JP6449374B2 - Magnetic resonance imaging system - Google Patents

Description

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

  MRI magnetically excites a nuclear spin of a subject placed in a static magnetic field with a radio frequency (RF) signal of Larmor frequency, and nuclear magnetic resonance (NMR) generated by this excitation. This is an imaging method for reconstructing an image from a signal.

  One of the high-speed imaging methods in the field of MRI is an imaging method called echo planar imaging (EPI). EPI 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 a 90 ° excitation pulse, before the magnetization in the xy plane decays and disappears due to transverse relaxation (T2 relaxation), it matches the phase encode (PE) step. Thus, a gradient magnetic field for readout (RO) is repeatedly applied at high speed. Thereby, a continuous gradient echo is generated, and all data necessary for image reconstruction can be collected.

  In EPI, SE EPI using spin echo method (SE: spin echo) that collects spin echo signals generated after 90 ° excitation pulse and 180 ° excitation pulse and echo signal generated after application of 90 ° excitation pulse. There are FE EPI using the field echo (FE) field to collect and FFE EPI using the FFE (Fast FE) method. In addition, EPI, which creates image data for one sheet by combining the echo train data obtained by applying 90 ° excitation pulses over multiple times, is called multi-shot EPI. EPI that reconstructs an image only by application is called single shot (SS) EPI.

  On the other hand, when imaging an imaging region affected by blood flow such as the neck, a flow compensation (FC) gradient magnetic field pulse is applied to correct the phase shift of the MR signal caused by the effect of blood flow The technology to do is known. That is, by applying the FC gradient magnetic field pulse, it is possible to cancel the phase shift caused by the flow of fluid such as cerebrospinal fluid (CSF) and the movement of respiration, etc. in addition to blood.

JP 2000-325327 A

  However, in the 3D-FE sequence, if multi-echo acquisition is performed by extending TR in order to improve contrast and SNR (signal to noise ratio), there is a problem that imaging time increases.

  On the other hand, if multi-echo acquisition is performed using the 3D-FE EPI sequence, high-speed imaging can be performed. However, in the EPI sequence, echo signals having both positive and negative polarities are acquired. For this reason, when there is a flow or motion, there is a problem that the phase shift of the MR signal increases due to the flow or motion, and the image quality deteriorates.

  Accordingly, an object of the present invention is to provide a magnetic resonance imaging apparatus capable of maintaining good image quality regardless of the presence or absence of flow or movement and performing imaging in a short time.

A magnetic resonance imaging apparatus according to an embodiment of the present invention includes a waveform setting unit and an imaging unit. The waveform setting means is a compensating gradient magnetic field for making the gradient magnetic field moment zero or close to zero at the acquisition timing of the magnetic resonance signal for imaging, and is applied in the reading direction of the magnetic resonance signal for imaging Sets the compensation gradient magnetic field waveform. The imaging means collects the magnetic resonance signals for imaging by applying a gradient magnetic field including a compensation gradient magnetic field set by the waveform setting means following the application of the excitation pulse in the readout direction. Image data is generated based on the magnetic resonance signal for imaging. The waveform setting means can select the application period of the compensation gradient magnetic field from a plurality of application periods having different ratios to the application period of the gradient magnetic field for imaging .

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 choice of the waveform of the gradient magnetic field in the RO direction selected in the pulse sequence and gradient magnetic field waveform selection part which are set in the imaging condition setting part shown in FIG. The figure which shows an example of the imaging condition for pre-scans set in the pre-scan condition setting part shown in FIG. The flowchart which shows the flow of the imaging by the magnetic resonance imaging apparatus shown in FIG.

  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.

  Furthermore, the magnetic resonance imaging apparatus 20 includes an ECG unit 38 that acquires an ECG (electro cardiogram) signal of the subject P as necessary. The ECG signal acquired by the ECG unit 38 is configured to be output to the computer 32 via the sequence controller 31.

  A pulse wave synchronization (PPG: peripheral pulse gating) signal that represents pulsation as pulse wave information can be acquired instead of an ECG signal that represents pulsation as heart rate information. The PPG signal is, for example, a signal obtained by detecting a fingertip pulse wave as an optical signal. When acquiring the PPG signal, a PPG signal detection unit is 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 selection unit 40A, a pre-scan condition setting unit 40B, and a phase difference 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 for collecting multi-echo data based on instruction information from the input device 33 and outputting the set imaging conditions to the sequence controller 31. More specifically, the imaging condition setting unit 40 continuously repeats the application of the gradient magnetic field pulse having a predetermined waveform in the readout (RO) direction following the application of the excitation pulse, thereby continuously. It has a function to set a sequence to collect multiple MR echo signals.

  The gradient magnetic field waveform selection unit 40A includes a plurality of imaging phase images that are sequentially acquired following the application of the phase difference information of the plurality of MR echo signals or the excitation pulses that are continuously acquired following the application of the excitation pulses. Based on conditions affecting the phase difference of MR echo signals, the waveform of gradient magnetic field pulses repeatedly applied in the RO direction of multiple MR echo signals for imaging is used for phase difference information of multiple MR echo signals or for imaging A function of setting an appropriate waveform corresponding to a condition that affects the phase difference between the plurality of MR echo signals.

  The phase difference between MR multi-echo signals generated by switching the RO gradient magnetic field changes mainly depending on the flow and the degree of movement in the imaging region. That is, if a flow having a high flow velocity such as blood exists in the imaging region, the phase difference of the MR signal becomes large. On the other hand, if the imaging region is stationary, the phase difference of the MR signal becomes small. Therefore, the imaging region is one of the conditions that affect the phase difference of the MR signal.

  However, if the imaging target is a stationary tissue or organ even if there is a flow or movement such as blood flow in the imaging region, MR signals from the part with flow or movement can be suppressed by adjusting the imaging parameters. Can do. In this case, the influence on the phase difference of the MR signal collected for imaging is reduced even if there is a flow or movement in the imaging region. That is, rather than the imaging region, the speed at which the MR signals for imaging are collected may be dominated by the phase difference of the MR signals. Therefore, the type of substance to be emphasized such as blood and tissue, the assumed speed range of the substance to be emphasized, and the imaging purpose are also important factors that influence the phase difference of MR signals.

  In addition, the interval between echo signal sequences (ETS: echo train space), flip angle (FA), repetition time (TR: TR: The phase difference of the MR signal changes depending on imaging parameters such as repetition time), echo time (TE), and type of prepulse. Therefore, the imaging parameter is one of the conditions that affect the phase difference of the MR signal.

  From the above, the conditions that affect the phase difference of the plurality of MR echo signals for imaging include the imaging parameters, the imaging region, the type of substance to be enhanced, the imaging conditions such as the speed range of the enhancement target, and the imaging purpose. It is done. Therefore, it is repeatedly applied in the RO direction as a condition that affects at least one of the imaging parameter, the imaging region, the type of enhancement target, the speed range of the enhancement target, and the imaging purpose, which affects the phase difference of a plurality of MR echo signals for imaging. The waveform of the gradient magnetic field pulse can be determined.

  The waveform of the gradient magnetic field pulse repeatedly applied in the RO direction can be prepared in advance as a plurality of options and stored in the gradient magnetic field waveform storage unit 44. That is, a plurality of appropriate gradient magnetic field pulse waveforms corresponding to a plurality of different conditions affecting the phase difference of the MR echo signal as described above can be stored in the gradient magnetic field waveform storage unit 44.

  Therefore, the gradient magnetic field waveform selection unit 40 sends an appropriate waveform corresponding to a condition that affects the phase difference of the MR signal designated as the imaging condition for imaging by operating the input device 33 to the gradient magnetic field waveform storage unit 44. By selecting from the stored gradient magnetic field pulse waveform options, it is possible to automatically set the gradient magnetic field pulse waveforms applied in the RO direction of the plurality of MR signals for imaging.

  FIG. 3 is a diagram illustrating an example of a pulse sequence set in the imaging condition setting unit 40 illustrated in FIG. 2 and a gradient magnetic field waveform option in the RO direction selected in the gradient magnetic field waveform selection unit 40A.

  In FIG. 3, the horizontal axis indicates time. In Fig. 3, (A) is an EPI sequence obtained by incorporating one of the gradient magnetic field pulse options in the RO direction, and (B), (C) and (D) are gradient magnetic field pulse options in the RO direction. The sequence (TUNE1, TUNE2, and TUNE3) obtained by incorporating one of the following is shown.

  3 (A), (B), (C), and (D), RF represents an RF pulse and an MR echo signal, Gss represents a gradient magnetic field pulse in the slice selection (SS) direction, Gro1, Gro2 And Gro3 indicate the gradient magnetic field pulse in the RO direction, Gpe indicates the gradient magnetic field pulse in the phase encode (PE) direction, and φ indicates the phase of the MR echo signal. However, in FIGS. 3B, 3C, and 3D, the gradient magnetic field pulse Gss in the SS direction is omitted because it is the same as that in FIG.

  Further, the straight line φ0 shows the phase change of the MR echo signal with zero speed, and the curve φ1 shows the phase change of the MR echo signal moving at a constant speed. Accordingly, the straight line φ0 also represents a change in the zero-order gradient magnetic field moment, and the curve φ1 represents a change in the first-order gradient magnetic field moment.

  Fig. 3 (A) shows that MR echo signals are continuously collected by repeatedly applying a positive RO gradient magnetic field pulse and a negative RO gradient magnetic field pulse in the RO direction following application of the excitation pulse. It is a sequence to do. This sequence corresponds to an EPI sequence in a narrow sense. If the narrowly-defined EPI sequence is called a normal EPI sequence, the normal EPI sequence has good data collection efficiency because all gradient magnetic field pulses applied in the RO direction are for the RO of the MR signal. Moreover, a favorable SNR can be obtained by adjusting the length of the gradient magnetic field pulse for RO.

  On the other hand, in a normal EPI sequence, MR echo signals are collected by both the positive-side RO gradient magnetic field pulse and the negative-side RO gradient magnetic field pulse. For this reason, the primary gradient magnetic field moment does not become zero at the collection timing of the MR echo signal. Therefore, when the MR signal acquisition target moves at a speed that cannot be ignored due to flow, movement, or the like, the phase shift of the MR echo signal from the moving part becomes large. As a result, there is a possibility that artifacts may occur in the image due to the influence of the flow or movement.

  Therefore, when imaging an object with flow or movement using a normal EPI sequence, measures such as measuring the phase distribution in advance by pre-scanning and correcting the phase of the MR signal based on the measured phase distribution, etc. Necessary.

  The sequences shown in FIGS. 3B, 3C, and 3D are all continuously applied by intermittently applying an RO gradient magnetic field pulse having one polarity in the RO direction following application of an excitation pulse. In this sequence, MR echo signals are collected. Therefore, it can be said that the sequences shown in FIGS. 3B, 3C, and 3D are EPI sequences in a broad sense. Here, an EPI sequence in a broad sense that collects MR echo signals with RO gradient magnetic field pulses having one polarity is referred to as a one-echo EPI sequence.

  In the one-echo EPI sequence, a gradient magnetic field pulse having a polarity different from that of the RO gradient magnetic field pulse is applied in the RO direction between adjacent RO gradient magnetic field pulses. This non-RO gradient magnetic field pulse applied intermittently and repeatedly in the RO direction functions as an FC gradient magnetic field pulse Gfc for reducing a phase shift due to the influence of flow or motion. An MR echo signal may be generated depending on the waveform of the FC gradient magnetic field pulse Gfc, but it may not be a sampling target.

  The gradient magnetic field pulse Gfc for FC is a gradient magnetic field pulse for bringing the first gradient magnetic field moment close to zero at the acquisition timing of the MR echo signal. Therefore, the waveform of the FC gradient magnetic field pulse Gfc can be tuned according to the flow and the speed of movement.

  However, in order to reconstruct image data by aligning the phases of MR echo signals from a stationary object, it is necessary to make the zeroth-order gradient magnetic field moment zero at the acquisition timing of MR echo signals. Therefore, it is necessary to perform control so that the area of the FC gradient magnetic field pulse Gfc matches the area of the RO gradient magnetic field pulse. That is, the primary gradient magnetic field moment at the MR echo signal acquisition timing can be adjusted by tuning the FC gradient magnetic field pulse Gfc to change the waveform while matching the area of the RO gradient magnetic field pulse.

  Note that a technique for making zero-order, first-order, or second-order or higher gradient magnetic field moments zero is called GMN (gradient moment nulling). The zero-order gradient magnetic field moment is the effect of the gradient magnetic field on the phase of the stationary spin, and the first gradient magnetic field moment is the effect of the gradient magnetic field on the phase of the spin moving at a constant speed. The magnetic field moment shows the effect of a gradient magnetic field on the phase of a spin moving at a constant acceleration, respectively.

  3 (B), (C) and (D) show examples of one-echo EPI sequences in which positive gradient magnetic field pulses for RO are applied while negative gradient magnetic field pulses for FC Gfc are applied. ing. The single-echo EPI sequences shown in FIGS. 3 (B), (C), and (D) show sequence variations (TUNE1, TUNE2, TUNE3) obtained by tuning the waveform of the FC gradient magnetic field pulse Gfc. ing.

  The TUNE1 single echo EPI sequence shown in FIG. 3B is a sequence in which the application period of the FC gradient magnetic field pulse Gfc is set to 1/4 of the application period of the RO gradient magnetic field pulse. In the one-echo EPI sequence of TUNE1, the application period of the FC gradient magnetic field pulse Gfc that does not contribute to acquisition of MR echo signals is relatively short. For this reason, the collection efficiency of MR signals can be improved.

  However, the effect of bringing the primary gradient magnetic field moment close to zero at the acquisition timing of the MR echo signal is small. Therefore, the phase φ0 of the MR echo signal whose velocity is zero can be made zero at the MR echo signal acquisition timing, but the phase φ1 of the MR echo signal moving at a constant velocity is not zero. On the other hand, as compared with the normal EPI sequence shown in FIG. 3A, the phase shift of the MR echo signal due to the influence of the flow or motion can be reduced.

  Therefore, the one-echo EPI sequence of TUNE1 is considered suitable for efficiently collecting MR echo signals from an object with a relatively low speed. That is, if the moving speed of the MR echo signal collection target is sufficiently low, the phase correction of the MR echo signal can be omitted. In addition, as compared with at least the normal EPI sequence shown in FIG. 3A, MR signal phase shift due to flow and motion effects and image quality degradation due to MR signal phase shift can be reduced.

  The TUNE2 one-echo EPI sequence shown in FIG. 3C is a sequence in which the application period of the FC gradient magnetic field pulse Gfc is set to ½ of the application period of the RO gradient magnetic field pulse. In the TUNE2 one-echo EPI sequence, the effect of the FC gradient magnetic field pulse Gfc for bringing the primary gradient magnetic field moment close to zero can be exhibited well.

  In other words, by applying the FC gradient magnetic field pulse Gfc, not only the zeroth-order gradient magnetic field moment but also the first-order gradient magnetic field moment can be brought close to zero at the acquisition timing of the MR echo signal. Accordingly, the phase φ1 of the MR echo signal that moves at a constant speed together with the phase φ0 of the MR echo signal whose speed is zero at the acquisition timing of the MR echo signal can be made zero.

  Therefore, the one-echo EPI sequence of TUNE2 is considered suitable for collecting MR echo signals from a subject with a high speed, such as when there is a lot of blood flow. In other words, even when the movement speed of the MR echo signal collection target is high, the phase shift of the MR signal and the phase shift of the MR signal due to the influence of the flow and motion are suppressed by the effect of suppressing the phase shift by the FC gradient magnetic field pulse Gfc. It is possible to reduce image quality degradation caused by the image quality. Further, the phase correction of the MR echo signal can be omitted.

  The TUNE3 one-echo EPI sequence shown in FIG. 3D is a V-shaped waveform of the FC gradient magnetic field pulse Gfc. In the TUNE3 single-echo EPI sequence, the temporal efficiency of the MR echo signal can be further improved as compared with other single-echo EPI sequences. In addition, the application period of the RO gradient magnetic field pulse can be secured. For this reason, SNR can be improved.

  Therefore, the one-echo EPI sequence of TUNE3 is considered to be suitable when a target with a relatively low speed is set as a target for collecting MR echo signals, and particularly when priority is given to SNR and data collection efficiency.

  Further, not only the single echo EPI sequence illustrated in FIGS. 3B, 3C and 3D but also single echo EPI sequences having waveforms of various gradient magnetic field pulses Gfc for FC are prepared in advance as options. I can leave. As described above, the combination of the gradient magnetic field pulse Gfc for FC and the waveform of the gradient magnetic field pulse for RO is associated with one or both of the imaging condition and the imaging purpose as options of the waveform of the gradient magnetic field pulse applied in the RO direction. It can be stored in the gradient magnetic field waveform storage unit 44.

  For example, if the imaging region is the neck, it is important to suppress the phase shift of the MR signal due to the influence of blood flow. Therefore, the gradient magnetic field waveform of TUNE2, which has a good effect of bringing the primary gradient magnetic field moment close to zero, can be associated as an appropriate waveform for imaging with the imaging region as a neck. Similarly, in the case of magnetic resonance angiography (MRA) for imaging blood vessels, it is important to suppress the phase shift of MR signals due to the influence of blood flow. Therefore, the gradient magnetic field waveform of TUNE2 that has an excellent effect of bringing the primary gradient magnetic field moment close to zero can be made an appropriate waveform when the imaging purpose is MRA.

  On the other hand, even when blood flow is present in the imaging region, if a substantial part other than blood is emphasized by suppressing a signal from the blood by a water suppression method, adjustment of imaging parameters, or the like, MR signal is collected. In this case, the primary GMN that brings the primary gradient magnetic field moment close to zero is not important. In other words, the application of the FC gradient magnetic field pulse Gfc is not important. Therefore, a waveform of a normal EPI sequence can be associated as an appropriate gradient magnetic field waveform when the imaging region is a stationary region.

  As a specific example, when imaging a knee, it is important to reduce the influence of blood flow. Therefore, if the MR signal from the blood can be suppressed by adjusting the TR, the imaging site and the MR signal collection target can be regarded as a stationary object. In this case, the waveform of a normal EPI sequence can be associated as an appropriate gradient magnetic field waveform with the TR value for suppressing the MR signal from blood and the imaging condition with the imaging region as the knee.

  On the other hand, if the MR signal from the blood cannot be sufficiently suppressed even by adjusting the imaging parameters in the knee imaging, it is important to reduce the influence of the blood flow. Thus, the waveform of the FC gradient magnetic field pulse Gfc corresponding to the blood flow velocity at the knee can be associated with the imaging condition with the imaging region as the knee.

  Also, bones and joints such as the spine and pelvis have almost no flow or movement. In addition, imaging of functional images such as diffusion images and perfusion images using bones and joints as imaging regions is not usually performed. Therefore, an EPI sequence gradient magnetic field waveform or a TUNE3 gradient magnetic field waveform with good MR signal temporal collection efficiency can be associated as an appropriate waveform with an imaging condition using a bone or a joint as an imaging region.

  Furthermore, when imaging the head, an appropriate gradient magnetic field waveform can be changed according to the imaging purpose. For example, when collecting longitudinal relaxation (T1) weighted image data and lateral relaxation (T2) weighted image data of brain tissue, it is important to suppress the influence of pulsation. On the other hand, the stationary part is the MR signal collection target. Therefore, the gradient magnetic field waveform of TUNE3 that can obtain the effect of the gradient magnetic field pulse Gfc for FC and has good data collection efficiency can be associated with the imaging condition in which the imaging region is the brain tissue.

  On the other hand, when imaging a functional image such as a Diffusion image or a Perfusion image on the head, it is difficult to make a uniform determination as to whether or not it corresponds to imaging of a portion having movement. However, the effect of motion can be reduced by adjusting TR. Therefore, the gradient value magnetic field waveform of the normal EPI sequence can be associated with the imaging condition and imaging purpose of the TR value and the diffusion image or perfusion image of the head.

  As described above, the waveforms of the gradient magnetic field pulses in the appropriate RO direction corresponding to various conditions that affect the phase shift of the MR signal such as the imaging conditions and the imaging purpose are shown in FIG. Various waveforms can be selected in advance and associated with each other. Then, when a plurality of gradient magnetic field pulse waveforms in the RO direction corresponding to a plurality of different conditions are stored in the gradient magnetic field waveform storage unit 44, the gradient in the RO direction based on the condition specific information input from the input device 33 is stored. Automatic setting of the magnetic field pulse waveform is possible.

  That is, in the gradient magnetic field waveform selection unit 40A, the waveform of the FC gradient magnetic field pulse Gfc corresponding to the imaging condition and imaging purpose, such as the imaging region and imaging parameters, which are indicators of the degree of phase shift of the MR signal due to flow and motion, Automatic selection from the gradient magnetic field waveform storage unit 44 becomes possible. In other words, the gradient magnetic field waveform selection unit 40A can tune the waveform of the FC gradient magnetic field pulse Gfc based on one or both of the imaging condition and the imaging purpose.

  Specifically, when an imaging condition such as an imaging region and an imaging purpose are input to the imaging condition setting unit 40 by an operation of the input device 33 through an imaging condition editing screen such as a pulse sequence displayed on the display device 34, a gradient magnetic field is obtained. The waveform selector 40A can be configured to automatically set the waveform of the gradient magnetic field pulse in the RO direction corresponding to the imaging condition and the imaging purpose.

  In this case, for example, when the default value of the waveform of the gradient magnetic field pulse in the RO direction is the waveform of a normal EPI sequence, and a specific imaging condition or imaging purpose that is affected by flow or motion is input to the imaging condition setting unit 40 The gradient magnetic field waveform selection unit 40A switches the waveform of the gradient magnetic field pulse to the waveform of the gradient magnetic field pulse accompanying the application of the gradient magnetic field pulse Gfc for FC according to the imaging condition and imaging purpose acquired from the gradient magnetic field waveform storage unit 44. can do. Alternatively, when the imaging condition and the imaging purpose are input to the imaging condition setting unit 40 without setting the default value of the gradient magnetic field pulse waveform in the RO direction, the gradient magnetic field waveform selection unit 40A responds to the imaging condition and the imaging purpose. The gradient magnetic field pulse waveform may be selected from the gradient magnetic field waveform storage unit 44 and set as an imaging condition for imaging.

  Up to this point, the description has been given of the case where the gradient magnetic field pulse waveform in the RO direction is automatically set based on the condition that affects the phase difference of a plurality of MR echo signals for imaging. Further, the gradient magnetic field pulse waveform in the RO direction can be automatically set based on the phase difference information of a plurality of MR signals.

  The pre-scan condition setting unit 40B has a function of setting pre-scan conditions for collecting a plurality of MR echo signals from which phase difference information is to be acquired. In addition, the phase difference acquisition unit 40C has a function of acquiring phase differences of a plurality of MR signals collected by prescan as phase difference information. Furthermore, the gradient magnetic field waveform selection unit 40A is configured to automatically set the waveform of the gradient magnetic field pulse in the RO direction based on the phase difference information when the phase difference information of the MR signal is acquired from the phase difference acquisition unit 40C. Has been.

  Here, two conditions and phase difference information are illustrated as pre-scan conditions and phase difference information acquired by pre-scanning.

  FIG. 4 is a diagram showing an example of prescan imaging conditions set in the prescan condition setting unit 40B shown in FIG.

  In FIGS. 4A and 4B, each horizontal axis represents time. 4A shows an example of simple pre-scan conditions, and FIG. 4B shows an example of pre-scan conditions for acquiring phase difference information with high accuracy.

  As shown in FIG. 4 (A), the pre-scan is performed in a normal EPI sequence in which the gradient magnetic field pulse for PE is not applied, that is, by applying the gradient magnetic field for RO repeatedly and repeatedly after applying the excitation pulse. It is possible to scan by a pulse sequence that collects MR signals. When pre-scanning is performed using a normal EPI sequence in which no gradient magnetic field pulse for PE is applied, MR signal projection data projected on the central axis of k-space is obtained. Then, the phase difference acquisition unit 40C can acquire the phase differences of the plurality of MR signals in the collected k space as phase difference information.

  In order to acquire the phase difference, it is necessary that at least two MR signals are collected as projection data. When collecting three or more MR signals, there are a method for obtaining a phase difference of each MR signal with respect to a reference MR signal and a method for sequentially obtaining a phase difference between two MR signals that are temporally adjacent to each other. . In consideration of the fact that the effect of the second or higher gradient magnetic field moment is reflected, the method of sequentially obtaining the phase difference between two MR signals that are temporally adjacent is preferable in terms of accuracy. When collecting three or more MR signals, a representative value such as an average value or an intermediate value of a plurality of phase differences can be defined as the phase difference of the MR signals. Then, the calculation accuracy of the phase difference can be improved by increasing the number of MR signals to be collected.

  When the phase difference of the MR signal is acquired by the pre-scan shown in FIG. 4A, the gradient magnetic field waveform selection unit 40A repeatedly in the RO direction of the plurality of MR signals for imaging according to the value of the phase difference. The waveform of the applied gradient magnetic field can be set.

  For example, when it is determined that the value of the phase difference is sufficiently small by the threshold processing, the phase shift due to the influence of motion or flow is small. Therefore, an imaging scan can also be executed with a normal EPI sequence. For this reason, the waveform of the gradient magnetic field applied for imaging can be set to the waveform of the gradient magnetic field in a normal EPI sequence.

  On the other hand, when the phase difference value is determined to be medium or large by the threshold processing, it is desirable to apply the FC gradient magnetic field pulse Gfc in order to reduce the phase shift due to the influence of motion or flow. Therefore, the waveform of the gradient magnetic field in the one-echo EPI sequence with the application of the FC gradient magnetic field pulse Gfc corresponding to the degree of the phase difference can be set to the gradient magnetic field waveform applied for imaging. That is, based on the result of the phase difference, switching from the normal EPI sequence to the one-echo EPI sequence and tuning of the FC gradient magnetic field pulse Gfc can be easily performed.

  On the other hand, in the pre-scan shown in FIG. 4B, a plurality of EPI sequences having different gradient magnetic field waveforms applied in the RO direction are sequentially executed. That is, a plurality of one-echo EPI sequences from TUNE1 to TUNEn (n is a natural number) having different waveforms of the normal EPI sequence and FC gradient magnetic field pulse Gfc are set as the prescan sequence. In each EPI sequence, the application of the PE gradient magnetic field pulse is not set.

  Therefore, in the prescan shown in FIG. 4B, a plurality of gradient magnetic fields having different waveforms are applied in the RO direction following the application of the excitation pulse, respectively, so that the plurality of gradient magnetic fields respectively correspond to the waveforms of different gradient magnetic fields. MR signals are collected as projection data.

  In this case, the phase difference acquisition unit 40C can acquire the phase difference between a plurality of MR signals for each waveform having different gradient magnetic fields as phase difference information. That is, the phase difference of the MR signal collected by the normal EPI sequence and the phase difference of the MR signal collected by applying the FC gradient magnetic field pulse Gfc of each waveform are acquired as the phase difference information. The method for calculating the phase difference itself is the same as in the pre-scan shown in FIG. Therefore, at least two MR signals are collected by each EPI sequence.

  The gradient magnetic field waveform selection unit 40A can set the waveform of the gradient magnetic field that minimizes the phase difference to the waveform of the gradient magnetic field applied in the RO direction of the plurality of MR signals for imaging.

  If phase difference information is acquired by executing such a pre-scan, the phase difference of the MR signal that actually reflects the parameter can be confirmed without considering many parameters that affect the phase of the MR signal. Is possible. Therefore, the gradient magnetic field waveform can be optimized more reliably.

  By the functions of the gradient magnetic field waveform selection unit 40A, the pre-scan condition setting unit 40B, and the phase difference acquisition unit 40C as described above, the imaging condition setting unit 40 collects phase difference information of a plurality of MR signals or for imaging. Collect multiple MR signals for imaging by applying a gradient magnetic field with an appropriate waveform corresponding to the conditions that affect the phase difference of multiple MR signals in the RO direction following the application of an excitation pulse. A function for setting imaging conditions is provided.

  On the other hand, the data processing unit 41 has a function of generating image data based on a plurality of MR signals for imaging collected by an imaging scan. More specifically, the data processing unit 41 acquires the MR signals collected by the imaging scan from the sequence controller 31 and arranges them in the k space formed in the k space data storage unit 42, a k space data storage unit The function of reconstructing image data by taking k-space data from 42 and performing image reconstruction processing including Fourier transform (FT), and the image data obtained by reconstruction are stored in the image data storage unit 43 A function of writing, and a function of performing necessary image processing on the image data fetched from the image data storage unit 43 and causing the display device 34 to display the image data.

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

  FIG. 5 is a flowchart showing the flow of imaging by the magnetic resonance imaging apparatus 20 shown in FIG.

  First, 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.

  In step S <b> 1, a method for determining the waveform of the gradient magnetic field Gro applied in the RO direction by operating the input device 33 is input to the imaging condition setting unit 40. That is, information indicating whether the waveform Gro of the gradient magnetic field applied in the RO direction is determined by executing pre-scan or based on the imaging condition and the imaging purpose is set on the imaging condition setting screen. Input to the unit 40.

  When the method to be determined by executing the pre-scan is selected, the imaging condition setting unit 40 determines YES in the determination in step S2. In step S3, the pre-scan condition setting unit 40B sets pre-scan conditions as shown in FIG. 4, and the pre-scan is executed according to the set conditions. As a result, the phase difference of the MR signal corresponding to the type of pre-scan is acquired by the phase difference acquisition unit 40C.

  Note that selection of the pre-scan type as shown in FIGS. 4A and 4B can also be performed by operating the input device 33 through the imaging condition setting screen. That is, by inputting prescan type selection information to the prescan condition setting unit 40B, a desired prescan type can be selected.

  Next, in step S4, the gradient magnetic field waveform selection unit 40A generates a gradient magnetic field waveform Gro for imaging, such that the phase difference of the MR signal becomes smaller, based on the phase difference of the MR signal acquired by the pre-scan. Set to gradient waveform Gro.

  On the other hand, in step S1, when the imaging condition setting unit 40 is instructed to set the gradient magnetic field waveform Gro based on the imaging condition and / or the imaging purpose by operating the input device 33, the imaging condition setting is performed in the determination of step S2. The unit 40 determines NO.

  In step S5, the gradient magnetic field waveform selection unit 40A selects an appropriate gradient corresponding to at least one of the imaging condition and the imaging purpose input to the imaging condition setting unit 40 by the operation of the input device 33 through the imaging condition setting screen. The magnetic field waveform Gro is selected from the gradient magnetic field waveform storage unit 44. Further, the gradient magnetic field waveform selection unit 40A sets the selected gradient magnetic field waveform Gro to the gradient magnetic field waveform Gro for imaging.

  Next, in step S <b> 6, the imaging condition setting unit 40 sets, as an imaging condition, a multi-echo data collection pulse sequence incorporating the gradient magnetic field waveform Gro set for imaging. That is, the imaging condition setting unit 40 sets imaging conditions for collecting imaging data by repeatedly applying a gradient magnetic field having a waveform set for imaging. Further, other imaging conditions are set by the imaging condition setting unit 40. For example, depending on the purpose of imaging, imaging conditions for synchronous imaging using a synchronous signal such as an ECG signal are set.

  Next, in step S <b> 7, an imaging scan based on the imaging conditions is executed by the components of the magnetic resonance imaging apparatus 20 for executing a scan of the sequence controller 31, the static magnetic field magnet 21, and the like.

  More specifically, 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. Then, 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 the RF coil 24 outputs an RF signal. Is generated.

  Therefore, an MR signal generated by nuclear magnetic resonance inside the subject P is received by the RF coil 24 and given to the receiver 30. The receiver 30 performs necessary signal processing on the MR signal, and then performs A / D (analog to digital) 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 raw data to the computer 32. Then, the data processing unit 41 generates image data based on the plurality of MR signals for imaging collected by the imaging scan.

  Here, the MR signal used to generate the image data is the phase difference information acquired by pre-scanning, or the tilt in the RO direction that is appropriately tuned according to conditions that affect the phase, such as imaging conditions and imaging purposes. Collected by applying magnetic field pulses. Therefore, it is possible to generate image data having good image quality with reduced influence of flow and movement. The generated image can be displayed on the display device 34 for diagnosis.

  In other words, the magnetic resonance imaging apparatus 20 as described above is capable of performing multi-functions according to the phase difference information acquired by the pre-scan or the conditions affecting the phase such as the imaging condition and the imaging purpose in order to reduce the influence of the flow and movement. The waveform of the FC gradient magnetic field pulse Gfc in the echo data collection sequence can be tuned. More specifically, the magnetic resonance imaging apparatus 20 pre-scans the switching between the normal EPI sequence and the one-echo EPI sequence and the waveform switching of the FC gradient magnetic field pulse Gfc applied in the one-echo EPI sequence. Can be automatically performed according to the phase difference information acquired by the above or conditions that affect the phase, such as imaging conditions and imaging purposes.

  For this reason, the magnetic resonance imaging apparatus 20 can optimize the waveform of the gradient magnetic field pulse Gfc for FC, which is important for suppressing the influence of flow and movement. That is, when the influence of the flow or motion is large, the ratio of the application time of the gradient magnetic field pulse Gfc for FC is increased so that the effect of the gradient magnetic field pulse Gfc for FC is increased, while the influence of the flow or motion is small The scan time can be shortened by reducing the application time ratio of the FC gradient magnetic field pulse Gfc. As a result, good image quality can be maintained regardless of whether there is a flow or movement, and imaging can be performed in a short time.

  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 38 ECG unit 40 imaging condition setting unit 40A gradient magnetic field waveform selection unit 40B pre-scan condition setting unit 40C phase difference acquisition unit 41 data processing unit 42 k-space data storage unit 43 image data storage unit 44 Magnetic field waveform storage part P

Claims (5)

Compensation gradient magnetic field for making the gradient magnetic field moment zero or close to zero at the acquisition timing of the magnetic resonance signal for imaging, and the waveform of the compensation gradient magnetic field applied in the readout direction of the magnetic resonance signal for imaging Waveform setting means for setting
The imaging magnetic resonance signal is collected by applying a gradient magnetic field including a compensation gradient magnetic field set by the waveform setting means in the readout direction following application of the excitation pulse, and the collected magnetic field for imaging is collected. An imaging means for generating image data based on the resonance signal;
With
The waveform setting means is capable of selecting the compensation gradient magnetic field application period from a plurality of application periods having different ratios relative to the imaging gradient magnetic field application period.
Magnetic resonance imaging device. The plurality of compensation gradient magnetic field application periods having different ratios relative to the imaging gradient magnetic field application period are such that the imaging gradient magnetic field pulse area and the compensation gradient magnetic field pulse area are equal to each other. Set to
The magnetic resonance imaging apparatus according to claim 1. A plurality of compensation gradient magnetic field application periods having different ratios relative to the imaging gradient magnetic field application period are respectively the product of the application time and intensity of the imaging gradient magnetic field pulse, and the pulse of the compensation gradient magnetic field. The product of application time and intensity is set to be equal,
The magnetic resonance imaging apparatus according to claim 1 or 2 . The waveform of the compensating gradient magnetic field is
Phase difference of multiple magnetic resonance signals collected by a gradient magnetic field for imaging applied continuously in the readout direction following application of an excitation pulse, depending on at least one of flow and motion at the imaging site Is set based on the phase difference information of the plurality of magnetic resonance signals that change as
The magnetic resonance imaging apparatus according to any one of claims 1 to 3 . The waveform of the compensating gradient magnetic field is
Imaging parameters, types of enhancement targets, and enhancement targets that are related to the phase difference of multiple magnetic resonance signals collected by the gradient magnetic field for imaging applied continuously in the readout direction following the application of the excitation pulse. Set based on at least one of speed range and imaging purpose;
The magnetic resonance imaging apparatus according to any one of claims 1 to 4 .

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