JP5666779B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention magnetically excites a subject's nuclear spin with a radio frequency (RF) signal of a Larmor frequency, and re-images the nuclear magnetic resonance (NMR) signal generated by this excitation. The present invention relates to a magnetic resonance imaging (MRI) apparatus, and more particularly, to a magnetic resonance imaging apparatus that performs imaging by an echo planar imaging (EPI) method.

  Magnetic resonance imaging is an imaging method in which a nuclear spin of a subject placed in a static magnetic field is magnetically excited with an RF signal having a Larmor frequency, and an image is reconstructed from an MR signal generated by the excitation.

  In the field of magnetic resonance imaging, there is an imaging method called echo planar imaging (EPI). EPI is one of high-speed imaging methods in MRI, and performs scanning by continuously reversing the gradient magnetic field at a high speed with respect to one nuclear magnetic excitation and continuously generating echoes. More specifically, in EPI, after applying an excitation pulse (FLIP PULSE), before the magnetization in the xy plane decays and disappears due to transverse relaxation (T2 relaxation), the phase encode (PE) step Generates continuous gradient echoes and collects all data necessary for image reconstruction. For EPI, we collect SE EPI using the spin echo method (SE: spin echo) that collects the spin echo signal generated after the excitation pulse and refocus pulse (FLOP PULSE) and the echo signal generated after applying the excitation pulse. There are FE EPI using the field echo method (FE) and FFE EPI using the FFE (Fast FE) method. In addition, EPI that creates image data for one image by combining echo train data obtained by applying excitation pulses over multiple times is called multi-shot EPI, whereas only one excitation pulse is applied. The EPI for reconstructing an image is called a single shot (SS) EPI. In addition, there is an EPI called Hybrid EPI.

  Further, diffusion weighted image (DWI) is known as an EPI application technique. The DWI is an image in which a phase shift due to a motion of an imaging target is emphasized by applying a strong gradient magnetic field called a motion probing gradient (MPG) pulse, and a diffusion effect of the imaging target is emphasized.

  On the other hand, a technology has been devised in which a digital filter (DF: digital filter) is provided to digitally filter the NMR signal so that an NMR signal can be acquired by applying a gradient magnetic field for reading whose intensity changes with time ( For example, see Patent Document 1).

Japanese National Patent Publication No. 8-508667

  However, with the various EPIs such as Single Shot SE EPI, Single Shot FE EPI, Multi Shot SE EPI, Multi Shot FE EPI, Multi Shot FFE EPI, and Hybrid EPI described above, there are differences in the spatial position of the imaging region, Reading during data sampling (RO: readout) due to the influence of eddy current generation and gradient amplifier output waveform imperfection depending on conditions such as design error, equipment adjustment difference, hardware individual difference, etc. In some cases, the strength of the gradient magnetic field varies. If the intensity of the gradient magnetic field changes during data sampling, the moment per point changes, which causes image quality degradation.

  For this reason, the gradient magnetic field during data sampling caused by various factors such as eddy current and nonlinearity of the output waveform of the gradient magnetic field amplifier in imaging based on various imaging methods regardless of the presence or absence of DF for the NMR signal as described above It is desired to further improve image quality by suppressing image quality degradation due to intensity change.

  The present invention has been made in order to cope with such a conventional situation, and can suppress deterioration in image quality due to a change in gradient magnetic field strength during data sampling and can further improve the image quality. The purpose is to provide.

  In order to achieve the above object, the magnetic resonance imaging apparatus according to the present invention is caused by the amount of fluctuation from the control value of the gradient magnetic field caused by eddy currents and the nonlinearity or imperfection of the output waveform of the gradient magnetic field power supply. Correction data acquisition means for acquiring correction data for correcting the influence on the magnetic resonance data due to the amount of fluctuation from the control value of the gradient magnetic field, and collecting the magnetic resonance data, and using the correction data, the magnetic resonance data Image data generating means for correcting data and generating image data based on the corrected magnetic resonance data, the correction data acquiring means: (a) control of the gradient magnetic field caused by the eddy current The amount of fluctuation from the value is estimated by performing a simulation, while the gradient waveform power supply output waveform is caused by nonlinearity or imperfection. The fluctuation amount from the control value of the gradient magnetic field is obtained using an output current waveform from the gradient magnetic field power source measured by an ammeter provided in the gradient magnetic field power source. (B) The gradient magnetic field caused by the eddy current The amount of fluctuation from the control value is determined using an output pulse waveform output from the eddy correction circuit to the gradient magnetic field power supply, while the gradient magnetic field is caused by non-linearity or incompleteness of the output waveform of the gradient magnetic field power supply. The fluctuation amount from the control value is estimated by performing a simulation, or (c) the fluctuation amount from the control value of the gradient magnetic field due to the eddy current and the nonlinearity or incompleteness of the output waveform of the gradient magnetic field power supply The amount of fluctuation from the control value of the gradient magnetic field due to the property is estimated by performing a simulation.

  In the magnetic resonance imaging apparatus according to the present invention, it is possible to suppress image quality deterioration due to gradient magnetic field strength change during data sampling and to further improve the image quality.

1 is a configuration diagram showing an embodiment of a magnetic resonance imaging apparatus according to the present invention. The figure which shows an example of the detailed structure of RF coil shown in FIG. The figure which shows the example of arrangement | positioning of the coil element provided in the body surface side of the subject shown in FIG. The figure which shows the example of arrangement | positioning of the coil element provided in the back side of the subject shown in FIG. FIG. 3 is a detailed circuit diagram of a reception system circuit in the receiver shown in FIG. 2. The functional block diagram of the computer shown in FIG. 3 is a flowchart showing a flow when imaging is performed by the EPI method with correction of a signal that removes the influence of gradient magnetic field fluctuations in a signal reception system by the magnetic resonance imaging apparatus shown in FIG. 1. The figure which shows the data obtained by the process of the flowchart shown in FIG. The figure which shows an example of the SS SE EPI DWI sequence set in the imaging condition setting part shown in FIG. The figure which shows the control waveform before the eddy correction | amendment of the gradient magnetic field for RO of the SS SE EPI DWI sequence shown in FIG. 9, and the waveform of the deformed gradient magnetic field for RO. The figure explaining the correction method with respect to the echo data sequence sampled by Ramp Sampling, applying the gradient magnetic field for RO of the deformed waveform shown in FIG. The flowchart which shows the flow in the case of imaging by EPI method with the correction | amendment of the signal which removes the influence of the gradient magnetic field fluctuation in a computer by the magnetic resonance imaging apparatus shown in FIG. The figure which shows the data obtained by the process of the flowchart shown in FIG.

  Embodiments of a magnetic resonance imaging apparatus according to the present invention will be described with reference to the accompanying drawings.

(Configuration and function)
FIG. 1 is a configuration diagram showing an embodiment of a magnetic resonance imaging apparatus according to the present invention.

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

  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. Further, the gradient magnetic field power supply 27 is provided with an ammeter 27a that measures currents output from the X-axis gradient magnetic field power supply 27x, the Y-axis gradient magnetic field power supply 27y, and the Z-axis gradient magnetic field power supply 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 may not be built in the gantry but may be provided near the bed 37 or 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 source 27x, a Y-axis gradient magnetic field power source 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 the transmitter 29 and the receiver 30. The RF coil 24 receives the RF signal from the transmitter 29 and transmits it to the subject P, and receives the NMR signal generated along with the excitation of the nuclear spin inside the subject P by the RF signal to the receiver 30. Has the function to give.

  2 is a diagram showing an example of a detailed configuration of the RF coil 24 shown in FIG. 1, FIG. 3 is a diagram showing an example of arrangement of the coil elements 24c provided on the body surface side of the subject P shown in FIG. 2, and FIG. It is a figure which shows the example of arrangement | positioning of the coil element 24c provided in the back side of the subject P shown in FIG.

  As shown in FIG. 2, the RF coil 24 includes a cylindrical whole-body (WB) coil 24a and a phased array coil 24b. The phased array coil 24b includes a plurality of coil elements 24c, and the plurality of coil elements 24c are arranged on the body surface side and the back surface side of the subject P, respectively.

  For example, as shown in FIG. 3, a total of 32 coil elements 24c of 4 rows in the x direction and 8 rows in the z direction are arranged on the body surface side of the subject so as to cover a wide range of imaging regions. Also, as shown in FIG. 4, a total of 32 coil elements 24c of 4 rows in the x direction and 8 rows in the z direction are arranged on the back side of the subject so as to cover a wide range of imaging regions similarly. . On the back side, a coil element 24c smaller than the other coil elements 24c is disposed near the body axis from the viewpoint of improving sensitivity in consideration of the presence of the spine of the subject P.

  On the other hand, the receiver 30 includes a duplexer 30a, an amplifier 30b, a switching synthesizer 30c, and a receiving system circuit 30d. The duplexer 30a is connected to the transmitter 29, the WB coil 24a, and the amplifier 30b for the WB coil 24a. The amplifiers 30b are provided as many as the number of the coil elements 24c and the WB coils 24a, and are individually connected to the coil elements 24c and the WB coils 24a. The switching synthesizer 30c is provided singly or in plural, and the input side of the switching synthesizer 30c is connected to the plurality of coil elements 24 or the WB coil 24a via the plurality of amplifiers 30b. A desired number of reception system circuits 30d are provided so as to be equal to or less than the number of each coil element 24c and WB coil 24a, and are provided on the output side of the switching synthesizer 30c.

  The WB coil 24a can be used as a coil for transmitting an RF signal. Each coil element 24c can be used as a coil for receiving NMR signals. Further, the WB coil 24a can be used as a receiving coil.

  For this reason, the duplexer 30a gives the RF signal for transmission output from the transmitter 29 to the WB coil 24a, while switching and synthesizing the NMR signal received by the WB coil 24a via the amplifier 24d in the receiver 30. It is comprised so that it may give to the container 30c. Further, the NMR signals received by the coil elements 24c are also output to the switching synthesizer 30c via the corresponding amplifiers 24d.

  The switching synthesizer 30c is configured to perform synthesis processing and switching of NMR signals received from the coil element 24c and the WB coil 24a, and to output to the corresponding reception system circuit 30d. In other words, the NMR signal received from the coil elements 24c and the WB coils 24a is combined and switched in the switching synthesizer 30c in accordance with the number of the receiving system circuits 30d, and photographing is performed using a desired plurality of coil elements 24c. A sensitivity distribution corresponding to the part is formed so that NMR signals from various imaging parts can be received.

  However, the NMR signal may be received only by the WB coil 24a without providing the coil element 24c. Further, the NMR signal received by the coil element 24c and the WB coil 24a may be directly output to the reception system circuit 30d without providing the switching synthesizer 30c. Further, more coil elements 24c can be arranged over a wide range.

  On the other hand, the sequence controller 31 of the control system 25 is connected to the gradient magnetic field power source 27, the transmitter 29, and the receiver 30. The sequence controller 31 has 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 source 27, the transmitter 29, and the receiver 30 are driven in accordance with the stored predetermined pulse sequence to drive the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, and the Z-axis. It has a function of generating a gradient magnetic field Gz and an RF signal.

  The sequence controller 31 is provided with a vortex correction circuit 31a. The eddy correction circuit 31a is a circuit that adjusts the output waveform of the pulse current so that the eddy current generated by the gradient magnetic field is reduced when the pulse current is output to the gradient magnetic field power supply 27 according to the pulse sequence acquired from the computer 32. . A pulse current having an output waveform corrected in the eddy correction circuit 31 a is actually applied to the gradient magnetic field power supply 27. That is, the eddy correction circuit 31a is a circuit that performs eddy current correction of the pulse current applied to the gradient magnetic field power supply 27 so that no eddy current is generated.

  Further, 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 give it to the computer 32. The

  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 and is required. And 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 by performing the signal processing of A and D.

  FIG. 5 is a detailed circuit diagram of the reception system circuit 30d in the receiver 30 shown in FIG.

The reception system circuit 30d as shown in FIG. 5, the frequency converter 30e, A / D converter (ADC: A / D converter) 30f, the detection circuit 30g, the digital filter (DF: digital filter) 30 h , DF control A system 30i is provided. In the frequency converter 30e, the frequency of the NMR signal output from the switching synthesizer 30c is converted to a frequency that can be converted from a Larmor frequency to a digital signal, and in the A / D converter 30f, the NMR signal is converted into a digital signal. . The NMR signal which has become a digital signal is converted into a baseband NMR signal having an I component which is a real component and a Q component which is an imaginary component in the detection circuit 30g and is output to the DF 30h. The DF 30 h has a function of digitally filtering the NMR signal with a desired filter coefficient and outputting it to the computer 32 through the sequence controller 31.

  The DF control system 30i can be configured by a calculation device that reads a circuit or a program. The DF control system 30i outputs the output current waveform of the gradient magnetic field power supply 27 acquired from the ammeter 27a, the output waveform of the pulse current applied to the gradient magnetic field power supply 27 acquired from the eddy correction circuit 31a, and / or the sequence controller 31 from the computer 32. Based on the information given through DF30h, the digital filtering filter coefficient in DF30h that corrects the shift of the echo data due to the RO gradient magnetic field fluctuation during sampling is calculated as correction data. It has a function of controlling the DF 30h. Further, the DF 30h can be controlled by giving a control signal for controlling the filter coefficient from the computer 32 to the DF 30h through the sequence controller 31.

  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 regardless of the program.

  FIG. 6 is a functional block diagram of the computer 32 shown in FIG.

  The computer 32 includes an imaging condition setting unit 40, a sequence controller control unit 41, a k-space database 42, an image reconstruction unit 43, an image database 44, an image processing unit 45, a data correction unit 46, a simulation unit 47, and simulation parameter storage. It functions as part 48. The imaging condition setting unit 40 includes a DF control unit 40a.

  The imaging condition setting unit 40 sets imaging conditions including a pulse sequence such as an EPI sequence or a PROPELLER (Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction) sequence based on instruction information from the input device 33, and the set imaging conditions are sequenced. It has a function given to the controller control unit 41. In particular, the imaging condition setting unit 40 can set imaging conditions for performing parallel imaging (PI).

  PI is an imaging method in which echo data is received using a plurality of coil elements 24c and phase encoding is skipped to reduce the number of phase encodings necessary for image reconstruction. In principle, the number of phase encodes can be reduced to a fraction of the number of coil elements 24c corresponding to the number of phase encodes necessary for image reconstruction. Scanning by the EPI method that continuously collects a plurality of echo signals is often performed by PI. When PI is performed, information necessary for PI is set as imaging conditions including information on the number of coil elements 24c used for collecting echo data and information relating each coil element 24c and imaging region.

  Further, the DF control unit 40a of the imaging condition setting unit 40 outputs the output current waveform of the gradient magnetic field power supply 27 acquired from the ammeter 27a, the output waveform of the pulse current applied to the gradient magnetic field power supply 27 acquired from the eddy correction circuit 31a, and And / or a function of calculating a filter coefficient of digital filtering in the DF 30h as correction data so that the shift of echo data due to the gradient magnetic field variation for RO during sampling is corrected based on the simulation result obtained from the simulation unit 47. The function of controlling the DF 30h by giving the calculated filter coefficient to the DF 30h of the receiver 29 through the sequence controller control unit 41 and the sequence controller 31.

  The sequence controller control unit 41 has a function of controlling driving by giving the sequence controller 31 imaging conditions including the pulse sequence acquired from the imaging condition setting unit 40 when receiving the scan start instruction information from the input device 33. . The sequence controller control unit 41 has a function of receiving raw data from the sequence controller 31 and arranging it in the k space formed in the k space database 42. For this reason, each raw data generated in the receiver 30 is stored in the k-space database 42 as k-space data.

  The image reconstruction unit 43 has a function of reconstructing image data by acquiring k-space data from the k-space database 42 and performing an image reconstruction process including Fourier transform (FT). A function of writing the received image data into the image database 44. For this reason, the image database 44 stores the image data reconstructed by the image reconstruction unit 43.

  The image processing unit 45 has a function of acquiring image data from the image database 44 and performing necessary image processing to generate display image data, and a function of causing the display device 34 to display the generated display image data. . For example, when an echo signal is collected by the PI, the image processing unit 45 performs unfolding processing, which is post-processing in PI, on the image data corresponding to each coil element 24c based on the PI condition. A function of generating the processed image data. The sensitivity distribution of each coil element 24c is used for the unfolding process. For this reason, the sensitivity distribution of each coil element 24 c is stored in the image processing unit 45.

  The data correction unit 46 acquires the output current waveform of the gradient magnetic field power supply 27 acquired from the ammeter 27a, the output waveform of the pulse current applied to the gradient magnetic field power supply 27 acquired from the eddy correction circuit 31a, and / or the simulation unit 47. A function for generating correction data for correcting the shift of echo data caused by the fluctuation of the gradient magnetic field for RO during sampling based on the simulation result, and the Regrid based on the correction data for the echo data read from the k-space database 42 By performing the processing, the function for correcting the shift of the echo data in the k space due to the influence of the eddy current and the non-linearity or incompleteness of the output pulse current from the gradient magnetic field power supply 27 and the corrected echo data are stored in the k space database 42. Has a writing function.

  The simulation unit 47 executes a simulation using the characteristic values of the gradient magnetic field power supply 27 and the magnetic resonance imaging apparatus 20 as simulation parameters, thereby simulating the waveform of the output pulse current from the gradient magnetic field power supply 27 and the intensity of the eddy current. And a function for obtaining a fluctuation amount from the control value of the gradient magnetic field moment for RO from the calculated simulation result and the control value of the gradient magnetic field for RO in the pulse sequence.

  In the simulation parameter storage unit 48, simulation parameters such as time constants of eddy currents necessary for the simulation in the simulation unit 47 are measured in advance and stored.

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

  First, the case where the NMR echo signal is corrected by hardware of the magnetic resonance imaging apparatus 20, for example, the DF 30h of the signal receiving system will be described.

  FIG. 7 is a flowchart showing a flow when imaging is performed by the EPI method with correction of a signal for removing the influence of gradient magnetic field fluctuations in the signal receiving system by the magnetic resonance imaging apparatus 20 shown in FIG. Reference numerals with numbers added to S indicate steps in the flowchart. FIG. 8 is a diagram showing data obtained by the processing of the flowchart shown in FIG.

  First, in step S1, the photographing condition setting unit 40 sets photographing conditions using an EPI sequence.

  FIG. 9 is a diagram showing an example of the SS SE EPI DWI sequence set in the imaging condition setting unit 40 shown in FIG.

  In FIG. 9, RF is an RF excitation pulse, ECHO is an echo signal, Gss is a gradient magnetic field for slice selection (SS), Gro is a gradient magnetic field for readout, and Gpe is a phase. The gradient magnetic field for encoding is shown respectively.

  As shown in FIG. 9, in the SS SE EPI DWI sequence, a refocus pulse is applied together with a slice selection gradient magnetic field pulse following the excitation pulse. In addition, a gradient magnetic field pulse called TUNE for adjusting the moment of the gradient magnetic field is applied between the excitation pulse and the refocus pulse in the RO direction and the PE direction, respectively. Further, after the refocus pulse is applied, an MPG pulse is applied in the SS direction.

  Next, a gradient magnetic field pulse in the PE direction called a BLIP pulse is repeatedly applied, and a phase encoding amount corresponding to the intensity of the BLIP pulse is sequentially added. On the other hand, a gradient magnetic field for RO whose polarity is alternately reversed is repeatedly applied. As a result, echo signals necessary for generating image data for one sheet are continuously generated in the data collection portion, and the generated echo signals are collected. That is, echo signals for generating image data for one sheet can be collected by one-time nuclear magnetic excitation.

  Next, in step S2, the hardware is controlled by the EPI sequence.

  For this purpose, 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.

  When the scan start instruction is given from the input device 33 to the sequence controller control unit 41, the sequence controller control unit 41 gives the imaging condition including the EPI sequence acquired from the imaging condition setting unit 40 to the sequence controller 31. The sequence controller 31 drives the gradient magnetic field power source 27, the transmitter 29, and the receiver 30 according to the EPI sequence received from the sequence controller control unit 41, thereby forming a gradient magnetic field in the imaging region in which the subject P is set. An RF signal is generated from the RF coil 24.

  At this time, the eddy correction circuit 31a performs eddy current correction of the pulse current waveform applied to the gradient magnetic field power supply 27 so that the eddy current is reduced by application of the gradient magnetic field. Further, the output pulse current waveform of the gradient magnetic field power supply 27 may change from the control waveform due to non-linearity or imperfection of the gradient magnetic field power supply 27. Furthermore, the waveform of the RO gradient magnetic field that is actually formed in the imaging region is further deformed by the influence of the spatially nonlinear magnetic field induced by the generation of eddy currents, the characteristics of each device, and the like. For this reason, the waveform of the gradient magnetic field actually formed in the imaging region is a waveform obtained by modifying the waveform of the RO gradient magnetic field set in the EPI sequence.

  FIG. 10 is a diagram showing a control waveform before the vortex correction of the RO gradient magnetic field in the SS SE EPI DWI sequence shown in FIG. 9 and a waveform of the deformed RO gradient magnetic field.

  In FIG. 10, ECHO is an echo signal, Gro is a control waveform of the gradient magnetic field for RO before eddy correction, Gro 'is eddy correction, non-linearity of the output pulse of the gradient magnetic field power supply 27, magnetic field induced by eddy current, etc. The waveform of the RO gradient magnetic field deformed by the influence is shown.

  As shown in FIG. 10, the control waveform Gro of the gradient magnetic field for RO before eddy correction is a waveform that rises linearly and then becomes a constant value and falls linearly. Then, sampling positions corresponding to one column of data in the k space are set at equal intervals. Note that the sampling position shown in FIG. 10 shows an example in the case of Ramp Sampling in which an echo signal is sampled while the gradient magnetic field is rising.

  However, the gradient magnetic field Gro ′ actually formed in the imaging region is deformed as shown in FIG. 10 due to eddy correction, the nonlinearity of the output pulse of the gradient magnetic field power supply 27, the magnetic field induced by the eddy current, and the like. . Therefore, an NMR signal is generated by nuclear magnetic resonance inside the subject P in a state where the gradient magnetic field of the deformed waveform Gro ′ is generated in the imaging region.

  Next, in step S <b> 3, the NMR signal sequence is continuously and sequentially received by the RF coil 24 in accordance with the EPI sequence and provided to the receiver 30. The receiver 30 receives the NMR signal from the RF coil 24, performs frequency conversion by the frequency converter 30e, A / D conversion by the A / D converter 30f, and detection by the detection circuit 30g, thereby obtaining an NMR signal of digital data. Generate raw data that is a column. That is, one echo data as shown in FIG. 8A is generated.

  However, since the echo data is sampled by Ramp Sampling by applying an RO gradient magnetic field having a deformed waveform Gro ′ as shown in FIG. 10, the gradient magnetic field moment (area) differs for each sampling position. For this reason, the sampling points of the echo data are unequal intervals. Therefore, it is necessary to correct the echo data so that they are equally spaced. In order to obtain the correction data used for this correction, it is necessary to obtain the fluctuation amount from the control value of the RO gradient magnetic field moment.

  The fluctuation amount from the control value of the gradient magnetic field moment for RO can be obtained based on the measured value of the gradient magnetic field acquired from the gradient magnetic field control system or the simulation result by software.

  When the fluctuation amount from the control value of the gradient magnetic field moment for RO is obtained by simulation, the simulation unit 47 of the computer 32 performs the RO based on the imaging conditions including the EPI sequence set in the imaging condition setting unit 40 in step S4. A simulation for simulating a device and a gradient magnetic field control system for determining the amount of variation from the control value of the gradient magnetic field moment for control is performed, and the amount of variation of the gradient magnetic field moment for RO is calculated based on the simulation result. Parameters such as the eddy current decay time constant, the scaling coefficient, and the coefficient of the approximate expression necessary for the simulation in the simulation unit 47 can be acquired from the simulation parameter storage unit 48.

  For example, the waveform of the output pulse current having nonlinearity after eddy correction processing from the gradient magnetic field power supply 27 can be calculated by simulation using the characteristic value for each gradient magnetic field power supply 27 as a simulation parameter. Based on the calculated waveform of the output pulse current from the gradient magnetic field power supply 27, the fluctuation amount from the control value of the RO gradient magnetic field moment can be obtained. Specifically, the difference between the control value of the RO gradient magnetic field in the EPI sequence and the waveform of the RO gradient magnetic field corresponding to the output pulse current calculated as a simulation may be calculated. As a result, it is possible to correct an error in echo data caused by the gradient magnetic field for RO that varies due to the nonlinearity of the output pulse current from the gradient magnetic field power supply 27.

  Furthermore, the intensity of the eddy current can be calculated by simulation using the characteristic value of the magnetic resonance imaging apparatus 20 as a simulation parameter. Then, the fluctuation amount of the gradient magnetic field moment for RO can be obtained based on the calculated intensity of the eddy current. Specifically, the difference between the control value of the RO gradient magnetic field in the EPI sequence and the gradient magnetic field induced by the eddy current calculated as a simulation may be calculated. In this case, it is possible to correct an error in echo data caused by fluctuation of the gradient magnetic field for RO due to eddy current.

  Further, the fluctuation amount of the gradient magnetic field moment for RO can be obtained by using both the waveform of the output pulse current from the gradient magnetic field power supply 27 and the intensity of the eddy current. In this case, it is possible to correct an error in echo data due to fluctuations in the RO gradient magnetic field caused by both the nonlinearity of the output pulse current from the gradient magnetic field power supply 27 and the eddy current.

  On the other hand, when obtaining the amount of variation from the control value of the gradient magnetic field moment for RO based on the measured value of the gradient magnetic field acquired from the gradient magnetic field control system, the amount of variation of the gradient magnetic field for RO is measured in step S5. .

  For example, the waveform of the gradient magnetic field pulse actually applied to the imaging region can be estimated based on the waveform of the output pulse current measured by the ammeter 27a of the gradient magnetic field power supply 27. Then, by obtaining the difference between the control value of the gradient magnetic field for RO in the EPI sequence and the estimated value of the gradient magnetic field pulse waveform based on the measurement value of the ammeter 27a, the amount of variation from the control value of the gradient magnetic field moment for RO is obtained. be able to. As a result, it is possible to correct an error in echo data caused by the gradient magnetic field for RO that varies due to the nonlinearity of the output pulse current from the gradient magnetic field power supply 27.

  Further, the eddy current intensity can be calculated from the eddy correction circuit 31a based on the history of the output pulse waveform for each data collection position. In other words, the eddy correction circuit 31a corrects the control value of the RO gradient magnetic field in the EPI sequence so that the eddy current corresponding to the control of the gradient magnetic field is reduced. The intensity of the current can be calculated. Parameters such as a time constant necessary for calculating the intensity of the eddy current can be obtained by measuring in advance. Then, by calculating the difference between the control value of the RO gradient magnetic field in the EPI sequence and the gradient magnetic field induced by the eddy current calculated based on the output pulse waveform from the eddy correction circuit 31a, the RO gradient magnetic field moment is calculated. The amount of variation from the control value can be obtained. In this case, it is possible to correct an error in echo data caused by fluctuation of the gradient magnetic field for RO due to eddy current.

  Similarly to the case of the simulation, the fluctuation amount of the gradient magnetic field moment for RO can be obtained using both the waveform of the output pulse current from the gradient magnetic field power supply 27 and the intensity of the eddy current.

  Calculation of the fluctuation amount of the gradient magnetic field moment for RO based on the measurement value obtained from such a gradient magnetic field control system can be performed by the DF control system 30 i of the receiver 30 or the DF control unit 40 a of the computer 32. When the fluctuation amount of the gradient magnetic field moment for RO is calculated by the simulation, the fluctuation amount of the gradient magnetic field moment for RO is calculated from the simulation unit 47 via the DF control unit 40a of the computer 32 or the sequence controller 31. To the DF control system 30i.

  Next, in step S6, correction data of echo data corresponding to the variation amount of the RO gradient magnetic field moment is created in the DF control system 30i of the receiver 30 or the DF control unit 40a of the computer 32. That is, the shift amount of each echo data resulting from the fluctuation amount of the gradient magnetic field moment for RO is obtained, and the obtained shift amount for each echo data is used as the correction data. As a result, the ratio between the correction data for each echo data is the ratio between the gradient magnetic field moments between the sampling positions.

  Next, in step S7, the filter coefficient of the DF 30h is calculated so that the echo data is corrected by the correction data created in the DF control system 30i of the receiver 30 or the DF control unit 40a of the computer 32, and the calculated filter The DF 30h is controlled by giving the coefficient to the DF 30h.

  Thereby, the echo data string after detection shifted due to the fluctuation of the gradient magnetic field moment for RO is sequentially corrected for each echo data string in the DF 30h. As a result, echo data strings after shift correction as shown in FIG. 8B are sequentially obtained in the DF 30h.

  FIG. 11 is a diagram for explaining a correction method for the echo data sequence sampled by Ramp Sampling by applying the RO gradient magnetic field of the modified waveform Gro ′ shown in FIG. 10.

  11 (a) and 11 (b), the horizontal axis indicates the RO direction. As shown in FIG. 11 (a), the echo data string after detection is affected by the fluctuation of the RO gradient magnetic field moment, and the sampling of echo data is performed even during the rise and fall of the RO gradient magnetic field by Ramp Sampling. Because of this, the intervals are unequal. Normally, when echo data is collected by Ramp Sampling, the echo data is displayed on the lattice points based on the linear change of the gradient magnetic field moment according to the control waveform of the rising and falling portions of the RO gradient magnetic field. Thus, phase soft correction is performed by filtering in DF30h.

  However, even if the phase shift of the echo data by Ramp Sampling is corrected, the echo data is still shifted because of the non-linearity or imperfection of the output pulse current from the eddy current or the gradient magnetic field power supply 27. The columns are unevenly spaced. If image data is reconstructed by using such unequally spaced echo data strings as they are, the image quality of the image data deteriorates.

  Therefore, in DF30h, in addition to the phase shift component of the echo data by Ramp Sampling, the asymmetric phase shift component of the echo data due to the influence of nonlinearity of the eddy current and the output pulse current from the gradient magnetic field power supply 27 is canceled. The phase shift correction of the echo data is performed. As a result, the echo data is translated in the RO direction by the shift corresponding to the fluctuation amount from the control value of the gradient magnetic field moment, and an equidistant echo data string on the lattice points as shown in FIG. 11B is obtained. It is done.

  In addition to the phase shift correction for the phase shift component of the echo data by Ramp Sampling, the phase shift correction for the phase shift component of the echo data due to the non-linearity of the eddy current and the output pulse current from the gradient magnetic field power supply 27 is performed in the DF 30h. May be. Further, when the ramp sampling is not performed, it is not necessary to perform the phase shift correction for the phase shift component of the echo data by the ramp sampling.

  In addition, when PI is performed, echo data sequences are collected in each of a plurality of reception channels, and thus such phase shift correction of the echo data sequences is sequentially performed for each reception channel.

  Next, in step S8, the echo data string after the shift correction sequentially obtained in the DF 30h is passed through the sequence controller 31 and the sequence controller control unit 41 from the receiver 30, as shown in FIG. These are sequentially arranged in the k space formed in the database 42.

  When all the echo data strings are arranged in the k space, in step S9, image data as shown in FIG. 8D is generated from the k space data, and the generated image data is displayed. That is, the image reconstruction unit 43 reconstructs image data by taking k-space data from the k-space database 42 and performing image reconstruction processing, and writes the obtained image data in the image database 44. The image processing unit 45 takes in the image data from the image database 44 and performs necessary image processing such as PI unfolding processing to generate display image data, and the generated display image data is displayed on the display device 34. To display. Thereby, a diagnostic image can be obtained.

  In the above-described example, the case where the echo data sequence is corrected in the DF 30h of the signal reception system has been described. However, the echo data sequence is corrected by control of hardware or firmware other than the DF 30h of the magnetic resonance imaging apparatus 20. Also good.

  Next, a case where echo data is corrected by software as pre-processing of image reconstruction processing in the computer 32 of the magnetic resonance imaging apparatus 20 will be described.

  FIG. 12 is a flowchart showing a flow when imaging is performed by the EPI method with correction of a signal for removing the influence of the gradient magnetic field fluctuation in the computer 32 by the magnetic resonance imaging apparatus 20 shown in FIG. The code | symbol which attached | subjected the number to the inside S shows each step of a flowchart. Steps similar to those in FIG. 7 are denoted by the same reference numerals and description thereof is omitted. FIG. 13 is a diagram showing data obtained by the processing of the flowchart shown in FIG.

  When correcting the echo data in the computer 32, in step S6, the data correction unit is performed in the same manner as the method for obtaining the correction data of the echo data in the DF control system 30i of the receiver 30 or the DF control unit 40a of the computer 32. 46, the correction data of the echo data is obtained.

  In step S10, echo data strings before shift correction as shown in FIG. 13 (a) are sequentially arranged in the k space as shown in FIG. 13 (b). For this reason, the echo data before the shift correction is arranged in the k-space database 42.

  Next, in step S11, the data correction unit 46 applies a Regrid process based on the correction data to the echo data arranged in the k-space database 42, so that an eddy current or an output pulse current from the gradient magnetic field power supply 27 is obtained. The shift of the echo data in the k space due to the influence of the nonlinearity of is corrected.

  The Regrid process is a known process performed for each echo data, and the shift of the echo data string is corrected by an interpolation process by interpolation using an n-order spline function such as a cubic spline function, Data shift is a process of translating echo data by phase shift processing.

  As in the case of correcting the echo data string in DF30h, if there is a shift component of the echo data by Ramp Sampling, the eddy current and the slope may be simultaneously or separately from the correction to the shift component of the echo data by Ramp Sampling. It is possible to correct the shift component of the echo data due to the influence of the nonlinearity of the output pulse current from the magnetic field power supply 27.

  By such echo data correction processing, corrected echo data as shown in FIG. 13C is obtained. In step S9, image data as shown in FIG. 13D is generated from the corrected k-space data, and the generated image data is displayed.

  That is, the magnetic resonance imaging apparatus 20 as described above obtains the shift of echo data caused by the fluctuation of the gradient magnetic field moment for RO during sampling based on the fluctuation amount from the control waveform of the gradient magnetic field for RO. The correction data is used for correction.

(effect)
For this reason, according to the magnetic resonance imaging apparatus 20, when sampling data is read, the raw data undergoes a phase shift or a point shift due to gradient magnetic field fluctuation factors such as eddy current correction and nonlinearity of the output waveform of the gradient magnetic field power supply. Even if it occurs, it can be corrected. For this reason, artifacts such as blurring, pixel shift, and ghost, which have been caused by factors that fluctuate the gradient magnetic field strength, such as eddy current correction at the time of reading sampling data and nonlinearity of the output waveform of the gradient magnetic field power source, are reduced Image quality can be improved.

  In addition, since the image quality of DWI data is improved also when DWI is performed, the image quality of an isotropic image or an apparent diffusion coefficient (ADC) image generated by post-processing of DWI data can be improved.

  Further, if the NMR signal is corrected as preprocessing using hardware or firmware such as DF30h, the NMR signal can be corrected without extending the time required for the image data generation processing. On the other hand, if the NMR signal is corrected in the computer 32 based on the simulation information obtained by the simulation in the computer 32, the eddy current correction and the gradient magnetic field power supply can be performed without changing the hardware of the conventional MRI apparatus. It is possible to correct the phase shift of the NMR signal caused by the deviation of the gradient magnetic field moment due to the non-linearity of the output waveform.

DESCRIPTION OF SYMBOLS 20 Magnetic resonance imaging apparatus 21 Magnet for static magnetic field 22 Shim coil 23 Gradient magnetic field coil 24 RF coil 24a WB coil 24b Phased array coil 24c Coil element 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 Computing device 36 Storage device 37 Bed 40 Imaging condition setting unit 41 Sequence controller control unit 42 k-space database 43 Image reconstruction unit 44 Image database 45 Image processing unit P Subject

Claims (9)

During sampling of magnetic resonance data from the subject, the amount of fluctuation from the control value of the gradient magnetic field caused by eddy current and the control value of the gradient magnetic field caused by nonlinearity or imperfection of the output waveform of the gradient magnetic field power supply Correction data acquisition means for acquiring correction data for correcting the influence on the magnetic resonance data due to a variation amount;
Image data generating means for collecting the magnetic resonance data, correcting the magnetic resonance data using the correction data, and further generating image data based on the corrected magnetic resonance data;
With
The correction data acquisition means includes
(A) While the fluctuation amount from the control value of the gradient magnetic field due to the eddy current is estimated by performing simulation, the gradient magnetic field due to nonlinearity or imperfection of the output waveform of the gradient magnetic field power supply (B) Control of the gradient magnetic field caused by the eddy current is obtained using an output current waveform from the gradient magnetic field power supply measured by an ammeter provided in the gradient magnetic field power supply. The amount of fluctuation from the value is obtained using an output pulse waveform output from the eddy correction circuit to the gradient magnetic field power supply, while the gradient magnetic field control is caused by nonlinearity or imperfection of the output waveform of the gradient magnetic field power supply. The fluctuation amount from the value is estimated by performing a simulation, or (c) the change from the control value of the gradient magnetic field caused by the eddy current. Estimated by performing a simulation of the amount of change from control values of the gradient magnetic field caused by the nonlinearity or imperfections of the amount and the gradient magnetic field power supply of the output waveform,
Magnetic resonance imaging device. The correction data acquisition means estimates the fluctuation amount from the control value of the gradient magnetic field caused by the eddy current by performing a simulation, while causing the nonlinearity or imperfection of the output waveform of the gradient magnetic field power source. 2. The magnetism according to claim 1, wherein the fluctuation amount from the control value of the gradient magnetic field is determined using an output current waveform from the gradient magnetic field power source measured by an ammeter provided in the gradient magnetic field power source. Resonance imaging device. The correction data acquisition means obtains the fluctuation amount from the control value of the gradient magnetic field caused by the eddy current using an output pulse waveform output from the eddy correction circuit to the gradient magnetic field power supply, while the gradient data The magnetic resonance imaging apparatus according to claim 1, wherein a fluctuation amount from the control value of the gradient magnetic field due to nonlinearity or imperfection of an output waveform of a magnetic field power supply is estimated by performing a simulation. The correction data acquisition means obtains the intensity of the eddy current generated by the gradient magnetic field using the history of the output pulse waveform for each data collection position output from the eddy correction circuit to the gradient magnetic field power supply, and The magnetic resonance imaging apparatus according to claim 3, wherein a fluctuation amount from a control value of the gradient magnetic field due to the eddy current is obtained using intensity. The magnetic resonance imaging apparatus according to claim 1, wherein the image data generation unit is configured to perform correction using the correction data on magnetic resonance data arranged in a k-space. The magnetic resonance imaging apparatus according to claim 1, wherein the image data generation unit is configured to perform correction of the magnetic resonance data using the correction data using hardware or firmware. The magnetic resonance imaging apparatus according to claim 6, wherein the image data generation unit is configured to perform correction of the magnetic resonance data using the correction data using a digital filter after A / D conversion in a receiver. The magnetic resonance imaging apparatus according to claim 6, wherein the image data generation unit is configured to perform a Regrid process based on the correction data with respect to the magnetic resonance data. The image data generating means is configured to collect the magnetic resonance data in a plurality of reception channels using a plurality of coil elements, and to correct the magnetic resonance data using correction data for each reception channel. Item 7. The magnetic resonance imaging apparatus according to Item 6.

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