Magnetic Resonance Imaging apparatus

Abstract

To acquire an accurate T2 value and an index on diffusion and turbulent movement in a short imaging time.SOLUTION: A magnetic resonance imaging device includes: an imaging condition setting unit for setting a first pulse sequence in which a first gradient magnetic field pulse is disposed between adjacent refocus pulses respectively, and a second pulse sequence in which a second gradient magnetic field pulse with a pulse shape different from that of the first gradient magnetic field pulse is disposed between the adjacent refocus pulses respectively, which are pulse sequences of an FSE type having an excitation pulse and a plurality of the refocus pulses following the excitation pulse; an imaging unit for applying the first pulse sequence and the second pulse sequence to a subject, and acquiring a first signal and a second signal respectively; an image generation unit for generating a first image and a second image from the first signal and the second signal respectively; and an analysis unit for calculating a T2 value of body fluid in which an influence of the movement including diffusion of the body fluid of the subject is removed, from the first image and the second image.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The embodiments disclosed in this specification and the drawings relate to a magnetic resonance imaging apparatus.

Background Art

[0002] A magnetic resonance imaging apparatus is an imaging device that excites the nuclear spins of a subject placed in a static magnetic field with a high-frequency (RF: Radio Frequency) signal at the Larmor frequency, and reconstructs a magnetic resonance signal (MR (Magnetic Resonance) signal) generated from the subject due to the excitation to generate an image.

[0003] A magnetic resonance imaging apparatus can image cerebrospinal fluid (CSF), interstitial fluid in the brain, etc. In recent years, cerebrospinal fluid and interstitial fluid in the brain are called Neurofluid, and are considered important for elucidating the clearance function (i.e., removal function) of waste products in the brain. Representative examples of imaging images of Neurofluid include T2-weighted images and diffusion-weighted images.

[0004] In a T2-weighted image, in order to well depict Neurofluid such as CSF, it is necessary to separate the brain parenchyma, and a pulse sequence called a CPMG (Carr-Purcell-Meiboom-Gill) sequence or an FSE (Fast Spin Echo) sequence is often used. On the other hand, in a diffusion-weighted image, imaging has been attempted that includes not only diffusion due to Brownian motion but also perfusion and turbulent motion.

[0005] The regions of interest of Neurofluid are the perivascular spaces in the brain (i.e., the spaces formed around the blood vessels in the brain), narrow cerebral sulci (i.e., the groove parts of the wrinkles of the brain), the brain surface, etc., and an imaging technique with high spatial resolution is required.

[0006] When attempting to acquire T2-weighted images and T2 mapping with high resolution using the above-described CPMG (or FSE) sequence, the intensity G and pulse length T of the readout gradient magnetic field pulse S need to be increased.

[0007] However, when increasing the intensity G and pulse length T of the readout gradient magnetic field pulse S the influence caused by the readout gradient magnetic field pulse itself due to diffusion and turbulent motion can no longer be ignored, and the accuracy of the T2 value in T2 mapping will decrease.

[0008] In evaluations related to the clearance of waste products in the brain, the T2 value of Neurofluid without the influence of diffusion and turbulent motion is important, and indices related to the diffusion and turbulent motion of Neurofluid are also important.

[0009] As described above, in conventional imaging techniques, the accuracy of the T2 value in T2 mapping has been reduced due to the influence of the diffusion and turbulent motion of Neurofluid. Also, when attempting to obtain an index related to diffusion and turbulent motion, it is necessary to perform imaging separate from the imaging for obtaining the T2 value, and the overall imaging time has been long.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0011] One of the problems to be solved by the embodiments disclosed in this specification and the drawings is to be able to obtain a highly accurate T2 value and an index related to diffusion or turbulent motion in a short imaging time. However, the problems to be solved by the embodiments disclosed in this specification and the drawings are not limited to the above problems. It is also possible to position the problems corresponding to the respective effects of the respective configurations shown in the embodiments described later as other problems.

Means for Solving the Problem

[0012] A magnetic resonance imaging apparatus according to an embodiment is an FSE-type pulse sequence having an excitation pulse and a plurality of refocusing pulses following the excitation pulse, and a first pulse sequence in which a first gradient magnetic field pulse is arranged between each of the adjacent refocusing pulses, and a second pulse sequence in which a second gradient magnetic field pulse having a different pulse shape from the first gradient magnetic field pulse is arranged between each of the adjacent refocusing pulses, an imaging condition setting unit for setting the first pulse sequence and the second pulse sequence, an imaging unit for applying the first pulse sequence and the second pulse sequence to a subject and respectively acquiring a first signal and a second signal, an image generation unit for generating a first image and a second image from the first signal and the second signal, and an analysis unit for calculating the T2 value of the body fluid from which the influence of the motion including the diffusion of the body fluid of the subject has been removed from the first image and the second image.

Brief Description of the Drawings

[0013]

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Embodiments for Carrying Out the Invention

[0014] Hereinafter, the magnetic resonance imaging apparatus 1 according to an embodiment of the present invention will be described with reference to the accompanying drawings.

[0015] (Outline of Configuration and Basic Operation) FIG. 1 is a block diagram showing the overall configuration of the magnetic resonance imaging apparatus 1 according to the present embodiment. The magnetic resonance imaging apparatus 1 of the embodiment includes a magnet gantry 100, a control cabinet 300, a console 400, a bed 500, and the like.

[0016] The magnet gantry 100 has a static magnetic field magnet 10, gradient magnetic field coils 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 top plate 51. Further, the magnetic resonance imaging apparatus 1 has an array coil 20 disposed close to the subject.

[0017] The control cabinet 300 includes gradient magnetic field power supplies 31 (31x for the X-axis, 31y for the Y-axis, 31z for the Z-axis), an RF receiver 32, an RF transmitter 33, and a sequence controller 34.

[0018] The static magnetic field magnet 10 of the magnet gantry 100 has a substantially cylindrical shape and generates a static magnetic field in a bore (i.e., the space inside the cylinder of the static magnetic field magnet 10), which is the imaging region of the subject (e.g., a patient). The static magnetic field magnet 10 incorporates a superconducting coil, and the superconducting coil is cooled to an extremely low temperature by liquid helium. 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 in the excitation mode, and then, when shifting to the persistent current mode, the static magnetic field power supply is disconnected. Once shifted to the persistent current mode, the static magnetic field magnet 10 continues to generate a large static magnetic field for a long time, for example, for more than one year. Note that the static magnetic field magnet 10 may be configured as a permanent magnet.

[0019] The gradient magnetic field coils 11 also have a substantially cylindrical shape and are fixed inside the static magnetic field magnet 10. These gradient magnetic field coils 11 apply gradient magnetic fields in the directions of the X-axis, Y-axis, and Z-axis to the subject by currents supplied from the gradient magnetic field power supplies (31x, 31y, 31z).

[0020] The bed body 50 of the bed 500 is capable of moving the top plate 51 in the vertical direction, and moves the subject placed on the top plate 51 to a predetermined height before imaging. Then, at the time of imaging, the top plate 51 is moved in the horizontal direction to move the subject into the bore.

[0021] The WB coil 12 is fixed in a substantially cylindrical shape so as to surround the subject inside the gradient magnetic field coil 11. The WB coil 12 transmits the RF pulse transmitted from the RF transmitter 33 toward the subject, while receiving the magnetic resonance signal emitted from the subject due to the excitation of hydrogen nuclei.

[0022] The array coil 20 is an RF coil and receives the magnetic resonance signal emitted from the subject at a position close to the subject. The array coil 20 is composed of, for example, a plurality of element coils. There are various types of the array coil 20 such as for the head, for the chest, for the spine, for the lower limbs, or for the whole body according to the imaging site of the subject, and in FIG. 1, the array coil 20 for the chest is illustrated.

[0023] The RF transmitter 33 transmits an RF pulse to the WB coil 12 based on an instruction from the sequence controller 34. On the other hand, the RF receiver 32 detects the magnetic resonance signal received by the WB coil 12 or the array coil 20, and sends the raw data obtained by digitizing the detected magnetic resonance signal to the sequence controller 34.

[0024] 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. Then, when the sequence controller 34 performs a scan and receives the raw data from the RF receiver 32, it sends the raw data to the console 400.

[0025] The sequence controller 34 includes a processing circuit (not shown). This processing circuit is composed of, for example, a processor that executes a predetermined program, or hardware such as 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 storage circuit 41, a display 42, and an input device 43.

[0026] The storage circuit 41 is a storage medium including an external storage device such as an HDD (Hard Disk Drive) or an optical disk device in addition to a ROM (Read Only Memory) and a RAM (Random Access Memory). The storage circuit 41 stores various kinds of information and data, and also stores various programs executed by the processor included in the processing circuit 40.

[0027] The input device 43 is, for example, a mouse, a keyboard, a trackball, a touch panel, etc., and includes various devices for an operator to input various kinds of information and data. The display 42 is a display device such as a liquid crystal display panel, a plasma display panel, or an organic EL panel.

[0028] The processing circuit 40 is, for example, a circuit including a CPU or a dedicated or general-purpose processor. The processor realizes various functions described later by executing various programs stored in the storage circuit 41. The processing circuit 40 may be composed of hardware such as an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). These hardware can also realize various functions described later. Further, the processing circuit 40 can also realize various functions by combining software processing by a processor and a program, and hardware processing.

[0029] Each of these components enables the console 400 to control the entire magnetic resonance imaging apparatus 1. Specifically, it receives imaging conditions and other various information and instructions from an operator such as a technician through operations on input devices 43 like a mouse and a keyboard. Then, based on the input imaging conditions, the processing circuit 40 causes the sequence controller 34 to execute a scan, and reconstructs an image based on the raw data transmitted from the sequence controller 34. The reconstructed image is either displayed on the display 42 or stored in the storage circuit 41.

[0030] (Detailed Configuration and Operation of the Embodiment) FIG. 2 is a block diagram of the magnetic resonance imaging apparatus 1 of the embodiment, and particularly, a functional block diagram focusing on the functions realized by the processing circuit 40.

[0031] In FIG. 2, among the components of the magnetic resonance imaging apparatus 1 shown in FIG. 1, components other than the console 400, that is, the magnet gantry 100, the control cabinet 300, and the examination table 500 are collectively referred to as the imaging unit 600.

[0032] As shown in FIG. 2, the processing circuit 40 of the magnetic resonance imaging apparatus 1 realizes functions such as an imaging condition setting function F01, an image generation function F02, an ADC map generation function F03, a diffusion / motion correction function F04, a T2 map generation function F05, and a T2 spectrum generation function F06.

[0033] The imaging condition setting function F01 determines the parameters of the pulse sequence based on the imaging conditions set or selected via the input device 43 or the like, and sets the pulse sequence with the determined parameters for the sequence controller 34 of the imaging unit 600.

[0034] The magnetic resonance imaging apparatus 1 according to the embodiment particularly has an FSE (Fast Spin Echo) type pulse sequence having an excitation pulse and a plurality of refocusing pulses following this excitation pulse, and a first pulse sequence in which a first gradient magnetic field pulse is arranged between each of the adjacent refocusing pulses, and a second pulse sequence in which a second gradient magnetic field pulse having a different pulse shape from the first gradient magnetic field pulse is arranged between each of the adjacent refocusing pulses can be set. The first and second pulse sequences will be described in more detail later.

[0035] The imaging unit 600 applies the set first pulse sequence and second pulse sequence to the subject, respectively acquires the first MR signal and the second MR signal, and sends them to the processing circuit 40 of the console 400 via the sequence controller 34.

[0036] The image generation function F02 of the processing circuit 40 generates a first image and a second image from the first MR signal and the second MR signal, respectively. For example, the first MR signal and the second MR signal are respectively reconstructed by a process such as Fourier transform to generate the first image and the second image.

[0037] The analysis function F07 calculates the T2 value of the body fluid from which the influence of the movement including the diffusion of the body fluid of the subject has been removed from the first image and the second image. Here, the body fluid of the subject is, for example, Neurofluid such as CSF (cerebrospinal fluid) or interstitial fluid of the brain. Further, the body fluid of the subject may include blood and lymph fluid, or may include other liquids inside the subject.

[0038] The analysis function F07 has, as its internal configuration, an ADC map generation function F03, a diffusion / motion correction function F04, a T2 map generation function F05, and a T2 spectrum generation function F06.

[0039] Before the specific descriptions of the ADC map generation function F03, the diffusion / motion correction function F04, the T2 map generation function F05, and the T2 spectrum generation function F06, the conventional problems in the imaging of neurofluids such as CSF and the key points of the solution means according to the present embodiment will be described with reference to FIGS. 3 to 5. Note that hereinafter, for the sake of simplicity of explanation, it will be described assuming that the imaging target is CSF, but it does not exclude neurofluids such as interstitial fluid in the brain or other body fluids.

[0040] FIG. 3 is a diagram for explaining the conventional problems in the imaging of CSF. Since body fluids such as CSF have a longer transverse relaxation time T2 compared to the brain parenchyma, T2-weighted images are often used as the imaging images of CSF. In particular, in order to well depict CSF, it is necessary to sufficiently separate the brain parenchyma, and pulse sequences called CPMG (Carr-Purcell-Meiboom-Gill) sequence or FSE (Fast Spin Echo) sequence have often been used conventionally. The CPMG sequence is an FSE pulse sequence that satisfies the so-called CPMG conditions. Here, the CPMG conditions are Condition (1): The interval ESP between two adjacent refocusing pulses is twice the interval between the excitation pulse and the first refocusing pulse, and the phase of the RF signals of the excitation pulse and the refocusing pulse is shifted by 90 degrees, and Condition (2): In all pairs of adjacent refocusing pulses, the integrated value of all the gradient magnetic fields between two adjacent refocusing pulses is the same. These are the conditions. When the CPMG conditions are satisfied, the spin echo SE and the stimulated echo STE are added in phase at the same position in the time direction, and the SN ratio is improved.

[0041] Figs. 3(a) to 3(c) show a conventionally used CPMG sequence. Fig. 3(a) shows an RF pulse train, Fig. 3(b) shows a phase-encoding gradient magnetic field pulse train, and Fig. 3(c) shows a readout gradient magnetic field pulse train. Note that a slice selection gradient magnetic field pulse is also generated corresponding to the generation timing of each RF pulse, but it is not shown in Fig. 3.

[0042] The application directions of the readout gradient magnetic field pulse, the phase-encoding gradient magnetic field pulse, and the slice selection gradient magnetic field pulse are, for example, the X direction, the Y direction, and the Z direction in Fig. 1, but are not limited thereto, and can take a desired application direction according to the inclination of the set FOV (Field Of View).

[0043] As shown in Fig. 3(a), the RF pulse train is composed of an excitation pulse (for example, an RF pulse with a flip angle of 90°) and a plurality of refocusing pulses (for example, RF pulses with a flip angle of 180°) following the excitation pulse. The excitation pulse and the plurality of refocusing pulses satisfy the above-described CPMG condition (1).

[0044] In the phase-encoding gradient magnetic field pulse G shown in Fig. 3(b), between two adjacent refocusing pulses, there is a pair of a phase-encoding pulse that determines the phase-encoding amount and a rewinder pulse with the same amplitude and opposite polarity. With this pair, it is possible to satisfy the above-described CPMG condition (2) while setting different phase-encoding amounts for each refocusing pulse. P

[0045] The readout gradient magnetic field pulse G shown in Fig. 3(c) r1Among them, the gradient magnetic field pulse between the excitation pulse and the first refocusing pulse is a gradient magnetic field pulse for rotating the phase of the transverse magnetization in the minus direction in advance so that the phase of the transverse magnetization becomes zero at the center of the readout gradient magnetic field pulse between the first and second refocusing pulses, and is called a prephasing pulse. Usually, the intensity of the prephasing pulse is set to be the same as that of the readout gradient magnetic field pulse, and the pulse length of the prephasing pulse is set to be half of the readout gradient magnetic field pulse.

[0046] As described above, when attempting to acquire high-resolution T2-weighted images or T2 mapping of CSF, it is necessary to increase the intensity and pulse length of the readout gradient magnetic field pulse. Correspondingly, the intensity and pulse length of the prephasing pulse also increase.

[0047] When the intensity and pulse length of the prephasing pulse and the readout gradient magnetic field pulse increase, signal attenuation due to phase dispersion of the MR signal caused by diffusion and turbulent motion of CSF becomes non-negligible.

[0048] That is, the pair of the prephasing pulse and the pulse that is half of the first readout gradient magnetic field pulse, which is shown by hatching in Fig. 3(c), functions equivalently to the MPG (Motion Probing Gradient) pulse in diffusion-weighted imaging and shows an effect equivalent to that of the MPG pulse. Hereinafter, such an effect, that is, the effect that phase dispersion occurs due to diffusion and turbulent motion of CSF and the signal intensity attenuates due to the application of a readout gradient magnetic field pulse with a large intensity and pulse length, shall be referred to as the MPG effect of the readout gradient magnetic field pulse.

[0049] Fig. 3(d) shows the MR signal emitted from the subject with the application of the readout gradient magnetic field pulse. The MR signal shows a peak at the center position of each readout gradient magnetic field pulse between two adjacent refocusing pulses.

[0050] Each peak value decays with the elapsed time from the excitation pulse, as shown by the dashed curve in Fig. 3(d), due to T2 relaxation. Usually, the transverse relaxation time T2 is calculated from the shape of this decay curve.

[0051] However, as described above, when the intensity and pulse length of the readout gradient magnetic field pulse increase, the decay due to the MPG effect is superimposed on the decay due to T2 relaxation. Therefore, even if the transverse relaxation time T2 is calculated from the shape of the obtained decay curve, it will not be the true T2 value, and only a T2 value with an error can be obtained. Note that in Fig. 3, the slice selection or slice encoding gradient magnetic field pulse is omitted. Also, a gradient magnetic field pulse for suppressing the FID signal (so-called spoiler or crusher gradient magnetic field pulse) may be inserted before and after the refocusing pulse, and it may also have the MPG effect. In this case as well, the MPG effect due to the readout gradient magnetic field pulse in Fig. 3 and the correction method therefor can be applied in the same manner.

[0052] To solve such problems, in the magnetic resonance imaging apparatus 1 according to the embodiment, in addition to the conventional pulse sequence (the first pulse sequence) shown in Fig. 3, a second pulse sequence is used.

[0053] Fig. 4 is a diagram illustrating the first pulse sequence and the second pulse sequence used in the embodiment. Figs. 4(a) to 4(d) are the same as the conventional pulse sequence (the first pulse sequence) shown in Fig. 3. A pair composed of the prephasing pulse shown by hatching in Fig. 4(c) and the first half of the readout gradient magnetic field pulse constitutes the MPG1 pulse that produces the above-described MPG effect.

[0054] In the second pulse sequence, the RF pulse (Fig. 4(a)), the slice selection gradient magnetic field pulse (not shown), and the phase encoding gradient magnetic field pulse (Fig. 4(b)) are the same as those in the first pulse sequence, and only the readout gradient magnetic field pulse is different from the first pulse sequence.

[0055] Figure 4(e) shows an example of the readout gradient magnetic field pulse G in the second pulse sequence. The readout gradient magnetic field pulse G in the second pulse sequence has a shape in which additional gradient magnetic field pulses with a predetermined shape are added to the leading edge side and the trailing edge side of the readout gradient magnetic field pulse G in the first pulse sequence, respectively. Also, the prephasing pulse in the second pulse sequence has a shape in which the same additional gradient magnetic field pulse as described above is added to the trailing edge side of the prephasing pulse in the first pulse sequence. An MPG2 pulse that produces the MPG effect in the second pulse sequence is constituted by a pair composed of the prephasing pulse indicated by hatching in Figure 4(e) and the first half portion of the readout gradient magnetic field pulse. r2 Figure 4(f) shows the MR signal emitted from the subject in accordance with the application of the readout gradient magnetic field pulse of the second pulse sequence. Similar to the first pulse sequence, each peak value of the MR signal decays with the elapsed time from the excitation pulse due to T2 relaxation. r2 As is clear from Figures 4(c) and 4(e), the time integral value of the MPG2 pulse in the second pulse sequence is larger than the time integral value of the MPG1 pulse in the first pulse sequence. For this reason, with respect to the MPG effect, the second pulse sequence is larger than the first pulse sequence, and the degree of attenuation of the MR signal peak value due to the elapsed time from the excitation pulse is larger for the second pulse sequence. r1 Figure 5 is a diagram for explaining the concept of a process for obtaining a true T2 value without the influence of diffusion or turbulent motion, using the MR signal s1(t) obtained in the first pulse sequence and the MR signal s2(t) obtained in the second pulse sequence.

[0056]

[0057]

[0058]

[0059] ​​​The solid line in the upper graph of FIG. 5 schematically shows the attenuation curves of the MR signal s1(t) by the first pulse sequence and the MR signal s2(t) by the second pulse sequence with respect to the elapsed time after the application of the excitation pulse. Each curve is the envelope of each peak of the MR signal generated between the refocusing pulses.

[0060] As described above, the MR signal s1(t) by the first pulse sequence is subject to attenuation due to T2 relaxation and attenuation due to diffusion and turbulent motion caused by the MPG1 pulse. The MR signal s1(t) can be expressed, for example, by the following (Equation 1). s1(t)=s0*exp(-t / T2)*exp(-b1*ADC) (Equation 1) In (Equation 1), t is the elapsed time from the excitation pulse, T2 is the transverse relaxation time T2 value of the imaging object, and ADC represents the apparent diffusion coefficient, which is an index of the diffusion and turbulent motion of the imaging object. Hereinafter, ADC will be simply referred to as the diffusion coefficient. b1 is an index indicating the MPG effect by the MPG1 pulse and is a known value that can be calculated from the time waveform of the MPG1 pulse.

[0061] Similarly, the MR signal s2(t) by the second pulse sequence can also be expressed, for example, by the following (Equation 2). s2(t)=s0*exp(-t / T2)*exp(-b2*ADC) (Equation 2) The difference between (Equation 1) and (Equation 2) is only b2. b2 is also an index indicating the MPG effect by the MPG2 pulse and is a known value that can be calculated from the time waveform of the MPG2 pulse.

[0062] Since b1 and b2 are known values, the diffusion coefficient ADC can be calculated from (Equation 1) and (Equation 2). Then, by substituting the calculated ADC and b1, which is a known b value, into (Equation 1), the following (Equation 3) can be obtained. s C (t)=s0*exp(-t / T2) (Equation 3) (Equation 3) is the corrected MR signal sC It represents (t). In (Equation 3), the attenuation term exp(-b1*ADC) of the diffusion coefficient ADC in (Equation 1) is removed, and only the attenuation term exp(-t / T2) due to T2 attenuation, which is not affected by the diffusion or turbulent movement of the imaging object, remains. Therefore, the true T2 value that is not affected by diffusion or turbulent movement can be calculated from the attenuation curve represented by (Equation 3).

[0063] Note that the first and second pulse sequences shown in FIG. 4 and the concept of the processing of the present embodiment shown in FIG. 5 implicitly assume that the entire imaging object is uniformly distributed with the same T2 value and the same diffusion coefficient ADC. However, in reality, the T2 value and the diffusion coefficient ADC should show different values for each position of the imaging object. Therefore, the diffusion coefficient ADC needs to be calculated for each pixel position of the reconstructed image, and the true T2 value from which the influence of diffusion and turbulent movement has been removed also needs to be calculated for each pixel position. In other words, it is necessary to calculate an ADC map in which the diffusion coefficient ADC is arranged for each pixel position and a true T2 map in which the true T2 value is arranged for each pixel position. Hereinafter, the calculation process of the ADC map and the true T2 map according to the embodiment will be described with reference to the flowchart shown in FIG. 6 and the explanatory diagrams in FIGS. 7 to 13.

[0064] First, in step ST100 of FIG. 6, the first and second pulse sequences are set. The process of step ST100 is performed by the imaging condition setting function F01 in FIG. 3. FIGS. 7 to 9 are diagrams showing an example of the first and second pulse sequences set in step ST100.

[0065] FIGS. 7(a) and 7(b) are diagrams respectively exemplifying the RF pulse and the phase encoding gradient magnetic field pulse G that are common to the first and second pulse sequences. P On the other hand, FIG. 7(c) is the readout gradient magnetic field pulse G r1 of the first pulse sequence, and FIG. 7(d) is the readout gradient magnetic field pulse G r2 of the second pulse sequence.

[0066] Figures 8(a) and 8(b) show the first and second pulse sequences over a longer time span than in FIG. 7. In FIG. 8(b), the solid line extending horizontally corresponds to a phase encoding amount of zero, and the value of the phase encoding amount is schematically shown by the distance from the solid line of the black square.

[0067] As shown in FIGS. 7(b), 8(a), and 8(b), in the first and second pulse sequences, a plurality of refocusing pulses are divided into a plurality of groups with different echo times TE, and the phase encoding amounts associated with each of the refocusing pulses are set such that the phase encoding amount changes for each refocusing pulse in a predetermined change pattern within the group.

[0068] For example, as shown in FIGS. 8(a) and 8(b), a plurality of refocusing pulses are divided into N groups from group 1 to group N, and each group is provided with M refocusing pulses (five refocusing pulses in the example of FIG. 8). And the phase encoding amounts associated with each of the refocusing pulses are set such that the phase encoding amount changes for each refocusing pulse in a predetermined change pattern within each group, for example, as “+2a, +a, 0, -a, -2a” (a is a predetermined unit phase encoding amount).

[0069] The elapsed time from the excitation pulse to the center of each group is considered to be the echo time TE representative of each group. That is, within groups 1, 2, 3, ···, N, MR signals corresponding to echo times TE1, TE2, TE3, ···, TE(N) are considered to be obtained.

[0070] Also, as shown in FIG. 8(a), in the first and second pulse sequences, one group (hereinafter referred to as a segment) composed of one excitation pulse and a plurality of subsequent refocusing pulses is set to repeat a predetermined number of times (for example, L times) at a repetition time TR.

[0071] Then, the phase encoding amount change pattern within each group is set such that the phase encoding amount has different change patterns between different segments. For example, as shown in FIG. 8(b), when the change pattern of the phase encoding amount of the first segment is “+2a, +a, 0, -a, -2a”, the change pattern of the phase encoding amount of the second segment is set to, for example, the change pattern “+2a + α, +a + α, +α, -a + α, -2a + α” obtained by uniformly shifting the phase encoding amount of the first segment in the positive direction by α.

[0072] In this way, by making the change patterns of the phase encoding amount within the group corresponding to the same echo time TE different between segments, the k - space in the phase encoding direction required to reconstruct the image for each echo time can be filled without overlapping with each other.

[0073] FIG. 9 is a diagram for explaining the concept of a method of arranging MR signals (i.e., k - space data) collected in a plurality of segments in the k - space. As shown in FIG. 9(c), the k - space data of the group corresponding to the same echo time TE (i.e., the group with the same number) is collected from each segment and filled into the k - space provided for each different echo time TE such as TE1, TE2, TE3, ···, TE(n).

[0074] In this way, by repeatedly collecting k - space data by repeating a plurality of segments composed of a plurality of groups with different echo times, a plurality of k - spaces corresponding to different echo times can be fully filled with k - space data sufficient to reconstruct a desired image.

[0075] Note that in the examples shown in FIGS. 7 to 9, the change pattern of the phase encoding amount between groups within the same segment is assumed to be the same pattern, but it is not limited to this. The change pattern of the phase encoding amount between groups within the same segment may be different patterns. The main point is that when collecting k-space data in a plurality of segments, a plurality of k-spaces corresponding to different echo times can be finally filled completely without duplication with sufficient k-space data to reconstruct a desired image. Returning to FIG. 6, in step ST100, the above-described first and second pulse sequences are set.

[0076] In the next step ST101, the first and second b-values (b1, b2) are respectively calculated from the shapes of the MPG pulses (MPG1, MPG2) of the readout gradient magnetic field pulses of the first and second pulse sequences by a known method (see FIG. 11(a)). The process of step ST101 is performed, for example, by the ADC map generation function F03.

[0077] In step ST102, the first and second pulse sequences set in step ST100 are applied to the subject to collect the first and second MR signals. The process of step ST102 is performed by the imaging unit 600.

[0078] In step ST103, as described in connection with FIGS. 7 to 9, the first and second MR signals are arranged in the k-space by aggregating them in groups of the same echo time TE, and the first and second k-space data sets for each echo time TE are generated. FIGS. 10(a) and 10(c) are diagrams illustrating the first and second k-space data sets generated for each echo time from TE1 to TE(N).

[0079] In the next step ST104, the first and second k-space data sets for each echo time are reconstructed to generate the first and second images for each echo time. FIGS. 10(a) to 10(d) are diagrams showing the concept of the process of reconstructing the first and second k-space data sets for each echo time to generate the first and second images for each echo time. The processes of step ST103 and step ST104 are performed, for example, by the image generation function F02 in FIG. 2.

[0080] Next, in step ST105, from the pixel values S1(x, y), S2(x, y) of the first and second images and the first and second b values (b1, b2), the diffusion coefficient ADC (ADC map) for each pixel position is calculated. The process of step ST105 is performed by the ADC map generation function F03 in FIG. 2.

[0081] FIG. 11 is a diagram for explaining the concept of the process of step ST105. As shown in FIG. 11(b), the pixel value S1(x, y) of the first image undergoes T2 decay (exp(-TE(n) / T2)) according to the T2 value of the tissue of the subject at the pixel position (x, y) and the echo time TE(n), and also undergoes decay due to diffusion and turbulent motion (exp(-b1*ADC(x, y))) according to the diffusion coefficient ADC at the pixel position (x, y) and the b1 value of the first pulse sequence for each readout. When M readouts are performed between TE(n + 1) and TE(n), it effectively undergoes decay corresponding to the (M*b1) value as the MPG effect (for example, in the case of FIG. 7, M = 5). Hereinafter, for convenience of explanation, the effective b value integrated between adjacent echo times will be used. Therefore, the pixel value S1(x, y) of the first image can be expressed by the following (Equation 4). S1(x, y)=S0(x, y)*exp(-TE(n) / T2)*exp(-b1*ADC(x, y)) (Equation 4)

[0082] Similarly, as shown in FIG. 11(c), the pixel value S2(x, y) of the second image is subject to T2 decay (exp(-TE(n) / T2)) according to the T2 value of the tissue of the subject at the pixel position (x, y) and the echo time TE(n), and is also subject to decay due to diffusion and turbulent motion (exp(-b2*ADC(x, y))) according to the diffusion coefficient ADC at the pixel position (x, y) and the b2 value of the second pulse sequence. Therefore, the pixel value S2(x, y) of the second image can be expressed by the following (Equation 5). S2(x, y)=S0(x, y)*exp(-TE(n) / T2)*exp(-b2*ADC(x, y)) (Equation 5)

[0083] By obtaining the ratio (S1(x, y) / S2(x, y)) of (Equation 4) and (Equation 5), only the term of decay due to diffusion and turbulent motion remains, and since the b values (b1, b2) are known, the diffusion coefficient ADC(x, y) for each pixel position can be calculated. Then, by arranging the diffusion coefficient ADC(x, y) at the corresponding pixel positions, an ADC map as shown in FIG. 11(d) can be generated.

[0084] Next, in step ST106, using the diffusion coefficient ADC (i.e., the ADC map) for each pixel position, the first image generated for each echo time is corrected to remove the influence of diffusion and turbulence. The process of step ST106 is performed by the diffusion and motion correction function F04 in FIG. 2.

[0085] FIG. 12 is a diagram for explaining the processing concept of step ST106. FIG. 12(a) shows the first image for each echo time TE(n) (n = 1 to N) before correction, which is affected by diffusion and turbulent motion. The first image before correction is subject to decay due to diffusion and turbulent motion (exp(-b1*ADC(x, y))) according to the b1 value of the first pulse sequence, as shown in (Equation 4).

[0086] Therefore, this attenuation is corrected for each pixel position using the ADC map (Fig. 12(b)) calculated in step ST105. As a result, as shown in Fig. 12(c), a corrected first image that is not affected by diffusion or turbulent movement is calculated for each echo time TE(n) (n = 1 to N). The pixel value S C (x, y) of the corrected first image is represented by the following (Equation 6). S C (x, y) = S0(x, y) * exp(-TE(n) / T2) (Equation 6)

[0087] Next, in step ST107, from the change of the pixel value of the corrected first image with respect to the echo time TE, a true T2 value from which the influence of diffusion and turbulence has been removed is calculated for each pixel position, and a T2 map is generated. The process of step ST107 is performed by the T2 map generation function F05 in Fig. 2.

[0088] Fig. 13 is a diagram for explaining the processing concept of step ST107. The curve shown by the dotted line in Fig. 13(b) is a schematic graph plotting the pixel value S C (x, y) of the corrected first image against the echo time TE. Also, the curve shown by the solid line in Fig. 13(b) is a schematic graph plotting the pixel value S1(x, y) of the first image before correction against the echo time TE.

[0089] Assuming that the change in the pixel value S C (x, y) of the corrected first image changes as an exponential function, if there are pixel values S C (x, y) for at least two echo times TE, the T2 value can be calculated. By increasing the sample points of the echo time TE, the calculation accuracy of the T2 value can be improved using methods such as curve fitting.

[0090] Also, there may be cases where a multi-component substance showing different T2 values is included at the same pixel position (i.e., within the same voxel). Even in such cases, for pixel values S corresponding to a plurality of echo times TE more than 2 CThe T2 value of each component can also be estimated from the change curve of (x, y).

[0091] Then, by arranging the T2 values (true T2 values not affected by diffusion or turbulent movement) calculated for each pixel position at the corresponding pixel positions, a true T2 map as shown in FIG. 13(c) can be generated.

[0092] Note that the pixel value S of the first image after correction C From the change curve of (x, y) with respect to the echo time TE (FIG. 14(a)), the T2 spectrum illustrated in FIG. 14(b) can also be calculated. The T2 spectrum is a plot with the T2 value on the horizontal axis and the intensity on the vertical axis. If affected by diffusion or turbulent movement, the peak of the T2 value appears at a position larger than the true T2 value, but for the true T2 value calculated from the change of the pixel value after correction, a peak occurs at the position of the original T2 value. The calculation process of the T2 spectrum is performed by the T2 spectrum generation function F06 in FIG. 2.

[0093] (First Modification Example) FIG. 15 is a sequence diagram of the first and second pulse sequences according to the first modification example of the embodiment. In the first modification example, the first pulse sequence (FIGS. 15(a) and 15(b)) is the same as that of the above-described embodiment, but as shown in FIG. 15(c), the readout gradient magnetic field pulse in the second pulse sequence is slightly different.

[0094] Specifically, in the readout gradient magnetic field pulse of the second pulse sequence, during a predetermined echo time TEa from the application of the excitation pulse, it is the same as the readout gradient magnetic field pulse of the first pulse sequence, that is, no additional gradient magnetic field pulse is added, and after the predetermined echo time TEa has elapsed, an additional gradient magnetic field pulse is added.

[0095] The main imaging target of the magnetic resonance imaging apparatus 1 according to the embodiment is a body fluid such as CSF having a relatively long T2 time. Therefore, in many cases, it is sufficient to process signals having an echo time TE longer than a predetermined echo time TEa. By not adding an additional gradient magnetic field pulse during the period from the application of the excitation pulse to the predetermined echo time TEa, the burden on the apparatus is reduced. Further, by limiting the reconstruction processing of the first and second images, the correction processing for the first image, the T2 value calculation processing using the corrected first image, etc. to the data collected with the additional gradient magnetic field pulse added, these processing times can be shortened.

[0096] (Second modification example) FIG. 16 is a diagram for explaining the processing concept of the second modification example of the embodiment. As shown in FIG. 16(b), in the second modification example, the interval of the echo time TE to be acquired is not equally spaced as in FIG. 14(a), but is made unevenly spaced by thinning out a part of the first and second images. As described above, the T2 value is calculated from the curve of the pixel value of the corrected first image with respect to the echo time TE. Usually, this curve changes greatly in the region where the echo time TE is short and does not change as the echo time TE becomes longer. Therefore, in the region where the echo time TE is short, data is acquired at equal intervals as in FIG. 14(a), and in the region where the echo time TE is long, the interval between adjacent echo times is made wider than in the region where the echo time TE is short. Then, by generating the first image from the data acquired in this way, the total number of images to be generated can be reduced without degrading the calculation accuracy of the T2 value, and the processing time required for the calculation and analysis of the T2 value can be shortened.

[0097] (Third modification example) The application direction of the readout gradient magnetic field pulse of the second pulse sequence in the above-described embodiment is set to be the same as the application direction of the readout gradient magnetic field pulse of the first pulse sequence.

[0098] In the third modification example, the imaging condition setting function F01 sets a third pulse sequence in which the application direction of the readout gradient magnetic field pulse is orthogonal to the application direction of the readout gradient magnetic field pulse of the second pulse sequence. Further, it sets a fourth pulse sequence in which the application direction of the readout gradient magnetic field pulse is orthogonal to both the application direction of the readout gradient magnetic field pulse of the second pulse sequence and the application direction of the readout gradient magnetic field pulse of the third pulse sequence.

[0099] Note that the readout gradient magnetic field pulses in the third and fourth pulse sequences are the same as the readout gradient magnetic field pulses in the second pulse sequence, with additional gradient magnetic field pulses added to the leading edge side and the trailing edge side of the readout gradient magnetic field pulses in the first pulse sequence, and have an MPG effect of a predetermined magnitude.

[0100] Also, the image generation function F02 according to the third modification example generates at least one of the second, third, and fourth diffusion-weighted images, the second, third, and fourth diffusion coefficient images, and the diffusion tensor image, based on the respective signals obtained by applying the above-described second, third, and fourth pulse sequences to the subject.

[0101] As described above, the magnetic resonance imaging apparatus 1 according to the embodiment and several modification examples have been described. However, the imaging condition setting function, the image generation function, and the analysis function in the description of the embodiment or its modification examples are examples of the imaging condition setting unit, the image generation unit, and the analysis unit in the claims. As described above, the magnetic resonance imaging apparatus according to the embodiment can acquire a highly accurate T2 value and an index related to diffusion and turbulent motion in a short imaging time.

[0102] Although some embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, as well as in the invention described in the claims and the equivalent scope thereof.

Explanation of Reference Numerals

[0103] 1 Magnetic Resonance Imaging Apparatus 40 Processing Circuit 41 Memory Circuit 42 Display 43 Input Device 400 Console 600 Imaging Unit F01 Imaging Condition Setting Function F02 Image Generation Function F03 ADC Map Generation Function F04 Diffusion / Motion Correction Function F05 T2 Map Generation Function F06 T2 Spectrum Generation Function

Claims

1. An FSE (Fast Spin Echo) type pulse sequence having an excitation pulse and a plurality of refocusing pulses following the excitation pulse, wherein a first readout gradient magnetic field pulse is arranged between each of the adjacent refocusing pulses. A first pulse sequence, and a second readout gradient magnetic field pulse having a pulse shape different from that of the first readout gradient magnetic field pulse is arranged between each of the adjacent refocusing pulses. An imaging condition setting unit for setting a second pulse sequence, An imaging unit that applies the first pulse sequence and the second pulse sequence to a subject and acquires a plurality of first signals having different echo times and a plurality of second signals having different echo times, respectively. An image generation unit that generates a plurality of first images and a plurality of second images from the plurality of first signals and the plurality of second signals, respectively. An analysis unit that calculates a T2 value of the body fluid from which the influence of the movement of the body fluid including the diffusion of the body fluid of the subject is removed from the plurality of first images and the plurality of second images. A magnetic resonance imaging apparatus comprising:

2. The image generation unit generates a plurality of the first images having different echo times and a plurality of the second images having different echo times. The magnetic resonance imaging apparatus according to claim 1.

3. The analysis unit calculates an index related to the movement of the body fluid and calculates the T2 value using the calculated index. The magnetic resonance imaging apparatus according to claim 2.

4. The second readout gradient magnetic field pulse is configured by adding additional gradient magnetic field pulses having a predetermined shape to the leading edge side and the trailing edge side of the first readout gradient magnetic field pulse, respectively. The magnetic resonance imaging apparatus according to claim 2.

5. In each of the first pulse sequence and the second pulse sequence, the plurality of refocusing pulses are divided into a plurality of groups having different echo times, and the phase encoding amount associated with each of the refocusing pulses is such that the phase encoding amount changes for each refocusing pulse in a predetermined change pattern within the group. Is set to The magnetic resonance imaging apparatus according to claim 2.

6. The change pattern of the phase encoding amount is set so that the same change pattern is repeated in each group. The magnetic resonance imaging apparatus according to claim 5.

7. The change pattern of the phase encoding amount is set to a different change pattern for each group. The magnetic resonance imaging apparatus according to claim 5.

8. Each of the first pulse sequence and the second pulse sequence is set to be repeated a predetermined number of times at a predetermined repetition time. During each repeated first pulse sequence and between each repeated second pulse sequence, the phase encoding amounts within the group corresponding to the same echo time do not overlap with each other and are set to be able to fill the k-space in the phase encoding direction necessary to reconstruct a desired image. The magnetic resonance imaging apparatus according to any one of claims 5 to 7.

9. The analysis unit calculates a first b value and a second b value respectively from the intensities and shapes of the first readout gradient magnetic field pulse and the second readout gradient magnetic field pulse. From the pixel values of the first image and the second image respectively generated from the first signal and the second signal, and the first b value and the second b value, an ADC map in which the index related to the movement of the body fluid is arranged for each pixel position is calculated. The magnetic resonance imaging apparatus according to any one of claims 2 to 8.

10. The analysis unit corrects each pixel value of the first image using the index in the ADC map, and corrects the first image so that the T2 value of the body fluid from which the influence of the movement of the body fluid is removed is reflected. The magnetic resonance imaging apparatus according to claim 9.

11. The analysis unit corrects each pixel value of the first image using the index in the ADC map for each of the plurality of first images having different echo times, and corrects the plurality of first images so that the T2 value of the body fluid from which the influence of the movement of the body fluid is removed is reflected. The magnetic resonance imaging apparatus according to claim 10.

12. The analysis unit calculates the T2 value for each pixel from the change in pixel values at the same pixel position between the plurality of first images having different corrected echo times, and generates a T2 map. The magnetic resonance imaging apparatus according to claim 11.

13. The analysis unit calculates the T2 spectrum for each pixel from the change in pixel values at the same pixel position between the plurality of first images having different corrected echo times. The magnetic resonance imaging apparatus according to claim 11.

14. The second readout gradient magnetic field pulse is applied in the same direction as the first readout gradient magnetic field pulse, The imaging condition setting unit further sets a third pulse sequence having a third readout gradient magnetic field pulse whose application direction is orthogonal to the application direction of the second readout gradient magnetic field pulse, and a fourth readout gradient magnetic field pulse whose application direction is orthogonal to both the application direction of the second readout gradient magnetic field pulse and the application direction of the third readout gradient magnetic field pulse. A fourth pulse sequence having, Based on the respective signals acquired by applying the second, third, and fourth pulse sequences to the subject, the image generation unit generates a plurality of second, third, and fourth diffusion-weighted images, a plurality of second, third, and fourth diffusion coefficient images, and at least one of a plurality of diffusion tensor images. The magnetic resonance imaging apparatus according to claim 1.

15. The additional gradient magnetic field pulse in the second pulse sequence is provided between each of the refocusing pulses after a predetermined echo time has elapsed from the excitation pulse among the plurality of refocusing pulses following the excitation pulse. The magnetic resonance imaging apparatus according to claim 4.

16. The image generation unit thins out a part of the plurality of first and second images having different echo times, makes the interval of the corresponding echo times uneven, and calculates the T2 value using the reduced number of first and second images obtained by the thinning. The magnetic resonance imaging apparatus according to claim 2.

17. The body fluid of the subject is at least one of cerebrospinal fluid and interstitial fluid of the brain. The magnetic resonance imaging apparatus according to any one of claims 1 to 16.

18. The FSE-type pulse sequence is a CPMG (Carr-Purcell-Meiboom-Gill) sequence. The magnetic resonance imaging apparatus according to any one of claims 1 to 17.

Images