JP6109508B2 - Magnetic resonance imaging system - Google Patents

Description

  Embodiments described herein relate generally to a magnetic resonance imaging (MRI) apparatus.

  The MRI apparatus magnetically excites the nuclear spin of the subject placed in a static magnetic field with a radio frequency (RF) signal of Larmor frequency, and generates magnetic resonance (MR) generated by this excitation. An image diagnostic apparatus that reconstructs an image from a signal.

  The waveform of the current output from the gradient magnetic field power supply of the MRI apparatus to the gradient coil is a trapezoidal wave that typically rises linearly and falls linearly after reaching a constant value. Therefore, the waveform of the gradient magnetic field applied from the gradient magnetic field coil to the imaging region is also a waveform corresponding to the waveform of the current output to the gradient magnetic field coil.

  In a high-speed imaging method such as an echo planar imaging (EPI) method, the MR echo signal may be sampled during the rising of the readout gradient magnetic field in order to shorten the imaging time. When sampling is performed in a period in which the gradient magnetic field strength is constant, MR data is sampled at equal intervals in time to collect MR data at equal intervals in the k space (phase space). be able to.

  On the other hand, when MR echo signals are sampled from when the gradient magnetic field for reading is rising, if sampling is performed at regular intervals in time, MR data at irregular intervals will be collected in the k space. . For this reason, after collecting MR data, a regridding process including an interpolation process based on the waveform of the current output from the gradient magnetic field power source to the gradient magnetic field coil is executed. Specifically, k-space data equivalent to MR data collected at unequal intervals in time so that the product of the current value output to the gradient coil and the sampling interval of MR data is constant is obtained by regridding processing. Generated from MR data collected at regular intervals in time. Thereby, k-space data at equal intervals on the k-space is obtained for generating image data.

JP 2010-172383 A

  In MRI, not only the high-speed imaging method as described above, it is desired to apply a gradient magnetic field having a waveform more suitable for imaging.

  Accordingly, an object of the present invention is to provide a magnetic resonance imaging apparatus capable of applying a gradient magnetic field having a waveform more suitable for MR imaging.

A magnetic resonance imaging apparatus according to an embodiment of the present invention includes an imaging unit and a condition setting unit. The imaging means performs magnetic resonance imaging of a subject using a gradient magnetic field generation system including a gradient magnetic field coil and a gradient magnetic field power supply. The condition setting means uses the gradient magnetic field power supply for the magnetic resonance imaging based on the waveform of the first gradient magnetic field applied to the imaging region in accordance with the waveform of the first current output from the gradient magnetic field power supply. At least one of the waveform of the output second current and the waveform of the second gradient magnetic field applied to the imaging region is set or presented. Further, the condition setting means is configured to reduce the index indicating the error between the waveform of the current output from the gradient magnetic field power supply and the waveform of the gradient magnetic field applied to the imaging region, so that the second current waveform is reduced. And at least one of the waveforms of the second gradient magnetic field.
The magnetic resonance imaging apparatus according to the embodiment of the present invention includes an imaging unit and a condition setting unit. The imaging means performs magnetic resonance imaging of a subject using a gradient magnetic field generation system including a gradient magnetic field coil and a gradient magnetic field power supply. The condition setting means uses the gradient magnetic field power supply for the magnetic resonance imaging based on the waveform of the first gradient magnetic field applied to the imaging region in accordance with the waveform of the first current output from the gradient magnetic field power supply. At least one of the waveform of the output second current and the waveform of the second gradient magnetic field applied to the imaging region is set or presented. Further, the condition setting means uses the gradient magnetic field waveform obtained by the simulation based on the first current waveform as the first gradient magnetic field waveform, and the waveform of the current output from the gradient magnetic field power source. And at least one of the waveform of the second current and the waveform of the second gradient magnetic field is determined so that an index representing an error between the waveform and the waveform of the gradient magnetic field applied to the imaging region is reduced. The

1 is a configuration diagram of a magnetic resonance imaging apparatus according to an embodiment of the present invention. The functional block diagram of the computer shown in FIG. The figure which shows an example of the EPI sequence set in the imaging condition setting part shown in FIG. The figure which shows an example of the 2nd current waveform and 2nd gradient magnetic field waveform which were produced | generated based on the 1st current waveform and the 1st gradient magnetic field waveform in the gradient magnetic field waveform setting part shown in FIG. The sequence chart which shows an example of the pulse sequence for acquisition of the waveform of the gradient magnetic field set in the imaging condition setting part shown in FIG. The circuit diagram which shows the detailed structural example of the gradient magnetic field generation system containing the gradient magnetic field power supply and gradient magnetic field coil shown in FIG. The circuit diagram which shows an example of the equivalent circuit model of the gradient magnetic field generation system shown in FIG. 3 is a flowchart showing a flow when imaging is performed in consideration of an error between an output current waveform from a gradient magnetic field power supply and a gradient magnetic field waveform by the magnetic resonance imaging apparatus shown in FIG. 1.

  A magnetic resonance imaging apparatus according to an embodiment of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a configuration diagram of a magnetic resonance imaging apparatus according to an embodiment of the present invention.

  The magnetic resonance imaging apparatus 20 includes a cylindrical static magnetic field magnet 21 that forms a static magnetic field, a shim coil 22, a gradient magnetic field coil 23, and an RF coil 24 provided inside the static magnetic field magnet 21.

  In addition, the magnetic resonance imaging apparatus 20 includes a control system 25. The control system 25 includes a static magnetic field power supply 26, a gradient magnetic field power supply 27, a shim coil power supply 28, a transmitter 29, a receiver 30, a sequence controller 31, and a computer 32. The gradient magnetic field power source 27 of the control system 25 includes an X-axis gradient magnetic field power source 27x, a Y-axis gradient magnetic field power source 27y, and a Z-axis gradient magnetic field power source 27z. In addition, the computer 32 includes an input device 33, a display device 34, an arithmetic device 35, and a storage device 36.

  The static magnetic field magnet 21 is connected to a static magnetic field power supply 26 and has a function of forming a static magnetic field in the imaging region by a current supplied from the static magnetic field power supply 26. In many cases, the static magnetic field magnet 21 is composed of a superconducting coil, and is connected to the static magnetic field power supply 26 at the time of excitation and supplied with current. It is common. In some cases, the static magnetic field magnet 21 is composed of a permanent magnet and the static magnetic field power supply 26 is not provided.

  A cylindrical shim coil 22 is coaxially provided inside the static magnetic field magnet 21. The shim coil 22 is connected to the shim coil power supply 28, and is configured such that a current is supplied from the shim coil power supply 28 to the shim coil 22 to make the static magnetic field uniform.

  The gradient magnetic field coil 23 includes an X-axis gradient magnetic field coil 23 x, a Y-axis gradient magnetic field coil 23 y, and a Z-axis gradient magnetic field coil 23 z, and is formed in a cylindrical shape inside the static magnetic field magnet 21. A bed 37 is provided inside the gradient magnetic field coil 23 as an imaging region, and the subject P is set on the bed 37. The RF coil 24 includes a whole body coil (WBC) for transmitting and receiving an RF signal built in the gantry, a bed 37 and a local coil for receiving an RF signal provided near the subject P.

  The gradient magnetic field coil 23 is connected to a gradient magnetic field power supply 27. The X-axis gradient magnetic field coil 23x, the Y-axis gradient magnetic field coil 23y, and the Z-axis gradient magnetic field coil 23z of the gradient magnetic field coil 23 are respectively an X-axis gradient magnetic field power supply 27x, a Y-axis gradient magnetic field power supply 27y, and a Z-axis gradient magnetic field coil 27z. It is connected to the magnetic field power supply 27z.

  The X-axis gradient magnetic field power source 27x, the Y-axis gradient magnetic field power source 27y, and the Z-axis gradient magnetic field power source 27z are supplied with currents supplied to the X-axis gradient magnetic field coil 23x, the Y-axis gradient magnetic field coil 23y, and the Z-axis gradient magnetic field coil 23z, respectively. In the imaging region, a gradient magnetic field Gx in the X-axis direction, a gradient magnetic field Gy in the Y-axis direction, and a gradient magnetic field Gz in the Z-axis direction can be formed, respectively.

  The RF coil 24 is connected to at least one of the transmitter 29 and the receiver 30. The transmission RF coil 24 has a function of receiving an RF signal from the transmitter 29 and transmitting it to the subject P, and the reception RF coil 24 is accompanied by excitation of the nuclear spin inside the subject P by the RF signal. It has a function of receiving the NMR signal generated in this way and giving it to the receiver 30.

  On the other hand, the sequence controller 31 of the control system 25 is connected to the gradient magnetic field power supply 27, the transmitter 29 and the receiver 30. The sequence controller 31 is control information necessary for driving the gradient magnetic field power supply 27, the transmitter 29, and the receiver 30, for example, 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 27. And the gradient magnetic field power supply 27, the transmitter 29 and the receiver 30 are driven according to the stored predetermined sequence to drive the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, and the Z-axis gradient magnetic field. It has the function of generating Gz and RF signals.

  The sequence controller 31 is configured to receive raw data, which is complex data obtained by detection of an NMR signal and A / D (analog to digital) conversion in the receiver 30, and supply the received raw data to the computer 32. The

  For this reason, the transmitter 29 is provided with a function of applying an RF signal to the RF coil 24 based on the control information received from the sequence controller 31, while the receiver 30 detects the NMR signal received from the RF coil 24. Then, by executing required signal processing and A / D conversion, a function of generating raw data that is digitized complex data and a function of supplying the generated raw data to the sequence controller 31 are provided.

  Further, the computer 32 is provided with various functions by executing the program stored in the storage device 36 of the computer 32 by the arithmetic unit 35. However, a specific circuit having various functions may be provided in the magnetic resonance imaging apparatus 20 instead of at least a part of the program.

  FIG. 2 is a functional block diagram of the computer 32 shown in FIG.

  The computing device 35 of the computer 32 functions as the imaging condition setting unit 40 and the data processing unit 41 by executing a program stored in the storage device 36. The imaging condition setting unit 40 includes a gradient magnetic field waveform setting unit 40A, a gradient magnetic field waveform simulation unit 40B, and a gradient magnetic field waveform acquisition unit 40C. The storage device 36 functions as a k-space data storage unit 42, an image data storage unit 43, and a gradient magnetic field waveform storage unit 44.

  The imaging condition setting unit 40 has a function of setting imaging conditions including a pulse sequence based on instruction information from the input device 33 and outputting the set imaging conditions to the sequence controller 31.

  FIG. 3 is a diagram showing an example of an EPI sequence set in the imaging condition setting unit 40 shown in FIG.

In FIG. 3, the horizontal axis represents time, RF represents an RF transmission pulse, G _SS represents a slice selection (SS) gradient magnetic field pulse, G _PE represents a phase encode (PE) gradient magnetic field pulse, and G _RO Indicates a readout (RO: readout) gradient magnetic field pulse, and ECHO indicates an MR received echo signal.

  As shown in FIG. 3, the EPI sequence repeats a PE gradient pulse and an RO gradient pulse after exciting a selected slice by applying an SS gradient pulse by applying a 90 ° RF pulse and a 180 ° RF pulse. This is a pulse sequence in which a plurality of MR reception echo signals are collected continuously from a selected slice by applying. The time from the time when the 90 ° RF pulse is applied to the time when the echo signal is collected at the center of the k-space is called the effective echo time (effective TE).

  The pulse sequence set in the imaging condition setting unit 40 is used for controlling the imaging system such as the gradient magnetic field power supply 27 and the transmitter 29 by the sequence controller 31. That is, current output instruction information having a waveform determined by a pulse sequence is output from the sequence controller 31 to the gradient magnetic field power supply 27 and the transmitter 29 as a control signal.

  A gradient magnetic field having a waveform corresponding to the pulse sequence is applied to the imaging region by a gradient magnetic field generation system including the gradient magnetic field coil 23 and the gradient magnetic field power supply 27. On the other hand, an RF magnetic field pulse having a waveform corresponding to the pulse sequence is applied to the imaging region by the RF magnetic field generation system including the transmitter 29 and the transmission RF coil 24. Then, MR imaging of the subject P can be performed using an imaging system including a gradient magnetic field generation system and an RF magnetic field generation system.

  Such a pulse sequence is based on the assumption that the waveform of the current output from the gradient magnetic field power supply 27 to the gradient magnetic field coil 23 and the waveform of the gradient magnetic field applied from the gradient magnetic field coil 23 to the imaging region are proportional. Designed. For example, assuming that the waveform of the current output from the gradient magnetic field power supply 27 is a trapezoidal wave as in the EPI sequence illustrated in FIG. 3, the waveform of the gradient magnetic field applied to the imaging region is also a trapezoidal wave. A pulse sequence is set. The same applies to a rectangular wave.

  However, it has been found by simulation and measurement of the gradient magnetic field that the waveform of the gradient magnetic field applied to the imaging region is not strictly similar to the waveform of the current output from the gradient magnetic field power supply 27. That is, the waveform of the current output from the gradient magnetic field power supply 27 and the waveform of the gradient magnetic field applied to the imaging region are not proportional, and the gradient magnetic field waveform is distorted with respect to the current waveform.

  This may be because the characteristics of the gradient magnetic field generation system are not exactly the ideal characteristics as design values. Typical causes include that the resistance of the gradient coil 23 varies from channel to channel, and that the response of the gradient coil 23 changes depending on the frequency of the current together with the resistance and inductance due to mutual induction.

  In the EPI sequence, the frequency of the current output to the gradient coil 23 corresponds to the echo interval (ETS: echo train space). Therefore, in the case of the EPI sequence, the strain amount of the gradient magnetic field changes depending on the ETS. In particular, in the EPI sequence, if the rising waveform of the gradient magnetic field is set to a 1/4 sine waveform, the amount of gradient magnetic field distortion becomes significant.

  As described above, when the waveform of the gradient magnetic field is distorted and an error from the current waveform becomes large, the gradient magnetic field power supply 27 consumes useless power. That is, the performance of the gradient magnetic field generation system is not fully utilized.

  In particular, in a pulse sequence for high-speed imaging such as an EPI sequence, MR data is collected even during the rising period of the RO gradient magnetic field pulse that is a trapezoidal wave in order to shorten the data collection time. In this case, if MR data collected at equal intervals in time is arranged in k space, it becomes spatially unequal, and therefore regridding processing for obtaining k space data at equal intervals in k space is executed.

  However, the regridding process is executed based on the waveform of the current output from the gradient magnetic field power supply 27. That is, in the regridding process, the distortion of the gradient magnetic field waveform with respect to the current waveform is not considered. For this reason, when the distortion of the gradient magnetic field waveform with respect to the current waveform cannot be ignored, it is difficult to generate accurate k-space data by the regridding process. In addition, when the error of the gradient magnetic field waveform with respect to the current waveform increases and the area of the gradient magnetic field waveform decreases, the image quality deteriorates and artifacts are generated.

  Therefore, the gradient magnetic field waveform setting unit 40A of the imaging condition setting unit 40 is based on the waveform of the first gradient magnetic field applied to the imaging region in accordance with the waveform of the first current output from the gradient magnetic field power supply 27. A function for setting or presenting at least one of the waveform of the second current output from the gradient magnetic field power supply 27 for MR imaging and the waveform of the second gradient magnetic field applied to the imaging region is provided. Specifically, the gradient magnetic field waveform setting unit 40A is configured to reduce the index indicating the error between the waveform of the current output from the gradient magnetic field power supply 27 and the waveform of the gradient magnetic field applied to the imaging region. Is configured to set or present at least one of the current waveform and the second gradient magnetic field waveform.

  FIG. 4 is a diagram showing an example of the second current waveform and the second gradient magnetic field waveform generated based on the first current waveform and the first gradient magnetic field waveform in the gradient magnetic field waveform setting unit 40A shown in FIG. It is.

  In each graph of FIG. 4, the horizontal axis represents time, and the vertical axis represents the relative amplitude of the current and gradient magnetic field scales. FIG. 4A shows a first current waveform initially set in the imaging condition setting unit 40 and a first gradient magnetic field waveform corresponding to the first current waveform. FIG. 4B shows the second current waveform and the second gradient magnetic field waveform generated as the correction waveform of the first current waveform and the first gradient magnetic field waveform in the gradient magnetic field waveform setting unit 40A.

  As shown in FIG. 4A, even if the RO gradient magnetic field pulse is defined as a trapezoidal wave in the pulse sequence and the first current is output from the gradient magnetic field power supply 27, the first gradient magnetic field applied to the imaging region. In this waveform, distortion occurs with respect to the first current waveform. The same applies to other waveforms such as a rectangular wave. Even if a current is output as a rectangular wave, the gradient magnetic field does not become an accurate rectangular wave.

  Therefore, as shown in FIG. 4B, by performing a correction process for reducing an error of the first gradient magnetic field waveform with respect to the first current waveform, as shown in FIG. A gradient magnetic field waveform can be obtained. Specifically, a second current waveform corrected to smoothly change a portion where the amplitude changes discontinuously is obtained. Therefore, if the first current waveform that is the initial waveform is a trapezoidal wave, the waveform in which the rising portion is blunted becomes the second current waveform.

  Since the second gradient magnetic field waveform is a waveform corresponding to the second current waveform, the second gradient magnetic field waveform is different from the first gradient magnetic field waveform.

  In the gradient magnetic field waveform setting unit 40A, either the second current waveform or the second gradient magnetic field waveform may be obtained, or a combination of the second current waveform and the second gradient magnetic field waveform may be obtained. May be. The obtained second current waveform and second gradient magnetic field waveform can be automatically set as imaging conditions. Alternatively, the second current waveform and the second gradient magnetic field waveform may be once presented on the display device 34 as candidates for imaging conditions, and the user may be prompted to confirm the operation by operating the input device 33.

  The first and second gradient magnetic field waveforms shown in FIG. 4 are waveforms calculated by simulation based on the first and second current waveforms, respectively. As illustrated in FIG. 4, the gradient magnetic field waveform corresponding to the current waveform can be obtained by a gradient magnetic field waveform simulation that appropriately models a gradient magnetic field generation system including the gradient coil 23 and the gradient magnetic field power supply 27. . Therefore, the waveform of the first gradient magnetic field that is the original data for setting or presenting at least one of the waveform of the second current and the waveform of the second gradient magnetic field is acquired by simulation based on the waveform of the first current. A gradient magnetic field waveform can be used.

  However, the waveform of the gradient magnetic field corresponding to the waveform of the current output from the gradient magnetic field power supply 27 can be obtained by other methods without being limited to the simulation. As a specific example, there is a method of outputting a pulse sequence to the sequence controller 31 to output a current having a waveform actually targeted from the gradient magnetic field power supply 27 and measuring a gradient magnetic field applied to the imaging region. As another specific example, by outputting a pulse sequence to the sequence controller 31, the current of the waveform actually targeted is output from the gradient magnetic field power supply 27, and the MR signal collected in a state where the gradient magnetic field is applied is output. A method of calculating a gradient magnetic field based on the above is mentioned.

  That is, the gradient magnetic field generating system applies a gradient magnetic field having the first gradient magnetic field waveform to the imaging region by actually outputting a current having the first current waveform from the gradient magnetic field power supply 27, and applies it to the imaging region. Based on the waveform of the first gradient magnetic field, at least one of the waveform of the second current and the waveform of the second gradient magnetic field can be set or presented.

  Therefore, the waveform of the first gradient magnetic field required for obtaining at least one of the waveform of the second current and the waveform of the second gradient magnetic field includes the waveform of the gradient magnetic field acquired by the gradient magnetic field waveform simulation, Based on the MR signal collected by actually outputting the waveform of the gradient magnetic field measured by actually outputting the waveform of one current from the gradient magnetic field power supply 27 or the waveform of the first current from the gradient magnetic field power supply 27. Waveforms acquired by various methods such as the waveform of the gradient magnetic field obtained in this way can be used.

  Therefore, the gradient magnetic field waveform simulation unit 40B has a function of obtaining a gradient magnetic field waveform corresponding to the waveform of the current output from the gradient magnetic field power supply 27 by the gradient magnetic field waveform simulation. The gradient magnetic field waveform acquisition unit 40C has a function of obtaining a gradient magnetic field waveform corresponding to the waveform of the current output from the gradient magnetic field power supply 27 by an arbitrary method other than the gradient magnetic field waveform simulation. The gradient magnetic field waveform obtained by the gradient magnetic field waveform simulation unit 40B or the gradient magnetic field waveform acquisition unit 40C is configured to be provided to the gradient magnetic field waveform setting unit 40A.

  The gradient magnetic field waveform can be measured using a gradient magnetic field measurement system including a search coil and an integrator. In that case, as shown in FIG. 2, the magnetic resonance imaging apparatus 20 includes a search coil 50, an integrator 51, an amplifier 52, and an A / D converter 53.

  The search coil 50 is installed in an imaging region to which a gradient magnetic field is applied. Then, the differential component of the gradient magnetic field is detected by the search coil 50. The differential component of the gradient magnetic field detected by the search coil 50 is output to the integrator 51. Then, the waveform of the gradient magnetic field is reproduced by integration processing of the differential component of the gradient magnetic field in the integrator 51.

  The waveform of the gradient magnetic field acquired by the integrator 51 is amplified by the amplifier 52 and then converted into digital information by the A / D converter 53. The digital magnetic field gradient magnetic field waveform is output from the A / D converter 53 to the gradient magnetic field waveform acquisition unit 40C of the computer 32.

  On the other hand, the gradient magnetic field waveform can also be calculated based on the MR signal collected according to the pulse sequence for acquiring the gradient magnetic field waveform. Therefore, the imaging condition setting unit 40 has a function of setting a pulse sequence for acquiring a gradient magnetic field waveform as a data acquisition condition and outputting the data acquisition condition including the set pulse sequence to the sequence controller 31.

  The gradient magnetic field waveform acquisition unit 40C acquires MR signals collected according to the pulse sequence for acquiring the gradient magnetic field waveform from the sequence controller 31 through the data processing unit 41, and based on the acquired MR signals, A function for obtaining a waveform is provided.

  FIG. 5 is a sequence chart showing an example of a pulse sequence for acquiring the gradient magnetic field waveform set in the imaging condition setting unit 40 shown in FIG.

  In FIG. 5, the horizontal axis represents time, RF represents an RF transmission pulse and a data acquisition period, and G represents a gradient magnetic field pulse. As shown in FIG. 5, after applying the SS gradient magnetic field pulse Gss together with the RF excitation pulse, a pulse sequence in which the encode gradient magnetic field pulse Gek and the gradient magnetic field Gmeasured to be measured are sequentially applied, and the intensity of the encode gradient magnetic field pulse Gek is set. The MR signal for acquiring the gradient magnetic field waveform can be collected by repeating the process while changing it stepwise.

  The MR signal acquisition period is the period during which the gradient magnetic field Gmeasured to be measured is applied, and time-series free induction decay (FID) at time t (t = t1, t2, t3, ..., tm) ) A signal is collected. Therefore, when the intensity of the encode gradient magnetic field pulse Gek is changed stepwise from Gek = Ge1, Ge2, Ge3,..., Gen, m × n MR data S (t, Gek) are collected.

Then, the gradient magnetic field waveform acquisition unit 40C can obtain the gradient magnetic field Gmeasured to be measured based on the MR data S (t, Gek). Specifically, k-space data D (t, Gek) whose phase angle is a phase difference between MR data S (t, Gek) that are temporally adjacent is obtained by the calculation of equation (1).
D (t, Gek) = S (t, Gek) * S ^* (t + Δt, Gek) (1)
However, in equation (1), data S ^* (t, Gek) is a conjugate complex number of data S (t, Gek), and Δt is a time difference between adjacent times.

Next, k-space data d (t) for each time t is obtained by adding the k-space data D (t, Gek) having the same intensity of the encode gradient magnetic field pulse Gek. Then, the gradient magnetic field difference ΔG (t) between adjacent times can be obtained from the equation (2).
ΔG (t) = arctan {d (t)} / (2πγzΔt) (2)
In Equation (2), arctan () is an arctangent function, γ is a magnetic rotation ratio, z is a slice position in the direction of the application axis of the gradient magnetic field Gmeasured, and Δt is a time difference between adjacent times.

  Then, by integrating the gradient magnetic field difference ΔG (t) obtained by the equation (2) until an arbitrary time T, the gradient G of the gradient magnetic field at time T, that is, the waveform of the gradient magnetic field Gmeasured to be measured is obtained. Can be sought.

  Note that the pulse sequence shown in FIG. 5 is an example, and the waveform of the gradient magnetic field can be obtained using another pulse sequence. For example, instead of the encode gradient magnetic field pulse Gek shown in FIG. 5, the collection of the time-series FID signal S (t) accompanied by the application of a rephase pulse is executed once, and the gradient magnetic field is generated based on the FID signal S (t). A waveform can also be obtained. In this case, the waveform of the gradient magnetic field Gmeasured can be similarly obtained by a calculation in which the k-space data addition process for each intensity of the encode gradient magnetic field pulse Gek is omitted.

  On the other hand, as described above, according to the gradient magnetic field waveform simulation in the gradient magnetic field waveform simulation unit 40B, the gradient magnetic field waveform corresponding to the current waveform can be obtained by calculation without actually outputting the current from the gradient magnetic field power supply 27. The gradient magnetic field waveform simulation is executed by using an equivalent circuit model of a gradient magnetic field generation system including as much as possible the components contributing to the generation of the gradient magnetic field, such as the gradient magnetic field power supply 27, the gradient magnetic field coil 26, and the sequence controller 31. Can do.

  FIG. 6 is a circuit diagram illustrating a detailed configuration example of the gradient magnetic field generation system including the gradient magnetic field power supply 27 and the gradient magnetic field coil 23 illustrated in FIG. 1.

  As shown in FIG. 6, the gradient magnetic field power supply 27 includes a breaker 122, a rectifier 123, a DC power supply 124, electrolytic capacitors 126, 126 ′, 126 ″, gradient magnetic field amplifiers 128, 128 ′, 128 ″, and a current. In other words, the X, Y, and Z axis gradient magnetic field power supplies 27x, 27y, and 27z shown in FIG. 1 include a breaker 122, a rectifier 123, and a constant voltage (CV). ) / DC power supply 124 having a constant current (CC) characteristic.

  Note that a DC power supply with CV / CC characteristics outputs a constant voltage when the load is light and the load becomes heavy, and when it is necessary to flow more than a certain current, control is performed to supply a constant current to the load. Power supply.

  Breaker 122 electrically disconnects between AC power supply 120 and rectifier 123 when the output current from external AC power supply 120 exceeds the rated current value. The rectifier 123 converts AC supply power from the AC power source 120 into DC power and supplies it to the DC power source 124.

  The DC power supply 124 charges the electrolytic capacitors 126, 126 ′, 126 ″ with a DC current supplied via the rectifier 123, and supplies the DC current to the gradient magnetic field amplifiers 128, 128 ′, 128 ″. The DC power source 124 operates as a constant voltage source when the load on the gradient magnetic field amplifiers 128, 128 ′, and 128 ″ is light, and operates as a constant current source when the load is heavy.

  Each gradient magnetic field amplifier 128, 128 ', 128 "has a + side input terminal (+ in), a-side input terminal (-in), a + side output terminal (+ out), and a-side output terminal (-out), respectively. The gradient magnetic field amplifiers 128, 128 ′, and 128 ″ receive power supply from the DC power supply 124 and receive control signals from the sequence controller 31 at the + side input terminals. The control signals input from the sequence controller 31 to the gradient magnetic field amplifiers 128 and 128'128 "are gradient magnetic field waveforms to be applied to the imaging region by the X, Y, and Z axis gradient magnetic field coils 23x, 23y, and 23z, respectively, according to the pulse sequence. Is a voltage signal having a waveform similar to.

  The current detectors 130, 130 ′, and 130 ″ detect current values of currents flowing into the negative side output terminals of the gradient magnetic field amplifiers 128, 128 ′, and 128 ″, respectively. The magnitude of the current detected by the current detectors 130, 130 ′, 130 ″ is equal to the current output from the + side output terminal of the gradient magnetic field amplifier 128, 128 ′, 128 ″. This is because currents output from the + side output terminals of the gradient magnetic field amplifiers 128, 128 ′, and 128 ″ flow through the gradient magnetic field coils 23x, 23y, and 23z, respectively, and − of the gradient magnetic field amplifiers 128, 128 ′, and 128 ″. This is because it returns to the side output terminal.

  Each of the current detectors 130, 130 ′, and 130 ″ generates a voltage signal indicating the current value of the detected current, and inputs the generated voltage signal to the − side input terminal of the gradient magnetic field amplifier 128, 128 ′, and 128 ″. To do.

  The gradient magnetic field amplifiers 128, 128 ′, and 128 ″ operate as current sources that output current so that the error signal between the +/− side input terminals becomes 0. Here, the gradient magnetic field amplifiers 128, 128 ′, and 128 The output current of "is negatively fed back by the current detectors 130, 130 ', 130". Therefore, feedback is performed so that a current proportional to the input voltage to the + side input terminal is output from the + side output terminal. Control is performed.

  FIG. 7 is a circuit diagram showing an example of an equivalent circuit model of the gradient magnetic field generation system shown in FIG.

  In FIG. 7, the X-axis gradient magnetic field power supply 27x corresponds to the breaker 122, rectifier 123, DC power supply 124, electrolytic capacitor 126, gradient magnetic field amplifier 128, and current detector 130 shown in FIG. Similarly, the Y-axis gradient magnetic field power supply 27y corresponds to the breaker 122, the rectifier 123, the DC power supply 124, the electrolytic capacitor 126 ′, the gradient magnetic field amplifier 128 ′, and the current detector 130 ′, and the Z-axis gradient magnetic field power supply 27z corresponds to the breaker. 122, a rectifier 123, a DC power supply 124, an electrolytic capacitor 126 ", a gradient magnetic field amplifier 128", and a current detector 130 ". Therefore, here, an equivalent circuit model of an X axis gradient magnetic field gradient magnetic field generation system will be described. .

  As shown in FIG. 7, the equivalent circuit model 140x is equivalent to the X-axis gradient magnetic field power source 27x, the resistance component 23xR corresponding to the resistance component of the X-axis gradient magnetic field coil 23x, and the inductance component of the X-axis gradient magnetic field coil 23x as the primary side. The coil 23xL to be connected is connected in series.

  The equivalent circuit model 140x has a series circuit of a resistor 141R and a virtual coil 141L as a first secondary circuit. Furthermore, the equivalent circuit model 140x has a series circuit of a resistor 142R and a virtual coil 142L as a second secondary circuit. The coil 23xL and the virtual coil 141L are magnetically coupled to each other. The coil 23xL and the virtual coil 142L are magnetically coupled to each other.

  As the frequency increases, the impedances of the X, Y, and Z axis gradient magnetic field coils 23x, 23y, and 23z do not simply increase as in a simple model represented by the sum of one resistance component and one inductance component. For example, when high-frequency current flows through a conductor, the current density is actually high on the surface of the conductor and decreases as it moves away from the surface. That is, due to the skin effect, the current concentrates on the surface as the frequency increases. As a result, the AC resistance of the conductor is increased.

  Therefore, in consideration of the skin effect and the like, in the polynomial representing the impedance of the gradient magnetic field generating system, the term including the resistance value of the resistor 23xR of the X-axis gradient magnetic field coil 23x may be a term that varies depending on the frequency. desirable. Therefore, for example, an equivalent circuit model in which the resistance value of the resistor 23xR is multiplied by the angular frequency ω, that is, an equivalent circuit model in which not only the imaginary part of impedance but also the real part has frequency dependence is used for the gradient magnetic field waveform simulation. Is desirable.

  In practice, an eddy current is generated when a pulse current is supplied to the X, Y, and Z axis gradient magnetic field coils 23x, 23y, and 23z. For this reason, the eddy magnetic field due to the eddy current is added to the gradient magnetic fields Gx, Gy, and Gz in the X, Y, and Z axis directions, and the gradient magnetic field distribution is distorted. Considering the eddy magnetic field, it is desirable to use an equivalent circuit model including the mutual inductance for the gradient magnetic field waveform simulation.

  Moreover, the gradient magnetic field generation system may be provided with a choke coil for cutting off a high-frequency current having a predetermined frequency or higher. In this case, it is desirable to use an equivalent circuit model including a plurality of mutual inductances for the gradient magnetic field waveform simulation.

  An equivalent circuit model 140x shown in FIG. 7 is an example of a model created in consideration of such points. Therefore, not only the equivalent circuit model 140x illustrated in FIG. 7 but also an equivalent circuit model according to the circuit configuration and characteristics of the gradient magnetic field generation system can be created.

In the case of the equivalent circuit model 140x shown in FIG. 7, the gradient magnetic field waveform Gx (t) in the X-axis direction can be calculated by Equation (3).
Gx (t) = aIout (t) + bI ₁ (t) + cI ₂ (t) (3)
In equation (3), a is the current sensitivity of the coil 23xL shown in FIG. 7, b is the current sensitivity of the virtual coil 141L, c is the current sensitivity of the virtual coil 142L, and Iout (t) is from the X-axis gradient magnetic field power supply 27x. , I ₁ (t) is a current flowing through the first secondary circuit, and I ₂ (t) is a current flowing through the second secondary circuit. The current sensitivity is a constant obtained by dividing the strength [T / m] of the gradient magnetic field generated by flowing a current through the coil by the current value [A] flowing through the coil.

If the gradient magnetic field waveform Gx (t) is calculated by Equation (3), an accurate gradient magnetic field waveform in which a magnetic field such as an eddy magnetic field that is not proportional to the output current Iout (t) from the X-axis gradient magnetic field power supply 27x is added. Can be requested. That is, the gradient magnetic field waveforms aIout (t) in which the virtual gradient magnetic field waveforms bI ₁ (t) and cI ₂ (t) generated by the virtual coils 141L and 142L are proportional to the output current Iout (t) from the X-axis gradient magnetic field power supply 27x. It is possible to obtain an accurate gradient magnetic field waveform Gx (t) added to ().

In Expression (3), the output current Iout (t) from the X-axis gradient magnetic field power supply 27x is known because it is determined by a pulse sequence. On the other hand, the current I ₁ (t) flowing through the virtual coil 141L and the current I ₂ (t) flowing through the virtual coil 142L can be calculated by equations (4-1) and (4-2), respectively.
R ₁ I ₁ (t) + L ₁ {dI ₁ (t) / dt} + M ₁ {dIout (t) / dt} = 0 (4-1)
R ₂ I ₂ (t) + L ₂ {dI ₂ (t) / dt} + M ₂ {dIout (t) / dt} = 0 (4-2)
However, in Equation (4-1) and Equation (4-2), R ₁ is the resistance value of the resistor 141R, R ₂ is the resistance value of the resistor 142R, L ₁ is the self-inductance value of the virtual coil 141L, and L ₂ is the virtual value. The self-inductance value of the coil 142L, M ₁ is the mutual inductance value of the coil 26xL and the virtual coil 141L, and M ₂ is the mutual inductance value of the coil 26xL and the virtual coil 142L.

Further, in the equivalent circuit model 140x, the impedance of the X-axis gradient magnetic field coil 26x as viewed from the X-axis gradient magnetic field power supply 44x includes the resistance value Rload of the resistor 23xR and the self-inductance value Lload of the coil 23xL in the X-axis gradient magnetic field coil 26x. (4-1) and the resistance values R ₁ and R _{2 of} the resistor 141R and the resistor 142R that are coefficients of the equation (4-2), the self-inductance values L ₁ and L _{2 of} the virtual coil 141L and the virtual coil 142L, It can be expressed by an equation including mutual inductance values M ₁ and M ₂ of the coil 26xL, the virtual coil 141L, and the virtual coil 142L. Specifically, the impedance of the X-axis gradient magnetic field coil 26x can be expressed by two equations for the real part and the imaginary part.

  In the equation representing the impedance of the X-axis gradient coil 26x, a design value or a measured value can be used as the resistance value Rload of the resistor 23xR and the self-inductance value Lload of the coil 23xL in the X-axis gradient coil 26x. Then, equations (4-1) and (4-2) are obtained by curve fitting between an equation representing the frequency characteristic of the impedance of the X axis gradient magnetic field coil 26x and a measured value of the frequency characteristic of the impedance of the X axis gradient magnetic field coil 26x. Can be obtained.

Note that the initial value of the current I ₁ (t) flowing through the coil 141L and the initial value of the current I ₂ (t) flowing through the coil 142L are both set if a sufficient time has elapsed since the past application of the gradient magnetic field. Can be zero. As a result, the unknown in equation (3) disappears, and the gradient magnetic field waveform Gx (t) can be obtained.

  Therefore, the waveform Gx (t) of the first gradient magnetic field corresponding to the first current Iout (t) is tilted based on the first current Iout (t) from the gradient magnetic field power supply 27 defined in the pulse sequence. It can be calculated by magnetic field waveform simulation. In other words, the gradient magnetic field waveform obtained by the simulation including the calculation of the gradient magnetic field generated by the virtual coil in the equivalent circuit of the gradient magnetic field generation system can be used as the first gradient magnetic field waveform.

  In addition, if it is possible to calculate the gradient magnetic field waveform corresponding to the current waveform by an equation including the term of the nonlinear component, the simulation can be performed using an arbitrary model. In other words, the gradient magnetic field generation system can be modeled by an arbitrary method so that the term of the nonlinear component of the gradient magnetic field is generated.

  When the first current waveform and the first gradient magnetic field waveform as the initial waveform are determined in the gradient magnetic field waveform setting unit 40A, an index representing an error between the first current waveform and the first gradient magnetic field waveform is reduced. By processing, it is possible to generate the second current waveform and the second gradient magnetic field waveform that are the final waveform for imaging.

As an index Dindex representing an error between the first current waveform and the first gradient magnetic field waveform, for example, an area Si of the first current waveform and an area Sg of the first gradient magnetic field waveform that are scaled with each other. Differences can be used. In this case, the error index Dindex can be defined by Equation (5).
Dindex = Si-Sg (5)

As another example, the ratio of the difference between the area Si of the first current waveform and the area Sg of the first gradient magnetic field waveform to the area Si of the first current waveform scaled with each other is used as the error index Dindex. You can also. In this case, the error index Dindex can be defined by Equation (6).
Dindex = (Si-Sg) / Si (6)

  As yet another example, the error waveform obtained as the difference between the first current waveform and the first gradient magnetic field waveform is frequency-resolved, and high frequency components that cause N / 2 artifacts appearing in the phase direction in the EPI method are obtained. The quantity can also be used as an error index Dindex.

  In this way, the index Dindex representing the error between the current waveform and the gradient magnetic field waveform can be defined by an arbitrary method. When the index Dindex representing the error is defined, it is possible to generate the second current waveform and the second gradient magnetic field waveform by calculation that reduces or minimizes the index Dindex.

  Examples of the calculation method of the second current waveform and the second gradient magnetic field waveform include a method of minimizing an error index by optimization calculation using an optimization algorithm such as a genetic algorithm. That is, at least one of the second current waveform and the second gradient magnetic field waveform can be obtained by an optimization algorithm that minimizes an error index between the first current waveform and the first gradient magnetic field waveform. . Since this method repeats the calculation of the gradient magnetic field waveform corresponding to the current, the calculation of the gradient magnetic field waveform by the gradient magnetic field waveform simulation is repeated.

  As another calculation method, there is a method of obtaining the second current waveform by curve fitting that minimizes an error with respect to the first gradient magnetic field waveform. Specifically, a current waveform with a dull rise period is generated using the exponential function coefficient as a parameter, and the second current waveform is approximated to the first gradient magnetic field waveform by optimizing the exponential function coefficient. Can be sought.

  As another calculation method, there is a method of repeating a loop calculation in which the first gradient magnetic field waveform is set as an initial gradient magnetic field waveform, the gradient magnetic field waveform is input as a current waveform to the gradient magnetic field waveform simulation, and the gradient magnetic field waveform is recalculated. . When this loop calculation is executed a predetermined number of times, the second current waveform and the second gradient magnetic field waveform with reduced errors can be obtained. That is, by repeating the calculation using the gradient magnetic field waveform applied from the gradient coil 23 to the imaging region as the waveform of the current output from the gradient magnetic field power supply 27, the second current waveform and the second gradient magnetic field waveform are repeated. At least one of them can be obtained.

  An allowable value of an error necessary for calculating the waveform of the second current and the waveform of the second gradient magnetic field can be arbitrarily determined according to the required image quality and imaging purpose. Further, since the amount of distortion of the gradient magnetic field waveform also changes depending on the imaging conditions such as the strength of the gradient magnetic field, the allowable value of error can be changed.

  As an example, there is a method of measuring in advance an upper limit of an error between a current waveform and a gradient magnetic field waveform that can avoid occurrence of artifacts such as N / 2 artifacts. Even if the allowable value of the error is determined so that the portion where the error between the current waveform and the gradient magnetic field waveform exceeds the upper limit of the error required for avoiding the artifact in the signal region is less than a predetermined ratio of the entire signal region. Good. Alternatively, when the error between the current waveform and the gradient magnetic field waveform converges by repeating the calculation of the gradient magnetic field waveform by the gradient magnetic field waveform simulation, the allowable value of the error can be determined based on the convergence value.

  Also, a plurality of error tolerance values can be set. In this case, a combination of the second current waveform and the second gradient magnetic field waveform is obtained for each error tolerance. Therefore, a combination of a plurality of second current waveforms and second gradient magnetic field waveforms corresponding to a plurality of allowable error values is presented as a candidate for imaging on the display device 34, and can be selected by the user by operating the input device 33. You may do it.

  When the second current waveform and the second gradient magnetic field waveform are obtained by simulation or the like, it is desirable to obtain the most distorted gradient magnetic field waveform among the imaging conditions using the corresponding pulse sequence. For example, the condition for minimizing the field of view (FOV) and outputting the current at the maximum current level corresponds to the most distorted gradient magnetic field waveform.

  Further, the integrated value of the second current waveform becomes smaller than the integrated value of the first current waveform that is the initial waveform as a result of the error reduction process. Therefore, the peak value of the second current waveform is actually adjusted so that the integrated value of the second current waveform is equal to the integrated value of the first current waveform so that the resolution of the image data does not deteriorate. Thus, it is desirable to set the imaging conditions for imaging. Therefore, the peak value and area of the second current waveform can be easily adjusted by scaling the second current waveform. The same applies to the relationship between the first gradient magnetic field waveform and the second gradient magnetic field waveform.

  The calculation of the second current waveform and the second gradient magnetic field waveform can be performed for each imaging, but may be performed when the magnetic resonance imaging apparatus 20 is designed or installed. Therefore, it is possible to create a library of the second current waveform and the second gradient magnetic field waveform that are appropriate for each imaging condition and apparatus that have been obtained. Also, the results of gradient magnetic field waveform simulations and the measurement results of gradient magnetic field waveforms can be made into a library.

  Specifically, a plurality of gradient magnetic field waveforms corresponding to a plurality of different current waveforms that can be output from the gradient magnetic field power supply 27 can be stored in the gradient magnetic field waveform storage unit 44. Similarly, the waveform of the second current and the waveform of the second gradient magnetic field corresponding to the waveform of the first current and the waveform of the first gradient magnetic field set as the initial waveform are also stored in the gradient magnetic field waveform storage unit 44. And can be made into a library.

  As a result, the gradient magnetic field waveform setting unit 40 </ b> A generates the second current waveform and the second gradient based on the first gradient magnetic field waveform corresponding to the first current waveform acquired from the gradient magnetic field waveform storage unit 44. It becomes possible to set or present at least one of the waveforms of the magnetic field. For this reason, it is possible to shorten the time required for the simulation and to omit the output of the current waveform from the gradient magnetic field power supply 27 for acquiring the gradient magnetic field waveform. Furthermore, the processing itself for generating the second current waveform and the second gradient magnetic field waveform can be omitted.

  On the other hand, the data processing unit 41 of the computer 32 acquires the MR signals collected by the imaging scan under the imaging conditions set in the imaging condition setting unit 40 from the sequence controller 31 and is formed in the k-space data storage unit 42. a function to arrange in k-space, a function to reconstruct image data by taking k-space data from the k-space data storage unit 42 and performing image reconstruction processing including Fourier transform (FT). A function of writing the obtained image data in the image data storage unit 43 and a function of performing necessary image processing on the image data captured from the image data storage unit 43 and displaying the image data on the display device 34 are provided.

  Next, the operation and action of the magnetic resonance imaging apparatus 20 will be described.

  FIG. 8 is a flowchart showing a flow when imaging is performed in consideration of an error between the output current waveform from the gradient magnetic field power supply 27 and the gradient magnetic field waveform by the magnetic resonance imaging apparatus 20 shown in FIG.

  First, in step S1, the imaging condition setting unit 40 sets imaging conditions including a pulse sequence such as an EPI sequence.

  Next, in step S11, the gradient magnetic field waveform setting unit 40A uses the gradient magnetic field control value in the pulse sequence set on the assumption that the gradient magnetic field waveform and the current waveform are similar to each other as an initial value for generating the gradient magnetic field. The first current waveform that is a current waveform is set.

  Next, in step S12, the gradient magnetic field waveform setting unit 40A obtains a first gradient magnetic field waveform corresponding to the initially set first current waveform. The first gradient magnetic field waveform can be obtained by a gradient magnetic field waveform simulation in which the first current waveform is input in the gradient magnetic field waveform simulation unit 40B.

  Note that the amount of distortion of the gradient magnetic field waveform with respect to the current waveform may change depending on the imaging conditions and the state of the magnetic resonance imaging apparatus 20 as in the case where the imaging cross section is an oblique cross section. For example, the distortion amount of the gradient magnetic field waveform may change depending on the application direction and strength of the gradient magnetic field.

  Therefore, a more accurate gradient magnetic field waveform corresponding to the state of the magnetic resonance imaging apparatus 20 and the imaging conditions can be obtained by a gradient magnetic field waveform simulation using information representing the state of the magnetic resonance imaging apparatus 20 and imaging conditions as parameters.

  Alternatively, the gradient magnetic field waveform may be acquired by performing a pre-scan for outputting the first current waveform from the gradient magnetic field power supply 27 without depending on the gradient magnetic field waveform simulation. Specifically, a gradient magnetic field waveform is detected using a gradient magnetic field waveform detection system including a search coil 50 and an integrator 51, or based on a FID signal collected during pre-scanning application of the gradient magnetic field. A gradient magnetic field waveform can be obtained by calculation. In this case, the first gradient magnetic field waveform corresponding to the first current waveform is acquired by the gradient magnetic field waveform acquisition unit 40C.

  Next, in step S13, the gradient magnetic field waveform setting unit 40A calculates an error between the first current waveform and the first gradient magnetic field waveform. Next, in step S14, the gradient magnetic field waveform setting unit 40A determines whether or not an error between the first current waveform and the first gradient magnetic field waveform is equal to or less than a threshold value determined in advance as an error tolerance. judge.

  If it is determined that the error between the first current waveform and the first gradient magnetic field waveform is not less than or equal to the threshold, the gradient magnetic field waveform setting unit 40A changes the current waveform in step S15. Specifically, the current waveform is changed by an optimization algorithm that minimizes an error using the current waveform as a parameter or curve fitting to a gradient magnetic field waveform by an exponential function. Or you may make it set the gradient magnetic field waveform itself which matched the scale to a current waveform.

  Then, a gradient magnetic field waveform corresponding to the current waveform changed in step S12 is acquired again by an arbitrary method. However, when the acquisition of the gradient magnetic field waveform is repeated, it is practical to obtain it by calculation using a gradient magnetic field waveform simulation.

  Then, the change of the current waveform and the acquisition of the changed gradient magnetic field waveform as described above are repeated until it is determined in step S14 that the error between the current waveform and the gradient magnetic field waveform is equal to or less than the threshold value. As a result, it is possible to obtain a combination of the second current waveform and the second gradient magnetic field waveform in which the deviation amount is equal to or less than the threshold value.

  Then, the obtained combination of the second current waveform and the second gradient magnetic field waveform is presented or set as an imaging condition in step S14. Further, the imaging condition setting unit 40 can perform a determination process as an imaging condition of the second current waveform and other necessary imaging conditions.

  When the setting of the imaging condition is completed in the imaging condition setting unit 40, MR imaging is executed by the imaging system including the sequence controller 31, the static magnetic field magnet 21 and the like in step S2.

  Specifically, the subject P is set on the bed 37 in advance, and a static magnetic field is formed in the imaging region of the static magnetic field magnet 21 (superconducting magnet) excited by the static magnetic field power supply 26. Further, a current is supplied from the shim coil power supply 28 to the shim coil 22, and the static magnetic field formed in the imaging region is made uniform.

  Then, when a scan start instruction is given from the input device 33 to the imaging condition setting unit 40, the imaging condition setting unit 40 outputs imaging conditions including a pulse sequence to the sequence controller 31. For this reason, the sequence controller 31 drives the gradient magnetic field power source 27, the transmitter 29, and the receiver 30 according to the pulse sequence to form a gradient magnetic field in the imaging region where the subject P is set, and from the RF coil 24 to the RF Generate a signal.

  For this reason, the MR signal generated by the magnetic resonance inside the subject P is received by the RF coil 24 and given to the receiver 30. The receiver 30 receives the MR signal from the RF coil 24, performs necessary signal processing, and then performs A / D conversion to generate raw data that is an MR signal of digital data. The receiver 30 gives the generated raw data to the sequence controller 31. The sequence controller 31 outputs the raw data to the data processing unit 41 of the computer 32.

  Then, the data processing unit 41 arranges MR signals collected by the imaging scan in the k space formed in the k space data storage unit 42. Subsequently, the data processing unit 41 takes in k-space data from the k-space data storage unit 42 and performs image reconstruction processing to generate image data. Then, the image data can be subjected to necessary image processing and displayed on the display device 34 or can be written and stored in the image data storage unit 43.

  FIG. 8 shows a flow in the case where the output current waveform of the gradient magnetic field power supply 27 is obtained in advance for each imaging so that an error from the gradient magnetic field waveform becomes small. You may make it obtain | require a current waveform that an error with a gradient magnetic field waveform becomes small at the time of installation.

  When obtaining the second current waveform for imaging so that an error from the gradient magnetic field waveform becomes small when designing the magnetic resonance imaging apparatus 20 or the pulse sequence, the magnetic resonance imaging including the gradient magnetic field power supply 27 and the gradient magnetic field coil 23 is used. Parameters representing the characteristics of the gradient magnetic field generation system can be estimated based on the design values of the apparatus 20. Then, a second current waveform for imaging that can reduce an error from the gradient magnetic field waveform by the gradient magnetic field waveform simulation using the estimated parameter value can be calculated for each pulse sequence. Further, the calculated second current waveform for imaging for each pulse sequence is stored in the gradient magnetic field waveform storage unit 44, and corresponds to the second current waveform and the second current waveform corresponding to the pulse sequence during imaging. The second gradient magnetic field waveform to be used can be used as the imaging condition.

  On the other hand, when the magnetic resonance imaging apparatus 20 is installed, a second current waveform for imaging that similarly reduces the error from the gradient magnetic field waveform can be obtained and stored in the gradient magnetic field waveform storage unit 44. . However, it is possible to obtain an appropriate second current waveform corresponding to variations in the design error of the gradient magnetic field generation system such as the inductance and capacitance of the gradient magnetic field coil 23. In this case, the second current waveform is calculated by a gradient magnetic field waveform simulation that simulates the variation in the characteristics of hardware such as the gradient coil 23. Alternatively, it is necessary to detect a gradient magnetic field waveform using a gradient magnetic field waveform detection system including a search coil 50 and an integrator 51 and to acquire a gradient magnetic field waveform based on an MR signal.

  When executing the gradient magnetic field waveform simulation, the gradient magnetic field waveform calculated by the gradient magnetic field waveform simulation, the gradient magnetic field waveform detected based on the gradient magnetic field waveform detected by the gradient magnetic field waveform detection system or the MR signal, and The parameters used in the gradient magnetic field waveform simulation can be adapted to the characteristics of the gradient magnetic field generation system. Then, by executing the gradient magnetic field waveform simulation under the condition that the gradient magnetic field waveform is most distorted for each pulse sequence, the second current waveform corresponding to the variation in the characteristics of the gradient magnetic field generation system and the second current waveform corresponding to the second current waveform are obtained. 2 gradient magnetic field waveforms can be obtained.

  That is, the magnetic resonance imaging apparatus 20 as described above obtains the first gradient magnetic field waveform corresponding to the first current waveform from the gradient magnetic field power supply 27 set as the initial waveform, and between the current waveform and the gradient magnetic field waveform. The second current waveform and the second gradient magnetic field waveform corrected so as to reduce the error are presented or set for imaging.

  For this reason, according to the magnetic resonance imaging apparatus 20, it is possible to apply a gradient magnetic field having a waveform closer to the current waveform output from the gradient magnetic field power supply 27 to the imaging region. Therefore, it is possible to avoid output of a current having a sharp rise for making the gradient magnetic field waveform close to a trapezoid or a rectangle. As a result, the load on the gradient magnetic field amplifier is reduced, and the performance of the gradient magnetic field generation system can be maximized.

  In particular, restrictions imposed on ETL (echo train length) in EPI can be relaxed. In addition, regridding processing in EPI can be performed more accurately. Therefore, image quality degradation and artifacts can be reduced.

  Although specific embodiments have been described above, the described embodiments are merely examples, and do not limit the scope of the invention. The novel methods and apparatus described herein can be implemented in a variety of other ways. Various omissions, substitutions, and changes can be made in the method and apparatus described herein without departing from the spirit of the invention. The appended claims and their equivalents include such various forms and modifications as are encompassed by the scope and spirit of the invention.

20 Magnetic Resonance Imaging Device 21 Magnet for Static Magnetic Field 22 Shim Coil 23 Gradient Magnetic Field Coil 24 RF Coil 25 Control System 26 Static Magnetic Field Power Supply 27 Gradient Magnetic Field Power Supply 28 Shim Coil Power Supply 29 Transmitter 30 Receiver 31 Sequence Controller 32 Computer 33 Input Device 34 Display Device 35 arithmetic unit 36 storage unit 37 bed 40 imaging condition setting unit 41 data processing unit 42 k-space data storage unit 43 image data storage unit 44 gradient magnetic field waveform storage unit 50 search coil 51 integrator 52 amplifier 53 A / D converter P Specimen

Claims (10)

An imaging means for performing magnetic resonance imaging of a subject using a gradient magnetic field generation system including a gradient coil and a gradient magnetic field power supply;
Based on the waveform of the first gradient magnetic field applied to the imaging region in accordance with the waveform of the first current output from the gradient magnetic field power supply, the second output from the gradient magnetic field power supply for the magnetic resonance imaging. Condition setting means for setting or presenting at least one of the waveform of the current and the waveform of the second gradient magnetic field applied to the imaging region;
Equipped with a,
The condition setting means includes the second current waveform and the second current waveform so that an index indicating an error between the waveform of the current output from the gradient magnetic field power supply and the waveform of the gradient magnetic field applied to the imaging region is small. magnetic resonance imaging apparatus that will be configured to determine at least one of said second gradient magnetic field waveform. An imaging means for performing magnetic resonance imaging of a subject using a gradient magnetic field generation system including a gradient coil and a gradient magnetic field power supply;
Based on the waveform of the first gradient magnetic field applied to the imaging region in accordance with the waveform of the first current output from the gradient magnetic field power supply, the second output from the gradient magnetic field power supply for the magnetic resonance imaging. Condition setting means for setting or presenting at least one of the waveform of the current and the waveform of the second gradient magnetic field applied to the imaging region;
With
The condition setting means uses a gradient magnetic field waveform obtained by simulation based on the first current waveform as the first gradient magnetic field waveform, and a waveform of a current output from the gradient magnetic field power supply. It is configured to determine at least one of the waveform and the second gradient of the waveform of the second current as an index representing the error becomes smaller between the waveform of a gradient magnetic field to be applied to the imaging area Magnetic resonance imaging device. Said condition setting means, wherein a first waveform of the gradient magnetic field, the first current waveform the inclination claim configured to use the measured waveform of the gradient magnetic field by the output from the magnetic field supply 1 The magnetic resonance imaging apparatus described. The condition setting means includes a gradient magnetic field waveform obtained based on a magnetic resonance signal collected by outputting the first current waveform from the gradient magnetic field power source as the first gradient magnetic field waveform. magnetic resonance imaging apparatus according to claim 1, wherein configured to use. The condition setting means uses a gradient magnetic field waveform obtained by a simulation including a calculation of a gradient magnetic field generated by a virtual coil in an equivalent circuit of the gradient magnetic field generation system as the waveform of the first gradient magnetic field. The magnetic resonance imaging apparatus according to claim 2 configured. A gradient magnetic field applying means for applying a gradient magnetic field having the waveform of the first gradient magnetic field to the imaging region by outputting a current having the waveform of the first current from the gradient magnetic field power supply;
The condition setting means sets or presents at least one of the waveform of the second current and the waveform of the second gradient magnetic field based on the waveform of the first gradient magnetic field applied to the imaging region. magnetic resonance imaging apparatus according to any one of constituted claims 1 to 5. A storage means for storing a plurality of gradient magnetic field waveforms corresponding to a plurality of different current waveforms that can be output from the gradient magnetic field power supply;
The condition setting means includes a waveform of the second current and a waveform of the second gradient magnetic field based on the waveform of the first gradient magnetic field corresponding to the waveform of the first current acquired from the storage means. magnetic resonance imaging apparatus according to any one of claims 1 to 5 arranged to set or display at least one. Said condition setting means, the optimization algorithm by said second current waveform and the magnetic of the second at least one of the ask configured claim 1 or 2, wherein the waveform of a gradient magnetic field for the index to the minimum Resonance imaging device. Said condition setting means, the first magnetic resonance imaging apparatus according to claim 1 or 2, wherein configured by curve fitting to minimize the error with respect to the waveform of a gradient magnetic field seek waveform of the second current. The condition setting means repeats the calculation using the gradient magnetic field waveform applied from the gradient magnetic field coil to the imaging region as the waveform of the current output from the gradient magnetic field power source, and the second current waveform and the magnetic resonance imaging apparatus according to any one of the second gradient magnetic field of claims configured to determine at least one waveform to claim 1 to 8.

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