JP2013075198A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus capable of changing an image contrast with a pulse for a prescribed purpose such as labeling, fat control, or inversion while keeping the phase continuity of spins in imaging using an SSFP sequence.SOLUTION: The magnetic resonance imaging apparatus includes a data collection means and an image data generating means. The data collection means collects data according to a sequence derived by adding a coherent control pulse, which has a center substantially at the center time between adjacent high frequency excitations and in which the zero-order moment amount is zero, on a Steady-State Free Precession pulse sequence for repeating high frequency excitations at regular intervals a plurality of times. The image data generation means generates image data from the collected data. A region for applying the coherent control pulse can be optionally set independently from an imaging region.

Description

  The present invention magnetically excites a nuclear spin of a subject with a radio frequency (RF) signal having a Larmor frequency, and reconstructs an image from a magnetic resonance (MR) signal generated by the excitation. The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance imaging apparatus that acquires a magnetic resonance image using steady-state free precession (SSFP).

  Magnetic Resonance Imaging (MRI) magnetically excites a subject's nuclear spin placed in a static magnetic field with an RF signal at the Larmor frequency, and reconstructs the image from the MR signal generated by this excitation. It is the imaging method to comprise. Magnetic resonance imaging devices operate according to a pulse sequence that defines imaging conditions. However, in recent magnetic resonance imaging devices, a pulse sequence (SSFP sequence) using a phenomenon called SSFP is used for cardiac cine imaging and coronary artery imaging. Often used.

  FIG. 1 is a diagram showing a conventional SSFP sequence.

  In FIG. 1, RF is an RF signal transmitted to the subject, Gss is a slice gradient magnetic field pulse applied to the subject for slice selection, and Gro is read out of echo data from the subject ( The RO gradient magnetic field pulse (also referred to as frequency encoding gradient magnetic field pulse) applied to the subject for RO (readout), Gpe is the PE gradient magnetic field pulse applied to the subject for phase encoding (PE). Respectively.

  The SSFP sequence collects echo data after repeating RF excitation and setting the magnetization spin in a static magnetic field to a steady state. Specifically, as shown in FIG. 1, in the SSFP sequence, + α / 2 excitation pulse or −α / 2 excitation pulse for start-up is applied, followed by + α at a constant repetition time (TR). Excitation pulses (flip pulses) and -α excitation pulses are alternately applied repeatedly. The magnetization spin in the static magnetic field is maintained in a steady state by applying the + α excitation pulse and the −α excitation pulse. In general, a ± α excitation pulse is applied together with the slice gradient magnetic field pulse Gss in order to selectively excite a desired slice. Further, an RO gradient magnetic field pulse Gro and a PE gradient magnetic field pulse Gpe are applied to add spatial information. In some cases, the + α / 2 excitation pulse or the −α / 2 excitation pulse for start-up is not applied.

  The integral value in TR of the slice gradient magnetic field pulse Gss, the RO gradient magnetic field pulse Gro, and the PE gradient magnetic field pulse Gpe is controlled to be zero. The application phase of the excitation pulse is controlled to shift by a certain angle. Usually, the constant angle is set to 180 ° with respect to the on-resonance spin.

  Since such an SSFP sequence does not spoil part of the signal, it is considered to be a great advantage that an image with a relatively high SN (signal to noise) ratio can be acquired at high speed. The SSFP sequence can obtain a constant image contrast by maintaining a steady state, but on the other hand, it is possible to obtain a different image contrast by taking advantage of being able to acquire an image with a high SN ratio at high speed. The demand is high.

  One example where different image contrast is required is a fat-suppressed SSFP image. Since fat has a relatively short longitudinal relaxation (T1) time, it is known that a strong signal can be obtained from fat in a steady state. Therefore, when an image of the abdomen is to be acquired by the SSFP sequence, it may be difficult to draw a structure or a lesion from a fat having a high signal intensity. Therefore, when an abdominal image is acquired by the SSFP sequence, it is required to suppress signals from fat.

  On the other hand, a technique for suppressing a signal from fat by providing a section called a preparation block in the SSFP sequence and applying a prepulse for fat suppression in the preparation block has been devised (see, for example, Non-Patent Document 1).

  FIG. 2 is a diagram showing a conventional SSFP sequence with a preparation block.

  FIG. 2 shows an RF signal applied to the subject in time series. As shown in FIG. 2, in the SSFP sequence with a preparation block, an excitation pulse sequence having a constant excitation angle ± α is applied following the application of a startup α- / 2 excitation pulse. In the middle, a + α / 2 flip-back pulse is applied. After applying the + α / 2 flip-back pulse, a section called a preparation block is provided, and after the preparation block, a + α / 2 startup pulse is applied. Then, after applying the + α / 2 start-up pulse, the application of the α excitation pulse sequence whose excitation angle is constant angle α is resumed.

  In the preparation block, a so-called RF prepulse such as a fat suppression pulse is applied to suppress the fat signal. This method is characterized in that a preparation block is provided in a state where the spin is returned to the longitudinal magnetization by the α / 2 flip back pulse. In the preparation block, a spoiler gradient magnetic field pulse for spoiling transverse magnetization is applied in addition to the normal RF prepulse. Thus, a preparation block is provided to change the image contrast in the SSFP sequence.

  Conventionally, the SSFP sequence is sometimes used for imaging by inversion recovery (IR). IR is an imaging method in which a 180 ° IR pulse is applied to obtain an image having a signal intensity that depends on recovery by T1 from a state in which spin is inverted.

  FIG. 3 is a diagram showing a conventional SSFP sequence with an IR pulse.

  In FIG. 3, RF represents an RF signal transmitted to the subject, Gss represents a slice gradient magnetic field pulse, Gro represents an RO gradient magnetic field pulse, and Gpe represents a PE gradient magnetic field pulse.

  When applying the IR pulse in the SSFP sequence, as shown in FIG. 3, the application of the α excitation pulse continuously applied by TR is temporarily stopped. Then, after the spoiler pulse is applied following the IR pulse, the application of the continuous excitation pulse is resumed. Note that a start-up pulse and a flip-back pulse may be used together.

  Furthermore, a global coherent free precession (GCFP) method has been proposed as a tagging technique applicable to SSFP sequences. The GCFP method is a technique that applies coherent spin labeling, and is a technique that captures only protons at high frequency, that is, tags, when protons of water molecules in blood cells pass through an MRI scan section.

Scheffler et al., Magnetic Resonance in Medicine, 45, 1075-1080, 2001

  In an SSFP sequence with a conventional preparation block, T1 relaxation of spin proceeds between an α / 2 flip-back pulse and an α / 2 start-up pulse. Further, in the preparation block, the continuity of the phase of the spin is destroyed by applying an RF pre-pulse or a spoiler gradient magnetic field pulse. It has been reported that the effects of the progress of spin T1 relaxation and the disruption of spin phase continuity are small when they are in a water component with a long T1. However, there is a problem that the effects of the progression of spin T1 relaxation and the disruption of spin phase continuity cannot be ignored in tissues with a short T1 in the living body.

  That is, in the course of the SSFP sequence, application of an α / 2 flip-back pulse or α / 2 start-up pulse and preparation block occur, and T1 relaxation occurs, and phase continuity is interrupted by the preparation block. The disruption of T1 relaxation and phase continuity is a factor that changes the image contrast obtained by the SSFP sequence and may result in image artifacts due to deviations from the SSFP state.

  Therefore, there is a need for a method for simultaneously realizing the phase continuity of the spin and the image contrast change due to the RF prepulse in the SSFP sequence.

  Further, in the conventional SSFP sequence with an IR pulse, the application of the α excitation pulse is interrupted, so that the spin immediately after the application of the excitation pulse is resumed is different from the steady state. As a result, there is a problem that an image contrast change and artifacts are generated.

Furthermore, in the GCFP method, the slice gradient magnetic field and the RO gradient magnetic field are common and fixed. In addition, the GCFP method has a drawback that the PE gradient magnetic field pulse is limited in the orthogonal direction to the slice and the radial acquisition cannot be performed. The present invention was made to cope with such a conventional situation, and uses the SSFP sequence. It is an object of the present invention to provide a magnetic resonance imaging apparatus capable of changing image contrast by a pulse for a desired purpose such as labeling, fat suppression, inversion, etc. while maintaining spin phase continuity. To do.

The magnetic resonance imaging apparatus according to the present invention has a zeroth-order moment substantially centered on a central time between adjacent high-frequency excitations on a Steady-State Free Precession pulse sequence in which a plurality of high-frequency excitations are repeated at regular intervals. Data collection means for collecting data according to a sequence to which a coherent control pulse is added so that the amount becomes zero;
Image data generating means for generating image data from the data, and the application area of the coherent control pulse can be arbitrarily set independently of the imaging area.

  The magnetic resonance imaging apparatus according to the present invention changes the image contrast by a pulse for a desired purpose such as labeling, fat suppression, and inversion while maintaining the phase continuity of spin in imaging using the SSFP sequence. Can do.

The figure which shows the conventional SSFP sequence. The figure which shows the SSFP sequence with the conventional preparation block. The figure which shows the SSFP sequence with the conventional IR pulse. 1 is a configuration diagram showing an embodiment of a magnetic resonance imaging apparatus according to the present invention. FIG. 5 is a functional block diagram of the computer shown in FIG. 4. FIG. 6 is a diagram showing an SSFP sequence with application of an RF coherent control pulse set in the imaging condition setting unit shown in FIG. 5. The figure which shows the inclination of the spin which changes by execution of the SSFP sequence which has coherent control block shown in FIG. FIG. 6 is a diagram showing an SSFP sequence with application of an RF coherent control pulse and a slice gradient magnetic field pulse set in the imaging condition setting unit shown in FIG. FIG. 6 is a diagram showing an SSFP sequence in which a slice that is selectively excited by a coherent control block in the imaging condition setting unit shown in FIG. 5 is set to a different slice from a slice that is selectively excited when an excitation pulse is applied. The figure which shows the example which set the SSFP sequence which has coherent control block which used radial collection together in the imaging condition setting part shown in FIG. The figure which shows the slice axis | shaft of the spin labeled in Radial collection by the SSFP sequence shown in FIG. The figure which shows the example which set the SSFP sequence which has several coherent control blocks accompanying the application of the pulse for labeling in the imaging condition setting part shown in FIG. FIG. 6 is a diagram for explaining a method of selectively rendering a spin with motion by difference processing in the image processing unit shown in FIG. 5. 6 is a flowchart showing an example of a procedure when imaging using an SSFP sequence involving application of a coherent control pulse for a desired purpose such as fat suppression, labeling, inversion, and the like by the magnetic resonance imaging apparatus shown in FIG. 4.

  Embodiments of a magnetic resonance imaging apparatus according to the present invention will be described with reference to the accompanying drawings.

  FIG. 4 is a block diagram showing an embodiment of the magnetic resonance imaging apparatus according to the present invention.

  The magnetic resonance imaging apparatus 20 includes a cylindrical static magnetic field magnet 21 that forms a static magnetic field and a shim coil 22, a gradient magnetic field coil 23, and an RF coil 24 that are provided inside the static magnetic field magnet 21. This is a built-in configuration.

  In addition, the magnetic resonance imaging apparatus 20 includes a control system 25. The control system 25 includes a static magnetic field power supply 26, a gradient magnetic field power supply 27, a shim coil power supply 28, a transmitter 29, a receiver 30, a sequence controller 31, and a computer 32. The gradient magnetic field power source 27 of the control system 25 includes an X-axis gradient magnetic field power source 27x, a Y-axis gradient magnetic field power source 27y, and a Z-axis gradient magnetic field power source 27z. In addition, the computer 32 includes an input device 33, a display device 34, an arithmetic device 35, and a storage device 36.

  The static magnetic field magnet 21 is connected to a static magnetic field power supply 26 and has a function of forming a static magnetic field in the imaging region by a current supplied from the static magnetic field power supply 26. In many cases, the static magnetic field magnet 21 is composed of a superconducting coil, and is connected to the static magnetic field power supply 26 at the time of excitation and supplied with current. It is common. In some cases, the static magnetic field magnet 21 is composed of a permanent magnet and the static magnetic field power supply 26 is not provided.

  A cylindrical shim coil 22 is coaxially provided inside the static magnetic field magnet 21. The shim coil 22 is connected to the shim coil power supply 28, and is configured such that a current is supplied from the shim coil power supply 28 to the shim coil 22 to make the static magnetic field uniform.

  The gradient magnetic field coil 23 includes an X-axis gradient magnetic field coil 23 x, a Y-axis gradient magnetic field coil 23 y, and a Z-axis gradient magnetic field coil 23 z, and is formed in a cylindrical shape inside the static magnetic field magnet 21. A bed 37 is provided inside the gradient magnetic field coil 23 as an imaging region, and the subject P is set on the bed 37. The RF coil 24 may not be built in the gantry but may be provided near the bed 37 or the subject P.

  The gradient magnetic field coil 23 is connected to a gradient magnetic field power supply 27. The X-axis gradient magnetic field coil 23x, the Y-axis gradient magnetic field coil 23y, and the Z-axis gradient magnetic field coil 23z of the gradient magnetic field coil 23 are respectively an X-axis gradient magnetic field power source 27x, a Y-axis gradient magnetic field power source 27y, and a Z-axis gradient magnetic field coil 27z. It is connected to the magnetic field power supply 27z.

  The X-axis gradient magnetic field power source 27x, the Y-axis gradient magnetic field power source 27y, and the Z-axis gradient magnetic field power source 27z are supplied with currents supplied to the X-axis gradient magnetic field coil 23x, the Y-axis gradient magnetic field coil 23y, and the Z-axis gradient magnetic field coil 23z, respectively. In the imaging region, a gradient magnetic field Gx in the X-axis direction, a gradient magnetic field Gy in the Y-axis direction, and a gradient magnetic field Gz in the Z-axis direction can be formed, respectively.

  The RF coil 24 is connected to the transmitter 29 and the receiver 30. The RF coil 24 receives the RF signal from the transmitter 29 and transmits it to the subject P, and receives the NMR signal generated by the excitation of the nuclear spin inside the subject P by the RF signal to the receiver 30. Has the function to give.

  On the other hand, the sequence controller 31 of the control system 25 is connected to the gradient magnetic field power source 27, the transmitter 29, and the receiver 30. The sequence controller 31 has control information necessary for driving the gradient magnetic field power supply 27, the transmitter 29, and the receiver 30, for example, operation control information such as the intensity, application time, and application timing of the pulse current to be applied to the gradient magnetic field power supply 27. And the gradient magnetic field power source 27, the transmitter 29, and the receiver 30 are driven according to the stored predetermined sequence to drive the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, and the Z-axis gradient magnetic field. It has the function of generating Gz and RF signals.

  The sequence controller 31 is configured to receive raw data, which is complex data obtained by detection of the NMR signal and A / D conversion in the receiver 30, and supply the raw data to the computer 32.

  For this reason, the transmitter 29 has a function of applying an RF signal to the RF coil 24 based on the control information received from the sequence controller 31, while the receiver 30 detects the NMR signal received from the RF coil 24. Then, by executing required signal processing and A / D conversion, a function of generating raw data that is digitized complex data and a function of supplying the generated raw data to the sequence controller 31 are provided.

  Further, the computer 32 is provided with various functions by executing the program stored in the storage device 36 of the computer 32 by the arithmetic unit 35. However, the computer 32 may be configured by providing a specific circuit regardless of the program.

  FIG. 5 is a functional block diagram of the computer 32 shown in FIG.

  The computer 32 functions as an imaging condition setting unit 40, a sequence controller control unit 41, a k-space database 42, an image reconstruction unit 43, an image database 44, and an image processing unit 45 by a program.

  The imaging condition setting unit 40 has a function of setting imaging conditions such as a pulse sequence based on instruction information from the input device 33 and giving the set imaging conditions to the sequence controller control unit 41. For this purpose, the shooting condition setting unit 40 has a function of causing the display device 34 to display shooting condition setting screen information. In particular, the imaging condition setting unit 40 is configured to be able to set an SSFP sequence as imaging conditions. Furthermore, the application of RF coherent control pulses with a desired purpose for changing the image contrast so that spin phase continuity is maintained can be added to the SSFP sequence.

  FIG. 6 is a diagram showing an SSFP sequence with application of an RF coherent control pulse set in the imaging condition setting unit 40 shown in FIG.

  In FIG. 6, RF indicates an RF pulse transmitted to the subject P. As shown in FIG. 6, in the SSFP sequence, a plurality of α excitation pulses are repeatedly applied at regular intervals (TR) after application of an α / 2 start-up pulse that excites spin by an angle + α / 2 or −α / 2. The Each α excitation pulse is an excitation pulse that tilts the spin by a certain angle α while adjusting the spin phases to 0, θ, 2 × θ, 3 × θ,.

  A coherent control block is provided in the TR period between any adjacent excitation pulses in the SSFP sequence. FIG. 6 shows a coherent between an (α, θ) excitation pulse in which the spin is tilted by a fixed angle α and the spin phase is θ and an (α, 2θ) excitation pulse in which the spin phase is 2 × θ. An example in which a control block is provided is shown.

  In the coherent control block, a (β, φ) RF coherent control pulse with a spin tilted by an arbitrary angle β and a spin phase φ is applied. The waveform, excitation angle, and excitation phase of the RF coherent control pulse can be arbitrarily determined according to the application purpose of the RF coherent control pulse. That is, the RF coherent control pulse can be an arbitrary pulse for changing the image contrast such as a labeling pulse, a fat suppression pulse, an inversion pulse, and the like.

  However, the center of the waveform of the RF coherent control pulse is controlled so as to be located substantially at the center between adjacent α excitation pulses. That is, the imaging conditions are set such that the center of the waveform of the RF coherent control pulse is located at a position that is temporally separated from the adjacent excitation pulse by TR / 2.

  Depending on the application purpose of the RF coherent control pulse, it is desirable to maintain a steady state of magnetization at the application timing of single or multiple excitation pulses before and after the coherent control block, but it is not necessary to collect imaging data There is. Therefore, any number of excitation pulses before and after the coherent control block may be dummy pulses that do not collect data for use in imaging. For example, when applying a fat suppression pulse as an RF coherent control pulse, the excitation pulse applied before the fat suppression pulse is applied and a sufficient fat suppression effect is obtained by applying the fat suppression pulse is referred to as a dummy pulse. can do.

  FIG. 7 is a diagram showing the spin gradient that changes due to the execution of the SSFP sequence having the coherent control block shown in FIG.

  In the SSFP sequence, as shown in FIG. 7, when a + α / 2 startup pulse is applied, the proton spin direction is tilted from the x-axis direction. Then, as time passes by TR, the spin direction of protons tilts symmetrically with respect to the y-axis. Here, when a -α excitation pulse for rotating the spin by a certain angle -α is applied, the spin direction is inclined in a direction symmetric with respect to the x axis. Further, when time passes by TR, the spin direction tilts in a direction symmetric with respect to the y-axis, and when a + α excitation pulse that rotates the spin by a fixed angle + α is applied, the spin direction tilts in a direction symmetric with respect to the x-axis. . That is, the spin direction returns to the original angle by applying the α excitation pulse twice.

  Therefore, the spin direction when the time of TR / 2 has elapsed after application of a certain α excitation pulse is the y-axis direction y (TR / 2). For this reason, if, for example, a 180 ° RF coherent control pulse is applied so as to excite the focused spin at the timing when the spin direction becomes the y-axis direction y (TR / 2), the spin direction of the focused spin is aligned with the x-axis. Although it tilts symmetrically, it becomes y ′ (TR / 2) on the y-axis. Therefore, if a 180 ° RF coherent control pulse is applied at the time when TR / 2 has elapsed after the α excitation pulse is applied, the spin direction is not affected by the 180 ° RF coherent control pulse, and continuity is maintained. can do.

  If the spin is affected by the RF coherent control pulse and continuity is not maintained, artifacts will occur in the image. Therefore, the application timing of the RF coherent control pulse does not have to be exactly the time when TR / 2 time has elapsed after the application of the α excitation pulse, and the spin direction is sufficiently high that the artifact can be sufficiently reduced. That is, any timing that can maintain continuity is sufficient. In other words, strictly speaking, the application timing of the RF coherent control pulse may be within an error range centered around the time when TR / 2 has elapsed after the application of the excitation pulse. As a specific example, the application timing of the RF coherent control pulse can be set within a range of TR / 2 ± TR / 4 after application of the α excitation pulse.

  As described above, in the coherent control block, the imaging condition is controlled so that the continuity of the phase of the spin of interest is maintained. For example, when paying attention to a stationary spin, the zero-order moment amount in the coherent control block is adjusted to be zero so that the phase of the stationary spin is maintained. Therefore, the imaging conditions are determined so that the application area of the gradient magnetic field pulse in the coherent control block is zero.

  Since FIG. 6 shows an example in which the gradient magnetic field pulse is not applied in the coherent control block, the 0th-order moment amount in the coherent control block is originally zero.

  FIG. 8 is a diagram showing an SSFP sequence involving application of an RF coherent control pulse and a slice gradient magnetic field pulse set in the imaging condition setting unit 40 shown in FIG.

  In FIG. 8, RF indicates an RF pulse transmitted to the subject P, and Gss indicates a slice gradient magnetic field pulse applied to the subject P.

  As shown in FIG. 8, a coherent control block can be provided in an SSFP sequence involving application of a slice gradient magnetic field pulse Gss. In the coherent control block, an RF coherent control pulse is applied at a timing separated by TR / 2 from two adjacent α excitation pulses.

  A slice gradient magnetic field pulse is also provided in the coherent control block, and the same slice as the slice that is selectively excited when the α excitation pulse is applied can be selectively excited. If the slice excitation position when applying the excitation pulse is the same as the slice excitation position when applying the RF coherent control pulse, the RF coherent control pulse is an inversion pulse such as a saturation recovery (SR) pulse or an IR pulse. The same effect is brought about. That is, an inversion pulse such as an SR pulse or an IR pulse can be applied in a state where spin is excited on the SSFP sequence.

  Therefore, the spin that is not excited by the inversion pulse maintains a steady state, while the spin that is excited by the inversion pulse exhibits a signal change of about T1 relaxation. When the inversion time is controlled, the interval between the inversion pulse and the inversion pulse may be changed.

  Here, if the phase of the applied inversion pulse is the same as the phase of the excitation pulse applied immediately before the application of the inversion pulse, there is an advantage that the robust effect against RF nonuniformity can be improved.

  In the example of FIG. 8, the pull-in portion G1 of the slice gradient magnetic field pulse Gss applied together with the excitation pulse and the pull-in portion G2 of the slice gradient magnetic field pulse Gss in the coherent control block are set differently in time so that they do not overlap. However, it is also possible to apply by overlapping in a common period. This point is common to all SSFP sequences described later provided with a coherent control block accompanied by application of a slice gradient magnetic field pulse Gss.

  FIG. 9 is a diagram showing an SSFP sequence in which the slice that is selectively excited by the coherent control block in the imaging condition setting unit 40 shown in FIG. 5 is set to a different slice from the slice that is selectively excited when the excitation pulse is applied.

  In FIG. 9, RF is an RF pulse transmitted to the subject P, Gss1 is a slice gradient magnetic field pulse applied to the subject P in order to select a slice for applying an excitation pulse, and Gss2 is a coherent control. A block, that is, a slice gradient magnetic field pulse applied to the subject P in order to select a slice for applying an RF coherent control pulse is shown.

  Although a coherent control block can be provided in the SSFP sequence accompanied by the application of the slice gradient magnetic field pulse Gss, as shown in FIG. 9, the slice for applying the excitation pulse and the slice for applying the RF coherent control pulse are in different directions. It can also be a slice. In this case, the application direction of the slice gradient magnetic field pulse Gss1 applied together with the excitation pulse and the application direction of the slice gradient magnetic field pulse Gss2 applied together with the application of the RF coherent control pulse are different from each other.

  As a typical example, the application direction of the slice gradient magnetic field pulse Gss2 applied with the application of the RF coherent control pulse is set to be perpendicular to the application direction of the slice gradient magnetic field pulse Gss1 applied with the excitation pulse, and the RO direction An example is given.

  Accordingly, the spins included in the two slices of the slice that is selectively excited when the excitation pulse is applied and the slice that is selectively excited when the RF coherent control pulse is applied, that is, the spins that have received two slice selective excitations are included in the coherent control block. Saturated or labeled with RF coherent control pulses. On the other hand, spins included only in slices that are selectively excited when an excitation pulse is applied, that is, spins that receive only slice selective excitation when an excitation pulse is applied, strictly maintain a steady state.

  For this reason, the RF coherent control pulse of the coherent control block functions as a saturation pulse or a spin labeling pulse. Such imaging conditions are suitable, for example, when suppressing signals in unnecessary areas or observing how a moving spin moves in a specific plane.

  FIG. 10 is a diagram illustrating an example in which an SSFP sequence having a coherent control block combined with radial collection is set in the imaging condition setting unit 40 illustrated in FIG.

  In FIG. 10, RF is an RF pulse transmitted to the subject P, Gss is a slice gradient magnetic field pulse applied to the subject P, and Gro1 and Gro2 are RO gradient magnetic fields applied to the subject P, respectively. Indicates a pulse.

  The SSFP sequence shown in FIG. 10 is a sequence when data collection is so-called radial collection. Radial acquisition is a collection method that changes the gradient magnetic field and collects data radially through the origin in k-space, and images liquids such as blood and cerebrospinal fluid (CSF) and moving organs. It is known that there are few artifacts.

  Also in this radial collection SSFP sequence, the coherent control block is configured such that the zero-order and first-order moments of the RO gradient magnetic field pulse are zero. In the coherent control block, it is possible to apply an inversion pulse for labeling a liquid such as blood or CSF, or a moving organ, or any other inversion pulse having a purpose other than labeling. When a coherent control block is provided in the SSFP sequence for radial collection, the phases of the stationary spin and the spin moving at a constant speed are always rephased. For this reason, it is possible to satisfactorily depict a moving part such as a liquid and obtain an image with little artifact.

  FIG. 11 is a diagram showing slice axes of spins that are labeled in the radial collection by the SSFP sequence shown in FIG.

  In FIG. 11, the RO1 axis indicates the starting point axis of Radial collection, and the RO2 axis is an axis orthogonal to the RO1 axis. The RO1 axis and the RO2 axis correspond to the application directions of the RO gradient magnetic field pulses Gro1 and Gro2, respectively. Therefore, the plane including the RO1 axis (surface excited by the RO gradient magnetic field pulse Gro1) and the plane including the RO2 axis (surface excited by the RO gradient magnetic field pulse Gro2) are each selectively sliced SL by the slice gradient magnetic field pulse Gss. It becomes a plane perpendicular to.

  In radial acquisition, k-space data is acquired on a slice SL that has been selectively excited so as to rotate around the intersection of the radial start point RO1 and the RO2 axis orthogonal to the RO1 axis. The slice to be labeled is set to a plane perpendicular to the slice SL that includes the starting point axis RO1 of Radial acquisition and is selectively excited by the slice gradient magnetic field pulse Gss. That is, as shown in FIG. 10, when the RO gradient magnetic field pulse Gro2 of the RO2 axial direction component is zero, a coherent inversion pulse is applied for labeling. On the other hand, when the RO gradient magnetic field pulse Gro2 of the RO2 axial direction component is not zero, data collection is performed.

  When spin labeling is performed under such imaging conditions, spins in the plane orthogonal to the slice SL in the radial acquisition start axis RO1 are labeled, while the spin orthogonal to the slice SL in the radial acquisition start axis RO1 is labeled. On the outside, a steady state of spin is maintained. That is, it is depicted where only the labeled spin has moved. Thus, according to Radial collection, it is possible to perform imaging while suppressing the influence of movement.

  Also, a plurality of coherent control blocks can be provided, and actions such as fat suppression and labeling can be obtained by RF coherent control pulses of a plurality of sets of coherent control blocks.

  For example, two pulses of a first fat saturation pulse and a second fat saturation pulse applied after the first fat saturation pulse can be added as a plurality of sets of RF coherent control pulses. In this case, between the application times of the first fat suppression pulse and the second fat suppression pulse so that the fat suppression effect is obtained at the application time of the excitation pulse for imaging that needs to have the fat suppression effect. If the time and the time between the application time of the second fat saturation pulse and the excitation pulse for desired imaging are controlled, the fat suppression effect obtained by the application of the two fat saturation pulses can be obtained in the data collection by the SSFP sequence. Can do. The same applies when three or more fat suppression pulses are applied as a plurality of sets of RF coherent control pulses.

  In particular, in order to improve the fat suppression effect, it is desirable to set a long pulse length of the fat suppression pulse. However, it may be difficult to set the pulse length of the fat suppression pulse sufficiently long due to the restriction of TR. . Therefore, if a plurality of fat saturation pulses having restricted frequency characteristics are applied, it can be expected that the same fat suppression effect as that obtained when applying a fat saturation pulse having a frequency characteristic to be originally set can be expected. In other words, by applying the fat suppression pulse in a plurality of times, it is possible to obtain the same fat suppression effect as when applying a fat suppression pulse having an appropriate frequency characteristic that can be set when TR is not considered. it can.

  As another example, a fat suppression pulse and a labeling pulse can be added as a plurality of sets of RF coherent control pulses. In this case, both fat suppression and labeling can be performed in data collection by the SSFP sequence. In addition, in order to obtain a fat suppression effect at the application time of the excitation pulse for imaging, fat suppression is performed by controlling the time between the application time of the fat suppression pulse and the excitation pulse that needs to obtain the fat suppression effect. It is necessary to obtain a fat suppression effect at the application timing of the excitation pulse that needs to obtain an effect.

  As yet another example, a plurality of labeling pulses can be added as a plurality of sets of coherent control pulses. For example, when a tag pulse for labeling is applied, there may be a case where necessary power to be applied to the tag pulse cannot be obtained due to restrictions on the hardware configuration. Therefore, by applying the Tag pulse in a plurality of times, the magnetization to be labeled can be reduced by 180 degrees. Further, as a labeling method involving application of a plurality of labeling pulses, there is a t-SLIP (Time-SLIP: Time Spatial Labeling Inversion Pulse) method.

  In the t-SLIP method, a t-SLIP pulse is applied and blood flowing into the imaging region is labeled. In other words, the t-SLIP sequence is an imaging sequence that involves the application of an ASL (arterial spin-labeling) pulse to selectively depict or suppress tagged blood by tagging blood flowing into the imaging section. is there. With this t-SLIP sequence, it is possible to selectively emphasize or suppress the signal intensity of only blood that has reached the imaging section after an inversion time (TI). If necessary, the t-SLIP pulse is applied after a certain delay time has elapsed from the R wave of the ECG (electrocardiogram) signal, and imaging is performed under electrocardiographic synchronization.

  The t-SLIP pulse is composed of a region non-selection inversion pulse and a region selection inversion pulse. The area non-selected inversion pulse can be switched ON / OFF. That is, the t-SLIP pulse includes at least a region selection inversion pulse and may be composed of only a region selection inversion pulse or may be composed of both a region selection inversion pulse and a region non-selection inversion pulse. .

  The area selection inversion pulse can be arbitrarily set independently of the imaging section. When the blood flowing into the imaging region is labeled with this region selection inversion pulse, the signal intensity of the portion where the blood reaches after TI increases. Note that when the region non-selection inversion pulse is turned OFF, the signal intensity of the portion where blood reaches after TI is lowered. For this reason, the moving direction and distance of blood can be grasped.

  Therefore, the region non-selective inversion pulse and the region selective inversion pulse in the t-SLIP method can be added to the SSFP sequence as a plurality of sets of coherent control pulses for labeling.

  In labeling by the t-SLIP method, when applying a labeling pulse after a predetermined delay time from a reference wave of an electrocardiogram signal in synchronization with the electrocardiogram information, an ECG for acquiring an ECG signal of the subject P is obtained. A unit 38 is provided in the magnetic resonance imaging apparatus 20. The ECG signal acquired by the ECG unit 38 is configured to be output to the computer 32 via the sequence controller 31. Also, a pulse wave synchronization (PPG) signal can be acquired instead of the ECG signal. The PPG signal is, for example, a signal obtained by detecting a fingertip pulse wave as an optical signal. When acquiring the PPG signal, a PPG signal detection unit is provided.

  FIG. 12 is a diagram illustrating an example in which an SSFP sequence having a plurality of coherent control blocks with application of a labeling pulse is set in the imaging condition setting unit 40 illustrated in FIG.

  In FIG. 12, RF is an RF pulse transmitted to the subject P, and Gss1 is a slice gradient magnetic field pulse applied to the subject P in order to select a slice for applying the excitation pulse and the region non-selection inversion pulse. Gss2 indicates a slice gradient magnetic field pulse applied to the subject P in order to select a slice for application of the region selection inversion pulse.

  As shown in FIG. 12, the first and second c2 oherent control blocks can be provided in the SSFP sequence. Application of the first (β1, φ1) RF coherent control pulse and the second (β2, φ2) RF coherent control pulse can be set in the first and second coherent control blocks, respectively. These first and second RF coherent control pulses may be, for example, a region selection inversion pulse and a region non-selection inversion pulse as the first and second labeling pulses, respectively.

  In the area selection inversion pulse, an application section can be arbitrarily selected independently of the imaging section, so that a slice gradient magnetic field pulse for selecting a slice different from the slice that is selectively excited for imaging is applied together with the area selection inversion pulse. The On the other hand, since the region non-selection inversion pulse is applied to the imaging region, a slice gradient magnetic field pulse for selecting a slice parallel to the slice selected for imaging is applied together with the region non-selection inversion pulse.

  Next, other functions of the computer 32 will be described.

  The sequence controller control unit 41 constructed in the computer 32 sets the imaging conditions including the pulse sequence acquired from the imaging condition setting unit 40 in the sequence controller 31 based on the imaging start instruction information from the input device 33 or other components. A function of controlling the driving by giving, and a function of receiving raw data as k-space (Fourier space) data from the sequence controller 31 and arranging the raw data in the k-space database 42.

  For this reason, each raw data generated in the receiver 30 is stored in the k-space database 42 as k-space data.

  The image reconstruction unit 43 includes a function of generating image data from k-space data by capturing k-space data from the k-space database 42 and performing image reconstruction processing such as two-dimensional or three-dimensional Fourier transform processing; A function of writing the generated image data into the image database 44;

  The image processing unit 45 has a function of reading image data from the image database 44 and performing necessary image processing, and a function of causing the display device 34 to display image data after image processing. In particular, the image processing unit 45 has a function of performing difference processing between image data obtained from data collected before and after the coherent control block provided on the SSFP sequence, and difference image data obtained by the difference processing. Is displayed on the display device 34 as image data for display. In other words, the image processing unit 45 has a function of generating image data by the difference method, but in particular, when labeling is performed, a need to apply the difference method to image data before and after labeling has become apparent.

  FIG. 13 is a diagram for explaining a method of selectively rendering a moving spin by the difference processing in the image processing unit 45 shown in FIG.

  In FIG. 13, RF indicates an RF pulse transmitted to the subject P, Gss1 indicates a slice gradient magnetic field pulse for excitation pulse, and Gss2 indicates a coherent control block, that is, a slice gradient magnetic field pulse for RF coherent control pulse. Show.

  As shown in FIG. 13, the data collection period before the coherent control block of the SSFP sequence shown in FIG. 9 is the first data collection period DAQ1, while the data collection period after the coherent control block is the second data collection period DAQ2. And Then, the first image data IMAGE1 is reconstructed using the k-space data collected in the first data collection period DAQ1, while the first image data IMAGE1 is reconstructed using the k-space data collected in the second data collection period DAQ2. When the image reconstruction unit 43 is configured so that the second image data IMAGE2 is reconstructed, the image database 44 stores the first image data IMAGE1 and the second data collection corresponding to the first data collection period DAQ1, respectively. Second image data IMAGE2 corresponding to the period DAQ2 is stored.

  Then, the image processing unit 45 performs difference processing between the first image data IMAGE1 and the second image data IMAGE2, so that difference image data in which only moving spins are selectively depicted | IMAGE1-IMAGE2 Is configured to generate | That is, in the first image data IMAGE1 obtained from the data before the coherent control block, the target spin is not labeled, but in the second image data IMAGE2 obtained from the data after the coherent control block, the inversion is performed. The focused spin is labeled by the pulse. On the other hand, the spin that has received only the slice selective excitation when the excitation pulse is applied maintains the steady state strictly before and after the coherent control block. Therefore, when the difference processing is performed between the first image data IMAGE1 and the second image data IMAGE2, the data from the spin in the steady state is canceled, and only the data from the labeled spin is the difference image data | IMAGE1-IMAGE2 Will remain as |.

  Note that a sufficient number of excitations is necessary so that the spin is in a steady state in the first data collection period DAQ1.

  Further, the difference processing between the image data obtained from the collected data before and after the coherent control block is performed not only in the shooting conditions shown in FIG. 9 but also in the shooting conditions shown in FIGS. be able to. For example, in the radial collection shown in FIG. 10, the first data collection period DAQ1 and the second data collection period DAQ2 are set, and the image data of the moving spin can be selectively obtained by subtracting the image data.

  Next, the operation and action of the magnetic resonance imaging apparatus 20 will be described.

  FIG. 14 shows an example of a procedure for performing imaging using an SSFP sequence with application of a coherent control pulse for a desired purpose such as fat suppression, labeling, inversion, and the like by the magnetic resonance imaging apparatus 20 shown in FIG. In the figure, reference numerals with numerals S indicate steps in the flowchart.

  First, in step S <b> 1, the shooting condition setting unit 40 causes the display device 34 to display shooting condition setting screen information. The user browses the imaging condition setting screen, and the imaging condition setting unit 40 is given from the input device 33 conditions such as an SSFP sequence selection instruction and an accompanying gradient magnetic field pulse application method as the imaging condition instruction information. Then, the shooting condition setting unit 40 sets an SSFP sequence as a shooting condition.

  Next, in step S2, the shooting condition instruction information in the coherent control block is given from the input device 33 as shooting condition instruction information to the shooting condition setting unit 40, and is set as the shooting condition in the coherent control block. That is, the waveform, excitation angle, and excitation phase of the RF coherent control pulse in the coherent control block are determined according to the application purpose of the coherent control pulse. Further, the method of applying the gradient magnetic field pulse in the coherent control block is also determined according to the imaging purpose.

  Next, in step S3, data collection is performed according to the shooting conditions set by the shooting condition setting unit 40.

  That is, the subject P is set on the bed 37 and a static magnetic field is formed in the imaging region of the static magnetic field magnet 21 (superconducting magnet) excited by the static magnetic field power supply 26. Further, a current is supplied from the shim coil power supply 28 to the shim coil 22, and the static magnetic field formed in the imaging region is made uniform.

  When a data collection instruction is given from the input device 33 to the sequence controller control unit 41, the sequence controller control unit 41 obtains the SSFP sequence provided with the coherent control block from the imaging condition setting unit 40 and gives it to the sequence controller 31. . The sequence controller 31 drives the gradient magnetic field power source 27, the transmitter 29, and the receiver 30 according to the SSFP sequence received from the sequence controller control unit 41, thereby forming a gradient magnetic field in the imaging region where the subject P is set, An RF signal is generated from the RF coil 24.

  Therefore, an NMR signal generated by nuclear magnetic resonance inside the subject P is received by the RF coil 24 and given to the receiver 30. The receiver 30 receives the NMR signal from the RF coil 24 and generates raw data. The receiver 30 gives the generated raw data to the sequence controller 31. The sequence controller 31 provides the raw data to the sequence controller control unit 41, and the sequence controller control unit 41 arranges the raw data as k-space data in the k-space formed in the k-space database 42.

  Here, since the coherent control block for applying the coherent control pulse is provided in the SSFP sequence, the k-space data from the excited spin of interest among the k-space data collected after the coherent control block. Is affected by a coherent control pulse. On the other hand, since the zero-order moment amount in the coherent control block is adjusted to be zero so that the phase of the spin not focused is maintained, the steady state of the spin not focused around the coherent control block is Strictly maintained.

  Next, in step S <b> 4, the image reconstruction unit 43 generates image data by taking k-space data from the k-space database 42 and performing image reconstruction processing. The generated image data is written and stored in the image database 44.

  Next, in step S5, the image processing unit 45 reads image data from the image database 44 and performs necessary image processing to generate image data for display. For example, the image processing unit 45 performs differential processing between image data obtained from data collected before and after the coherent control block provided on the SSFP sequence, thereby selectively rendering a spin with motion. Generated image data is generated.

  Next, in step S <b> 6, the image processing unit 45 gives the image data generated by the image processing to the display device 34. Thereby, the display device 34 displays an image in which the image contrast of the portion including the spin of interest is changed by the coherent control pulse in the coherent control block.

  That is, the magnetic resonance imaging apparatus 20 as described above can add a coherent spin to the SSFP sequence which is one of the imaging conditions. Therefore, according to the magnetic resonance imaging apparatus 20, it is possible to realize a change in image contrast due to a coherent control pulse while maintaining spin phase continuity in the SSFP sequence.

  Furthermore, the slice gradient magnetic field pulse and the excitation pulse in the coherent control block are set by setting the application method of the slice gradient magnetic field pulse in the coherent control block to be different from the application method of the slice gradient magnetic field pulse when applying the excitation pulse for excitation. Spins excited by both slice gradient magnetic field pulses at the time of application of can be selectively labeled. Also in this case, the steady state of the spin excited only by the slice gradient magnetic field pulse at the time of applying the excitation pulse can be strictly maintained.

  In addition, by providing a coherent control block at the time of radial collection in the SSFP sequence, it is possible to selectively label the moving spins and depict the movement of the spins.

DESCRIPTION OF SYMBOLS 20 Magnetic resonance imaging apparatus 21 Magnet for static magnetic field 22 Shim coil 23 Gradient magnetic field coil 24 RF coil 25 Control system 26 Static magnetic field power supply 27 Gradient magnetic field power supply 28 Shim coil power supply 29 Transmitter 30 Receiver 31 Sequence controller 32 Computer 33 Input apparatus 34 Display apparatus 35 computing device 36 storage device 37 bed 40 imaging condition setting unit 41 sequence controller control unit 42 k-space database 43 image reconstruction unit 44 image database 45 image processing unit P subject

Claims (7)

A coherent control pulse that is centered around the center time between adjacent high frequency excitations and that the zeroth moment is zero on a Steady-State Free Precession pulse sequence that repeats multiple high frequency excitations at regular intervals. Data collection means for collecting data according to a sequence with
Image data generating means for generating image data from the data;
Have
The application region of the coherent control pulse can be arbitrarily set independently of the imaging region,
A magnetic resonance imaging apparatus. The data collection means is configured to collect the data by adding a region selection pulse for labeling as the coherent control pulse.
The magnetic resonance imaging apparatus according to claim 1. The data collecting means is configured to collect the data as a dummy pulse that does not collect data for use in imaging, using a single or a plurality of excitation pulses before and after the coherent block to which the coherent control pulse is applied. ,
The magnetic resonance imaging apparatus according to claim 1. The data collection means includes
When the coherent control pulse is applied, the gradient magnetic field of the coherent control pulse is excited so that the same slice as the slice excited by each gradient magnetic field pulse applied during the plurality of high frequency excitations is excited. Collect the data by adding pulses,
The coherent control pulse is an inversion pulse;
The magnetic resonance imaging apparatus according to claim 1. The pull-in part of the slice gradient magnetic field pulse applied together with the excitation pulse immediately before the coherent control pulse and the pull-in part of the slice gradient magnetic field pulse applied together with the coherent control pulse can be overlapped.
The magnetic resonance imaging apparatus according to claim 1. The image data generation means obtains first image data from first data collected before application of the coherent control pulse, and second image data from second data collected after application of the coherent control pulse. The magnetic resonance imaging apparatus according to claim 1, wherein each of the first and second image data is generated, and a difference process is performed between the first image data and the second image data. The magnetic resonance imaging apparatus according to claim 1, wherein the data collection unit is configured to collect the data by adding a plurality of sets of coherent control pulses.