JP5678163B2 - 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. It is related to the magnetic resonance imaging (MRI) apparatus to be configured, and in particular, it is possible to reduce the influence of B1 non-uniformity by performing universal B1 shimming regardless of the characteristics of each subject. The present invention relates to a magnetic resonance imaging apparatus.

  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 recent years, B1 inhomogeneity has become a problem as the magnetic field of devices increases. B1 inhomogeneity is also called RF magnetic field inhomogeneity, and in a high magnetic field device with a high resonance frequency, RF pulses with a short wavelength are attenuated in vivo and the nonuniformity is increased. As a result, the echo signal also becomes non-uniform. That is, in the high magnetic field MRI apparatus, the signal intensity distribution of the RF transmission pulse is greatly different between the center and the peripheral portion of the imaging region due to the influence of the difference in the dielectric constant of the subject.

  For this reason, it is necessary to perform B1 shimming in order to reduce the influence of B1 nonuniformity. That is, if correction for making the signal intensity distribution of the RF transmission pulse uniform is not performed, sensitivity unevenness may occur between the central portion and the peripheral portion of the imaging region. Therefore, in order to reduce the influence of B1 nonuniformity, research reports have been made that it is effective to transmit an RF transmission pulse whose amplitude and phase are corrected in a region where sensitivity unevenness occurs (for example, Non-Patent Document 1). Non-Patent Document 2 and Non-Patent Document 3).

G. McKinnon et al., “RF Shimming With a Conventional 3T Body Coil”, Proc. Intl. Soc. Mag. Reson. Med. 15 (2007), p173. D. Weyers et al., “Shading Reduction at 3.0T using an Elliptical Drive”, Proc. Intl. Soc. Mag. Reson. Med. 14 (2006), p2023. J. Nistler et al., “Homogeneity Improvement Using A 2 Port Birdcage coil”, Proc. Intl. Soc. Mag. Reson. Med. 15 (2007), p1063.

  However, the area where sensitivity non-uniformity occurs due to the influence of B1 non-uniformity and the phase and amplitude of the appropriate RF transmission pulse depend on characteristics such as the weight, height, volume, water distribution, fat distribution, and muscle distribution of the subject. Depending on. In contrast, the conventionally proposed B1 shimming method obtains in advance the phase and amplitude of a transmission RF pulse according to the characteristics of a specific subject, and sends the transmission RF pulse having the determined phase and amplitude to the subject. It is said that the transmission is performed in a region where the sensitivity unevenness occurs according to the characteristics. For this reason, when the subject changes, the sensitivity unevenness region and the amplitude and phase of an appropriate RF transmission pulse also change, and there is a problem that good B1 shimming cannot be performed constantly. In other words, if the conventional method is used as it is, the amplitude and phase of the transmission RF pulse are adjusted with respect to a predetermined common sensitivity unevenness region, even though the sensitivity unevenness region is different for each subject. .

  Further, every time the subject changes, it is necessary for the user to obtain an appropriate phase and amplitude of the transmission RF pulse, and it is necessary to manually and frequently adjust the phase and amplitude of the transmission RF pulse.

  The above problems hinder the implementation of the B1 shimming function in the high magnetic field device. Thus, it is desired to develop a technique for performing B1 shimming universally and easily even if the subject changes.

  The present invention has been made to cope with such a conventional situation, and it is possible to reduce the influence of B1 nonuniformity by performing universal B1 shimming regardless of the characteristics of each subject. An object is to provide a magnetic resonance imaging apparatus.

Magnetic resonance imaging apparatus according to the present invention, an RF coil for transmitting a RF signal to a subject, said storing means for storing at least one of the amplitude and phase of the patient imaging region and wherein each high-frequency signal of the RF coil And input means for inputting imaging conditions including the imaging part of the subject, and at least one of the amplitude and phase of the high-frequency signal corresponding to the imaging part of the subject and the characteristics of the RF coil is read from the storage means. Imaging means for acquiring image data by performing imaging using at least one of the amplitude and the phase as an imaging condition, and the features of the RF coil are the number, size, and shape of coils used for transmitting RF signals. And at least one of the types .

  In the magnetic resonance imaging apparatus according to the present invention, the influence of B1 nonuniformity can be reduced by performing universal B1 shimming regardless of the characteristics of each subject.

1 is a configuration diagram showing an embodiment of a magnetic resonance imaging apparatus according to the present invention. The figure which shows the detailed structural example of the sequence controller, transmitter, and RF coil which are shown in FIG. The functional block diagram of the computer shown in FIG. The figure explaining the method of determining the area | region for setting ROI based on the data collected by the preparation scan in the ROI setting part shown in FIG. The figure which shows an example of several ROI set in the ROI setting part shown in FIG. The figure explaining the method of searching the optimal amplitude and phase of RF transmission signal by the lattice search method in the amplitude phase setting part shown in FIG. The figure explaining the method of searching the optimal amplitude and phase of RF transmission signal by a bisection method in the amplitude phase setting part shown in FIG. 2 is a flowchart showing a procedure for collecting image data of a subject with B1 shimming by adjusting the phase and amplitude of an RF transmission signal by the magnetic resonance imaging apparatus shown in FIG. 1.

  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, 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 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 / or 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 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 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.

  In addition, 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 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.

  FIG. 2 is a diagram illustrating a detailed configuration example of the sequence controller 31, the transmitter 29, and the RF coil 24 illustrated in FIG.

  As shown in FIG. 2, the sequence controller 31 includes an amplitude control unit 31A, a phase control unit 31B, and a signal generator 31C. The transmitter 29 includes a 90-degree signal distributor 29A, an amplitude / phase balancer 29B, a pair of pre-transmission amplifiers 29C, a pair of power amplifiers 29D, a pair of isolators 29E, and a pair of 50Ω terminations 29F. It functions as a parallel drive RF amplifier that amplifies two RF signals obtained in parallel. An RF front end 38 is provided between the transmitter 29 and the RF coil 24. The RF front end 38 is provided with a transmission / reception selector switch 38A, an I signal transmission / reception channel 38B, a Q signal transmission / reception channel 38C, a 90-degree phase synthesizer 38D, a 50Ω termination 38E, and a pre-reception amplifier 38F. The RF coil 24 includes a plurality of coil elements including a coil element 24A for transmitting / receiving an I signal and a coil element 24B for transmitting / receiving a Q signal.

  The amplitude controller 31A, the phase controller 31B, and the signal generator 31C of the sequence controller 31 are each configured to be controlled by a control signal from the computer 32. The amplitude control unit 31A has a function of adjusting the amplitude of the I signal by giving a control signal to the amplitude phase balancer 29B of the transmitter 29. The phase control unit 31B has a function of adjusting the phase of the I signal by giving a control signal to the amplitude / phase balancer 29B of the transmitter 29. The signal generator 31C has a function of generating an RF signal and outputting it to the 90-degree signal distributor 29A of the transmitter 29.

  An amplitude / phase balancer 29B and a pre-transmission amplifier 29C for Q signal are connected in parallel to the 90-degree signal distributor 29A of the transmitter 29. A pre-transmission amplifier 29C for I signal, a power amplifier 29D and an isolator 29E are connected in series at the subsequent stage of the amplitude / phase balancer 29B, and a power amplifier 29D for Q signal is connected at the rear stage of the pre-transmission amplifier 29C for Q signal. And an isolator 29E are connected in series. A 50Ω termination 29F is connected to each isolator 29E for I and Q signals.

  The 90 degree signal distributor 29A distributes the RF signal acquired from the signal generator 31C into an I signal having a phase of 0 degrees and a Q signal having a phase of 90 degrees, and outputs the distributed I signal to the amplitude phase balancer 29B. On the other hand, it has a function of outputting the Q signal to the corresponding pre-transmission amplifier 29C. The pre-transmission amplifier 29C has a function of setting the amplitude and phase of the I signal to predetermined values in accordance with the control signals from the amplitude control unit 31A and the phase control unit 31B, and the adjusted I signal to the corresponding pre-transmission amplifier 29C. Has a function to output. Each pre-transmission amplifier 29C and each power amplifier 29D have a function of amplifying signals. Each isolator 29E has a function of isolating an input signal and an output signal in series in order to prevent signal wraparound and circuit protection by a 50Ω termination 38E.

  The transmission / reception selector switch 38A of the RF front end 38 has an I signal transmission / reception channel 38B between the I signal isolator 29E side and the 90-degree phase synthesizer 38D side, and a Q signal transmission / reception channel 38C at the Q signal isolator 29E side. And the 90-degree phase synthesizer 38D side have a function of switching between the circuit connection configuration at the time of RF signal transmission and the circuit connection configuration at the time of reception of the echo signal by switching each of them. The I signal transmission / reception channel 38B is connected to a coil element 24A that transmits an I signal and receives an echo signal corresponding to the I signal. The Q signal transmission / reception channel 38 </ b> C is connected to a coil element 24 </ b> B that transmits a Q signal and receives an echo signal corresponding to the Q signal. The 90-degree phase synthesizer 38D combines the echo signals output from the I signal transmission / reception channel 38B and the Q signal transmission / reception channel 38C after performing phase correction for canceling the phase difference of 90 degrees, thereby combining a single signal. A function of obtaining an echo signal and a function of outputting the received echo signal to the reception pre-stage amplifier 38F. The pre-reception amplifier 38F has a function of amplifying the echo signal synthesized by the 90-degree phase synthesizer 38D and outputting it to the receiver 30.

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

  The computer 32 functions as 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, and an amplitude / phase balance database 46 by a program. The imaging condition setting unit 40 includes an ROI setting unit 40A and an amplitude / phase setting unit 40B.

  The imaging condition setting unit 40 has a function of setting imaging conditions including a pulse sequence based on instruction information from the input device 33 and providing the set imaging conditions to the sequence controller control unit 41. For this purpose, the shooting condition setting unit 40 has a function of causing the display device 34 to display shooting condition setting screen information.

  The ROI setting unit 40A of the imaging condition setting unit 40 has a function of setting a single region or a plurality of regions of interest (ROI). Therefore, the ROI setting unit 40A is configured so that the image database 44 can be referred to as necessary.

  Further, the amplitude phase setting unit 40B is a subject whose signal intensity variation between data collected for each ROI set in the ROI setting unit 40A and / or data variation within a single ROI is reduced. A function of determining an absolute value or a relative value of one or both of the phase and the amplitude of the RF transmission signal for excitation of P is provided. Therefore, the amplitude / phase setting unit 40B is configured so that the image database 44 can be referred to. The amplitude phase setting unit 40B is configured to set the determined phase and amplitude of the RF transmission signal as imaging conditions for the imaging scan.

  That is, particularly when the magnetic resonance imaging apparatus 20 is a high magnetic field apparatus, the signal intensity of the RF transmission signal changes for each imaging region due to the influence of so-called B1 nonuniformity. That is, the signal intensity of the RF transmission signal is spatially nonuniform. In addition, the spatial non-uniformity of the RF transmission signal also changes depending on characteristics such as the body weight, height, volume, water distribution, fat distribution, muscle distribution, physique, and age of the subject. For this reason, variation in the intensity of the signal collected for each ROI, for each position in the ROI, and for each characteristic of the subject changes due to the influence of B1 nonuniformity.

  Therefore, the amplitude phase setting unit 40B reduces signal intensity variation between data collected in each ROI and / or data variation within a single ROI in order to reduce the influence of B1 non-uniformity. The appropriate RF transmission signal phase and amplitude are determined. The ROI setting unit 40A can set one or a plurality of ROIs to be subjected to signal intensity variation reduction either manually based on instruction information from the input device 33 or automatically according to a predetermined condition. . For example, it is possible to provide a ROI setting mode in which ROI is manually set by a user operating the input device 33 and a ROI setting mode in which ROI is automatically set.

  When setting the ROI, it is necessary to determine an area for setting the ROI. The area for setting the ROI can be set as a default value and stored in the ROI setting unit 40A. However, based on the data collected by performing the positioning preparatory scan for determining the ROI setting area. It can also be determined automatically or manually.

  FIG. 4 is a diagram for explaining a method of determining an area for setting ROI based on data collected by the preparation scan in the ROI setting unit 40A shown in FIG.

  In FIG. 4, the horizontal axis indicates the position in one direction, and the vertical axis indicates the signal intensity of the image data. For example, when a cylindrical human body equivalent phantom is imaged by a preparatory scan, the signal intensity of the reconstructed image data on a cross section parallel to the central axis is as shown by the solid line in FIG. Note that setting the signal strength of the image data so as to be automatically measured in the ROI setting unit 40A leads to automation of processing and reduction of labor of user work. As shown in FIG. 4, due to the influence of B1 non-uniformity, the signal intensity of the image data increases at the center of the human body equivalent phantom, but decreases at the periphery of the human body equivalent phantom.

  On the other hand, the ideal value of the signal strength of the image data assuming no influence of B1 nonuniformity takes a constant value in the range where the human body equivalent phantom exists as shown by the one-dot chain line in FIG. It is zero outside the human body equivalent phantom. Therefore, it is necessary to determine the amplitude and phase of the RF transmission pulse so that the measured value of the image data indicated by the solid line in FIG. 4 approaches the ideal value indicated by the alternate long and short dash line.

  The region for setting the ROI can be set to a range that is equal to or greater than a certain threshold that is considered to contain a human body equivalent phantom with reference to the signal intensity of the image data. As a result, the existence area of the subject P such as a human body equivalent phantom can be extracted as an area for setting the ROI. This area is set manually by the user by displaying a signal intensity profile of image data as shown in FIG. 4 on the display device 34 and operating the input device 33 such as a mouse to specify the range. However, the ROI setting unit 40A can automatically perform the determination based on the magnitude relationship between the threshold value and the data signal value.

  The threshold value can be defined as an absolute value with respect to the signal intensity, but a low signal portion considered to be a background noise region is automatically extracted from image data and defined as a relative value with respect to the low signal portion. You can also.

  ROI can also be set manually or automatically. The conditions for setting the ROI include the number of ROIs, the position, and the shape, the overlap rate or the overlap amount between adjacent ROIs, and the ratio of the ROI area to the area of the ROI setting region. If adjacent ROIs are overlapped, continuous image data can be obtained by an imaging scan. Therefore, it is possible to set a plurality of overlapped ROIs for the preparation scan in accordance with the ROI for the imaging scan. Parameters for setting the ROI can be stored in the ROI setting unit 40A as default values, and can be changed by operating the input device 33.

  FIG. 5 is a diagram showing an example of a plurality of ROIs set in the ROI setting unit 40A shown in FIG.

  As shown in FIG. 5, the outline of the cross section of the human body equivalent phantom is extracted based on the profile of the image data as a region for setting the ROI. Then, for example, one ROI A1 having a desired size is overlapped at the center and four desired sizes of ROI B1, ROI B2, ROI B3, and ROI B4 are overlapped with each other at a desired overlap ratio. Can be set. Information on the plurality of set ROIs is given from the ROI setting unit 40A to the amplitude / phase setting unit 40B.

  However, as shown in FIG. 4, there is a deviation in signal intensity due to the influence of B1 non-uniformity between the central portion and the peripheral portion of the image data. For this reason, the signal intensity varies for each ROI. There is also a variation in signal strength within a single ROI. Therefore, the amplitude phase setting unit 40B obtains the amplitude and phase of the RF transmission signal as the imaging conditions for the imaging scan such that the variation between the signal intensities within a single ROI and / or between a plurality of ROIs is minimized. Configured.

  As a method of minimizing variations in signal strength within and between ROIs, define an index that represents the variation in signal strength, and determine the amplitude and phase of the RF transmission signal so that the defined index is minimized. And a method of obtaining the amplitude and phase of an RF transmission signal so as to obtain a more uniform B1 distribution and setting it as an imaging condition.

  First, a method for obtaining the amplitude and phase of an RF transmission signal that minimizes an index representing signal intensity variation will be described.

For example, as shown in FIG. 5, when one ROI A1 is set in the central portion and i pieces of ROI B1, ROI B2, ROI B3,..., ROI Bi are set in the peripheral portion, the data between the ROIs The signal strength deviation D can be defined as in equation (1).
[Equation 1]
D = Ia1- (Ib1 + Ib2 + Ib3 +… + Ibi) / i (1)
However, Ia1 is a representative value of the signal intensity for ROI A1, and Ibk (1 ≦ k ≦ i) is a representative value of the signal intensity for ROI Bk. The representative value of the signal strength can be an average value, an intermediate value, a maximum value, or a minimum value of the signal strength within each ROI. Further, representative values such as an average value and an intermediate value may be obtained by removing singular values and error values.

  That is, as shown in Equation (1), the representative value of the signal strength for the central ROI A1 with relatively large signal strength and the average value of the signal strength for the peripheral ROI Bk with relatively small signal strength and Can be defined as the deviation D of the signal strength of the data between multiple ROIs. However, the deviation D of the signal strength of the data between the ROIs can be arbitrarily defined according to the conditions such as the number and shape of the ROIs or the imaging purpose regardless of the equation (1). Further, the deviation of the signal intensity of data within a single ROI can be similarly defined as in equation (1) by setting a plurality of regions and points within the ROI.

  The signal intensity in each ROI can be a value automatically measured by the ROI setting unit 40A, but can also be automatically measured by the amplitude / phase setting unit 40B.

  When the deviation D of the signal strength of the data between multiple ROIs or within a single ROI is obtained as a numerical value, the amplitude and phase of the RF transmission signal that minimizes the deviation D can be calculated by an arbitrary search method. Is possible. Typical methods for searching for the amplitude and phase of an RF transmission signal that minimizes the deviation D include a lattice search method and a bisection method. If necessary, both the lattice search method and the bisection method can be used.

  In the lattice search method, when the phase and amplitude of the RF transmission signal are changed two-dimensionally, that is, the deviation Ddata group of the signal strength between ROIs or within the ROI using the phase and amplitude as parameters, and the deviation Ddata group This is a method for searching for the amplitude and phase of the RF transmission signal that minimizes the deviation D based on the above. For this reason, when the lattice search method is used, a preparation scan in which the phase and amplitude of the RF transmission signal are changed is executed. The imaging conditions for the preparatory scan in which the phase and amplitude of the RF transmission signal are changed are set in the imaging condition setting unit 40. However, considering relaxation time and the like, the RF transmission signal of each slice cross section is taken as multi-slice imaging. Gradually changing the phase or amplitude at a predetermined interval leads to shortening of the photographing time. Furthermore, sharing the preparation scan for collecting data for extracting the contour of the subject P described above and the preparation scan in which the phase and amplitude of the RF transmission signal are respectively changed leads to shortening of the imaging time.

  The amplitude and phase of the RF transmission signal set as the imaging condition in the imaging condition setting unit 40 are output to the amplitude control unit 31A and the phase control unit 31B of the sequence controller 31 through the sequence controller control unit 41, respectively. The amplitude and phase of the I signal of the RF transmission signal can be adjusted to set values by controlling the amplitude phase balancer 29B by the amplitude control unit 31A and the phase control unit 31B.

  FIG. 6 is a diagram for explaining a method of searching for the optimum amplitude and phase of the RF transmission signal by the lattice search method in the amplitude / phase setting unit 40B shown in FIG.

  In FIG. 6, the horizontal axis represents the amplitude of the RF transmission signal as a ratio of the amplitude Aq of the Q signal and the amplitude Ai of the I signal. The vertical axis represents the phase of the RF transmission signal as a ratio of the phase Φq of the Q signal and the phase Φi of the I signal.

  For example, while the phase Φq of the Q signal is set to 0 degree, the phase Φi of the I signal is changed at a constant interval from 90 degrees in the range of + X to -Y, while the amplitude Ai of the I signal is controlled by controlling the amplitude Ai of the I signal. It is assumed that the preparatory scan is executed by changing the ratio between the amplitude Aq and the amplitude Ai of the I signal at a constant interval in the range of Aqmax: Aimin to Aqmin: Aimax. There is a report that Aqmax: Aimin has a limit of 4: 1 and Aqmin: Aimax has a limit of 1: 4. Then, the deviation Ddata (Aq / Ai, Φi) of the signal intensity corresponding to the amplitude ratio Aq / Ai of the Q signal and the I signal and the phase Φi of the I signal on each lattice point in the two-dimensional space as shown in FIG. can get. In the case of multi-slice imaging, there are as many grid points as the number of slices.

  Here, the smallest deviation Ddata_min among the deviations Ddata on the lattice points is searched. Next, surrounding points that surround the lattice point with the smallest deviation Ddata_min are extracted. Then, it can be considered that the point having the minimum deviation Dmin of the signal intensity exists within the range surrounded by the points around the lattice point having the minimum deviation Ddata_min. Therefore, the minimum deviation Dmin and the corresponding point, that is, the amplitude ratio Aqopt / Aiopt and I signal of the Q signal and I signal, are interpolated using the respective deviations Ddata at the points around the lattice point that becomes the minimum deviation Ddata_min. The phase Φi = 90 + ΔΦopt can be estimated. As the interpolation processing, any interpolation processing such as linear interpolation, spline interpolation, and quadratic interpolation can be performed, and the user can select from these interpolation method candidates. For example, three-dimensional spline interpolation is practical.

  By fitting using a function of any order using the deviation Ddata on all grid points, the minimum deviation Dmin and the corresponding Q signal to I signal amplitude ratio Aqopt / Aiopt and the I signal phase Φi = 90 + ΔΦopt can also be obtained. However, as described above, fitting is performed in a narrow range using only the points around the lattice point having the minimum deviation Ddata_min, so that approximation can be established and calculation accuracy can be improved.

  On the other hand, the bisection method determines the magnitude relationship between the signal intensity deviations D1 and D2 between or within the ROI of two image data collected by changing either the amplitude or phase of the RF transmission signal. Searching for the amplitude and phase of the RF transmission signal that minimizes the deviation D by repeating the operation of collecting two image data by changing either the amplitude or phase of the RF transmission signal again in the vicinity of the smaller side Is the method. For this reason, when the bisection method is used, a preparation scan for collecting image data by changing either one of the phase and amplitude of the RF transmission signal, and a signal intensity deviation D1, between the ROIs of the two image data The process of determining the magnitude relationship of D2 is repeatedly executed.

  FIG. 7 is a diagram for explaining a method of searching for the optimum amplitude and phase of the RF transmission signal by the bisection method in the amplitude / phase setting unit 40B shown in FIG.

  In FIG. 7, the horizontal axis represents the amplitude A of the I signal of the RF transmission signal, and the vertical axis represents the signal strength deviation D (A) between the ROIs corresponding to the amplitude A of the I signal. For example, when the amplitude A of the RF transmission signal is changed, two amplitudes A1, A2 that are sufficiently separated from each other and are considered to have the amplitude Aopt corresponding to the minimum deviation Dmin are determined as the base points A1, A2. The amplitude corresponding to the midpoint (A1 + A2) / 2 of A2 is obtained. Next, the midpoints (3A1 + A2) / 4 and (A1 + 3A2) / 4 between the midpoint (A1 + A2) / 2 and the two base points A1 and A2 on both sides adjacent to each other are obtained.

  Then, the amplitude of the RF transmission signal is set to two amplitudes corresponding to the two points (3A1 + A2) / 4 and (A1 + 3A2) / 4 on both sides of the middle point (A1 + A2) / 2 by the preparation scan. Collect image data. Next, each deviation D ((3A1 + A2) / 4), D ((A1 + 3A2) of the signal intensity of image data corresponding to two points (3A1 + A2) / 4, (A1 + 3A2) / 4, respectively / 4) is calculated. Next, the two deviations D ((3A1 + A2) / 4) and D ((A1 + 3A2) / 4) are compared to specify the side having the smaller value. Then, it can be considered that the amplitude Aopt corresponding to the minimum deviation Dmin exists on the side of taking the value of the deviation D (A) smaller than the midpoint (A1 + A2) / 2 of the two base points A1, A2.

  Therefore, two points A1 or A2 and (A1 + A2) / 2 on both sides with the point (3A1 + A2) / 4 or (A1 + 3A2) / 4 corresponding to the value of the small deviation D (A) as the midpoint Image data collection and comparison determination processing corresponding to the two different amplitudes described above are performed as new base points. By repeating such image data collection and comparison determination processing, the minimum deviation Dmin and the corresponding amplitude Aopt can be obtained.

  It should be noted that when the search for amplitude is completed to some extent, it is desirable to search for phase by the same method. Then, the phase Φopt and the amplitude Aopt of the RF transmission signal for minimizing the deviation D can be obtained by alternately repeating the search for the amplitude and the search for the phase while reflecting the search result. In this case, for example, a sequence for multi-slice imaging in which RF transmission signals are transmitted with different amplitudes to collect two slice image data, and two slice image data are transmitted with RF transmission signals having different phases. Prepare multiple sets of multi-slice shooting sequences to collect data, perform deviation D calculation and deviation D comparison judgment processing between each multi-slice shooting, and reflect the processing results to the next multi-slice shooting Will lead to shortened shooting time. In multi-slice imaging, it is considered that the minimum deviation Dmin and the corresponding amplitude Aopt and phase Φopt can be obtained with practical accuracy by repeating about four times for each of amplitude and phase.

  Next, a method for obtaining the amplitude and phase of the RF transmission signal so as to obtain a more uniform B1 distribution and setting as an imaging condition will be described.

  In this case, the B1 distribution (RF magnetic field distribution) is acquired in advance by a preparation scan in which the phase and amplitude of the RF transmission signal are changed. The B1 distribution can be acquired by a known method before shipment or every imaging. For example, the B1 distribution component can be extracted by removing a component due to an influence other than the B1 distribution by calculating between signals acquired by applying a 30 degree RF pulse and a 60 degree RF pulse. The subject P for obtaining the B1 distribution may be a phantom, but it is desirable for accuracy to be a human body. In addition, the B1 distribution varies depending on the body shape such as the weight and height of the subject P, the RF coil 24 used for transmitting the RF transmission signal, and the imaging region. For this reason, it is desirable in terms of accuracy to obtain the body shape of the subject P, the RF coil 24 to be used, and the imaging region.

  The B1 distribution obtained in this way has the same characteristics as the signal intensity distribution. Then, the optimum phase Φopt and amplitude Aopt of the RF transmission signal can be obtained based on the B1 distribution. That is, the amplitude and phase of the RF transmission signal corresponding to the imaging conditions such as the body shape of the subject P and the like, which can obtain a more uniform B1 distribution, can be set as the imaging conditions.

  Up to this point, the method for obtaining the phase Φopt and amplitude Aopt of the RF transmission signal that minimizes the deviation in signal intensity based on the data collected by the preparatory scan and the phase of the RF transmission signal that provides the most uniform B1 distribution Although the method for obtaining Φopt and amplitude Aopt has been described, the optimum RF transmission signal that reduces the influence of B1 non-uniformity obtained in advance according to the imaging conditions such as the characteristics of the subject P, the imaging region, the characteristics of the RF coil 24, etc. The phase Φopt and the amplitude Aopt can be stored in the computer 32 and stored in the computer 32, and the phase Φopt and the amplitude Aopt of the appropriate RF transmission signal can be used according to the imaging conditions.

  Therefore, in the data amplitude phase balance database 46 of the computer 32, the optimum phase Φopt and amplitude Aopt of the RF transmission signal corresponding to the imaging conditions such as the characteristics of the subject P, the imaging region, the characteristics of the RF coil 24, and the like are stored. . The characteristics of the subject P include the body type such as the weight and height of the subject P, the volume, the water distribution, the fat distribution, the muscle distribution, the physique, and the age. The features of the RF coil 24 include the number, size, shape, and type of surface coils used for RF signal transmission.

  The phase Φopt and the amplitude Aopt of the RF transmission signal for each imaging condition obtained in the amplitude phase setting unit 40B by executing the preparatory scan in the past by the method described above can be written and stored in the data amplitude phase balance database 46. . In this case, the phase Φopt and the amplitude Aopt of the appropriate RF transmission signal have different values for each device. On the other hand, the phase Φopt and the amplitude Aopt of an appropriate RF transmission signal for each imaging condition estimated or obtained in advance by another method are not stored in the data amplitude phase balance database 46 at the time of shipment of the apparatus, regardless of the above-described method involving a preparation scan. You can also save it. In this case, the phase Φopt and the amplitude Aopt of the RF transmission signal can be set to values according to the model of the magnetic resonance imaging apparatus 20. However, the phase Φopt and the amplitude Aopt of the appropriate RF transmission signal from the viewpoint of improving accuracy may be obtained for each imaging and stored in the data amplitude phase balance database 46.

  Then, the amplitude phase setting unit 40B refers to the data amplitude phase balance database 46, and the RF transmission signal corresponding to the imaging conditions such as the characteristics of the subject P, the imaging region, and the characteristics of the RF coil 24 input from the input device 33. Are read from the data amplitude phase balance database 46 and set as imaging conditions for the imaging scan.

  In addition, instead of the phase Φopt and the amplitude Aopt of the RF transmission signal, a function or table representing the B1 distribution for each photographing condition is stored in the data amplitude phase balance database 46, and the amplitude phase setting is performed based on the B1 distribution corresponding to the photographing condition. The unit 40B may be configured to set the optimum phase Φopt and amplitude Aopt of the RF transmission signal.

  Next, other functions of the computer 32 will be described.

  The sequence controller control unit 41 has a function of controlling driving by giving the imaging conditions including the pulse sequence received from the imaging condition setting unit 40 to the sequence controller 31 based on information from the input device 33 or other components. . 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 of the subject P by taking in k-space data from the k-space database 42 and performing image reconstruction processing including Fourier transformation (FT). It has a function of writing the image data obtained by the configuration 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 takes in image data from the image database 44 and performs necessary image processing to generate two-dimensional image data for display, and causes the display device 34 to display the generated image data for display. It has a function.

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

  Here, a case will be described in which a plurality of ROIs are set and the amplitude and phase of the RF transmission signal are determined so that the signal intensity deviation between the ROIs is minimized. When determining the amplitude and phase of the RF transmission signal so that the deviation in signal strength within a single ROI is minimized, or when determining the amplitude and phase of the RF transmission signal so that a uniform B1 distribution is obtained Can be imaged in the same way.

  FIG. 8 is a flowchart showing a procedure for collecting image data of the subject P with B1 shimming by adjusting the phase and amplitude of the RF transmission signal by the magnetic resonance imaging apparatus 20 shown in FIG. Reference numerals with numerals indicate the steps of the flowchart.

  First, in step S1, a plurality of ROIs are created manually or automatically in the ROI setting unit 40A of the computer 32. When the ROI is automatically created, a preparatory scan is executed, and image data is obtained by signal processing and image reconstruction processing described later. Then, an area for setting the ROI is extracted with threshold processing of the signal intensity of the image data.

  Next, in step S2, the data amplitude phase balance database 46 is referred to by the amplitude phase setting unit 40B, and the phase and amplitude of the appropriate RF transmission signal corresponding to the imaging conditions such as the characteristics of the subject P and ROI are the data amplitude phase. It is determined whether or not it is stored in the balance database 46.

  If the appropriate phase and amplitude of the RF transmission signal are not stored in the data amplitude phase balance database 46, a preparation scan in which the amplitude and phase of the RF transmission signal are changed is executed in step S3. Even if the phase and amplitude of the appropriate RF transmission signal are stored in the data amplitude phase balance database 46, an instruction to calculate the phase and amplitude of the appropriate RF transmission signal from the input device 33 by the preparation scan is an amplitude phase setting. If it is input to the section 40B, a preparatory scan is executed in step S3.

  For this purpose, the imaging condition setting unit 40 sets imaging conditions for a preparatory scan in which the amplitude and phase of the RF transmission signal are changed. Data collection is performed according to the set shooting conditions.

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

  Then, when an instruction to start the preparation scan is given from the input device 33 to the sequence controller control unit 41, the sequence controller control unit 41 acquires the imaging conditions in which the amplitude and phase of the RF transmission signal are changed from the imaging condition setting unit 40. This is given 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 imaging conditions 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.

  Specifically, the RF signal generated by the signal generator 31C of the sequence controller 31 is output to the 90-degree signal distributor 29A of the transmitter 29, and is distributed to the I signal and the Q signal by the 90-degree signal distributor 29A. The amplitude and phase of the I signal are adjusted in accordance with control signals from the amplitude control unit 31A and the phase control unit 31B in the amplitude phase balancer 29B. The I signal and the Q signal are transmitted from the I signal transmission / reception channel 38B and the Q signal transmission / reception channel 38C to the corresponding coil elements 24A and 24B via the pre-transmission amplifier 29C, the power amplifier 29D, the isolator 29E, and the transmission / reception selector switch 38A, respectively. Is output. As a result, RF transmission signals whose amplitude and phase are adjusted are transmitted toward the subject P from the coil elements 24A and 24B.

  Therefore, an NMR signal generated by nuclear magnetic resonance inside the subject P is received by the coil elements 24A and 24B of the RF coil 24 and passes through the transmission / reception selector switch 38A, the 90-degree phase synthesizer 38D, and the pre-reception amplifier 38F. To the receiver 30. The receiver 30 receives the NMR signal from the RF coil 24, performs necessary signal processing, and then performs A / D conversion to generate raw data that is an NMR signal of digital data. The receiver 30 gives the generated raw data to the sequence controller 31. The sequence controller 31 provides the raw data to the sequence controller control unit 41, and the sequence controller control unit 41 arranges the raw data as k-space data in the k-space formed in the k-space database 42.

  Next, the image reconstruction unit 43 retrieves k-space data from the k-space database 42 to reconstruct the image data, and writes the obtained image data into the image database 44. Thus, the image database 44 stores image data for each of a plurality of ROIs corresponding to different amplitudes and phases of RF transmission signals.

  Next, in step S4, the amplitude / phase setting unit 40B of the computer 32 obtains the deviation between the ROIs of the representative values of the signal strengths as an index representing the variation in the signal strength of the image data between the ROIs according to a predetermined definition. Then, based on the deviations of the plurality of image data corresponding to the amplitudes and phases of different RF transmission signals, the amplitude and phase of the RF transmission signals that minimize the deviation are calculated by an arbitrary search method.

  Next, in step S5, the amplitude / phase setting unit 40B sets the calculated amplitude and phase of the RF transmission signal as imaging conditions for the imaging scan.

  On the other hand, if it is determined in step S2 that the phase and amplitude of the appropriate RF transmission signal corresponding to the imaging condition such as the characteristics of the subject P and the ROI are stored in the data amplitude phase balance database 46, step In S6, the amplitude phase setting unit 40B sets the corresponding phase and amplitude of the appropriate RF transmission signal as imaging conditions for the imaging scan.

  Next, in step S7, an imaging scan is executed using the phase and amplitude of the RF transmission signal set as the imaging condition. The flow of the imaging scan is the same as the flow of the preparation scan.

  Next, the image reconstruction process in step S8 is performed. For this reason, the image database 44 stores image data collected by an imaging scan and in which the influence of B1 nonuniformity is reduced by B1 shimming by adjusting the phase and amplitude of an appropriate RF transmission signal. Then, the image processing unit 45 takes in the image data from the image database 44 and performs necessary image processing to generate two-dimensional image data for display. Further, the generated image data for display is displayed on the display device 34.

  That is, the magnetic resonance imaging apparatus 20 as described above changes the amplitude and phase of the RF transmission signal transmitted from the RF coil 24 in order to reduce the influence of B1 nonuniformity. In particular, the magnetic resonance imaging apparatus 20 can set a desired single or multiple ROIs. Then, for example, the amplitude and phase of the RF transmission signal are determined so that variation in signal strength of data within and / or between ROIs is reduced. Alternatively, the amplitude and phase of the RF transmission signal are determined so that a more uniform B1 distribution can be obtained.

(effect)
For this reason, according to the magnetic resonance imaging apparatus 20, the degree of freedom in setting a region to be corrected for variations in signal intensity can be improved, and operability related to the B1 shimming target region can be improved. Further, B1 shimming can be performed by simply obtaining the optimum amplitude and phase of the RF transmission signal adaptively to the imaging conditions such as the characteristics of the subject P. For this reason, it is possible to transmit the NMR signal from the RF coil 24 to the data processing system without deteriorating the NMR signal, and the B1 shimming function can be mounted on the apparatus.

(Modification)
In the above-described embodiment, the deviation of the signal intensity of the image data corresponding to each ROI is used as an indicator of variation. However, the deviation of the signal intensity of the k-space data corresponding to each ROI can be used as an indicator of variation. .

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, 24B Coil element 25 Control system 26 Static magnetic field power supply 27 Gradient magnetic field power supply 28 Shim coil power supply 29 Transmitter 29A 90 degree signal distributor 29B Amplitude phase Balancer 29C Pre-transmission amplifier 29D Power amplifier 29E Isolator 29F 50Ω termination 30 Receiver 31 Sequence controller 31A Amplitude controller 31B Phase controller 31C Signal generator 32 Computer 33 Input device 34 Display device 35 Computing device 36 Storage device 37 Bed 38 RF front End 38A Transmission / reception selector switch 38B I signal transmission / reception channel 38C Q signal transmission / reception channel 38D 90-degree phase synthesizer 38E 50Ω termination 38F Reception preamplifier 40 Imaging condition setting unit 40A ROI setting unit 0B amplitude phase setting unit 41 the sequence controller control unit 42 k-space database 43 image reconstruction unit 44 image database 45 the image processing unit 46 amplitude phase balance database P subject

Claims (4)

An RF coil that transmits a high-frequency signal to the subject ;
Storage means for storing at least one of an amplitude and a phase of a high-frequency signal for each feature of the imaging region of the subject and the RF coil ;
Input means for inputting imaging conditions including an imaging region of the subject;
Image data is obtained by reading at least one of the amplitude and phase of a high-frequency signal corresponding to the imaging part of the subject and the characteristics of the RF coil from the storage unit and performing imaging using at least one of the amplitude and phase as an imaging condition. An imaging means for obtaining
With
The RF coil is characterized by at least one of the number, size, shape, and type of coils used for transmitting RF signals.
Magnetic resonance imaging apparatus characterized by. The storage means further stores at least one of the amplitude and phase for each feature of the subject,
The imaging means performs imaging using at least one of the amplitude and phase according to the imaging region , the characteristics of the RF coil, and the characteristics of the subject as the imaging conditions.
Magnetic resonance imaging apparatus according to claim 1, characterized in that. The characteristic of the subject is the body shape of the subject.
The magnetic resonance imaging apparatus according to claim 2. The RF coil is a transmission / reception RF coil for both transmission and reception,
A switch for switching a transmission signal to the transmission / reception RF coil and a reception signal from the transmission / reception RF coil;
The magnetic resonance imaging apparatus according to claim 1, further comprising:

Images