Magnetic Resonance Imaging Apparatus and Imaging Time Reduction Method

Abstract

To reduce imaging time by CEST imaging.SOLUTION: An MRI device includes a sequence control part and an allocation part. The sequence control part executes a first sequence for collecting a first magnetic resonance signal corresponding to a low-frequency region of a K space and a second magnetic resonance signal corresponding to a high-frequency region of the k space, and a second sequence for collecting a third magnetic resonance signal corresponding to at least the low-frequency region by changing conditions of multiple saturation pulses in CEST (Chemical Exchange Saturation Transfer) imaging for executing application of multiple saturation pulses and collection of multiple magnetic resonance signals corresponding to the multiple saturation pulses. The allocation part allocates the third magnetic resonance signal and the second magnetic resonance signal to one k space generated in accordance with the second sequence.SELECTED DRAWING: Figure 4

Description

Technical Field

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

Background Art

[0002] As a conventional technique, an imaging method using magnetic resonance imaging (MRI) with chemical exchange saturation transfer (hereinafter referred to as CEST (Chemical Exchange Saturation Transfer)) has been proposed. Protons of a solute dissolved in water are chemically exchanged with protons of water. The rate of such chemical exchange depends on temperature and pH. On the other hand, the resonance frequency of a proton changes depending on the state of the proton. Such a change in the resonance frequency of a proton is called a chemical shift. From these facts, by transmitting a saturation RF pulse set to a frequency specific to the protons of the solute, a phenomenon occurs in which protons of water that should not originally be saturated become saturated. Such a phenomenon is called chemical exchange saturation transfer (CEST). An imaging method using CEST is called CEST imaging.

[0003] In CEST imaging, when acquiring a single image related to CEST (hereinafter referred to as a CEST image), imaging is performed by repeating the application of a saturation pulse and the collection of a magnetic resonance signal (hereinafter referred to as an MR (Magnetic Resonance) signal) accompanying the application of the saturation pulse. Specifically, in order to obtain the influence of the CEST effect (for example, Z spectrum) at an accurate chemical shift, for example, it is necessary to acquire an MR image while applying a plurality of saturation pulses at intervals of 0.1 ppm from -8 ppm to 8 ppm with the resonance frequency of free water set to 0 ppm. That is, in order to obtain one Z spectrum, it is necessary to acquire 161 MR images, and the imaging time becomes long. Further, if all the k-space data cannot be acquired in one collection of an MR signal, it is necessary to re-excite and collect the MR signal, so that the imaging time becomes even longer.

Prior Art Documents

Patent Document

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] One of the problems to be solved by the embodiments disclosed in this specification and the drawings is to shorten the imaging time by CEST imaging. However, the problems to be solved by the embodiments disclosed in this specification and the drawings are not limited to the above problems. The problems corresponding to the respective effects of each configuration shown in the embodiments described later can also be regarded as other problems.

Means for Solving the Problems

[0006] The magnetic resonance imaging apparatus according to this embodiment includes a sequence control unit and an assignment unit. The sequence control unit executes CEST (Chemical Exchange Saturation Transfer) imaging for applying a plurality of saturation pulses and collecting a plurality of magnetic resonance signals corresponding to the plurality of saturation pulses, and collects a first magnetic resonance signal corresponding to a low-frequency region of k-space and a second magnetic resonance signal corresponding to a high-frequency region of k-space in a first sequence, and a second sequence for collecting at least a third magnetic resonance signal corresponding to the low-frequency region, by changing the conditions of the plurality of saturation pulses. The assignment unit assigns the third magnetic resonance signal and the second magnetic resonance signal to one k-space generated corresponding to the second sequence.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Embodiment for Carrying Out the Invention

[0008] Hereinafter, embodiments of a magnetic resonance imaging apparatus (hereinafter referred to as an MRI (Magnetic Resonance Imaging) apparatus) and an imaging time shortening method will be described with reference to the drawings. Note that the embodiments are not limited to the following embodiments. In addition, the content described in each embodiment can be similarly applied to other embodiments in principle. In the following embodiments, parts denoted by the same reference numerals perform the same operations, and duplicate explanations will be omitted as appropriate.

[0009] (Embodiment) FIG. 1 is a block diagram showing an MRI apparatus 100 according to the embodiment. As shown in FIG. 1, the MRI apparatus 100 includes a static magnetic field magnet 101, a static magnetic field power supply 102, a gradient magnetic field coil 103, a gradient magnetic field power supply 104, a bed 105, a bed control circuit 106, a transmission coil 107, a transmission circuit 108, a reception coil 109, a reception circuit 110, a sequence control circuit 120, and a computer 130 (also referred to as an image processing apparatus). Note that the MRI apparatus 100 does not include a subject P (for example, a human body). Also, the configuration shown in FIG. 1 is merely an example. For example, each part in the sequence control circuit 120 and the computer 130 may be appropriately integrated or separated.

[0010] The static magnetic field magnet 101 is a magnet formed in a hollow substantially cylindrical shape and generates a static magnetic field in the internal space. The static magnetic field magnet 101 is, for example, a superconducting magnet or the like, and is excited by receiving a current supply from the static magnetic field power source 102. The static magnetic field power source 102 supplies a current to the static magnetic field magnet 101. Note that the static magnetic field magnet 101 may be a permanent magnet, and in this case, the MRI apparatus 100 may not include the static magnetic field power source 102. Also, the static magnetic field power source 102 may be provided separately from the MRI apparatus 100.

[0011] The gradient magnetic field coil 103 is a coil formed in a hollow substantially cylindrical shape and is disposed inside the static magnetic field magnet 101. The gradient magnetic field coil 103 is formed by combining three coils corresponding to the X, Y, and Z axes that are orthogonal to each other, and these three coils are individually supplied with a current from the gradient magnetic field power source 104 to generate a gradient magnetic field in which the magnetic field strength changes along the X, Y, and Z axes. The gradient magnetic fields along the X, Y, and Z axes generated by the gradient magnetic field coil 103 are, for example, the slice gradient magnetic field Gs, the phase encoding gradient magnetic field Ge, and the readout gradient magnetic field Gr. The gradient magnetic field power source 104 supplies a current to the gradient magnetic field coil 103.

[0012] The bed 105 includes a top plate 105a on which the subject P is placed, and under the control of the bed control circuit 106, the top plate 105a is inserted into the cavity (imaging opening) of the gradient magnetic field coil 103 with the subject P placed thereon. Usually, the bed 105 is installed such that the longitudinal direction is parallel to the central axis of the static magnetic field magnet 101. The bed control circuit 106 drives the bed 105 under the control of the computer 130 to move the top plate 105a in the longitudinal direction and the vertical direction.

[0013] The transmission coil 107 is disposed inside the gradient magnetic field coil 103, receives an RF pulse supply from the transmission circuit 108, and generates a high-frequency magnetic field. The transmission circuit 108 supplies an RF pulse corresponding to the Larmor frequency determined by the type of the target atom and the magnetic field strength to the transmission coil 107.

[0014] The receiving coil 109 is disposed inside the gradient magnetic field coil 103 and receives a magnetic resonance signal (hereinafter referred to as an MR (Magnetic Resonance) signal) emitted from the subject P due to the influence of a high-frequency magnetic field. When the receiving coil 109 receives an MR signal, it outputs the received MR signal to the receiving circuit 110.

[0015] Note that the above-described transmitting coil 107 and receiving coil 109 are merely examples. The transmitting coil 107 and the receiving coil 109 may be configured by combining one or more of a coil having only a transmitting function, a coil having only a receiving function, or a coil having both transmitting and receiving functions.

[0016] The receiving circuit 110 detects the MR signal output from the receiving coil 109 and generates MR data based on the detected MR signal. Specifically, the receiving circuit 110 generates MR data by digitally converting the MR signal output from the receiving coil 109. Further, the receiving circuit 110 transmits the generated MR data to the sequence control circuit 120. Note that the receiving circuit 110 may be provided on the gantry device side including the static magnetic field magnet 101, the gradient magnetic field coil 103, and the like.

[0017] The sequence control circuit 120 performs imaging of the subject P by driving the gradient magnetic field power supply 104, the transmitting circuit 108, and the receiving circuit 110 based on the sequence information transmitted from the computer 130. Here, the sequence information is information defining a procedure for performing imaging, and is also referred to as sequence conditions. The sequence information defines the strength of the current supplied by the gradient magnetic field power supply 104 to the gradient magnetic field coil 103 and the timing of supplying the current, the strength of the RF pulse supplied by the transmitting circuit 108 to the transmitting coil 107 and the timing of applying the RF pulse, the timing at which the receiving circuit 110 detects the MR signal, and the like.

[0018] For example, the sequence control circuit 120 is an integrated circuit such as an application specific integrated circuit (ASIC) or a field programmable gate array (FPGA), or an electronic circuit such as a central processing unit (CPU) or a micro processing unit (MPU). The sequence control circuit 120 corresponds to a sequence control unit.

[0019] When the sequence control circuit 120 drives the gradient magnetic field power supply 104, the transmission circuit 108, and the reception circuit 110 to image the subject P and receives MR data from the reception circuit 110, the received MR data is transferred to the computer 130.

[0020] The computer 130 performs overall control of the MRI apparatus 100 and image generation. The computer 130 includes a storage circuit 132, an input device 141, a display 143, and a processing circuit 150. The processing circuit 150 includes an interface function 131, a control function 133, a setting function 134, an allocation function 136, and an image generation function 138.

[0021] Each processing function performed by the interface function 131, the control function 133, the setting function 134, the allocation function 136, and the image generation function 138 is stored in the storage circuit 132 in the form of a program executable by a computer. The processing circuit 150 is a processor that reads a program from the storage circuit 132 and executes it to realize the functions corresponding to the respective programs. In other words, the processing circuit 150 in the state of having read each program has each function shown in the processing circuit 150 of FIG. 1.

[0022] In FIG. 1, the processing functions performed by the interface function 131, the control function 133, the setting function 134, the allocation function 136, and the image generation function 138 are described as being realized by a single processing circuit 150. However, it is also possible to configure the processing circuit 150 by combining a plurality of independent processors, and each processor realizes the function by executing a program. In other words, each of the above-described functions may be configured as a program, and it may be the case where a single processing circuit 150 executes each program, or it may be the case where a specific function is implemented in a dedicated independent program execution circuit.

[0023] The term "processor" used in the above description means, for example, a circuit such as a CPU, a GPU (Graphical Processing Unit), an application-specific integrated circuit, or a programmable logic device (for example, a simple programmable logic device (SPLD), a complex programmable logic device (CPLD), and a field programmable gate array FPGA). The processor realizes the function by reading and executing the program stored in the storage circuit 132.

[0024] Note that instead of storing the program in the storage circuit 132, it may be configured to directly incorporate the program into the circuit of the processor. In this case, the processor realizes the function by reading and executing the program incorporated in the circuit. Note that the bed control circuit 106, the transmission circuit 108, the reception circuit 110, etc. are also similarly constituted by the above-described electronic circuits such as the processor.

[0025] The processing circuit 150 transmits sequence information to the sequence control circuit 120 and receives MR data from the sequence control circuit 120 through the interface function 131. Also, when receiving MR data, the processing circuit 150 having the interface function 131 stores the received MR data in the memory circuit 132. The processing circuit 150 that realizes the interface function 131 corresponds to the interface section. The MR data stored in the memory circuit 132 is arranged in the k-space by the control function 133. The memory circuit 132 stores k-space data.

[0026] The memory circuit 132 stores MR data received by the processing circuit 150 having the interface function 131, the conditions of the saturation pulse set by the processing circuit 150 having the setting function 134, the k-space data arranged in the k-space by the processing circuit 150 having the allocation function 136, the image data generated by the processing circuit 150 having the image generation function 138, and the like. For example, the memory circuit 132 is realized by a semiconductor memory element such as a RAM (Random Access Memory), a flash memory, a hard disk, an optical disk, or the like.

[0027] The input device 141 receives various instructions and information inputs from the user. The input device 141 is, for example, a pointing device such as a mouse or a trackball, a selection device such as a mode switch, or an input device such as a keyboard. The display 143 displays a GUI (Graphical User Interface) for receiving input of imaging conditions and images generated by the processing circuit 150 having the image generation function 138 under the control of the processing circuit 150 having the control function 133. The display 143 is, for example, a display device such as a liquid crystal display.

[0028] The processing circuit 150 performs overall control of the MRI apparatus 100 by the control function 133, and controls imaging, image generation, image display, etc. For example, the processing circuit 150 having the control function 133 receives input of imaging conditions (imaging parameters, etc.) on the GUI, and generates sequence information according to the received imaging conditions and the conditions of the saturation pulse set by the setting function 134. Further, the processing circuit 150 having the control function 133 transmits the generated sequence information to the sequence control circuit 120. The processing circuit 150 that realizes the control function 133 corresponds to the control unit.

[0029] The processing circuit 150 reads out k-space data from the memory circuit 132 by the image generation function 138, and generates an image by performing reconstruction processing such as Fourier transform on the read k-space data. The processing circuit 150 that realizes the image generation function 138 corresponds to the image generation unit. The setting function 134 and the allocation function 136 in the processing circuit 150 will be described later. The processing circuit 150 that realizes the setting function 134 corresponds to the setting unit. Further, the processing circuit 150 that realizes the allocation function 136 corresponds to the allocation unit.

[0030] The chemical exchange saturation transfer (CEST) effect according to this embodiment will be briefly described. Protons in free water (bulk water) and protons of a compound are exchanged. For example, protons of a compound such as an amide group (-NH), a hydroxyl group (-OH), and an amino group (-NH2) are exchangeable protons. The CEST effect is described using quantities called "Z spectrum" and "MTRasym spectrum". Imaging related to the CEST effect is an MR imaging method (hereinafter referred to as CEST imaging) that utilizes the exchange of protons such as an amide group (-NH), a hydroxyl group (-OH), and an amino group (-NH2) and protons in free water.

[0031] In a magnetic resonance imaging method related to the CEST effect (hereinafter referred to as CEST imaging), the sequence control circuit 120 applies a saturation pulse, which is a frequency-selective RF (Radio Frequency) pulse, at a frequency away from the resonance frequency of free water (off-resonance frequency) and at the resonance frequency of exchangeable protons (for example, protons of a compound) to the subject P before collecting the MR signal. The saturation pulse is also referred to as a presaturation pulse. Specifically, CEST imaging is a magnetic resonance imaging method that executes application of a plurality of saturation pulses and collection of a plurality of magnetic resonance signals corresponding to the plurality of saturation pulses.

[0032] CEST imaging as a comparative example will be described with reference to FIG. 2. FIG. 2 is a diagram showing an example of CEST imaging as a comparative example. Hereinafter, for the sake of specific description, it is assumed that the frequencies of the plurality of saturation pulses used in CEST imaging are at intervals of 0.1 ppm in the range from -8 ppm to 8 ppm. For example, when the static magnetic field strength is 3 T, the frequency of the saturation pulse corresponding to 0 ppm is the resonance frequency of free water (hereinafter referred to as the central frequency) based on the static magnetic field strength, which is 128 MHz. At this time, the frequency of the saturation pulse corresponding to +8 ppm is (128 MHz + 128×8 Hz). On the other hand, the frequency of the saturation pulse corresponding to -8 ppm is (128 MHz - 128×8 Hz).

[0033] As shown in FIG. 2, according to CEST imaging as a comparative example, an MR image is acquired with the application of a plurality of saturation pulses at intervals of 0.1 ppm from -8 ppm to 8 ppm with the central frequency set to 0 ppm. Based on 161 MR images, a Z spectrum showing the correction of the (static magnetic field) inhomogeneity and the influence of the chemical exchange saturation transfer (CEST) effect at the accurate chemical shift after the correction is generated. Therefore, in the comparative example, there is a problem that the imaging time for the subject P becomes long with respect to the generation of the Z spectrum. 0 (Static magnetic field) inhomogeneity correction and a Z spectrum showing the influence of the chemical exchange saturation transfer (CEST) effect at the accurate chemical shift after the correction are generated. For this reason, in the comparative example, there is a problem that the imaging time for the subject P becomes long with respect to the generation of the Z spectrum.

[0034] In view of such problems, in the present embodiment, in generating a plurality of MR images related to the generation of a Z spectrum, the aim is to shorten the imaging time. In each of the plurality of MR images related to the generation of the Z spectrum, the contrast component is important. In the MR image, the contrast component is important in the MR signal in the low-frequency region in the k-space data from which the MR image is generated.

[0035] FIG. 3 is a diagram showing an example of a contrast component and a contour component with respect to k-space data kD and an MR image MI. As shown in FIG. 3, the contrast component CCI in the MR image corresponds to the low-frequency region kLR of the k-space data. Also, the contour component OCI in the MR image corresponds to the high-frequency region kHR of the k-space data. In the present embodiment, imaging is performed with respect to the low-frequency region kLR of the k-space that is dominant in the generation of the Z spectrum according to the conditions of the saturation pulse, and for the high-frequency region kHR of the k-space, the MR signal acquired by other imaging is reused. Thereby, in the present embodiment, a pulse sequence of CEST imaging capable of shortening the imaging time is provided for the generation of the Z spectrum.

[0036] As described above, the overall configuration of the MRI apparatus 100 according to the embodiment has been described. Based on such a configuration, the MRI apparatus 100 according to the embodiment executes CEST imaging (CEST imaging) with a shorter imaging time compared to the comparative example, and executes a process of generating a Z spectrum (hereinafter referred to as a CEST imaging shortening process). Hereinafter, the procedure regarding the CEST imaging shortening process will be described with reference to FIG. 4. FIG. 4 is a flowchart showing an example of the procedure of the CEST imaging shortening process.

[0037] Hereinafter, for the sake of specific description, it is assumed that the static magnetic field strength is 3T, and the range of the frequencies to which a plurality of saturation pulses are applied in CEST imaging is in the range from -8 ppm to 8 ppm. At this time, the frequency of the saturation pulse corresponding to 0 ppm, that is, the center frequency, is 128 MHz. Note that the range of the frequencies to which a plurality of saturation pulses are applied in CEST imaging can be appropriately set by the setting function 134.

[0038] (CEST Imaging Shortening Process) (Step S401) The processing circuit 150 sets, by the setting function 134, a center frequency and a plurality of frequencies set at intervals of a predetermined frequency (hereinafter referred to as frequency interval) from the center frequency as the conditions of the saturation pulse in the first sequence. The first sequence is a pulse sequence for collecting a first MR signal corresponding to the low-frequency region of the k-space and a second MR signal corresponding to the high-frequency region of the k-space.

[0039] For example, the setting function 134 sets, in the k-space, a region that contributes to the contrast of the MR image generated based on the MR signal, that is, a region that reflects the contrast component in the MR image, as the low-frequency region. The frequency interval is set by a user's instruction via the input device 141 or is set in advance.

[0040] Hereinafter, for the sake of specific explanation, it is assumed that the frequency interval is 1 ppm. At this time, the frequency interval is 128 Hz. At this time, the frequencies of the saturation pulses for which the first sequence is executed are a total of 17 types of frequencies, namely 128 MHz and 16 frequencies of (128 MHz ± 128 × (1 to 8) Hz). Note that the frequency interval is not limited to 1 ppm and can be arbitrarily set by the setting function 134.

[0041] (Step S402) The setting function 134 sets a plurality of frequencies included in the frequency interval as the conditions of the saturation pulse in the second sequence. The second sequence is a pulse sequence for collecting at least a third MR signal corresponding to the low-frequency region of the k-space. The plurality of frequencies are, for example, the corresponding frequencies every 0.1 ppm. For example, the plurality of frequencies between 0 ppm and 1 ppm are 9 types of (128 MHz + 128 × (1 to 9) × 0.1 Hz). The interval of the frequencies of the saturation pulses for which the second sequence is executed is not limited to 0.1 ppm and can be arbitrarily set by the setting function 134.

[0042] In addition, the conditions of the saturation pulse in the second sequence may be frequencies excluding the 10 frequencies adjacent to the center frequency as a starting point. Also, the conditions of the saturation pulse in the second sequence may be frequencies excluding ±1.0 ppm from the center frequency as a starting point. Further, the conditions of the saturation pulse in the second sequence may be frequencies excluding 10% of the total number of points of the Z spectrum from the center frequency as a starting point.

[0043] (Step S403) In CEST imaging, the sequence control circuit 120 executes the first sequence and the second sequence while changing the conditions of a plurality of saturation pulses. That is, the sequence control circuit 120 executes the first sequence and the second sequence using the set conditions. Specifically, the sequence control circuit 120 executes the first sequence and the second sequence until the application of the saturation pulse according to the set conditions of the saturation pulse and the collection of the MR signal according to the application of the saturation pulse are completed. The sequence control circuit 120 collects the first MR signal and the second MR signal, or the third MR signal, according to the frequency of the saturation pulse.

[0044] (Step S404) The processing circuit 150 generates a plurality of first MR images corresponding to the frequencies of a plurality of saturation pulses related to the first sequence based on the first MR signal and the second MR signal collected by the first sequence by the image generation function 138. Specifically, the assignment function 136 assigns the first MR signal and the second MR signal to one k-space generated to correspond to the first sequence. Next, the image generation function 138 performs a Fourier transform on the data of the k-space to which the first MR signal and the second MR signal are assigned, and generates a first MR image. More specifically, the image generation function 138 arranges the first MR signal and the second MR signal corresponding to a total of 17 frequencies of 128 MHz and 16 frequencies of (128 MHz ± 128×(1 to 8) Hz) in the k-space respectively, and performs a Fourier transform on the k-space data arranged in the k-space. Thereby, the image generation function 138 generates a plurality of first MR images corresponding to the above 17 frequencies.

[0045] (Step S405) The processing circuit 150 assigns the third MR signal and the second MR signal to one k-space generated to correspond to the second sequence by the assignment function 136. Specifically, the assignment function 136 assigns the second MR signal collected by the first sequence to the high-frequency region of the k-space related to the second sequence based on the conditions of the saturation pulse. More specifically, the assignment function 136 identifies the frequency of the saturation pulse of the first sequence that is closest to the frequency of the saturation pulse in the second sequence. Next, the assignment function 136 assigns the second MR signal related to the identified frequency to the high-frequency region of the second sequence, and assigns the third MR signal to the low-frequency region of the second sequence.

[0046] As a result, the low-frequency region of the k-space related to the second sequence is filled with the third MR signal, and the high-frequency region of the k-space related to the second sequence is filled with the second MR signal. In other words, the MR signal assigned to the high-frequency region of the k-space related to the second sequence is shared as the second MR signal with the MR signal filled in the high-frequency region of the k-space related to the first sequence. Such sharing of k-space data is also referred to as view sharing.

[0047] (Step S406) The processing circuit 150 generates a plurality of second MR images corresponding to the frequencies of the plurality of saturation pulses related to the second sequence based on the second MR signal and the third MR signal assigned to the k-space related to the second sequence by the image generation function 138. Specifically, the image generation function 138 generates a plurality of second MR images (in this embodiment, 2×8×9 = 144 types of second MR images) for all frequencies ranging from 9 types of frequencies (128 MHz ± 128(i + ×0.1×j (j is a natural number from 1 to 9)) Hz) between i (i is an arbitrary natural number from 0 to 7) ppm and ±(i + 1) ppm.

[0048] (Step S407) The processing circuit 150 generates a Z spectrum based on a plurality of first MR images and a plurality of second MR images by the image generation function 138. Since existing methods are available for generating the Z spectrum, the description thereof is omitted. Thus, the CEST imaging shortening process ends. The execution order of step S404 is not limited to FIG. 4, and it may be executed at any processing stage as long as it is between step S403 and step S407.

[0049] FIG. 5 is a schematic diagram showing an overview of the CEST imaging shortening process. The vertical axis of the Z spectrum graph ZS shown in FIG. 5 is the ratio of the signal value M of the MR signal when the saturation pulse is not applied to the signal value M of the MR signal when the saturation pulse is applied (hereinafter referred to as the signal ratio) (M / M 0 ). The horizontal axis of the Z spectrum graph ZS shown in FIG. 5 indicates the frequency of the saturation pulse in ppm. The triangular marks in the Z spectrum graph ZS shown in FIG. 5 indicate the signal ratio with respect to the frequency of the saturation pulse for the first sequence. The round marks in the Z spectrum graph ZS shown in FIG. 5 indicate the signal ratio with respect to the frequency of the saturation pulse for the second sequence. 0 ) is shown.

[0050] As shown in FIG. 5, in the first sequence, MR signals (first MR signal and second MR signal) are collected for the entire k-space (low-frequency region kLR and high-frequency region kHR). As a result, as shown in FIG. 5, the low-frequency region kLR of the k-space kS1 related to the first sequence is filled with the first MR signal, and the high-frequency region kHR of the k-space kS2 is filled with the second MR signal.

[0051] On the other hand, as shown in FIG. 5, in the second sequence, an MR signal (third MR signal) corresponding to the low-frequency region kLR of the k-space is collected. As a result, the imaging time in CEST imaging is shortened. As shown in FIG. 5, the low-frequency region kLR of the k-space kS2 related to the second sequence is filled with the third MR signal, and the high-frequency region kHR of the k-space kS2 is filled by diverting and using the second MR signal.

[0052] In the generation of MR images used for Z-spectrum, in the case of each of the plurality of circled marks (second sequence) shown in FIG. 5, the assignment function 136 identifies the closest (or neighboring) triangular mark (first sequence) using the conditions of the saturation pulse (frequency of the saturation pulse) of the circled mark, and as described above, the second MR signal is diverted (view sharing) to the high-frequency region kHR of the k-space related to the second sequence.

[0053] The MRI apparatus 100 according to the embodiment described above executes, in CEST imaging, a first sequence that collects a first MR signal corresponding to the low-frequency region kLR of the k-space and a second MR signal corresponding to the high-frequency region kHR of the k-space according to the conditions of the saturation pulse, and a second sequence that collects at least a third MR signal corresponding to the low-frequency region kLR, and assigns the second MR signal to the high-frequency region kHR of the k-space related to the second sequence based on the conditions of the saturation pulse. Specifically, the present MRI apparatus 100 sets a center frequency and a plurality of frequencies (triangular marks shown in FIG. 5) set at a frequency interval from the center frequency as the conditions of the saturation pulse in the first sequence, sets the plurality of frequencies included in the frequency interval as the conditions of the saturation pulse in the second sequence, and executes the first sequence and the second sequence according to the set conditions of the saturation pulse.

[0054] Accordingly, according to this MRI apparatus 100, as shown in FIG. 5, in the frequency interval, a second sequence for collecting only the third MR signal in the low-frequency region kLR can be executed, and the imaging time in CEST imaging can be significantly reduced. Further, in the second sequence, since the MR signal corresponding to the low-frequency region kLR that contributes to the contrast in the MR image is collected, the contrast of a plurality of second MR images regarding the Z spectrum can be maintained. In addition, at the center frequency, while acquiring accurate data by executing the first sequence, the k-space data in the high-frequency region kHR, which is data of low importance, is diverted to the k-space regarding the second sequence. From the above, according to this MRI apparatus 100, it is possible to significantly shorten the very time-consuming CEST imaging while suppressing the influence regarding the generation of the Z spectrum.

[0055] (First Modification Example) The difference between the first modification example and the embodiment lies in executing the first sequence in the vicinity of the center frequency. The processing circuit 150 sets, by the setting function 134, a plurality of frequencies included in a predetermined range centered on the center frequency as the conditions of the saturation pulse in the first sequence. Specifically, the setting function 134 sets a plurality of frequencies included in a predetermined range centered on the center frequency, for example, ±1 ppm, as the conditions of the saturation pulse in the first sequence. The plurality of frequencies are, for example, frequencies from -1 ppm to +1 ppm in increments of 0.1 ppm. Note that the predetermined range is not limited to the range from -1 ppm to +1 ppm, and can be arbitrarily set via the input device 141.

[0056] FIG. 6 is a schematic diagram showing an overview of the CEST imaging shortening process according to this modified example. In the graph ZS of the Z spectrum shown in FIG. 6, the triangular marks indicate the signal ratio with respect to the frequency of the saturation pulse for the first sequence. The circular marks in the graph ZS of the Z spectrum shown in FIG. 6 indicate the signal ratio with respect to the frequency of the saturation pulse for the second sequence. As shown in FIG. 6, the first sequence is executed for the frequencies of 21 saturation pulses in the vicinity of the center frequency of 0 ppm. For other frequencies of the saturation pulse, it is the same as in the embodiment.

[0057] The MRI apparatus 100 according to the first modified example of the embodiment described above sets a plurality of frequencies included in a predetermined range centered on the center frequency as the conditions of the saturation pulse in the first sequence. Thus, according to this modified example, since the MR signals for the entire k-space are collected in the frequency range including the center frequency, a more accurate first MR image can be generated. Therefore, in addition to the effects in the embodiment, the accuracy of the Z spectrum in the vicinity of the center frequency can be improved.

[0058] (Second Modified Example) The second modified example is to determine the minimum value of the signal ratio in the Z spectrum, and set the frequency corresponding to the determined minimum value (hereinafter referred to as the minimum frequency) as the condition of the saturation pulse, and perform CEST imaging.

[0059] The sequence control circuit 120 executes the first sequence while changing the frequency of the saturation pulse from the center frequency until the signal intensity (signal ratio) in the first MR image generated based on the first MR signal and the second MR signal becomes the minimum value. Thereby, the minimum value of the signal ratio is determined in the vicinity of the center frequency.

[0060] The processing circuit 150 sets, by the setting function 134, a plurality of frequencies included in a predetermined range centered on the minimum frequency, for example, ±1 ppm, and a plurality of frequencies set at a predetermined frequency interval from the minimum frequency, as the conditions for the saturation pulse in the first sequence. The minimum frequency in this modification corresponds to the center frequency in the first modification. That is, in this modification, the center frequency in the first modification is determined as the minimum frequency.

[0061] The setting function 134 sets a plurality of frequencies included in the above frequency interval as the conditions for the saturation pulse in the second sequence. Note that the setting function 134 may set, as the conditions for the saturation pulse in the second sequence, frequencies excluding 10 adjacent frequencies starting from the minimum frequency, frequencies excluding ±1.0 ppm starting from the minimum frequency, or frequencies excluding 10% of the total number of points of the Z spectrum starting from the minimum frequency.

[0062] FIG. 7 is a flowchart showing an example of the procedure of the process of setting the minimum frequency (hereinafter referred to as the minimum frequency setting process). The minimum frequency setting process corresponds to the process before step S401 in FIG. 4. That is, in step S401 of the CEST imaging shortening process in this modification, the process of step S707 in the minimum frequency setting process is executed.

[0063] (Minimum frequency setting process) (Step S701) The processing circuit 150 sets, by the setting function 134, two frequencies adjacent to the center frequency as the frequencies of the saturation pulse. For example, the two frequencies are 128 MHz ± 12.8 Hz (±0.1 ppm). The sequence control circuit 120 executes the first sequence with respect to the center frequency and the two frequencies adjacent to the center frequency.

[0064] (Step S702) The processing circuit 150 generates three MR images corresponding to the center frequency and two frequencies based on the MR signals collected by the first sequence by the image generation function 138. The image generation function 138 calculates the signal-to-noise ratio in each of the three MR images.

[0065] (Step S703) The setting function 134 identifies an adjacent frequency adjacent to the low-signal frequency corresponding to a signal-to-noise ratio lower than the signal-to-noise ratio of the center image. The low-signal frequency is the frequency corresponding to the signal-to-noise ratio lower than the signal-to-noise ratio of the center image among the two signal-to-noise ratios corresponding to the two frequencies. For example, when the signal-to-noise ratio lower than the signal-to-noise ratio corresponding to the center frequency of 128 MHz is the signal-to-noise ratio related to the MR image corresponding to the frequency of +0.1 ppm, the setting function 134 identifies the frequency of +0.1 ppm as the low-signal frequency. Next, the setting function 134 identifies the adjacent frequency in the direction from the center frequency to the low-signal frequency. At this time, the adjacent frequency becomes the frequency of +0.2 ppm.

[0066] If the signal-to-noise ratio lower than the signal-to-noise ratio of the center image does not exist in the signal-to-noise ratios related to the two MR images corresponding to the two frequencies, the minimum frequency setting process ends. At this time, the minimum frequency is the center frequency itself.

[0067] (Step S704) The sequence control circuit 120 executes the first sequence using the adjacent frequency as the frequency of the saturation pulse. The image generation function 138 generates an MR image corresponding to the adjacent frequency based on the MR signals acquired by the first sequence. The image generation function 138 calculates the signal-to-noise ratio in the MR image.

[0068] (Step S705) The setting function 134 compares the signal ratio related to the low signal frequency with the signal ratio related to the adjacent frequency. If the signal ratio related to the low signal frequency is greater than the signal ratio related to the adjacent frequency (Yes in step S705), the process of step S706 is executed. If the signal ratio related to the low signal frequency is less than or equal to the signal ratio related to the adjacent frequency (No in step S705), the process of step S707 is executed.

[0069] (Step S706) The setting function 134 sets the adjacent frequency as the low signal frequency in the direction from the center frequency to the low signal frequency, and specifies the adjacent frequency adjacent to the low signal frequency. That is, the setting function 134 changes the adjacent frequency to the low signal frequency, and specifies the adjacent frequency adjacent to the changed low signal frequency. Thereby, a new adjacent frequency is set in the direction from the center frequency to the low signal frequency. After this step, the process of step S704 is executed.

[0070] (Step S707) The setting function 134 sets the low signal frequency as the minimum frequency. The setting function 134 sets a plurality of frequencies included in a predetermined range centered on the minimum frequency and a plurality of frequencies set at a predetermined frequency interval from the minimum frequency as the conditions of the saturation pulse in the first sequence. After this step, the processes after step S402 of the CEST imaging shortening process are executed.

[0071] That is, the sequence control circuit 120 executes the first sequence and the second sequence until the application of the saturation pulse related to the plurality of frequencies in the conditions of the saturation pulse is completed. Among the frequencies of the plurality of set saturation pulses, for the frequencies of the saturation pulses for which the signal ratio has already been calculated (at least the center frequency and two frequencies in step S701), the execution of the first pulse sequence is not required.

[0072] The MRI apparatus 100 according to the second modification of the above-described embodiment executes the first sequence while changing the frequency of the saturation pulse from the center frequency until the signal intensity in the MR image generated based on the first MR signal and the second MR signal reaches a minimum value. A plurality of frequencies set at a predetermined frequency interval from the minimum frequency corresponding to the minimum value and a plurality of frequencies included in a predetermined range centered on the minimum frequency are set as the conditions of the saturation pulse in the first sequence, and the plurality of frequencies included in the frequency interval are set as the conditions of the saturation pulse in the second sequence.

[0073] FIG. 8 is a schematic diagram showing an overview of the minimum frequency setting process and the CEST imaging shortening process. The triangular marks in the graph ZS of the Z spectrum shown in FIG. 8 indicate the signal ratio with respect to the frequency of the saturation pulse for the first sequence. The circular marks in the graph ZS of the Z spectrum shown in FIG. 8 indicate the signal ratio with respect to the frequency of the saturation pulse for the second sequence. As shown in FIG. 8, the first sequence is executed for the frequencies of 21 saturation pulses in the vicinity of the minimum value. For other frequencies of the saturation pulse, it is the same as in the embodiment. In this modification, by determining the minimum value of the signal ratio, B 0 (static magnetic field) inhomogeneity can be considered to determine the minimum frequency corresponding to the center frequency.

[0074] In addition, according to the MRI apparatus 100 according to the second modification of the embodiment, since the conditions (frequencies) of the saturation pulse for the first sequence and the second sequence can be determined by the minimum frequency, the accuracy in the vicinity of the center frequency can be improved in the Z spectrum. From these facts, according to this modification, in addition to the effects in the embodiment, B 0 (static magnetic field) inhomogeneity is taken into account, so the accuracy of the Z spectrum in the vicinity of the center frequency can be improved.

[0075] (Third Modification) The difference between the third modification example and the embodiment lies in setting the vicinity of the frequency related to the CEST effect as the condition of the saturation pulse in the first sequence. For example, the setting function 134 sets a plurality of frequencies included in a predetermined range centered on the frequency corresponding to the peak of the signal intensity due to the CEST effect (hereinafter referred to as the peak frequency) as the condition of the saturation pulse in the first sequence. Specifically, the setting function 134 sets the peak frequency according to the substance that reflects the material properties in the CEST effect (hereinafter referred to as the CEST substance).

[0076] The CEST substance is, for example, a substance that causes chemical exchange of protons present in the Region Of Interest (ROI), such as an amide group, a substance used in the Ratiometric method (for example, iopamidol), and the like. The material properties are, for example, temperature, pH (hydrogen ion index), and the like.

[0077] FIG. 9 is a schematic diagram showing an overview of the CEST imaging shortening process when the substance that causes chemical exchange of protons present in the region of interest is an amide group. The triangle marks in the graph ZS of the Z spectrum shown in FIG. 9 indicate the signal ratio with respect to the frequency of the saturation pulse for the first sequence. The circle marks in the graph ZS of the Z spectrum shown in FIG. 9 indicate the signal ratio with respect to the frequency of the saturation pulse for the second sequence. As shown in FIG. 9, the peak frequency of the amide group corresponds to the downwardly convex peak in the graph ZS of the Z spectrum, and the literature value is 3.5 ppm.

[0078] Therefore, as shown in FIG. 9, the processing circuit 150 sets, by the setting function 134, a plurality of frequencies included in a predetermined range (for example, ±1 ppm) starting from the peak frequency as the condition of the saturation pulse in the first sequence. The plurality of frequencies are, for example, frequencies from 2.5 ppm to 4.5 ppm in steps of 0.1 ppm. Note that the predetermined range is not limited to the range from 2.5 ppm to 4.5 ppm, and can be arbitrarily set via the input device 141. Regarding other frequencies of the saturation pulse, it is the same as in the embodiment.

[0079] FIG. 10 is a schematic diagram showing an overview of CEST imaging shortening processing when a substance whose physical properties are reflected in CEST has two peak frequencies. As an example of a substance having two peak frequencies shown in FIG. 10, ioversol will be described. The triangles in the graph ZS of the Z spectrum shown in FIG. 10 indicate the signal ratio with respect to the frequency of the saturation pulse for the first sequence. The circles in the graph ZS of the Z spectrum shown in FIG. 9 indicate the signal ratio with respect to the frequency of the saturation pulse for the second sequence. As shown in FIG. 10, the two peak frequencies correspond to the downwardly convex peaks in the graph ZS of the Z spectrum, and are 4.2 ppm and 5.6 ppm in the literature values.

[0080] Therefore, as shown in FIG. 10, the processing circuit 150 sets, by the setting function 134, a plurality of frequencies included in a predetermined range (for example, ±0.5 ppm) starting from the peak frequencies as the conditions of the saturation pulse in the first sequence. The plurality of frequencies are, for example, the frequencies from 3.7 ppm in 0.1 ppm increments to 4.8 ppm and the frequencies from 5.1 ppm in 0.1 ppm increments to 5.7 ppm. Note that the predetermined range is not limited to the above range and can be arbitrarily set via the input device 141. Regarding the other frequencies of the saturation pulse, it is the same as in the embodiment.

[0081] The MRI apparatus 100 according to the third modification of the embodiment described above sets a plurality of frequencies included in a predetermined range centered on the peak frequency of the signal intensity due to the CEST effect as the conditions of the saturation pulse in the first sequence. Further, the MRI apparatus 100 sets the peak frequency of the signal intensity due to the CEST effect according to the substance that reflects the physical properties in the CEST effect. Thereby, according to this modification, since the MR signals for the entire k-space are collected in the frequency range including the peak frequency, the S / N of the first MR image in the vicinity of the peak frequency can be improved. For this reason, in addition to the effects in the embodiment, the accuracy of the Z spectrum in the vicinity of the peak frequency can be improved.

[0082] (Fourth Modification Example) The difference between the fourth modification example and the embodiment lies in setting the low-frequency region based on the length of the region of interest set in the positioning image used for the positioning of CEST imaging. That is, the process in this modification example is performed before the execution of the CEST imaging shortening process.

[0083] Before the execution of the first sequence and the second sequence, that is, before the execution of the CEST imaging shortening process, the sequence control circuit 120 executes a positioning sequence regarding the positioning of the region of interest. The region of interest may be the imaging range. The sequence control circuit 120 collects MR signals (hereinafter referred to as positioning MR signals) corresponding to the positioning sequence by executing the positioning sequence.

[0084] The processing circuit 150 generates an MR image (hereinafter referred to as a positioning image) based on the positioning MR signals by the image generation function 138. The processing circuit 150 causes the control function 133 to display the positioning image on the display 143.

[0085] The input device 141 inputs a region of interest with respect to the positioning image displayed on the display 143 according to the user's instruction. The input device 141 inputs the length regarding the region of interest with respect to the positioning image displayed on the display 143. The length regarding the region of interest corresponds to, for example, the length of the imaging object, the length (width) of the region of interest, or the length of the structure of the imaging object.

[0086] The processing circuit 150 sets the region of interest and the length regarding the region of interest in the positioning image by the setting function 134. The setting function 134 sets the low-frequency region based on the set length regarding the region of interest. Specifically, the setting function 134 sets the reciprocal of the length regarding the region of interest as the width of the low-frequency region (hereinafter referred to as the low-frequency width) in the phase encoding direction in the k-space with reference to the origin of the k-space.

[0087] FIG. 11 is a diagram showing an example of the relationship between a positioning image LI in which the length related to the region of interest is set and the low-frequency width of k-space CESTk related to the first sequence and the second sequence. As shown in FIG. 11, when the length TL of the imaging object is input with respect to the positioning image LI, the setting function 134 sets the reciprocal of the length TL of the imaging object as the low-frequency width TLW with reference to the origin of k-space CESTk. At this time, in the k-space related to the second sequence, the high-frequency region THW where the second MR signal is diverted has the width as shown in FIG. 11.

[0088] Also, as shown in FIG. 11, when the length RL of the region of interest ROI is input with respect to the positioning image LI, the setting function 134 sets the reciprocal of the length RL of the imaging object as the low-frequency width RLW with reference to the origin of k-space CESTk. At this time, in the k-space related to the second sequence, the high-frequency region RHW where the second MR signal is diverted has the width as shown in FIG. 11.

[0089] Also, as shown in FIG. 11, when the length SL of the structure of the imaging object is input with respect to the positioning image LI, the setting function 134 sets the reciprocal of the length SL of the imaging object as the low-frequency width SLW with reference to the origin of k-space CESTk. At this time, in the k-space related to the second sequence, the high-frequency region SHW where the second MR signal is diverted has the width as shown in FIG. 11.

[0090] The MRI apparatus 100 according to the fourth modification of the above-described embodiment executes a positioning sequence related to the positioning of the region of interest ROI before the execution of the first sequence and the second sequence, and sets the length related to the region of interest ROI in the positioning image generated based on the MR signals collected by the positioning sequence, and sets a low-frequency region based on the length related to the region of interest ROI. Thus, according to this modification, depending on the lengths of the imaging target, ROI, and structure in the positioning image LI, a low-frequency region (non-diverted portion that does not divert the MR signal) where the contrast component is dominant in the frequency space and a high-frequency region (diverted portion that diverts the MR signal) where the contrast component is non-dominant in the frequency space can be set by an input according to the user's desire. Thereby, the low-frequency region can be set according to the region that the user focuses on, and the accuracy of the spectrum can be improved.

[0091] (Fifth Modification) The difference between the fifth modification and the embodiment lies in setting the low-frequency regions related to the first sequence and the second sequence according to the period between two adjacent saturation pulses (repetition period (TR: Repetition Time, time to repeat)) in CEST imaging. Specifically, the setting function 134 sets the low-frequency region based on the time interval (TR) between the application timings of two adjacent saturation pulses among the plurality of saturation pulses. That is, the setting function 134 sets the low-frequency region based on the length of the time of the data collection period in the time interval between the application timings of the two saturation pulses.

[0092] FIG. 12 is a diagram showing an example of the first sequence where the saturation pulse is at the center frequency (0 ppm). As shown in FIG. 12, it is assumed that the first sequence fills the entire k-space with MR signals by applying RF pulses three times (3 shots). At this time, the first MR signal is filled in the low-frequency region kLR of the k-space. Also, the second MR signal is filled in the high-frequency region kHR of the k-space. The second MR signal filled in the high-frequency region kHR of the k-space is diverted to the high-frequency region of the k-space related to the second sequence.

[0093] As shown in FIG. 12, a saturation pulse for applying a frequency of 0 ppm is applied before data collection. As shown in FIG. 12, the interval of the saturation pulses, i.e., TR, is preset according to a desired emphasized image regarding the longitudinal relaxation time T 1 of the imaging target. Therefore, the size of the low-frequency region kLR capable of collecting the first MR image is determined by the imaging method (such as the fast spin echo method, the gradient method, etc.) of data collection in the first sequence. That is, the setting function 134 sets the low-frequency region of the k-space that can be imaged as the low-frequency region at the time interval between the application timings of two adjacent saturation pulses. Since the effects in this modified example are the same as those in the embodiment, the description is omitted.

[0094] (Sixth Modified Example) The difference between this modified example and the embodiment is that as the frequency of the saturation pulse approaches at least one of the center frequency and the peak frequency, the low-frequency region in the second sequence is set to be expanded. FIG. 13 is a schematic diagram showing an overview in the CEST imaging shortening process when the substance causing the chemical exchange of the protons existing in the region of interest is an amide group. The triangular marks in the graph ZS of the Z spectrum shown in FIG. 13 indicate the signal ratio with respect to the frequency of the saturation pulse for the first sequence. The large and small round marks in the graph ZS of the Z spectrum shown in FIG. 13 indicate the signal ratio with respect to the frequency of the saturation pulse for the second sequence.

[0095] As shown in FIG. 13, the conditions of the saturation pulse for the first sequence are set in the same manner as in the third and fifth modified examples. For example, as shown in FIG. 13, the setting function 134 sets a plurality of frequencies included in a predetermined range (for example, ±0.5 ppm) starting from the peak frequency (3.5 ppm) as the conditions of the saturation pulse in the first sequence. The plurality of frequencies are, for example, frequencies from 3.0 ppm to 4.0 ppm in 0.1 ppm increments. Note that the predetermined range is not limited to the above range and can be arbitrarily set via the input device 141. As shown in FIG. 13, the k-space kS1 for the first sequence is filled with the first MR signal and the second MR signal collected by the 3-shot RF pulse.

[0096] On the other hand, as shown in FIG. 13, regarding the frequencies of the small round-marked saturation pulses that are separated from both ends (for example, 3.0 ppm and 4.0 ppm) of the frequency of the saturation pulse for the first sequence by a predetermined frequency or more, the low-frequency region kLR1 in the second sequence is set as a region that can be collected by applying a 1-shot RF pulse, for example, as in the fifth modified example. Here, the predetermined frequency is, for example, 1 ppm. At this time, the third MR signal is filled in the low-frequency region of the k-space kS2 for the second sequence, and the second MR signal is diverted and filled in the high-frequency region (k-space diversion part) of the k-space kS21.

[0097] The setting function 134 sets by expanding the low-frequency region in the second sequence as the frequency of the saturation pulse approaches at least one of the center frequency and the peak frequency. Specifically, in the range of the frequency of the saturation pulse corresponding to a predetermined frequency (in FIG. 13, for example, from 2.0 ppm to 3.0 ppm and from 4.0 ppm to 5.0 ppm), the setting function 134 sets the low-frequency region kLR2 in the k-space kS22 as a region that can be collected by a two-shot RF pulse as shown in FIG. 13. In other words, as the frequency of the saturation pulse in the second sequence approaches at least one of the center frequency and the peak frequency, the setting function 134 sets the low-frequency region so as to reduce the high-frequency region, that is, the diverted portion of the k-space. Thereby, the imaging time in CEST imaging is shortened. The effects in this modified example are the same as those in the embodiment, the first modified example, the third modified example, etc., and thus the description is omitted.

[0098] (Seventh Modified Example) This modified example corresponds to an application example of the sixth modified example. In this modified example, as the frequency of the saturation pulse by the first sequence moves away from both ends of the applied frequency range, the filling rate of the high-frequency region imaged in the second sequence is increased.

[0099] FIG. 14 is a schematic diagram showing an overview of the CEST imaging shortening process when the substance causing the chemical exchange of the protons existing in the region of interest is an amide group. The triangular marks in the graph ZS of the Z spectrum shown in FIG. 14 indicate the signal ratio with respect to the frequency of the saturation pulse for the first sequence. The large and small circular marks in the graph ZS of the Z spectrum shown in FIG. 14 indicate the signal ratio with respect to the frequency of the saturation pulse for the second sequence. The setting of the conditions of the saturation pulse in the first sequence is the same as that in the sixth modified example, and thus the description is omitted.

[0100] As shown in FIG. 14, in the second sequence, as moving away from both ends of the frequency of the saturation pulse for the first sequence (for example, 3.0 ppm and 4.0 ppm), the existing high-speed imaging method (for example, the filling rate of Compressed Sensing (= 1 - decimation collection rate)) is reduced from k-space kS2A to k-space kS2F in the high-frequency region kHR of the k-space of the second sequence. Note that the existing high-speed imaging method is not limited to compressed sensing, and may be parallel imaging or the like. In other words, as approaching the frequency of the saturation pulse of interest (for example, the ppm of the peak frequency), that is, in the order of k-space kS2F, kS2A, and kS1, the new filling rate of the k-space approaches 100%. Thereby, the ratio of the k-space data (second MR signal) diverted from the first sequence to the region where the MR signal is not collected in the high-frequency region for the second sequence decreases.

[0101] The setting function 134 sets a partial region of the high-frequency region so as to increase the ratio (filling rate) of a partial region of the high-frequency region to the high-frequency region kHR from k-space kS2F to kS2A in the second sequence as the frequency of the saturation pulse approaches at least one of the center frequency and the peak frequency. That is, the setting function 134 changes the filling rate of the k-space in the second sequence according to the degree of separation from the frequency of the saturation pulse of interest (the distance indicated by the frequency), and reduces the ratio of the second MR signal diverted from the first sequence in the k-space for the second sequence according to the degree. Thereby, the imaging time in CEST imaging is shortened.

[0102] The sequence control circuit 120 further collects a fourth MR signal corresponding to the set partial region in the second sequence.

[0103] The allocation function 136 allocates the fourth MR signal to a partial region in the high-frequency region. The allocation function 136 allocates the second MR signal corresponding to the other region to another region different from the partial region to which the fourth MR signal is allocated. Since the effects in this modified example are the same as those in the embodiment and the sixth modified example, etc., the description is omitted.

[0104] (Eighth Modified Example) In this modified example, regarding a plurality of second sequences with different saturation pulse frequencies, a partial region in the high-frequency region of the k-space is complementarily set, and the fourth MR signal corresponding to the partial region in the plurality of second sequences is complementarily allocated to another region different from the partial region. The partial region in the high-frequency region has, for example, arbitrary sparsity. The partial region corresponds to the sparsity in compressed sensing.

[0105] The setting function 134 complementarily sets a partial region in the high-frequency region of the k-space regarding a plurality of second sequences with different saturation pulse frequencies. The partial region corresponds to the region where the fourth MR signal is collected by the second sequence. The setting function 134 sets a partial region having sparsity so as to complement each other the regions where the MR signal is not collected between the k-spaces regarding different second sequences in each of the plurality of second sequences.

[0106] The sequence control circuit 120 further collects the fourth MR signal corresponding to the partial region in the second sequence. That is, the sequence control circuit 120 collects the third MR signal and the fourth MR signal in the second sequence.

[0107] The assignment function 136 assigns the fourth MR signal corresponding to a partial region in a plurality of second sequences to another region different from a partial region in the high-frequency region related to the second sequence in a complementary manner. Specifically, the assignment function 136 assigns the fourth MR signal collected with another region in the high-frequency region of the k-space in each of the plurality of second sequences as a partial region by a second sequence with a saturation pulse condition (frequency) close to that region. Thereby, the assignment function 136 assigns the fourth MR signal collected by a second sequence with a different saturation pulse frequency to another region in the k-space in each of the plurality of second sequences.

[0108] FIG. 15 is a schematic diagram showing an overview of the CEST imaging shortening process. Hereinafter, a specific explanation will be given with reference to FIG. 15. The triangular marks in the graph ZS of the Z spectrum shown in FIG. 15 indicate the signal ratio with respect to the frequency of the saturation pulse for the first sequence. The circular marks in the graph ZS of the Z spectrum shown in FIG. 15 indicate the signal ratio with respect to the frequency of the saturation pulse for the second sequence. Since the first sequence shown in FIG. 15 is the same as that in FIG. 5 in the embodiment, the description thereof will be omitted.

[0109] As shown in FIG. 15, in a plurality of second sequences, an MR signal (third MR signal) corresponding to the low-frequency region kLR of the k-space is collected. Also, as shown in FIG. 15, in the high-frequency region kHR in each of the plurality of second sequences, an MR signal (fourth MR signal) is collected for a partial region having a preset sparsity. That is, the fourth MR signal is acquired so that the fourth MR signal is shared (diverted) to the k-space in another second sequence with a different saturation pulse frequency for a partial region of the contour component (high-frequency region) in the second sequence.

[0110] Specifically, as shown in FIG. 15, in the high-frequency region kHR of k-space (kS2A, kS2B, kS2C) for three second sequences with different saturation pulse frequencies, the fourth MR signal is acquired to have different sparsities. Focusing on one of the three k-spaces, kS2A, for other regions in the high-frequency region kHR of the k-space kS2A where no MR signal is acquired, the fourth MR signal collected under conditions close to the conditions (frequency, ppm) of the saturation pulse of the second sequence corresponding to the k-space kS2A is assigned.

[0111] More specifically, as shown in FIG. 15, for other regions in the high-frequency region kHR of the k-space kS2A, the fourth MR signal in the high-frequency region kHR of the k-space kS2B and the fourth MR signal in the high-frequency region kHR of the k-space kS2C are interpolated. Thereby, the k-space kSC in which the fourth MR signal in other second sequences is interpolated in the high-frequency region kHR of the k-space kS2A is generated by the assignment function 136. The effects in this modification are the same as those in the embodiment and the like, so the description is omitted.

[0112] When realizing the technical idea in the embodiment by an imaging time shortening method, the imaging time shortening method executes CEST (Chemical Exchange Saturation Transfer) imaging that applies a plurality of saturation pulses and collects a plurality of magnetic resonance signals corresponding to the plurality of saturation pulses. A first sequence that collects a first magnetic resonance signal corresponding to the low-frequency region of k-space and a second magnetic resonance signal corresponding to the high-frequency region of the k-space, and a second sequence that collects at least a third magnetic resonance signal corresponding to the low-frequency region are executed by changing the conditions of the plurality of saturation pulses, and the third magnetic resonance signal and the second magnetic resonance signal are assigned to one k-space generated to correspond to the second sequence. The procedure and effects of the CEST imaging shortening process executed by the imaging time shortening method are the same as those in the embodiment, so the description is omitted.

[0113] According to at least the embodiment, modification, etc. described above, the imaging time by CEST imaging can be shortened.

[0114] Although some embodiments 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, changes, and combinations of embodiments 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

[0115] 100 Magnetic Resonance Imaging Apparatus 101 Static Magnetic Field Magnet 102 Static Magnetic Field Power Supply 103 Gradient Coil 104 Gradient Power Supply 105 Bed 105a Top Plate 106 Bed Control Circuit 107 Transmission Coil 108 Transmission Circuit 109 Reception Coil 120 Sequence Control Circuit 130 Computer 131 Interface Function 132 Memory Circuit 133 Control Function 134 Setting Function 136 Allocation Function 138 Image Generation Function 141 Input Device 143 Display 150 Processing Circuit

Claims

1. In a Chemical Exchange Saturation Transfer (CEST) imaging that performs application of a plurality of saturation pulses and collection of a plurality of magnetic resonance signals corresponding to the plurality of saturation pulses, a first sequence that collects a first magnetic resonance signal corresponding to a low-frequency region of k-space and a second magnetic resonance signal corresponding to a high-frequency region of the k-space, and a second sequence that collects at least a third magnetic resonance signal corresponding to the low-frequency region, are executed with conditions of the plurality of saturation pulses being changed; a sequence control unit, an allocation unit that allocates the third magnetic resonance signal and the second magnetic resonance signal to one k-space generated corresponding to the second sequence, comprising: the sequence control unit executes the first sequence while changing the frequency of the saturation pulse from the resonance frequency of free water based on the static magnetic field strength until the signal intensity in the magnetic resonance image generated based on the first magnetic resonance signal and the second magnetic resonance signal becomes a minimum value, a setting unit that sets a plurality of frequencies set at a predetermined frequency interval from the frequency corresponding to the minimum value and a plurality of frequencies included in a predetermined range centered on the frequency corresponding to the minimum value as the conditions of the plurality of saturation pulses in the first sequence, and sets the plurality of frequencies included in the interval as the conditions of the plurality of saturation pulses in the second sequence; the sequence control unit executes the first sequence and the second sequence until application of the saturation pulse regarding the plurality of frequencies in the conditions is completed, a magnetic resonance imaging apparatus.

2. In a Chemical Exchange Saturation Transfer (CEST) imaging that performs application of a plurality of saturation pulses and collection of a plurality of magnetic resonance signals corresponding to the plurality of saturation pulses, a first sequence that collects a first magnetic resonance signal corresponding to a low-frequency region of k-space and a second magnetic resonance signal corresponding to a high-frequency region of the k-space, and a second sequence that collects at least a third magnetic resonance signal corresponding to the low-frequency region, are executed with conditions of the plurality of saturation pulses being changed; a sequence control unit, an allocation unit that allocates the third magnetic resonance signal and the second magnetic resonance signal to one k-space generated corresponding to the second sequence, In the k-space, a setting unit that sets, as the low-frequency region, a region that contributes to the contrast of a magnetic resonance image generated based on a magnetic resonance signal; comprising; Before executing the first sequence and the second sequence, the sequence control unit executes a positioning sequence regarding the positioning of the region of interest; the setting unit; In a positioning image generated based on the magnetic resonance signal collected by the positioning sequence, sets a length regarding the region of interest; sets the low-frequency region based on the length regarding the region of interest; A magnetic resonance imaging apparatus. **Claim 3**: In Chemical Exchange Saturation Transfer (CEST) imaging that performs application of a plurality of saturation pulses and collection of a plurality of magnetic resonance signals corresponding to the plurality of saturation pulses, a first sequence that collects a first magnetic resonance signal corresponding to a low-frequency region of k-space and a second magnetic resonance signal corresponding to a high-frequency region of the k-space, and a second sequence that collects at least a third magnetic resonance signal corresponding to the low-frequency region, are executed by changing conditions of the plurality of saturation pulses, a sequence control unit; An allocation unit that allocates the third magnetic resonance signal and the second magnetic resonance signal to one k-space generated corresponding to the second sequence; A setting unit that sets the low-frequency region based on a time interval between application timings of two adjacent saturation pulses among the plurality of saturation pulses; A magnetic resonance imaging apparatus comprising. **Claim 4**: In Chemical Exchange Saturation Transfer (CEST) imaging that performs application of a plurality of saturation pulses and collection of a plurality of magnetic resonance signals corresponding to the plurality of saturation pulses, a first sequence that collects a first magnetic resonance signal corresponding to a low-frequency region of k-space and a second magnetic resonance signal corresponding to a high-frequency region of the k-space, and a second sequence that collects at least a third magnetic resonance signal corresponding to the low-frequency region, are executed by changing conditions of the plurality of saturation pulses, a sequence control unit; An allocation unit that allocates the third magnetic resonance signal and the second magnetic resonance signal to one k-space generated corresponding to the second sequence; A setting unit that expands and sets the low-frequency region in the second sequence as the frequency of the saturation pulse approaches at least one of the resonance frequency of free water based on the magnetic field strength and the frequency corresponding to the peak of the signal intensity due to the CEST effect; A magnetic resonance imaging apparatus comprising the same. **Claim 5**: In CEST (Chemical Exchange Saturation Transfer) imaging that performs application of a plurality of saturation pulses and collection of a plurality of magnetic resonance signals corresponding to the plurality of saturation pulses, a first sequence that collects a first magnetic resonance signal corresponding to a low-frequency region of k-space and a second magnetic resonance signal corresponding to a high-frequency region of the k-space, and a second sequence that collects at least a third magnetic resonance signal corresponding to the low-frequency region, are executed by changing the conditions of the plurality of saturation pulses; a sequence control unit; An allocation unit that allocates the third magnetic resonance signal and the second magnetic resonance signal to one k-space generated corresponding to the second sequence; A setting unit that sets a partial region so as to increase the ratio of a partial region of the high-frequency region to the high-frequency region in the k-space in the second sequence as the frequency of the saturation pulse approaches at least one of the resonance frequency of free water based on the magnetic field strength and the frequency corresponding to the peak of the signal intensity due to the CEST effect; Comprising: The sequence control unit further collects a fourth magnetic resonance signal corresponding to the partial region in the second sequence; The allocation unit allocates the second magnetic resonance signal corresponding to the other region to another region different from the partial region to which the fourth magnetic resonance signal is allocated in the high-frequency region; A magnetic resonance imaging apparatus. **Claim 6**: The setting unit sets the resonance frequency of free water based on the static magnetic field strength and a plurality of frequencies set at a predetermined frequency interval from the resonance frequency as the conditions of the plurality of saturation pulses in the first sequence, and sets the plurality of frequencies included in the interval as the conditions of the plurality of saturation pulses in the second sequence; The sequence control unit executes the first sequence and the second sequence using the set conditions; The magnetic resonance imaging apparatus according to any one of claims 1 to 5. **Claim 7** The setting unit sets a plurality of frequencies included in a predetermined range centered on the resonance frequency as the conditions in the first sequence. The magnetic resonance imaging apparatus according to claim 6.

8. The setting unit sets a plurality of frequencies included in a predetermined range centered on the frequency corresponding to the peak of the signal intensity due to the CEST effect as the conditions of the plurality of saturation pulses in the first sequence. The magnetic resonance imaging apparatus according to any one of claims 1 to 7.

9. The setting unit sets the frequency corresponding to the peak of the signal intensity due to the CEST effect according to the substance that reflects the material characteristics in the CEST effect. The magnetic resonance imaging apparatus according to claim 8.

10. The setting unit complementarily sets a partial region of the high-frequency region of the k-space for a plurality of the second sequences having different frequencies of the saturation pulses. The sequence control unit further collects a fourth magnetic resonance signal corresponding to the partial region in the second sequence. The allocation unit complementarily allocates the fourth magnetic resonance signals corresponding to the partial regions in the plurality of second sequences to other regions different from the partial region of the high-frequency region related to the second sequence. The magnetic resonance imaging apparatus according to any one of claims 1 to 4.

11. In CEST (Chemical Exchange Saturation Transfer) imaging that performs application of a plurality of saturation pulses and collection of a plurality of magnetic resonance signals corresponding to the plurality of saturation pulses, a first sequence that collects a first magnetic resonance signal corresponding to the low-frequency region of the k-space and a second magnetic resonance signal corresponding to the high-frequency region of the k-space, and a second sequence that collects at least a third magnetic resonance signal corresponding to the low-frequency region are executed while changing the conditions of the plurality of saturation pulses. The third magnetic resonance signal and the second magnetic resonance signal are allocated to one k-space generated corresponding to the second sequence. While changing the frequency of the saturation pulse from the resonance frequency of free water based on the static magnetic field strength, the first sequence is executed until the signal intensity in the magnetic resonance image generated based on the first magnetic resonance signal and the second magnetic resonance signal becomes a minimum value. A plurality of frequencies set at intervals of a predetermined frequency from the frequency corresponding to the minimum value, and a plurality of frequencies included in a predetermined range centered on the frequency corresponding to the minimum value are set as conditions for the plurality of saturation pulses in the first sequence. A plurality of frequencies included in the interval are set as conditions for the plurality of saturation pulses in the second sequence. The first sequence and the second sequence are executed until the application of the saturation pulses for the plurality of frequencies in the conditions is completed. An imaging time shortening method comprising the above. In CEST (Chemical Exchange Saturation Transfer) imaging that performs application of a plurality of saturation pulses and collection of a plurality of magnetic resonance signals corresponding to the plurality of saturation pulses, a first sequence that collects a first magnetic resonance signal corresponding to a low-frequency region of k-space and a second magnetic resonance signal corresponding to a high-frequency region of the k-space, and a second sequence that collects at least a third magnetic resonance signal corresponding to the low-frequency region are executed with different conditions for the plurality of saturation pulses. The third magnetic resonance signal and the second magnetic resonance signal are assigned to one k-space generated corresponding to the second sequence. In the k-space, a region that contributes to the contrast of a magnetic resonance image generated based on the magnetic resonance signal is set as the low-frequency region. Before executing the first sequence and the second sequence, a positioning sequence regarding positioning of a region of interest is executed. In a positioning image generated based on the magnetic resonance signal collected by the positioning sequence, the length regarding the region of interest is set. The low-frequency region is set based on the length regarding the region of interest. An imaging time shortening method comprising the above. In CEST (Chemical Exchange Saturation Transfer) imaging that performs application of a plurality of saturation pulses and collection of a plurality of magnetic resonance signals corresponding to the plurality of saturation pulses, a first sequence that collects a first magnetic resonance signal corresponding to a low-frequency region of k-space and a second magnetic resonance signal corresponding to a high-frequency region of the k-space, and a second sequence that collects at least a third magnetic resonance signal corresponding to the low-frequency region are executed with different conditions for the plurality of saturation pulses. Assign the third magnetic resonance signal and the second magnetic resonance signal to one k-space generated corresponding to the second sequence. Set the low-frequency region based on the time interval between the application timings of two adjacent saturation pulses among the plurality of saturation pulses. An imaging time shortening method comprising the above.

14. In CEST (Chemical Exchange Saturation Transfer) imaging that performs application of a plurality of saturation pulses and collection of a plurality of magnetic resonance signals corresponding to the plurality of saturation pulses, a first sequence that collects a first magnetic resonance signal corresponding to a low-frequency region of k-space and a second magnetic resonance signal corresponding to a high-frequency region of the k-space, and a second sequence that collects at least a third magnetic resonance signal corresponding to the low-frequency region are executed by changing the conditions of the plurality of saturation pulses. Assign the third magnetic resonance signal and the second magnetic resonance signal to one k-space generated corresponding to the second sequence. As the frequency of the saturation pulse approaches at least one of the resonance frequency of free water based on the magnetic field strength and the frequency corresponding to the peak of the signal intensity due to the CEST effect, expand and set the low-frequency region in the second sequence. An imaging time shortening method comprising the above.

15. In CEST (Chemical Exchange Saturation Transfer) imaging that performs application of a plurality of saturation pulses and collection of a plurality of magnetic resonance signals corresponding to the plurality of saturation pulses, a first sequence that collects a first magnetic resonance signal corresponding to a low-frequency region of k-space and a second magnetic resonance signal corresponding to a high-frequency region of the k-space, and a second sequence that collects at least a third magnetic resonance signal corresponding to the low-frequency region are executed by changing the conditions of the plurality of saturation pulses. Assign the third magnetic resonance signal and the second magnetic resonance signal to one k-space generated corresponding to the second sequence. As the frequency of the saturation pulse approaches at least one of the resonance frequency of free water based on the magnetic field strength and the frequency corresponding to the peak of the signal intensity due to the CEST effect, set the partial region so as to increase the ratio of the partial region of the high-frequency region to the high-frequency region in the k-space in the second sequence. In the second sequence, further collect a fourth magnetic resonance signal corresponding to the partial region, In the high-frequency region, assign the second magnetic resonance signal corresponding to the other region to another region different from the partial region to which the fourth magnetic resonance signal is assigned, An imaging time shortening method comprising the above.

Images