JP5060569B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a medical magnetic resonance imaging (MRI) apparatus, and more particularly, to a magnetic resonance imaging apparatus capable of performing multi-slice imaging or oblique imaging while linearly moving a subject to be imaged in one direction. .

  Magnetic resonance imaging (MRI) magnetically excites a subject's nuclear spin placed in a static magnetic field with a high-frequency signal of its Larmor frequency, and re-images using the MR signal generated by this excitation. An imaging method to be configured.

  In recent years, in this field of magnetic resonance imaging, in order to obtain an image with a wider field of view than the region that can be imaged from the system, imaging is performed while continuously moving the top plate on which the subject is placed in the longitudinal direction. Has been proposed. For example, the imageable region is a slice having a thickness of several centimeters, and the wider region indicates, for example, a 40 cm field of view of the abdomen.

  As a basic technique of such an imaging method, a technique described in Patent Document 1 is known. The magnetic resonance imaging apparatus described in Patent Document 1 performs high-frequency excitation for selective excitation according to the position in the slice direction of a specific cross section (single slice) to be imaged when the subject is imaged while moving the subject. A high-frequency oscillator that adjusts the carrier frequency of the pulse is provided.

Specifically, when the center frequency corresponding to the magnetic field intensity of the magnetic resonance imaging apparatus is f ₀ and the carrier frequency of the RF pulse is f ₀ + Δf, Δf is changed according to the movement of the subject P. As an example, assuming that an axial image is obtained while moving the couch top, the change frequency Δf is
[Equation 1]
Δf = (γ · Gs · V · TR) / 2π [Hz]
Given in. Here, γ is the magnetic rotation ratio, Gs is the intensity of the gradient magnetic field pulse [T / m], V is the moving speed of the top plate [m / s], and TR is the repetition time [s].

  As a result, when the subject moves, the position of the selected excitation slice can always be continuously tracked on the specific section, so that throughput can be improved as the subject moves.

  Another example is shown in the paper “AH Herlihy et al.,“ Continuous scanning using single shot fast spin on a short bore neo-scanner, ”ISMRM '98, p. 1942”. The imaging method described in this paper is a technique for performing continuous imaging by controlling the carrier frequency of selective excitation in conjunction with the top plate motion in the single shot FSE method.

The carrier frequency in the case of this single shot FSE method is

It is controlled based on. Here, Δf0 is the carrier frequency offset of the first excitation pulse, Δf _k (k ≧ 1) is the carrier frequency offset of the kth refocus pulse, and ETS is the echo interval [s].

JP-A-8-71056

  However, in the above-described conventional MR imaging method involving movement of the subject (actually the top plate), single slice or sequential when the direction of movement of the subject coincides with the direction of the slice selection axis. Multi-slice imaging methods are shown, but imaging methods required in actual clinical practice such as multi-slice imaging and oblique imaging have not been proposed yet, and rapid imaging methods based on efficient data collection Is not established.

  Further, there has been no specific proposal for reducing artifacts typically caused by a constant speed movement of the bed top (ie, the subject).

  Therefore, the present invention provides an imaging method and an image quality improvement method that exhibit extremely useful functions in actual clinical practice, such as multi-slice imaging, oblique imaging, and an artifact reduction method due to subject movement. It can also be used for imaging that involves movement, thereby enhancing the functionality of this moving imaging method, and even when the area that can be imaged is small, it is possible to image a clinical region in a larger area at high speed. An object of the present invention is to provide a magnetic resonance imaging apparatus capable of improving throughput.

Magnetic resonance imaging apparatus of the present invention includes a scanning means for collecting echo data by a region of the multi-slice of the subject (magnetically) selective excitation while moving the object, the echo data collected by the scanning unit Reconstructing means for reconstructing an MR image based on the image data . The scanning means is configured to selectively excite another slice after the selective excitation of one slice and until the end of the collection of echo data for the selectively excited slice, and from the magnetic resonance imaging apparatus side. A position moving means for moving the position of the multi-slice in accordance with the movement of the subject within the fixed imaging range is provided.
In one embodiment, the reconstruction unit reconstructs an MR image based on echo data excluding the first and last portions of the echo data collected by the scanning unit.
In another embodiment, the scanning means does not perform data acquisition for selective excitation of the first part of the scan that does not become multi-slice, and does not perform data collection for the selective excitation after the first part. Data collection.
Accordingly, it is possible to perform multi-slice imaging useful in clinical settings while moving the subject, and to increase data collection efficiency.

  Preferably, the magnetic resonance imaging apparatus includes a couch having a top plate on which the subject is placed and a mechanism for moving the top plate in the longitudinal direction.

  Further, preferably, the position moving means is means for changing the carrier frequency of the RF pulse of selective excitation for the multi-slice for each slice.

  The scanning unit may be configured to selectively excite one slice and then selectively excite another slice that is not adjacent to the one slice.

With respect to the position moving means, the position of the slice in the slice selection direction set in a direction different from the moving direction of the subject is within the imaging range fixedly set from the magnetic resonance imaging apparatus side. It is good also as a structure moved according to a movement of. This makes it possible to perform oblique imaging useful in clinical settings while moving the subject, thereby improving the convenience of the apparatus. In this configuration, as a preferred first example, the position moving means changes the carrier frequency of the RF pulse for performing selective excitation in accordance with the geometric relationship between the moving direction of the subject and the slice selecting direction. Formed in the means to do. In this configuration, as a preferred second example, the acquisition means for acquiring MR signals from the slices that have been selectively excited, and the acquisition means based on the geometric relationship between the position of the subject and the direction of the gradient magnetic field are used. The magnetic resonance imaging apparatus further includes phase correction means for correcting the phase of the collected MR signal, and the reconstruction means reconstructs the MR signal phase-corrected by the phase correction means into an MR image . Accordingly, it is possible to prevent or suppress a situation in which a phase shift of the signal occurs in the slice due to the movement of the subject and an artifact due to this occurs. Note that the slice is preferably a single slice or a multi-slice.

  As for the above scanning means, when the slice located at the beginning of the movement direction in the multi-slice exceeds the imaging range, another slice is added as a part of the multi-slice at the tail of the movement direction of the multi-slice. Also good.

  The above scanning means includes a scanning means for collecting echo data by selectively exciting a predetermined region of the subject under a pulse sequence accompanied by a preparation pulse while moving the subject. It is good also as a structure provided with a means to link the application position of the pulse to the subject to the movement of the predetermined region. Thereby, unnecessary signal suppression and the like can be appropriately performed.

  The scanning means includes means for selectively exciting a predetermined region of the subject under a predetermined pulse sequence while collecting the subject and collecting MR signals. A phase compensation pulse for gradient moment nulling of the first order or second order may be added to at least a part of the gradient magnetic field in the moving direction. Thereby, it is possible to compensate for the signal phase disturbance accompanying the movement of the subject and prevent or suppress the occurrence of artifacts caused by the phase disturbance.

  The scanning means includes means for selectively exciting a predetermined region of the subject under a predetermined pulse sequence and collecting an echo signal while moving the subject. Alternatively, at least part of the gradient magnetic field in the moving direction may be composed of a high-speed spin echo pulse sequence that satisfies the VIPS condition. Thereby, it is possible to compensate for the signal phase disturbance caused by the movement of the subject, to prevent or suppress the occurrence of the artifact caused by the phase disturbance, and to avoid the lengthening of the data collection time.

  According to the present invention, an imaging method and an image quality improvement method that exhibit extremely useful functions in actual clinical practice, such as multi-slice imaging, oblique imaging, and an artifact reduction method due to movement of the subject, Since it can also be used for imaging with continuous movement, it is possible to improve the functionality of this moving imaging method, and even when the area that can be imaged is narrow, it is possible to image a clinical region in a larger area at high speed, and Throughput can be improved.

1 is a block diagram showing an outline of a configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention. The figure explaining the multislice imaging which concerns on 1st Embodiment by the relationship of time, a top-plate (subject) movement, and the selection excitation position of multislice. The timing chart which shows the pulse sequence based on FE method which can be used for the multi-slice imaging accompanying the movement of a subject. The figure explaining the relationship between the subject movement, the fixed imaging range, and the position of the multi-slice in multi-slice imaging, using several times as parameters. The positional relationship between the moving direction of the subject and the slice selection direction in the oblique imaging according to the second embodiment (FIG. 5B) will be described in comparison with the case where the oblique angle = 0 (FIG. 5A). Figure. The elements on larger scale which show the detail of the oblique imaging of FIG. The figure which illustrates the application position when using a presaturation pulse based on a modification together.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

(First embodiment)
A first embodiment of the present invention will be described. First, FIG. 1 shows a schematic configuration of an MRI (magnetic resonance imaging) apparatus commonly used in the following embodiments.

  The MRI apparatus includes a bed unit on which the subject P is placed, a static magnetic field generation unit that generates a static magnetic field, a gradient magnetic field generation unit for adding position information to the static magnetic field, a transmission / reception unit that transmits and receives high-frequency signals, A control / arithmetic unit responsible for overall system control and image reconstruction, and an electrocardiogram measurement unit that measures an ECG signal as a signal representing the cardiac time phase of the subject P are provided.

The static magnetic field generation unit includes, for example, a superconducting magnet 1 and a static magnetic field power supply 2 that supplies current to the magnet 1, and an axial direction of a cylindrical opening (diagnostic space) into which the subject P is loosely inserted. The static magnetic field H ₀ is generated (here, it is made to coincide with the Z-axis direction of the XYZ orthogonal coordinate system). In addition, the shim coil 14 is provided in this magnet part. The shim coil 14 is supplied with a current for homogenizing a static magnetic field from a shim coil power supply 15 under the control of a host computer to be described later.

  The bed portion has a bed 17, and the subject P is placed on the top plate 17 </ b> A of the bed in a state where the subject P is laid on its back, for example. The top plate 17A is removably moved in the longitudinal direction (Z-axis direction) by the operation of a driving mechanism (not shown) inside the bed 17. A drive command is given to the drive mechanism of the bed 17 from a host computer, which will be described later, and the drive function operates in response to this command. For this reason, when the top plate 17A moves in the longitudinal direction as shown by the arrow, the subject P on the top plate is also continuously moved in the Z-axis direction and removably inserted into the opening of the magnet 1.

  The gradient magnetic field generator includes a gradient magnetic field coil unit 3 incorporated in the magnet 1. The gradient coil unit 3 includes three sets (types) of x, y, and z coils 3x to 3z for generating gradient magnetic fields in the X, Y, and Z axis directions orthogonal to each other. The gradient magnetic field unit also includes a gradient magnetic field power supply 4 that supplies current to the x, y, and z coils 3x to 3z. The gradient magnetic field power supply 4 supplies a pulse current for generating a gradient magnetic field to the x, y, z coils 3x to 3z under the control of a sequencer 5 described later.

By controlling the pulse current supplied from the gradient magnetic field power source 4 to the x, y, z coils 3x to 3z, the gradient magnetic fields in the three-axis X, Y, and Z-axis directions, which are physical axes, are synthesized and orthogonal to each other. The logical axis directions of the slice direction gradient magnetic field Gs, the phase encode direction gradient magnetic field Ge, and the readout direction (frequency encode direction) gradient magnetic field Gr can be arbitrarily set and changed. Slice direction, phase encoding direction, and gradient magnetic fields in the readout direction are superimposed on the static magnetic field H _0.

  The transmission / reception unit includes an RF coil 7 disposed in the vicinity of the subject P in the imaging space in the magnet 1, and a transmitter 8T and a receiver 8R connected to the RF coil 7. The transmitter 8T and the receiver 8R operate under the control of the sequencer 5 described later. The transmitter 8T supplies the RF coil 7 with an RF current pulse having a Larmor frequency for exciting nuclear magnetic resonance (NMR). The receiver 8R takes in the MR signal (high frequency signal) received by the RF coil 7 and performs various signal processing such as preamplification, intermediate frequency conversion, phase detection, low frequency amplification, filtering, etc. Digital data of the MR signal (raw data) is generated by / D conversion.

  The control / arithmetic unit further includes a sequencer (also referred to as a sequence controller) 5, a host computer 6, an arithmetic unit 10, a storage unit 11, a display device 12, and an input device 13. Among these, the host computer 6 functions to command the pulse sequence information to the sequencer 5 according to software procedures stored in advance based on various modes as will be described later, and to control the overall operation of the apparatus including the bed 17. Have

  The sequencer 5 includes a CPU and a memory, stores pulse sequence information sent from the host computer 6, controls the operations of the gradient magnetic field power source 4, the transmitter 8T, and the receiver 8R according to this information, The digital data of the MR signal output from the receiver 8R is once inputted and transferred to the arithmetic unit 10. Here, the pulse sequence information is all information necessary for operating the gradient magnetic field power source 4, the transmitter 8T, and the receiver 8R according to a series of pulse sequences, for example, x, y, z coils 3x to 3z. Includes information on the intensity, application time, application timing, and the like of the pulse current applied to.

  In addition, the arithmetic unit 10 inputs the digital amount of original data output from the receiver 8R through the sequencer 5, places the original data in a Fourier space (also referred to as k space or frequency space) in its internal memory, and this original data. The data is subjected to two-dimensional or three-dimensional Fourier transform for each set to reconstruct the image data in real space. The arithmetic unit 10 is also capable of performing data composition processing, difference arithmetic processing, and the like regarding images.

  The storage unit 11 has a memory, and can store not only the reconstructed image data but also the image data subjected to the above-described combining process and difference process. The storage unit also includes a ROM or RAM (not shown) as a recording medium in which a pulse sequence used in this MR imaging is recorded in the form of program data and is readable by a computer.

  The display device 12 displays an image. In addition, information relating to imaging conditions, pulse sequences, image synthesis, and difference calculation desired by the operator can be input to the host computer 6 via the input unit 13.

  Further, the electrocardiogram measurement unit attaches to the body surface of the subject and detects an ECG signal as an electrical signal, and performs various processing including digitization processing on the sensor signal to perform the host computer 6 and the sequencer. And an ECG unit 18 for outputting to the PC. By using the measurement signal from the electrocardiograph, the synchronization timing by the ECG gate method (electrocardiogram synchronization method) can be set appropriately, and the ECG gate method imaging scan based on the synchronization timing can be used to collect data. ing.

  Next, the principle of the multi-slice method executed by this magnetic resonance imaging apparatus will be described with reference to FIGS.

  FIG. 2 shows the principle of the multi-slice method in which the number of multi-slices is set to 3 and the top plate, that is, the subject is moved continuously along the body axis (Z-axis) direction. And the slice selection direction are the same.

  Multi-slice data collection is performed according to the FE method. The pulse sequence based on the FE (gradient field echo) method executed for each slice is set as shown in FIG. That is, one echo is collected by one selective excitation of each slice, and this echo collection is repeated for the number of matrices in the phase encoding direction.

  Information on this pulse sequence is transferred from the host computer 6 to the sequencer 5. Based on this information, the sequencer 5 instructs the gradient magnetic field power source 4, the transmitter 8T, and the receiver 8R to perform a multi-slice scan in which another slice is selectively excited after the excitation for one slice and before the data collection. To do. In parallel with this, the host computer 6 sends a control signal for moving the top board to the bed 7. Thus, at a predetermined speed so that the imaging portion (for example, the abdomen of the subject) gradually enters the imaging space of the magnet 1 along the longitudinal direction (Z-axis direction) of the top plate 7A in time with the start of imaging. It is moved continuously. Therefore, multi-slice imaging is performed while moving the subject P.

  In FIG. 2, the direction from the left to the right indicates the time axis direction, and the direction from the bottom to the top of the figure indicates the moving direction of the top board. For easy understanding, it is assumed that the so-called multi-slice number = 3. A small rectangle shown in the figure indicates a selective excitation position, and represents that a spatial region at a certain time is selectively excited.

Specifically, as the subject P moves, a range (for example, an abdomen) that the subject P wants to capture sequentially enters the imaging range D assumed by the apparatus side. The assumed imaging range D may be determined from the uniform region of the static magnetic field or the sensitivity region of the RF coil, or may be determined from limitations of imaging parameters such as a repetition time TR for obtaining a desired contrast in a predetermined time. . In response to this movement, the sequencer 5 starts scanning based on the multi-scan method at a predetermined timing under the FE method (see FIG. 2, t = t ₀ ). As a result, the predetermined slice ns = 3 located in the imaging range D is selectively excited in consideration of the moving position at that time, and then every other slice ns = 1 located in the same range D The selective excitation is performed in consideration of the moving position at the time point, and then the remaining slices ns = 2 located in the same range D are selectively excited in consideration of the moving position at the time point (see t = t ₁ ). In response to these excitations, echoes for a certain phase encoding amount are collected.

  When this is finished, the selective excitation is performed again by returning to the first predetermined slice ns = 3. At this time, since the position of the slice ns = 3 is moved due to the movement of the subject compared to the first time point, the offset amount Δf of the carrier frequency f of the selective excitation RF pulse is changed. The amount of change is controlled so that the position of the slice ns = 3 at the time of excitation always follows a fixed desired slice position determined in advance for the subject. Since the moving speed of the top plate 17A is predetermined to a predetermined value, this known speed is used for the change control.

  Similarly, the following selective excitation is sequentially performed for the other slices ns = 2, 1 that have already been selectively excited once.

Thereafter, while repeating the same scan, the slice ns = 1 that was located on the exit side in the moving direction of the subject at time t = t ₂ is selectively excited at the exit side head of the imaging range D. . At this time, the other slices ns = 2 and 3 still have a distance to the head position of the fixed imaging range D viewed from the apparatus side.

Therefore, the slice ns = 1 located at the head cannot be excited because it protrudes from the imaging range D at the next selective excitation timing (t = t ₄ ). For this reason, at such timing (t = t ₄ ), a new slice ns = 4 is selectively excited at the head on the entry side of the imaging range D. Of course, the same applies to the other slices. For example, after the slice ns = 2 is selectively excited at the head position on the outgoing side, another slice ns = 5 that is newly selectively excited on the incoming side is generated (t = t). ₅ ).

  That is, the imaging parameters and the moving speed of the top plate are matched so that the end slice reaches the exit side of the imaging range D and at the same time the new slice starts collecting data on the opposite side. To be maintained.

FIG. 4 shows the imaging range D, the movement of the subject P in the body axis direction, and the positions of the multi-slices that are selectively excited at several points t ₁ , t ₃ , t ₄ , t ₅ of the horizontal axis time in FIG. The relationship is schematically illustrated. The number of multi-slices that are selectively excited almost simultaneously within the imaging range D is maintained at 3. However, the three slices are positioned so that the same position is excited when viewed from the subject as the subject P moves. Therefore, when viewed from the MRI side, which is the imaging apparatus, the positions of the three slices are updated so as to sequentially move in the same direction as the top plate movement direction.

  In FIG. 2, since the time axis direction and the top plate movement direction are taken as described above, the upward-pointing arrow indicates that the selected excitation position is always a constant position when viewed from the subject P, that is, in the same slice. It shows that data is collected while tracking. During the tracking period from the entrance side to the exit side of the imaging range D, the data is changed for each slice while changing the phase encoding amount from −NE / 2 to NE / 2 based on an arbitrary phase encoding order. Acquisition is performed and a series of data sets required for image reconstruction is acquired. That is, while the subject P moves in the imaging range D at a fixed position as viewed from the apparatus, the phase encoding amount is changed for each slice by a necessary amount, and all data corresponding to this is collected.

  As shown in FIG. 2, for a slice adjacent to a certain slice, data collection is started with a delay of about TD = TR · NE / MS (here, MS = 3), and the data collection is completed with a delay of TD. Is set to As a result, it is possible to collect data for an arbitrary total number of slices while maintaining the “multi-slice method” of the number MS at all times during scanning.

  Thus, by changing the carrier frequency of the selective excitation pulse for each slice of the multi-slice in conjunction with the movement of the bed top plate, the top plate was moved at a constant speed and assumed based on the above-mentioned reason. MR data can be collected from a subject region (for example, 40 cm in the abdomen) larger than the imaging range (for example, 15 cm), and efficient data collection unique to MR imaging based on the multi-slice method can be utilized. Therefore, by performing such multi-slice imaging for a predetermined time, for example, the entire lower abdomen that requires a field of view wider than the imaging range D is quickly scanned.

Here, regarding MR imaging based on the multi-slice method performed while moving the subject P performed as described above, the data collection condition is quantitatively expressed. First,
MS: Number of multi-slices
NE: Matrix size in the phase encoding direction,
TR: repetition time,
TS (= TR · NE): scan time (time required for data collection of each slice),
ns: serial number of the slice (0 to NS: NS> MS),
ne: phase encoding number (-NE / 2 to NE / 2-1),
D: width of the imaging range in the subject movement direction as seen from the apparatus side,
SI: Slice interval (= slice thickness ST + slice gap SG),
Δf (ns, ne): carrier frequency offset (slice number, phase encode number) of the selective excitation RF pulse,
And

At this time,
[Equation 3]
Top plate speed V = D / TS [m / s]
Slice interval SI = D / MS [m]
The conditions are set so that At this time, the slice center d (n, t) of the nth slice is about time t.
[Equation 4]
d (n, t) = V · t−n · SI [m]
Generality is maintained even if it exists in.

So in general,
[Equation 5]
ns = MS · ns1 + ns2 (0 ≦ ns2 <MS)
When the data collection timing T (ns, ne) of each slice and the offset Δf (ns, ne) of the carrier frequency are

As shown in FIG. 2, the multi-slices can be collected by being updated almost uniformly in order.

  As the maximum value NS of the multi-slice serial number is sufficiently larger than the number of slices MS, the top plate can be continuously fed, so that the whole can be scanned more efficiently.

  As shown in FIG. 2, data collection up to “NS-1” images at the beginning and end of imaging does not become a complete multi-slice mode of MS images (the portion F surrounded by a dotted line in FIG. The data collection up to “NS-1” sheets at the beginning of is shown). The data for the first and last portions of “NS-1” may be combined and reconstructed into an image. However, in order to make the MT (Magnetization Transfer) effect constant, it is executed as a pulse sequence. The first and last data may be discarded without imaging.

  Further, in the multi-slice method of the present embodiment, for the excitation of the first portion (dotted line portion F in FIG. 2) that does not become MS multi-slice, the data acquisition is not actually performed, It is also preferable to actually collect data for the excitation after the portion.

  Furthermore, in the above-described embodiment, the pulse sequence used for the multi-slice imaging has been described as the pulse train of the most basic FE method (see FIG. 3) in order to avoid complicated explanation. However, this pulse sequence may be a multi-echo type sequence. If multi-echo acquisition is performed as in an EPI (echo planar imaging) system sequence, the data acquisition time can be shortened by a value obtained by dividing the matrix size in the phase encoding direction by the number of multi-echoes. In the case of multi-echo acquisition using a plurality of RF pulses, such as a high-speed spin echo, the offset amount of the carrier frequency of all the RF pulses is measured by the method disclosed in the above-mentioned paper by AH Herlihy, for example. What is necessary is just to comprise so that it may change in connection with the movement position of a sample, ie, each slice.

(Second Embodiment)
Subsequently, a magnetic resonance imaging apparatus according to the second embodiment of the present invention will be described with reference to FIGS. The hardware of this apparatus is configured in the same way as that shown in FIG.

  In the first embodiment described above, the case where the moving direction of the subject and the slice selection direction are the same has been described. However, the second embodiment is called so-called oblique imaging (scanning) in which the two directions do not match. An imaging method will be described. This oblique imaging is performed in an actual clinical field, for example, by matching the cross section in the direction of the OM line of the subject (patient).

  In the oblique imaging according to this embodiment, the scan condition is set so as to set the carrier frequency for selective excitation corresponding to the geometrical relationship between the moving direction of the subject P and the slice selection direction.

Specifically, in oblique imaging, the moving direction M and the slice selection direction N of the subject are represented as shown in FIG. FIG. 6A shows the situation described in the first embodiment in which both directions M and N coincide with each other for reference. The relationship between the moving speed V of the top plate and the carrier frequency offset Δf described in detail in the first embodiment will be described more precisely. The moving speed V ′ of the subject in the slice direction and the carrier frequency Of the offset Δf. V ′ and V have a relationship of V ′ = V · IE. Here, as shown in FIG. 5, when the unit vector representing the moving direction M of the subject is represented by Ep and the unit vector representing the slice selection direction N is represented by Es, IE = [Ep, Es], and between the unit vectors Represents the inner product of. Therefore, “V · IE” is again expressed as V, and various imaging parameters are set and executed so as to satisfy the same relationship as described above. The offset Δf of the carrier frequency is also given in accordance with this, and the setting is automatically calculated from the scan condition given before the scan by the host computer 6 or the sequencer 5, and the carrier wave of “center frequency f ₀ + offset frequency Δf” Is sent from the sequencer 5 to the transmitter 8T.

  By setting the offset Δf of the carrier frequency in this way, the position change of the oblique slice accompanying the movement of the subject P can be automatically tracked, and this slice can always be selectively excited.

  Incidentally, as shown in FIG. 6, for a slice that does not pass through the magnetic field center of the static magnetic field due to the magnet 1, a deviation occurs between the “magnetic center Cm” and the “center Cr in real space” of the slice. When the subject is stationary, which is normal imaging, such deviation during data collection is a constant value, so the center of the reconstructed image is only the “magnetic center Cm”. The effect is small, and correction processing can be performed as necessary, for example, according to the method described in US Pat. No. 5,084,818.

  However, when the subject moves as in the present embodiment, the method is different, and inconsistent data collection is performed for each phase encoding, so phase correction is necessary. For this reason, the arithmetic unit 10 of this embodiment is configured to perform this phase correction in the following manner.

This phase correction is to adjust the phase so that the phase of the magnetized spin does not rotate at the center position Cr in the real space of the slice. As shown in FIG. 6, let C be a vector from the magnetic center Cm toward the center Cr in real space. This vector C is included in the plane where the unit vector Ee in the phase encoding direction and the vector Er in the reading direction exist. When the phase encoding amount due to the gradient magnetic field is K = (kr, ke) in the k space, the phase at the center Cr in the real space is
[Equation 7]
exp (2πi [K, C])
So,
[Equation 8]
exp (-2πi [K, C])
Phase correction is performed by multiplying the data by.

  Thereby, even when oblique imaging is performed while moving the subject, the MR signals are collected in a state in which the contradiction on the phase for each collection that has occurred due to the movement is reliably eliminated. Therefore, a high-quality MR image can be obtained by normal reconstruction processing based on data with the correct spin phase.

(Other embodiments)
The multi-slice imaging and oblique imaging of the first and second embodiments described above can be further modified as follows.

(Modification 1)
In the above-described multi-slice imaging and oblique imaging, various types of prepulses can be used in combination. 7A and 7B, in multi-slice imaging, a pre-saturation pulse as a pre-pulse can be applied every time each of the three multi-slices is selectively excited once. Information on the presaturation pulse is incorporated in advance in the pulse sequence. For this reason, the sequencer 5 that has received the pulse sequence information drives the transmitter 8T and the gradient magnetic field power supply 4 in accordance with the presaturation pulse information, so that the presaturation pulse is applied. Since this presaturation pulse needs to excite a spatial position that is considerably in close contact with the currently scanned multi-slice, as shown in FIG. It is changing.

  On the other hand, the characteristics required for the pre-pulse vary depending on the imaging target. When there is no request for the close contact between the multi-slice and the pre-saturation area as the main photographing part, as shown in FIG. 7B, the pre-saturation area is separated so as not to interfere with any slice regardless of the multi-slice movement. It may be fixed in a different position.

(Modification 2)
Another variation is a gradient moment nulling that compensates for phase disturbance of the MR signal due to motion (in the field of GMN: MRI, this gradient moment nulling is also called rephase, flow rephase, or flow compensation) Concerning the combined use.

  The subject also moves by moving the top plate at a constant speed. This state is considered to be influenced by the primary moment (velocity moment) due to the flow of the entire subject. For this reason, the phase compensation pulse for gradient moment nulling of the first order or the second order or more in the slice direction gradient magnetic field corresponding to the moving direction of the object in the pulse sequence used in the first and second embodiments described above. Is incorporated. In the case of oblique imaging, gradient moment nulling in the readout direction or phase encoding direction may be incorporated.

  As a result, even when the subject moves at a constant speed during the scan, signal phase disturbance due to this movement does not occur. Therefore, the generation of artifacts due to such phase disturbance is eliminated, and a high-quality MR image is obtained. Can be provided.

(Modification 3)
According to the gradient moment nulling technique described above, a configuration in which a phase compensation pulse is additionally applied is required. Therefore, the application time of the gradient magnetic field is increased, so that it cannot always be used for almighty.

  Therefore, when the pulse sequence used in the imaging method described in the first and second embodiments is a high-speed SE method sequence using an RF refocusing pulse, the VIPS (Velocity Independent Phase) is applied in the moving direction of the subject. A modification can be provided that is configured to satisfy the -shift stabilization) method.

  This VIPS method is known, for example, from a paper “Proc. Intl. Soc. Magn. Reson. Med. 7th (1999), p. 1910”, for example, a spin echo based on a pulse sequence of a fast SE method. Is an imaging method that satisfies the “φ / 2 condition”.

  This eliminates the need to add a phase compensation pulse such as gradient moment nulling, so that the above-described multi-slice imaging and oblique imaging can be performed while maintaining good data collection efficiency. You can enjoy various advantages.

  In addition, this invention is not limited to the thing of embodiment mentioned above, In the range which does not deviate from the summary of invention of a claim, it can change suitably.

DESCRIPTION OF SYMBOLS 1 Static magnetic field magnet 3 Gradient magnetic field coil 4 Gradient magnetic field power supply 5 Sequencer 6 Host computer 8T Transmitter 8R Receiver 10 Arithmetic unit 11 Storage unit 12 Display 13 Input device 17 Bed 17A Top plate P Subject (imaging object)

Claims (9)

A scanning unit that selectively excites a multi-slice region of the subject while moving the subject and collects echo data; a reconstruction unit that reconstructs an MR image based on the echo data collected by the scanning unit; A magnetic resonance imaging apparatus comprising:
The scanning unit is configured to selectively excite another slice after the selective excitation of one slice and until the end of the collection of echo data for the selectively excited slice, and is fixed from the magnetic resonance imaging apparatus side. A position moving means for moving the position of the multi-slice in accordance with the movement of the subject within a set imaging range ,
The reconstruction means reconstructs an MR image based on echo data excluding the first and last portions of the echo data collected by the scanning means.
A magnetic resonance imaging apparatus. A scanning unit that selectively excites a multi-slice region of the subject while moving the subject and collects echo data; a reconstruction unit that reconstructs an MR image based on the echo data collected by the scanning unit; A magnetic resonance imaging apparatus comprising:
The scanning unit is configured to selectively excite another slice after the selective excitation of one slice and until the end of the collection of echo data for the selectively excited slice, and is fixed from the magnetic resonance imaging apparatus side. in the imaging range which is to set, in response to said movement of the object have a position moving means for moving a position of the multi-slice, portion of the selective excitation that does not multislice a first part of the scan Data collection is performed for selective excitation after the first part without performing data collection for
A magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 1 or 2 ,
The magnetic resonance imaging apparatus according to claim 1, wherein the scanning unit is configured to selectively excite another slice that is not adjacent to the one slice after selectively exciting the one slice. The magnetic resonance imaging apparatus according to claim 1 or 2 ,
The position moving means is configured to set the position of the slice in the slice selection direction set in a direction different from the moving direction of the subject within the imaging range fixedly set from the magnetic resonance imaging apparatus side. The magnetic resonance imaging apparatus is moved according to the movement of the magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 4 .
The position moving means is means for changing a carrier frequency of an RF pulse for performing selective excitation in accordance with a geometric relationship between a moving direction of the subject and the slice selecting direction. apparatus. The magnetic resonance imaging apparatus according to claim 4 or 5 ,
Acquisition means for acquiring MR signals from the selectively excited slices;
Phase correcting means for correcting the phase of the MR signal collected by the collecting means based on the geometric relationship between the position of the subject and the direction of the gradient magnetic field;
Further comprising
The magnetic resonance imaging apparatus characterized in that the reconstruction means reconstructs the MR signal phase-corrected by the phase correction means into an MR image . The magnetic resonance imaging apparatus according to any one of claims 1 to 6,
The scanning means adds another slice as a part of the multi-slice when the slice located at the head of the multi-slice in the movement direction exceeds the imaging range. A magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 1 or 2,
  When echo data is collected as oblique imaging by the scanning means, the phase of the magnetized spin does not rotate at the center position in the real space of the slice with respect to the MR signal collected as echo data from the selectively excited slice. A magnetic resonance imaging apparatus further comprising phase correction means for performing phase correction. The magnetic resonance imaging apparatus according to claim 1 or 2,
  The magnetic resonance imaging apparatus characterized in that the scanning means collects echo data by a sequence using gradient moment nulling.

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