Magnetic resonance imaging apparatus and magnetic resonance imaging method

Abstract

To set an excitation thickness and an excitation position of a saturation pulse applied to the outside in a phase encode direction of an imaging region in a subject easily and appropriately.SOLUTION: A magnetic resonance imaging apparatus includes an acquisition unit, a measurement unit, and a calculation unit. The acquisition unit acquires one-dimensional projection data in a phase encode direction of a subject that is an object of the imaging. The measurement unit measures a subject thickness in the phase encode direction using the one-dimensional projection data. The calculation unit calculates an excitation thickness and an excitation position of a saturation pulse applied to the outside in the phase encode direction of an imaging region in the subject when the imaging is performed on the basis of the subject thickness.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Conventionally, a local excitation technique has been widely used as a method for imaging a small imaging region (Field of View: FOV) in a subject with high resolution using a magnetic resonance imaging (MRI) apparatus.

[0003] For example, two-dimensional RF (Radio Frequency) excitation, oblique excitation, and OVS (Outer Volume Suppression) are well-known common local excitation techniques. Two-dimensional RF (Radio Frequency) excitation is a method of local excitation by applying RF pulses while performing phase encoding. Oblique excitation is a method of local excitation by changing the application angles of the excitation pulse and the refocusing pulse. OVS is a method of local excitation by applying a saturation pulse outside the phase encoding direction of the imaging region to suppress signals outside the imaging region.

[0004] Among these, in the OVS method, the excitation thickness and excitation position of the saturation pulse change depending on the size of the subject and the position of the imaging region, so the operator of the MRI system needs to set the excitation thickness and excitation position of the saturation pulse while visually checking the structure of the subject, which makes the workflow complicated. On the other hand, although it is possible to reduce the complexity of the workflow by setting the excitation thickness of the saturation pulse to be large, increasing the excitation thickness of the saturation pulse deteriorates the excitation profile and results in insufficient signal suppression in areas close to the imaging region, so it is desirable to set the excitation thickness as thin as possible. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-261487 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-098031 [Patent Document 3] International Publication No. 2014 / 185521 Summary of the Invention [Problem to be solved by the invention]

[0006] One of the problems to be solved by the embodiments disclosed in this specification and the drawings is to easily and appropriately set the excitation thickness and excitation position of a saturation pulse applied to the outside of the phase encoding direction of an imaging region within a subject. However, the problems to be solved by the embodiments disclosed in this specification and the drawings are not limited to the above problem. Problems corresponding to the effects of each configuration shown in the embodiments described below can also be positioned as other problems. [Means for solving the problem]

[0007] An MRI apparatus according to an embodiment includes an acquisition unit, a measurement unit, and a calculation unit. The acquisition unit acquires one-dimensional projection data in a phase encoding direction of a subject to be imaged. The measurement unit uses the one-dimensional projection data to measure a subject thickness in the phase encoding direction. The calculation unit calculates, based on the subject thickness, an excitation thickness and an excitation position of a saturation pulse to be applied to an imaging region within the subject outside the phase encoding direction when the actual imaging is performed. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing an example of the configuration of an MRI apparatus according to the first embodiment. [Figure 2] FIG. 2 is a diagram showing an outline of the processing performed by the MRI apparatus according to the first embodiment. [Figure 3] FIG. 3 is a diagram illustrating an example of processing performed by the MRI apparatus according to the first embodiment. [Figure 4] FIG. 4 is a diagram illustrating an example of processing performed by the measurement function according to the first embodiment. [Figure 5] FIG. 5 is a diagram illustrating an example of processing performed by the measurement function according to the first embodiment. [Figure 6] FIG. 6 is a diagram illustrating an example of processing performed by the calculation function according to the first embodiment. [Figure 7] FIG. 7 is a flowchart showing the processing procedure of the processing performed by each processing function of the MRI apparatus according to the first embodiment. [Figure 8] FIG. 8 is a diagram showing an example of processing performed by the MRI apparatus according to the second embodiment. [Figure 9] FIG. 9 is a diagram showing an example of processing performed by the MRI apparatus according to the second embodiment. [Figure 10] FIG. 10 is a diagram showing an example of processing performed by the MRI apparatus according to the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of an MRI apparatus and an MRI method according to the present application will be described in detail with reference to the drawings.

[0010] (First embodiment) FIG. 1 is a diagram showing an example of the configuration of an MRI apparatus according to the first embodiment.

[0011] As shown in FIG. 1, the MRI apparatus 100 includes a static magnetic field magnet 1, a gradient magnetic field coil 2, a gradient magnetic field power supply 3, a whole-body radio frequency (RF) coil 4, a local RF coil 5, a transmission circuit 6, a reception circuit 7, an RF shield 8, a gantry 9, a bed 10, an input interface 11, a display 12, a memory circuit 13, and processing circuits 14 to 17.

[0012] The static magnetic field magnet 1 generates a static magnetic field in an imaging space in which a subject S is placed. Specifically, the static magnetic field magnet 1 is formed in a hollow, approximately cylindrical shape (including those in which the cross section perpendicular to the central axis is elliptical), and generates a static magnetic field in an imaging space formed on its inner periphery. For example, the static magnetic field magnet 1 is a superconducting magnet or a permanent magnet. The superconducting magnet referred to here is composed of, for example, a container filled with a coolant such as liquid helium, and a superconducting coil immersed in the container.

[0013] The gradient coil 2 is disposed inside the static magnetic field magnet 1 and generates a gradient magnetic field in an imaging space in which the subject S is placed. Specifically, the gradient coil 2 is formed in a hollow, approximately cylindrical shape (including a shape in which the cross section perpendicular to the central axis is elliptical) and has an X coil, a Y coil, and a Z coil corresponding to the X axis, Y axis, and Z axis, which are orthogonal to each other. The X coil, Y coil, and Z coil generate a gradient magnetic field in the imaging space that changes linearly along each axis direction based on a current supplied from the gradient magnetic field power supply 3. Here, the Z axis is set to be along the magnetic flux of the static magnetic field generated by the static magnetic field magnet 1. Furthermore, the X axis is set to be along the horizontal direction perpendicular to the Z axis, and the Y axis is set to be along the vertical direction perpendicular to the Z axis. Here, the X axis, Y axis, and Z axis form an apparatus coordinate system unique to the MRI apparatus 100.

[0014] The gradient magnetic field power supply 3 generates a gradient magnetic field in the imaging space by supplying current to the gradient magnetic field coil 2. Specifically, the gradient magnetic field power supply 3 generates a gradient magnetic field in the imaging space that changes linearly along the frequency encoding direction, phase encoding direction, and slice selective excitation direction, which are orthogonal to each other, by supplying current individually to the X coil, Y coil, and Z coil of the gradient magnetic field coil 2. Here, the axis along the frequency encoding direction, the axis along the phase encoding direction, and the axis along the slice selective excitation direction form a logical coordinate system for defining a slice region or volume region to be imaged.

[0015] Here, the gradient magnetic fields along the frequency encoding direction, phase encoding direction, and slice selective excitation direction are superimposed on the static magnetic field generated by the static magnetic field magnet 1 to impart spatial position information to the nuclear magnetic resonance (NMR) signal generated from the subject S. Specifically, the gradient magnetic field in the frequency encoding direction changes the frequency of the NMR signal depending on the position in the frequency encoding direction, thereby imparting position information in the frequency encoding direction to the NMR signal. The gradient magnetic field in the phase encoding direction changes the phase of the NMR signal depending on the position in the phase encoding direction, thereby imparting position information in the phase encoding direction to the NMR signal. When two-dimensional MR images (slice images) are captured, the gradient magnetic field in the slice selective excitation direction changes the frequency of the NMR signal depending on the position in the slice selective excitation direction, thereby determining the position, thickness, and number of slices to be captured. When three-dimensional MR images (volume images) are captured, the gradient magnetic field in the slice selective excitation direction changes the phase of the NMR signal depending on the position in the slice selective excitation direction, thereby imparting position information in the slice selective excitation direction to the NMR signal.

[0016] The whole-body RF coil 4 is disposed on the inner periphery side of the gradient magnetic field coil 2, applies RF pulses (excitation pulses, etc.) to the subject S disposed in the imaging space, and receives NMR signals (echo signals, etc.) generated from the subject S due to the influence of the RF pulses. Specifically, the whole-body RF coil 4 is formed in a hollow, approximately cylindrical shape (including a shape in which the cross section perpendicular to the central axis is elliptical), and applies RF pulses to the subject S disposed in the imaging space located on the inner periphery side thereof based on RF pulse signals supplied from the transmitting circuitry 6. The whole-body RF coil 4 then receives the NMR signals generated from the subject S due to the influence of the RF pulses, and outputs the received NMR signals to the receiving circuitry 7. For example, the whole-body RF coil 4 is a birdcage coil or a TEM (Transverse Electromagnetic) coil.

[0017] The local RF coil 5 is placed near the subject S during imaging and receives NMR signals generated from the subject S. Specifically, a local RF coil 5 is provided for each region of the subject S, is placed near the region to be imaged when imaging the subject S, receives NMR signals generated from the subject S due to the influence of RF pulses applied by the whole-body RF coil 4, and outputs the received NMR signals to the receiving circuit 7. For example, the local RF coil 5 is a surface coil or a phased array coil configured by combining multiple surface coils as coil elements. Note that the local RF coil 5 may further have a transmission function of applying RF pulses to the subject.

[0018] The transmission circuit 6 outputs an RF pulse signal corresponding to a resonance frequency (Larmor frequency) specific to the target nucleus placed in the static magnetic field to the whole-body RF coil 4 or the local RF coil 5. Specifically, the transmission circuit 6 has a pulse generator, an RF generator, a modulator, and an amplifier. The pulse generator generates a waveform of the RF pulse signal. The RF generator generates an RF signal at the resonance frequency. The modulator generates an RF pulse signal by modulating the amplitude of the RF signal generated by the RF generator with the waveform generated by the pulse generator. The amplifier amplifies the RF pulse signal generated by the modulator and outputs it to the whole-body RF coil 4 or the local RF coil 5.

[0019] The receiving circuit 7 generates NMR data based on the NMR signal output from the whole-body RF coil 4 or the local RF coil 5, and outputs the generated NMR data to the processing circuit 15. Specifically, the receiving circuit 7 includes a selector, a pre-amplifier, a phase detector, and an A / D (Analog / Digital) converter. The selector selectively inputs the NMR signal output from the whole-body RF coil 4 or the local RF coil 5. The pre-amplifier amplifies the NMR signal output from the selector. The phase detector detects the phase of the NMR signal output from the pre-amplifier. The A / D converter generates NMR data by converting the analog signal output from the phase detector into a digital signal, and outputs the generated NMR data to the processing circuit 15. Note that, of the processes described here as being performed by the receiving circuit 7, not all of them necessarily need to be performed by the receiving circuit 7; some of the processes (e.g., processing by an A / D converter) may be performed by the whole-body RF coil 4 or the local RF coil 5.

[0020] The RF shield 8 is disposed between the gradient magnetic field coil 2 and the whole-body RF coil 4, and shields the gradient magnetic field coil 2 from RF pulses generated by the whole-body RF coil 4. Specifically, the RF shield 8 is formed in a hollow, approximately cylindrical shape (including a cylindrical shape having an elliptical cross section perpendicular to the central axis), and is disposed in the space on the inner periphery of the gradient magnetic field coil 2 so as to cover the outer periphery of the whole-body RF coil 4.

[0021] The gantry 9 has a hollow bore 9a formed in a substantially cylindrical shape (including one in which the cross section perpendicular to the central axis has an elliptical shape), and houses the static magnetic field magnet 1, the gradient magnetic field coil 2, the whole-body RF coil 4, and the RF shield 8. Specifically, the gantry 9 houses each of the following in a state in which the whole-body RF coil 4 is arranged on the outer periphery of the bore 9a, the RF shield 8 is arranged on the outer periphery of the whole-body RF coil 4, the gradient magnetic field coil 2 is arranged on the outer periphery of the RF shield 8, and the static magnetic field magnet 1 is arranged on the outer periphery of the gradient magnetic field coil 2. Here, the space within the bore 9a of the gantry 9 becomes an imaging space in which the subject S is placed during imaging.

[0022] The bed 10 includes a top plate 10a on which the subject S is placed, and when imaging the subject S, the top plate 10a on which the subject S is placed is moved into the imaging space. For example, the bed 10 is installed so that the longitudinal direction of the top plate 10a is parallel to the central axis of the static magnetic field magnet 1.

[0023] Here, an example will be described in which the MRI apparatus 100 has a so-called tunnel-type structure in which the static magnetic field magnet 1, the gradient magnetic field coil 2, and the whole-body RF coil 4 are each formed in a substantially cylindrical shape, but the embodiment is not limited to this. For example, the MRI apparatus 100 may have a so-called open-type structure in which a pair of static magnetic field magnets, a pair of gradient magnetic field coils, and a pair of RF coils are arranged to face each other across an imaging space in which a subject S is placed. In such an open-type structure, the space sandwiched between the pair of static magnetic field magnets, the pair of gradient magnetic field coils, and the pair of RF coils corresponds to the bore in the tunnel-type structure.

[0024] The input interface 11 accepts input operations of various instructions and information from an operator. Specifically, the input interface 11 is connected to the processing circuitry 17, converts the input operations received from the operator into electrical signals, and outputs the electrical signals to the processing circuitry 17. For example, the input interface 11 may be realized by a trackball for setting imaging conditions and a region of interest (ROI), a switch button, a mouse, a keyboard, a touchpad for performing input operations by touching the operation surface, a touchscreen in which a display screen and a touchpad are integrated, a non-contact input circuit using an optical sensor, and a voice input circuit. Note that in this specification, the input interface 11 is not limited to those having physical operation components such as a mouse and a keyboard. For example, an electrical signal processing circuit that receives an electrical signal corresponding to an input operation from an external input device provided separately from the device and outputs the electrical signal to a control circuit is also included as an example of the input interface 11.

[0025] The display 12 displays various types of information. Specifically, the display 12 is connected to the processing circuit 17, and converts various types of information data sent from the processing circuit 17 into electrical signals for display and outputs the signals. For example, the display 12 is realized by a liquid crystal monitor, a CRT (Cathode Ray Tube) monitor, a touch panel, or the like.

[0026] The memory circuitry 13 stores various data. Specifically, the memory circuitry 13 is connected to the processing circuits 14 to 17 and stores various data input and output by each processing circuit. For example, the memory circuitry 13 is realized by a semiconductor memory element such as a RAM (Random Access Memory) or a flash memory, a hard disk, an optical disk, or the like.

[0027] The processing circuitry 14 has a bed control function 14a. The bed control function 14a controls the operation of the bed 10 by outputting control electrical signals to the bed 10. For example, the bed control function 14a receives an instruction from the operator via the input interface 11 to move the tabletop 10a in the longitudinal direction, vertical direction, or horizontal direction, and operates a moving mechanism of the tabletop 10a of the bed 10 so as to move the tabletop 10a in accordance with the received instruction.

[0028] The processing circuitry 15 has an acquisition function 15a. The acquisition function 15a executes various pulse sequences to acquire NMR data of the subject S. Specifically, the acquisition function 15a executes various pulse sequences by driving the gradient power supply 3, the transmission circuitry 6, and the reception circuitry 7 in accordance with sequence execution data output from the processing circuitry 17. Here, the sequence execution data is data representing a pulse sequence, and is information that specifies the timing and strength of the current that the gradient power supply 3 supplies to the gradient coil 2, the timing and strength of the RF pulse signal that the transmission circuitry 6 supplies to the whole-body RF coil 4, and the timing at which the reception circuitry 7 samples the NMR signal. The acquisition function 15a then receives the NMR data output from the reception circuitry 7 as a result of executing the pulse sequence and stores it in the memory circuitry 13. At this time, the NMR data stored in the memory circuitry 13 is assigned position information along each of the frequency encoding direction, phase encoding direction, and slice selection excitation direction by the aforementioned gradient magnetic fields, and is stored as k-space data representing two-dimensional or three-dimensional k-space.

[0029] The processing circuitry 16 has a generating function 16a. The generating function 16a generates an MR image from the NMR data collected by the collecting function 15a of the processing circuitry 15. Specifically, under the control of the processing circuitry 17, the generating function 16a reads the NMR data collected by the collecting function 15a of the processing circuitry 15 from the storage circuitry 13 and performs reconstruction processing such as Fourier transform on the read NMR data to generate a two-dimensional or three-dimensional MR image. The generating function 16a then stores the generated MR image in the storage circuitry 13.

[0030] The processing circuitry 17 has an imaging control function 17a, an acquisition function 17b, a measurement function 17c, and a calculation function 17d. The imaging control function 17a controls each component of the MRI apparatus 100 to perform overall control of the MRI apparatus 100. Specifically, the imaging control function 17a displays a GUI (Graphical User Interface) on the display 12 to receive input operations of various instructions and information from an operator, and controls each component of the MRI apparatus 100 in response to input operations received via the input interface 11. For example, the imaging control function 17a receives input of imaging conditions from the operator and sets a pulse sequence for acquiring NMR data of the subject S based on the input imaging conditions. The imaging control function 17a then generates sequence execution data representing the set pulse sequence and outputs the data to the processing circuitry 15, thereby causing the acquisition function 15a of the processing circuitry 15 to execute various pulse sequences. Furthermore, for example, the imaging control function 17a controls the generation function 16a of the processing circuitry 16 to reconstruct an MR image from the k-space data collected by the processing circuitry 15. Furthermore, for example, the imaging control function 17a reads out an MR image stored in the storage circuitry 13 in response to a request from an operator, and displays the read-out MR image on the display 12. The acquisition function 17b, the measurement function 17c, and the calculation function 17d will be described later.

[0031] Here, the processing circuits 14 to 17 are each realized by, for example, a processor. In this case, the processing functions possessed by each processing circuit are stored in the storage circuitry 13 in the form of, for example, a program executable by a computer. Then, each processing circuit realizes the processing function corresponding to each program by reading and executing each program from the storage circuitry 13. In other words, each processing circuit has the processing function shown in FIG. 1 when each program is read.

[0032] The above describes an example of the configuration of the MRI apparatus 100 according to this embodiment. With this configuration, the MRI apparatus 100 according to this embodiment has a function of imaging a subject using the OVS method, which is one of local excitation techniques for imaging a small imaging region within the subject in high detail. The OVS method is a method for local excitation by applying a saturation pulse outside the imaging region in the phase encoding direction to suppress signals outside the imaging region.

[0033] Generally, in the OVS method, the excitation thickness and excitation position of the saturation pulse change depending on the size of the subject and the position of the imaging region, so the operator of the MRI system must set the excitation thickness and excitation position of the saturation pulse while visually checking the structure of the subject, which makes the workflow complicated. On the other hand, although it is possible to reduce the complexity of the workflow by setting a large excitation thickness of the saturation pulse, increasing the excitation thickness of the saturation pulse deteriorates the excitation profile and results in insufficient signal suppression in areas close to the imaging region, so it is desirable to set the excitation thickness as thin as possible.

[0034] For this reason, the MRI apparatus 100 according to this embodiment is configured to enable easy and appropriate setting of the excitation thickness and excitation position of the saturation pulse applied outside the phase encoding direction of the imaging region within the subject.

[0035] The following describes in detail the configuration of such an MRI apparatus 100. Note that, hereinafter, a saturation pulse applied to the outside of the imaging region in the subject in the phase encoding direction when imaging is performed using the OVS method is referred to as an OVS pulse.

[0036] FIG. 2 is a diagram showing an outline of the processing performed by the MRI apparatus 100 according to the first embodiment.

[0037] As shown in FIG. 2, when the actual imaging is performed using the OVS method, the MRI apparatus 100 measures the thickness of the subject in the phase encoding direction using one-dimensional (1-dimensional: 1D) projection data in the phase encoding direction of the subject that is the subject of the actual imaging ((A) of FIG. 2).

[0038] Here, the 1D projection data in the phase encoding direction is data obtained by projecting the subject along one axis along the phase encoding direction, with the frequency encoding direction and the slice selective excitation direction as the projection directions, and includes a signal profile that indicates the distribution of the subject's signals in the phase encoding direction.

[0039] Then, the MRI apparatus 100 automatically calculates the excitation thickness and excitation position of the OVS pulse based on the measured subject thickness ((B) of FIG. 2).

[0040] Specifically, acquisition function 17b of processing circuitry 17 acquires 1D projection data in the phase encoding direction of the subject to be imaged. Furthermore, measurement function 17c of processing circuitry 17 measures the subject thickness in the phase encoding direction using the 1D projection data acquired by acquisition function 17b. Furthermore, calculation function 17d of processing circuitry 17 calculates the excitation thickness and excitation position of the OVS pulse based on the subject thickness measured by measurement function 17c. Here, acquisition function 17b is an example of an acquisition unit. Furthermore, measurement function 17c is an example of a measurement unit. Furthermore, calculation function 17d is an example of a calculation unit.

[0041] FIG. 3 is a diagram showing an example of processing performed by the MRI apparatus 100 according to the first embodiment.

[0042] For example, as shown in FIG. 3, the acquisition function 17b acquires 1D projection data in a pre-scan performed before the actual imaging by exciting a slice imaging range including a slice of the subject to be imaged in the actual imaging and collecting 1D projection data in the phase encoding direction ((A) of FIG. 3).

[0043] For example, the acquisition function 17b controls the collection function 15a of the processing circuitry 15 to perform slab excitation of a slice imaging range at the position of the center coordinate of the imaging region to acquire 1D projection data in the phase encoding direction, thereby acquiring 1D projection data in the phase encoding direction. Here, when multiple slices of the subject are imaged in the actual imaging, the slice imaging range is set to include the multiple slices. When only one slice is imaged in the actual imaging, the slice imaging range may include only one slice.

[0044] Thereafter, before the actual imaging is performed, the measurement function 17c measures the object thickness in the phase encoding direction using the 1D projection data acquired by the acquisition function 17b (Figure 3(B)), and the calculation function 17d calculates the excitation thickness and excitation position of the OVS pulse based on the object thickness measured by the measurement function 17c (Figure 3(C)).

[0045] For example, the measurement function 17c acquires a signal profile indicating the distribution of signals from the object in the phase encoding direction from the 1D projection data, and measures the object thickness outside the imaging region in the phase encoding direction based on the signal profile.

[0046] 4 and 5 are diagrams showing an example of processing performed by the measurement function 17c according to the first embodiment.

[0047] For example, as shown in Figure 4, measurement function 17c measures the subject thickness thick_sub1 outside one side (upper side in Figure 4) of the imaging area in the phase encoding direction and the subject thickness thick_sub2 outside the other side (lower side in Figure 4) of the imaging area based on the signal profile obtained from the 1D projection data.

[0048] At this time, for example, the measurement function 17c identifies the position in the phase encoding direction where the signal of the subject reaches a predetermined value based on the signal profile acquired from the 1D projection data, and measures the distance between that position and the edge position of the imaging region in the phase encoding direction, thereby measuring the thickness of the subject outside the imaging region in the phase encoding direction.

[0049] For example, the measurement function 17c removes background noise from the 1D projection data, acquires a signal profile from the 1D projection data, and identifies a position where the subject signal becomes zero in the phase encoding direction based on the signal profile.Then, the measurement function 17c measures the distance between the identified position where the subject signal becomes zero and the edge position of the imaging region in the phase encoding direction, thereby measuring the thickness of the subject outside the imaging region in the phase encoding direction.

[0050] 5, based on a signal profile acquired from 1D projection data, the measurement function 17c identifies a coordinate y1_zero where the subject signal is zero and a coordinate y1_edge of the edge of the imaging region on one side of the imaging region in the phase encoding direction from the center coordinate. Then, the measurement function 17c measures the distance abs(y1_zero-y1_edge) between the identified coordinates y1_zero and y1_edge to measure the subject thickness thick_sub1 on the outside of one side of the imaging region in the phase encoding direction. The measurement function 17c also performs a similar process on the other side of the imaging region in the phase encoding direction from the center coordinate to measure the subject thickness thick_sub2 on the outside of the other side of the imaging region in the phase encoding direction.

[0051] Here, the measurement function 17c is configured to remove background noise contained in the 1D projection data and then identify a position where the subject signal in the phase encoding direction becomes zero, but the embodiment is not limited to this. For example, the measurement function 17c may use a threshold set to a value large enough to remove background noise to identify a position where the subject signal in the phase encoding direction becomes a threshold. In this case, the measurement function 17c does not need to remove background noise contained in the 1D projection data.

[0052] Furthermore, for example, the calculation function 17d calculates the excitation thickness and excitation position of the OVS pulse based on the object thickness, the center position of the imaging region, and the size of the imaging region measured by the measurement function 17c.

[0053] FIG. 6 is a diagram showing an example of processing performed by the calculation function 17d according to the first embodiment.

[0054] For example, as shown in Figure 6, the calculation function 17d calculates the excitation thickness thick_sut1 of the first OVS pulse applied outside one side (upper side in Figure 6) of the imaging area in the phase encoding direction, and the excitation thickness thick_sut2 of the second OVS pulse applied outside the other side (lower side in Figure 6) of the imaging area.

[0055] For example, the calculation function 17d calculates the excitation thickness thick_sut1 of the first OVS pulse so that it is equal to the object thickness thick_sub1 on one side of the imaging region measured by the measurement function 17c, and calculates the excitation thickness thick_sut2 of the second OVS pulse so that it is equal to the object thickness thick_sub2 on the other side of the imaging region measured by the measurement function 17c.

[0056] Furthermore, for example, when the coordinate in the frequency encoding direction is represented by x or X, the coordinate in the phase encoding direction is represented by y or Y, the size of the imaging area in the phase encoding direction is FOVy, the center coordinate of the imaging area is (x, y), the excitation center coordinate of the first OVS pulse is (X1, Y1), and the excitation center coordinate of the second OVS pulse is (X2, Y2), the calculation function 17d calculates the coordinate Y1 in the phase encoding direction of the excitation center coordinate of the first OVS pulse and the coordinate Y2 in the phase encoding direction of the excitation center coordinate of the second OVS pulse by the following equations:

[0057] Y1=y+FOVy / 2+thick_sat1 / 2 Y2=y-FOVy / 2-thick_sat1 / 2

[0058] Furthermore, for example, the calculation function 17d receives the excitation width OVSx in the frequency encoding direction of the OVS pulse from the operator, and calculates the coordinate X1 in the frequency encoding direction of the first OVS pulse and the coordinate X2 in the frequency encoding direction of the second OVS pulse using the equations: X1=OVSx / 2, X2=OVSx / 2.

[0059] Then, after the calculation function 17d calculates the excitation thickness and excitation position of the OVS pulse, the imaging control function 17a of the processing circuit 17 applies the OVS pulse based on the calculated excitation thickness and excitation position, thereby performing actual imaging of the subject using the OVS method.

[0060] The processing functions of the processing circuitry 17 have been described above. As mentioned above, the processing circuitry 17 is realized by, for example, a processor. In this case, the processing functions of the processing circuitry 17 are stored in the storage circuitry 13 in the form of programs executable by a computer. The processing circuitry 17 then reads out and executes each program from the storage circuitry 13, thereby realizing the processing function corresponding to each program.

[0061] FIG. 7 is a flowchart showing the processing procedure of the processing performed by each processing function of the MRI apparatus 100 according to the first embodiment.

[0062] 7, in this embodiment, first, the acquisition function 17b acquires 1D projection data in the phase encoding direction of the subject to be imaged (step S101). The process of step S101 is realized, for example, by the processing circuitry 17 reading out from the storage circuitry 13 a predetermined program corresponding to the acquisition function 17b and executing it.

[0063] Thereafter, the measurement function 17c measures the object thickness in the phase encoding direction using the 1D projection data acquired by the acquisition function 17b (step S102). The process of step S102 is realized, for example, by the processing circuitry 17 reading out from the storage circuitry 13 and executing a predetermined program corresponding to the measurement function 17c.

[0064] Thereafter, the calculation function 17d calculates the excitation thickness and the excitation position of the OVS pulse based on the object thickness measured by the measurement function 17c (step S103). The process of step S103 is realized, for example, by the processing circuitry 17 reading out from the storage circuitry 13 a predetermined program corresponding to the calculation function 17d and executing it.

[0065] Thereafter, the imaging control function 17a applies an OVS pulse based on the excitation thickness and excitation position calculated by the calculation function 17d, thereby performing actual imaging of the subject using the OVS method (step S104). The processing of step S104 is realized, for example, by the processing circuitry 17 reading out from the storage circuitry 13 and executing a predetermined program corresponding to the imaging control function 17a.

[0066] As described above, in the first embodiment, the acquisition function 17b acquires 1D projection data in the phase encoding direction of the object to be imaged. Furthermore, the measurement function 17c measures the object thickness in the phase encoding direction using the 1D projection data acquired by the acquisition function 17b. Furthermore, the calculation function 17d calculates the excitation thickness and excitation position of the OVS pulse based on the object thickness measured by the measurement function 17c.

[0067] This configuration eliminates the need for the operator of the MRI apparatus to set the excitation thickness and excitation position of the OVS pulse, leading to improved workflow. Furthermore, it becomes possible to apply an OVS pulse with the minimum excitation thickness regardless of the size of the subject or the position of the imaging region, thereby enabling appropriate local excitation. Therefore, according to the first embodiment, the excitation thickness and excitation position of the OVS pulse can be easily and appropriately set.

[0068] The first embodiment has been described above, but the above-described first embodiment can also be implemented by appropriately modifying some of the processing functions of the processing circuitry 17 of the MRI apparatus 100. Therefore, several modifications of the first embodiment will be described below as other embodiments. Note that the following embodiments will be described mainly focusing on differences from the first embodiment, and detailed description of overlapping content will be omitted.

[0069] (Second embodiment) For example, in the first embodiment described above, the calculation function 17d calculates the excitation thickness and excitation position of the OVS pulse based on the subject thickness, the center position of the imaging area, and the size of the imaging area measured by the measurement function 17c. However, it is generally known that the RF profile of an OVS pulse is not rectangular but has side lobes, and signal folding occurs in the side lobe portions.

[0070] Therefore, for example, the calculation function 17d may calculate the excitation thickness and excitation position of the OVS pulse in consideration of the side lobes of the OVS pulse, thereby suppressing signal aliasing caused by the side lobes of the OVS pulse. Such an example will be described below as a second embodiment.

[0071] For example, the calculation function 17d calculates the side lobe width of the OVS pulse, and calculates the excitation thickness and excitation position of the OVS pulse so that the OVS pulse is applied at a distance from the imaging region at least the size of the side lobe width.

[0072] 8 and 9 are diagrams showing an example of processing performed by the MRI apparatus 100 according to the second embodiment.

[0073] For example, as shown in FIG. 8, the calculation function 17d calculates the excitation thickness thick_sut1 of the first OVS pulse applied to the outside of one side of the imaging region in the phase encoding direction and the excitation thickness thick_sut2 of the second OVS pulse applied to the outside of the other side of the imaging region in the same manner as in the first embodiment, and then calculates the RF profile of the OVS pulse for each OVS pulse and calculates the side lobe width of the RF profile.

[0074] Then, for example, as shown in FIG. 9, when the coordinate in the frequency encoding direction is represented by x or X, the coordinate in the phase encoding direction is represented by y or Y, the size of the imaging area in the phase encoding direction is FOVy, the center coordinate of the imaging area is (x, y), the excitation center coordinate of the first OVS pulse is (X1, Y1), and the excitation center coordinate of the second OVS pulse is (X2, Y2), the calculation function 17d calculates the coordinate Y1 in the phase encoding direction of the excitation center coordinate of the first OVS pulse and the coordinate Y2 in the phase encoding direction of the excitation center coordinate of the second OVS pulse by the following equations:

[0075] Y1=y+FOVy / 2+thick_sat1 / 2+side lobe width of the first OVS pulse Y2 = y - FOVy / 2 - thick_sat1 / 2 - side lobe width of the second OVS pulse

[0076] As a result, the first OVS pulse and the second OVS pulse are applied at a distance from the imaging region that is equal to the side lobe width. Note that the distance between the OVS pulse and the imaging region does not necessarily have to match the side lobe width of the OVS pulse, and may be, for example, the distance obtained by adding a predetermined width to the side lobe width of the OVS pulse so as to more reliably suppress signal aliasing.

[0077] In the second embodiment described above, the acquisition function 17b calculates the side lobe width of the OVS pulse, and calculates the excitation thickness and excitation position of the OVS pulse so that the OVS pulse is applied at a distance from the imaging region that is at least the size of the side lobe width.

[0078] Therefore, according to the second embodiment, it is possible to suppress signal aliasing caused by the side lobes of the OVS pulse, and it is also possible to prevent the need to retake imaging when signal aliasing occurs, thereby suppressing an extension of the time required for imaging.

[0079] (Third embodiment) Furthermore, for example, in the first embodiment described above, the acquisition function 17b acquires 1D projection data by exciting a slice imaging range including a slice of the subject to be imaged in the main imaging in a pre-scan performed before the main imaging, and collecting 1D projection data in the phase encoding direction, but the method of collecting 1D projection data is not limited to this.

[0080] For example, the acquisition function 17b may generate 1D projection data in the phase encoding direction from imaging data typically collected in a pre-scan, thereby acquiring the 1D projection data used to calculate the excitation thickness and excitation position of the OVS pulse. Such an example will be described below as a third embodiment.

[0081] For example, the acquisition function 17b acquires 1D projection data by generating 1D projection data in the phase encoding direction from imaging data for positioning the subject that is collected before the actual imaging is performed.

[0082] FIG. 10 is a diagram showing an example of processing performed by the MRI apparatus 100 according to the third embodiment.

[0083] For example, as shown in FIG. 10, the acquisition function 17b acquires from the memory circuitry 13 positioning images (Slice 1 to Slice n) of multiple slices of the subject collected in a pre-scan performed before the actual imaging ((A) of FIG. 10).

[0084] Here, a positioning image (also called a locator image) is an image of a subject used to determine the position of an imaging region in actual imaging, and is, for example, multi-slice data or volume data of the subject.

[0085] Then, the acquisition function 17b accumulates the signals of the acquired multiple positioning images in the slice selection excitation direction, and then projects them in the frequency encoding direction as the projection direction, thereby virtually generating 1D projection data in the phase encoding direction ((B) of Figure 10).

[0086] Thereafter, similarly to the first embodiment, the measurement function 17c measures the object thickness in the phase encoding direction using the 1D projection data acquired by the acquisition function 17b (FIG. 10(C)), and the calculation function 17d calculates the excitation thickness and excitation position of the OVS pulse based on the object thickness measured by the measurement function 17c (FIG. 10(D)).

[0087] Although the acquisition function 17b generates 1D projection data in the phase encoding direction from a positioning image of the subject here, other types of imaging data that are generally collected in a pre-scan may be used instead of the positioning image. For example, the acquisition function 17b may generate 1D projection data in the phase encoding direction from sensitivity map data for brightness correction or parallel imaging that is collected in a pre-scan.

[0088] As described above, in the third embodiment, the acquisition function 17b acquires 1D projection data used to calculate the excitation thickness and excitation position of the OVS pulse by generating 1D projection data in the phase encoding direction from imaging data such as positioning images and sensitivity maps that are generally collected in a pre-scan before the actual imaging is performed.

[0089] Therefore, according to the third embodiment, it is not necessary to add new data collection for calculating the excitation thickness and excitation position of the OVS pulse to the pre-scan, and it is possible to suppress an extension of the time required for imaging.

[0090] (Other embodiments) In the above-described embodiment, the processing circuits 14 to 17 are each implemented by a single processor, but the embodiment is not limited thereto. For example, each processing circuit may be configured by combining multiple independent processors, and each processor may execute a program to implement each processing function. Furthermore, the processing functions of each processing circuit may be implemented by being appropriately distributed or integrated among a single or multiple processing circuits. Furthermore, in the above description, a single storage circuit 13 stores a program corresponding to each processing function, but the embodiment is not limited thereto. For example, multiple storage circuits may be distributed among the processing circuits, and each processing circuit may read a corresponding program from a separate storage circuit.

[0091] In the above-described embodiment, the acquiring unit, measuring unit, and calculating unit in this specification are respectively realized by the acquiring function, measuring function, and display control function of a processing circuit, but the embodiment is not limited to this. For example, the acquiring unit, measuring unit, and calculating unit in this specification may be realized by hardware only, software only, or a combination of hardware and software, in addition to being realized by the acquiring function, measuring function, and calculating function described in the embodiment.

[0092] Although the above description describes an example in which a "processor" reads and executes a program corresponding to each processing function from a storage circuit, the embodiment is not limited to this. The term "processor" refers to a circuit such as a central processing unit (CPU), a graphics processing unit (GPU), an application-specific integrated circuit (ASIC), or a programmable logic device (e.g., a simple programmable logic device (SPLD), a complex programmable logic device (CPLD), or a field programmable gate array (FPGA)). If the processor is a CPU, for example, the processor realizes each processing function by reading and executing a program stored in a storage circuit. On the other hand, if the processor is an ASIC, instead of storing a program in a storage circuit, the processing function is directly incorporated into the processor circuit as a logic circuit. Note that each processor in this embodiment is not limited to being configured as a single circuit, but may be configured as a single processor by combining multiple independent circuits to realize its processing function. Furthermore, the multiple components in FIG. 1 may be integrated into a single processor to realize the processing functions.

[0093] Here, the program executed by the processor is provided in advance in a read-only memory (ROM) or a storage circuit. The program may be provided in a format installable or executable by these devices and recorded on a computer-readable storage medium such as a compact disk (CD)-ROM, a flexible disk (FD), a recordable CD-R, or a digital versatile disk (DVD). The program may also be provided or distributed by being stored on a computer connected to a network such as the Internet and downloaded via the network. For example, the program may be composed of modules including the above-mentioned functional units. In actual hardware, a CPU reads and executes the program from a storage medium such as a ROM, whereby each module is loaded into a main memory device and generated on the main memory device.

[0094] In the above-described embodiments, the components of each device shown in the drawings are conceptual functional units and do not necessarily have to be physically configured as shown. In other words, the specific form of distribution or integration of each device is not limited to that shown in the drawings, and all or part of the devices can be functionally or physically distributed or integrated in any unit depending on various loads, usage conditions, etc. Furthermore, all or any part of the processing functions performed by each device can be realized by a CPU and a program analyzed and executed by the CPU, or can be realized as hardware using wired logic.

[0095] Furthermore, among the processes described in the above embodiments, all or part of the processes described as being performed automatically can be performed manually, or all or part of the processes described as being performed manually can be performed automatically using a known method.In addition, the information including the processing procedures, control procedures, specific names, various data and parameters shown in the above documents and drawings can be changed as desired unless otherwise specified.

[0096] According to at least one of the embodiments described above, it is possible to easily and appropriately set the excitation thickness and excitation position of a saturation pulse applied to the outside of the imaging region in the phase encoding direction within the subject.

[0097] Although several embodiments have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations of embodiments can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as defined in the claims. [Explanation of symbols]

[0098] 100 Magnetic Resonance Imaging (MRI) equipment 17 Processing circuit 17b Acquisition function 17c Measurement Function 17d Calculation Function

Claims

1. an acquisition unit that acquires one-dimensional projection data in a phase encoding direction of a subject that is a target of actual imaging; a measurement unit that measures a subject thickness in the phase encoding direction using the one-dimensional projection data; a calculation unit that calculates, based on the object thickness, an excitation thickness and an excitation position of a saturation pulse that is applied to an outer side of the imaging region in the object in the phase encoding direction when the main imaging is performed; A magnetic resonance imaging apparatus comprising:

2. the acquisition unit acquires the one-dimensional projection data by exciting a slice imaging range including a slice of the subject to be imaged in the main imaging and collecting one-dimensional projection data in the phase encoding direction.

2. The magnetic resonance imaging apparatus according to claim 1.

3. the measurement unit acquires a signal profile indicating a distribution of signals from the subject in the phase encoding direction from the one-dimensional projection data, and measures a thickness of the subject outside the imaging region in the phase encoding direction based on the signal profile.

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

4. the measurement unit identifies a position where a signal from the subject reaches a predetermined value in the phase encoding direction based on the signal profile, and measures the distance between the identified position and a position of an edge of the imaging region in the phase encoding direction, thereby measuring the thickness of the subject.

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

5. the calculation unit calculates an excitation thickness and an excitation position of the saturation pulse based on the object thickness, the center position of the imaging region, and the size of the imaging region.

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

6. the calculation unit calculates a side lobe width of the saturation pulse, and calculates an excitation thickness and an excitation position of the saturation pulse so that the saturation pulse is applied at a distance from the imaging region that is at least the size of the side lobe width.

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

7. the acquisition unit acquires the one-dimensional projection data by generating one-dimensional projection data in the phase encoding direction from imaging data for positioning the subject that is collected before the main imaging is performed.

2. The magnetic resonance imaging apparatus according to claim 1.

8. acquiring one-dimensional projection data in a phase encoding direction of a subject to be imaged; measuring a thickness of the object in the phase encoding direction using the one-dimensional projection data; calculating, based on the object thickness, an excitation thickness and an excitation position of a saturation pulse to be applied to an outer side of the imaging region in the object in the phase encoding direction when the main imaging is performed; A magnetic resonance imaging method comprising:

Images