magnetic resonance imaging equipment

Abstract

To shorten an imaging time of non-contrast enhanced imaging using a magnetic resonance imaging apparatus.SOLUTION: A magnetic resonance imaging apparatus includes: a scanner that includes a static magnetic field magnet configured to generate a static magnetic field, a gradient magnetic field coil configured to generate a gradient magnetic field, and a WB coil configured to apply an RF pulse to an object; and processing circuitry. The processing circuitry is configured to: set a pulse sequence in which a sequence element is repeated, the sequence element including at least an inversion pulse and a data acquisition sequence executed after a delay time from the inversion pulse; and cause the scanner to execute the pulse sequence by using virtual gating processing.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

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

[0003] Among imaging methods using magnetic resonance imaging devices, non-contrast imaging methods that image blood flow or blood vessels without injecting a contrast agent are known. Another non-contrast imaging method is known as the Time-SLIP (time-spatial labeling inversion pulse) method, which controls blood flow visualization by applying an RF pulse (e.g., an inversion pulse) prior to data acquisition. The Time-SLIP method is an imaging method that can visualize blood flow in target blood vessels with high contrast while suppressing the background of the imaging area. The Time-SLIP method can image not only blood flowing through arteries and veins, but also fluids such as cerebrospinal fluid (CSF).

[0004] On the other hand, electrocardiogram (ECG) gating and respiratory gating are used to suppress body movement artifacts caused by body movements due to heartbeat and respiration. ECG gating is an imaging method that acquires data within a predetermined period of a desired cardiac phase in synchronization with an ECG signal, for example, an R wave. Respiratory gating is an imaging method that acquires data within a predetermined period of a predetermined phase, for example, during expiration, of the inspiration-expiration cycle in synchronization with a respiratory signal indicating respiratory movement detected by a respiratory sensor or the like.

[0005] In conventional magnetic resonance imaging systems, ECG gating and respiratory gating were used in conjunction with Time-SLIP imaging to suppress motion artifacts. However, when Time-SLIP imaging was used in conjunction with gating imaging such as ECG gating and respiratory gating, the imaging time tended to be longer. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] U.S. Patent No. 10,488,484 Summary of the Invention [Problem to be solved by the invention]

[0007] One of the problems to be solved by the embodiments disclosed in this specification and the drawings is to shorten the imaging time for non-contrast imaging using a magnetic resonance imaging apparatus. 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 each embodiment described below can also be positioned as other problems. [Means for solving the problem]

[0008] A magnetic resonance imaging apparatus according to one embodiment includes a scanner including a static magnetic field magnet that generates a static magnetic field, a gradient magnetic field coil that generates a gradient magnetic field, and a WB coil that irradiates an RF pulse onto a subject, and a processing circuit, wherein the processing circuit sets a pulse sequence in which sequence elements including at least an inversion pulse and a data acquisition sequence that is executed after a delay time from the inversion pulse are repeated, and causes the scanner to execute the pulse sequence using virtual gating processing. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a configuration diagram showing an example of the overall configuration of a magnetic resonance imaging apparatus according to an embodiment; [Figure 2] FIG. 2 is a block diagram including a configuration related to pulse sequence generation of the magnetic resonance imaging apparatus according to the embodiment. [Figure 3] The first figure shows the concept of the Time-SLIP method. [Figure 4] The second diagram shows the concept of the Time-SLIP method. [Figure 5] Sequence diagram showing an example of a conventional Time-SLIP pulse sequence using real gating (respiratory gating or ECG gating). [Figure 6] 4 is a flowchart showing an example of processing performed by the magnetic resonance imaging apparatus according to the present embodiment. [Figure 7] FIG. 10 is a diagram showing an example of Cartesian sampling. [Figure 8] FIG. 10 is a diagram showing an example of two-dimensional radial sampling. [Figure 9] FIG. 10 is a diagram showing an example of three-dimensional radial sampling. [Figure 10] 10 is a flowchart showing an example of a process for executing a Time-SLIP pulse sequence using virtual gating. [Figure 11] Sequence diagram of the Time-SLIP pulse sequence using virtual gating. [Figure 12] FIG. 1 is a diagram illustrating a concept for determining a waiting time. [Figure 13] FIG. 10 is a diagram showing that the recovery time of the vertical time varies depending on the magnitude of the flip angle. [Figure 14] FIG. 10 is a first diagram comparing the imaging time of virtual gating according to this embodiment with the imaging time of conventional respiratory gating. [Figure 15] FIG. 2 is a second diagram comparing the imaging time of virtual gating according to this embodiment with the imaging time of conventional respiratory gating. DETAILED DESCRIPTION OF THE INVENTION

[0010] A magnetic resonance imaging apparatus 1 according to an embodiment of the present invention will now be described with reference to the accompanying drawings.

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

[0012] The magnetic gantry 100 has a static magnetic field magnet 10, a gradient magnetic field coil 11, a WB (Whole Body) coil 12, etc., and these components are housed in a cylindrical housing. The bed 500 has a bed body 50 and a tabletop 51. The magnetic resonance imaging apparatus 1 also has an array coil 20 disposed close to the subject. The subject is also fitted with, for example, a respiratory sensor 700 and an electrocardiograph 710 for respiratory gating processing and ECG (electrocardiogram) gating processing, which will be described later.

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

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

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

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

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

[0018] The array coil 20 is an RF coil that receives magnetic resonance signals emitted from a subject at a position close to the subject. The array coil 20 is composed of, for example, a plurality of element coils. There are various types of array coils 20 depending on the imaging region of the subject, such as for the head, chest, spine, lower limbs, or whole body, and Fig. 1 shows an array coil 20 for the chest.

[0019] The RF transmitter 33 transmits RF pulses to the WB coil 12 based on instructions from the sequence controller 34. On the other hand, the RF receiver 32 detects magnetic resonance signals received by the WB coil 12 and the array coil 20, and sends raw data obtained by digitizing the detected magnetic resonance signals to the sequence controller 34.

[0020] The sequence controller 34 scans the subject by driving the gradient magnetic field power supply 31, the RF transmitter 33, and the RF receiver 32 under the control of the console 400. After performing the scan and receiving raw data from the RF receiver 32, the sequence controller 34 sends the raw data to the console 400.

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

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

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

[0024] The processing circuit 40 is a circuit including, for example, a CPU or a dedicated or general-purpose processor. The processor executes various programs stored in the storage circuit 41 to realize various functions described below. The processing circuit 40 may be configured with hardware such as an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). The various functions described below can also be realized by such hardware. The processing circuit 40 can also realize various functions by combining software processing by a processor and a program with hardware processing.

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

[0026] Furthermore, the magnetic resonance imaging apparatus 1 of the embodiment generates an image of the subject by reconstructing MR signals obtained by executing a scan using a pulse sequence described below. In particular, in this embodiment, a scan is executed using a pulse sequence that can shorten the imaging time compared to conventional methods. FIG. 2 is a block diagram of the magnetic resonance imaging apparatus 1 according to the embodiment, and is a functional block diagram that particularly focuses on the functions realized by the processing circuitry 40. As shown in FIG.

[0027] 2, among the components of the magnetic resonance imaging apparatus 1 shown in FIG. 1, components other than the console 40, i.e., the control cabinet 300, the magnet gantry 100, and the bed 500, are collectively referred to as a scanner 600. Also shown in FIG. 2 are a respiratory sensor 700 and an electrocardiograph 710 connected to the magnetic resonance imaging apparatus 1.

[0028] As shown in FIG. 2, the processing circuitry 40 of the magnetic resonance imaging apparatus 1 realizes the following functions: a user interface control function F01, an imaging sequence setting function F02, a gating selection function F03, a virtual gating period calculation function F04, and a reconstruction function F05.

[0029] The user interface control function F01 executes control for users such as doctors and imaging technicians to input and select various data related to imaging via the user interface 401. The user interface 401 is configured to include, for example, the input device 43 and display 42 described above.

[0030] The user inputs or selects data related to the imaging conditions for the imaging to be performed via the user interface 401. The imaging conditions here include, for example, the type of imaging method, the type of imaging target, FOV (field of view), resolution, etc. In addition, the user may input or select specific values ​​of various parameters of the pulse sequence that realize these imaging conditions.

[0031] The types of imaging methods include, for example, types of k-space sampling methods such as two-dimensional Cartesian sampling, three-dimensional Cartesian sampling, two-dimensional radial sampling, and three-dimensional radial sampling.

[0032] Furthermore, the type of imaging method may include the type of pulse sequence, such as the fast low angel shot (FLASH) method, the spoiled gradient-recalled acquisition in the steady state (SPGR) method, the balanced-steady-state free precession (SSFP) method, the fast spin echo (FSE) method, the fast advanced spin echo (FASE) method, the echo planar imaging (EPI) method, the time of flight (TOF) method, the phase contrast (PC) method, the fresh blood imaging (FBI) method, the time-spatial labeling inversion pulse (Time-SLIP) method, the diffusion weighted imaging (DWI) method, and the arterial spin labeling (ASL) method.

[0033] In addition, the types of imaging targets that can be set via the user interface 401 include, for example, types of regions of the body of the object, such as the head, chest, abdomen, and lower limbs; types of organs, such as the brain, heart, lungs, liver, and kidneys; and types of tissues, such as blood, cerebrospinal fluid, and bones.

[0034] Meanwhile, the imaging protocol storage area 411 of the storage circuitry 41 stores standard imaging protocols or recommended imaging protocols corresponding to imaging conditions such as the type of imaging method and the type of imaging target. Here, the imaging protocol may include, for example, the type of pulse sequence used for imaging, the values ​​of various parameters in the pulse sequence, and a combination of multiple pulse sequences when they are executed consecutively. Here, the values ​​of various parameters in the pulse sequence include, for example, the values ​​or temporal changes of parameters related to transmit RF pulses, such as the flip angle α of the excitation pulse and the repetition period TR of the excitation pulse, and the values ​​or temporal changes of parameters related to gradient magnetic fields, such as the magnitude and application time of each gradient magnetic field in the X, Y, and X directions.

[0035] The user interface control function F01 reads out, from the imaging protocol storage area 411 of the storage circuitry 41, a standard imaging protocol that corresponds to the imaging conditions input or selected by the user.

[0036] The user interface control function F01 may display the read imaging protocol on the display 42 of the user interface 401. The user can accept and confirm the displayed imaging protocol as is. In addition, the user may change the displayed imaging protocol as necessary and confirm the changed imaging protocol.

[0037] The user interface control function F01 sends the confirmed imaging protocol to the imaging sequence setting function F02. The imaging sequence setting function F02 sets data on the pulse sequence corresponding to the received imaging protocol as an imaging sequence in the sequence controller 34 of the scanner 600. The sequence controller 34 executes the pulse sequence in accordance with data relating to the set pulse sequence, and acquires magnetic resonance signals (MR signals).

[0038] The acquired MR signals are sent to a reconstruction function F05 of the processing circuitry 40. The reconstruction function F05 performs reconstruction processing such as Fourier transform processing on the MR signals to generate a magnetic resonance image. The generated magnetic resonance image is displayed on a display 42, for example.

[0039] The gating selection function F03 and the virtual gating period calculation function F04 are functions related to the virtual gating process that is characteristic of the magnetic resonance imaging apparatus 1 of this embodiment. The virtual gating process will be described in detail below.

[0040] (Virtual gating processing) The pulse sequence targeted by the virtual gating process according to this embodiment (hereinafter simply referred to as virtual gating) is a pulse sequence in which sequence elements including at least an inversion pulse and a data acquisition sequence executed a predetermined delay time after this inversion pulse are repeated.

[0041] A typical example of this type of pulse sequence is a pulse sequence called the Time-SLIP method. The magnetic resonance imaging apparatus 1 of this embodiment executes a pulse sequence according to the Time-SLIP method as an example of the pulse sequence, using virtual gating.

[0042] Before explaining virtual gating, the Time-SLIP method will be briefly explained using Figures 3 and 4. In addition, the execution of the Time-SLIP method using real gating (respiratory gating or ECG gating), which has been conventionally performed, will be briefly explained using Figure 5.

[0043] FIG. 3 is a diagram illustrating an imaging method called "Flow-Out" among the Time-SLIP methods. As will be described later, the Flow-Out type Time-SLIP method uses two inversion pulses, a region-non-selective pulse and a region-selective pulse, and the main imaging target is the fluid that flows out from the region to which the region-selective pulse is applied (i.e., the selected region). On the other hand, FIG. 4 is a diagram illustrating an imaging method called "Flow-In" among the Time-SLIP methods. The Flow-In type Time-SLIP method uses only one inversion pulse, a region-selective pulse, and the main imaging target is the fluid that flows in from outside into the region to which the region-selective pulse is applied (i.e., the selected region including the imaging region).

[0044] Figure 3(a) shows a sequence diagram of the Time-SLIP (Flow-Out) method. In the Time-SLIP (Flow-Out) method, two inversion pulses are applied, and a data acquisition sequence is applied after a predetermined delay time from the two inversion pulses.

[0045] Of the two inversion pulses, for example, non-selective inversion pulse A is applied first, and immediately thereafter, selective inversion pulse B is applied. The two pulses, non-selective pulse A and selective pulse B, are collectively called a time-slip pulse. Note that the order in which the non-selective pulse A and selective pulse B are applied may be reversed.

[0046] The delay time from the Time-SLIP pulse to the leading edge of the data acquisition sequence is called BBTI ("black blood inversion time" or "black blood traveling time").

[0047] In a data acquisition sequence, MR signals are acquired by multiple excitation pulses. Various types of data acquisition sequences can be used, as described below. If MR signals necessary for image formation cannot be acquired from the entire k-space using a single data acquisition sequence, the entire k-space is divided into multiple regions (i.e., multiple segments), and the sequence diagram shown in Figure 3(a) is repeated the same number of times as the number of segments to acquire MR signals necessary to fill the entire k-space.

[0048] The unit of repetition, the period from the leading edge of a Time-SLIP pulse to the leading edge of the next Time-SLIP pulse, is called a sequence element. In this case, the Time-SLIP pulse sequence is made up of multiple sequence elements repeated over time for the number of segments.

[0049] Fig. 3(c) is a diagram schematically showing the application region of the region non-selective pulse A, the application region of the region selective pulse B, and the imaging region. Fig. 3(b) shows the behavior of longitudinal magnetization that changes due to the application of the region non-selective pulse A and the region selective pulse B, corresponding to the time axis of the sequence diagram in Fig. 3(a).

[0050] The region-nonselective pulse A is applied to the imaging region R indicated by the thick solid square frame in Fig. 3(c). IM and the imaging area R IMThe region non-selective pulse A is applied to the entire region including the region outside the region (the region enclosed by the rectangular frame indicated by the thin dashed line in Fig. 3(c)). By applying the region non-selective pulse A, the longitudinal magnetization of the background within the imaging region, the blood vessels within the imaging region, and the blood in the artery α and vein β outside the imaging region are reversed from positive to negative, as shown in Fig. 3(b). These longitudinal magnetizations recover over time, as shown by the dashed lines in Fig. 3(b), cross the null point from a negative value, and then become positive.

[0051] On the other hand, region-selective pulse B is applied to a desired selected region outside the imaging region. Specifically, region-selective pulse B is applied to a selected region upstream of the imaging region, including a specific blood vessel flowing into the imaging region from outside the imaging region. In the example shown in Figure 3(c), region-selective pulse B is applied to a region including artery α upstream of the imaging region. This application causes only the blood in artery α to return from negative to positive, as shown by the thick solid line in Figure 3(b), and then flows into the imaging region while maintaining a large positive value.

[0052] By collecting data near the time when the longitudinal magnetization of the background in the imaging region crosses the zero point (the time BBTI has elapsed since the Time-SLIP pulse), the blood in the artery α flowing in from outside the imaging region can be visualized with high contrast against the background of the imaging region. Meanwhile, blood in the vein β also flows in the imaging region, but because the region-selective pulse B is not applied to the vein β, the longitudinal magnetization of the blood in the vein β flowing in the imaging region is maintained at a small value. Therefore, the Time-SLIP (Flow-Out) method makes it possible to visualize only the blood in a specific blood vessel among multiple blood vessels in the imaging region.

[0053] Figure 4 is a diagram explaining an imaging method called "Flow-In" in the Time-SLIP method. In the Time-SLIP method (Flow-In), one inversion pulse, i.e., one region-selective pulse B, is applied as a Time-SLIP pulse. This region-selective pulse B is applied to the imaging region R indicated by the thick solid square frame in Figure 4(c). IM and the imaging area R IMThe region-selective pulse B is applied to a selected region including a part outside the region (the region enclosed by the rectangular frame indicated by the thin dashed line in Fig. 4(c)). As shown in Fig. 4(b), application of the region-selective pulse B reverses the longitudinal magnetization of the background within the imaging region, the blood in the blood vessels within the imaging region, and the blood in the vein γ within the selected region outside the imaging region from positive to negative. These longitudinal magnetizations recover over time, cross the null point from a negative value, and then become positive.

[0054] On the other hand, the region-selective pulse B is not applied to the artery α located outside the selected region and imaging region. Therefore, the blood in the artery α flows into the imaging region while maintaining a large value of longitudinal magnetization. Therefore, by collecting data near the time when the longitudinal magnetization of the background in the imaging region crosses the zero point (the time BBTI has elapsed since the Time-SLIP pulse), the blood in the artery α flowing in from outside the selected region can be visualized with high contrast relative to the background of the imaging region and the blood flowing into the imaging region from the vein γ in the selected region.

[0055] FIG. 5 is a diagram illustrating a conventional Time-SLIP method using actual gating (respiratory gating or ECG gating). In respiratory gating, the sequence elements shown in FIG. 3(a) or FIG. 4(a) are repeatedly executed in synchronization with a respiratory gating signal detected by a respiratory sensor 700. In ECG gating, the sequence elements shown in FIG. 3(a) or FIG. 4(a) are repeatedly executed in synchronization with an ECG gating signal, for example, an R-wave signal detected by an electrocardiograph 710. Although FIG. 5 illustrates the Time-SLIP method using respiratory gating, the essential parts of the explanation are the same for ECG gating.

[0056] Fig. 5(a) shows a respiratory cycle in which exhalation and inhalation are repeated, and Fig. 5(b) shows an example of a respiratory gating signal detected by the respiratory sensor 700. For example, the respiratory cycle period of a normal adult is said to be 12 to 18 times per minute (i.e., 3 to 5 seconds). Furthermore, since the respiratory cycle changes from moment to moment depending on the subject's surrounding environment and the subject's psychological state, the respiratory cycle period does not necessarily show a constant value.

[0057] FIG. 5(c) is a diagram showing a sequence diagram of a pulse sequence in the Time-SLIP method using respiratory gating under the setting of a first BBTI value (=BBTI(1)). FIG. 5(d) is a diagram showing a sequence diagram of a pulse sequence in the Time-SLIP method using respiratory gating under the setting of a second BBTI value (=BBTI(2)). Data collection is usually performed around the middle of the exhalation in the respiratory cycle. Therefore, when the BBTI is longer than the respiratory cycle, data collection may occur every two respiratory cycles, as shown in FIG. 5(d).

[0058] In respiratory gating, a Time-SLIP pulse starts in synchronization with the respiratory gating signal. When data is acquired by dividing k-space into multiple segments, sequence elements defined by the period from one Time-SLIP pulse to the next are repeated in synchronization with the respiratory gating signal. This data acquisition method enables k-space data to be acquired at the same time phase of the respiratory cycle in the data acquisition sequence of all sequence elements.

[0059] On the other hand, as can be seen from Figures 5(c) and 5(d), in the Time-SLIP method using respiratory gating, a dead time D occurs between the trailing edge of the data acquisition sequence and the next Time-SLIP pulse. The dead time D can also be called waiting time. The dead time D varies depending on the magnitude of the BBTI. The shorter the BBTI, the longer the dead time D. As a result, the overall imaging time becomes longer. The main aim of this embodiment is to reduce this dead time D and shorten the overall imaging time.

[0060] The main purpose of the above-mentioned actual gating such as respiratory gating and ECG gating is to suppress artifacts caused by body movements such as respiratory movements and pulsation. As shown in Fig. 5(c) and Fig. 5(d) above, when k-space data is acquired by dividing the data into a plurality of segments, the magnitude of the MR signal (i.e., k-space data) varies between segments due to body movements, and this variation causes body movement artifacts.

[0061] Respiratory gating suppresses fluctuations in k-space data between segments due to body movement by collecting data at the same respiratory phase between segments, and ECG gating suppresses fluctuations in k-space data between segments due to body movement by collecting data at the same cardiac phase between segments.

[0062] It is known that motion artifacts depend on the k-space sampling method. Specifically, as will be described later, it is known that motion artifacts are likely to occur when data is acquired using Cartesian sampling, and motion artifacts are likely to occur when data is acquired using radial sampling.

[0063] Furthermore, the likelihood of motion artifacts occurring varies depending on the imaging target region. When the imaging target region is the chest or abdomen, motion artifacts are likely to occur due to large motions caused by breathing and heartbeat. In contrast, when the imaging target region is a region away from the chest or abdomen, such as the head, motion artifacts are less likely to occur because they are less affected by motions caused by breathing and heartbeat.

[0064] From the above viewpoint, the magnetic resonance imaging apparatus 1 of this embodiment is configured to select between execution of a pulse sequence using real gating such as respiratory gating or ECG gating, and execution of a pulse sequence using virtual gating instead of real gating without using respiratory gating or ECG gating, depending on the k-space data sampling method and the region to be imaged.

[0065] Fig. 6 is a flowchart showing an example of processing by the magnetic resonance imaging apparatus 1 according to this embodiment. In step ST100 of Fig. 6, imaging conditions or an imaging sequence are set by a user interface control function F01 and an imaging sequence setting function F02. Note that the following description mainly relates to the Time-SLIP method. Therefore, the imaging conditions or imaging sequence set in step ST100 are imaging conditions or an imaging sequence related to the Time-SLIP method.

[0066] In step ST101, the type of sampling of the data acquisition sequence executed in the pulse sequence of the Time-SLIP method is determined based on the imaging conditions set in step ST100. Specifically, if the data acquisition sequence is executed by radial sampling, the process proceeds to step ST102, where a pulse sequence using virtual gating is executed. On the other hand, if the data acquisition sequence is performed by Cartesian sampling rather than radial sampling, the process proceeds to step ST103. 7 to 9 are diagrams illustrating some examples of Cartesian sampling and radial sampling.

[0067] As examples of Cartesian sampling, Fig. 7(a) illustrates two-dimensional (2D) Cartesian sampling, and Fig. 7(b) illustrates three-dimensional (3D) Cartesian sampling. In 2D Cartesian sampling, the readout direction and the phase encoding direction are set to be orthogonal to each other, for example, the kx direction and the ky direction. In 3D Cartesian sampling, the slice encoding direction is set to be orthogonal to both the readout direction (kx direction) and the phase encoding direction (ky direction), i.e., the kz direction.

[0068] FIG. 8 shows two examples of two-dimensional radial sampling. 2D radial sampling (1) shown in FIG. 8(a) is an example of so-called narrow-sense radial sampling. FIG. 8(b) is a sequence diagram of this narrow-sense radial sampling. In narrow-sense radial sampling, MR signals are sampled for each excitation pulse along a line extending from one end of k-space (the position indicated by the black circle in FIG. 8(a)) through the center of k-space to the other end of k-space (the position indicated by the tip of the arrow in FIG. 8(a)). Then, by changing the gradient magnetic field Gx in the X direction and the gradient magnetic field Gy in the Y direction, the rotation angle of the line is sequentially changed from 0 degrees to 180 degrees, and data is collected for the entire k-space.

[0069] On the other hand, 2D radial sampling (2) shown in Figure 8(c) is an example of radial sampling corresponding to the two-dimensional UTE (ultra-short TE) method. Figure 8(d) is a sequence diagram corresponding to the radial sampling of this 2DUTE method. The UTE method is an imaging method that realizes an extremely short echo time TE in order to image tissues or imaging areas with very short transverse relaxation times.

[0070] In the radial sampling of the 2DUTE method, MR signals are sampled along a line extending from the center of k-space to the edge of k-space (the position indicated by the tip of the arrow in Figure 8(c)) for each excitation pulse. Then, by changing the gradient magnetic field Gx in the X direction and the gradient magnetic field Gy in the Y direction, the rotation angle of the line is changed sequentially from 0 to 360 degrees, and data is collected over the entire k-space.

[0071] 9A and 9B are diagrams showing two examples of three-dimensional radial sampling. 3D radial sampling (1) shown in FIG. 9A is a so-called Koosh ball type radial sampling. Koosh ball type radial sampling is a sampling method used in the three-dimensional UTE method. In this Koosh ball type radial sampling, MR signals are sampled along a number of lines directed from the center of k-space to points distributed almost uniformly over the entire surface of a virtual sphere outside the k-space, thereby acquiring k-space data for the entire three-dimensional k-space.

[0072] The sequence diagram of the 3D UTE method using the Cush Ball type radial sampling is the 2D UTE sequence diagram shown in Figure 8(d) with the addition of a Z-direction gradient magnetic field Gz that changes in the same way as Gx and Gy.

[0073] On the other hand, the 3D radial sampling (2) shown in Fig. 9(b) is a Stack of Stars (SoS) type radial sampling. The SoS type radial sampling acquires three-dimensional k-space data by acquiring multiple two-dimensional radial sampling planes (e.g., kx-ky planes) in a direction perpendicular to the planes (e.g., kz direction).

[0074] 6, if the set imaging conditions or the type of the set data acquisition sequence corresponds to the above-mentioned various radial samplings, the process proceeds to step ST102. The specific processing of step ST102 will be described later.

[0075] On the other hand, if the set imaging conditions or the set type of data acquisition sequence is performed by Cartesian sampling rather than the above-mentioned various radial samplings, the process proceeds to step ST103.

[0076] In step ST103, it is determined based on the set imaging conditions whether the imaging target region is a region with little body movement, such as the head or lower limbs, or a region with great body movement, such as the chest or abdomen.

[0077] When the imaging target region is the head or lower limbs, body movement due to breathing or heartbeat is small, so body movement artifacts are unlikely to occur even when the data acquisition sequence is performed using Cartesian sampling. Therefore, in this case, the process proceeds to step ST102, where a pulse sequence using virtual gating, specifically a pulse sequence based on the Time-SLIP method, is executed. In other words, when the imaging target region is the head or lower limbs, virtual gating can be applied to all of the sampling methods shown in Figures 7 to 9, including Cartesian sampling.

[0078] On the other hand, when the data acquisition sequence is performed using Cartesian sampling and the imaging target area is a region where body movement due to breathing or heartbeat is large, such as the abdomen or chest, a pulse sequence based on the Time-SLIP method using actual gating such as respiratory gating or ECG gating, for example the pulse sequence shown in Figure 5, is executed to suppress body movement artifacts. The processing of steps ST101 and ST103 in FIG. 6 is performed by the gating selection function F03 in FIG.

[0079] Next, specific processing relating to the execution of a Time-SLIP pulse sequence using virtual gating will be described with reference to the flowchart of FIG. 10 and the sequence diagram of FIG.

[0080] Figure 11(a) is an overall sequence diagram of a Time-SLIP pulse sequence using virtual gating, and Figure 11(c) is an expanded sequence diagram of one sequence element of the Time-SLIP pulse sequence.

[0081] As described above, a sequence element is defined by the behavior of RF pulses and gradient magnetic fields during the period from the leading edge of a Time-SLIP pulse to the leading edge of the next Time-SLIP pulse. The sequence element includes at least a Time-SLIP pulse and a data acquisition sequence executed after a delay time BBTI from the Time-SLIP pulse.

[0082] The Time-SLIP pulse may include both a non-region-selective pulse A and a region-selective pulse B (Flow-Out method), or it may include only a region-selective pulse B (Flow-In method). Figure 11 shows the Time-SLIP pulse sequence for the Flow-Out method.

[0083] In the execution of a pulse sequence using virtual gating, the repetition period of the virtual gating, T IR-IR It is important to calculate the virtual gating repetition period T IR-IR is the repetition period T of the inversion pulse (i.e., the Time-SLIP pulse). IR-IR and the repetition period of the sequence element is T IR-IR It is also.

[0084] Virtual gating repetition period T IR-IR As can be seen from Figure 11, the duration of the Time-SLIP pulse D IR , BBTI, the period of the data collection sequence T ack , and a waiting time WT. The waiting time WT can be considered as the time required for the longitudinal magnetization inverted by the excitation pulse train of the data acquisition sequence to fully recover, or the time required for the transverse magnetization generated by the excitation pulse train of the data acquisition sequence to fully disappear.

[0085] Steps ST200 to ST205 in FIG. 10 are performed in a virtual gating repetition period T IR-IR The processing from step ST200 to step ST205 is performed by the virtual gating period calculation function F04 shown in FIG.

[0086] In step ST200, the BBTI set by the user is acquired via the user interface 401. For example, in the case of the Flow-Out method, the value of BBTI is set by the user in consideration of the distance between the imaging region and a selected region (a region to which the region selection pulse B is applied) set upstream of the imaging region, the velocity of blood flowing from the selected region into the imaging region, the waiting time for longitudinal magnetization of the background of the imaging region, etc.

[0087] In step ST201, sequence parameters are acquired. Specifically, in step ST201, values ​​of various parameters in the Time-SLIP pulse sequence set by the user interface control function F01 and the imaging sequence setting function F02 are acquired.

[0088] More specifically, (a) the duration of the inversion pulse, i.e., the Time-SLIP pulse, D IR , (b) Total number of lines of k-space data N L , (c) Number of segments N when k-space data is acquired by dividing k-space into multiple segments S The values ​​of various parameters such as (d) the repetition period TR of the excitation pulse used in the data acquisition sequence, and (e) the flip angle α of the excitation pulse are acquired as sequence parameters.

[0089] In step ST202, tissue parameters of the imaging target are acquired. The tissue parameters are the transverse relaxation time T2 and the longitudinal relaxation time T1 of the imaging target. The values ​​of these tissue parameters can be estimated to some extent from, for example, the imaging target region and imaging conditions. For example, if the imaging target region is the lung field, the transverse relaxation time T2 can be estimated to be very short (for example, 1 ms or less). Furthermore, if the imaging target region is the head and the imaging target tissue is CSF, the transverse relaxation time T2 can be estimated to be very long (for example, about 1000 ms).

[0090] The flip angle of the excitation pulse obtained in step ST201, and the transverse relaxation time T2 and longitudinal relaxation time T1 obtained (or estimated) in step ST202 are used to determine the waiting time WT.

[0091] Next, in step ST203, the period T ack Calculate the period T of the data collection sequence. ack As can be seen from the sequence diagram in FIG. 11(c), the number N of excitation pulses per segment (i.e., the number N of acquired lines of k-space data per segment) is the product of the repetition period TR of the excitation pulse. Also, the number N of acquired lines of k-space data per segment is the total number N of lines of k-space data. L the number of segments N S Therefore, the period of the data collection sequence, T ack is the total number of lines N of the k-space data acquired in step ST201. L and the number of segments N S Using T ack =N*TR=(N L / N S )*TR (Formula 1) It can be calculated as follows.

[0092] Next, in step ST204, a waiting time WT is determined. The waiting time WT is the time from the trailing edge of the data acquisition sequence to the leading edge of the next Time-SLIP pulse. As described above, the waiting time WT can be considered as the time required for the longitudinal magnetization inverted by the excitation pulse train of the data acquisition sequence to fully recover, or the time required for the transverse magnetization generated by the excitation pulse train of the data acquisition sequence to fully disappear. If the waiting time WT can be determined to be short, the overall length of the Time-SLIP pulse sequence can be shortened, and the imaging time based on the Time-SLIP method can be shortened.

[0093] 12 is a diagram showing the concept for determining the waiting time WT. In step S1, it is determined whether the transverse relaxation time T2 of the tissue to be imaged is very short. If the transverse relaxation time T2 is very short, for example, T2<1 ms, the waiting time WT is determined to be a predetermined minimum value WTmin, for example, 5 ms.

[0094] In step S2, the Time-SLIP method is used to determine whether the imaging method is capable of depicting blood flowing in from outside the imaging region while sufficiently suppressing the background. In this imaging method, the contrast between blood and the background is considered to be sufficiently high. In this case, the waiting time WT is also determined to be a predetermined minimum value WTmin, for example, 5 ms.

[0095] In step S3, it is determined whether the imaging method is one in which the transverse relaxation time T2 of the tissue to be imaged is standard but the flip angle is very small. As shown in FIG. 13, when the flip angle is small, for example, when the flip angle is 5°, the longitudinal recovery time is shorter than when the flip angle is 20°. In this case, the waiting time WT can also be set to a short value. For example, even when the transverse relaxation time is standard, such as T2 = 40 to 80 ms, if the flip angle α is smaller than 5°, the waiting time WT is set to a relatively small value of 10 ms or less, for example, WT = 5 to 10 ms.

[0096] On the other hand, as shown in step S4, when the transverse relaxation time T2 of the tissue to be imaged is relatively long, for example, when the tissue to be imaged is synovial fluid or cerebrospinal fluid, the transverse relaxation time T2 exhibits a relatively long value such as T2=200 to 1000 ms. In such a case, the waiting time WT must also be determined to be a relatively long value, for example, about twice the waiting time T2, i.e., in this case, the waiting time WT is determined to be a value of about 400 to 2000 ms.

[0097] Returning to FIG. 10, once the waiting time WT is determined in step ST204 as described above, the virtual gating period (i.e., the repetition period T of the Time-SLIP pulse) is calculated in the next step ST205. IR-IR Specifically, the virtual gating period T IR-IR Calculate. T IR-IR =D IR +BBTI+T ack +WT (Formula 2) Finally, in step ST206, a Time-SLIP pulse sequence as shown in FIG. 11 is executed using virtual gating.

[0098] When executing the Time-SLIP pulse sequence, the calculated virtual gating period T IR-IR Therefore, a virtual gating signal as shown in FIG. 11(b) may be actually generated, and the Time-SLIP pulse sequence may be executed in synchronization with this virtual gating signal.

[0099] On the other hand, without actually generating the virtual gating signal, the sequence elements of the Time-SLIP sequence shown in FIG. 11(c) are calculated with the calculated virtual gating period T IR-IR It may be executed repeatedly.

[0100] 14 and 15 are diagrams comparing the imaging time of the Time-SLIP method using virtual gating according to this embodiment with the imaging time of the conventional Time-SLIP method using respiratory gating.

[0101] Fig. 14(a) shows a respiratory cycle, and Fig. 14(b) shows a Time-SLIP pulse sequence using conventional respiratory gating. In the Time-SLIP pulse sequence shown in Fig. 14(b), three segments, segment 1 to segment 3, are extracted and shown.

[0102] On the other hand, Figure 14(c) is a diagram showing a Time-SLIP pulse sequence using virtual gating according to this embodiment, and similar to Figure 14(b), shows three segments, segment 1 to segment 3, extracted.

[0103] In conventional respiratory-gated Time-SLIP pulse sequences, a data acquisition sequence is executed during the same time phase of the respiratory cycle to suppress motion artifacts caused by respiratory motion. This results in a dead time D between the trailing edge of the data acquisition sequence and the next Time-SLIP pulse. The length of the dead time D depends on the length of the respiratory cycle and the BBTI, but it can become long enough to be significant. Therefore, the existence of the dead time D is a major factor in lengthening the imaging time of conventional respiratory-gated Time-SLIP techniques.

[0104] On the other hand, the time-SLIP pulse sequence using virtual gating according to this embodiment uses a waiting time WT instead of the dead time D. The waiting time WT is a value that is determined independently of respiration or heart rate, and is determined to a value that takes into consideration the T2 value of the tissue to be imaged, the flip angle of the excitation pulse in the data acquisition sequence, etc. The waiting time WT is determined to be a very short value, for example, about 5 to 10 ms, except when imaging tissue with a very long transverse relaxation time T2, such as cerebrospinal fluid.

[0105] Therefore, the imaging time by the Time-SLIP pulse sequence using virtual gating according to this embodiment is significantly shorter than the conventional imaging time using respiratory gating.

[0106] An example of calculation showing the degree of reduction in imaging time is shown in Figure 15. The following parameter values ​​are assumed as prerequisites for the calculation. Time-SLIP pulse duration D IR Duration 100ms Total number of lines in k-space N L 16,000 lines Number of segments N S 64 Number of lines per segment (N L / N S ) 250 lines Excitation pulse repetition period TR 4ms In this case, the data collection time Tack per segment is 1000 ms (4 ms * 250 lines).

[0107] Now, further assuming that BBTI is set to 1000 ms, the total time Ta (=D IR +BBTI+Tack) is 2100ms.

[0108] On the other hand, assuming that the repetition period of respiratory gating, Tresp, i.e., the period of the respiratory cycle, is 5 seconds (5000 ms), the dead time D (dead time) in a Time-SLIP pulse sequence using conventional respiratory gating is Tresp-Ta=5000-2100=2900 ms.

[0109] Furthermore, the total imaging time Ttotal of the Time-SLIP pulse sequence using conventional respiratory gating is Ttotal=Tresp*number of segments, which is Ttotal=5 seconds*64=320 seconds (5 minutes 20 seconds).

[0110] In contrast, if we consider a case in which blood flowing in from outside the imaging area is visualized using the Time-SLIP method with the background sufficiently suppressed, the waiting time WT can be set to the specified minimum value WTmin (=5 ms).

[0111] Therefore, the repetition period T of the virtual gating in the Time-SLIP pulse sequence using the virtual gating of this embodiment is IR-IR (i.e., the repetition period of the Time-SLIP pulse T IR-IR ) is T IR-IR =Ta+WT=2100+5=2105ms.

[0112] As a result, when the virtual gating of this embodiment is used, the total imaging time Ttotal is T IR-IR *Since this is the number of segments, Ttotal = 2105ms * 64 = 135 seconds (2 minutes 15 seconds).

[0113] As described above, when using the above numerical example, the total imaging time with conventional respiratory gating is 320 seconds (5 minutes 20 seconds), whereas with the virtual gating of this embodiment, the total imaging time is 135 seconds (2 minutes 15 seconds), making it possible to significantly reduce the imaging time.

[0114] In conventional respiratory gating, the magnitude of acquired MR signals may fluctuate due to fluctuations in the respiratory cycle. In contrast, in the virtual gating of this embodiment, the period of virtual gating is always constant. Therefore, fluctuations in the magnitude of MR signals due to fluctuations in the acquisition period do not occur, and it is possible to suppress the occurrence of artifacts due to amplitude fluctuations.

[0115] As described above, the magnetic resonance imaging apparatus of this embodiment can reduce the imaging time for non-contrast imaging using the magnetic resonance imaging apparatus.

[0116] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents. [Explanation of symbols]

[0117] 1. Magnetic resonance imaging device 40 Processing circuit 41 Memory circuit 42 Display 43 Input Devices 400 Console 600 scanner F01 User interface control function F02 Imaging sequence setting function F03 Gating selection function F04 Virtual gating period calculation function F04 Reconfiguration function

Claims

1. a scanner including a static magnetic field magnet that generates a static magnetic field, a gradient magnetic field coil that generates a gradient magnetic field, and a WB (Whole Body) coil that irradiates an RF pulse onto a subject; processing circuitry; The processing circuitry In non-contrast imaging, a pulse sequence is set in which a sequence element including at least an inversion pulse and a data acquisition sequence executed after a delay time from the inversion pulse is repeated; calculating a repetition period from the inversion pulse to the next inversion pulse, the repetition period being a constant repetition period shorter than that of an actual gating process synchronized with the biological signal of the subject; a process of repeating the sequence elements at the calculated repetition period, wherein the scanner executes the pulse sequence using a virtual gating process that is not synchronized with the biological signal; Magnetic resonance imaging device.

2. The processing circuitry selecting, based on the type of the data collection sequence, whether to use the virtual gating process or an actual gating process including at least one of a respiratory gating process and an ECG gating process to execute the pulse sequence; 2. The magnetic resonance imaging apparatus according to claim 1.

3. The processing circuitry selecting the virtual gating process when the data acquisition sequence is a sequence using radial sampling, and selecting the real gating process when the data acquisition sequence is a sequence using Cartesian sampling; 3. The magnetic resonance imaging apparatus according to claim 2.

4. The sequence using radial sampling includes at least one of an Ultra-Short TE sequence (UTE sequence) and a Stack of Stars sequence (SoS sequence), 4. The magnetic resonance imaging apparatus according to claim 3.

5. The processing circuitry selecting, based on an imaging target region of a subject, whether to use the virtual gating process or real gating process including at least one of respiratory gating process and ECG gating process to execute the pulse sequence; 2. The magnetic resonance imaging apparatus according to claim 1.

6. The processing circuitry When the imaging target region is the head or the lower extremities, the virtual gating process is selected, and when the imaging target region is the abdomen or the chest, the real gating process is selected.

6. A magnetic resonance imaging apparatus according to claim 5.

7. a repetition period of the sequence element in the virtual gating process is a repetition period of the inversion pulse, The processing circuitry Calculating the repetition period of the inversion pulse using at least a delay time from the inversion pulse to the leading edge of the data collection sequence, a period of the data collection sequence, and a waiting time from the trailing edge of the data collection sequence to the next inversion pulse; 2. The magnetic resonance imaging apparatus according to claim 1.

8. The processing circuitry The repetition period T of the inversion pulse in the virtual gating process ＩＲ－ＩＲ the duration of the inversion pulse D ＩＲ , a delay time BBTI from the inversion pulse to the leading edge of the data acquisition sequence, and a period T of the data acquisition sequence. ａｃｋ and the waiting time WT from the trailing edge of the data acquisition sequence to the next inversion pulse, T ＩＲ－ＩＲ =D ＩＲ +BBTI+T ａｃｋ +WT、 Calculated by 8. The magnetic resonance imaging apparatus according to claim 7.

9. When the processing circuitry generates an image from data collected over multiple segments, The period T of the data collection sequence ａｃｋ is the total number of lines N that fill the k-space to generate the image. Ｌ , the number N of the plurality of segments Ｓ and the repetition time TR of the excitation pulses in the data acquisition sequence, T ａｃｋ =TR*(N) Ｌ / N Ｓ ) Calculated by 8. The magnetic resonance imaging apparatus according to claim 7.

10. When generating an image from data collected over a plurality of segments, the processing circuit calculates a repetition period of the inversion pulse so that the repetition period is constant over the entire period of data collection over the plurality of segments.

8. The magnetic resonance imaging apparatus according to claim 7.

11. The processing circuitry Calculating the waiting time based on the T2 value of the imaging subject.

8. The magnetic resonance imaging apparatus according to claim 7.

12. The processing circuitry Calculating the waiting time to be 2 to 5 times the T2 value of the imaging subject.

8. The magnetic resonance imaging apparatus according to claim 7.

13. The processing circuitry The magnetic resonance imaging apparatus according to claim 7 , wherein the waiting time is calculated based on a flip angle of an excitation pulse in the data acquisition sequence.

14. The processing circuitry When the data acquisition sequence is applied at a timing when the background of the imaging region is suppressed by applying the inversion pulse, the waiting time is set to a predetermined minimum time.

8. The magnetic resonance imaging apparatus according to claim 7.

15. a user interface through which a user can input or select data; the user interface is configured to allow input or selection of whether the data collection sequence is a sequence using the radial sampling or a sequence using the Cartesian sampling.

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

16. a user interface through which a user can input or select data; The user interface is configured to allow the imaging target region of the subject to be input or selected.

6. A magnetic resonance imaging apparatus according to claim 5.

17. The pulse sequence set by the processing circuit is based on the Time-SLIP method.

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

Images