Magnetic resonance imaging device and magnetic resonance imaging program

Abstract

To reduce N / 2 artifacts in a comparatively simple way.SOLUTION: A magnetic resonance imaging apparatus includes a setting unit, an imaging unit, an addition unit, and a generation unit. The setting unit sets a first pulse sequence in which collection of first magnetic resonance signal set is started after a first delay time from a first excitation pulse, and a second pulse sequence in which collection of a second magnetic resonance signal set is started after a second delay time different from the first delay time from a second excitation pulse. The imaging unit applies the first pulse sequence and the second pulse sequence to a subject, and collects the first magnetic resonance signal set and the second magnetic resonance signal set respectively. The addition unit adds and averages a first dataset based on the collected first magnetic resonance signal set and a second dataset based on the collected second magnetic resonance signal set, and generates a composite dataset. The generation unit generates a magnetic resonance image on the basis of the composite dataset.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The embodiments disclosed in the present specification and drawings relate to a magnetic resonance imaging apparatus and a magnetic resonance imaging program. [Background technology]

[0002] A magnetic resonance imaging device is an imaging device that excites the nuclear spins of a subject placed in a static magnetic field with radio frequency (RF) signals at the Larmor frequency, and generates images by reconstructing magnetic resonance (MR) signals generated from the subject as a result of the excitation.

[0003] One of the imaging methods of magnetic resonance imaging is a high-speed imaging method called Echo Planar Imaging (EPI), in which magnetic resonance signals, i.e., MR (Magnetic Resonance) signals, are acquired by applying a phase encoding amount, for example, stepwise, while alternately switching the polarity of the readout gradient field between positive and negative.

[0004] EPI is an imaging technique that can acquire the MR signals necessary for image generation in an extremely short time, but it is also an imaging technique that is prone to artifacts. For example, by acquiring MR signals in the phase encoding direction while alternately changing the polarity of the readout gradient magnetic field, different amounts of phase error occur between odd-numbered lines and even-numbered lines in the phase encoding direction. This is known to result in an artifact called the N / 2 (N-half) artifact (or N-half ghost artifact). The N / 2 artifact is an artifact in which a ghost of the true image appears at a position shifted, for example, by 1 / 2 of the field of view (FOV) in the phase encoding direction.

[0005] Although various techniques have been proposed to suppress the N / 2 artifact, it is difficult to completely eliminate the N / 2 artifact, and further improvements are desired. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 5-68674 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-116815 Summary of the Invention [Problem to be solved by the invention]

[0007] One of the problems to be solved by the embodiments disclosed in this specification and the drawings is to reduce N / 2 artifacts in a relatively simple manner. However, the problems to be solved by the embodiments disclosed in this specification and the drawings are not limited to the above problem. Problems corresponding to the effects of the configurations shown in the embodiments described below can also be positioned as other problems. [Means for solving the problem]

[0008] According to one embodiment, a magnetic resonance imaging apparatus includes a setting unit, an imaging unit, an adder, and a generator. The setting unit sets a first pulse sequence that starts acquiring a first set of magnetic resonance signals after a first delay time from a first excitation pulse, and a second pulse sequence that starts acquiring a second set of magnetic resonance signals after a second delay time from a second excitation pulse that is different from the first delay time. The imaging unit applies the first pulse sequence and the second pulse sequence to a subject to acquire the first set of magnetic resonance signals and the second set of magnetic resonance signals, respectively. The adder averages a first data set based on the acquired first set of magnetic resonance signals and a second data set based on the acquired second set of magnetic resonance signals to generate a composite data set. The generator generates a magnetic resonance image based on the composite data set. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a configuration diagram showing an example of the overall configuration of a magnetic resonance imaging apparatus according to an embodiment; [Figure 2] A pulse sequence diagram for a typical GRE (gradient echo)-based EPI imaging method (GRE-EPI) and a diagram showing the placement of acquired data in k-space. [Figure 3] A diagram for explaining the N / 2 artifact that occurs in the EPI method. [Figure 4] FIG. 1 is a diagram illustrating the concept of a first conventional example of an N / 2 artifact suppression technique. [Figure 5] FIG. 10 is a diagram illustrating the concept of a second conventional example of N / 2 artifact suppression technology. [Figure 6] FIG. 1 is a block diagram of a magnetic resonance imaging apparatus according to a first embodiment including a configuration related to suppression of N / 2 artifacts. [Figure 7] 5 is a flowchart showing an example of N / 2 artifact suppression processing in the magnetic resonance imaging apparatus according to the first embodiment. [Figure 8] 4 is a sequence diagram showing a first example of first and second pulse sequences in the first embodiment. [Figure 9] 6 is a sequence diagram showing a second example of the first and second pulse sequences in the first embodiment. [Figure 10] 5A and 5B are diagrams schematically showing the effect of N / 2 artifact suppression processing in the magnetic resonance imaging apparatus according to the first embodiment. [Figure 11] 10 is a flowchart showing an example of N / 2 artifact suppression processing in the magnetic resonance imaging apparatus according to the second embodiment. [Figure 12] 10 is a sequence diagram of four pulse sequences of the EPI method according to the second embodiment. [Figure 13] 10A and 10B are diagrams schematically showing the effect of N / 2 artifact suppression processing in the magnetic resonance imaging apparatus 1 according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

[0011] (First embodiment) 1 is a block diagram showing the overall configuration of a magnetic resonance imaging apparatus 1 according to the first embodiment. The magnetic resonance imaging apparatus 1 of the embodiment is configured to include a magnet gantry 100, a bed 500, a control cabinet 300, a console 400, etc.

[0012] The magnetic gantry 100 has a static magnetic field magnet 10, a gradient magnetic field coil 11, a WB (Whole Body) coil 12, etc., and these components are housed in a cylindrical housing. The bed 500 has a bed body 50 and a tabletop 51. The magnetic resonance imaging apparatus 1 also has a local coil 20 disposed close to the subject.

[0013] The control cabinet 300 includes a static magnetic field power supply 30, gradient magnetic field power supplies 31 (for X-axis 31x, Y-axis 31y, and Z-axis 31z), an RF receiver 32, an RF transmitter 33, and a sequence controller .

[0014] The static magnetic field magnet 10 of the magnetic gantry 100 has a roughly cylindrical shape and generates a static magnetic field within a bore (the space inside the cylinder of the static magnetic field magnet 10), which is the imaging region of a subject (e.g., a patient). The static magnetic field magnet 10 incorporates a superconducting coil, which is cooled to an extremely low temperature by liquid helium. In the excitation mode, the static magnetic field magnet 10 generates a static magnetic field by applying a current supplied from a static magnetic field power supply (not shown) to the superconducting coil. After that, when the static magnetic field magnet 10 transitions to the persistent current mode, the static magnetic field power supply is disconnected. Once transitioned to the persistent current mode, the static magnetic field magnet 10 continues to generate a strong static magnetic field for a long period of time, for example, for more than one year. The static magnetic field magnet 10 may also be configured as a permanent magnet.

[0015] The gradient magnetic field coil 11 also has a roughly cylindrical shape and is fixed inside the static magnetic field magnet 10. This gradient magnetic field coil 11 applies gradient magnetic fields to the subject in the X-axis, Y-axis, and Z-axis directions by currents supplied from gradient magnetic field power supplies (31x, 31y, 31z).

[0016] The bed body 50 of the bed 500 has a top plate 51 that can be moved up and down, and the subject placed on the top plate 51 is moved to a predetermined height before imaging. Then, during imaging, the top plate 51 is moved horizontally to move the subject into the bore.

[0017] The WB coil 12 is fixed in a roughly cylindrical shape so as to surround the subject inside the gradient magnetic field coil 11. The WB coil 12 transmits RF pulses transmitted from the RF transmitter 33 toward the subject, and also receives MR signals emitted from the subject due to excitation of hydrogen nuclei.

[0018] The local coil 20, also called a surface coil or an RF coil, receives magnetic resonance signals emitted from the subject at a position close to the surface of the subject's body. The local coil 20 is composed of, for example, a plurality of element coils. There are various types of local coils 20 depending on the imaging region of the subject, such as for the head, chest, spine, lower limbs, or whole body, but FIG. 1 illustrates a local coil 20 for the chest.

[0019] The RF transmitter 33 transmits RF pulses to the WB coil 12 based on instructions from the sequence controller 34. Meanwhile, the RF receiver 32 detects MR signals received by the WB coil 12 and the local coil 20, digitizes the detected MR signals, and sends them to the sequence controller 34.

[0020] The sequence controller 34 scans the subject by driving the gradient magnetic field power supply 31, the RF transmitter 33, and the RF receiver 32 under the control of the console 400. The sequence controller 34 receives MR signals collected by the scan from the RF receiver 32 and sends them to the console 400.

[0021] The sequence controller 34 includes a processing circuit (not shown). This processing circuit is configured with hardware such as a processor that executes a predetermined program, an FPGA (Field Programmable Gate Array), or an ASIC (Application Specific Integrated Circuit). The console 400 is configured as a computer having a processing circuit 40, a memory circuit 41, an input device 43, and a display 42.

[0022] The memory circuitry 41 is a storage medium including a ROM (Read Only Memory), a RAM (Random Access Memory), and external storage devices such as an HDD (Hard Disk Drive), an optical disk device, etc. The memory circuitry 41 stores various information and data, as well as various programs executed by the processor included in the processing circuitry 40.

[0023] The input device 43 includes various devices such as a mouse, keyboard, trackball, touch panel, etc., which are used by the operator to input various information and data. The display 42 is a display device such as a liquid crystal display panel, a plasma display panel, an organic EL panel, etc.

[0024] The processing circuit 40 is a circuit including, for example, a CPU or a dedicated or general-purpose processor. The processor executes various programs stored in the storage circuit 41 to realize various functions described below. The processing circuit 40 may be configured with hardware such as an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit). The various functions described below can also be realized by such hardware. The processing circuit 40 can also realize various functions by combining software processing by a processor and a program with hardware processing.

[0025] The console 400 controls the entire magnetic resonance imaging apparatus 1. Specifically, it accepts imaging conditions and other various information and instructions through the operation of a mouse, keyboard, etc. (input device 43) by an operator such as a medical technician. The processing circuitry 40 then causes the sequence controller 34 to execute a scan based on the input imaging conditions, while reconstructing an image based on the raw data transmitted from the sequence controller 34. The reconstructed image is displayed on a display 42 or stored in a memory circuitry 41.

[0026] The magnetic resonance imaging apparatus 1 according to the first embodiment is capable of further reducing the N / 2 artifacts that occur, for example, in the EPI imaging method in a relatively simple manner compared to conventional methods. Prior to explaining this, however, we will briefly explain the EPI imaging method and the N / 2 artifacts, as well as examples of conventional techniques for suppressing the N / 2 artifacts.

[0027] Figure 2(a) shows an example of a pulse sequence diagram for a typical GRE (gradient echo)-based EPI imaging method (GRE-EPI). The first row in Figure 2(a) shows an RF pulse. In the GRE-based EPI imaging method, one excitation pulse is applied as the RF pulse. The flip angle of the excitation pulse is set to, for example, 90°.

[0028] The second row of Fig. 2(a) shows the slice selection gradient magnetic field pulse Gss. The application direction of the slice selection gradient magnetic field pulse Gss can be any direction, but in the following description, it is assumed that the application direction of the slice selection gradient magnetic field pulse Gss is the Z direction (see Fig. 1) corresponding to the head-to-foot direction of the subject.

[0029] The third row of Fig. 2(a) shows the phase encoding gradient magnetic field pulse Gpe. The application direction of the phase encoding gradient magnetic field pulse Gpe can be any direction, but in the following description, it is assumed that the application direction of the phase encoding gradient magnetic field pulse Gpe is the Y direction (see Fig. 1) corresponding to the dorsoventral direction of the subject.

[0030] 2(a) shows a readout gradient magnetic field pulse Gro. In the following description, it is assumed that the readout gradient magnetic field pulse Gro is applied in the X direction (see FIG. 1) corresponding to the left-right direction of the subject.

[0031] As is well known, in the EPI method, the polarity of the readout gradient magnetic field pulse Gro is repeatedly applied while alternating between positive and negative, while a phase encoding gradient magnetic field pulse Gpe having a short pulse width and blip shape is applied between each pulse of the readout gradient magnetic field pulse Gro.

[0032] The phase encoding gradient magnetic field pulse Gpe (prephasing pulse) with a wide pulse width and negative polarity applied immediately after the excitation pulse is intended to shift the initial value of the phase encoding amount to the maximum negative value. Also, the readout gradient magnetic field pulse Gro (prephasing pulse) with a pulse width half that of the excitation pulse is intended to shift the initial value of the frequency encoding amount to the maximum negative value.

[0033] The fifth row of Fig. 2(a) shows MR signals. N sets of MR signals, for example, from repetition number (1) to repetition number (N), are read out by each pulse of the readout gradient magnetic field pulse Gro. The read-out MR signals, i.e., the MR signals acquired by the pulse sequence of the EPI method described above, are arranged in k-space represented by the kx direction and ky direction as shown in Fig. 2(b).

[0034] Corresponding to the application order and application direction of the phase encoding gradient magnetic field pulse Gpe and the readout gradient magnetic field pulse Gro described above, for example, the MR signal of repetition number (1) is arranged on line (1) corresponding to the maximum negative value in the phase encoding direction (ky direction) from negative to positive in the frequency encoding direction (kx direction), and the MR signal of repetition number (2) is arranged on line (2) in the phase encoding direction (ky direction) from positive to negative in the frequency encoding direction (kx direction).

[0035] In this way, the MR signals from repetition number (1) to repetition number (N) are arranged in k-space from line (1) to line (N) in the phase encoding direction (ky direction) while alternating the direction of the frequency encoding direction (kx direction). In FIG. 2(b), odd-numbered lines are indicated by solid lines, and even-numbered lines are indicated by dashed lines.

[0036] Fig. 3 is a diagram for schematically explaining the N / 2 artifact that occurs in the EPI method. Fig. 3(a) is a diagram illustrating an example of MR signals (i.e., k-space data) arranged in k-space, and is the same as Fig. 2(b). Fig. 3(b) is a diagram for schematically showing the N / 2 artifact. In Fig. 3(b), the circle shown in the center is the true image, and the upper and lower semicircles are N / 2 artifacts, i.e., ghosts of the true image.

[0037] The N / 2 artifact is caused by variations in the amplitude and phase of MR signals between odd and even lines, which are caused by various imperfections such as eddy magnetic fields and inhomogeneity of the static magnetic field.

[0038] Fluctuations in the amplitude and phase of MR signals between odd and even lines cause fluctuations of 2 / N cycles in the phase encoding direction (ky direction) of k-space. As a result, in the real space generated by Fourier transforming the k-space data, ghosts (i.e., N / 2 artifacts) occur at N / 2 cycles, which is the reciprocal of the 2 / N cycle, in the Y direction corresponding to the phase encoding direction.

[0039] The N / 2 artifact appears in the Y direction at a position where the true image is shifted in both the positive and negative directions by half the FOV (i.e., by 1 / 2 of N, the total number of lines in the Y direction). Note that when parallel imaging is used in conjunction with EPI imaging to thin out lines in the Y direction, the position where the N / 2 artifact appears is not limited to this. However, for the sake of convenience, the following description will be given assuming that the N / 2 artifact appears at a position shifted by 1 / 2 of N, the total number of lines in the Y direction, from the true image. The N / 2 artifact is a ghost image that is unnecessary for image diagnosis, and techniques for suppressing the N / 2 artifact have been studied. 4 and 5 are diagrams showing a typical example of a conventional N / 2 artifact suppression technique.

[0040] 4 is a diagram illustrating the concept of a first conventional example of an N / 2 artifact suppression technique disclosed in, for example, Patent Document 1. In the first conventional example, a reference scan is performed to acquire MR signals separately from a scan (hereinafter referred to as a main scan) for generating a diagnostic image. In the first conventional example, a technique is adopted in which phase correction data is generated from the MR signals acquired in the reference scan, and data acquired in the main scan is corrected using the phase correction data.

[0041] Specifically, in the reference scan, as shown in Figure 4(a), the same pulse sequence as that used in the EPI method for the main scan is applied to the subject, except that no gradient magnetic field for phase encoding is applied, and MR signals numbered (1) to (N) are acquired. Then, these MR signals are one-dimensionally Fourier transformed in the X direction, and the linear expression for the phase error θn(X) shown below is calculated from the phase amount of the obtained complex signal at each position in real space in the X direction by, for example, linear approximation using the least squares method. θn(X)=αn·X+βn (n=1~N) (Formula 1)

[0042] Then, the N / 2 artifact is suppressed by correcting the phase of the data collected in the main scan using the linear approximation formula shown in (Formula 1), that is, the linear phase correction amount.

[0043] 5 is a diagram illustrating the concept of a second conventional example of N / 2 artifact suppression technology disclosed in, for example, Patent Document 2. In the second conventional example, data collected in a scan (main scan) for generating a diagnostic image and data collected in a reference scan performed separately from the main scan are complex added together, and an image of real space is generated based on the complex added data.

[0044] Here, the polarity of the readout gradient magnetic field pulse is reversed between the pulse sequence of the main scan (FIG. 5(a)) and the pulse sequence of the reference scan (FIG. 5(b)) used in the second conventional example. That is, of the two MR signals to be complex added, one MR signal is acquired by a readout gradient magnetic field pulse of positive polarity, and the other MR signal is acquired by a readout gradient magnetic field pulse of negative polarity.

[0045] In this way, in the second conventional example, it is said that by complex adding two MR signals acquired by readout gradient magnetic field pulses of different polarities, the phase error is canceled out and the N / 2 artifact can be suppressed.

[0046] However, in the first conventional example described above, the approximation formula for the phase error used for correction is linear, so it is not possible to correct a nonlinear phase error of second order or higher. As a result, in the first conventional example, a nonlinear phase error remains, and it is not possible to completely suppress the N / 2 artifact.

[0047] On the other hand, in the second conventional example described above, two pieces of data collected using readout gradient magnetic field pulses of opposite polarities, positive and negative, are complex-added. However, because the influence of disturbances caused by eddy currents and the like differs depending on the polarity of the readout gradient magnetic field pulse, complex-adding two MR signals does not completely cancel out the phase error. Furthermore, because the polarities of the readout gradient magnetic field pulses are different, the polarity of the phase error in the readout direction is reversed, and the direction of the position error in real space is reversed. Therefore, when two pixel values are averaged, the pixel values are averaged, making it easy for blurring to occur in the image. In contrast to this, the method for suppressing N / 2 artifacts according to the present embodiment, which will be described below, improves upon the problems of the first and second conventional examples described above.

[0048] Fig. 6 is a block diagram of a magnetic resonance imaging apparatus 1 according to a first embodiment, including a configuration related to suppression of N / 2 artifacts. As shown in Fig. 6, the processing circuitry 40 of the magnetic resonance imaging apparatus 1 implements the following functions: an imaging condition setting function F01, an addition function F02, and a generation function F05. Here, the addition function F02 is configured by a conversion function F03 and an averaging function F04. As described above, each of these functions is implemented, for example, by a processor included in the processing circuitry 40 executing a predetermined program.

[0049] 1, the components other than the console 400 (the magnet gantry 100 including the static magnetic field magnet, the control cabinet 300 including the sequence controller 34, and the bed 500) constitute an imaging section 600. Note that the memory circuitry 41, the input device 43, and the display 42 in FIG. 6 are the same as those shown in FIG. 1, and are therefore denoted by the same reference numerals.

[0050] Among the above-described configurations, the imaging condition setting function F01 sets a first pulse sequence and a second pulse sequence. Here, the first pulse sequence is a pulse sequence that starts acquiring a first set of MR signals after a first delay time from a first excitation pulse, for example, a first pulse sequence according to the EPI method. On the other hand, the second pulse is a pulse sequence that starts acquiring a second set of MR signals after a second delay time from a second excitation pulse that is different from the first delay time, for example, a second pulse sequence that also follows the EPI method like the first pulse sequence. Specific examples of the first and second pulse sequences will be described later.

[0051] The imaging section 600 applies the set first and second pulse sequences to the subject, and acquires a set of first and second MR signals corresponding to the first and second pulse sequences, respectively.

[0052] The addition function F02 generates a composite data set by averaging a first data set based on a first set of acquired MR signals and a second data set based on a second set of acquired MR signals. The addition averaging function F04 in the addition function F02 performs different averaging for each of the following three cases:

[0053] In the first case, the first and second data sets to be averaged are each defined in k-space. In this case, the spatial coordinates of the first and second data sets are expressed, for example, as (kx, ky). In this case, the conversion function F03 sends the sets of digitized first and second MR signals output from the sequence controller 34 to the averaging function F04 as the first and second data sets substantially as they are. The averaging function F04 generates a composite data set by complex-adding the first and second data sets defined in k-space.

[0054] The generating function F05 generates a magnetic resonance image based on the synthetic data set. In a first case, the generated synthetic data set is a data set defined in k-space. Thus, in the first case, the generating function F05 generates the magnetic resonance image by transforming the synthetic data set into a data set in real space, for example by a two-dimensional Fourier transform.

[0055] In the second case, the first and second data sets to be averaged are each defined in a hybrid space that combines k-space and real space. The hybrid space is, for example, a space defined in the X direction by real space and the Y direction by k-space. In this case, the spatial coordinates of the first and second data sets are expressed, for example, as (X, k-y). In this case, the transformation function F03 generates the first and second data sets by, for example, performing a one-dimensional Fourier transform on the X-direction data set of the first and second digitized MR signals output from the sequence controller 34 to convert it into a real-space data set and leaving the Y-direction data set essentially unchanged. The averaging function F04 generates a composite data set by complex-adding the first and second data sets defined in the hybrid space.

[0056] In the second case, the generated synthetic data set is a hybrid space data set in which the X direction is defined by real space and the Y direction is defined by k-space. Therefore, in the second case, the generation function F05 generates a magnetic resonance image in which both the X and Y directions are transformed into real space data sets by performing a one-dimensional Fourier transform only on the Y direction data set of the synthetic data set.

[0057] In the third case, the first and second data sets to be averaged are each defined in real space. In this case, the spatial coordinates of the first and second data sets are expressed, for example, as (X, Y). In this case, the conversion function F03 generates the first and second data sets by converting each of the sets of digitized first and second MR signals output from the sequence controller 34 into two-dimensional real space data sets, for example, by performing a two-dimensional Fourier transform. The averaging function F04 may generate a composite data set by complex-adding the first and second data sets defined in real space, or may generate a composite data set by adding the absolute values of the first and second data sets.

[0058] In the third case, the generated synthetic data set is a data set that has been transformed into real space in both the X and Y directions. In the third case, the synthetic data set is essentially a magnetic resonance image. Therefore, the generation function F05 outputs the input synthetic data set as a magnetic resonance image essentially as is.

[0059] The magnetic resonance images generated in each of the first to third cases are provided to a user, for example, by being output to a display 42. Alternatively, the magnetic resonance images may be stored in the storage circuitry 41 or in an external image server.

[0060] FIG. 7 is a flowchart showing an example of N / 2 artifact suppression processing in the magnetic resonance imaging apparatus 1 according to the first embodiment. First, in step ST100, the imaging condition setting function F01 sets a first pulse sequence and a second pulse sequence.

[0061] 8(a) and 8(b) are sequence diagrams showing a first example of the first and second pulse sequences. As shown in FIG. 8(a), the first pulse sequence starts with a first delay time t d1 After that, the acquisition of the first set of MR signals from number (1) to number (N) is started. In addition, the second pulse sequence starts with a first delay time t from the excitation pulse (second excitation pulse) as shown in FIG. 8(b). d1 A second delay time t different from d2 After that, start acquiring a second set of MR signals numbered (1) to (N).

[0062] The first and second pulse sequences are, for example, pulse sequences of the GRE-EPI method (GRE-EPI) shown in FIG. 2. The first delay time t d1 corresponds to the delay time from the first excitation pulse to the beginning of the MR signal read out by the first readout gradient magnetic field pulse among the plurality of readout gradient magnetic field pulses in the EPI method. Similarly, the second delay time t d2 corresponds to the delay time from the second excitation pulse to the beginning of the MR signal read out by the first readout gradient magnetic field pulse of a plurality of readout gradient magnetic field pulses in the EPI method. The difference between the first and second pulse sequences is the first and second delay times t d1 , t d2 are different from each other.

[0063] Here, the first delay time t d1 and the second delay time t d2 The difference Δt dis preferably set to be short enough so that the level difference due to transverse relaxation between the MR signals acquired by the first and second pulse sequences does not become too large. For example, in the EPI method, the interval between adjacent readout gradient magnetic field pulses, i.e., the time interval between the MR signals for one line read out by the readout gradient magnetic field pulses, is set to t ESP (or simply ESP (echo space)), Δt d It is preferable to set Δt to ESP or less. d may be set to approximately half of ESP.

[0064] 9(a) and 9(b) are sequence diagrams showing a second example of the first and second pulse sequences. The first and second pulse sequences in the second example are pulse sequences of the SE (spin echo) EPI method (SE-EPI). In the pulse sequence of the SE EPI method, an inversion pulse with a flip angle of, for example, 180° is applied after an excitation pulse, and then a set of MR signals numbered (1) to (N) is acquired in response to the application of a readout gradient magnetic field pulse.

[0065] In the SE-based EPI method, the first pulse sequence (FIG. 9(a)) is a pulse sequence starting from the excitation pulse (first excitation pulse) with a first delay time t d1 The acquisition of the first set of MR signals begins after the second excitation pulse (FIG. 9(b)). d1 A second delay time t different from d2 After this, start acquiring a second set of MR signals.

[0066] Here, as with the GRE-based EPI, the first delay time t d1 corresponds to the delay time from the first excitation pulse to the beginning of the MR signal read out by the first readout gradient magnetic field pulse among the plurality of readout gradient magnetic field pulses, and the second delay time t d2corresponds to the delay time from the second excitation pulse to the beginning of the MR signal read out by the first readout gradient magnetic field pulse of the plurality of readout gradient magnetic field pulses.

[0067] Also, the first delay time t d1 and the second delay time t d2 The difference Δt d As with the GRE EPI, it is preferable to set the value of the EPI equal to or less than the ESP, for example, to approximately half the ESP.

[0068] Returning to FIG. 7, in step ST101, the imaging section 600 applies the first and second pulse sequences set in step ST100 to the subject, and acquires a set of first and second MR signals. Then, in step ST102, the conversion function F03 of the processing circuitry 40 converts the first and second sets of MR signals into first and second data sets, respectively. Next, in step ST103, the averaging function F04 of the processing circuit 40 averages the first and second data sets to generate a combined data set. Then, in step ST104, the generating function F05 of the processing circuitry 40 generates a magnetic resonance image based on the combined data set.

[0069] As described above, there are three cases in step ST103: arithmetic averaging in k-space, arithmetic averaging in hybrid space, and arithmetic averaging in real space. As described above, the processing performed by the conversion function F03 and the processing performed by the generation function F05 are slightly different depending on these three cases.

[0070] FIG. 10 is a diagram schematically showing the effect of the N / 2 artifact suppression processing of the magnetic resonance imaging apparatus 1 according to the first embodiment described above. Figure 10 illustrates the case where averaging is performed in real space, where the first and second data sets are the first and second MR images transformed into real space. As shown in the upper and lower figures on the left side of Figure 10, N / 2 artifacts occur in the first and second MR images.

[0071] The N / 2 artifact in the first MR image and the N / 2 artifact in the second MR image are common in that they appear at a position where the true image (the central circle) is shifted by 1 / 2 of the FOV in the phase encoding direction, and the shape and location of the N / 2 artifact are common between the first MR image and the second MR image.

[0072] On the other hand, the delay time t from the excitation pulse to the acquisition of the set of MR signals that form each MR image is d is different between the first MR image and the second MR image. Therefore, the amplitude and phase of corresponding pixel values between the N / 2 artifacts in the first MR image and the N / 2 artifacts in the second MR image are different from each other.

[0073] As a result, by averaging the first MR image and the second MR image, the pixel values of the N / 2 artifacts are smoothed, and the N / 2 artifacts are reduced as shown in the right diagram of Figure 10.

[0074] (Second embodiment) 11 is a flowchart showing an example of N / 2 artifact suppression processing in the magnetic resonance imaging apparatus 1 according to the second embodiment. The difference between the second embodiment and the first embodiment is the number of data to be averaged, and the configuration of the magnetic resonance imaging apparatus 1 shown in FIGS. 1 and 6 itself is the same between the second embodiment and the first embodiment.

[0075] In step ST200 of FIG. 11, three or more pulse sequences are set, each having a different delay time from an excitation pulse to the start of acquisition of a set of MR signals.

[0076] The plurality of pulse sequences to be set are, for example, pulse sequences of the GRE-based EPI method (GRE-EPI) and pulse sequences of the SE-based EPI method (SE-EPI), as in the first embodiment.

[0077] 12(a) to 12(d) show sequence diagrams of four pulse sequences of the GRE-based EPI method as examples of a plurality of pulse sequences set in the second embodiment. In each of the first pulse sequence (FIG. 12(a)) to the fourth pulse sequence (FIG. 12(d)), the delay time t from the excitation pulse to the head of the MR signal read by the first readout gradient magnetic field pulse among the plurality of readout gradient magnetic field pulses is set to t. dn (n=1-4) are set to different values.

[0078] The maximum delay time among multiple delay times (in this example, the delay time t d4 ) is below ESP (i.e., t ESP It is preferable to set the delay time t dn When (n=1-4) are arranged in descending or ascending order, the difference in delay time between adjacent ones Δt dm (m=1-3) are approximately equal values (=Δt d ) so that the delay time t dn It is preferable to set (n=1-4).

[0079] Returning to FIG. 11, in step ST201, the imaging section 600 applies the three or more pulse sequences set in step ST200 to the subject, and acquires three or more sets of MR signals corresponding to the three or more pulse sequences. Then, in step ST202, the conversion function F03 of the processing circuitry 40 converts the three or more sets of MR signals into three or more data sets, respectively. Next, in step ST203, the averaging function F04 of the processing circuit 40 averages the three or more data sets to generate a composite data set. Then, in step ST204, the generating function F05 of the processing circuitry 40 generates a magnetic resonance image based on the combined data set.

[0080] In the second embodiment, there are three cases of averaging performed in step ST203: averaging in k-space, averaging in hybrid space, and averaging in real space, and the conversion function F03 and generation function F05 perform processing corresponding to these three cases, just like in the first embodiment.

[0081] FIG. 13 is a diagram schematically showing the effect of the N / 2 artifact suppression process of the magnetic resonance imaging apparatus 1 according to the second embodiment described above.

[0082] The delay time td from the excitation pulse to the acquisition of the set of MR signals that form each MR image is different among the first to fourth MR images. Therefore, the amplitude and phase of the corresponding pixel values of the N / 2 artifacts in the first to fourth MR images are different from each other. As a result, when the four MR images, namely the first to fourth MR images, are averaged, the pixel values of the N / 2 artifacts are smoothed, and the N / 2 artifacts are reduced as shown in the lower diagram of FIG. 13.

[0083] In the second embodiment, although the imaging time is longer than in the first embodiment, the number of data sets to be averaged (for example, the number of MR images) increases, and therefore the effect of suppressing N / 2 artifacts is enhanced.

[0084] Whether the number of data sets to be averaged is two as in the first embodiment or three or more as in the second embodiment, the signal-to-noise ratio of the true image improves according to the number of data sets to be averaged, as in the conventional method. In the first and second embodiments described above, by averaging two or three or more data sets, it is possible to improve the signal-to-noise ratio of the true image and suppress N / 2 artifacts at the same time.

[0085] As mentioned above, the N / 2 artifact suppression technique of the first conventional example disclosed in Patent Document 1 has difficulty in suppressing N / 2 artifacts caused by second-order or higher nonlinear phase errors. However, the N / 2 artifact suppression methods of the first and second embodiments described above can also suppress N / 2 artifacts caused by second-order or higher nonlinear phase errors.

[0086] The N / 2 artifact suppression methods of the first and second embodiments described above may be combined with the linear phase correction processing disclosed in Patent Document 1, etc. In this case, the N / 2 artifacts caused by the nonlinear phase error remaining in the first conventional example can be suppressed by the magnetic resonance imaging apparatus 1 according to this embodiment.

[0087] As mentioned above, the second conventional N / 2 artifact suppression technique disclosed in Patent Document 2 performs complex addition of two data collected by readout gradient magnetic field pulses of opposite polarities, positive and negative. However, because the influence of disturbances caused by eddy currents and the like differs depending on the polarity of the readout gradient magnetic field pulse, complex addition of two MR signals cannot completely cancel out the phase error. In addition, because the polarities of the readout gradient magnetic field pulses are different, the polarity of the phase error in the readout direction is reversed, and the direction of the position error in real space is reversed. Therefore, when two pixel values are averaged, the pixel values are averaged, which makes the image more likely to blur.

[0088] In contrast, in the first and second embodiments described above, the polarity of the readout gradient magnetic field pulse is not inverted between the multiple pulse sequences that are averaged, and therefore the above-mentioned problem in the second conventional example does not occur.

[0089] Furthermore, the second conventional example disclosed in Patent Document 2 requires a pulse sequence in which the polarities of the readout gradient magnetic field pulses are reversed, and therefore there is a constraint that the number of data sets used for averaging must be an even number. In contrast to this, in the first and second embodiments, there is no such restriction on the number of data sets to be averaged, and the number may be either an even number or an odd number.

[0090] As described above, according to at least one embodiment of the magnetic resonance imaging apparatus and magnetic resonance imaging program, N / 2 artifacts can be reduced by a relatively simple method.

[0091] The imaging condition setting function, the addition function, and the generation function in the description of each embodiment are examples of the setting unit, the addition unit, and the generation unit in the description of the claims, respectively.

[0092] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents. [Explanation of symbols]

[0093] 1. Magnetic resonance imaging device 40 Processing circuit 600 Imaging unit F01 Imaging condition setting function F02 Addition function F03 Conversion function F04 Additive averaging function F05 Generation Function

Claims

1. a setting unit that sets a first pulse sequence that starts acquisition of a first set of magnetic resonance signals after a first delay time from a first excitation pulse, and a second pulse sequence that is different from the first pulse sequence and starts acquisition of a second set of magnetic resonance signals after a second delay time from a second excitation pulse that is different from the first delay time; an imaging unit that applies the first pulse sequence and the second pulse sequence to a subject to acquire the first set of magnetic resonance signals and the second set of magnetic resonance signals, respectively; an adder that generates a composite data set by averaging a first data set based on the acquired first set of magnetic resonance signals and a second data set based on the acquired second set of magnetic resonance signals; a generator for generating a magnetic resonance image based on the synthetic data set; A magnetic resonance imaging apparatus comprising:

2. the first pulse sequence and the second pulse sequence are pulse sequences corresponding to an EPI (Echo Planar Imaging) method; 2. The magnetic resonance imaging apparatus according to claim 1.

3. the first delay time is a delay time from the first excitation pulse to a leading edge of a magnetic resonance signal read out by a first readout gradient magnetic field pulse among a plurality of readout gradient magnetic field pulses in the EPI method; the second delay time is a delay time from the second excitation pulse to a leading edge of a magnetic resonance signal read out by a first readout gradient magnetic field pulse among a plurality of readout gradient magnetic field pulses in the EPI method; 3. The magnetic resonance imaging apparatus according to claim 2.

4. the setting unit sets the first pulse sequence and the second pulse sequence such that a difference between the first delay time and the second delay time is equal to or less than an ESP (echo space), where ESP is a time interval between adjacent readout gradient magnetic field pulses in the EPI method.

4. The magnetic resonance imaging apparatus according to claim 2 or 3.

5. the setting unit sets the first pulse sequence and the second pulse sequence such that, when a time interval between adjacent readout gradient magnetic field pulses in the EPI method is defined as an ESP (echo space), a difference between the first delay time and the second delay time is approximately ½ of the ESP.

4. The magnetic resonance imaging apparatus according to claim 2 or 3.

6. the first data set and the second data set are data sets in k-space corresponding to the first magnetic resonance signal set and the second magnetic resonance signal set, respectively; the adder averages the first data set and the second data set in the k-space to generate the composite data set.

6. A magnetic resonance imaging apparatus according to claim 1.

7. the first data set and the second data set are data sets obtained by transforming the first magnetic resonance signal set and the second magnetic resonance signal set into real space, respectively; the adder averages the first data set and the second data set in the real space to generate the composite data set.

6. A magnetic resonance imaging apparatus according to claim 1.

8. the first data set and the second data set are data sets obtained by transforming the first magnetic resonance signal set and the second magnetic resonance signal set into a hybrid space in which a k-space and a real space are combined, respectively; the adder averages the first data set and the second data set in the hybrid space to generate the composite data set.

6. A magnetic resonance imaging apparatus according to claim 1.

9. a setting unit that sets three or more pulse sequences with different delay times from an excitation pulse to the start of acquisition of a magnetic resonance signal set; an imaging unit that applies the three or more pulse sequences to a subject and acquires three or more sets of magnetic resonance signals respectively corresponding to the three or more pulse sequences; an adder that generates composite data by averaging three or more data sets based on the three or more acquired magnetic resonance signal sets; a generator for generating a magnetic resonance image based on the composite data; A magnetic resonance imaging apparatus comprising:

10. Each of the three or more pulse sequences is a pulse sequence corresponding to an EPI (Echo Planar Imaging) method.

10. The magnetic resonance imaging apparatus according to claim 9.

11. the setting unit sets the three or more pulse sequences such that, when a time interval between adjacent readout gradient magnetic field pulses in the EPI method is an ESP (echo space), differences in the delay times are substantially equal among the three or more pulse sequences.

11. The magnetic resonance imaging apparatus according to claim 10.

12. setting a plurality of pulse sequences with different delay times from an excitation pulse to the start of acquisition of a set of magnetic resonance signals; applying the plurality of pulse sequences to a subject and acquiring a plurality of sets of magnetic resonance signals corresponding to the plurality of pulse sequences; averaging a plurality of data sets based on each of the plurality of acquired magnetic resonance signal sets to generate composite data; generating a magnetic resonance image based on the composite data; A magnetic resonance imaging program that causes a computer to run

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