JP2009160378A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus, obtaining a three-dimensional cineradiographic image with high resolving power in a certain moving region in a free breathing state. <P>SOLUTION: This magnetic resonance imaging apparatus includes: a data acquisition unit, which acquires a plurality of data for imaging from an object and a plurality of projection data for obtaining the breathing position of the object by continuously applying a high-frequency pulse string at fixed intervals to the object; a correction unit, which performs motion correction of the plurality of data using the breath levels of the object obtained based on the plurality of projection data; a data sorting unit, which sorts the data after the motion correction into a cardiac time phase order based on electrocardiographic information; and a data reconstruction unit, which reconstructs three dimensional image data based on the sorted data after the motion correction. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention magnetically excites a subject's nuclear spin with a radio frequency (RF) signal of a Larmor frequency, and re-images the nuclear magnetic resonance (NMR) signal generated by this excitation. The present invention relates to a magnetic resonance imaging (MRI) apparatus, and in particular, magnetic resonance imaging capable of performing three-dimensional (3D) cine imaging in a moving part such as the heart under free breathing. Relates to the device.

  Magnetic resonance imaging is an imaging method in which a nuclear spin of a subject placed in a static magnetic field is magnetically excited with an RF signal having a Larmor frequency, and an image is reconstructed from an MR signal generated by the excitation.

Conventionally, cine imaging of the heart has been performed in the field of MRI (see, for example, Patent Document 1). The cine imaging of the heart by MRI is to perform two-dimensional (2D) imaging under electrocardiographic synchronization while a patient subject holds a breath for about 10 seconds. Usually, since a short-axis image is acquired so as to cover the entire left ventricle of the heart, it is necessary to repeat breath-hold imaging about 10 times. For this reason, k-space (Fourier space) data to be collected is divided into several regions (segmented), and a plurality of k-space data is sequentially collected for each segment during one breath hold, that is, segmented. A technique for collecting data by a sequence has been devised.
Japanese Patent Laid-Open No. 2007-82753

  However, in conventional cine imaging of the heart by MRI, there are problems that image quality degradation such as image blurring occurs and the slice position shifts when an image of a patient with difficulty in breath holding is taken. As a result, there is a concern that the accuracy of diagnosis such as cardiac function analysis is reduced. In addition, in conventional cine imaging of the heart by MRI, there is a problem that the burden on the patient increases because breath holding imaging is repeated about 10 times.

  Furthermore, in conventional cine imaging of the heart by MRI, it is difficult to improve temporal resolution and spatial resolution due to the restriction of breath holding time, so 3D imaging is considered impractical in practice.

  In contrast, the parallel imaging method, one of the high-speed imaging methods devised in recent years, has improved temporal resolution and spatial resolution, and 3D imaging has been attempted at the research level.

  However, this requires a long breath-holding time of several tens of seconds, and the resolution must be sacrificed to some extent.

  The present invention has been made to deal with such a conventional situation, and an object thereof is to provide a magnetic resonance imaging apparatus capable of acquiring a high-resolution 3D cine image in the heart under free breathing. .

  In order to achieve the above object, a magnetic resonance imaging apparatus according to the present invention applies a high-frequency pulse train to a subject continuously at a constant interval to thereby obtain a plurality of data for imaging from the subject and the subject. Data collection means for collecting a plurality of projection data for obtaining a breathing position, and a correction means for performing motion correction of the plurality of data using the breathing positions of the subject obtained based on the plurality of projection data. And a data rearranging means for rearranging the plurality of data after the motion correction based on the electrocardiographic information of the subject in order of cardiac time phases, and a plurality of data rearranged in the order of the cardiac time phases after motion correction. And image reconstruction means for reconstructing three-dimensional image data based on the image data.

  In order to achieve the above-described object, the magnetic resonance imaging apparatus according to the present invention applies a high-frequency pulse train to a subject continuously at a constant interval to thereby obtain a plurality of data for imaging from the subject and the subject. Data collecting means for collecting a plurality of projection data for determining the respiratory position of the specimen, data sorting means for rearranging the plurality of data in order of cardiac time phase based on the electrocardiogram information of the subject, and the plurality of data Correction means for performing motion correction of a plurality of data rearranged in order of the cardiac time phase using the respiratory position of the subject obtained based on the projection data of the subject, and rearranging in order of the cardiac time phase after the motion correction And image reconstructing means for reconstructing three-dimensional image data based on the plurality of pieces of data.

  In the magnetic resonance imaging apparatus according to the present invention, it is possible to acquire a high-resolution 3D cine image at a site such as the heart that moves under free breathing.

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

  FIG. 1 is a configuration diagram showing an embodiment of a magnetic resonance imaging apparatus according to the present invention.

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

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

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

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

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

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

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

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

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

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

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

  Further, the magnetic resonance imaging apparatus 20 includes an ECG unit 38 that acquires an ECG (electro cardiogram) signal of the subject P. The ECG signal acquired by the ECG unit 38 is configured to be output to the computer 32 via the sequence controller 31.

  Note that a pulse wave synchronization (PPG) signal can be acquired instead of the ECG signal. The PPG signal is, for example, a signal obtained by detecting a fingertip pulse wave as an optical signal. When acquiring the PPG signal, a PPG signal detection unit is provided.

  Further, the computer 32 is provided with various functions by executing the program stored in the storage device 36 of the computer 32 by the arithmetic unit 35. However, a specific circuit having various functions may be provided in the magnetic resonance imaging apparatus 20 regardless of the program.

  FIG. 2 is a functional block diagram of the computer 32 shown in FIG.

  The computer 32 includes an imaging condition setting unit 40, a sequence controller control unit 41, a k-space database 42, an ECG trigger detection unit 43, an ECG database 44, a breathing position calculation unit 45, a gating unit 46, a data correction unit 47, data according to a program. It functions as a rearrangement unit 48, an image reconstruction unit 49, an image database 50, and an image processing unit 51.

  The imaging condition setting unit 40 has a function of setting imaging conditions including a pulse sequence based on instruction information from the input device 33 and giving the set imaging conditions to the sequence controller control unit 41. For this purpose, the shooting condition setting unit 40 has a function of causing the display device 34 to display shooting condition setting screen information. In particular, the imaging condition setting unit 40 has a function of setting a pulse sequence for acquiring a 3D cine image of a part such as the heart that moves in free breathing with high resolution.

  FIG. 3 is a diagram showing an example of a pulse sequence set in the imaging condition setting unit 40 shown in FIG.

  In FIG. 3, BREATH is the amount of heart motion due to respiration shown for reference, ECG is an ECG signal, RF is an RF pulse, Gss is a slice selection gradient magnetic field pulse (slice encoding ( SE is also called gradient magnetic field pulse for slice sncode), Gro is gradient magnetic field pulse for readout (RO) (also called gradient magnetic field pulse for frequency encode), Gpe is phase encoding ( PE (phase encode) gradient magnetic field pulse, Gproj represents a frequency encoding gradient magnetic field pulse for collecting projection data for obtaining the respiratory position of the subject P, respectively.

  The pulse sequence shown in FIG. 3 is a balanced free precession (SSFP) sequence. That is, by applying RF pulses continuously at a constant repetition time (TR), a steady state of magnetization is maintained, and an NMR signal is generated. However, a different gradient echo sequence such as a FLASH (fast low angle shot) sequence, FISP (fast imaging with steady-state precession) sequence, or PSIF (time reversed FISP) sequence may be used. .

  As shown in FIG. 3, a pulse sequence for acquiring a 3D cine image of a region that moves under free breathing has an imaging part (IMAGING PART) and a projection part (PROJECTION PART). More specifically, the imaging part and the projection part are executed alternately and repeatedly.

  Note that the pulse sequence is executed independently of the ECG signal without being synchronized. In other words, it is not ECG synchronous imaging. However, during the execution of the pulse sequence, the ECG unit 38 always collects and records electrocardiogram information. The collected electrocardiogram information may be the waveform of the ECG signal itself or time stamp information such as an R wave.

  In the imaging part of the pulse sequence, a readout gradient magnetic field pulse is applied as a frequency encoding pulse in one direction together with a two-phase phase encoding pulse of a slice selection gradient magnetic field pulse Gss and a phase encoding gradient magnetic field pulse Gpe. Thus, NMR signals are collected as imaging data during application of the readout gradient magnetic field pulse.

  The imaging part can be a sequence using a segment k-space method in order to realize 3D imaging with high temporal resolution by speeding up imaging. The segment k-space method is a method in which k-space (frequency space; also referred to as Fourier space) is segmented by dividing it into several regions, and k-space data is captured sequentially for each segment. For this reason, the intensity of the phase encoding gradient magnetic field pulse Gpe is set to a plurality of different values for each segment, and by collecting a plurality of data in all the segments, all the data in the k-space is collected. Buried. A plurality of phase encoding gradient magnetic field pulses Gpe having intensities set to a plurality of values corresponding to a certain segment are repeatedly applied over at least one heartbeat. Hereinafter, a case where the pulse sequence is a segmented sequence by the segment k-space method will be described.

  In the example of FIG. 3, the (n + 1) th projection part is executed following the imaging part for collecting data in the nth segment, and the (n + 1) th segment is followed by the (n + 1) th projection part. The pulse sequence is set so that the imaging part for collecting the data within is executed. Further, after the imaging part for collecting data in the (n + 1) th segment, the (n + 2) th projection part is executed. In this manner, the N imaging parts corresponding to the N segments and the projection parts provided between the adjacent imaging parts are alternately and sequentially repeated.

  In addition, the slice including the imaging target is selectively excited by applying the slice selection gradient magnetic field pulse Gss in the imaging part. In the example of FIG. 3, the slice selection gradient magnetic field pulse Gss is applied at the same timing as the RF pulse.

  On the other hand, in the projection part, a frequency encoding gradient magnetic field pulse Gproj for collecting projection data for obtaining the breathing position of the subject P is applied in the same direction as the direction of breathing motion or in a direction close to the direction of breathing motion. Is done. An NMR signal is collected as projection data for determining the breathing position of the subject P by applying the frequency encoding gradient magnetic field pulse Gproj for collecting the projection data.

  Further, the projection part is executed in a state where the slice is selected by the slice selection gradient magnetic field pulse Gss in the imaging part. That is, the pulse sequence so that the slice selection gradient magnetic field pulse Gss is shared by the imaging part and the projection part, and the projection part is executed in a state where the last selected slice is selected in the imaging part immediately before the projection part. Is set. For this reason, as shown in FIG. 3, when the SSFP sequence is used, the steady state of magnetization can be maintained even in the projection part.

  Further, the projection part is executed in a state excited by application of an RF pulse in the imaging part. That is, the RF pulse is also shared by the imaging part and the projection part, and the pulse sequence is set so that the projection part is executed in a state where it is excited by the application of the last RF pulse of the imaging part immediately before the projection part. The RF pulses of the imaging part and the projection part are applied at a constant repetition time, and the steady state is maintained. Therefore, no extra idle time is set between the imaging part and the projection part and between the projection part and the imaging part.

  FIG. 4 is a diagram showing a setting example of the application direction of the slice selected in the imaging part and the projection part of the pulse sequence shown in FIG. 3 and the frequency encoding gradient magnetic field pulse Gproj for collecting projection data.

  FIG. 4 shows a typical imaging section and the direction of each gradient magnetic field pulse set when imaging the heart of the subject P. When imaging the heart of the subject P, the volume including the heart is used as an imaging slab during the imaging plan. The imaging slab is set to a short-axis cross section including the left ventricle of the heart. Therefore, the slice selection gradient magnetic field pulse Gss is in the direction perpendicular to the short axis cross section of the heart, the readout gradient magnetic field pulse Gro in the imaging part is in the short axis direction of the heart, and the phase encoding gradient magnetic field pulse Gpe is The slice selection gradient magnetic field pulse Gss is set in a direction perpendicular to both the application direction of the readout gradient magnetic field pulse Gro and the application direction of the readout gradient magnetic field pulse Gro.

  On the other hand, the frequency encoding gradient magnetic field pulse Gproj for collecting projection data is applied in the same direction as the direction of respiration movement or in a direction close to the direction of respiration movement. Therefore, in the example of FIG. 4, the application direction of the frequency encoding gradient magnetic field pulse Gproj for collecting projection data is set to the body axis direction of the subject P. Therefore, the application directions of the readout gradient magnetic field pulse Gro in the imaging part and the frequency encoding gradient magnetic field pulse Gproj for collecting projection data in the projection part are different from each other. Conversely, the slice selection gradient magnetic field pulse Gss and the RF pulse are shared between the imaging part and the projection part. Therefore, the conditions including the application timing of the slice selection gradient magnetic field pulse Gss and the RF pulse are the same between the imaging part and the projection part.

  The imaging conditions including the pulse sequence set in this way are configured to be given from the imaging condition setting unit 40 to the sequence controller control unit 41.

  The sequence controller control unit 41 has a function of controlling driving by giving imaging conditions including a pulse sequence to the sequence controller 31 in accordance with imaging start instruction information from the input device 33. The sequence controller control unit 41 has a function of receiving raw data from the sequence controller 31 and storing it in the k-space database 42.

  For this reason, each raw data generated in the receiver 30 is stored in the k-space database 42 as k-space data. That is, imaging data corresponding to a plurality of segments collected by execution of the pulse sequence set in the imaging condition setting unit 40 and projection data for obtaining the respiratory position of the subject P are sequentially written in the k-space database 42. Saved.

  The ECG trigger detection unit 43 has a function of acquiring an ECG signal separately acquired by the ECG unit 38 during data collection by executing a pulse sequence via the sequence controller 31 and the sequence controller control unit 41, and the acquired ECG signal. And a function of detecting a trigger signal based on electrocardiographic information such as an R wave. However, the ECG unit 38 may be configured to detect a trigger signal based on electrocardiographic information such as an R wave. In this case, a trigger signal based on the electrocardiogram information detected by the ECG unit 38 is acquired by the ECG trigger detection unit 43 via the sequence controller 31 and the sequence controller control unit 41. The ECG trigger detection unit 43 is configured to write a trigger signal based on the obtained electrocardiogram information in the ECG database 44.

  For this reason, the ECG database 44 stores trigger signals based on electrocardiographic information collected during data collection by the execution of the pulse sequence.

  The breathing position calculation unit 45 reads a plurality of time-series projection data collected intermittently by executing the projection part of the pulse sequence from the k-space database 42, and N based on the read time-series projection data. It has a function of calculating the respiration position of the subject P at the timing when the k-space data in the segments n (n = 1, 2, 3,..., N) are collected.

  Specifically, by performing FT (Fourier transform) on each of a plurality of time-series projection data in the readout direction, a plurality of projection data in real space showing respiratory motion can be created. Then, by referring to the projection data, it is possible to obtain the amount of movement due to respiration of the imaging region such as the heart of the subject P at the timing when each projection data is collected. The amount of movement of the subject P due to respiration in the imaging region can be obtained as a relative movement amount of the imaging region with respect to a certain reference position. As a method of obtaining the relative movement amount of the imaging part with respect to the reference position, for example, the correlation is obtained by taking a cross-correlation between the projection data corresponding to the reference position and the projection data for which the relative movement amount is to be obtained. A method for obtaining a typical position shift amount can be mentioned.

  Since each projection data is collected before or after each segment n corresponding to each segment n, the timing at which each projection data is collected can be regarded as the timing at which the corresponding segment n is collected. Therefore, the breathing position at the timing when each projection data is collected, that is, the amount of motion due to breathing can be regarded as the amount of motion at the timing when a plurality of corresponding k-space data in each segment n is collected.

  However, the breathing position data at the timing when each projection data is collected corresponds to the breathing position data before and after the timing when a plurality of k-space data in each segment n is collected. Therefore, if the respiratory position data is interpolated by calculating an average value or an interpolated value between the respiratory position data at the timing when each projection data is collected, an arbitrary position k (Kro, Kpe, Kse in each segment n is obtained. ), The respiratory position r (Kro, Kpe, Kse) at the timing when the k-space data S (Kro, Kpe, Kse) is collected can be calculated with higher accuracy. That is, the time change of the respiratory position of the subject P can be calculated.

  The gating unit 46 functions as necessary. The gating unit 46 then k-space data S (Kro, Kpe, Kse) at the position k (Kro, Kpe, Kse) in each segment n collected by executing the imaging part of the pulse sequence from the k-space database 42. , And applying gating based on each respiration position r (Kro, Kpe, Kse) at the timing when each k-space data S (Kro, Kpe, Kse) calculated in the respiration position calculation unit 45 is collected Only k-space data S (Kro, Kpe, Kse) excluding k-space data collected when the amount of movement or breathing position r (Kro, Kpe, Kse) is outside the predetermined threshold range Is provided to the data correction unit 47.

  As a result, k-space data S (Kro, Kpe, Kse) excluding k-space data, which has a large amount of movement due to free breathing of the subject P and is inappropriate to be used for generating cine image data, is extracted and extracted. The k-space data S (Kro, Kpe, Kse) can be selectively given to the data correction unit 47 for generating cine image data.

  The data correction unit 47 uses the k-space data S (Kro, Kpe, Kse) extracted by the gating unit 46 or the k-space data S (Kro, Kpe) collected from the execution of the imaging part read from the gating unit 46. , Kse) using the respiratory position data r (Kro, Kpe, Kse) of the subject P corresponding to each k-space data S (Kro, Kpe, Kse) acquired from the respiratory position calculation unit 45. It has a function of obtaining k-space data S ′ (Kro, Kpe, Kse) after motion correction by performing correction. That is, the k-space data S (Kro, Kpe, Kse) in each segment n is obtained from the respiration position data r (Kro, Kpe, Kse) calculated based on the projection data collected corresponding to each segment n. The motion is corrected.

The k-space data S (Kro, Kpe, Kse) before motion correction is corrected by using the respiratory position data r (Kro, Kpe, Kse) for the k-space data S (Kro, Kpe, Kse) before motion correction. ) Is converted into k-space data S ′ (Kro, Kpe, Kse) after motion correction can be expressed as shown in Equation (1).
[Equation 1]
S '(Kro, Kpe, Kse) = S (Kro, Kpe, Kse)
× exp [-I × (Gro ・ Gproj) × Kro × {r (Kro, Kpe, Kse) -r0} / FOVro]
× exp [-I × (Gpe ・ Gproj) × Kpe × {r (Kro, Kpe, Kse) -r0} / FOVpe]
× exp [-I × (Gse ・ Gproj) × Kse × {r (Kro, Kpe, Kse) -r0} / FOVse]
… (1)
In Expression (1), FOVro, FOVpe, and FOVse are imaging fields of view (FOV: field of view) in the frequency encoding direction, phase encoding direction, and slice encoding direction, respectively, and r0 is a reference position of the respiratory position. The symbol “·” represents the inner product.

  As a result of the motion correction according to Equation (1), a plurality of k-space data S ′ (Kro, Kpe, Kse) in each time-series segment n after motion correction is fixed to the reference position. This is equivalent to the k-space data collected from the imaging region. That is, the imaging region is actually moved by breathing in the slab, but can be handled as being fixed at the reference position.

  The data rearrangement unit 48 acquires time-series k-space data S ′ (Kro, Kpe, Kse) for each segment n after motion correction from the data correction unit 47, while triggering based on electrocardiographic information from the ECG database 44. After acquiring the signal, the trigger signal based on the electrocardiogram information, the time series k-space data S ′ (Kro, Kpe, Kse) after the data correction and the data acquisition time and the trigger signal acquisition time after the motion correction A function for associating k-space data S ′ (Kro, Kpe, Kse) with cardiac time phases, and each k-space data S ′ (Kro, Kpe, Kse) after motion correction associated with cardiac time phases And a function of rearranging (sorting) in order from the earliest. The k-space data after the motion correction is arranged at each position on a plurality of k-spaces corresponding to the cardiac time phase of the heart.

  FIG. 5 is a diagram for explaining a method of rearranging k-space data after motion correction in the data rearrangement unit 48 shown in FIG.

  FIG. 5A shows a time-series k space used for generating cine image data and a k-space data string arranged in the k space. As shown in FIG. 5 (a), k-space data on each k-space corresponding to each cardiac time phase t = t1, t2, t3,... Is required for generating cine image data. In FIG. 5 (a), the k-space data is shown two-dimensionally, but actually k-space data for a plurality of slices is required. Since the pulse sequence is a segmented sequence, each k space is divided into N segments n (n = 1, 2, 3,..., N). A plurality of k-space data strings after motion correction at each position k = kn1, kn2, kn3,... In each segment n are sequentially arranged in the k-space.

  Note that the segmentation of the k space in FIG. 5A is an example, and there is an arbitrary division method.

  FIG. 5 (b) shows the k-space data Sn ′ {k (Kro, Kpe, Kse)} after motion correction collected for each segment n asynchronously with the ECG signal by executing the imaging part of the pulse sequence. It is a figure which shows the example matched with the phase. That is, by obtaining the delay time TDn (k) from the acquisition time of the trigger signal such as the R wave at the collection time of the k space data Sn (k) at each position k in each segment n, the time series after motion correction Each k-space data Sn ′ (k) can be associated with a cardiac phase. As a result, as shown in FIG. 5B, the timing of the R wave and the collection timing of each k-space data Sn ′ (k) after the motion correction can be correlated.

  As shown in FIG. 5B, since the k-space data Sn ′ (k) is sequentially collected for each segment n asynchronously with the ECG signal, the time-series k at the same position k in the segment n in the k-space. Spatial data Sn ′ (k) is not necessarily collected at the same cardiac phase t = t1, t2, t3,. For example, after k-space data S1 ′ (k11) at position k11 in segment n = 1 is collected at cardiac time phase t1 ′ close to cardiac time phase t1, k-space data S1 at position k21 in segment n = 2 If '(k21) was collected at cardiac phase t2' close to cardiac phase t2, k-space data S1 '(k11) at position k11 within segment n = 1 collected at cardiac phase t1' Is used as the original data for the cine image data of the cardiac phase t1, while the k-space data S1 ′ (k21) at the position k21 in the segment n = 2 collected in the cardiac phase t2 ′ is the cardiac phase It will be used as the original data for the cine image data at t2. On the other hand, the k-space data S1 ′ (k21) at the position k21 in the segment n = 2 collected at the cardiac phase t1 '' close to the cardiac phase t1 at another data collection timing is the cine image of the cardiac phase t1. It will be used as the original data.

  Therefore, if k-space data Sn ′ (k) after motion correction at a position k on a k-space of a certain segment n is rearranged in order with the earliest cardiac phase t, the k-space after motion compensation with the earliest cardiac phase t. The data Sn ′ (k) is used as the original data for the cine image data of the cardiac phase t1, and then the k-space data Sn ′ (k) after the motion correction with the early cardiac phase is used as the cine image of the cardiac phase t2. It can be used as original data for data. Similarly, k-space data Sn ′ (k) after motion correction can be used as the original data for the corresponding cardiac phase cine image data in the order of early cardiac phase. In other words, all k-space data groups Sn ′ (k) (n = 1, 2, 3,... At the position k in each k-space of each segment n after motion correction associated with the earliest cardiac time phase. , N; k = k11, k12, k13,…, k21, k22, k23,…, k31, k32, k33,…, kn1, kn2, kn3,…, kN1, kN2, kN3,…) This is the original data for the cine image data of phase t1. Similarly, the k-space data group Sn ′ (k) after all the motion corrections is the original data for the corresponding cine image data of the cardiac phase t in the order of early cardiac phase.

  The image reconstruction unit 49 sequentially acquires k-space data after motion correction and after the rearrangement from the data rearrangement unit 48, and performs image reconstruction processing including FT on the real space data. It has a function of reconstructing image data for each cardiac phase of a subject P, and a function of writing time-series image data for each cardiac time phase obtained by reconstruction into the image database 50.

  For this reason, the image database 50 stores the image data reconstructed by the image reconstruction unit 49. The stored image data is data collected by a 3D pulse sequence for acquiring a 3D cine image of a moving part such as the heart set in the imaging condition setting unit 40, and thus becomes 3D cine image data. .

  The image processing unit 51 takes in 3D cine image data from the image database 50 and performs image processing such as MIP (Maximum Intensity Projection) processing and MPR (multi-planar reconstruction) processing to generate 2D cine image data for display. And a function for displaying the generated cine image data for display on the display device 34.

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

  FIG. 6 is a flowchart showing a procedure for capturing a 3D cine image of the heart of the subject P under free breathing by the magnetic resonance imaging apparatus 20 shown in FIG. 1. Each step is shown.

  First, in step S1, a pulse sequence is set in the imaging condition setting unit 40, and imaging under free breathing is started according to the set pulse sequence.

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

  The selection information of the segmented 3D pulse sequence having the imaging part and the projection part as shown in FIG. 3 is set by the operation of the input device 33 through the imaging condition setting screen displayed on the display device 34. The segmented 3D pulse sequence selected and given to the unit 40 is set as the imaging condition in the imaging condition setting unit 40. At this time, as shown in FIG. 4, the application direction of the readout gradient magnetic field pulse Gro in the imaging part is set to the short axis direction of the heart via the imaging condition setting screen, while the frequency for collecting the projection data is set. The application direction of the encoding gradient magnetic field pulse Gproj is set to the body axis direction of the subject P.

  Then, an instruction to start imaging a 3D cine image in the heart of the subject P using the segmented 3D pulse sequence is given from the input device 33 to the sequence controller control unit 4140.

  Then, in step S2, a segmented 3D pulse sequence having a projection part under the free breathing of the subject P is executed, and segmented time-series k-space data and time-series projection data corresponding to each segment are obtained. Collected sequentially.

  That is, the sequence controller control unit 41 acquires a segmented 3D pulse sequence having an imaging part and a projection part from the imaging condition setting unit 40 and supplies the segmented 3D pulse sequence to the sequence controller 31. The sequence controller 31 drives the gradient magnetic field power source 27, the transmitter 29, and the receiver 30 according to the segmented 3D pulse sequence received from the sequence controller control unit 41, thereby forming a gradient magnetic field in the imaging region where the subject P is set. In addition, an RF excitation pulse signal is generated from the RF coil 24.

  Therefore, an NMR signal generated by nuclear magnetic resonance inside the subject P is received by the RF coil 24 and given to the receiver 30. The receiver 30 receives the NMR signal from the RF coil 24, performs necessary signal processing, and then performs A / D conversion to generate raw data that is an NMR signal of digital data. The receiver 30 gives the generated raw data to the sequence controller 31. The sequence controller 31 provides the raw data to the sequence controller control unit 41, and the sequence controller control unit 41 writes the raw data as k-space data in the k-space database 42.

  Here, since the k-space data is collected by the segmented 3D pulse sequence, the k-space data for each segment is stored in the k-space database 42.

  On the other hand, in step S 3, the ECG signal waveform of the subject P during imaging is monitored by the ECG unit 38. Here, the ECG unit 38 detects a trigger signal based on electrocardiographic information such as an R wave from the collected ECG signal as necessary. Then, the collected ECG signal or trigger signal is given to the ECG trigger detection unit 43 via the sequence controller 31 and the sequence controller control unit 41. When the ECG trigger detection unit 43 receives an ECG signal instead of a trigger signal, the ECG trigger detection unit 43 detects a trigger signal based on electrocardiographic information such as an R wave. Furthermore, the trigger signal based on the obtained electrocardiogram information is written and stored in the ECG database 44 from the ECG trigger detection unit 43.

  Next, the motion amount of the subject P is calculated at the collection timing of the k-space data for each segment n, and the motion correction for each segment n of the k-space data based on the motion amount is performed.

  Therefore, in step S4, 1 is substituted for n and the first segment n = 1 is selected.

  Next, in step S5, the breathing position of the subject P at the timing when each k-space data in the first segment n = 1 is collected is calculated based on the projection data.

  That is, the respiration position calculation unit 45 reads a plurality of time-series projection data collected intermittently by the execution of the projection part of the pulse sequence from the k-space database 42, and applies FT in the readout direction to perform the respiration characteristics. Create time-series projection data in real space showing the movement of Then, by sequentially taking the cross-correlation between the plurality of time-series projection data collected at each time and the projection data collected at the time when the imaging region is at the reference position, each time when the projection data is collected, that is, The relative movement amount from the reference position of the imaging region before and after the timing when the k-space data in each segment n is collected can be obtained as the respiratory position. Furthermore, the respiratory position of the subject P at an arbitrary time can be obtained by interpolating the obtained time-series respiratory position data of the subject P using an average value or an interpolated value.

  Therefore, the respiratory position of the subject P at each time when each k-space data in the first segment n = 1 is collected can be obtained.

  Next, in step S6, the movement of each k-space data using the breathing position of the subject P at each time when each k-space data in the first segment n = 1 is collected in the data correction unit 47. Correction is performed. That is, each k-space data in the first segment n = 1 before the motion correction is converted into k-space data after the motion correction by the motion correction based on the breathing position. This motion correction conversion process can be expressed by equation (1) as described above.

  If the k-space data collected when the movement of the subject P due to respiration is not used as the original data for generating cine image data, the respiration position is set in advance prior to the movement correction. The gating unit 46 can execute a gating process for excluding k-space data corresponding to a breathing position outside the threshold range. In this case, k-space data extracted by the gating process is a target for motion correction.

  The k-space data in the first segment is equivalent to data collected in such a manner that the imaging object actually moved by respiration is fixed at the reference position in the imaging slab by such motion correction. .

  Next, in step S7, it is determined whether or not the segment n subject to motion correction is the last segment N. If the segment is not the last segment N, n + 1 is added to n in step S8. Is substituted to select the next segment n. Then, for the next segment n, the calculation of the breathing position and the motion correction based on the breathing position are performed again in step S5 and step S6. By repeating such processing from segment 1 to segment N, motion correction of k-space data in all segments n is performed.

  When the motion correction for the last segment N is completed, it is determined in step S7 that the segment n targeted for motion correction is the last segment N.

  Then, in step S9, the data rearrangement unit 48 rearranges the 3D k-space data of each segment after motion correction in the order of cardiac phases.

  For this purpose, the 3D k-space data after the segmented time-series motion correction is synchronized with the ECG signal based on a trigger signal based on the ECG signal. That is, the data rearrangement unit 48 acquires an ECG trigger signal associated with the collection time from the ECG database 44.

  Next, the data rearrangement unit 48 obtains each delay time from the ECG trigger signal at the time when each k-space data is collected. Each obtained delay time is associated with k-space data after motion correction corresponding to the cardiac phase. Then, all k-space data of all segments n are associated with the cardiac phase, and cardiac phase information of each k-space data Sn ′ (k) after motion correction as shown in FIG. 5B is obtained. .

  Next, the data rearrangement unit 48 sorts the k-space data after the motion correction in the order of the heart time phase by sorting the k-space data after the motion correction associated with the cardiac time phase. . As a result, a set of k-space data after motion correction for each cardiac phase as shown in FIG. 5 (a) is obtained. Although not shown in FIG. 5 (a), k-space data after motion correction is acquired by a 3D sequence, and thus is obtained for a plurality of slices. Then, the 3D acquired data for each cardiac phase after the rearrangement and motion correction is accumulated as original data for generating 3D cine image data.

  Next, in step S10, the image reconstruction unit 49 performs image reconstruction processing on 3D acquired data for each cardiac phase after rearrangement and motion correction. Thereby, 3D cine image data in the heart of the subject P is generated. The generated 3D cine image data is written in the image database 50.

  Then, the image processing unit 51 reads 3D cine image data from the image database 50 and performs image processing such as MIP processing and MPR processing to generate 2D cine image data for display. Further, the generated 2D cine image data for display is output and displayed on the display device 34. Thereby, the user can observe the cine image in the heart of the subject P created based on the data collected under free breathing.

  In other words, the magnetic resonance imaging apparatus 20 as described above has a plurality of time periods in which the electrocardiogram is asynchronously moved under free breathing by a pulse sequence such as a 3D segmented sequence provided with a projection part for collecting projection data for detecting the respiratory position of the subject P. Collect phase data, perform data motion correction using the breathing position calculated based on the projection data, and rearrange in order of cardiac phase using a trigger signal based on separately acquired ECG information Thus, the cine image data in a moving part such as the heart is reconstructed.

  For this reason, according to the magnetic resonance imaging apparatus 20, a 3D cine image of a portion having movement such as the heart under free breathing is acquired with high temporal resolution and high spatial resolution by monitoring and correcting the amount of movement due to respiration. be able to. This makes it possible to increase the resolution of the 3D cine image and remove arrhythmia.

  In the flowchart shown in FIG. 6, an example of performing motion correction and rearrangement of the imaging data after the collection of the projection data and the imaging data by executing the pulse sequence is shown. During the collection of the projection data and the imaging data, That is, it is also possible to perform motion correction and rearrangement of imaging data in real time during data acquisition in parallel with data collection.

  In the flowchart shown in FIG. 6, an example is shown in which k-space data is rearranged in the cardiac phase sequence after motion correction of the k-space data. However, the k-space data is rearranged in the cardiac phase sequence. After performing, k-space data motion correction may be performed.

  FIG. 7 shows a procedure for performing 3D cine image capturing by performing k-space data motion correction after rearranging k-space data in the order of cardiac phases by the magnetic resonance imaging apparatus 20 shown in FIG. In the figure, reference numerals with numerals S indicate steps in the flowchart. In FIG. 7, steps similar to those in FIG.

  When k-space data motion correction is performed after the k-space data is rearranged in the cardiac phase order, k-space data and projection data are collected by executing a segmented 3D pulse sequence in step S2. After that, in step S9, the data rearrangement unit 48 rearranges the collected 3D k-space data of each segment in the order of cardiac phases.

  Then, after rearrangement of the k-space data, motion correction of the k-space data rearranged in steps S4 to S8 is performed. However, while the projection data is time-series data, k-space data to be subjected to motion correction is rearranged, and k-space data in the nth segment is not time-series k-space data at the time of data collection. Therefore, in step S6 ′, based on the data correspondence information before and after the rearrangement of the k-space data, the k-space data before the rearrangement in the nth segment is calculated based on the projection data of the corresponding segment. The motion is corrected based on the above.

  Therefore, if the k-space data is rearranged after the motion correction of the k-space data as in the procedure of the flowchart shown in FIG. 6, the k-space data that is not used as the original data of the cine image data by the gating process. Since synchronization processing and rearrangement processing are not required, the amount of data processing can be reduced. For this reason, the real-time property of the imaging data motion correction process and the rearrangement process after motion correction is improved, and as described above, the motion correction and rearrangement of the imaging data is performed in real time in parallel with the data collection during imaging. It becomes easy.

  In addition, when the pulse sequence is a sequence for radial acquisition, the k-space data in the frequency domain is non-rotated by repeatedly rotating a band-like region called a blade formed by a plurality of parallel data acquisition trajectories every time. In the case of a sequence according to the PROPELLER (periodically rotated overlapping parallel lines with enhanced reconstruction) method of collecting and filling orthogonally, the motion of the heartbeat is shown by FT k-space data collected by the execution of these sequences. Time series projection data can be obtained. Therefore, when data collection is performed using a pulse sequence that can obtain projection data indicating electrocardiographic information such as heartbeat motion, the same as the ECG signal can be obtained without obtaining an ECG signal or a PPG signal. It is possible to detect a trigger signal based on electrocardiographic information from projection data having periodicity. That is, electrocardiogram information can be detected from the time phase when each projection data is collected, and a trigger signal can be set from the detected electrocardiogram information. In this case, the ECG trigger detection unit 43 of the computer 32 creates a projection data indicating electrocardiogram information from the function of reading the k-space data collected from the k-space database 42 and the k-space data as described above. A function for detecting electrocardiogram information from data may be provided.

  In addition, the pulse sequence can be a slice non-selected radial acquisition 3D sequence. In this case, the radial acquisition 3D sequence collects a plurality of projection data for imaging and a plurality of projection data for obtaining a respiratory position. Further, since both the imaging data and the breathing position detection are projection data, the projection data in the same direction may be shared by the imaging and the breathing detection.

1 is a configuration diagram showing an embodiment of a magnetic resonance imaging apparatus according to the present invention. The functional block diagram of the computer shown in FIG. The figure which shows an example of the pulse sequence set in the imaging | photography condition setting part shown in FIG. The figure which shows the example of a setting of the application direction of the magnetic field pulse for frequency encoding for the slice selected in the imaging part and projection part of the pulse sequence shown in FIG. 3, and collection of projection data. The figure explaining the method of rearrangement of the k space data after the motion correction in the data rearrangement part shown in FIG. The flowchart which shows the procedure at the time of imaging the 3D cine image of the subject's heart under free breathing by the magnetic resonance imaging apparatus shown in FIG. The flowchart which shows the procedure in the case of performing 3D cine image pick-up by performing motion correction of k space data, after rearranging k space data to the cardiac phase order by the magnetic resonance imaging apparatus shown in FIG.

Explanation of symbols

20 Magnetic Resonance Imaging Device 21 Magnet for Static Magnetic Field 22 Shim Coil 23 Gradient Magnetic Field Coil 24 RF Coil 25 Control System 26 Static Magnetic Field Power Supply 27 Gradient Magnetic Field Power Supply 28 Shim Coil Power Supply 29 Transmitter 30 Receiver 31 Sequence Controller 32 Computer 33 Input Device 34 Display Device 35 arithmetic unit 36 storage unit 37 bed 38 ECG unit 40 imaging condition setting unit 41 sequence controller control unit 42 k-space database 43 ECG trigger detection unit 44 ECG database 45 respiration position calculation unit 46 gating unit 47 data correction unit 48 data rearrangement Unit 49 image reconstruction unit 50 image database 51 image processing unit P subject

Claims (9)

Data collecting means for collecting a plurality of data for imaging and a plurality of projection data for obtaining a breathing position of the subject by applying a high-frequency pulse train to the subject continuously at regular intervals;
Correction means for performing motion correction of the plurality of data using the respiration position of the subject obtained based on the plurality of projection data;
Data rearranging means for rearranging the plurality of data after the motion correction in order of cardiac phases based on the electrocardiogram information of the subject;
Image reconstruction means for reconstructing three-dimensional image data based on a plurality of data rearranged in order of cardiac phases after the motion correction;
A magnetic resonance imaging apparatus comprising: Data collecting means for collecting a plurality of data for imaging and a plurality of projection data for obtaining a breathing position of the subject by applying a high-frequency pulse train to the subject continuously at regular intervals;
Data rearranging means for rearranging the plurality of data in order of cardiac phase based on electrocardiographic information of the subject;
Correction means for performing motion correction of the plurality of data rearranged in order of the cardiac phase using the breathing position of the subject obtained based on the plurality of projection data;
Image reconstruction means for reconstructing three-dimensional image data based on a plurality of data rearranged in order of cardiac phases after the motion correction;
A magnetic resonance imaging apparatus comprising: The data collection means is configured to apply a gradient magnetic field to the subject so that a gradient magnetic field pulse is shared between an imaging part for collecting the imaging data and a projection part for collecting projection data. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. The data collection means is configured to apply the high-frequency pulse train so that a high-frequency pulse is shared between an imaging part for collecting the imaging data and a projection part for collecting projection data. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. The data collection means is configured to collect the plurality of data for each of a plurality of segments set in a k-space, and collect a plurality of projection data corresponding to each of the plurality of segments. The magnetic resonance imaging apparatus according to claim 1 or 2. Further comprising data extraction means for extracting data collected in a state where the respiration position of the subject is within a predetermined threshold range from the plurality of data;
The said image reconstruction means is comprised so that the said three-dimensional image data may be reconfigure | reconstructed based on the data rearranged in order of the extracted cardiac time phase after the motion correction. The magnetic resonance imaging apparatus described. The data collection means is configured to collect the plurality of data using a pulse sequence capable of obtaining projection data indicating electrocardiogram information of the subject,
3. The magnetic resonance according to claim 1, wherein the data rearranging unit is configured to rearrange the plurality of data in order of cardiac phases based on electrocardiographic information detected from the projection data. Imaging device. The data collection means is configured to collect the plurality of data using a Steady-state Free Precession sequence that applies the high-frequency pulse train so that a steady state of magnetization is maintained in the subject. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. 3. The magnetic resonance imaging apparatus according to claim 1, wherein the correction unit is configured to perform the motion correction during acquisition of the plurality of data for imaging and the plurality of projection data.

Images