JP5068606B2 - Magnetic resonance imaging equipment, program - Google Patents

Description

  The present invention relates to a magnetic resonance imaging (MRI) apparatus and a program. In particular, in a static magnetic field space, an RF pulse (Radio Frequency) is repeatedly transmitted and imaged at a repetition time (TR: Time of Repetition) such that the spin of the imaging region of the subject is in an SSFP (Standy State Free Precession) state. A scan for collecting magnetic resonance signals from the subject is performed by repeatedly transmitting gradient pulses in the slice selection direction, the phase encoding direction, and the frequency encoding direction in the region so that the time integration value becomes zero within the time TR. The present invention relates to a magnetic resonance imaging apparatus and a program for generating an image of the subject based on the magnetic resonance signals collected by performing the scan.

  A magnetic resonance imaging apparatus excites spins in a subject by a nuclear magnetic resonance (NMR) phenomenon in a static magnetic field space, and based on a magnetic resonance (MR) signal generated along with the excitation, Generate an image for.

  Magnetic resonance imaging apparatuses are used in various fields such as the medical field and the industrial field, and an imaging region of a subject is imaged by various imaging methods according to the imaging purpose.

  For example, as an imaging method used in a magnetic resonance imaging apparatus, there is known, for example, a patented SSFP pulse sequence called FIESTA (Fast Imaging Employing Steady State Precession) and FISP (Fast Imaging with Steady-State Presence) (Patent 1). reference).

  This Balanced SSFP pulse sequence is a gradient echo system, and is a pulse sequence that completely rewinds the phase shift of transverse magnetization caused by the gradient magnetic field applied in the TR.

  Specifically, in the Balanced SSFP pulse sequence, an RF pulse is repeatedly transmitted to the subject with a TR shorter than both the longitudinal relaxation time and the lateral relaxation time of the spin included in the subject, and the spin To SSFP state. Then, after receiving the magnetic resonance signal generated by the spin in the SSFP state, an image is generated for the subject based on the magnetic resonance signal. Here, in the TR, each gradient magnetic field is applied so that the time integral value of the gradient magnetic field applied in each of the slice selection direction, the phase encoding direction, and the frequency encoding direction becomes zero. That is, the phase shifted by each gradient magnetic field is reset by rewinding the transverse magnetization after collecting the magnetic resonance signals. For this reason, in this imaging method, since the magnetic resonance signals including the FID (Free Induction Decay) signal and the echo signal are collected, the signal intensity is increased, and a high-contrast image is captured at high speed. Can be realized.

  However, in the Balanced SSFP pulse sequence described above, band artifacts may occur in the image due to the influence of magnetic field inhomogeneity, and the image quality may deteriorate.

  For this reason, in order to suppress the occurrence of band artifacts in an image, a phase cycling method has been proposed (see, for example, Patent Document 2 and Patent Document 3).

JP 2003-10148 A US Pat. No. 6,307,368 JP 2006-122222 A

  In the phase cycling method, an angle of one round (360 °) of a phase for transmitting an RF pulse is divided by the number of additions (Nex), and a phase increase angle at the time of transmitting the RF pulse is determined. Then, a plurality of scans with different RF pulse phase increase angles are performed so as to correspond to the number of additions (Nex). Then, an image is generated for each of a plurality of scans performed so as to correspond to the number of additions (Nex). Thereafter, the plurality of images are combined to generate a combined image.

  In each of the images generated in the scan performed so as to correspond to the number of additions (Nex) in the phase cycling method, positions where band artifacts are generated are shifted from each other. For this reason, in the synthesized image generated by synthesizing these images, generation of band artifacts is suppressed, and thus excellent image quality is realized.

  However, even when the phase cycling method is applied, it may be difficult to sufficiently suppress the occurrence of band artifacts. In particular, when the number of additions (Nex) is small, such as when the number of additions (Nex) is 2, this problem may become apparent.

  Therefore, the present invention provides a magnetic resonance imaging apparatus and a program that can improve image quality.

  The present invention repeatedly transmits an RF pulse at a repetition time such that the spin of the imaging region of the subject is in the SSFP state in the static magnetic field space, and the slice selection direction, the phase encoding direction, and the frequency encoding direction in the imaging region. A scanning unit that scans the imaging region so as to collect a magnetic resonance signal from the imaging region by transmitting a gradient pulse so that a time integration value becomes zero within the repetition time. An image generation unit that generates an image of the imaging region based on the magnetic resonance signal, wherein the scan unit transmits the RF pulse at a first flip angle. The scan is performed, and the first magnetic resonance signal is used as the magnetic resonance signal. And performing the scan by transmitting the RF pulse so as to have a second flip angle different from the first flip angle, and collecting a second magnetic resonance signal as the magnetic resonance signal. The image generation unit generates the first image as the image based on the first magnetic resonance signal, and generates the second image as the image based on the second magnetic resonance signal. The first image and the second image are combined by performing addition processing to generate a combined image.

  Preferably, the scanning unit transmits the RF pulse so that the first flip angle is 45 ° or more and the second flip angle is less than 45 °.

  Preferably, the scan unit performs the scan by a phase cycling method.

  Preferably, a display unit for displaying the composite image is provided.

  The present invention repeatedly transmits an RF pulse at a repetition time such that the spin of the imaging region of the subject is in the SSFP state in the static magnetic field space, and the slice selection direction, the phase encoding direction, and the frequency encoding direction in the imaging region. The scanning unit scans the imaging region so as to collect magnetic resonance signals from the imaging region by transmitting a gradient pulse so that the time integration value becomes zero within the repetition time. , A program for causing a computer to function based on the magnetic resonance signal so that the image generation unit generates an image of the imaging region, wherein the scanning unit transmits the RF pulse at a first flip angle. To perform the scan, and use the first magnetic resonance as the magnetic resonance signal. And performing the scan by transmitting the RF pulse at a second flip angle different from the first flip angle, and collecting a second magnetic resonance signal as the magnetic resonance signal, The image generation unit generates the first image as the image based on the first magnetic resonance signal, and generates the second image as the image based on the second magnetic resonance signal. The computer is caused to function by performing addition processing on the image and the second image and generating a composite image.

  Preferably, the computer is caused to function so that the scanning unit transmits the RF pulse so that the first flip angle is 45 ° or more and the second flip angle is less than 45 °.

  Preferably, the computer is caused to function so that the scan unit performs the scan by a phase cycling method.

  Preferably, the computer is caused to function so that the display unit displays the composite image.

  According to the present invention, it is possible to provide a magnetic resonance imaging apparatus and a program with improved image quality.

  Hereinafter, an example of an embodiment according to the present invention will be described with reference to the drawings.

<Embodiment 1>
(Device configuration)
FIG. 1 is a configuration diagram illustrating a configuration of a magnetic resonance imaging apparatus 1 according to the first embodiment of the present invention.

  As shown in FIG. 1, the magnetic resonance imaging apparatus 1 of the present embodiment includes a scanning unit 2 and an operation console unit 3.

  In the magnetic resonance imaging apparatus 1, the scanning unit 2 transmits an RF pulse to an imaging region of a subject in an imaging space B in which a static magnetic field is formed, and a magnetic resonance signal generated in the imaging region where the RF pulse is transmitted. Perform a scan to get. Thereafter, in the magnetic resonance imaging apparatus 1, the operation console unit 3 generates an image of the imaging region based on the magnetic resonance signal obtained by performing the scan.

  The scanning unit 2 will be described.

  As shown in FIG. 1, the scanning unit 2 includes a static magnetic field magnet unit 12, a gradient coil unit 13, an RF coil unit 14, an object moving unit 15, an RF drive unit 22, a gradient drive unit 23, The data collection unit 24 is included, and based on the control signal output from the operation console unit 3, the imaging region of the subject SU is scanned. Here, the scanning unit 2 is formed to have a cylindrical shape, for example, and accommodates the subject SU with the columnar space at the center thereof as the imaging space B. The scanning unit 2 is placed on the subject moving unit 15 in the imaging space B where the static magnetic field is formed by the static magnetic field magnet unit 12 when scanning the imaging region of the subject SU. The RF coil unit 14 transmits an RF pulse so as to excite spins in the imaging region of the subject SU, and the gradient coil unit 13 applies a gradient magnetic field to the imaging region of the subject SU to which the RF pulse has been transmitted. Then, the RF coil unit 14 receives a magnetic resonance signal generated in the imaging region of the subject SU.

  Although details will be described later, in the present embodiment, when the scanning unit 2 performs scanning on the imaging region of the subject SU, the spin of the imaging region of the subject SU is in the SSFP state in the imaging space B. The RF pulse is repeatedly transmitted at such a repetition time (TR). Here, after the elapse of the idling time in which the RF pulse is repeatedly transmitted every repetition time (TR) so that the spin in the imaging region of the subject SU is in the SSFP state in the imaging space B, the spin is in the SSFP state at the idling time. The RF pulse is repeatedly transmitted at every repetition time (TR) to the imaging region of the subject SU. At the same time, the gradient magnetic field is rewinded by transmitting a gradient pulse repeatedly in the slice selection direction, phase encoding direction, and frequency encoding direction so that the time integration value becomes zero in the imaging region. Then, magnetic resonance signals are collected from the imaging region. That is, a scan is executed by a balanced SSFP pulse sequence called FIESTA, FISP, etc., which cancels the entire gradient during 1TR in the SSFP state.

  Here, the scan unit 2 performs the first scan by transmitting the RF pulse so as to become the first flip angle FA1 in the Balanced SSFP pulse sequence, and the first magnetic resonance signal is used as the first magnetic resonance signal. Collect the resonance signal. At the same time, in the Balanced SSFP pulse sequence, the second scan is performed by transmitting the RF pulse so that the second flip angle FA2 is different from the first flip angle FA1, and the magnetic resonance signal is obtained. A second magnetic resonance signal is collected. Specifically, the scanning unit 2 transmits the RF pulse so that the first flip angle FA1 is 45 ° or more and the second flip angle FA2 is less than 45 °.

  Each component of the scanning unit 2 will be described sequentially.

  The static magnetic field magnet unit 12 is composed of a superconducting magnet (not shown), and forms a static magnetic field in the imaging space B in which the subject SU is accommodated. Here, the static magnetic field magnet unit 12 forms a static magnetic field along the body axis direction (z direction) of the subject SU placed on the subject moving unit 15. That is, it is a horizontal magnetic field type. In addition, the static magnetic field magnet unit 12 may be a vertical magnetic field type, and may form a static magnetic field along a direction in which a pair of permanent magnets face each other.

  The gradient coil unit 13 forms a gradient magnetic field in the imaging space B in which the static magnetic field is formed by the static magnetic field magnet unit 12 and adds spatial position information to the magnetic resonance signal received by the RF coil unit 14. Here, the gradient coil unit 13 includes three systems so as to correspond to the three axial directions orthogonal to each other in the x direction, the y direction, and the z direction. These transmit gradient pulses so as to form gradient magnetic fields in the frequency encoding direction, the phase encoding direction, and the slice selection direction according to the imaging conditions. Specifically, the gradient coil unit 13 applies a gradient magnetic field in the slice selection direction of the subject SU, and selects a slice of the subject SU to be excited when the RF coil unit 14 transmits an RF pulse. The gradient coil unit 13 applies a gradient magnetic field in the phase encoding direction of the subject SU, and phase encodes the magnetic resonance signal from the slice excited by the RF pulse. The gradient coil unit 13 applies a gradient magnetic field in the frequency encoding direction of the subject SU, and frequency encodes the magnetic resonance signal from the slice excited by the RF pulse.

  In the imaging space B where a static magnetic field is formed, the RF coil unit 14 transmits an RF pulse, which is an electromagnetic wave, to the imaging region of the subject SU to form a high-frequency magnetic field, and proton spins in the imaging region of the subject SU. Excited. Then, the RF coil unit 14 receives an electromagnetic wave generated from protons in the imaging region of the excited subject SU as a magnetic resonance signal. In the present embodiment, the RF coil unit 14 includes a transmission coil 14a and a reception coil 14b as shown in FIG. Here, the transmission coil 14a is, for example, a bird gauge type body coil, is arranged so as to surround the imaging region of the subject SU, and transmits an RF pulse. On the other hand, the receiving coil 14b is a surface coil and receives a magnetic resonance signal.

  The subject moving unit 15 includes a cradle 15a and a cradle moving unit 15b. Based on a control signal output from the operation console unit 3, the subject moving unit 15 moves the cradle 15a between the inside and outside of the imaging space B. The cradle moving part 15b moves. Here, the cradle 15a is a table having a placement surface on which the subject SU is placed. As shown in FIG. 1, the cradle 15a is moved in the horizontal direction xz and the up / down direction y by the cradle moving unit 15b. To the imaging space B where a static magnetic field is formed. The cradle moving unit 15b moves the cradle 15a and accommodates it inside the imaging space B from the outside. The cradle moving unit 15b includes, for example, a roller drive mechanism, and drives the roller by an actuator to move the cradle 15a in the horizontal direction xz. The cradle moving unit 15b includes, for example, an arm-type drive mechanism, and moves the cradle 15a in the vertical direction y by changing the angle between the two intersecting arms.

  The RF drive unit 22 drives the RF coil unit 14 to transmit an RF pulse in the imaging space B, thereby forming a high frequency magnetic field in the imaging space B. Based on the control signal from the operation console unit 3, the RF drive unit 22 modulates a signal from the RF oscillator (not shown) into a signal having a predetermined timing and a predetermined envelope using a gate modulator (not shown). After that, the signal modulated by the gate modulator is amplified by an RF power amplifier (not shown) and output to the RF coil unit 14 to transmit an RF pulse.

  The gradient driving unit 23 applies a gradient pulse to the gradient coil unit 13 based on a control signal from the operation console unit 3 and drives it to generate a gradient magnetic field in the imaging space B in which a static magnetic field is formed. The gradient drive unit 23 includes three systems of drive circuits (not shown) corresponding to the three systems of gradient coil units 13.

  The data collection unit 24 collects magnetic resonance signals received by the RF coil unit 14 based on the control signal from the operation console unit 3. Here, in the data collection unit 24, the phase detector (not shown) detects the magnetic resonance signal received by the RF coil unit 14 using the output of the RF oscillator (not shown) of the RF drive unit 22 as a reference signal. Thereafter, using an A / D converter (not shown), the magnetic resonance signal, which is an analog signal, is converted into a digital signal and output.

  The operation console unit 3 will be described.

  As illustrated in FIG. 1, the operation console unit 3 includes a control unit 30, a data processing unit 31, an operation unit 32, a display unit 33, and a storage unit 34.

  Each component of the operation console unit 3 will be described sequentially.

  The control unit 30 includes a computer and a memory that stores a program that causes the computer to execute predetermined data processing, and controls each unit. Here, the control unit 30 receives operation data from the operation unit 32, and based on the operation data input from the operation unit 32, the subject moving unit 15, the RF drive unit 22, the gradient drive unit 23, and data collection A scan is executed by outputting a control signal to each of the units 24. At the same time, a control signal is output to the data processing unit 31, the display unit 33, and the storage unit 34 to perform control.

  In the present embodiment, as described above, the control unit 30 controls the scan unit 2 by causing the computer to function as a program so that the scan is performed in the Balanced SSFP pulse sequence.

  The data processing unit 31 includes a computer and a memory that stores a program that executes predetermined data processing using the computer, and performs data processing based on a control signal output from the control unit 30. To do. Here, the data processing unit 31 uses the magnetic resonance signal obtained when the scan unit 2 performs the scan as raw data, and generates an image of the imaging region of the subject SU. Then, the data processing unit 31 outputs the generated image to the display unit 33. Specifically, the image is reconstructed by performing inverse Fourier transform on the magnetic resonance signals collected so as to correspond to the k space.

  In the present embodiment, as described above, the data processing unit 31 reconstructs an image from the magnetic resonance signals collected by the scan executed in the Balanced SSFP pulse sequence.

  Here, the data processing unit 31 collects as a magnetic resonance signal in the first scan performed by transmitting the RF pulse so as to be the first flip angle FA1 of 45 ° or more in the Balanced SSFP pulse sequence. A first image is generated from the first magnetic resonance signal. In addition, the data processing unit 31 collects the RF pulse as a magnetic resonance signal in the second scan performed by transmitting the RF pulse so that the second flip angle FA2 is less than 45 ° in the Balanced SSFP pulse sequence. A second image is generated from the second magnetic resonance signal. Thereafter, a synthesized image is generated by synthesizing the first image and the second image. Specifically, the data processing unit 31 synthesizes the first image and the second image by addition processing to generate a synthesized image. Then, the image data of the generated composite image is output to the display unit 33.

  The operation unit 32 is configured by operation devices such as a keyboard and a pointing device. The operation unit 32 is input with operation data by an operator and outputs the operation data to the control unit 30.

  The display unit 33 includes a display device such as an LCD (Liquid Crystal Display) or a CRT (Cathode Ray Tube), and displays an image on the display screen based on a control signal from the control unit 30. For example, the display unit 33 displays a plurality of images of input items for which operation data is input to the operation unit 32 by the operator on the display screen. In addition, the display unit 33 receives data of the image of the subject SU generated based on the magnetic resonance signal from the subject SU from the data processing unit 31 and displays the image on the display screen.

  In the present embodiment, as described above, the display unit 33 causes the data processing unit 31 to display the combined image obtained by combining the first image and the second image on the display screen.

  The storage unit 34 includes a memory and stores various data. The storage unit 34 is accessed by the control unit 30 as necessary for the stored data.

(Operation)
Hereinafter, a magnetic resonance imaging method for imaging the imaging region of the subject SU using the magnetic resonance imaging apparatus 1 of the present embodiment will be described.

  FIG. 2 is a flowchart showing an operation when imaging the imaging region of the subject SU in the first embodiment according to the present invention.

  As shown in FIG. 2, first, the subject SU is placed (S11).

  Here, the subject SU is placed on the placement surface of the cradle 15 a in the subject moving unit 15. Then, after the receiving coil 14b of the RF coil unit 14 is installed so as to correspond to the imaging region of the subject SU, the operation unit 32 sends an operation signal to the control unit 30 based on the imaging conditions input to the operation unit 32 by the operator. Output to. For example, scan parameters such as TR, TE, and flip angle α are input by the operator, and the control unit 30 sets the scan conditions based on the input scan parameters. In the present embodiment, as described above, the scan conditions are set so that the scan is performed using a Balanced SSFP pulse sequence called FIESTA or the like.

  Next, as shown in FIG. 2, the imaging region of the subject SU is carried into the imaging space B (S21).

  Here, based on the imaging conditions input to the operation unit 32, the cradle 15a on which the subject SU is placed in the subject moving unit 15 is moved into the imaging space B where the static magnetic field is formed. Then, the control unit 30 controls.

  Next, as shown in FIG. 2, the first scan IS1 is performed (S31a).

  Here, in the imaging space B in which the static magnetic field is formed, an RF pulse is transmitted to the imaging region of the subject SU so as to excite spins in the imaging region of the subject SU, and the subject to which the RF pulse is transmitted is transmitted. A gradient pulse is transmitted so as to apply a gradient magnetic field to the imaging region of the specimen SU. In this way, the first scan IS1 is performed so as to obtain a magnetic resonance signal generated in the subject SU.

  In the present embodiment, as described above, the first scan IS1 is performed in the Balanced SSFP pulse sequence.

  FIG. 3 is a pulse sequence diagram of the first scan IS1 performed in the first embodiment according to the present invention.

  In FIG. 3, RF is a time axis for transmitting RF pulses, Gslice is a time axis for transmitting gradient pulses in the slice selection direction, and Gread indicates a time axis for transmitting gradient pulses in the frequency encoding direction. Gwarp indicates a time axis for transmitting a gradient pulse in the phase encoding direction, and in each case, the horizontal axis indicates time t and the vertical axis indicates pulse intensity.

  As shown in FIG. 3, when the first scan IS1 is performed, the RF pulse RF is repeatedly transmitted to the imaging region of the subject SU. Here, the scan unit 2 repeatedly transmits each RF pulse RF at a repetition time TR such that the longitudinal magnetization and transverse magnetization of the spin are in a steady state in the subject SU.

  Along with this, gradient pulses are repeatedly transmitted within the time TR in each of the slice selection direction, the frequency encoding direction, and the phase encoding direction.

  Here, gradient pulses in the slice selection direction, the frequency encoding direction, and the phase encoding direction are transmitted to the subject SU so that the time integration value within the repetition time TR becomes zero. That is, as shown in FIG. 3, the transverse magnetization is rewinded so as to reset the phase shifted by the gradient magnetic field within the repetition time TR after acquiring the magnetic resonance signal so as to correspond to the echo time.

  When performing the first scan IS1, first, in the idling time, the RF pulse is repeatedly repeated so that the spin in the imaging region of the subject SU is in the SSFP state in the imaging space B in which the static magnetic field is formed. Transmit repeatedly for each TR. That is, the RF pulse is blanked to form the SSFP state. Thereafter, an RF pulse is repeatedly transmitted to the imaging region in which the spin is in the SSFP state at the idling time for each repetition time TR, and a magnetic resonance signal generated in the imaging region is received. That is, after the RF pulse is blanked, the magnetic resonance signal is collected.

  In the present embodiment, in the Balanced SSFP pulse sequence, the first scan IS1 is performed by repeatedly transmitting the RF pulse so as to become the first flip angle FA1, and the subject SU is used as a magnetic resonance signal. First magnetic resonance signals are sequentially collected from the imaging region. Specifically, the first scan IS1 is performed with the first flip angle FA1 set to 45 ° or more.

  For example, as shown in FIG. 3, the first scan IS1 is performed with the first flip angle FA1 set to 60 °. In addition, here, as shown in FIG. 3, the first scan IS1 is performed at a repetition time TR with the phase increase angle for increasing the phase for transmitting the RF pulse being 180 °. That is, the RF pulse is repeatedly transmitted so that the phase Φ of the RF pulse in each TR sequentially changes to 0 °, 180 °, 0 ° (360 °), 180 ° (540 °),. Thus, the first scan IS1 is performed. Then, the magnetic resonance signals generated by performing the first scan IS1 in this way are sequentially collected as the first magnetic resonance signals so as to correspond to the k space.

  Next, as shown in FIG. 2, the second scan IS2 is performed (S31b).

  Here, in the above Balanced SSFP pulse sequence, the second scan IS2 is performed by repeatedly transmitting the RF pulse so that the second flip angle FA2 is different from the first flip angle FA1, and the magnetic resonance is performed. As a signal, second magnetic resonance signals are sequentially collected from the imaging region of the subject SU. Specifically, the second scan IS2 is performed by setting the second flip angle FA2 to less than 45 °.

  FIG. 4 is a pulse sequence diagram of the second scan IS2 performed in the first embodiment according to the present invention.

  As shown in FIG. 4, the second scan IS2 is performed with the second flip angle FA2 set to 6 °. Then, as shown in FIG. 4, the second scan IS2 is performed at a repetition time TR with the phase increase angle for increasing the phase for transmitting the RF pulse being 180 °. That is, the second scan IS2 is performed so as to satisfy the same scan conditions except that the flip angle is different from that of the first scan IS1. Then, magnetic resonance signals generated by performing the second scan IS2 in this way are sequentially collected as second magnetic resonance signals so as to correspond to the k space.

  Next, as shown in FIG. 2, a first image I1 is generated (S41a).

  Here, in the above-described Balanced SSFP pulse sequence, the first collected as a magnetic resonance signal in the first scan IS1 performed by transmitting the RF pulse so as to become the first flip angle FA1 of 45 ° or more. The data processing unit 31 reconstructs the first image I1 from the magnetic resonance signals.

  FIG. 5 is a diagram illustrating the first image I1 generated from the magnetic resonance signals collected in the first scan IS1 performed at the flip angle of 60 ° in the first embodiment according to the present invention.

  As indicated by a region R1 surrounded by a dotted line in FIG. 5, a band artifact occurs in the first image I1.

  Next, as shown in FIG. 2, a second image I2 is generated (S41b).

  Here, in the above SSFP pulse sequence, the second collected as a magnetic resonance signal in the second scan IS2 performed by transmitting the RF pulse so as to be the second flip angle FA2 of less than 45 °. The data processing unit 31 generates the second image I2 from the magnetic resonance signal by reconstructing the second image I2.

  FIG. 6 is a diagram showing a second image I2 generated from the magnetic resonance signal collected in the second scan IS2 performed at the flip angle of 6 ° in the first embodiment according to the present invention.

  As indicated by a region R2 surrounded by a dotted line in FIG. 6, a band artifact has occurred in the second image I2.

  Next, as shown in FIG. 2, a composite image CI is generated (S51).

  Here, the first image I1 and the second image I2 generated as described above are combined by the data processing unit 31 performing addition processing to generate a composite image CI. For example, the composite image CI is generated by subjecting the first image I1 and the second image I2 to an averaging process of pixel values between the pixels. That is, the composite image CI is generated so that the average value of the pixel values of the pixels at the same position in the first image I1 and the second image I2 becomes the pixel value of each pixel of the composite image CI.

  FIG. 7 is a diagram showing the composite image CI generated in the first embodiment according to the present invention.

  As indicated by a region R3 surrounded by a dotted line in FIG. 7, band artifacts that have occurred in the first image I1 and the second image I2 are suppressed in the composite image CI.

  FIG. 8 shows the first signal SG1 in the first scan IS1 performed by transmitting the 60 ° flip angle RF pulse and the 6 ° flip angle RF pulse in the first embodiment of the present invention. 2 shows the second signal SG2 in the second scan IS2 performed by transmitting and the synthesized signal SG3 obtained by adding and synthesizing the first signal SG1 and the second signal SG2. FIG.

  In FIG. 8, the relationship between the signal intensity I of the first signal SG1 or the second signal SG2 and the phase shift amount Φ caused by the effect of static magnetic field inhomogeneity is expressed by the following equations (1), (2 ), (3), (4), (5), (6), (7), (8), the simulation results are shown, the horizontal axis is the phase shift amount B, and the vertical axis The axis is the signal intensity I. Here, the result of each signal strength I of the first signal SG1 and the second signal SG2 is obtained by synthesizing myFID and mxFID shown in the following mathematical formula. FIG. 8 further shows the signal intensity I of the combined signal SG3 obtained by adding and combining the signal intensity I of the first signal SG1 and the second signal SG2.

  In equations (1), (2), (3), and (4), mySSFP is the value of transverse magnetization that occurs in the y direction in the steady state immediately before transmitting the RF pulse, and mxSSFP is the RF pulse. Is the value of the transverse magnetization that occurs in the x direction in the steady state just before transmitting. Also, myFID is the value of the transverse magnetization in the y direction of the FID signal generated immediately after the RF pulse is transmitted and the spin is excited, and mxFID is the x direction of the FID signal generated immediately after the RF pulse is transmitted and the spin is excited. Is the value of transverse magnetization at. In the equations (1), (2), (3), and (4), E1, E2, C3, and C4 are represented by equations (5), (6), (7), and (8). Further, A is the flip angle of the RF pulse, and B is the phase shift amount that the transverse magnetization receives due to the influence of the non-uniform static magnetic field. Then, for the case where the RF transmission phase transitions as 0 ° → 180 ° → 0 → 180 ° →..., The following equation was obtained by solving the Bloch equation in the rotating coordinate system.

[Equation 1]
mySSFP = Mo (1-E1) (E2 · sinA · cosB + E2 · E2 · sinA) / (C3-C4) (1)

[Equation 2]
mxSSFP = Mo (1-E1) (E2 · sinA · sinB) / (C3-C4) (2)

[Equation 3]
myFID = −Mo (1−E1) ((1 + E2 · cosB) · sinA) / (C3−C4) (3)

[Equation 4]
mxFID = mxSSFP (4)

[Equation 5]
C3 = (1−E1 · cosA) (1 + E2 · cosB) (5)

[Equation 6]
C4 = E2 (E1-cosA) (E2 + cosB) (6)

[Equation 7]
E1 = exp {-TR / T1} (7)

[Equation 8]
E2 = exp {−TR / T2} (8)

  The first signal SG1 is a signal obtained in the first scan IS1 performed by transmitting an RF pulse with a flip angle of 60 °, and has a small phase shift amount as shown by a thin solid line in FIG. In the range CE, the signal intensity is substantially uniform, whereas in the range SE where the phase shift amount is larger than the phase shift amount in the center range CE, the signal intensity is drastically decreased. For this reason, in the first image I1, a band artifact occurs in a portion where the phase shift amount is large, as indicated by a region R1 surrounded by a dotted line in FIG.

  On the other hand, the second signal SG2 is a signal obtained in the second scan IS2 performed by transmitting an RF pulse having a flip angle of 6 °, and has a small amount of phase shift as shown by a dotted line in FIG. In the center range CE, the signal strength is substantially uniform, whereas in the range SE where the phase shift amount is larger than the phase shift amount in the center range CE, the signal strength increases rapidly. That is, the signal strength increases in the off-resonance state. For this reason, in the second image I2, a band artifact occurs in a portion where the phase shift amount becomes large due to the influence of the static magnetic field inhomogeneity, as indicated by a region R2 surrounded by a dotted line in FIG.

  However, as shown by a thick solid line in FIG. 8, a synthesized signal SG3 obtained by adding and averaging the first signal SG1 and the second signal SG2 is a center range CE with a small amount of phase shift, In the range SE where the phase shift amount is larger than the phase shift amount of the center range CE, the signal intensity changes substantially uniformly. For this reason, in the synthesized image CI obtained above, the occurrence of band artifacts is suppressed and the image quality is improved, as indicated by the region R3 surrounded by the dotted line in FIG.

  In particular, in the present embodiment, the first flip angle FA1 of the RF pulse in the first scan IS1 is set to 45 ° or more, and the second flip angle FA2 of the RF pulse in the second scan IS2 is set to be less than 45 °. Therefore, in the range where the signal intensity decreases in the first scan IS1, the signal intensity increases in the second scan IS2. Therefore, it is possible to effectively suppress the occurrence of band artifacts in the composite image CI.

  Next, as shown in FIG. 2, the composite image CI is displayed (S61).

  Here, as described above, the display unit 33 displays the composite image CI generated by the data processing unit 31 on the display screen.

  As described above, in the present embodiment, the first scan IS1 is performed by transmitting the RF pulse so as to be the first flip angle FA1 in the Balanced SSFP pulse sequence, and the first scan IS1 is used as the magnetic resonance signal. The second scan IS2 is performed by collecting the magnetic resonance signal and transmitting the RF pulse in the Balanced SSFP pulse sequence so that the second flip angle FA2 is different from the first flip angle FA1, A second magnetic resonance signal is collected as the magnetic resonance signal. Thereafter, the first image I1 is generated based on the first magnetic resonance signal, and the second image I2 is generated based on the second magnetic resonance signal. Then, the synthesized image C1 is generated by synthesizing the first image I1 and the second image I2. For this reason, as described above, generation of band artifacts is suppressed in the composite image CI, and excellent image quality is realized.

  In the present embodiment, the RF pulse is transmitted so that the first flip angle FA1 is 45 ° or more and the second flip angle FA2 is less than 45 °. In this way, by specifying the first flip angle FA1 and the second flip angle FA2, the first flip angle FA1 and the second flip angle FA2 in the range of the flip angle of 0 ° to 90 °. Therefore, the occurrence of band artifacts can be more effectively suppressed.

  Therefore, in this embodiment, the image quality can be improved.

<Embodiment 2>
Hereinafter, Embodiment 2 according to the present invention will be described.

  In the present embodiment, the first scan IS1 and the second scan IS2 performed in the first embodiment are performed by the phase cycling method. Except for this point, the present embodiment is the same as the first embodiment. Therefore, the description about the overlapping part is abbreviate | omitted.

  FIG. 9 is a diagram illustrating the phase increase angle of the RF pulse transmitted when the first scan IS1 and the second scan IS2 are performed by the phase cycling method in the second embodiment according to the present invention.

  In the phase cycling method, as shown in FIG. 9, the angle of one round (360 °) is divided by the number of additions (Nex) to determine the phase increase angle. Then, the scan is performed a plurality of times while changing the phase increase angle, and an image is obtained for each scan. Thereafter, the plurality of images are combined to generate a combined image.

  Specifically, when performing the first scan IS1 for transmitting the RF pulse of the first flip angle FA1, for example, as shown in FIG. 9, the phase cycling method is performed under the condition of 2Nex.

  In this case, first, the first scan IS1a for a plurality of additions is performed with the phase increase angle, which is the phase angle increased for each TR, set to 0 °.

  FIG. 10 is a pulse sequence diagram of the first scan IS1a under the condition of 2Nex in the first scan IS1 in the second embodiment according to the present invention.

  As shown in FIG. 10, when the first scan IS1a is performed for a plurality of addition times in the first scan IS1, the phase of the RF pulse in each TR is 0 °, 0 °,. Repeatedly transmit the RF pulse so that it sequentially changes to.

  Next, in the first scan IS1, as shown in FIG. 9, the second scan IS1b in a plurality of additions is performed with the phase increase angle being 180 °.

  FIG. 11 is a pulse sequence diagram of the second scan IS1b in the second scan IS1 in the first scan IS1 in the second embodiment according to the present invention.

  As shown in FIG. 11, when the second scan IS1b in a plurality of addition times is performed in the first scan IS1, the phase of the RF pulse in each TR is 0 °, 180 °, 360 ° ( The RF pulse is repeatedly transmitted so as to sequentially shift to 0 °), 540 ° (180 °),. That is, the second scan IS1b is performed in the same manner as in the first embodiment.

  The addition number Nex generates the first image I1a based on the magnetic resonance signal obtained in the execution of the first scan IS1a, and the addition number Nex is the execution of the second scan IS1b. The second image I1b is generated based on the magnetic resonance signal obtained in step (1).

  After that, between the first image I1a and the second image I1b, pixel values of pixels corresponding to each other are combined by performing an averaging process, and the combined image is the first image I1. And

  Next, the second scan IS2 for transmitting the RF pulse of the second flip angle FA2 is performed under the condition of 2Nex similarly to the first scan IS1.

  That is, as shown in FIG. 9, as in the case of the first scan IS1, the phase increase angle, which is the phase angle to be increased for each TR, is set to 0 °, and the number of additions in the second scan IS2 is first The second scan IS2a is performed.

  FIG. 12 is a pulse sequence diagram of the first scan IS2a at a plurality of addition times in the second scan IS2 in the second embodiment according to the present invention.

  As shown in FIG. 12, in the second scan IS2, when the first scan IS2a in a plurality of addition times is performed, the phase of the RF pulse in each TR is 0 °, 0 °,.・ Repeatedly transmit RF pulses so as to change sequentially.

  Next, in the second scan IS2, as shown in FIG. 9, the second scan IS2b in a plurality of additions is performed with the phase increase angle being 180 °.

  FIG. 13 is a pulse sequence diagram of the second scan IS2b in the second scan IS2 in the second scan IS2 according to the present invention at a plurality of addition times.

  As shown in FIG. 13, in the second scan IS2, when the second scan IS2b is performed at a plurality of addition times, the phase of the RF pulse in each TR is 0 °, 180 °, 360 °. The RF pulse is repeatedly transmitted so as to sequentially shift to (0 °), 540 ° (180 °),.

  In the second scan IS2, the first image I2a is generated based on the magnetic resonance signal obtained in the first scan IS2a, and the second scan IS2b is performed. A second image I2b is generated based on the magnetic resonance signal obtained in this manner.

  After that, between the first image I2a and the second image I2b, the pixel values of the pixels corresponding to each other are combined by performing an averaging process, and the combined image is the second image I2. And

  Then, in the same manner as in the first embodiment, the first image I1 and the second image I2 are combined to generate a combined image CI, and then displayed on the display screen.

  As described above, in the present embodiment, as in the first embodiment, a plurality of scans IS1 and IS2 that transmit RF pulses of a plurality of different flip angles FA1 and FA2 are performed in a Balanced SSFP pulse sequence, In each of the scans IS1 and IS2, a first image I1 and a second image I2 are generated. Then, the synthesized image C1 is generated by synthesizing the first image I1 and the second image I2. For this reason, as described above, in this composite image CI, occurrence of band artifacts is suppressed, and excellent image quality is realized.

  Further, in the present embodiment, each of the first and second scans IS1 and IS2 is performed so as to correspond to a plurality of addition times (Nex) in the phase cycling method. In the phase cycling method, between the images obtained at a plurality of addition times (Nex) (between the first image I1a and the second image I1b in the first scan IS1, or the second In the second scan IS2, between the first image I2a and the second image I2b), positions where band artifacts occur are shifted from each other. For this reason, in each of the first image I1 and the second image I2 generated by synthesizing these images, occurrence of band artifacts is suppressed, and excellent image quality is realized. Therefore, the synthesized image C1 generated by synthesizing the first image I1 and the second image I2 in which the occurrence of the band artifact is suppressed further suppresses the occurrence of the band artifact, and thus a higher quality image. Can be obtained.

  FIG. 14 shows the first signal SG1 in the first scan IS1 performed by transmitting the 60 ° flip angle RF pulse and the 6 ° flip angle RF pulse in the second embodiment of the present invention. 2 shows the second signal SG2 in the second scan IS2 performed by transmitting and the synthesized signal SG3 obtained by adding and synthesizing the first signal SG1 and the second signal SG2. FIG.

  In FIG. 14, as in the case of FIG. 8, the result of calculating the relationship between the signal intensity of the first signal SG1 or the second signal SG2 and the amount of phase shift caused by the influence of static magnetic field inhomogeneity is shown. The horizontal axis represents the phase shift amount, and the vertical axis represents the signal intensity.

  Here, FIG. 14A shows the first signal SG1, the second signal SG2, and the combined signal SG3 obtained by the first scans IS1a and IS2a under the condition of 2Nex. That is, obtained by the first scan IS1, which is performed by repeatedly transmitting the RF pulse so that the phase of the RF pulse in each TR sequentially changes at 0 °, 0 °,. In the same manner, the first signal SG1 obtained and the second signal SG2 obtained by the second scan IS2 are shown and obtained by synthesizing the first signal SG1 and the second signal SG2. The synthesized signal SG3 is shown.

  On the other hand, FIG. 14B shows the first signal SG1, the second signal SG2, and the combined signal SG3 obtained by the second scan IS1b, IS2b under the condition of 2Nex. That is, it is implemented by repeatedly transmitting the RF pulse so that the phase of the RF pulse in each TR sequentially changes at 0 °, 180 °, 0 °, 180 °,... The first signal SG1 obtained by the second scan IS1 and the second signal SG2 obtained by the second scan IS2 in the same manner, and the first signal SG1 and the second signal SG2 Is a synthesized signal SG3 obtained by synthesizing.

  As shown by a thin solid line in FIG. 14A, the first signal SG1 obtained by the first scan IS1 that transmits the RF pulse having a flip angle of 60 ° has a center range CE and a small phase shift amount. In contrast, the signal strength is rapidly decreased, whereas the signal strength is uniform in the range SE in which the phase shift amount is larger than the phase shift amount in the center range CE. On the other hand, as shown by a dotted line in FIG. 14A, the second signal SG2 obtained by the second scan IS2 that transmits the RF pulse having the flip angle of 6 ° has the center range CE with a small phase shift amount. In this case, the signal intensity rapidly increases, whereas the signal intensity is uniform in the range SE in which the phase shift amount is larger than the phase shift amount in the center range CE. However, as indicated by a thick solid line in FIG. 14A, the synthesized signal SG3 obtained by adding and synthesizing the first signal SG1 and the second signal SG2 has a small phase shift amount. In the range CE and the range SE in which the phase shift amount is larger than the phase shift amount of the center range CE, the signal intensity changes substantially uniformly.

  Further, as shown by a thin solid line in FIG. 14B, the first signal SG1 obtained by the first scan IS1 that transmits the RF pulse having a flip angle of 60 ° has a central range with a small phase shift amount. In CE, the signal strength is uniform, whereas in the range SE where the phase shift amount is larger than the phase shift amount in the center range CE, the signal strength rapidly decreases. On the other hand, as indicated by the dotted line in FIG. 14B, the second signal SG2 obtained by the second scan IS2 that transmits the RF pulse having the flip angle of 6 ° has the center range CE with a small phase shift amount. In this case, the signal intensity is uniform, whereas in the range SE where the phase shift amount is larger than the phase shift amount of the center range CE, the signal intensity rapidly increases. However, as shown by a thick solid line in FIG. 14B, the synthesized signal SG3 obtained by adding and synthesizing the first signal SG1 and the second signal SG2 has a central range with a small amount of phase shift. In the CE and the range SE in which the phase shift amount is larger than the phase shift amount in the center range CE, the signal intensity changes substantially uniformly.

  For this reason, in each of the first image I1 and the second image I2 generated as described above, occurrence of band artifacts is suppressed, and excellent image quality is realized. And since the synthetic | combination image CI produced | generated by synthesize | combining the 1st image I1 in which generation | occurrence | production of this band artifact is suppressed, and the 2nd image I2 will further suppress generation | occurrence | production of a band artifact, High quality images can be obtained.

  Therefore, in this embodiment, the image quality can be improved.

  In implementing the present invention, the present invention is not limited to the above-described embodiment, and various modifications can be employed.

  For example, in the above-described embodiment, a case has been described in which scans that transmit RF pulses having two different flip angles are performed, and each of the images obtained by the scans is combined. However, the present invention is not limited to this. For example, the present invention may be applied to a case where scans that transmit RF pulses having three or more different flip angles are performed and images obtained by the scans are combined.

  In the above embodiment, the case where the phase cycling method is applied under the condition of 2Nex where the number of additions is 2 has been described, but the present invention is not limited to this. The phase cycling method may be applied under the condition that the number of additions is 3 or more.

  Further, in the above embodiment, after performing addition processing for each corresponding pixel value between the first image I <b> 1 and the second image I <b> 2, addition averaging is performed for each of the added pixel values. Although the case where the composite image CI is generated by performing the processing has been described, the present invention is not limited to this. For example, even when only the addition process is performed in the above and the averaging process is not performed, the band artifact can be effectively reduced.

FIG. 1 is a configuration diagram illustrating a configuration of a magnetic resonance imaging apparatus 1 according to the first embodiment of the present invention. FIG. 2 is a flowchart showing an operation when imaging the imaging region of the subject SU in the first embodiment according to the present invention. FIG. 3 is a pulse sequence diagram of the first scan IS1 performed in the first embodiment according to the present invention. FIG. 4 is a pulse sequence diagram of the second scan IS2 performed in the first embodiment according to the present invention. FIG. 5 is a diagram illustrating the first image I1 generated from the magnetic resonance signals collected in the first scan IS1 performed at the flip angle of 60 ° in the first embodiment according to the present invention. FIG. 6 is a diagram showing a second image I2 generated from the magnetic resonance signal collected in the second scan IS2 performed at the flip angle of 6 ° in the first embodiment according to the present invention. FIG. 7 is a diagram showing the composite image CI generated in the first embodiment according to the present invention. FIG. 8 shows the first signal SG1 in the first scan IS1 performed by transmitting the 60 ° flip angle RF pulse and the 6 ° flip angle RF pulse in the first embodiment of the present invention. The second signal SG2 in the second scan IS2 performed by transmitting the signal, and the synthesized signal SG3 obtained by adding and averaging the first signal SG1 and the second signal SG2 FIG. FIG. 9 is a diagram illustrating the phase increase angle of the RF pulse transmitted when the first scan IS1 and the second scan IS2 are performed by the phase cycling method in the second embodiment according to the present invention. FIG. 10 is a pulse sequence diagram of the first scan IS1a under the condition of 2Nex in the first scan IS1 in the second embodiment according to the present invention. FIG. 11 is a pulse sequence diagram of the second scan IS1b in the second scan IS1 in the first scan IS1 in the second embodiment according to the present invention. FIG. 12 is a pulse sequence diagram of the first scan IS2a at a plurality of addition times in the second scan IS2 in the second embodiment according to the present invention. FIG. 13 is a pulse sequence diagram of the second scan IS1b in the second scan IS2 in the second scan IS2 according to the present invention at a plurality of addition times. FIG. 14 shows the first signal SG1 in the first scan IS1 performed by transmitting the 60 ° flip angle RF pulse and the 6 ° flip angle RF pulse in the second embodiment of the present invention. The second signal SG2 in the second scan IS2 performed by transmitting the signal, and the synthesized signal SG3 obtained by adding and averaging the first signal SG1 and the second signal SG2 FIG.

Explanation of symbols

1: Magnetic resonance imaging apparatus (magnetic resonance imaging apparatus)
2: Scan part (scan part),
3: Operation console part,
12: Static magnetic field magnet section,
13: Gradient coil part,
14: RF coil section,
15: Cradle,
22: RF drive unit,
23: Gradient drive unit,
24: Data collection unit,
30: control unit,
31: Data processing unit,
32: Operation unit,
33: Display unit (display unit),
34: Storage unit
B: Imaging space (static magnetic field space)

Claims (8)

In the static magnetic field space, the RF pulse is repeatedly transmitted at a repetition time such that the spin of the imaging region of the subject is in the SSFP state, and in the imaging region, the slice selection direction, the phase encoding direction, and the frequency encoding direction, A scanning unit that scans the imaging region so as to collect magnetic resonance signals from the imaging region by transmitting a gradient pulse so that a time integration value becomes zero within the repetition time;
An image generation unit that generates an image of the imaging region based on the magnetic resonance signal,
The scan unit performs the scan by transmitting the RF pulse to have a first flip angle, collects the first magnetic resonance signal as the magnetic resonance signal, and transmits the RF pulse to the first pulse. Performing the scan by transmitting a second flip angle different from the one flip angle, collecting a second magnetic resonance signal as the magnetic resonance signal,
The image generation unit generates the first image as the image based on the first magnetic resonance signal, and generates the second image as the image based on the second magnetic resonance signal. A magnetic resonance imaging apparatus that combines one image and the second image by performing addition processing to generate a combined image. The scanning unit transmits the RF pulse so that the first flip angle is 45 ° or more and the second flip angle is less than 45 °.
The magnetic resonance imaging apparatus according to claim 1. The scan unit performs the scan by a phase cycling method.
The magnetic resonance imaging apparatus according to claim 1 or 2. A display unit for displaying the composite image;
The magnetic resonance imaging apparatus according to claim 1. In the static magnetic field space, the RF pulse is repeatedly transmitted at a repetition time such that the spin of the imaging region of the subject is in the SSFP state, and in the imaging region, the slice selection direction, the phase encoding direction, and the frequency encoding direction, A scanning unit scans the imaging region so as to collect magnetic resonance signals from the imaging region by transmitting a gradient pulse so that a time integration value becomes zero within the repetition time, and the magnetic resonance A program that causes a computer to function based on a signal so that the image generation unit generates an image of the shooting area;
The scanning unit performs the scan by transmitting the RF pulse at a first flip angle, collects the first magnetic resonance signal as the magnetic resonance signal, and also transfers the RF pulse to the first flip Performing the scan by transmitting at a second flip angle different from the angle, collecting the second magnetic resonance signal as the magnetic resonance signal,
The image generation unit generates the first image as the image based on the first magnetic resonance signal, and generates the second image as the image based on the second magnetic resonance signal. A program that causes the computer to function by combining an image and the second image by performing addition processing to generate a combined image. Causing the computer to function so that the scan unit transmits the RF pulse so that the first flip angle is 45 ° or more and the second flip angle is less than 45 °;
The program according to claim 5. Causing the computer to function so that the scan unit performs the scan by a phase cycling method;
The program according to claim 5 or 6. Causing the computer to function so that the display unit displays the composite image;
The program according to any one of claims 5 to 7.