JP7267752B2 - Magnetic resonance imaging apparatus and k-space trajectory correction method - Google Patents

Description

TECHNICAL FIELD Embodiments of the present invention relate to a magnetic resonance imaging apparatus and a method of correcting a k-space trajectory.

In magnetic resonance imaging (MRI), there is Stack Of Stars acquisition in which radial scans are performed in multiple planes to acquire data along oblique k-space trajectories in k-space. . In Stack of Stars acquisition, if there is imperfection in hardware such as coils, the actual acquisition position of k-space data in k-space will be offset (shifted) from the set acquisition position, The image quality of the MR image deteriorates due to the offset.

JP 2009-095656 A JP 2012-090957 A

Dana C. Peters et al. Centering the Projection Reconstruction Trajectory: Reducing Gradient Delay Errors. Magn Reson Med. 2003 Jul; 50(1): 1-6.

An object of the present invention is to improve the image quality of MR images obtained by non-Cartesian scanning.

The magnetic resonance imaging apparatus according to the embodiment collects data for calibration by non-Cartesian scanning by limiting slice encoding to a number of encodes smaller than the number of encodes in data collection for image reconstruction by non-Cartesian scanning. a sequence control unit that executes and acquires k-space data for a first k-space trajectory; and a correction unit that corrects a second k-space trajectory related to data acquisition for the image reconstruction based on the k-space data. ,

FIG. 1 is a diagram showing the configuration of a magnetic resonance imaging apparatus according to this embodiment. FIG. 2 shows a three-dimensional k-space representation of the Stack of Stars collection. FIG. 3 is a diagram showing k-space notation of an example of encoding of a calibration scan according to this embodiment. FIG. 4 is a diagram showing k-space representation of one encoding mode in the calibration scan according to the present embodiment. FIG. 5 is a diagram showing k-space notation of another encoding mode in the calibration scan according to this embodiment. FIG. 6 is a diagram showing k-space representation of another encoding mode in the calibration scan according to the present embodiment. FIG. 7 is a diagram showing k-space notation of another encoding mode in the calibration scan according to this embodiment. FIG. 8 is a diagram showing k-space representation of another encoding mode in the calibration scan according to this embodiment. FIG. 9 is a diagram showing k-space notation of another encoding mode in the calibration scan according to this embodiment. FIG. 10 is a diagram showing k-space representation of another encoding mode in the calibration scan according to this embodiment. FIG. 11 is a diagram showing a typical flow of MR examination according to this embodiment. FIG. 12 is a diagram schematically showing the offset between the measured k-space trajectory and the set k-space trajectory. FIG. 13 is a diagram showing another flow of MR examination according to this embodiment.

A magnetic resonance imaging apparatus and a k-space trajectory correction method according to this embodiment will be described below with reference to the drawings.

FIG. 1 is a diagram showing the configuration of a magnetic resonance imaging apparatus 1 according to this embodiment. As shown in FIG. 1, the magnetic resonance imaging apparatus 1 includes a pedestal 11, a bed 13, a gradient magnetic field power supply 21, a transmission circuit 23, a reception circuit 25, a bed driving device 27, a sequence control circuit 29, and a host computer 50. have

The gantry 11 has a static magnetic field magnet 41 and a gradient magnetic field coil 43 . The static magnetic field magnet 41 and the gradient magnetic field coil 43 are housed in the housing of the pedestal 11 . A hollow bore is formed in the housing of the pedestal 11 . A transmitting coil 45 and a receiving coil 47 are arranged in the bore of the pedestal 11 .

The static magnetic field magnet 41 has a hollow, substantially cylindrical shape, and generates a static magnetic field inside the substantially cylindrical shape. A permanent magnet, a superconducting magnet, a normal conducting magnet, or the like is used as the static magnetic field magnet 41, for example. Here, the central axis of the static magnetic field magnet 41 is defined as the Z-axis, the axis vertically orthogonal to the Z-axis is defined as the Y-axis, and the axis horizontally orthogonal to the Z-axis is defined as the X-axis. The X-axis, Y-axis and Z-axis constitute an orthogonal three-dimensional coordinate system.

The gradient magnetic field coil 43 is a coil unit attached inside the static magnetic field magnet 41 and formed in a hollow, substantially cylindrical shape. The gradient magnetic field coil 43 receives current supply from the gradient magnetic field power supply 21 and generates a gradient magnetic field. More specifically, the gradient magnetic field coil 43 has three coils corresponding to mutually orthogonal X-, Y-, and Z-axes. The three coils form gradient magnetic fields with magnetic field strengths varying along the X, Y, and Z axes. The gradient magnetic fields along the X, Y, and Z axes are synthesized to form a slice selection gradient magnetic field Gs, a phase encoding gradient magnetic field Gp, and a frequency encoding gradient magnetic field Gr which are orthogonal to each other in desired directions. The slice selection gradient magnetic field Gs is used to arbitrarily determine an imaging section (slice). A phase-encoding magnetic field gradient Gp is utilized to vary the phase of the MR signal according to spatial position. A frequency-encoding magnetic field gradient Gr is utilized to vary the frequency of the MR signal according to spatial location. In the following description, it is assumed that the direction of gradient of the slice selection gradient magnetic field Gs is the Z-axis, the direction of gradient of the phase-encoding gradient magnetic field Gp is the Y-axis, and the direction of gradient of the frequency-encoding gradient magnetic field Gr is the X-axis.

The gradient magnetic field power supply 21 supplies current to the gradient magnetic field coils 43 according to the sequence control signal from the sequence control circuit 29 . The gradient magnetic field power supply 21 supplies a current to the gradient magnetic field coil 43 to generate a gradient magnetic field along each of the X-axis, the Y-axis, and the Z-axis. The gradient magnetic field is superimposed on the static magnetic field formed by the static magnetic field magnet 41 and applied to the subject P. FIG.

The transmission coil 45 is arranged, for example, inside the gradient magnetic field coil 43, receives current supply from the transmission circuit 23, and generates a high-frequency magnetic field pulse (hereinafter referred to as an RF magnetic field pulse).

The transmission circuit 23 supplies current to the transmission coil 45 in order to apply an RF magnetic field pulse for exciting target protons present in the subject P to the subject P via the transmission coil 45 . The RF magnetic field pulse oscillates at a resonant frequency characteristic of the protons of interest and excites the protons of interest. Magnetic resonance signals (hereinafter referred to as MR signals) are generated from the excited target protons and detected by the receiving coil 47 . The transmission coil 45 is, for example, a whole-body coil (WB coil). A whole body coil may be used as a transmit/receive coil.

The receiving coil 47 receives MR signals emitted from target protons present in the subject P under the action of RF magnetic field pulses. The receiving coil 47 has a plurality of receiving coil elements capable of receiving MR signals. The received MR signal is supplied to the receiving circuit 25 via wire or wireless. Although not shown in FIG. 1, the receive coil 47 has a plurality of receive channels implemented in parallel. The receive channel has a receive coil element for receiving the MR signal, an amplifier for amplifying the MR signal, and the like. An MR signal is output for each reception channel. The total number of reception channels and the total number of reception coil elements may be the same, or the total number of reception channels may be larger or smaller than the total number of reception coil elements.

The receiving circuit 25 receives MR signals generated from the excited target protons via the receiving coil 47 . The receiving circuit 25 processes the received MR signal to generate a digital MR signal. Digital MR signals can be represented in k-space defined by spatial frequencies. Therefore, the digital MR signal is hereinafter referred to as k-space data. K-space data is a type of raw data that is used for image reconstruction. The k-space data is supplied to the host computer 50 via wire or wireless.

Note that the transmission coil 45 and the reception coil 47 described above are merely examples. Instead of the transmitting coil 45 and the receiving coil 47, a transmitting/receiving coil having a transmitting function and a receiving function may be used. Also, the transmission coil 45, the reception coil 47, and the transmission/reception coil may be combined.

A bed 13 is installed adjacent to the gantry 11 . The bed 13 has a top board 131 and a base 133 . A subject P is placed on the top plate 131 . The base 133 supports the top plate 131 so as to be slidable along each of the X-axis, Y-axis, and Z-axis. The bed driving device 27 is accommodated in the base 133 . The bed driving device 27 moves the top plate 131 under the control of the sequence control circuit 29 . The couch drive 27 may include any motor, such as, for example, a servomotor or a stepper motor.

The sequence control circuit 29 has a CPU (Central Processing Unit) or MPU (Micro Processing Unit) processor and memories such as ROM (Read Only Memory) and RAM (Random Access Memory) as hardware resources. The sequence control circuit 29 synchronously controls the gradient magnetic field power supply 21, the transmission circuit 23, and the reception circuit 25 based on the imaging protocol determined by the imaging protocol setting function 511 of the processing circuit 51, and generates a pulse according to the imaging protocol. A sequence is executed to MR image the subject P and acquire k-space data about the subject P.

Specifically, the sequence control circuit 29 performs Stack Of Non-Cartesian collection. Stack-of-Non-Cartesian Acquisition is a data acquisition method in which a non-Cartesian scanning method (k-space filling method) is performed on multiple slices (cross-sections or slabs). Scanning methods other than the Cartesian method include, for example, the radial method and the spiral method. A stack of non-Cartesian collection based on the radial method is called a stack of stars collection.

FIG. 2 shows a three-dimensional k-space representation of the Stack of Stars collection. As shown in FIG. 2, k-space is a three-dimensional space of spatial frequencies defined by mutually orthogonal kz-, ky-, and kx-axes. In stack-of-stars acquisition, slice encoding 61 based on Cartesian scans along the kz-axis is performed for slice selection in the z-axis direction. A radial scan is performed for each slice encode 61 . In the radial scan, data acquisition is performed along a plurality of k-space trajectories (spokes) 63 radially set through approximately the center of the kx-ky plane for each slice encode 61 . That is, the radial scan k-space locus 63 is set obliquely on the kx-ky plane. Note that the positions (or angles around the center of the k-space) and the number of k-space trajectories 63 may be set to be the same over a plurality of slice encodes 61, or may be set to different values.

The oblique k-space locus 63 in the kx-ky plane can be realized by parallelly performing phase encoding by the phase encoding gradient magnetic field Gp and frequency encoding by the frequency encoding gradient magnetic field Gr. In other words, the readout gradient magnetic field of the MR signal is realized by synthesizing the phase-encoding gradient magnetic field Gp and the frequency-encoding gradient magnetic field Gr. A pulse sequence is designed to achieve an oblique k-space trajectory 63 . Since the oblique k-space trajectory 63 cannot be realized unless the application of the gradient magnetic field or the like is performed accurately according to the pulse sequence, the gradient magnetic field coil 43 , transmission coil 45, etc., are required to be complete. In reality, these hardware are imperfect. Hardware imperfections cause a delay in the application start time of the readout gradient magnetic field, a decrease in sharpness (blurring) in the rise or fall of the readout gradient magnetic field, an extension of the duration of application, and the like. This causes a mismatch between the echo time and the center time of the readout gradient, which in turn shifts the measured k-space trajectory relative to the intended set-up k-space trajectory. The shift causes deterioration of the image quality of the reconstructed image. Note that the set k-space trajectory means the ideal k-space trajectory when data is acquired faithfully to the pulse sequence, and the actually measured k-space trajectory means data acquisition by actual hardware for the pulse sequence. We mean the k-space trajectory when we do.

In order to specify the temporal and/or positional relationship between the set k-space trajectory and the actually measured k-space trajectory, the sequence control circuit 29 according to the present embodiment, prior to data acquisition for image reconstruction, Perform data collection for calibration. The sequence control circuit 29 performs data acquisition for calibration by non-Cartesian scanning, limited to slice encoding with a smaller number of encodes than the number of encodes in data acquisition for image reconstruction by non-Cartesian scanning, and k Acquire k-space data for spatial trajectories. Data collection for image reconstruction is hereinafter referred to as main scan, and data collection for calibration is referred to as calibration scan.

As shown in FIG. 1, host computer 50 is a computer having processing circuitry 51 , memory 52 , display 53 , input interface 54 and communication interface 55 .

The processing circuit 51 has a processor such as a CPU as a hardware resource. The processing circuit 51 functions as the core of the magnetic resonance imaging apparatus 1 . For example, the processing circuit 51 has an imaging protocol setting function 511, a k-space trajectory correction function 512, an image reconstruction function 513, an image processing function 514, and a display control function 515 by executing various programs.

In the imaging protocol setting function 511, the processing circuit 51 sets the imaging protocol for MR imaging according to a user's instruction via the input interface 54 or automatically. An imaging protocol is a set of various imaging parameters for one MR imaging. As the imaging parameters according to the present embodiment, various imaging parameters set directly or indirectly for performing MR imaging such as imaging time, type of k-space filling method, type of pulse sequence, TR, and TE can be applied. be.

In the k-space trajectory correction function 512, the processing circuit 51 corrects the k-space trajectory for the main scan based on the k-space data collected by the calibration scan. More specifically, the processing circuit 51 calculates the deviation between the measured acquisition position of the k-space trajectory for the calibration scan and the set acquisition position, and corrects the k-space trajectory for the main scan based on the calculated deviation. . There are, for example, the following two methods for correcting the k-space trajectory. According to the first method, the processing circuit 51 calculates the deviation between the measured acquisition position of the k-space trajectory for data acquisition for calibration and the set acquisition position, and corrects the k-space trajectory for the main scan. Therefore, the k-space data collected by the main scan is corrected based on the calculated deviation. According to the second method, the processing circuit 51 calculates the deviation between the measured acquisition position of the k-space trajectory for data acquisition for calibration and the set acquisition position, and corrects the k-space trajectory for the main scan. Therefore, the pulse sequence of the main scan is corrected based on the calculated shift.

In an image reconstruction function 513, processing circuitry 51 reconstructs an MR image based on k-space data acquired by non-Cartesian scanning. The reconstruction method is not particularly limited.

In the image processing function 514, the processing circuit 51 performs various image processing on the MR image. For example, the processing circuit 51 performs image processing such as volume rendering, surface rendering, pixel value projection processing, MPR (Multi-Planer Reconstruction) processing, and CPR (Curved MPR) processing.

The processing circuit 51 in the display control function 515 displays various information on the display 53 . For example, the processing circuit 51 displays on the display 53 an MR image generated by the image reconstruction function 513, an MR image generated by the image processing function 514, an imaging protocol setting screen, and the like.

The memory 52 is a storage device such as an HDD (Hard Disk Drive), an SSD (Solid State Drive), or an integrated circuit storage device that stores various information. Also, the memory 52 may be a drive device or the like that reads and writes various information from/to a portable storage medium such as a CD-ROM drive, a DVD drive, or a flash memory. For example, the memory 52 stores k-space data, control programs, and the like.

The display 53 displays various information. For example, the display 53 displays an MR image generated by the image reconstruction function 513, an MR image generated by the image processing function 514, an imaging protocol setting screen, and the like. As display 53, for example, a CRT display, liquid crystal display, organic EL display, LED display, plasma display, or any other display known in the art can be used as appropriate.

The input interface 54 includes an input device that receives various commands from the user. A keyboard, a mouse, various switches, a touch screen, a touch pad, etc. can be used as input devices. Input devices are not limited to those having physical operation parts such as mice and keyboards. For example, an electrical signal processing circuit that receives an electrical signal corresponding to an input operation from an external input device provided separately from the magnetic resonance imaging apparatus 1 and outputs the received electrical signal to various circuits is also input. Examples of interfaces 54 are included.

The communication interface 55 connects the magnetic resonance imaging apparatus 1, workstations, PACS (Picture Archiving and Communication System), HIS (Hospital Information System), RIS (Radiology Information System), etc. via a LAN (Local Area Network) or the like. This is the interface to connect. The network IF transmits and receives various information to and from connected workstations, PACS, HIS and RIS.

In addition, the above configuration is an example, and the present invention is not limited to this. For example, sequence control circuit 29 may be incorporated in host computer 50 . Also, the sequence control circuit 29 and the processing circuit 51 may be mounted on the same substrate.

An operation example of the magnetic resonance imaging apparatus 1 according to this embodiment will be described below. First, the calibration scan will be explained. In order to make the following explanation concrete, the stack of non-Cartesian collection is assumed to be the stack of stars collection.

FIG. 3 is a diagram showing the k-space notation of the encoding mode in a normal calibration scan. The vertical axis in FIG. 3 is defined as the kz axis. The horizontal axis is defined in the kx-ky plane. The arrow 65 shown in FIG. 3 indicates slice encoding in the kz direction. As described above, in each slice encoding, a radial scan consisting of a plurality of radial k-space trajectories is performed. Let arrows 65 represent multiple k-space trajectories for radial scans for that slice encode.

As shown in FIG. 3, in the normal calibration scan, the same slice encoding as in the main scan is performed. That is, in a normal calibration scan, data acquisition is performed for the same slice encoding as the slice encoding for the main scan. For the kx-ky plane, data acquisition is performed for the same k-space trajectory as the radial scan of the main scan. In a normal calibration scan, for example, data is collected from a phantom. Acquisition delays in the gradient magnetic field application directions (kx, ky, and kz directions) of the three gradient magnetic field coils are formulated based on the k-space data collected by the calibration scan. A delay time D for intermediate angles of each axis is interpolated using, for example, the following correction formula (1). where _D1 is the kx-axis delay time, _D2 is the ky-axis delay time, and θ is the angle of the k-space trajectory.

D=D ₁ cos ² θ+D ₂ sin ² θ (1)

However, 1. 1. Deterioration due to interpolation generation of oblique angle correction formula (1); Degradation of the image quality of the reconstructed image is caused by two factors, ie, degradation due to the difference between the time of phantom measurement and the time of collection of the human body. In addition, in the stack of stars acquisition, since the oblique angle is minutely measured for each object, a large amount of time is required for the calibration scan.

FIG. 4 is a diagram showing k-space notation of one mode of slice encoding in the calibration scan according to the present embodiment. A dotted arrow 65 shown in FIG. 4 indicates slice encoding that is performed in a normal calibration scan but not performed in a calibration scan according to the present embodiment. A solid-line arrow 67 shown in FIG. 4 indicates slice encoding that is performed in the normal calibration scan and in the calibration scan according to the present embodiment.

As shown in FIG. 4, in the calibration scan according to this embodiment, slice encoding corresponding to a single slice is performed. For example, data acquisition is limited to a single slice encode at the center of the kz direction (kz=0). The slice encode where the data acquisition takes place does not have to be perfectly central with respect to the kz direction, but rather from the slice encode where the intensity of the MR signal acquired is high, in other words, the slice encode with spatial frequency components that dominate the image. It should be selected. Specifically, data acquisition may be performed for one slice encode that falls within the central 1/4 range of the spatial frequency range in the kz direction. Hereinafter, the central 1/4 range of the spatial frequency range will be referred to as the central portion, and the areas other than the central portion will be referred to as the end portions. Note that slice encoding for data acquisition may be set to one slice encoding that falls within the central ⅛ range of the spatial frequency range. In this way, since data collection is limited to a single slice encoding, the time required for calibration scanning can be reduced compared to normal calibration scanning in which data is collected for all slice encodings. . In addition, since the time resolution of the calibration scan is enhanced, it is less likely to be affected by respiration of the subject P.

FIG. 5 is a diagram showing k-space notation of another encoding mode in the calibration scan according to this embodiment. In the calibration scan shown in FIG. 5, data acquisition is limited to two or more slice encodes that fit in the center in the kz direction. In other words, no data acquisition is performed for the ends in the kz direction. For example, data acquisition may be performed for 3 to 5 slice encodes included in the center. Note that data collection may be performed for all slice encodes targeted for the main scan in the central portion, or data collection may be performed by limiting to arbitrary slice encodes among all slice encodes targeted for the main scan. may In this way, data collection is limited to slice encoding in the central part, so the time required for calibration scanning can be reduced compared to normal calibration scanning in which data is collected for all slice encodings. . In addition, since the time resolution of the calibration scan is enhanced, it is less likely to be affected by respiration of the subject P.

FIG. 6 is a diagram showing k-space representation of another encoding mode in the calibration scan according to the present embodiment. In the calibration scan shown in FIG. 6, slice encoding is thinned out at predetermined intervals. For example, as shown in FIG. 6, slice encoding may be thinned out every one encoding, or slice encoding may be thinned out every two or more encodes. Also, the thinning interval does not need to be constant, and may be increased, for example, in the kz direction. In this way, data collection is performed only for thinned slice encodes, so the time required for calibration scans can be reduced compared to normal calibration scans in which data is collected for all slice encodes. can. In addition, since the time resolution of the calibration scan is enhanced, it is less likely to be affected by respiration of the subject P.

FIG. 7 is a diagram showing k-space notation of another encoding mode in the calibration scan according to this embodiment. A thin arrow 69 shown in FIG. 7 indicates slice encoding in which data collection is performed in the normal calibration scan and slice encoding in which dummy shots are performed in the calibration scan according to the present embodiment. In the calibration scan shown in FIG. 7, data acquisition is limited to the central slice encode in the kz direction, and dummy shots or conditioning shots are applied to the two end slice encodes. A dummy shot or a conditioning shot is a gradient magnetic field application that is not used to generate calibration parameters. Alternatively, data acquisition may be limited to a plurality of slice encodes at the center in the kz direction, and dummy shots or conditioning shots may be applied to one or more slice encodes at the ends. Data collection may be performed instead of dummy shots or conditioning shots.

In the above calibration scans of FIGS. 4 to 7, it is assumed that data acquisition is performed for the same slice encoding or k-space trajectory as in the main scan. However, this embodiment is not limited to this. As shown below, data acquisition is performed for slice encoding or k-space trajectories that are different from the main scan.

FIG. 8 is a diagram showing k-space representation of another encoding mode in the calibration scan according to this embodiment. A thick arrow 71 shown in FIG. 8 indicates slice encoding that is not performed in the main scan, but is performed in the calibration scan. In the calibration scan shown in FIG. 8, data acquisition is performed a plurality of times while changing the direction of the k-space trajectory with respect to the target slice encoding of the main scan. Although the positions of arrows 67 and 71 are shifted in the kz direction in FIG. 8, they are actually slice encoded at the same position.

For example, as shown in FIG. 8, two data acquisitions are made with different k-space orientations for a single slice encode centered in the kz direction. More specifically, the sequence control circuit 29 first radially scans the kz center slice encode along a plurality of radial k-space trajectories in a forward direction, as indicated by arrows 67 . Next, as indicated by an arrow 71, the sequence control circuit 29 radially scans the kz center slice encode in the opposite direction along the k-space trajectory in the same direction as the k-space trajectory of the first radial scan. In this case, the k-space trajectory of the first radial scan is the same k-space trajectory as the main scan, and the k-space trajectory of the second radial scan is a different k-space trajectory from the main scan. In this way, by performing data acquisition a plurality of times while changing the direction of the k-space trajectory in the same direction and performing calibration using the difference, it is possible to improve the accuracy of the calibration parameters. Note that data may be acquired along a k-space trajectory different from that of the main scan first, and then data may be acquired along the same k-space trajectory as that of the main scan.

Turning data acquisition is not limited to a single slice encode centered in the kz direction. It can be appropriately applied to the encoding modes shown in FIGS. For example, data acquisition may be performed while changing the orientation for each of a plurality of slice encodes in the center in the kz direction. Also, the number of data acquisitions for the k-space trajectory in the same direction is not limited to two, and may be three or more.

FIG. 9 is a diagram showing k-space notation of another encoding mode in the calibration scan according to this embodiment. In the calibration scan shown in FIG. 9, data collection is performed for slice encodes targeted for data collection in the main scan, and for slice encodes whose positions are close to the slice encodes. For example, with respect to slice encoding to be data-collected in the main scan, data is collected for the slice encoding, the slice encoding shifted by a predetermined step width in the +kz direction, and the slice encoding shifted by a predetermined step width in the -kz direction. done. The step width in the kz direction of the calibration scan is set to a value shorter than the step width of the main scan. In this way, by performing dense data collection for a plurality of slice encodes at different positions with respect to one slice encode, it is possible to improve the accuracy of the calibration parameters for the one slice encode.

It should be noted that the slice encode for data acquisition of the calibration scan does not need to be set on both the +kz direction and the -kz direction, and may be set on either one side. Also, in the calibration scan, data acquisition does not have to be performed for the slice encoding for which data is to be acquired in the main scan. Also, the slice encoding in the +kz direction or the -kz direction is not limited to single slice encoding, and may be multiple slice encoding arranged at the predetermined step width.

FIG. 10 is a diagram showing k-space representation of another encoding mode in the calibration scan according to this embodiment. In the calibration scan shown in FIG. 10, data acquisition is performed on the k-space trajectory and the k-space trajectory near the k-space trajectory with respect to the k-space trajectory targeted for data acquisition in the main scan. For example, assume that the k-space trajectory of the data acquisition target for the main scan is kz=ky=0 and is parallel to the kx axis. In this case, in the calibration scan, data is collected for a plurality of k-space trajectories that are set on a plane that is substantially orthogonal to the k-space trajectory of the data acquisition target of the main scan, and that have the same direction and different positions as the k-space trajectory. is done. For example, with respect to the k-space trajectory of the data acquisition target of the main scan, the k-space trajectory, the k-space trajectory shifted by a predetermined step width in the +ky direction, and the k-space trajectory shifted by a predetermined step width in the −ky direction. Data will be collected on That is, the step width in the kz direction of the calibration scan is set to a value shorter than or the same as the step width of the main scan. In this way, by densely collecting data on a plurality of k-space trajectories at different positions with respect to one k-space trajectory, it is possible to improve the accuracy of the calibration parameters for the one k-space trajectory. .

Note that the k-space trajectory of the calibration scan target may be set not only in the ±ky directions but also in the ±kx directions. Also, the k-space trajectory may be set only in one of the ±kx directions or only in one of the ±ky directions. Also, in the calibration scan, data acquisition may not be performed for slice encoding for the main scan. The k-space trajectory set in the ±kx direction or ±ky direction is not limited to single slice encoding, and may be a plurality of k-space trajectories arranged with a predetermined step width. Also, the encoding modes shown in FIGS. 4 to 10 can be appropriately combined. For example, the encoding mode shown in FIG. 9 and the encoding mode shown in FIG. 10 may be combined.

Next, the flow of MR examination including calibration scan and main scan will be described.

FIG. 11 is a diagram showing a typical flow of MR examination according to this embodiment. In the MR examination shown in FIG. 11, the processing circuit 51 corrects the k-space data acquired by the main scan afterward in order to correct the k-space trajectory of the main scan.

As shown in FIG. 11, the sequence control circuit 29 first executes a calibration scan (step SA1). A pulse sequence for the calibration scan is generated according to the encoding mode or the like shown in FIG. 4-10 by implementing the imaging protocol setting function 511 . A calibration scan is performed on the subject P to be subjected to the main scan.

After step SA1 is performed, the sequence control circuit 29 executes the main scan (step SA2). The main scan is executed according to a preset pulse sequence. That is, the pulse sequence of the main scan is not calibrated. The main scan collects k-space data on the measured k-space trajectory. There is a discrepancy between the measured k-space trajectory and the set k-space trajectory due to hardware imperfections.

In parallel with step SA2, the processing circuit 51 implements the k-space trajectory correction function 512 to calculate calibration parameters based on the k-space data collected in step SA1 (step SA3). In step SA3, the processing circuit 51 calculates a correction function for correcting the deviation between the actually measured k-space trajectory and the set k-space trajectory, and calculates calibration parameters based on the correction function. The mode of deviation between the measured k-space trajectory and the set k-space trajectory may be offset, rotation, extension, shortening, or a combination of these. A calibration parameter is a parameter that indicates a temporal and/or positional relationship between a set k-space trajectory and an actually measured k-space trajectory.

FIG. 12 is a diagram schematically showing an offset between an actually measured k-space trajectory and a set k-space trajectory, taking as an example only an offset as a deviation. As shown in FIG. 12, the set k-space trajectory T1 passes through the center of the kx-ky plane, and the measured k-space trajectory T2 does not pass through the center of the kx-ky plane. The processing circuit 51 calculates an offset DO between the set k-space trajectory T1 and the actually measured k-space trajectory T2. For example, the acquisition position P1 of the set k-space trajectory T1 and the acquisition position P2 of the actually measured k-space trajectory T2 are set. Acquisition position P1 or P2 corresponds to the central time of the data acquisition window of the k-space trajectory T1 or T2, in other words, the central time of the readout gradient magnetic field. The processing circuit 51 calculates the distance and direction between the collection position P1 and the collection position P2 as the offset DO. Offset DO is an example of a calibration parameter. Further, the processing circuit 51 converts the offset DO into _D1 and _D2 of the above correction formula (1), and delays according to the above correction formula (1) based on _D1 and _D2 and the direction θ of the offset DO. Time D may be calculated. Delay time D is an example of a calibration parameter. The processing circuit 51 calculates a calibration parameter for slice encoding to be scanned for calibration according to the above algorithm. The calibration parameters are applied to slice encoding for the calibration scan and slice encoding for the main scan but not for the calibration scan.

When steps SA2 and SA3 are performed, the processing circuit 51 corrects the k-space data as one mode of correcting the k-space trajectory by implementing the k-space trajectory correction function 512 (step SA4). At step SA4, the processing circuit 51 corrects the k-space data collected at step SA2 based on the calibration parameters calculated at step SA3. Specifically, the corrected k-space data is generated by shifting the acquisition position of the k-space data acquired in step SA2 by the offset calculated in step SA3. The processing circuit 51 sets the calibration parameters not only for k-space data for slice encoding at the same position as the slice encoding for the calibration scan, but also for k-space data for slice encoding at a position different from the slice encoding for the calibration scan. also apply. As a result, corrected k-space data is calculated for all slice encodes to be scanned.

When step SA4 is performed, the processing circuit 51 reconstructs an MR image by implementing the image reconstruction function 513 (step SA5). In the radial scan, sample points are arranged at non-equidistant intervals in the k-space data, so the processing circuit 51 uses a reconstruction method using the Jackson method, the gridding method, or the like. The processing circuit 51 restores the corrected non-uniformly spaced k-space data generated in step SA4 to equally spaced k-space data by the Jackson method. The processing circuit 51 applies FFT (Fast Fourier Transfer) to the equally spaced k-space data to generate an MR image. This makes it possible to generate an MR image in which image quality deterioration due to hardware imperfections is improved.

When step SA5 is performed, the processing circuit 51 displays an MR image by implementing the display control function 515 (step SA6). At step SA6, the processing circuit 51 displays the MR image on the display 53, for example. As a result, the user can observe MR images in which image quality deterioration due to hardware imperfections has been improved.

With the above, the MR examination according to the present embodiment is completed.

Note that the flow of the MR examination according to this embodiment is not limited to the flow shown in FIG. An MR examination may be performed along any sequence as long as the k-space trajectory of the main scan can be corrected based on the k-space data acquired by the calibration scan.

FIG. 13 is a diagram showing another flow of MR examination according to this embodiment. As shown in FIG. 13, the sequence control circuit 29 first executes a calibration scan (step SB1). Step SB1 is similar to step SA1.

When step SB1 is performed, the processing circuit 51 implements the k-space trajectory correction function 512 to calculate calibration parameters based on the k-space data collected in step SB1 (step SB2). The processing circuit 51 calculates a calibration parameter for slice encoding to be scanned for calibration according to the above algorithm. The calibration parameters are applied to slice encoding for the calibration scan and slice encoding for the main scan but not for the calibration scan. Step SB2 is similar to step SA3.

When step SB2 is performed, the processing circuit 51 implements the k-space trajectory correction function 512 to correct the pulse sequence of the main scan as one mode of k-space trajectory correction (step SB3). At step SB3, the processing circuit 51 preliminarily corrects the pulse sequence of the main scan performed at step SB4 based on the calibration parameters calculated at step SB2. For example, a pulse sequence is calculated such that the k-space trajectory of the acquired k-space data matches the intended k-space trajectory when shifted by the offset calculated in step SB2. Specifically, the application start time of the readout gradient magnetic field is corrected according to the offset calculated in step SB2. A delay time D corresponding to the offset is calculated according to the correction formula (1), and the application start time of the readout gradient magnetic field is postponed by the delay time D. This corrects the k-space trajectory of the main scan. Note that the processing circuit 51 sets the calibration parameters obtained for the slice encode for the calibration scan target not only to the pulse sequence for the slice encode at the same position as the slice encode for the calibration scan target, but also to the slice encode for the calibration scan target. It also applies to pulse sequences related to slice encoding at different positions. As a result, corrected pulse sequences are calculated for all slice encodes to be scanned.

When step SB3 is performed, the sequence control circuit 29 executes the main scan (step SB4). The main scan in step SB4 is executed according to the pulse sequence generated in step SB3. K-space data is collected by performing the main scan. As described above, the pulse sequence for the main scan is calibrated. The actually measured k-space trajectory of the k-space data acquired by the main scan in step SB4 matches the intended k-space trajectory.

When step SB4 is performed, the processing circuit 51 reconstructs the MR image by implementing the image reconstruction function 513 (step SB5). Step SB5 is similar to step SA5.

When step SB5 is performed, the processing circuit 51 displays the MR image by realizing the display control function 515 (step SB6). As a result, the user can observe an MR image in which image quality deterioration due to hardware imperfections has been improved, as in step SA6.

This completes the description of another flow of the MR examination according to the present embodiment.

Various modifications are possible for the above MR examination. For example, a pulse sequence including pre-pulse application may be executed in the main scan. A pre-pulse is an RF pulse for adjusting the contrast of an image. Any pre-pulse may be used as the pre-pulse according to this embodiment. There is no need to precisely adjust the contrast in the calibration scan, and there is no need to calculate the calibration parameters (shift and delay time). For this reason, the processing circuit 51 according to the present embodiment implements the imaging protocol setting function 511 so that even when a pulse sequence including pre-pulse application is executed in the main scan, the pulse sequence for the calibration scan is: A pulse sequence that does not include application of the pre-pulse is generated and executed. For example, the pulse sequence for the calibration scan may be generated by excluding the pre-pulse application from the pulse sequence including the pre-pulse application for the main scan. Alternatively, the pulse sequence for the calibration scan may be generated independently of the pulse sequence for the main scan.

For the calibration scan, it is not necessary to remove all pre-pulse applications. For example, the application of at least two pre-pulses may be incorporated into the pulse sequence of the calibration scan. In this case, by applying the first pre-pulse and subsequent application of the gradient magnetic field, data acquisition is performed forward on the k-space trajectory in the direction of interest. It is preferable to collect data by reversing the directional k-space trajectory (reversing the forward direction by 180 degrees). Processing circuitry 51 provides calibration parameters based on the k-space data for the first pre-pulse and the k-space data for the second pre-pulse. For example, the calibration parameters are set based on an intermediate value or an average value between the k-space data regarding the first pre-pulse and the k-space data regarding the second pre-pulse. Alternatively, calculating a first calibration parameter based on k-space data regarding the first pre-pulse, calculating a second calibration parameter based on k-space data regarding the second pre-pulse, and calculating the first calibration parameter based on k-space data regarding the second pre-pulse and the second calibration parameter, the calibration parameter may be calculated based on an intermediate value, an average value, or the like. By using k-space data relating to a pair of k-space trajectories in the same direction but in opposite directions, more accurate calibration parameters can be calculated.

In a calibration scan, data acquisition may be performed multiple times after applying one pre-pulse. In this case, the processing circuit 51 does not have to use all of the k-space data related to multiple data acquisitions for calculating the calibration parameters, and may use some of them. For example, in calculating the calibration parameters, the processing circuit 51 does not use the k-space data from the application of the prepulse to the predetermined data acquisition because the MR signal is unstable immediately after the application of the prepulse. That is, the processing circuit 51 calculates the calibration parameters based on the k-space data acquired after the predetermined number of data acquisitions from the application of the pre-pulse. The predetermined number can be arbitrarily set by the user, for example, the first or second. This can improve the accuracy of the calibration parameters.

Also, in the above description, the non-Cartesian scan is assumed to be the radial scan. However, the present embodiment is not limited to this, and non-Cartesian scanning can also be applied to spiral scanning that acquires data along a spiral k-space trajectory. Hardware imperfections in spiral scanning are manifested in the tangential deviation of each acquisition position of the spiral k-space trajectory. Similar to the radial scan, the processing circuit 51 calculates the deviation between the actually measured acquisition position and the set acquisition position of the k-space trajectory related to the calibration scan, and based on the calculated deviation, the spiral k Correct the spatial trajectory. The acquisition position may be set at, for example, a position where the strength of the readout gradient magnetic field is zero.

For example, both the main scan and the calibration scan may be spiral scans. In this case, the spiral scan of the calibration scan is performed in accordance with the encoding modes shown in FIGS. 4 to 10, etc., limited to slice encoding with a smaller number of encodes in the kz direction than the main scan. This makes it possible to shorten the calibration scan time and improve the time resolution. The processing circuit 51 corrects the spiral k-space trajectory of the main scan based on the k-space data of the spiral k-space trajectory of the calibration scan.

The main scan may be spiral scan and the calibration scan may be radial scan. In this case, the processing circuit 51 corrects the spiral k-space trajectory of the main scan based on the k-space data of the radial k-space trajectory of the calibration scan. Thus, the processing circuit 51 according to this embodiment can also correct the k-space trajectory for spiral scanning.

In the above description, it is assumed that the calibration scan is performed separately from the main scan. However, this embodiment is not limited to this. For example, a calibration scan and a main scan may be performed for each predetermined number of slice encodes or predetermined books of k-space trajectories.

A calibration scan may be incorporated into the main scan. For example, in the main scan, data acquisition is sequentially performed for a plurality of k-space trajectories. An arbitrary k-space trajectory among the plurality of k-space trajectories is set as a k-space trajectory to be calibrated. After data collection is performed on the k-space trajectory to be calibrated in the main scan, data collection is performed in the reverse direction on the k-space trajectory to be calibrated as a calibration scan. For example, in the main scan, 0°, 45°, 90°, 135°, 180°, 225°, and 270° k-space trajectories are set for data acquisition, and 135° k-space trajectory is set for calibration. shall be In this case, for example, if 0°, 45°, and 90° k-space trajectories are sequentially collected in the main scan, and data is collected for 135°, then a 315° k-space trajectory is collected as a calibration scan. Data will be collected. The processing circuit 51 calculates calibration parameters based on the k-space data relating to the k-space trajectory of the calibration target collected in the calibration scan, and calculates the calibration parameters collected in the main scan based on the calculated calibration parameters. Correct the k-space data for the k-space trajectory of interest. Thereby, the k-space trajectory of the main scan can be corrected.

As described above, the magnetic resonance imaging apparatus 1 according to this embodiment has the sequence control circuit 29 and the processing circuit 51 . The sequence control circuit 29 performs a non-Cartesian calibration scan by limiting slice encoding with a smaller number of encodes than the number of encodes in the non-Cartesian main scan, and collects k-space data of the first k-space trajectory. . Processing circuitry 51 corrects the second k-space trajectory for the main scan based on the k-space data collected from the calibration scan.

With the above configuration, it is possible to correct the deviation of the k-space trajectory related to the main scan due to hardware imperfections that appear prominently in the non-Cartesian scan. Image quality can be improved. Also, by reducing the number of encodings for the calibration scan compared to the number of encodings for the main scan, the scan time for the calibration scan can be reduced. In addition, along with this, the time resolution of the entire k-space data acquired by the calibration scan can be increased, so that deterioration of the k-space data caused by body movements caused by respiration of the subject P, for example, can be reduced. be possible.

According to at least one embodiment described above, it is possible to improve the image quality of MR images obtained by non-Cartesian scanning.

The term "processor" used in the above description includes, for example, a CPU, a GPU (Graphics Processing Unit), or an application specific integrated circuit (ASIC)), a programmable logic device (for example, a simple programmable logic device (Simple Programmable Logic Device: SPLD), Complex Programmable Logic Device (CPLD), and Field Programmable Gate Array (FPGA)). The processor realizes its functions by reading and executing the programs stored in the memory circuit. It should be noted that instead of storing the program in the memory circuit, the program may be directly installed in the circuit of the processor. In this case, the processor implements its functions by reading and executing the program embedded in the circuit. Also, functions corresponding to the program may be realized by combining logic circuits instead of executing the program. Note that each processor of the present embodiment is not limited to being configured as a single circuit for each processor, and may be configured as one processor by combining a plurality of independent circuits to realize its function. good. Furthermore, a plurality of components in FIG. 1 may be integrated into one processor to realize its functions.

While several embodiments of the invention have been described, these embodiments have been presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and modifications can be made without departing from the scope of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, as well as the scope of the invention described in the claims and equivalents thereof.

1 magnetic resonance imaging apparatus 11 pedestal 13 bed 21 gradient magnetic field power supply 23 transmission circuit 25 reception circuit 27 bed driving device 29 sequence control circuit 41 static magnetic field magnet 43 gradient magnetic field coil 45 transmission coil 47 reception coil 50 host computer 51 processing circuit 52 memory 53 display 54 input interface 55 communication interface 131 tabletop 133 base 511 imaging protocol setting function 512 k-space trajectory correction function 513 image reconstruction function 514 image processing function 515 display control function

Claims (18)

Slice encoding with a smaller number of encodings than the number of encodings in data acquisition for image reconstruction by non-Cartesian scanning, limited to slice encoding in the center in the array direction of slices , for calibration by non-Cartesian scanning a sequence controller that performs data acquisition of the first k-space trajectory and acquires k-space data for the first k-space trajectory;
a correction unit that corrects a second k-space trajectory related to data acquisition for image reconstruction based on the k-space data;
A magnetic resonance imaging apparatus comprising: In order to correct the second k-space trajectory, the correction unit calculates a deviation between the measured acquisition position of the second k-space trajectory and the set acquisition position regarding data acquisition for the calibration, 2. The magnetic resonance imaging apparatus according to claim 1, wherein the k-space data acquired by data acquisition for said image reconstruction is corrected based on said calculated shift. In order to correct the second k-space trajectory, the correction unit calculates a deviation between the measured acquisition position of the second k-space trajectory and the set acquisition position regarding data acquisition for the calibration, 2. A magnetic resonance imaging apparatus according to claim 1, wherein a pulse sequence for data acquisition for said image reconstruction is corrected based on said calculated deviation. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the target slice encoding for data acquisition for said calibration is a single slice encoding. 5. The magnetic resonance imaging apparatus according to claim 4 , wherein said sequence control unit executes data acquisition for said calibration a plurality of times for said single slice encoding. 5. The magnetic resonance imaging apparatus according to claim 4, wherein said single slice encode is positioned substantially at the center of the array of slices. 2. The magnetic resonance imaging apparatus according to claim 1, wherein target slice encodes for data acquisition for said calibration include a plurality of slice encodes positioned at said central portion with respect to the slice encode direction. 2. The magnetic resonance imaging apparatus according to claim 1, wherein a step width in the slice encoding direction in data acquisition for said calibration is shorter than a step width in data acquisition for said image reconstruction. In a plane orthogonal to the slice encoding direction, the sequence control unit performs correction of the second k-space trajectory with respect to a predetermined position, a third k-space trajectory at a single or a plurality of positions different from the predetermined position. 2. The magnetic resonance imaging apparatus of claim 1, wherein the k-space data is acquired for trajectories. 2. The magnetic resonance imaging apparatus according to claim 1, wherein said data acquisition for calibration does not include application of pre-pulses even when a pulse sequence for data acquisition for said image reconstruction includes application of pre-pulses. . The correction unit is
Based on the k-space data, calculate a calibration parameter for a slice encode whose position matches the target slice encode in the data acquisition for the calibration;
Using the calibration parameters for slice encoding whose position does not match target slice encoding in data acquisition for the calibration;
The magnetic resonance imaging apparatus according to claim 1. Data acquisition for calibration by non-Cartesian scanning is performed by limiting slice encoding with a number of encodes smaller than the number of encodes in data acquisition for image reconstruction by non-Cartesian scanning, and the first k-space trajectory a sequence controller that acquires k-space data;
a correction unit that corrects a second k-space trajectory related to data acquisition for image reconstruction based on the k-space data ;
The target slice encoding for data collection for the calibration is a single slice encoding,
The sequence control unit performs data collection for the calibration multiple times for the single slice encoding.
Magnetic resonance imaging equipment. Data acquisition for calibration by non-Cartesian scanning is performed by limiting slice encoding with a number of encodes smaller than the number of encodes in data acquisition for image reconstruction by non-Cartesian scanning, and the first k-space trajectory a sequence controller that acquires k-space data;
a correction unit that corrects a second k-space trajectory related to data acquisition for image reconstruction based on the k-space data ;
A step width in the slice encoding direction in the data acquisition for the calibration is shorter than the step width in the data acquisition for the image reconstruction.
Magnetic resonance imaging equipment. Data acquisition for calibration by non-Cartesian scanning is performed by limiting slice encoding with a number of encodes smaller than the number of encodes in data acquisition for image reconstruction by non-Cartesian scanning, and the first k-space trajectory a sequence controller that acquires k-space data;
a correction unit that corrects a second k-space trajectory related to data acquisition for image reconstruction based on the k-space data ;
In a plane orthogonal to the slice encoding direction, the sequence control unit performs correction of the second k-space trajectory with respect to a predetermined position, a third k-space trajectory at a single or a plurality of positions different from the predetermined position. collecting k-space data about the trajectory;
Magnetic resonance imaging equipment. Slice encoding with a smaller number of encodings than the number of encodings in data acquisition for image reconstruction by non-Cartesian scanning, limited to slice encoding in the center in the array direction of slices , for calibration by non-Cartesian scanning to collect k-space data for the first k-space trajectory;
correcting a second k-space trajectory for data acquisition for said image reconstruction based on said k-space data;
A method for correcting a k-space trajectory comprising: Data acquisition for calibration by non-Cartesian scanning is performed by limiting slice encoding with a number of encodes smaller than the number of encodes in data acquisition for image reconstruction by non-Cartesian scanning, and the first k-space trajectory an acquisition step of acquiring k-space data;
a correcting step of correcting a second k-space trajectory for data acquisition for said image reconstruction based on said k-space data ;
The target slice encoding for data collection for the calibration is a single slice encoding,
The collecting step performs data collection for the calibration multiple times for the single slice encoding.
Correction method for k-space trajectories. Data acquisition for calibration by non-Cartesian scanning is performed by limiting slice encoding with a number of encodes smaller than the number of encodes in data acquisition for image reconstruction by non-Cartesian scanning, and the first k-space trajectory collecting k-space data;
correcting a second k-space trajectory for data acquisition for said image reconstruction based on said k-space data ;
A step width in the slice encoding direction in the data acquisition for the calibration is shorter than the step width in the data acquisition for the image reconstruction.
Correction method for k-space trajectories. Data acquisition for calibration by non-Cartesian scanning is performed by limiting slice encoding with a number of encodes smaller than the number of encodes in data acquisition for image reconstruction by non-Cartesian scanning, and the first k-space trajectory an acquisition step of acquiring k-space data;
a correcting step of correcting a second k-space trajectory for data acquisition for said image reconstruction based on said k-space data ;
The acquiring step includes, in a plane orthogonal to the slice encoding direction, obtaining a third k-space trajectory at a single or a plurality of positions different from the predetermined position for correction of the second k-space trajectory with respect to the predetermined position. Collecting k-space data for
Correction method for k-space trajectories.

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