axis direction is assumed to be the slice selection direction. FIG. 3 is a schematic plan view in a gantry showing an example of a cause of an artifact artifact in a positional relationship with an imaging region. FIG. 9 is a schematic diagram showing an example of a warning display executed by the notification unit and the display device when there is a possibility that an artifact artifact may occur. 9 is a flowchart illustrating an example of the flow of processing of the MRI apparatus when setting a pulse sequence of the SE method. 9 is a flowchart illustrating an example of the processing flow of the MRI apparatus when setting a pulse sequence of the FSE method.

  Hereinafter, embodiments of an MRI apparatus and an MRI method will be described with reference to the accompanying drawings. 1 and 2 explain the hardware configuration of the MRI apparatus, FIGS. 3 to 9 (particularly FIGS. 5 and 7) explain the principle of the present embodiment, and FIGS. 10 and 11 show the operation of the MRI apparatus. An example will be described with a flowchart. In each of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

<Configuration of this embodiment>
FIG. 1 is a block diagram illustrating an example of the overall configuration of an MRI apparatus 10 according to the present embodiment. Here, as an example, the components of the MRI apparatus 10 will be described by dividing the components into a bed apparatus 20, a gantry 30, and a control apparatus 40.

  First, the couch device 20 includes a support 21, a top 22, and a top moving mechanism 23 disposed in the support 21.

The subject P is placed on the upper surface of the top plate 22. Further, on the upper surface of the top plate 22, a plurality of connection ports 25 to which an RF coil device mounted on the subject P is connected are arranged.
Here, as an example, the arm RF coil device 100 for detecting the MR signal is attached to the subject P.

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

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

  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 where the subject P is placed and a static magnetic field is applied. Note that the static magnetic field magnet 31 may be constituted by a permanent magnet without providing the static magnetic field power supply 42.

  The shim coil unit 32 is formed, for example, in a cylindrical shape, and is arranged inside the static magnetic field magnet 31 with the same axis as that of the static magnetic field magnet 31. The shim coil unit 32 forms an offset magnetic field for equalizing a 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 unit 33 is formed, for example, in a cylindrical shape, and is arranged inside the shim coil unit 32. The gradient magnetic field coil unit 33 has 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 device coordinate systems unless otherwise specified. Here, as an example, the X axis, the Y axis, and the Z axis of the device coordinate system are defined as follows.
First, the vertical direction is defined as the Y-axis direction, and the top plate 22 is arranged such that the normal direction of the upper surface thereof is the Y-axis direction.
The horizontal movement direction of the top plate 22 is defined as the Z-axis direction, and the gantry 30 is disposed 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.

Here, as an example, the origin of the apparatus coordinate system is set to the center of the magnetic field.
The magnetic field center is, for example, a geometric center position of the static magnetic field magnet 31.

  The X-axis gradient magnetic field coil 33x forms a gradient magnetic field Gx in the X-axis direction in the imaging region according to a current supplied from an X-axis gradient magnetic field power supply 46x described later. 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 in the imaging region according to a current supplied from a Z-axis gradient magnetic field power supply 46z described later.

  The gradient magnetic field Gss in the slice selection direction, the gradient magnetic field Gpe in the phase encoding direction, and the gradient magnetic field Gro in the readout direction (frequency encoding direction) can be arbitrarily determined by synthesizing the gradient magnetic fields Gx, Gy, and Gz in the three axis directions of the apparatus coordinate system. The direction can be set.

  The imaging region is, for example, at least a part of an acquisition range of an MR signal used to generate one image or one set of images, and is an image region. For example, the imaging region is three-dimensionally defined in the device coordinate system as a part of the imaging space. For example, when an MR signal is acquired in a wider area than an area to be imaged in order to prevent aliasing artifacts, the imaging area is a part of the acquisition area of the MR signal. On the other hand, there is a case where the entire acquisition range of the MR signal becomes an image, and the acquisition range of the MR signal matches the imaging region.

  The “one set of images” is a plurality of images in a case where MR signals of a plurality of images are collectively collected in one pulse sequence, such as multi-slice imaging. Here, as an example, the imaging region is referred to as a slice if the region is thin, and is referred to as a slab if the region has a certain thickness.

  The RF coil unit 34 is formed, for example, in a cylindrical shape, and is arranged inside the gradient magnetic field coil unit 33. The RF coil unit 34 includes, for example, a whole-body coil that transmits RF pulses and receives MR signals.

  Third, 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, an RF transmitter 48, an RF receiver 50, a sequence controller 58, a calculation device 60, an input device, 72, a display device 74, and a storage device 76.

  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 supply respective currents for forming the gradient magnetic fields Gx, Gy, Gz to the X-axis gradient magnetic field coil 33x and the Y-axis gradient magnetic field coil. 33y and the Z-axis gradient magnetic field coil 33z.

  The RF transmitter 48 generates an RF current pulse having a Larmor frequency that causes nuclear magnetic resonance based on the control information input from the sequence controller 58, and transmits this to the RF coil unit 34. An RF pulse corresponding to the RF current pulse is transmitted from the RF coil unit 34 to the subject P.

  The whole-body coil of the RF coil unit 34 and the arm RF coil device 100 attached to the subject P detect and detect an MR signal generated by exciting a nuclear spin in the subject P by an RF pulse. The obtained 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 arithmetic unit 60 (the image reconstruction unit 62 thereof).

  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 arithmetic device 60. The control information here is, for example, sequence information describing operation control information such as the intensity, application time, and application timing of a pulse current to be applied to the gradient magnetic field power supply 46.

  The sequence controller 58 generates gradient magnetic fields Gx, Gy, Gz and RF pulses by driving the gradient magnetic field power supply 46, the RF transmitter 48, and the RF receiver 50 according to the stored predetermined sequence.

  The arithmetic device 60 has a system control unit 61, a system bus SB, an image reconstructing unit 62, an image database 63, an image processing unit 64, a safety standard determining unit 65, and a notification unit 66.

  The system control unit 61 functions as an imaging condition setting unit that sets the imaging conditions for the main scan. Further, the system control unit 61 performs system control of the entire MRI apparatus 10 via wiring such as a system bus SB in an imaging operation and image display after imaging.

  The above-mentioned imaging conditions mean, for example, what kind of pulse sequence, under what conditions RF pulses or the like are transmitted, and under what conditions MR signals are collected from the subject P. Examples of imaging conditions include an imaging region as positional information in an imaging space, a flip angle, a repetition time TR (Repetition Time), a number of slices, an imaging region, and a type of pulse sequence such as a spin echo method or parallel imaging. No.

  The above-mentioned imaging part means, for example, which part of the subject P, such as the head, the chest, and the abdomen, is imaged as the 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 acquiring an MR signal for a positioning image or a calibration scan.
Scanning 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. Scan An example of the calibration scan includes a sequence for calculating the center frequency of the RF pulse in the main scan. The pre-scan refers to a calibration scan that is performed before the main scan.

  Further, the system control unit 61 causes the display device 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 inputs the set imaging condition to the sequence controller 58. Further, after imaging, the system control unit 61 causes the display device 74 to display an image indicated by the generated display image data.

The input device 72 provides a user with a function of setting imaging conditions and image processing conditions.
The image reconstruction unit 62 arranges and stores 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. k-space means frequency space.

  The image reconstruction unit 62 generates image data of the subject P by performing image reconstruction processing including two-dimensional or three-dimensional Fourier transform on the k-space data. The image reconstruction unit 62 stores the generated image data in the image database 63.

  The image processing unit 64 fetches image data from the image database 63, performs predetermined image processing on the image data, and stores the image data after the image processing in the storage device 76 as display image data.

  The storage device 76 stores the above-described display image data with the imaging conditions used for generating the display image data, information on the subject P (patient information), and the like attached thereto as supplementary information.

  Each time the system control unit 61 sets all the conditions of the pulse sequence of the main scan, the safety standard determination unit 65 determines whether the pulse sequence satisfies the safety standard. Specifically, the safety criterion determination unit 65 calculates the magnetic field time change rate (hereinafter referred to as dB / dt value) and the SAR of the pulse sequence.

  The SAR means a specific absorption ratio as energy of an RF pulse absorbed by 1 kg of the living tissue. For example, safety standards are set so that the SAR for any 10 seconds and 6 minutes does not exceed the first or second upper limit value, and the upper limit value differs depending on parts such as the whole body and the head.

  After setting the imaging region of the main scan, the notifying unit 66 notifies that there is a possibility that an artifact artifact may occur before executing the main scan.

  The arithmetic device 60, the input device 72, the display device 74, and the storage device 76 may be configured as one computer, and may be installed in, for example, a control room.

In the above description, the components of the MRI apparatus 10 are classified into the gantry 30, the bed apparatus 20, and the control apparatus 40, but this is only an example of interpretation.
For example, the top moving mechanism 23 may be considered as a part of the control device 40.

  Alternatively, the RF receiver 50 may be located inside the gantry 30 instead of outside the gantry 30. In this case, for example, an electronic circuit board corresponding to the RF receiver 50 is provided in the gantry 30. The MR signal converted from an electromagnetic wave to an analog electric signal by an RF coil device or the like is amplified by a preamplifier in the electronic circuit board, output as a digital signal outside the gantry 30, and input to the image reconstruction unit 62. You. For output to the outside of the gantry 30, it is desirable to transmit the optical digital signal using, for example, an optical communication cable because the influence of external noise is reduced.

  FIG. 2 is a schematic plan view illustrating an example of the configuration of the arm RF coil device 100 as an example of a wearable RF coil device that detects an MR signal. As shown in FIG. 2, the arm RF coil device 100 includes a cover member 110, a cable 112, and a connector 114.

  The cover member 110 is formed of a flexible material so that deformation such as bending can be performed. As such a deformable material, for example, a flexible printed circuit (FPC) described in JP-A-2007-229004 can be used.

  In the cover member 110, for example, eight element coils 120 corresponding to the subject P from the shoulder to the hand are arranged. The arm RF coil device 100 includes a control circuit (not shown) and a storage element (not shown) storing identification information of the arm RF coil device 100 in the cover member 110.

  When the connector 114 is connected to the connection port 25 on the top plate 22, the identification information of the arm RF coil device 100 is input from this control circuit to the system control unit 61 via the wiring in the MRI device 10.

<Principle of the present embodiment>
Hereinafter, the principle of the present embodiment will be described, but prior to the description, the pulse sequence of the SE (Spin Echo: Spin Echo) method and the FSE (Fast Spin Echo) method to which the pulse sequence correction algorithm of the present embodiment is applied. Will be described briefly.

FIG. 3 is a schematic timing chart showing an example of a pulse sequence of the SE method.
In FIG. 3, each horizontal axis corresponds to the elapsed time t, and the upper RF indicates an RF pulse, and the lower Gss indicates a slice selection direction gradient magnetic field (the same applies to FIGS. 4 to 6 described later).

  Gpe below Gss indicates a gradient magnetic field in the phase encoding direction, Gro indicates a gradient magnetic field in the reading direction (frequency encoding direction), and SIGNAL at the bottom indicates a generated MR signal.

  The area between the two vertical broken lines in FIG. 3 is the acquisition (one cycle) of the MR signal for one phase encoding step. At the beginning of each cycle, for example, a pre-pulse such as a fat suppression pre-pulse is transmitted.

  Next, at the same time as the application of the slice selection direction gradient magnetic field pulse Gsse, an excitation RF pulse having a flip angle of, for example, 90 ° is transmitted to the imaging region. However, the above-mentioned pre-pulse is not essential, and in that case, the application of the slice selection direction gradient magnetic field pulse Gsse and the transmission of the excitation RF pulse are performed first.

Next, after applying the phase encoding direction gradient magnetic field pulse, the reading direction gradient magnetic field pulse is applied.
Next, at the timing when half of the echo time TE has elapsed from the transmission timing of the excitation RF pulse, a refocus RF pulse having a flip angle of 180 ° is transmitted to the imaging region simultaneously with the application of the slice selection direction gradient magnetic field pulse Gssr. You.

  Thereafter, at the timing when the echo time TE elapses from the transmission timing of the excitation RF pulse, the MR signal is detected under the application of the readout gradient pulse.

  Up to this point is the acquisition (1 cycle) of the MR signal for one phase encoding step. By repeating this one cycle by the same number as the number of phase encoding steps, MR signals for one image are collected.

  FIG. 4 is a schematic timing chart showing an example of the pulse sequence of the FSE method in the same notation as FIG. Here, as an example, the number of phase encoding steps and the number of frequency encoding steps are both 256, and one set in which MR signals for four phase encoding steps are collected is repeated 64 times. Shall be collected.

In this case, at the beginning of each set, a pre-pulse such as a fat suppression pre-pulse is transmitted.
Next, at the same time as the application of the slice selection direction gradient magnetic field pulse Gsse, an excitation RF pulse having a flip angle of, for example, 90 ° is transmitted to the imaging region. However, the pre-pulse is not essential, and in that case, the application of the slice selection direction gradient magnetic field pulse Gsse and the transmission of the excitation RF pulse are performed first.

Next, the readout direction gradient magnetic field pulse is applied with an application time width that is half of the readout direction gradient magnetic field pulse applied when the MR signal is detected.
Next, at the timing when half of the echo time TE has elapsed from the transmission timing of the excitation RF pulse, a refocus RF pulse having a flip angle of 180 ° is transmitted to the imaging region simultaneously with the application of the slice selection direction gradient magnetic field pulse Gssr. You.

Next, a gradient magnetic field pulse in the phase encoding direction is applied.
Next, the MR signal is detected under the application of the readout gradient pulse.

  At the end of each cycle, a compensating gradient magnetic field whose polarity is inverted from that of the phase encoding direction gradient magnetic field pulse applied earlier in each cycle is applied in the phase encoding direction.

Thereby, the influence of the gradient magnetic field in the phase encoding direction is eliminated before the next MR signal acquisition cycle.
Up to this point, the acquisition of the MR signal for one phase encoding step has been completed.

Thereafter, the same operation is repeated three times except that the pre-pulse and the excitation RF pulse are not applied, and an MR signal for three phase encode steps is acquired.
This is the acquisition of one set of MR signals, and the pre-pulse and the excitation RF pulse are applied only at the beginning of each set.

FIG. 5 is a schematic timing chart showing an example of a method of increasing the amplitude of the slice selection direction gradient magnetic field pulse Gssr to equalize the amplitudes of the slice selection direction gradient magnetic field pulses Gssr and Gssr when the respective amplitudes are different. .
The upper part of FIG. 5 shows the state before the deformation of the waveform, and the lower part of FIG. 5 shows the state after the deformation of the waveform.

  As will be described in more detail later with reference to FIG. 7, in off-center imaging, it is desirable that the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr be equal. In the off-center, where the linearity of the gradient magnetic field is deteriorated, the image quality is less likely to be degraded by making the amplitudes of the gradient magnetic field pulses Gsse and Gssr in the slice selection direction closer to each other so that the non-linear gradient magnetic field strength distributions are close to each other. Because.

  Therefore, in the present embodiment, when at least a part of the imaging region is out of the DSV, the waveform of the refocus RF pulse and the slice selection direction gradient magnetic field pulse Gssr applied at the same time is deformed, thereby changing the slice selection direction gradient. The amplitudes of the magnetic field pulses Gsse and Gssr are made uniform.

  In order to equalize the amplitudes of both, it is conceivable to modify the waveforms of the excitation RF pulse and the slice selection direction gradient magnetic field pulse Gsse applied at the same time. However, in principle, it is desirable to modify the waveforms of the refocusing RF pulse and the slice selection direction gradient magnetic field pulse Gssr applied at the same time.

  More specifically, since various characteristics such as chemical shift are determined by the first excitation RF pulse, the pulse sequence is often set based on the first excitation RF pulse. Therefore, if the waveform of the first excitation RF pulse is changed, the entire pulse sequence itself is reset, which is complicated.

  However, by changing the waveforms of the excitation RF pulse and the slice selection gradient pulse Gsse without changing the conditions of the refocus RF pulse and the slice selection gradient pulse Gssr, each of the slice selection direction gradient magnetic field pulses Gsse and Gssr is changed. The case where the amplitudes are made uniform is also within the scope of the technical idea of the present embodiment.

  FIG. 5 shows that, in the SE method or the FSE method, the amplitude of the slice selection direction gradient magnetic field pulse Gsse applied simultaneously with the excitation RF pulse is larger than the amplitude of the slice selection direction gradient magnetic field pulse Gssr applied simultaneously with the refocusing RF pulse. If it is higher.

  In this case, the system control unit 61 increases the amplitude of the slice selection direction gradient magnetic field pulse Gssr to the amplitude of the slice selection direction gradient magnetic field pulse Gsse so that the time integral value of the absolute value of the gradient magnetic field intensity does not change. Keeping the time integrated value of the absolute value of the gradient magnetic field intensity unchanged does not mean that the area of each oblique line region of the slice selection direction gradient magnetic field pulse Gssr is equal between the upper stage and the lower stage in FIG.

  If the amplitude of the slice selection direction gradient magnetic field pulse Gssr is increased so that the time integral value of the gradient magnetic field intensity does not change, the application time width becomes shorter. Therefore, the system control unit 61 shortens the transmission time width of the refocus RF pulse transmitted simultaneously with the slice selection direction gradient magnetic field pulse Gssr so as to match the application time width.

  However, the deformation of the reconverging RF pulse is also executed so that the time integral value of the absolute value of the pulse intensity does not change. Therefore, the amplitude of the refocusing RF pulse increases as the application time width decreases. In the following description, when referring to "intensity of gradient magnetic field" and "pulse intensity", they refer to "absolute value of gradient magnetic field intensity" and "absolute value of pulse intensity", and all intensities are positive values. To think.

  As shown in FIG. 5, when the amplitude of the gradient magnetic field pulse Gssr in the slice selection direction at the same time as the refocus RF pulse is increased, the application time width is shortened, so that the echo time TE does not change before and after the waveform deformation.

  However, as the amplitude of the slice selection direction gradient magnetic field pulse Gssr increases, the rising gradient (Rising Gradient) and the falling gradient (Falling Gradient) increase, so that the dB / dt value increases. Further, since the amplitude of the refocus RF pulse increases, the SAR increases.

  Therefore, after the waveform is deformed, the safety standard may not be satisfied from the two viewpoints of the SAR and the dB / dt value. In that case, the pulse sequence is reset so as to satisfy the safety standard (see steps S9 to S11 in FIG. 10 described later and steps S29 and S30 in FIG. 11).

FIG. 6 is a schematic timing chart showing an example of a method of reducing the amplitude of the slice selection direction gradient magnetic field pulse Gssr to equalize the amplitudes of the slice selection direction gradient magnetic field pulses Gssr and Gssr when the respective amplitudes are different. .
The upper part of FIG. 6 shows a state before the waveform is deformed, and the lower part of FIG. 6 shows a state after the waveform is deformed.

  FIG. 6 shows that in the SE method and the FSE method, the amplitude of the slice selection direction gradient magnetic field pulse Gsse applied simultaneously with the excitation RF pulse is larger than the amplitude of the slice selection direction gradient magnetic field pulse Gssr applied simultaneously with the refocus RF pulse. Lower than the case.

  In this case, processing that reduces the amplitude of the slice selection direction gradient magnetic field pulse Gssr to the amplitude of the slice selection direction gradient magnetic field pulse Gsse so that the time integral value of the gradient magnetic field intensity does not change can be considered. Thereby, the application time width of the slice selection direction gradient magnetic field pulse Gssr becomes long.

  In that case, the transmission time width of the refocusing RF pulse transmitted at the same time as this is made longer so as to match the application time width of the slice selection direction gradient magnetic field pulse Gssr. Since the refocus RF pulse is also deformed so that the time integral value of the pulse intensity does not change, the amplitude decreases as the application time width increases.

  As shown in FIG. 6, when the amplitude of the gradient magnetic field pulse Gssr in the slice selection direction at the same time as the refocus RF pulse is reduced, the slope of the rise and fall becomes gentle, so that the dB / dt value becomes small. Further, since the amplitude of the refocus RF pulse is reduced, the SAR is reduced. Therefore, after the waveform deformation, there is no possibility that the safety standard is not satisfied from the two viewpoints of the SAR and the dB / dt value.

  However, as the application time width of the refocus RF pulse and the slice selection direction gradient magnetic field pulse Gssr becomes longer (2α in the example of FIG. 6), the echo time after the waveform deformation becomes longer as shown in FIG.

  This is because when the interval between the peak timing of the amplitude of the excitation RF pulse and the peak timing of the amplitude of the refocusing RF pulse corresponds to half the echo time, after the transmission of the excitation RF pulse is completed, This is because the time width before the start of transmission (Tune Time in the figure) is fixed in principle. In addition, it is undesirable in principle that the echo time becomes long after the waveform is deformed. This is because problems such as a change in desired contrast and an increase in the time required to execute the pulse sequence occur.

  Therefore, in the present embodiment, as an example, when the amplitude of the slice selection direction gradient magnetic field pulse Gsse applied earlier is lower than the amplitude of the subsequent slice selection direction gradient magnetic field pulse Gssr, the system control unit 61 The amplitude of the excitation RF pulse is increased without performing the waveform deformation as shown in FIG.

  That is, the application time width is shortened so that the time integral value of the intensity of the excitation RF pulse does not change. Specifically, for example, the system control unit 61 can use an RF pulse having an asymmetric waveform with respect to the time at which the amplitude is at the peak. In this case, since the condition of the reference excitation RF pulse changes, the system control unit 61 resets the entire pulse sequence of the main scan.

  FIG. 7 is a schematic diagram conceptually showing the effect of equalizing the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr as shown in FIG. 5, assuming that the Z-axis direction is the slice selection direction.

  7 is a schematic diagram showing the positions of the magnetic field centers MFC and DSV in the gantry 30. The middle part of FIG. 7 is an example of the magnetic field intensity distribution when the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr are different. The lower part of FIG. 7 is an example of the magnetic field intensity distribution when the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr are made uniform as in the present embodiment.

  As shown in the upper part of FIG. 7, in the gantry 30 (in the static magnetic field magnet 31), the magnetic field center MFC is located at the geometric center. That is, assuming that the gantry length in the Z-axis direction is GL, the magnetic field center MFC is located on the axis of the cylindrical bore in the gantry 30 at the position of GL / 2 from both the entrance side and the back side of the gantry 30. I do. Then, a spherical region having a predetermined radius centered on the magnetic field center MFC becomes DSV.

7, the vertical axis indicates the magnetic field strength, the horizontal axis indicates the position in the Z-axis direction, and the DSV between the two broken lines vertically drawn from the upper to lower rows in FIG. The magnetic field intensity distribution in the region of FIG.
As shown in the middle and lower parts of FIG. 7, the linearity of the magnetic field intensity distribution when the gradient magnetic field is applied is almost ensured in the DSV regardless of whether or not the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr are equal. You.

Here, as an example, the middle part of FIG. 7 shows a case where the amplitude of the slice selection direction gradient magnetic field pulse Gsse is larger than the slice selection direction gradient magnetic field pulse Gssr, as in the upper part of FIG. The gradient of the direction gradient magnetic field pulse Gsse has a steeper straight line.
However, regardless of whether or not the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr are equal, the magnetic field intensity distribution when the gradient magnetic field is applied becomes non-linear at the off-center.

  Here, as shown in the middle part of FIG. 7, when the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr are different, the magnetic field intensity distribution in the region outside the DSV shows different nonlinearities when each pulse is applied. .

  More specifically, for example, in imaging in DSV, both the excitation RF pulse and the refocusing RF pulse are commonly set after the pre-scan so that the center frequency is, for example, the Larmor frequency at the center of the magnetic field. Since the Larmor frequency is proportional to the magnetic field strength, when a gradient magnetic field that changes linearly in the Z-axis direction is superimposed and applied to the static magnetic field, the Larmor frequency changes linearly along the Z-axis direction.

  The magnetic field strength Lah in the middle of FIG. 7 means the magnetic field strength that sets the center frequency of the RF pulse to the Larmor frequency. Therefore, the position at which the magnetic field strength Lah is located at the center in the thickness direction of the imaging region (slice or slab). Here, as an example, it is assumed that the range from the magnetic field strength Lah-β to the magnetic field strength Lah + β is excited (selected) by the RF pulse.

  As shown in the middle part of FIG. 7, when the gradient magnetic field distributions at the off-center show different nonlinearities when the gradient magnetic field pulses Gsse and Gssr are applied in the slice selection direction, the following problem occurs.

  That is, the region excited by the excitation RF pulse when the slice selection direction gradient magnetic field pulse Gsse is applied (the region between the two vertical dashed lines in the middle of FIG. 7) and the slice selection direction gradient magnetic field pulse Gssr are applied again. The region excited by the converging RF pulse (the region between the two vertical broken lines in the middle of FIG. 7) causes a mismatch in the magnetic field moment in a region where the gradient magnetic field has poor linearity. This magnetic field moment shift is a main factor of image quality deterioration at off-center.

  On the other hand, in the present embodiment, as shown in the lower part of FIG. 7, the amplitudes of the gradient magnetic field pulses Gsse and Gssr in the slice selection direction are equalized, so that the off-center magnetic field intensity distribution has substantially the same nonlinearity when each pulse is applied. Shows sex. Therefore, the area to be excited is substantially the same between when the excitation RF pulse is transmitted and when the reconverging RF pulse is transmitted, so that the above-described problem of image quality deterioration hardly occurs.

  FIG. 8 is a schematic plan view in the gantry 30 showing an example of a factor of an artifact artifact in a positional relationship with an imaging region. FIG. 8 shows an example in which the right hand of the subject P has been selected as an imaging region (FOV: Field Of View) when the arm RF coil device 100 is mounted.

FIG. 9 is a schematic diagram illustrating an example of a warning display executed by the notification unit 66 and the display device 74 when there is a possibility that an artifact artifact may occur.
Hereinafter, a method for dealing with an artifact artifact according to the present embodiment will be described with reference to FIGS. 8 and 9.

  An artifact artifact is one in which an MR signal from an object that has undergone magnetic resonance outside the imaging region is mixed in the image, depending on various conditions such as the positional relationship between the imaging region and the center of the magnetic field and the arrangement of element coils.

  More specifically, the intensity of the gradient magnetic field in the slice selection direction linearly increases along the slice selection direction, and ideally, no region having the same magnetic field intensity exists in the slice selection direction. However, at the off-center, the magnetic field strength distribution becomes nonlinear as described above. Then, even though the magnetic field strength increases in the slice selection direction from the magnetic field center to the position Ss1 in a non-linear manner, the magnetic field strength decreases as the distance from the magnetic field center further increases from the position Ss1.

  Therefore, for example, when the Z-axis direction is the slice selection direction, a region having the same magnetic field strength as the imaging region may exist at a position Ss2 such as the end of the gantry 30 that is considerably away from the center of the magnetic field. If an element coil that is not turned off is present at the position Ss2, it resonates at the Larmor frequency to generate an MR signal, and this MR signal may be mixed into an image as an artifact artifact.

  Here, an artifact artifact in the case where an off-center image is taken without aligning the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr will be considered. In this case, as described above, since the off-center gradient magnetic field distribution differs between the transmission of the excitation RF pulse and the transmission of the refocusing RF pulse, the region excited by the excitation RF pulse and the refocusing RF pulse As a result, the artifacts degrade as a result.

  However, when the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr are made uniform in off-center imaging as in the present embodiment, the off-center of the excitation RF pulse and the refocus RF pulse are transmitted when the excitation RF pulse is transmitted. The gradient magnetic field distribution becomes almost the same. Then, since the region excited by the excitation RF pulse and the region excited by the reconverging RF pulse do not shift, the anefact artifact also appears largely without deterioration.

  As a countermeasure, if the element coil 120 unnecessary for detecting the MR signal from the imaging region is turned off, the turned off element coil 120 does not resonate when the MR signal is detected, and the factor of an artifact artifact Does not. Here, “turning off” means, for example, turning off the function of detecting the MR signal by interrupting the loop of the current path of the element coil 120.

  In the case of FIG. 8, for example, the six element coils 120 indicated by broken lines outside the FOV are turned off, and the MR signal is detected only by the element coils 120 inside the FOV, so that an artifact artifact can be avoided.

  If a large number of imaging regions set as the main scan include a region outside the DSV and a slice or slab in which an element coil outside the imaging region is selected, the notification unit 66 may be configured as shown in FIG. The following warning display (notification) is executed. In the present embodiment, when imaging a slice or a slab including a region outside the DSV, the amplitudes of the gradient magnetic field pulses Gse and Gssr in the slice selection direction are made uniform, and thus, as described above, an artifact is likely to be large.

  The warning display is performed by, for example, displaying the effect on the display device 74 as character information. In the example of FIG. 9, since the FOV includes an area outside the DSV in the imaging of the hand, a message indicating that the function of detecting the MR signal of the element coil outside the FOV is turned off is displayed.

<Operation description of this embodiment>
For example, in the case of an examination using an RF coil device for the spine, each positioning image of the head, chest, and abdomen is sequentially captured while the top plate 22 is shifted, and each imaging region of the head, chest, and abdomen (in plan view) FOV) may be determined first.

  In this case, after determining the imaging regions of the head, chest, and abdomen, the pulse sequence of the full scan is determined by the algorithm of the present embodiment so that the safety standards of the SAR and the dB / dt value are satisfied. Then, after returning the top board to the position for imaging the head, the main scan of the head, the main scan of the chest, and the main scan of the abdomen are sequentially performed.

  As described above, the order in which the imaging of the positioning image and the determination of the imaging region are collectively executed before the main scan may be performed. However, for each of the head, chest, and abdomen imaging regions, the execution of the main scan from the positioning image imaging is performed. May be executed.

  That is, the MRI apparatus 10 captures the positioning image of the head, determines the imaging region of the head, and then converts the pulse sequence of the main scan of the head according to the algorithm of the present embodiment so as to satisfy a safety standard such as SAR. After the determination, the main scan of the head is executed. Next, the MRI apparatus 10 executes a series of operations from the imaging of the positioning image to the main scan for the chest, and then executes a series of operations from the imaging of the positioning image to the main scan for the abdomen.

  In the case of imaging substantially on the body axis using the spinal RF coil device, as described above, it is easy to fit the imaging region in the DSV by moving the top plate 22. In this case, good image quality can be obtained without applying the above algorithm for aligning the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr between the transmission of the excitation RF pulse and the transmission of the refocus RF pulse. No algorithm needs to be applied.

  Therefore, in the following flowcharts of FIGS. 10 and 11, a case of one-arm inspection will be described as an example. In this case, for example, the hand separates from the body axis and tends to be out of the DSV in the X-axis direction. Therefore, it is highly likely that the algorithm of the present embodiment for equalizing the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr is applied.

  In a case where only some of the slices or slabs of the entire slice or slab include a region outside the DSV, in the present embodiment, the sequence change for equalizing the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr is performed for all slices or slabs. Execute in common. The reason why this is desirable is as follows.

  That is, if the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr are equalized for only one slice, the method of generating the magnetic field by the eddy current is different from that at the time of acquiring MR signals of other slices. In this case, for example, the correction for compensating the magnetic field due to the eddy current is different only for one slice, and a problem arises in that the image quality is not the same as other slices.

  However, the present embodiment is not limited to the above aspect. When at least a part of some slices (slabs) out of all slices or slabs is outside the DSV, only slices (slabs) including a region outside the DSV are subjected to the slice selection direction gradient magnetic field pulses Gse and Gssr. It is not necessary to make the amplitudes uniform and to make the amplitudes of both pulses uniform in collecting MR signals of slices (slabs) in the DSV.

FIG. 10 is a flowchart illustrating an example of a processing flow of the MRI apparatus 10 when a pulse sequence of the SE method is set.
Hereinafter, the operation of the MRI apparatus 10 when the SE method is executed according to the step numbers shown in FIG.

  [Step S1] The system control unit 61 (see FIG. 1) acquires imaging conditions input to the MRI apparatus 10 via the input device 72, and, based on the acquired imaging conditions, imaging conditions ( (Including the condition of the pulse sequence of the SE method for one arm). In addition, the center frequency and the like of the RF pulse are set by a prescan or the like. Thereafter, the process proceeds to step S2.

  [Step S2] The MRI apparatus 10 captures a positioning image. Specifically, the subject P is placed on the top plate 22, and a static magnetic field is formed in the imaging space by the static magnetic field magnet 31 excited by the static magnetic field power supply 42. Also, a current is supplied from the shim coil power supply 44 to the shim coil unit 32, and the static magnetic field formed in the imaging space is made uniform.

  Then, when an imaging start instruction is input from the input device 72 to the system control unit 61, the system control unit 61 inputs imaging conditions including a pulse sequence to the sequence controller 58. The sequence controller 58 drives the gradient magnetic field power supply 46, the RF transmitter 48, and the RF receiver 50 according to the input pulse sequence to form a gradient magnetic field in the imaging region including the imaging region of the subject P, An RF pulse is generated from the RF coil unit.

  Therefore, an MR signal generated by nuclear magnetic resonance in the subject P is detected by the RF coil device for arm 100 (see FIG. 2) and input to the RF receiver 50. The RF receiver 50 performs the above-described processing on the MR signal to generate raw data of the MR signal, and inputs the raw data to the image reconstruction unit 62.

The image reconstruction unit 62 arranges and stores the raw data of the MR signal as k-space data.
The image reconstruction unit 62 reconstructs the image data by performing an image reconstruction process including a Fourier transform on the k-space data, and stores the obtained image data in the image database 63.

  The image processing unit 64 takes in the image data from the image database 63, performs predetermined image processing on the image data, generates display image data of the positioning image, and stores the display image data in the storage device 76. The system control unit 61 causes the display device 74 to display the positioning image indicated by the display image data.

Thereafter, the user determines an imaging area (FOV in a plan view) of the main scan with reference to the positioning image displayed on the display device 74.
After the imaging region of the main scan is determined, the process proceeds to step S3. Here, for simplicity of description, it is assumed that the imaging area of the main scan determined in step S2 is not changed in subsequent steps.

  [Step S3] The system control unit 61 tentatively sets all conditions of the pulse sequence of the SE method based on the imaging region of the main scan determined in step S2 so as to satisfy safety standards such as SAR and dB / dt values. To set.

  After determining whether or not the entire imaging region of the determined main scan is within the DSV, if the determination result is positive, the system control unit 61 may set the pulse sequence as follows. Good.

  Specifically, in the case of imaging within the DSV, the amplitude of the slice selection direction gradient magnetic field pulse Gssr at the time of transmission of the refocus RF pulse and the amplitude of the slice selection direction gradient magnetic field pulse Gsse at the time of transmission of the excitation RF pulse are separated. However, a sufficiently good image quality can be obtained. Further, when the amplitudes of both pulses are far from each other, an artifact artifact is less likely to occur.

  Therefore, when it is estimated that an artifact artifact is likely to occur even in imaging in the DSV, the system control unit 61 intentionally increases the difference between the amplitude of the excitation RF pulse and the amplitude of the refocus RF pulse, Prevent an artifact artifacts.

The following situations are conceivable in the DSV when it is estimated that an artifact is likely to occur.
For example, a case is considered in which an RF coil device is attached to the whole body of the subject P, and a region on the body axis (not including both arms) of the subject P is sequentially imaged from the head to the foot while moving the top plate 22. . In this case, if all the element coils are set to the ON state, for example, in the imaging of the abdomen or the chest, the MR signal near the foot may be mixed as an artifact.

Note that the sequence setting method for avoiding an artifact artifact as described above when the entire imaging region is within the DSV is only an option and is not essential.
Thereafter, the process proceeds to step S4.

[Step S4] The system control unit 61 determines whether there is a slice or slab including a region outside the DSV in the imaging region of the main scan determined up to step S3.
When there is a slice or slab including a region outside the DSV, the process proceeds to step S5.

  If there is no slice or slab including a region outside the DSV, good image quality can be obtained without adjusting the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr, and the process proceeds to step S13.

  [Step S5] Even if the system control unit 61 makes the amplitude of the slice selection direction gradient magnetic field pulse Gssr at the time of transmission of the refocus RF pulse equal to the amplitude of the slice selection direction gradient magnetic field pulse Gsse at the time of transmission of the excitation RF pulse, It is determined whether the echo time can be maintained before and after the change.

  That is, as shown in FIG. 5, when the amplitude of the slice selection direction gradient magnetic field pulse Gssr is smaller than the amplitude of the slice selection direction gradient magnetic field pulse Gsse, the echo time can be equal before and after the change, and the process proceeds to step S8.

  On the other hand, as shown in FIG. 6, when the amplitude of the slice selection direction gradient magnetic field pulse Gssr is higher than the amplitude of the slice selection direction gradient magnetic field pulse Gsse, the process proceeds to step S6. In this case, if the amplitude of the slice selection direction gradient magnetic field pulse Gssr is reduced to the slice selection direction gradient magnetic field pulse Gsse, the echo time becomes longer.

  If the pulse sequence conditions set in step S3 have the same amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr, the process proceeds to step S8. In step S8, the pulse sequence is substantially changed. Is not executed, and the process proceeds to step S9.

  [Step S6] The system control unit 61 shortens the transmission time width of the excitation RF pulse and increases the amplitude so that the time integral value of the intensity of the excitation RF pulse does not change, so that the slice selection direction gradient magnetic field pulse Gsse, The amplitudes of Gssr are made uniform.

In this case, the system control unit 61 can increase the amplitude while maintaining the time integrated value of the intensity by using, for example, an RF pulse having an asymmetric waveform with respect to the time when the intensity is at the peak.
Thereafter, the process proceeds to step S7.

[Step S7] The system control unit 61 resets the entire pulse sequence of the main scan SE method based on the excitation RF pulse conditions changed in step S6 and the imaging region determined in step S2.
Thereafter, the process proceeds to step S9.

  [Step S8] The system control unit 61 increases the amplitude of the slice selection direction gradient magnetic field pulse Gssr to the amplitude of the slice selection direction gradient magnetic field pulse Gsse so that the time integration value of the gradient magnetic field intensity does not change. Thereby, the application time width of the slice selection direction gradient magnetic field pulse Gssr becomes short (see FIG. 5).

  Therefore, the transmission time width of the refocus RF pulse transmitted simultaneously with this is shortened so as to match the application time width of the slice selection direction gradient magnetic field pulse Gssr. At this time, the system control unit 61 increases the amplitude of the refocus RF pulse so that the time integral value of the pulse intensity does not change.

  After changing only the conditions of the refocus RF pulse and the slice selection direction gradient magnetic field pulse Gssr in the pulse sequence as described above, the system control unit 61 proceeds to step S9.

  [Step S9] The safety criterion determination unit 65 calculates the estimated SAR value based on the pulse sequence condition of the main scan after the change in step S8 or step S7, so that the pulse sequence of the main scan after the change is the SAR. It is determined whether safety standards are satisfied.

  Similarly, the safety criterion determination unit 65 calculates the estimated value of the dB / dt value based on the condition of the pulse sequence of the main scan after the change, so that the pulse sequence of the main scan after the change has the dB / dt value. It is determined whether safety standards are satisfied.

If the pulse sequence of the main scan after the change satisfies the safety standards in both the SAR and the dB / dt value, the process proceeds to step S12.
Otherwise, the process proceeds to step S10 to change the conditions of the pulse sequence so as to satisfy the safety standard.

[Step S10] The system control unit 61 increases the transmission time width of the excitation RF pulse so that the time integration value of the intensity of the excitation RF pulse does not change, and reduces the amplitude thereof, so that the slice selection direction gradient magnetic field pulse Gsse, The amplitudes of Gssr are made uniform.
Thereafter, the process proceeds to step S11.

  [Step S11] The system control unit 61 resets the entire pulse sequence of the SE method of the main scan based on the excitation RF pulse conditions changed in step S10 and the imaging region determined in step S2.

  At the time of this resetting, the system control unit 61 suppresses the artifact artifact by turning off the element coil 120 that is separated from the imaging region particularly in the acquisition of the MR signal for the off-center slice or slab (FIG. 8 and FIG. 9).

Specifically, the system control unit 61 causes the sequence controller 58 to input a control signal for turning off the MR signal detection function of the element coil 120 distant from the imaging region to the arm RF coil device via the connection port 25. .
Thereafter, the process returns to step S5.

  [Step S12] In step S6 or step S8, a changing process for aligning the amplitudes of the slice selection direction gradient magnetic field pulses Gssr and Gsse is performed, and the process reaches step S12 only when it is determined in step S9 that the safety criterion is satisfied. .

  In other words, when the process reaches step S12, a region outside the DSV is included in the imaging region, and the correction for equalizing the amplitudes of the gradient magnetic field pulses Gsse and Gssr in the slice selection direction is also performed, so that an artifact artifact occurs. Easy to do. Therefore, in step S11, the element coil 120 that is distant from the imaging region is turned off for the off-center slice or slab.

  For this reason, the notification unit 66 performs a warning display as described in FIG. That is, the notification unit 66 causes the display device 74 to display a message indicating that the element coil which may be a cause thereof is turned off for a slice or a slab including an area outside the DSV since an artifact artifact is likely to occur.

Then, the system control unit 61 determines (determines) the pulse sequence condition of the main scan under the condition provisionally set (the latest condition changed in step S8).
Thereafter, the process proceeds to step S14.

[Step S13] The process reaches step S13 only when the imaging region does not include a region outside the DSV. In this case, good image quality can be obtained without performing a correction for equalizing the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr. Therefore, the system control unit 61 determines (determines) the pulse sequence of the main scan under the conditions set in step S3.
Thereafter, the process proceeds to step S14.

[Step S14] Acquisition of MR signals from the imaging region is executed according to the determined conditions of the pulse sequence of the main scan. The operation of collecting the MR signal is the same as that of the positioning image. Thereafter, as in the case of the positioning image, image reconstruction processing is executed based on the MR signals collected in the main scan to generate main scan image data, and the main scan image is generated based on the image data. It is displayed on the display device 74.
The above is an explanation of an example of the flow in the case of the SE method.

FIG. 11 is a flowchart illustrating an example of a processing flow of the MRI apparatus 10 when a pulse sequence of the FSE method is set.
Hereinafter, the operation of the MRI apparatus 10 when executing the FSE method according to the step numbers shown in FIG.

  [Steps S21 to S24] The processing is the same as the processing of steps S1 to S4 in FIG. 10 except that the conditions of the FSE method are input as the conditions of the pulse sequence. Thereafter, the process proceeds to step S25.

  [Step S25] Even if the system control unit 61 makes the amplitude of the slice selection direction gradient magnetic field pulse Gssr at the time of transmission of the refocus RF pulse equal to the amplitude of the slice selection direction gradient magnetic field pulse Gsse at the time of transmission of the excitation RF pulse, It is determined whether the echo train interval can be maintained.

  Note that the echo train interval of the FSE method is, for example, the timing at which the intensity of the refocusing RF pulse reaches the peak and the timing at which the intensity of the next refocusing RF pulse reaches the peak in the collection of one set of MR signals in FIG. Time interval.

  As shown in FIG. 5, when the amplitude of the slice selection direction gradient magnetic field pulse Gssr is smaller than the amplitude of the slice selection direction gradient magnetic field pulse Gsse, the echo time can be made equal before and after the change. It can be equal (see the pulse sequence of the FSE method in FIG. 4). In this case, the process proceeds to step S28.

  On the other hand, as shown in FIG. 6, when the amplitude of the slice selection direction gradient magnetic field pulse Gssr is higher than the amplitude of the slice selection direction gradient magnetic field pulse Gsse, the echo time becomes longer after the change, and the echo train interval also becomes longer after the change. It will be long. In this case, the process proceeds to step S26.

  If the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr are already the same under the conditions of the pulse sequence set in step S23, the process proceeds to step S28, and the pulse sequence is changed in step S28. Instead, the process proceeds to step S29.

  [Step S26] As described above, the system control unit 61 slightly increases the amplitude by shortening the transmission time width of the excitation RF pulse so that the time integration value of the intensity of the excitation RF pulse does not change. Thereafter, the process proceeds to step S27.

  [Step S27] The system control unit 61 resets the entire pulse sequence of the FSE method in the main scan based on the excitation RF pulse condition changed in step S26 and the imaging region determined in step S22.

  Here, as an example, the system control unit 61 resets the pulse sequence without changing the echo train interval. Here, each application time of the phase encoding gradient magnetic field pulse inserted between the refocusing RF pulse and the next refocusing RF pulse, and the compensating gradient magnetic field pulse in the phase encoding direction in which the polarity is inverted. The width is almost fixed.

  For this reason, when the application time of the gradient magnetic field pulse in the two phase encoding directions is subtracted from the time interval between the end of the transmission of the refocusing RF pulse and the start of the transmission of the next refocusing RF pulse, the remaining MR signal is detected. The time span is possible.

  Therefore, the combination of the upper limit value of the sampling time (the upper limit value of the application time width of the readout gradient magnetic field pulse at the time of detecting the MR signal) and the upper limit value of the transmission time width of the refocus RF pulse is determined by the echo train interval. Trade-off conditions). This is because there is a trade-off between the upper limit of the sampling time and the upper limit of the transmission time width of the refocus RF pulse. That is, if the sampling time is lengthened, the transmission time width of the refocus RF pulse and the application time width of the slice selection direction gradient magnetic field pulse Gssr become shorter.

The system control unit 61 resets the pulse sequence so as to satisfy a trade-off condition between the upper limit of the sampling time determined by the echo train interval and the upper limit of the transmission time width of the refocus RF pulse.
Thereafter, the process returns to step S25.

  [Step S28] The system control unit 61 increases the amplitude of the slice selection direction gradient magnetic field pulse Gssr to the amplitude of the slice selection direction gradient magnetic field pulse Gsse so that the time integration value of the gradient magnetic field intensity does not change. The system control unit 61 shortens the transmission time width of the simultaneously transmitted reconvergent RF pulse so as to match the application time width of the slice selection direction gradient magnetic field pulse Gssr which becomes shorter. At this time, the system control unit 61 increases the amplitude of the refocus RF pulse so that the time integral of the pulse intensity does not change.

  The system control unit 61 changes only the conditions of the refocus RF pulse and the slice selection direction gradient magnetic field pulse Gssr in the pulse sequence as described above, and then proceeds to step S29.

  [Step S29] The safety standard determination unit 65 determines whether or not the pulse sequence of the main scan changed in step S28 satisfies all the safety standards of the SAR and the dB / dt values as in step S9 in FIG. .

  If the pulse sequence of the main scan after the change satisfies the safety standards in both the SAR and the dB / dt value, the process proceeds to step S31; otherwise, the process proceeds to step S30.

  [Step S30] First, the system control unit 61 executes one of the following first change processing and second change processing.

  In the first change process, similarly to step S10 in FIG. 10, the amplitude of the excitation RF pulse is slightly reduced by increasing the transmission time width of the excitation RF pulse so that the time integral value of the intensity of the excitation RF pulse does not change.

  Here, as an example, when the dB / dt value does not satisfy the safety standard in the determination in step S29, the first change process is selected. If the transmission time width of the excitation RF pulse is made longer, the rising and falling gradients become gentler and the dB / dt value becomes lower, because the application time width of the simultaneously applied slice selection direction gradient magnetic field pulse Gsse becomes longer. Because.

  The second change process is a process for increasing the echo train interval. Here, as an example, it is assumed that the second change process can be selected only when the dB / dt value satisfies the safety standard but the SAR does not satisfy the safety standard and the execution time of the entire pulse sequence can be extended. . This is because in the FSE method, the SAR is reduced by increasing the interval of the echo train.

  Next, based on the excitation RF pulse condition or the echo train interval changed in the first or second change processing and the imaging area determined in step S22, the system control unit 61 performs the FSE method of the main scan. Reset the entire pulse sequence.

  At the time of this resetting, the system control unit 61 turns off the element coil 120 which is separated from the imaging region particularly in the acquisition of the MR signal for the off-center slice or slab, similarly to step S11 in FIG. Suppress artifacts (see FIGS. 8 and 9).

Further, at the time of this resetting, the system control unit 61 satisfies the pattern (trade-off condition) of the combination of the upper limit of the sampling time determined by the echo train interval and the upper limit of the transmission time width of the refocusing RF pulse. , Reset the pulse sequence.
Thereafter, the process returns to step S25.

[Step S31] When the process reaches step S31, an area outside the DSV is included in the imaging area, and correction is performed to equalize the amplitudes of the gradient magnetic field pulses Gsse and Gssr in the slice selection direction, so that an artifact is generated. easy.
For this reason, the notification unit 66 executes a warning display as in step S12 of FIG.

In addition, the system control unit 61 determines (determines) the pulse sequence condition of the main scan under the provisionally set condition (the latest condition determined to be variable in step S29).
Thereafter, the process proceeds to step S33.

  [Step S32] When the process reaches step S32, since the area outside the DSV is not included in the imaging area, the system control unit 61 determines (determines) the pulse sequence of the main scan under the conditions set in step S23. Thereafter, the process proceeds to step S33.

[Step S33] After the main scan is executed as described above according to the determined conditions of the main scan pulse sequence, image reconstruction processing and image display of the main scan are executed.
The above is the description of the operation of the MRI apparatus 10 of the present embodiment.

<Effect of this embodiment>
In the present embodiment, when at least a part of the imaging region is outside the DSV, the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr are made uniform, so that the off-center magnetic field intensity distribution can be substantially reduced at the time of applying both pulses. It is based on an extremely innovative technical idea of doing the same (see FIGS. 5 and 7).

  Therefore, there is almost no difference between the region excited by the excitation RF pulse when the slice selection direction gradient magnetic field pulse Gsse is applied and the region excited by the refocus RF pulse when the slice selection direction gradient magnetic field pulse Gssr is applied. Therefore, in off-center imaging, slice selection is performed satisfactorily, so that image quality can be improved as compared with the related art.

  As a result, the image quality is poor after off-center imaging, and there is no time loss in which the user manually changes the imaging conditions and performs imaging again.

  Here, the effect of the present embodiment, in which the region excited by the excitation RF pulse and the region excited by the refocusing RF pulse are not so shifted off center, is more likely to cause an artifact artifact.

  Therefore, in the present embodiment, when an artifact artifact is likely to occur, a warning display is executed as shown in FIG. Further, in the present embodiment, since the element coil 120 which is located outside the FOV and may cause an artifact artifact is turned off, the artifact artifact can be avoided.

  As described above, in the present embodiment, the imaging conditions are automatically optimized by the MRI apparatus 10, so that sufficient image quality can be obtained even at an off-center while suppressing an artifact artifact. As a result, there is no need for the user himself to manually search for the optimal imaging conditions, and the convenience for the user is greatly improved.

  According to the above-described embodiment, the image quality can be improved in an imaging region far from the center of the magnetic field by using an MRI technique different from the related art.

<Supplementary information of this embodiment>
Hereinafter, modified examples and supplementary points of the above embodiment will be described.
[1] In the above-described embodiment, in the SE method and the FSE method, an example is described in which the amplitudes of the gradient magnetic field pulses Gsse and Gssr in the slice selection direction are equalized when transmitting the excitation RF pulse and when transmitting the refocusing RF pulse. Embodiments of the present invention are not limited to such an aspect.
The above-described pulse sequence changing algorithm can be applied to other pulse sequences of a spin echo system such as a spin echo single shot EPI (Echo Planar Imaging) and a spin echo multi shot EPI.

  [2] In the above embodiment, an example has been described in which the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr are completely equal between the transmission of the excitation RF pulse and the transmission of the refocusing RF pulse. Embodiments of the present invention are not limited to such an aspect. The amplitudes of both pulses need not be completely equal.

  To be more effective, in the vicinity of the off-center imaging region, the slice selection direction gradient magnetic field distribution at the time of transmitting the excitation RF pulse and the slice selection direction at the time of transmitting the refocusing RF pulse, to the extent that the image quality is improved more than before. What is necessary is just to make the direction gradient magnetic field distribution close to each other. That is, it is also within the scope of the technical idea of the present embodiment to execute a sequence change that brings the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr closer to each other to the extent that the image quality is improved as compared with the related art.

  [3] The example in which the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr are made uniform when at least a part of one slice or one slab is out of the DSV has been described. Embodiments of the present invention are not limited to such an aspect.

  Even if a slice or slab part of which is outside the DSV exists in a region that does not cause a problem in diagnosis, the image quality is not improved by aligning the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr. This is because it may be.

  Therefore, for example, when at least one slice or slab whose entirety is outside the DSV is included in a large number of slices or slabs as the imaging region, even if the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr are aligned. Good.

  Alternatively, the input device 72, the display device 74, and the system control unit 61 may be configured as follows as conditions for proceeding to step S5 and step S25 as YES after each determination in step S4 in FIG. 10 and S24 in FIG. Good.

  That is, the display device 74 superimposes and displays the outer edge of the image of the slice or slab including the area outside the DSV on the positioning image. Thereafter, when the user selects, via the input device 72, a process for equalizing the amplitudes of the slice selection direction gradient magnetic field pulses Gsse and Gssr, the system control unit 61 proceeds to steps S5 and S25.

  According to the MRI apparatus and the MRI method of at least one embodiment described above, the image quality can be improved in an imaging region apart from the center of the magnetic field.

[4] Correspondence between claim terms and embodiments will be described. Note that the correspondence shown below is one interpretation shown for reference, and does not limit the present invention.
The function of each component in the gantry 30 and the whole of the control device 40 (see FIG. 1) to collect MR signals from the subject P by imaging with application of a gradient magnetic field and an RF pulse is described in claims. It is an example of a signal collection unit.

The system control unit 61 that sets the conditions of the pulse sequence is an example of the imaging condition setting unit described in the claims.
The input device 72 that receives the input of the condition of the pulse sequence and transmits the input condition to the system control unit 61 is an example of the input unit described in the claims.

The slice selection direction gradient magnetic field pulse Gsse in FIGS. 5 and 6 is an example of the first gradient magnetic field pulse described in the claims.
The slice selection direction gradient magnetic field pulse Gssr in FIGS. 5 and 6 is an example of the second gradient magnetic field pulse described in the claims.
The DSV is an example of a region having a predetermined radius from the center of the magnetic field described in the claims.

  [5] While some embodiments of the present invention have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. These embodiments can be implemented in other various forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and equivalents thereof.

10: MRI apparatus,
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: arithmetic device 61: system control unit, 65: safety standard stabilizing unit, 66: notification unit, 72: input device

Claims (5)

Whether the entire imaging region is inside a region having a predetermined magnetic field uniformity, or a determination unit that determines whether at least a part of the imaging region is outside the region,
When the entire imaging region is determined to be inside the region having the predetermined magnetic field uniformity, the amplitude of the first gradient magnetic field pulse in the slice selection direction applied simultaneously with the excitation RF pulse and the refocus RF pulse An imaging condition setting unit that changes the condition of the pulse sequence so as to separate the amplitude of the second gradient magnetic field pulse in the slice selection direction to be applied;
A magnetic resonance imaging apparatus comprising: An input unit for receiving an input of a pulse sequence condition;
Whether the entire imaging region is inside a region having a predetermined magnetic field uniformity, or a determination unit that determines whether at least a part of the imaging region is outside the region,
When the entire imaging region is determined to be inside the region having the predetermined magnetic field uniformity, the amplitude of the first gradient magnetic field pulse in the slice selection direction applied simultaneously with the excitation RF pulse and the refocus RF pulse The pulse sequence is set such that the difference between the applied amplitude and the second gradient magnetic field pulse in the slice selection direction is larger than the difference between the respective amplitudes set based on the conditions of the pulse sequence received by the input unit. An imaging condition setting unit for changing the conditions of
A magnetic resonance imaging apparatus comprising: An input unit for receiving an input of a pulse sequence condition;
When the imaging region is inside a region having a predetermined magnetic field uniformity, the amplitude of the first gradient magnetic field pulse in the slice selection direction applied simultaneously with the excitation RF pulse and the slice selection applied simultaneously with the refocus RF pulse The condition of the pulse sequence is changed so that the difference between the amplitude of the second gradient magnetic field pulse in the direction and the amplitude of each pulse set based on the condition of the pulse sequence received by the input unit is larger. An imaging condition setting unit;
With
The imaging condition setting unit changes the pulse sequence condition when the pulse sequence condition input by the input unit determines that there is a possibility that an artifact artifact may occur in the imaging region.
Magnetic resonance imaging device. The determination that the an artifact artifact may occur is performed based on an ON state of an element coil included in the RF coil device attached to the subject.
The magnetic resonance imaging apparatus according to claim 3. Whether the entire imaging region is inside a region having a predetermined magnetic field uniformity, or a determination unit that determines whether at least a part of the imaging region is outside the region,
If it is determined that the entire imaging region is inside the region having the predetermined magnetic field uniformity, the amplitude of the first gradient magnetic field pulse in the slice selection direction applied simultaneously with the excitation RF pulse and the refocus RF pulse An imaging condition setting unit that adjusts the difference between the applied amplitude of the second gradient magnetic field pulse in the slice selection direction to be large and sets the imaging condition;
With the device.

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