JP2014100392A - Magnetic resonance imaging apparatus and magnetic resonance imaging method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus that can implement high-speed imaging by effectively acquiring signals with a quality of acquired images maintained while images for confirming a region under dominance of a specific blood vessel are being captured by applying RF pulses as prepulses to suppress signals from the specific region.SOLUTION: A prepulse combined image 505 with RF pulses set as prepulses to suppress signals from a specific region and a normal image 502 are used to generate a mask image 507 for masking on the normal image 502, so that the blood vessel (blood flow) 506 to be suppressed is visualized clearly. At this time, during prepulse combined imaging 504, data is extracted only from space frequency bands 503 contributing to the blood vessel to be suppressed in a k-space, and the remaining bands are complemented by normal imaging 501. In addition, the mask image 507 is binarized by predetermined thresholds to determine the blood vessel 506 to be suppressed. When signals are acquired as 3D data, space encode in a projection direction is cut during prepulse combined imaging.

Description

  The present invention relates to a Magnetic Resonance Imaging (hereinafter, MRI) apparatus, and more particularly to a TOF (Time-Of-Flight) imaging technique for confirming a dominant region of a specific blood vessel.

  In MRI, there is a technique called TOF imaging in which a blood vessel is drawn using a phenomenon in which a signal intensity generated from a flowing tissue such as a blood flow is different from a signal intensity generated from surrounding stationary tissue.

  In normal MRI, an RF pulse is applied together with a gradient magnetic field in order to selectively excite an arbitrary slice having a thickness in a one-dimensional direction. In recent years, a method of selectively exciting a two-dimensional or three-dimensional region is being used (for example, see Non-Patent Document 1). The RF pulse used at this time is called a two-dimensional selective excitation pulse (hereinafter referred to as a 2D RF pulse).

  There is a method of applying this 2DRF pulse as a pre-saturation pulse, performing TOF imaging, suppressing the magnetization of blood in a specific blood vessel, and confirming the dominant region of the blood vessel (see, for example, Non-Patent Document 2). At this time, the 2DRF pulse applied as the pre-saturation pulse is referred to as a Beam Sat pulse.

  At this time, in order to depict only the suppressed blood vessels, a technique called Selective TOF MRA is used in which a Beam Sat pulse is applied (ON) to perform TOF imaging, and the imaging is performed by non-application (OFF) to obtain images. There is.

Three-Dimensional Spectral-Spatial Excitation, Glen Morrell, Albert Macovski, Magn. Reson. Med. , 1997 37 p378-386 Selective TOF MRA using Beam Saturation pulse, Takashi Nishihara, et al. , Proc. ISMRM 2012, p2497

  In Selective TOF MRA, in addition to normal TOF imaging, imaging (Beam Sat combined TOF imaging) in which Beam Sat pulses are applied as pre-saturation pulses for the number of blood vessels for examining the dominant region is required.

  The time required for TOF imaging is usually about 5 minutes. Therefore, for example, when examining the dominant regions of the two left and right internal carotid arteries, it is necessary to perform Beam Sat combined TOF imaging for each internal carotid artery. For this reason, in addition to the time required for normal TOF imaging, a total of 10 minutes imaging time of 5 minutes × 2 times becomes longer. When the imaging time becomes long, not only the examination efficiency is bad, but also a positional shift depending on the body movement of the subject occurs, and the examination accuracy also deteriorates.

  Further, as described above, in the selective TOF MRA, a difference between an image obtained by applying (ON) a Beam Sat pulse and an image obtained by non-application (OFF) is obtained. At this time, if there is a positional shift as described above, the accuracy of the difference does not increase.

  Also, the magnetization suppressed by the Beam Sat pulse is recovered by T1 recovery. Therefore, in peripheral blood vessels far from the suppressed position, the suppression rate decreases due to this effect. For this reason, even in the difference image, the region far from the suppression position is an image in which the influence of T1 recovery is mixed.

  The present invention has been made in view of the above circumstances, and applies an RF pulse that suppresses a signal from a specific region as a pre-saturation pulse, and obtains the image quality of an image obtained in imaging for confirming a dominant region of a specific blood vessel. An object of the present invention is to provide an MRI technique that realizes high-speed imaging by effectively acquiring a signal while maintaining it.

  The present invention generates a mask image from data obtained by prepulse combined imaging, which is imaging that applies an RF pulse that suppresses a signal from a specific region as a prepulse, masks the image obtained by normal imaging, Clarify blood vessels (blood flow). At this time, in pre-pulse combined imaging, data in only the spatial frequency band contributing to the blood vessel to be suppressed is acquired in the k space. The remaining bandwidth is supplemented with data obtained by normal imaging. Further, at the time of generating a mask image, binarization is performed using a predetermined threshold value, thereby specifying a blood vessel to be suppressed. Furthermore, when acquiring a signal as 3D data, pre-pulse combined imaging is executed with the spatial encoding in the projection direction cut.

  According to the present invention, an RF pulse that suppresses a signal from a specific region is applied as a presaturation pulse, and in imaging for confirming a dominant region of a specific blood vessel, the signal is effectively maintained while maintaining the image quality of the obtained image. To obtain high-speed imaging.

(A) is a block diagram of the magnetic resonance imaging apparatus of the first embodiment. (B) is a functional block diagram of the control unit of the first embodiment. (A) is explanatory drawing for demonstrating a slice selection pulse sequence, (b) is explanatory drawing for demonstrating a two-dimensional space selective excitation sequence. (A) And (b) is explanatory drawing for demonstrating Selective TOF MRA. It is a flowchart of the imaging process of 1st embodiment. It is explanatory drawing for demonstrating the data and image which are obtained at each step of 1st embodiment. It is explanatory drawing for demonstrating the threshold value setting screen of 1st embodiment. (A) And (b) is explanatory drawing for demonstrating the image after a projection by the presence or absence of the encoding of a projection direction, respectively. It is explanatory drawing for demonstrating the projection direction setting screen of 2nd embodiment. It is a flowchart of the imaging process of 2nd embodiment.

<< First Embodiment >>
Hereinafter, a first embodiment to which the present invention is applied will be described. Hereinafter, in all the drawings for explaining the embodiments of the present invention, those having the same function are basically denoted by the same reference numerals, and the repeated explanation thereof is omitted.

  First, the configuration of the MRI apparatus of this embodiment will be described. FIG. 1 is a block diagram of an MRI apparatus 100 of this embodiment. The MRI apparatus 100 of the present embodiment is an apparatus that obtains a tomographic image of the subject 101 using an NMR phenomenon. As shown in FIG. 1, a static magnetic field generating magnet 102, a gradient magnetic field coil 103 and a gradient magnetic field power source 106, a transmission RF coil (transmission coil) 104, an RF transmission unit 107, a reception RF coil (reception coil) 105, and a signal The detection unit 108, the signal processing unit 109, the sequencer 110, the control unit 120, the display unit 121, the operation unit 122, the storage unit 123, and the subject 101 are mounted, and the subject 101 generates a static magnetic field. And a bed 111 that is taken in and out of the magnet 102.

  The static magnetic field generating magnet 102 functions as a static magnetic field generating unit that generates a static magnetic field. The static magnetic field generating magnet 102 generates a uniform static magnetic field in a direction perpendicular to the body axis of the subject 101 in the vertical magnetic field method, and in the body axis direction in the horizontal magnetic field method. A permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the.

  The gradient magnetic field coil 103 and the gradient magnetic field power source 106 function as a gradient magnetic field application unit that applies a gradient magnetic field to the subject 101 arranged in a static magnetic field. The gradient magnetic field coil 103 is a coil wound in the three-axis directions of X, Y, and Z that are the real space coordinate system (stationary coordinate system) of the MRI apparatus. Each of the gradient magnetic field coils is connected to a gradient magnetic field power source 106 that drives the gradient coil, and a current is supplied thereto. Specifically, the gradient magnetic field power supply 106 of each gradient coil is driven according to a command from a sequencer 110 described later, and supplies a current to each gradient coil. Thereby, gradient magnetic fields Gx, Gy, and Gz are generated in the three-axis directions of X, Y, and Z.

  For example, when imaging a two-dimensional slice plane, a slice gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging section) to set the slice plane for the subject 101. A phase encoding gradient magnetic field pulse (Gp) and a frequency encoding (lead-out) gradient magnetic field pulse (Gf) are applied in the remaining two directions orthogonal to the slice plane and orthogonal to each other, and the echo signal in each direction is applied. Location information is encoded.

  The transmission coil 104 and the RF transmission unit 107 function as a high-frequency magnetic field transmission unit that transmits a high-frequency magnetic field pulse (RF pulse) that excites the magnetization of the subject 101 with a predetermined flip angle. The transmission coil 104 is a coil that irradiates the subject 101 with an RF pulse, is connected to the RF transmission unit 107, and is supplied with an RF pulse current from the RF transmission unit 107. By irradiating the subject 101 with an RF pulse from the transmission coil 104, an NMR phenomenon is induced in the atomic nuclear spin that constitutes the biological tissue of the subject 101.

  Specifically, the RF transmission unit 107 is driven according to a command from a sequencer 110 described later, amplitude-modulates and amplifies the high-frequency pulse, and supplies it to the transmission coil 104 arranged close to the subject 101. The supplied high frequency pulse is applied to the subject 101 from the transmission coil 104.

  The reception coil 105 and the signal detection unit 108 function as a signal reception unit that receives an echo signal generated by the subject 101. The reception coil 105 is a coil that receives an NMR signal (echo signal) emitted by the NMR phenomenon of the nuclear spin that constitutes the biological tissue of the subject 101, and is connected to the signal detection unit 108 and receives the received echo signal as a signal. The data is sent to the detection unit 106. The signal detection unit 108 performs processing for detecting an echo signal received by the reception coil 105.

  Specifically, when an echo signal of the response of the subject 101 induced by the RF pulse irradiated from the transmission coil 104 is received by the receiving coil 105 disposed in the vicinity of the subject 101, a signal detection unit 108 is sent. The signal detection unit 108 amplifies the received echo signal according to a command from the sequencer 110 described later, divides the signal into two orthogonal signals by quadrature detection, and each of them is a predetermined number (for example, 128, 256, 512, etc.). ) Sampling, A / D conversion of each sampling signal is converted into a digital quantity, and it is sent to the signal processing unit 109 described later. Thus, the echo signal is obtained as time-series digital data (hereinafter referred to as echo data) composed of a predetermined number of sampling data.

  The signal processing unit 109 performs various types of signal processing on the echo data, and sends the processed echo data to the control unit 120.

  The sequencer 110 mainly transmits various commands for data collection necessary for the reconstruction of the tomographic image of the subject 101 to the gradient magnetic field power source 106, the RF transmission unit 107, and the signal detection unit 108. To control. Specifically, the sequencer 110 operates under the control of the control unit 120 to be described later, and controls the gradient magnetic field power source 106, the RF transmission unit 107, and the signal detection unit 108 according to the imaging sequence, and the RF pulse to the subject 101. And the application of the gradient magnetic field pulse and the detection of the echo signal from the subject 101 are repeatedly executed to collect echo data necessary for reconstruction of the image of the imaging region of the subject 101.

  The control unit 120 performs control of the sequencer 110, various data processing, display of processing results, storage, and the like, and has a CPU and a memory therein. In the present embodiment, an image is reconstructed from the echo signal received by the signal receiving unit described above, and a command for controlling the operations of the gradient magnetic field applying unit, the high frequency magnetic field transmitting unit, and the signal receiving unit is given to the sequencer 110 according to the imaging sequence. give. The imaging sequence is generated by imaging parameters set by the user and a pulse sequence specified by the user.

  Specifically, the control unit 120 of the present embodiment controls the sequencer 110 to collect echo data, and the collected echo data is stored in the memory based on the encoding information applied to the echo data. Store in an area corresponding to k-space. A group of echo data stored in an area corresponding to the k space of the memory is also referred to as k space data. The k-space data is subjected to processing such as signal processing and image reconstruction by Fourier transform, and the resulting image of the subject 101 is displayed on the display unit 121 described later and recorded in the storage unit 123. To do.

  The display unit 121 and the operation unit 122 are interfaces for exchanging various control information of the MRI apparatus 100, information necessary for arithmetic processing, and arithmetic processing results with the user. The MRI apparatus 100 according to the present embodiment accepts input from the user via the display unit 121 and the operation unit 122. The operation unit 122 is arranged close to the display unit 121, and an operator interactively controls various processes of the MRI apparatus 100 through the operation unit 122 while looking at the display unit 121. For example, the display unit 121 displays the reconstructed image of the subject 101. The operation unit 122 includes at least one of a trackball, a mouse, a keyboard, and the like serving as an input device.

  The storage unit 123 stores information necessary for the operation of the MRI apparatus 100, data being processed, and the like. For example, it is composed of an optical disk, a magnetic disk, or the like.

  In FIG. 1, the transmission coil 104 and the gradient magnetic field coil 103 are opposed to the subject 101 in the static magnetic field space of the static magnetic field generating magnet 102 into which the subject 101 is inserted. If it is a horizontal magnetic field system, it will be installed so as to surround the subject 101. The receiving coil 105 is installed so as to face or surround the subject 101.

  The imaging target nuclide of the current MRI apparatus 100 is a hydrogen nucleus (proton) which is a main constituent material of the subject 101 as being widely used in clinical practice. In the MRI apparatus 100, information on the spatial distribution of the proton density and the spatial distribution of the relaxation time of the excited state is imaged so that the form or function of the human head, abdomen, limbs, etc. can be expressed two-dimensionally or three-dimensionally. Take an image. At this time, in MRI, an RF pulse is applied together with a gradient magnetic field in order to excite only protons in a specific region.

  In the present embodiment, high-speed imaging is realized without degrading the obtained image quality in imaging for confirming a dominant region of a specific blood vessel. In the present embodiment, a case where Selective TOF MRA imaging is used as an imaging for confirming a dominant region of a specific blood vessel will be described as an example.

  As described above, in general, in selective TOF MRA imaging, an image (normal image) obtained by TOF imaging without a pre-pulse (normal TOF imaging) and TOF imaging (Beam Sat combined TOF using a Beam Sat pulse as a pre-pulse) are applied. The difference from the image (suppressed image) obtained in (imaging) is taken, a difference image is obtained, and an image in which the dominant region of the blood vessel (specific blood vessel) to be suppressed is clarified is obtained.

  In the present embodiment, the time required for the entire imaging is shortened by increasing the efficiency of Beam Sat combined TOF imaging. Furthermore, the image obtained by clarifying the dominant region of the specific blood vessel is obtained by appropriately performing image processing on the data obtained by the efficient Beam Sat combined TOF imaging.

  First, a two-dimensional selective excitation pulse used as a Beam Sat pulse for selecting and exciting a region in two dimensions or three dimensions will be described. Prior to the description of the two-dimensional selective excitation pulse, a pulse used in a normal slice selective excitation sequence will be described for comparison.

  FIG. 2A shows a pulse sequence (slice selection pulse sequence) 900 for exciting an arbitrary slice having a thickness in a one-dimensional direction. Hereinafter, in the pulse sequence diagram of the present specification, RF, Gx, Gy, and Gz indicate application timings of the RF pulse, the gradient magnetic field in the x-axis direction, the gradient magnetic field in the y-axis direction, and the gradient magnetic field in the z-axis direction, respectively. . As shown in the figure, simultaneously with the RF pulse 901, a slice selection gradient magnetic field 902 is applied in any one direction of Gx, Gy, and Gz. Here, the case of applying in the Gz direction is illustrated. Thereby, a predetermined slice in which only the position in the z-axis direction is specified is selectively excited.

  Next, a description will be given of a pulse sequence for exciting a region selected two-dimensionally or three-dimensionally. Here, as an example, a two-dimensional space selective excitation sequence (2DRF sequence) for selectively exciting a region in two dimensions will be described. FIG. 2B shows this 2DRF sequence.

  As shown in the figure, the 2DRF sequence includes a two-dimensional selective excitation pulse (hereinafter, 2DRF pulse) 201 and an oscillating gradient magnetic field pulse 202. Here, as an example, a case where the oscillating gradient magnetic field pulse 202 is applied in the Gx and Gy directions is shown. By these 2DRF pulse 201 and oscillating gradient magnetic field pulse 202, only a predetermined cylindrical region is selectively excited. The 2D RF pulse 201 is a pencil beam type excitation RF pulse for exciting a cylindrical region.

  In general, when a crusher gradient magnetic field pulse is applied after an RF pulse, a signal in a specific region is suppressed. In the combined imaging of Beam TOS of Selective TOF MRA, a sequence 200 in which a crusher gradient magnetic field pulse 203 is applied after a 2D RF pulse 201 and an oscillating gradient magnetic field pulse 202 is used as a pre-pulse sequence. Thereby, the signal of the area | region excited by the 2DRF pulse 201 and the vibration gradient magnetic field pulse 202 is suppressed, and the blood flow which flows in from the said area | region is selectively suppressed.

  Next, blood vessels rendered by normal TOF imaging and Beam Sat combined TOF imaging will be described. In normal TOF imaging, as shown in FIG. 3A, a blood vessel 301 in a slab 310 that is an imaging region is drawn. On the other hand, in Beam Sat combined TOF imaging, as shown in FIG. 3B, the signal is reduced in the slab 311 for the blood vessel 302 suppressed by the Beam Sat pulse. Reference numeral 312 denotes a region that is suppressed by the 2D RF pulse that is the Beam Sat pulse, the oscillating gradient magnetic field pulse 202, and the crusher gradient magnetic field pulse 203.

  In Selective TOF MRA, normal TOF imaging and Beam Sat combined TOF imaging are performed, and a reconstructed image of the slab 310 and the slab 311 is obtained. Moreover, only the suppressed blood vessel can be drawn by taking the difference between the two. The blood vessel image is evaluated by a projection image obtained by projecting the 3D data after the difference in two dimensions. At this time, a maximum value projection image (MIP image) or the like is used.

  In the present embodiment, in this selective TOF MRA, the image quality is improved and the imaging time is shortened. In order to realize this, as shown in FIG. 1B, a normal image acquisition unit 410 that performs normal TOF imaging and obtains a normal image, and a suppression image acquisition unit 420 that performs Beam Sat combined TOF imaging and obtains a suppression image. And a display image generation unit 430 that performs image processing on both images and generates a display image.

  As described above, in the present embodiment, an imaging sequence (TOF sequence) that realizes TOF imaging is used as an imaging sequence used for main imaging for acquiring an image. Therefore, the normal image acquisition unit 410 acquires k-space data according to the TOF sequence, and obtains a normal image by reconstructing the k-space data. On the other hand, the suppression image acquisition unit 420 performs Beam Sat pulse combined imaging, and acquires a suppression image from the result.

  At this time, in this embodiment, the suppression image acquisition unit 420 does not acquire all k-space data, but contributes k-space data (partial k) that is a spatial frequency band that contributes to the depiction of the structure of the suppression target blood vessel. (Spatial data) only. The remaining k-space data is supplemented with k-space data (total k-space data) acquired at the time of normal image acquisition. Then, the suppressed image is obtained by reconstructing the k-space data after complementation. Note that the imaging sequence executed by the suppressed image acquisition unit 420 for the main imaging is the same as the TOF sequence executed by the normal image acquisition unit 410 except for the k-space data acquisition band.

  Since blood vessels are relatively fine structures in the image, in the measurement data (low data) arranged in the k-space, the data in the middle to high frequency range is dominant, Contributes to drawing. In the TOF sequence, when Kx is the frequency encoding direction, Ky is the phase encoding direction, and Kz is the slice encoding direction, in the TOF imaging combined with Beam Sat pulse of this embodiment, the data acquired in the Ky and Kz directions is represented in the spatial frequency band. Is only mid- to high-frequency data.

  The data band to be acquired is determined in advance and stored in the storage unit 123. Moreover, you may comprise so that an operator may set. The band is empirically determined according to the diameter of the imaging object (target blood vessel). In the present embodiment, the band of data to be acquired is determined so that data contributing to the depiction of the blood vessel structure can be clearly acquired.

  The display image generation unit 430 performs threshold processing on the difference image between the normal image and the suppression image to binarize, and extracts the suppression target blood vessel. By performing the threshold processing, the influence of the lack of suppression due to the lack of the acquisition bandwidth and the T1 relaxation is reduced. The threshold used for the threshold processing is determined in advance and stored in the storage unit 123.

  As described above, by acquiring only the k-space data of the contribution band of the present embodiment, one blood vessel suppression image can be obtained in a shorter time than when acquiring the k-space data of the entire band.

  Hereinafter, the flow of the imaging process by the control unit 120 of the present embodiment, the acquired k-space data, and the generated image will be described. FIG. 4 is a processing flow of the imaging process of the present embodiment. FIG. 5 is a diagram for explaining k-space data and an image obtained by each processing. Here, as described above, a case where three-dimensional TOF imaging is used for main imaging will be described as an example. A Beam Sat pulse is used as the pre-pulse. The imaging conditions are set by the user in advance.

  First, the normal image acquisition unit 410 performs normal TOF imaging, and acquires k-space data (total k-space data) 501 in the entire spatial frequency band (step S1001). Here, only the kx and ky planes are shown. Then, the normal image acquisition unit 410 reconstructs the normal image 502 from the acquired all k-space data 501 (step S1002). The reconstruction is performed by a general Fourier transform or the like.

  Next, the suppression image acquisition unit 420 executes Beam Sat combined TOF imaging, and acquires data (partial k-space data) 503 in a predetermined frequency band (step S1003). Here, only the kx and ky planes are shown. Further, as described above, data in the middle to high spatial frequency band is acquired. The imaging conditions of main imaging (TOF imaging) in Beam Sat combined TOF imaging are the same as those for normal TOF imaging executed in step S1001 except for the encoding amount in the Ky and Kz directions.

  Next, the suppression image acquisition unit 420 supplements the shortage of the partial k-space data 503 acquired in step S1003 with the entire k-space data 501 acquired in step S1001 (step S1004). Here, the above-described compensation is realized by replacing the k-space data of the contribution band in the entire k-space data 501 with the partial k-space data 503. The compensated k-space data is called replacement k-space data.

  Then, the suppression image acquisition unit 420 reconstructs an image (suppression image) 505 from the replacement k-space data 504 (step S1005). For reconstruction, for example, Fourier transform is used. In this suppressed image 505, the k-space data of the entire band is obtained by using the beam-sat combined TOF imaging, and the signal intensity of the blood vessel (blood flow) 506 suppressed by the Beam Sat pulse is compared with an image reconstructed from the k-space data. There is little suppression. However, compared to the normal image 502, the value of the blood flow signal is suppressed more than other tissues.

  Next, the display image generation unit 430 takes the difference between the normal image 502 reconstructed in step S1003 and the suppression image 505 reconstructed in step S1005, and generates a difference image 507 (step S1006). In the difference image 507, only the blood vessel (blood flow) 506 suppressed on the suppression image 505 is a high signal.

  The display image generation unit 430 binarizes the difference image 507 using a predetermined threshold value, and generates a mask image 508 (step S1007). Here, the mask image 508 is generated by giving a predetermined color to the pixel 509 having a pixel value equal to or larger than the threshold value of the difference image 507. The generated mask image 508 is a color mask image in which pixels 509 having a threshold value or more are colored. Thereby, in the mask image 508, only the area | region suppressed by the Beam Sat pulse is colored.

  The display image generation unit 430 creates a display element image 510 by superimposing the mask image 508 on the normal image 502 (step S1008). In the display element image 510, only the pixel 509 in the region of the normal image 502 that is suppressed by the Beam Sat pulse is colored. As a result, information on the dominant region of the blood vessel is added to the display element image 510 that is three-dimensional data.

  Here, the blood vessel image is generally evaluated by an image projected in two dimensions. For this reason, the display image generation unit 430 projects the display element image 510 in a predetermined projection direction, and generates a display image 511 (step S1009). For example, a maximum value projection process (MIP) is used for the projection process, and an MIP image is obtained as the display image 511. Volume rendering may be performed as a projection process to generate a volume rendering (VR) image.

  Note that the main imaging of the present embodiment may be two-dimensional imaging such as two-dimensional TOF imaging. In this case, step S1009 is not performed. Even in the case of three-dimensional imaging, step S1009 may not be performed.

  When the threshold used in step S1007 is small, regions other than the suppression target blood vessels, for example, the brain parenchyma region, may be colored in the mask image 508. However, on the normal image 502, the signal value of the unnecessary area to be colored is low. In the present embodiment, the normal image 502 and the mask image 508 are superimposed and a projection process is performed. For this reason, even on the display element image 510, a low signal is obtained. This does not affect the rendering of the dominant region on the display image 511 obtained by projecting the display element image 510.

  In the above processing flow, the display element image 510 is projected to obtain the display image 511. However, the present invention is not limited to this. For example, the display image generation unit 430 may be configured to perform projection processing on the normal image 502 and the suppression image 505 before performing subsequent image processing.

  In this case, when the normal image 502 is obtained, the display image generation unit 430 performs a projection process of projecting in a predetermined projection direction, and obtains a normal image after projection. Moreover, when the suppression image 505 is obtained, a projection process is performed in the same direction, and a suppression image after projection is obtained. Then, a mask image is generated from the normal image after projection and the suppression image after projection by the same procedure as described above. Then, a mask image is superimposed on the projected normal image to obtain a display image 511.

  In the present embodiment, the case where TOF imaging is used as the main imaging has been described as an example, but the imaging method used for the main imaging is not limited thereto. Further, the pre-pulse is not limited to the Beam Sat pulse. Any imaging sequence or prepulse that suppresses the magnetization of blood flow in a specific blood vessel and makes the dominant region clear can be used.

  Further, the threshold value used when creating the mask image 508 may be set while the user looks at the display image 511 (or the display element image 510). In this case, the control unit 120 further includes a reception unit. The reception unit displays a UI screen (threshold setting screen) on which a threshold can be set on the display unit 121 and receives a setting from the user. An example of the UI screen (threshold setting screen) displayed at this time is shown in FIG.

  The threshold setting screen 600 includes an image display area 601 for displaying the display image 511, a threshold input area 602 for the user to input a threshold, and a confirmation button 603 for accepting the intention to confirm the set threshold. The reception unit causes the display image generation unit 430 to generate the mask image 508, the display element image 510, and the display image 511 using the threshold value every time the user inputs the threshold value via the threshold value input area 602. Then, the display of the image display area 601 is updated with the generated display image 511. The reception unit sets the threshold value input to the threshold value input area 602 as the threshold value used for processing when the pressing of the confirmation button 03 is received.

  The user sets an optimum threshold while viewing the display in the image display area 601. Thereby, the user can easily set a threshold value that is optimal for the diameter of the blood vessel to be imaged (for example, matched with the diameter).

  As described above, the MRI apparatus according to the present embodiment is an MRI apparatus 100 that obtains a display image that can identify a dominant region of a specific blood vessel, and the subject's interest including the dominant region according to a predetermined imaging sequence. A normal image acquisition unit 410 that acquires an image of a region as a normal image, and a part of the imaging sequence is executed while applying a pre-pulse that selectively suppresses magnetization in the specific blood vessel, and a blood flow signal of the specific blood vessel A suppression image acquisition unit 420 that obtains a suppression image in which the suppression is suppressed, and a display image generation unit 430 that generates the display image from the normal image and the suppression image.

The suppression image acquisition unit 420 may execute the imaging sequence so as to acquire only partial k-space data that is k-space data of a contribution band that is a spatial frequency band that contributes to depiction of the structure of the specific blood vessel.
The suppression image acquisition unit 420 may reconstruct the suppression image after supplementing k-space data other than the partial k-space data with k-space data obtained by the normal image acquisition unit 410.
The display image generation unit 430 obtains a difference image from the normal image and the suppression image, and creates a mask image that can identify pixels having a pixel value equal to or greater than a predetermined threshold in the difference image. The display image may be generated by creating and superimposing the mask image on the normal image.
The imaging sequence is a sequence for measuring a three-dimensional space, and the display image generation unit 430 projects a superimposed image obtained by superimposing the mask image on the normal image in a predetermined direction. The display image may be generated.

  For this reason, according to the present embodiment, in the TOF imaging with Beam Sat combined with Selective TOF MRA, the acquisition time is efficiently limited according to the purpose, thereby reducing the imaging time. Specifically, only k-space data in the spatial frequency domain necessary for rendering the tissue to be selectively suppressed is collected. For example, when the imaging target is a blood vessel, data is collected only in the middle and high spatial frequency regions of k space. Therefore, it is possible to reduce the time required for Beam Sat combined TOF imaging as compared with the case of acquiring all the data in the k space.

  In the method according to the present embodiment, the beam Sat combined TOF imaging is made more efficient, so that the amount of suppression can be suppressed by the amount of data collection. However, according to the present embodiment, in the difference image between the normal image and the suppression image, a mask image obtained by binarizing a region that becomes a high signal using a threshold is obtained and superimposed on the normal image. By masking a normal image with a mask image binarized using a threshold value, the influence of T1 recovery can be eliminated. Therefore, according to the present embodiment, an image (elementary image) in which a necessary area is appropriately suppressed can be obtained. Finally, this elementary image is projected, and a high-quality image in which the dominant region of a specific blood vessel is clarified can be obtained.

  Therefore, according to the present embodiment, the selective TOF MRA can be efficiently performed in a short time, and the visibility of the dominant region of each blood vessel is further improved. That is, according to the present embodiment, the BeamSat TOF measurement can be efficiently shortened without deterioration in image quality.

<< Second Embodiment >>
Next, a second embodiment to which the present invention is applied will be described. In the first embodiment, in Beam Sat combined TOF imaging, the imaging time is shortened by acquiring only data in the spatial frequency band corresponding to the blood vessel to be suppressed. On the other hand, in this embodiment, in Beam Sat combined TOF imaging, encoding time is shortened by omitting encoding in the direction that is the projection direction when creating a projection image.

  In the three-dimensional TOF imaging, encoding is performed to give position information in three directions (for example, the x direction, the y direction, and the z direction). On the other hand, an image obtained by three-dimensional TOF imaging is generally often used for diagnosis after being projected in a specific projection direction to form a two-dimensional image. For this reason, position information is not necessary for projection direction data. In the present embodiment, using this characteristic, after determining the projection direction, encoding that gives position information for the direction is omitted, and the imaging time is shortened.

  For example, when the projection direction is the y direction, as shown in FIG. 7A, an image 702 obtained by performing a projection process from the raw image 701 captured with the encoding in the y direction, and FIG. As shown in (), the image 712 obtained by performing the projection process from the elementary image 711 obtained by imaging without applying encoding in the y direction is substantially the same. FIG. 7A and FIG. 7B illustrate a case where imaging is performed in the axial (AX) direction where the TOF effect is most obtained. The y direction is a direction parallel to the axial cross section.

  The MRI apparatus 100 of this embodiment is basically the same as that of the first embodiment. The functional blocks of the control unit 120 of this embodiment are the same as those of the first embodiment. However, the imaging sequence used in the main imaging of the present embodiment is a sequence that realizes three-dimensional imaging (three-dimensional TOF imaging). Moreover, the process by the suppression image acquisition part 420 and the process by the display image generation part 430 differ.

  The suppressed image acquisition unit 420 according to the present embodiment determines a TOF imaging sequence used for Beam Sat combined TOF imaging according to a predetermined projection direction before performing Beam Sat combined TOF imaging. In the TOF imaging sequence to be determined, the encoding step in the projection direction is omitted (cut), and for the other two directions, only necessary spatial frequency band data is collected according to the suppression target blood vessel as in the first embodiment. Is.

  In the present embodiment, the projection direction may be determined in advance according to the imaging region, or may be configured to be set by the user. When the user sets, the suppression image acquisition unit 420 generates a projection direction setting screen as a UI screen and displays it on the display unit 121. The user sets the projection direction via the projection direction setting screen.

  An example of the projection direction setting screen 610 is shown in FIG. As shown in the figure, the projection direction setting screen 610 includes a normal image display area 611 that displays a normal image 502, a projection direction input area 612 that receives an input of the projection direction from the user, and a projection in the projection direction input by the user. The post-projection normal image display area 614 for displaying the post-projection normal image 502p and the confirmation button 613 for accepting the intention of confirmation from the user are provided.

  Each time the user inputs a projection direction via the projection direction input area 612, the display image generation unit 430 generates a normal image 502p after projection projected in the projection direction, and displays it in the post-projection normal image display area 614. indicate. When the user views the normal image 502p after projection displayed in the post-projection normal image display area 614 and determines that the projection direction is desired, the user presses the confirm button 613. The suppression image acquisition unit 420 employs the projection direction input to the projection direction input area 612 at the timing when the confirmation button 613 is pressed.

  In general, the projection is often performed in a direction parallel to the head-to-tail direction and the axial section. Accordingly, one or more selectable projection directions may be registered in advance according to the imaging region, and the user may select from among them. Further, the projection direction may be stored in advance in the storage unit 123 in accordance with the imaging region and automatically selected.

  Encoding in the projection direction may be performed to such an extent that a minimum necessary S / N can be obtained. For example, 1 encoding (encoding amount 0) is used. About other 2 directions, it determines so that the data which contribute to the structure drawing of the suppression target object can be acquired like 1st embodiment. For example, when the suppression target is blood, it is determined to acquire data in a medium to high frequency region that contributes to the depiction of blood vessel structure.

  The suppression image acquisition unit 420 supplements the partial k-space data acquired by the imaging sequence with the k-space data acquired by the normal image acquisition unit 410 as in the first embodiment, and suppresses the suppression image from the k-space data after the supplement. 505 is reconfigured. Then, a post-projection suppressed image that is projected in a projection direction in which encoding is omitted is obtained.

  The display image generation unit 430 obtains the display image 511 from the normal image after projection and the suppressed image after projection in the same procedure as in the first embodiment. Here, a difference image is generated from the normal image after projection and the suppression image after projection, and as in the first embodiment, a mask image is obtained by binarization using a threshold value, and the normal image after projection is obtained. A display image is obtained by superimposing the mask image.

  A flow of imaging processing by the control unit 120 of this embodiment will be described. FIG. 9 is a processing flow of the imaging process of the present embodiment. Here, a case where the user sets the projection direction via the projection direction setting screen 610 will be described as an example.

  First, the normal image acquisition unit 410 performs normal TOF imaging, and acquires k-space data (total k-space data) 501 in the entire spatial frequency band (step S1101). Then, the normal image acquisition unit 410 reconstructs the normal image 502 from the acquired all k-space data 501 (step S1102).

  Next, the suppressed image acquisition unit 420 displays the projection direction setting screen 610 on the display unit 121, receives the setting of the projection direction, and determines the projection direction (step S1103). At this time, the display image generation unit 430 holds the normal image 502p after projection generated at the time of setting the projection direction in the storage unit 123.

  Next, the suppression image acquisition unit 420 cuts the encoding in the projection direction determined in step S1103, executes Beam Sat combined TOF imaging, and acquires data (partial k-space data) 503 in a predetermined frequency band. (Step S1104). Here, for the projection direction, partial k-space data to which position information is not given is obtained. As in the first embodiment, the imaging conditions of the main imaging (TOF imaging) in the Beam Sat combined TOF imaging are the same as those of the normal TOF imaging executed in step S1101 except for the encoding amount.

  Next, the suppressed image acquisition unit 420 supplements the insufficient bandwidth of the partial k-space data 503 acquired in step S1103 with the entire k-space data 501 acquired in step S1101, and generates replacement k-space data 504 (step). S1105). Then, the suppression image acquisition unit 420 reconstructs an image (suppression image) 505 from the replacement k-space data 504 (step S1106).

  Next, the display image generation unit 430 obtains a post-projection suppressed image projected in the projection direction determined in step S1103 from the suppressed image 505 (step S1107).

  Next, the display image generation unit 430 takes a difference between the normal image 502p after projection obtained in step S1103 and the suppression image after projection obtained in step S1107, and generates a difference image (step S1108). Then, the display image generation unit 430 uses a predetermined threshold value and binarizes the difference image by the same method as in the first embodiment to generate a mask image (step S1109).

  Then, the display image generation unit 430 creates a display element image by superimposing the mask image on the projected normal image 502p (step S1110).

  If the projection direction is determined in advance, in step S1103, only the process of generating the projected normal image 502p that is projected from the normal image in the predetermined projection direction is performed. Further, the suppression image acquisition unit 420 omits encoding in a predetermined projection direction, and performs Boma Sat combined TOF imaging.

  Also in the present embodiment, as in the first embodiment, the user may set the threshold used when creating the mask image via the threshold setting screen 600.

  As described above, the MRI apparatus according to the present embodiment is an MRI apparatus 100 that obtains a display image that can identify a dominant region of a specific blood vessel, and the subject's interest including the dominant region according to a predetermined imaging sequence. A normal image acquisition unit 410 that acquires an image of a region as a normal image, and a part of the imaging sequence is executed while applying a pre-pulse that selectively suppresses magnetization in the specific blood vessel, and a blood flow signal of the specific blood vessel A suppression image acquisition unit 420 that obtains a suppression image in which the suppression is suppressed, and a display image generation unit 430 that generates the display image from the normal image and the suppression image.

The suppression image acquisition unit 420 may execute the imaging sequence so as to acquire only partial k-space data that is k-space data of a contribution band that is a spatial frequency band that contributes to depiction of the structure of the specific blood vessel.
The suppression image acquisition unit 420 may reconstruct the suppression image after supplementing k-space data other than the partial k-space data with k-space data obtained by the normal image acquisition unit 410.
The display image generation unit 430 obtains a difference image from the normal image and the suppression image, and creates a mask image that can identify pixels having a pixel value equal to or greater than a predetermined threshold in the difference image. The display image may be generated by creating and superimposing the mask image on the normal image.

The imaging sequence is a sequence for measuring a three-dimensional space, and the display image generation unit 430 projects the normal image and the suppression image in a predetermined projection direction prior to generation of the display image. And the display image may be generated using each obtained two-dimensional image.
The suppressed image acquisition unit 420 may execute the imaging sequence without encoding the projection direction.

  Thus, according to the present embodiment, as in the first embodiment, in the selective TOF MRA, only the k-space data in the spatial frequency domain necessary for rendering the tissue to be selectively suppressed is acquired in the beam sat combined TOF imaging. To do. For this reason, compared with the case where all the data of k space are acquired, the time concerning Beam Sat combined TOF imaging can be shortened. Further, by masking a normal image with a mask image binarized using a threshold value, the influence of T1 recovery can be eliminated. Therefore, according to the present embodiment, an image (elementary image) in which a necessary area is appropriately suppressed can be obtained.

  Furthermore, according to the present embodiment, spatial encoding in the projection direction is not necessary. For example, if the encoding in the projection direction is cut and the number of times of data collection in that direction is set to one, the imaging time of the Beam Sat combined TOF imaging is reduced to 1 / (the number of encodings in the projection direction) of the first embodiment. it can. For example, if the number of encodings in the projection direction of 256 steps is normally 1 encoding, it can be reduced to 1/256. For this reason, it is possible to obtain an image in which the dominant region of a specific blood vessel is clarified at a higher speed without degrading the image quality.

  Therefore, according to the present embodiment, the selective TOF MRA can be efficiently performed in a short time, and the visibility of the dominant region of each blood vessel is further improved. That is, according to the present embodiment, the BeamSat TOF measurement can be efficiently shortened without deterioration in image quality.

  The number of encodings in the projection direction (data collection count) is not limited to one. It is sufficient that the number is less than the number of encodings in the direction at the time of normal TOF imaging. The number of times data is collected is the same as the total number of data. If the number is increased, the SNR of the obtained data is improved.

  Further, in the present embodiment, in the Beam Sat combined TOF, only encoding in the projection direction is omitted, and the rest may be the same as that of the conventional Selective TOF MRA. That is, in Beam Sat combined TOF imaging, encoding in the shadow direction is omitted, and in the other two directions, measurement is performed with all of the encoding equivalent to normal TOF imaging being performed, and all k-space data is acquired. Then, a difference image between the obtained image and the normal image is obtained, and the difference image is projected in the projection direction to obtain an MIP image.

  In each of the above-described embodiments, each function of the control unit 120 is realized by a CPU included in the control unit 120 loading a program stored in advance in the storage unit 123 into a memory and executing the program.

  DESCRIPTION OF SYMBOLS 100: MRI apparatus, 101: Subject, 102: Static magnetic field generation magnet, 103: Gradient magnetic field coil, 104: Transmission coil, 105: Reception coil, 106: Gradient magnetic field power supply, 106: Signal detection part, 107: RF transmission part , 108: signal detection unit, 109: signal processing unit, 110: sequencer, 111: bed, 120: control unit, 121: display unit, 122: operation unit, 123: storage unit, 200: sequence, 201: 2DRF pulse, 202: Vibration gradient magnetic field pulse, 203: Crusher gradient magnetic field pulse, 301: Blood vessel, 302: Blood vessel, 310: Slab, 311: Slab, 410: Normal image acquisition unit, 420: Suppression image acquisition unit, 430: Display image generation unit 501: Total k-space data 502: Normal image 502 p: Normal image after projection 503: Partial k-space data 50 : Replacement k-space data, 505: suppression image, 507: difference image, 508: mask image, 509: pixel, 510: display element image, 511: display image, 600: threshold setting screen, 601: image display area, 602: Threshold input area, 603: Confirm button, 610: Projection direction setting screen, 611: Normal image display area, 612: Projection direction input area, 613: Confirm button, 614: Normal image display area after projection, 701: Elementary image, 702 : Image, 711: Elementary image, 712: Image, 901: RF pulse, 902: Slice selection gradient magnetic field

Claims (16)

A magnetic resonance imaging apparatus for obtaining a display image capable of identifying a dominant region of a specific blood vessel,
A normal image acquisition unit that acquires an image of the region of interest of the subject including the dominant region as a normal image according to a predetermined imaging sequence;
A suppressed image acquisition unit that executes a part of the imaging sequence while applying a prepulse that selectively suppresses magnetization in the specific blood vessel, and obtains a suppressed image that suppresses a blood flow signal of the specific blood vessel;
A magnetic resonance imaging apparatus comprising: a display image generation unit configured to generate the display image from the normal image and the suppression image. The magnetic resonance imaging apparatus according to claim 1,
The suppressed image acquisition unit executes the imaging sequence so as to acquire only partial k-space data that is k-space data of the contribution band,
The magnetic resonance imaging apparatus, wherein the contribution band is a spatial frequency band that contributes to depiction of the structure of the specific blood vessel. The magnetic resonance imaging apparatus according to claim 1 or 2,
The suppression image acquisition unit reconstructs the suppression image after supplementing k-space data other than the partial k-space data with k-space data obtained by the normal image acquisition unit. apparatus. The magnetic resonance imaging apparatus according to any one of claims 1 to 3,
The display image generation unit obtains a difference image from the normal image and the suppression image, and creates a mask image having an aspect in which pixels having a pixel value equal to or greater than a predetermined threshold in the difference image can be identified. The display image is generated by superimposing the mask image on the normal image. The magnetic resonance imaging apparatus according to claim 4,
The imaging sequence is a sequence for measuring a three-dimensional space,
The magnetic resonance imaging apparatus, wherein the display image generating unit generates the display image by projecting a superimposed image obtained by superimposing the mask image on the normal image in a predetermined direction. The magnetic resonance imaging apparatus according to any one of claims 1 to 4,
The imaging sequence is a sequence for measuring a three-dimensional space,
Prior to generation of the display image, the display image generation unit obtains a two-dimensional image obtained by projecting the normal image and the suppression image in a predetermined projection direction, and uses each obtained two-dimensional image, A magnetic resonance imaging apparatus that generates the display image. The magnetic resonance imaging apparatus according to claim 6,
The magnetic resonance imaging apparatus, wherein the suppression image acquisition unit executes the imaging sequence without encoding the projection direction. The magnetic resonance imaging apparatus according to any one of claims 1 to 7,
The magnetic resonance imaging apparatus, wherein the imaging sequence is a sequence based on a time-of-flight (TOF) method. A magnetic resonance imaging apparatus according to any one of claims 1 to 8,
The magnetic resonance imaging apparatus, wherein the pre-pulse is a two-dimensional space selective excitation pulse that selectively excites an area limited by two specific directions. The magnetic resonance imaging apparatus according to claim 4,
A display device;
A threshold setting unit that displays the display image on the display device and receives the setting of the threshold;
The display image generation unit creates the mask image using the threshold every time the threshold is set. A magnetic resonance imaging method for obtaining a display image capable of identifying a dominant region of a specific blood vessel,
A normal image acquisition step of obtaining an image of the region of interest of the subject including the dominant region as a normal image from all k-space data obtained by executing according to a predetermined imaging sequence;
A suppression image acquisition step of obtaining a suppression image of the region of interest from partial k-space data obtained by applying a pre-pulse that selectively suppresses magnetization in the specific blood vessel and executing a part of the imaging sequence;
And a display image generation step of generating the display image from the normal image and the suppression image. The magnetic resonance imaging method according to claim 11, comprising:
The partial k-space data is k-space data of a contribution band that is a spatial frequency band contributing to the depiction of the structure of the specific blood vessel,
The suppressed image acquisition step includes
A replacement step of replacing the k-space data of the contribution band in the total k-space data with the partial k-space data to obtain replacement k-space data;
Reconstructing an image from the replacement k-space data and obtaining a suppressed image,
The display image generation step includes
A difference image generation step of subtracting the suppression image from the normal image to generate a difference image;
A mask image generation step of generating a mask image in which a pixel having a pixel value equal to or greater than a predetermined threshold in the difference image can be identified;
And a superimposed image generating step of superimposing the mask image on the normal image and using the obtained superimposed image as the display image. A magnetic resonance imaging method according to claim 12, comprising:
The imaging sequence is a sequence for measuring a three-dimensional space,
The magnetic resonance imaging method, wherein the display image generation step further includes a projection image generation step of obtaining a projection image by projecting the display image in a predetermined direction. The magnetic resonance imaging method according to claim 11 or 12,
The imaging sequence is a sequence for measuring a three-dimensional space,
The normal image is a two-dimensional image projected in a predetermined projection direction,
The magnetic resonance imaging method, wherein the suppression image is a two-dimensional image projected in the projection direction. The magnetic resonance imaging method according to claim 14, comprising:
The magnetic resonance imaging method, wherein the partial k-space data is data obtained by executing the imaging sequence without encoding the projection direction. On the computer,
A normal image acquisition function for acquiring, as a normal image, an image of a region of interest of a subject including a dominant region of a specific blood vessel according to a predetermined imaging sequence;
A suppressed image acquisition function for executing a part of the imaging sequence while applying a prepulse that selectively suppresses magnetization in the specific blood vessel, and obtaining a suppressed image in which a blood flow signal of the specific blood vessel is suppressed,
A program for realizing a display image generation function for obtaining a display image capable of identifying a dominant region of the specific blood vessel from the normal image and the suppression image.

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