JP2008183021A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the photographing efficiency when a wide region exceeding a uniform static magnetic field region is photographed. <P>SOLUTION: In the MRI apparatus, movement of a table is controlled by synchronizing the movement with the pulse sequence. A first signal is measured while moving the table, and during the period, the table is periodically stopped to measure a second signal. Specifically, the first signal is an echo signal which can reconstruct a morphologic image and the second signal is an echo signal which can reconstruct a functional image. The second signal may be an echo signal with which adjustment parameter and correction parameter can be computed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging technique. In particular, the present invention relates to a technique for performing imaging while moving a table on which a subject is placed.

  A magnetic resonance imaging (MRI) apparatus is a medical diagnostic imaging apparatus that applies a high-frequency magnetic field and a gradient magnetic field to a subject placed in a static magnetic field, measures echo signals generated from the subject by nuclear magnetic resonance, and images them. is there. In the MRI apparatus, since the uniform region of the static magnetic field is a spherical region having a diameter of about 45 cm, the size of the region that can be imaged by one measurement is usually limited to about 40 cm. Therefore, when imaging a wider area such as the whole body, table moving imaging is performed in which imaging is performed while moving the bed (table) in the body axis direction of the subject. Such table moving photography is roughly divided into two types, multi-station photography and moving table photography.

  In multi-station imaging, a whole body is divided into a plurality of regions (stations), imaging is performed, and images of the stations are combined to create a whole body image (see, for example, Patent Document 1). Since individual photographing is performed with the table fixed, the conventional photographing method can be used as it is. Also, the adjustment parameters that must be set according to the object to be imaged, such as the optimal irradiation intensity of the resonance frequency and high-frequency magnetic field, and the gain of the receiving coil, can be acquired before the start of each station imaging, as in conventional imaging. it can. As described above, the multi-station imaging to which the conventional imaging technique can be applied as it is is already used in clinical practice.

  On the other hand, moving table shooting is a method of shooting while continuously moving a table (see, for example, Non-Patent Document 1). This method has better shooting time efficiency than the multi-station shooting method because there is no time loss associated with table movement. However, since the photographing is performed while the subject is moving, the conventional photographing technique cannot be applied as it is, and it has not been used clinically yet. In moving table imaging, imaging may be interrupted depending on the situation such as the subject moving after imaging starts. Such a case is caused by the difference based on the difference (movement distance) between the position of the moving unit at the time when the instruction to pause the imaging is received and the position of the moving unit when the imaging is resumed. It has been reported that the position of the moving means or the photographing position at the time of resumption is controlled so as to compensate for data loss (see, for example, Patent Document 2).

  Further, in the MRI apparatus, there is a method of shortening the imaging time by measuring signals in parallel using a plurality of probes (parallel imaging) (for example, see Non-Patent Document 2). Parallel imaging is an imaging method in which position information is given by using a difference in sensitivity distribution of the receiving coil in addition to giving position information to in-subject magnetization by a normal gradient magnetic field. Because it does not depend on the pulse sequence, it can be used not only for imaging with an echo planar sequence (EPI), but also for imaging methods such as MRA (MR Angiography) that are clinically useful but difficult to speed up. be able to.

  An example of the arrangement of the measurement data in the parallel shooting in the hybrid space (kx-z space) 401 is shown in FIG. In normal imaging, measurement data is measured densely arranged in the kx-z space. However, in parallel imaging, measurement data is thinned out, measured, and arranged. In FIG. 12, data 402 drawn with a solid line is measurement data, and a dotted line 403 and an alternate long and short dash line 404 indicate data that is measured in normal imaging but not in parallel imaging. In parallel imaging, the imaging time is shortened by omitting some data measurement. In the example shown in FIG. 12, since every other data is measured, it is possible to photograph at approximately twice the speed.

  By the way, images captured by the MRI apparatus include a morphological image in which the internal structure of the subject is reflected in the luminance distribution and a functional image in which the activity state of the imaging target is reflected in the luminance distribution.

  The functional image is effective for diagnosis, evaluation, follow-up observation and the like. However, since the function is to be imaged, the structure is generally not drawn sufficiently, and the spatial resolution is often low due to the difficulty of photographing. Therefore, in order to grasp the accurate position of the acquired activity state (function) information, it is often displayed superimposed on a separately captured morphological image. Main functional images include diffusion-weighted images and spectral images. In addition, a fat suppression image by STIR (short TI inversion recovery) is also used for tumor screening and is a kind of functional image (see, for example, Non-Patent Document 3).

US Pat. No. 5,924,987 International Publication No. 2006/109472 Pamphlet Walker R, et al. Turbo STIR Magnetic Resonance Imaging as a Whole-BodyScreening Tool for Metastases in Patients With Breast Carcinoma: PreliminaryClinical Experience.J Magn Reson Imaging 2000; 11: 343-350. Pruessmann KP, Markus MW, Scheidegger B, Boesiger P. SENSE: Sensitivity Encoding for Fast MRI. Magnetic Resonance in Medicine 1999; 42: 952--962. Kruger DG, Riederer SJ, Grimm RC, Rossman PJ. Continuously movingtable data acquisition method for long FOV contrast-enhanced MRA and whole-bodyMRI.Magn Reson Med 2002; 47: 224--231.

  Many functional images cannot be acquired by the moving table imaging method. For example, when a diffusion weighted image is captured, the image is captured so that the signal intensity changes depending on the size of the diffusion coefficient to be captured. For this reason, if the object to be photographed moves due to table movement during photographing, the movement is emphasized rather than diffusion, which is a very small movement, and it becomes difficult to image the diffusion itself. In addition, spectral image capturing requires high static magnetic field uniformity. In addition to the performance of the magnet, the static magnetic field uniformity is affected by the magnetic susceptibility distribution that substantially depends on the shape of the object to be imaged. For this reason, in moving table imaging in which the imaging target moves from moment to moment during imaging, it is difficult to always maintain a high static magnetic field uniformity, and moving table imaging is difficult. Therefore, when acquiring a functional image of a wide area exceeding the uniform area of the static magnetic field, it must be performed by multi-station imaging, which takes time.

  In moving table shooting, it is difficult to acquire adjustment parameters that must be set during shooting during shooting, so it is necessary to move and acquire the shooting target once in the same way as in multi-station before starting moving table shooting. . For this reason, although the moving table photographing itself has better photographing efficiency than the multi-station photographing, additional time is required for acquiring various information.

  In addition, since parallel imaging is an imaging method that uses the difference in sensitivity distribution of the probe, image reconstruction requires information on sensitivity distribution of the probe in addition to the magnetic resonance signal. In parallel imaging, simultaneous equations are constructed from an aliased image reconstructed from magnetic resonance signals and a sensitivity distribution image, and an image without aliasing is reconstructed by solving this. The sensitivity distribution image of the probe is created using data 405 measured simultaneously with or separately from the data 402 shown in FIG. In general, since the sensitivity distribution of the probe changes smoothly in space, it can be created with data only in the low spatial frequency region. Usually, the number of data 405 necessary for obtaining the sensitivity distribution is about 32 or more. However, the sensitivity distribution acquisition data 405 needs to be measured separately from the image reconstruction data, and extra time is required. In the case of a 256-pixel double-speed parallel image, the required number of data is 128. Therefore, the actual speed-up rate is not double-speed but 256 / (128 + 32/2) = 1.78-times or less. Therefore, even in the case of multi-station shooting, in parallel shooting, it is necessary to measure sensitivity distribution data in advance, which is a factor that hinders speeding up.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to improve imaging efficiency when imaging a wide area exceeding a uniform area of a static magnetic field in an MRI apparatus.

In the magnetic resonance imaging apparatus, the movement of the table is controlled in synchronization with the pulse sequence. While moving the table, the first signal is measured, and during that time, the table is periodically stopped to measure the second signal.
Specifically, transmitting means for transmitting a high-frequency signal to a subject placed in a static magnetic field, receiving means for receiving a nuclear magnetic resonance signal generated from the subject, and placing the subject on the subject A magnetic resonance imaging apparatus comprising: a transport unit that moves relative to a transmission unit; and a control unit that controls operations of the transmission unit and the reception unit according to a predetermined pulse sequence and controls the transport unit. The pulse sequence includes a first pulse sequence for causing the receiving unit to receive a first magnetic resonance signal that is alternately executed, and a second pulse for causing the receiving unit to receive a second magnetic resonance signal. A sequence, wherein the control means moves the transport means while performing control according to the first pulse sequence, and moves the second pulse sequence to the second pulse sequence. While performing control I provides a magnetic resonance imaging apparatus, characterized by controlling so as to stop the conveying means.

  ADVANTAGE OF THE INVENTION According to this invention, the imaging efficiency at the time of imaging | photography the wide area | region exceeding the uniform area | region of a static magnetic field can be improved in an MRI apparatus.

<< First Embodiment >>
Hereinafter, a first embodiment to which the present invention is applied will be described in detail with reference to the drawings.

  FIG. 1 is a block diagram showing a schematic configuration of the MRI apparatus 100 of the present embodiment. The MRI apparatus 100 according to the present embodiment includes a magnet 101 that generates a static magnetic field, a coil 102 that generates a gradient magnetic field, a sequencer 104, a gradient magnetic field power source 105, a high-frequency magnetic field generator 106, and a high-frequency magnetic field. The probe 107 to detect, the receiver 108, the computer 109, the display 110, the storage medium 111, the table movement control part 150, and the table 152 are provided.

  A subject (for example, a living body) 103 is placed on a table 152 in a static magnetic field space generated by a magnet 101. The movement of the table 152 is controlled by the table movement control unit 150 in accordance with an instruction from the sequencer 104. In the present embodiment, the table 152 moves in the body axis direction. In the present embodiment, the z axis is in the body axis direction, the x axis is in a direction perpendicular to the z axis on a plane parallel to the table 152, and the y axis is in the direction perpendicular to the x axis and z axis. In this embodiment, the reading gradient magnetic field is applied in the z direction, and the phase encoding gradient magnetic field is applied in the x direction.

  The sequencer 104 sends commands to the gradient magnetic field power source 105 and the high frequency magnetic field generator 106 to generate a gradient magnetic field and a high frequency magnetic field, respectively. The high frequency magnetic field is applied to the subject 103 through the probe 107. A signal generated from the subject 103 is received by the probe 107 and detected by the receiver 108.

  The computer 109 controls the operation of each component of the MRI apparatus 100 according to a predetermined program. Among these, the program describing the generation timing and intensity of the high-frequency magnetic field and gradient magnetic field to be controlled by the sequencer 104 and the timing of signal reception is called a pulse sequence. The pulse sequence implemented in the present embodiment is not particularly limited. For example, an SE pulse sequence such as a spin echo method (SE) or a fast spin echo method, a gradient echo (GrE) pulse sequence, an echo planar spectroscopic imaging ( EPSI), diffusion-weighted echo planar imaging (DWEPI), and the like.

  Further, the computer 109 receives the signal detected by the receiver 108, performs signal processing such as image reconstruction, and displays the result on the display 110. If necessary, the detected signal and measurement conditions may be stored in the storage medium 111.

  Furthermore, the sequencer 104 of the present embodiment causes the table movement control unit 150 to control the operation of the table 152 in synchronization with the pulse sequence. In the present embodiment, control is performed such that the first signal is measured while moving the table 152, and the table is periodically stopped to measure the second signal.

  Hereinafter, in this embodiment, the first signal is an echo signal obtained by a pulse sequence that can reconstruct a morphological image, and the second signal is an echo signal obtained by a pulse sequence that can reconstruct a functional image. An example will be described. Here, as a pulse sequence for acquiring an echo signal that can reconstruct a morphological image, for example, there is a pulse sequence for acquiring a T1-weighted image by GrE. In addition, examples of the pulse sequence for acquiring an echo signal that can reconstruct a functional image include a pulse sequence for acquiring a diffusion weighted image by DWEPI and a pulse sequence for acquiring a fat suppression image.

  Next, imaging of whole body morphological images and functional images by the MRI apparatus 100 of the present embodiment will be described. In the present embodiment, the morphological image is taken while moving the table 152, and during that time, the table 152 is periodically stopped and the functional image is taken. That is, moving table shooting for shooting a morphological image and multi-station shooting for shooting a functional image are alternately performed. Then, the respective images are fused after being reconstructed. Hereinafter, in the present embodiment, shooting performed while moving the table 152 is referred to as moving table shooting C, and shooting performed while the table 152 is stopped is referred to as multi-station shooting S. Further, data (echo signal) measured during each photographing is referred to as C (i) and S (i) using a photographing number i (i is a natural number of 1 or more) that specifies the number of photographing.

  First, the moving table photographing C of the present embodiment, which is photographing performed by moving the table 152, will be described. FIG. 2 is a diagram for explaining the relationship between the table 152 and the field of view (FOV). A subject is placed on the table 152. Here, the range (imaging range) 154 to be imaged is the whole body of the subject.

  As described above, in the MRI apparatus, since a uniform magnetic field space is limited, an area that can be imaged by one measurement is limited. In the present embodiment, an FOV determined from echoes obtained by one measurement is referred to as a sub visual field 151. When the shooting range 154 is wider than the sub-view field 151, it is necessary to move the sub-view field 151 relative to the shooting range 154 to cover the entire shooting range 154. In general, since the sub visual field 151 is fixed to a magnet that generates a static magnetic field, this is realized by moving the table 152.

  In the present embodiment, a case where the imaging range 154 in the body axis direction (z-axis direction) is larger than the sub visual field 151 in the same direction as shown in the figure will be described as an example. Therefore, in this embodiment, the table 152 is moved in the body axis direction (z-axis direction) 153, the relative position of the sub visual field 151 in the entire imaging range 154 is moved, and the entire imaging range 154 is imaged. When the photographing of the photographing range 154 is finished, the image is reconstructed using the data acquired in each sub visual field 151, and an image of the whole photographing range 154 is obtained.

  FIG. 3 shows the relationship between the elapsed time t during photographing and the center position z of the sub visual field 151 on the fixed coordinate system on the table 152 in this embodiment. In the figure, the shooting number (i) is described with i only for each measurement. The moving speed of the table 152 is equal to the slope of the graph shown in FIG. Multi-station shooting is performed when the inclination is zero. As described above, in this embodiment, C (i) is data that can reconstruct a morphological image, and S (i) is data that can reconstruct a functional image. C (i) and S (i) are arranged in separate measurement spaces, and images are reconstructed respectively.

  Next, imaging control performed by the computer 109 of this embodiment will be described. FIG. 4 is a flowchart of processing by the computer 109 from photographing to image display according to the present embodiment. In this embodiment, the number of repetitions N is calculated prior to shooting from the shooting range 154, the sub visual field 151, the overlap amount of the sub visual field 151, and the like. Details of the calculation of the number of repetitions N will be described later. Further, in the present embodiment, a case where an image of a coronal section (Coronal) of a subject, that is, an image on the xz plane in FIG. 2 will be described as an example. The moving direction 153 of the table 152 and the direction in which the readout gradient magnetic field is applied are the same direction.

  First, when an instruction to start shooting is received, the shooting number i is initialized (step 201). Next, the shooting number i is incremented by 1 (step 202).

  The movement of the table 152 is started (step 203), and C (i) is measured (step 204). The measurement result is inverse Fourier transformed in the z direction and is arranged in the kx-z space which is a hybrid space. When the measurement of C (i) is completed, the movement of the table 152 is stopped (step 205), and the measurement of S (i) is performed (step 206). The measurement result is inverse Fourier transformed in the z direction and is arranged in the kx-z space which is a hybrid space. As described above, C (i) and S (i) are arranged in different hybrid spaces. Details of the hybrid space in which each is arranged will be described later.

  The measurement of C (i) and the measurement of Si are repeated N times. That is, when the measurement of S (i) is completed, it is determined whether or not the measurement number i has reached the number of repetitions N (step 207). If the number of repetitions is N or less, the process returns to step 202 to repeat the measurement.

  When the measurement number i reaches N, the table 152 starts moving (step 208), the last C (i) is measured, that is, C (N + 1) is measured (step 209), and the measurement result is z It is inverse Fourier transformed in the direction and arranged in the kx-z space which is a hybrid space.

  Then, the measurement results C (i) (i = 1 to N + 1) and S (i) (i = 1 to N) are used to reconstruct the image Ic and the image Is of the shooting range 154, respectively (step 210, 211). Next, the reconstructed image Ic and image Is are fused (step 212) and displayed on the display 110 (step 213). FIG. 5 shows an image reconstructed or merged from the data obtained by the above processing. 5A is a morphological image reconstructed from C (i), FIG. 5B is a functional image reconstructed from S (i), and FIG. 5C is FIG. 6 is an example of an image obtained by fusing a) and FIG.

  Next, the hybrid space (kx-z space) in which the measured C (i) and S (i) are arranged in the above process will be described. Here, moving table photographing C will be described using the method described in Non-Patent Document 1 and N as an example.

  FIG. 6 is a diagram for explaining an arrangement of measurement data in the hybrid space (kx-z space) by the moving table imaging C and the multi-station imaging S of the present embodiment. Here, FIG. 6B shows the arrangement of the measurement data C (i) by the moving table photographing C, and FIG. 6C shows the arrangement of the measurement data S (i) by the multi-station photographing S. 6 (a) is a combination of both.

  As shown in FIG. 6B, in the moving table photographing C, since the table 152 moves at a predetermined speed during photographing, the position of the acquired measurement data C (i) in the z direction moves accordingly. The control of the moving speed of the table 152 will be described later. On the other hand, as shown in FIG. 6C, in the multi-station shooting S, since the table 152 is stopped during shooting, the positions in the z direction of the measurement data S (i) of the same shooting number i are the same. .

  The sub visual fields of the moving table photographing and the multi-station photographing S of this embodiment are determined as follows. The sub visual field fs of the multi-station imaging S is determined from the uniformity of the static magnetic field space as usual. On the other hand, the sub visual field fc of the moving table photographing C is usually made smaller than the sub visual field fs of the multi-station photographing.

  This is due to the following reason. In general, when the sub visual field in the z direction is increased, the uniformity of the static magnetic field is deteriorated at both ends of the sub visual field, the gradient magnetic field nonlinearity is increased, and the data distortion (mainly positional distortion) is also increased. Since this distortion appears as a distortion at a simple position of the image in the multi-station imaging S, it can be easily corrected. On the other hand, in the moving table photographing C, since it appears as blurring or ghosting, it cannot be easily corrected. Therefore, as shown in FIG. 6, the sub-field fc of the moving table imaging C is made smaller than the sub-field fs of the multi-station imaging S, and the moving table imaging C is less likely to be affected by non-uniform static magnetic field and gradient magnetic field nonlinearity. Like that.

In FIG. 6, as an example, if the overlap in multi-station shooting is o, it is half of the sub visual field fs in multi-station shooting minus the overlap o. That is,
fc = (fs−o) / 2 (Formula 1).
However, since the functional image reconstructed from the data acquired by the multi-station photographing S and the morphological image reconstructed from the data obtained by the moving table photographing C are separately reconstructed, fc and fs are arbitrary. Can be set.

  The typical size of the sub field of view is about 30 to 40 cm in DWEPI used for multi-station imaging S, and about 20 to 30 cm in a pulse sequence for acquiring a T1-weighted image by GrE used for moving table imaging C. The overlap is about 5 cm.

  As shown in FIG. 6, since the moving station photographing C is performed while the table is moving, unmeasured areas are generated at both ends of the kx-z space by the main photographing. In this embodiment, in order to prevent that both ends of the reconstructed image are not correctly reconstructed due to lack of information on the portion, in this embodiment, control is performed so that shooting starts at C (1) and ends at C (N + 1). Yes. With this control, the moving station shooting C covers a wider range than the multi-station shooting S, and a correct image can be obtained for the entire shooting range 154.

The number of repetitions N is calculated as the smallest integer that satisfies the following equation, where fa is the field of view of the entire imaging range 154 and fs and o are the sub-fields and overlap of the above-described multi-station imaging (FIG. 6). (See (c).)
N ≧ (fa−o) / (fs−o) (Formula 2)

Next, the control of the table 152 will be described. What is measured while moving the table 152 is C (i). The sub-field of the moving table imaging C is fc, and nx echo signals are arranged in the x direction (that is, the number of pixels in the x direction is nx), and the measurement time per echo signal (of the imaging sequence) If tc is a value obtained by dividing the repetition time by the number of echoes measured in one imaging sequence), in order to arrange the measured C (i) in the kx-z space without any gap, nx number of echo signals in the x direction. It is only necessary to control the table 152 to advance by the sub visual field fc or a movement amount smaller than fc during the acquisition. Accordingly, the moving speed vc of the table 152 is controlled as follows.
vc ≦ fc / (tc × nx) (Formula 3)

  The time Tc for moving the table 152 at the above speed is tc × nx. On the other hand, the table 152 is stopped during the multi-station shooting S. In multi-station imaging S, mx number of echo signals are arranged in the x direction (that is, mx number of pixels in the x direction are assumed to be mx), and the measurement time per echo signal is ts. Is ts × mx. As described above, in this embodiment, the control of moving the table 152 at the speed vc during Tc and stopping during Ts is repeated.

  In DWEPI performed as multi-station imaging S of the present embodiment, the number of echoes in the kx direction is typically 64 to 256. In the T1-weighted image acquisition sequence by GrE performed as the moving table photographing C, the number of echoes in the kx direction is typically 128 to 256.

  In many cases, the moving speed of the table 152 is not strictly constant. For example, as shown in FIG. 7, the moving speed of the table 152 immediately after the start of movement 251 and immediately before the stop 252 is generally smaller than vc. In order to prevent deterioration in image quality due to a non-constant moving speed, data may be arranged using the position of the table 152 obtained by monitoring. Or you may comprise so that an echo signal may not be measured during the period when speed is not constant. In this case, assuming that the distances that the table 152 moves to 251 and 252 while the speeds immediately after the start and stop of the table 152 are not constant, fc ′ = vc × nx × tc, fc = fc ′ If it is + (pa + pb) / 2, even if there is a period during which the moving speed decreases, data can be acquired without any gaps.

  In addition, the timing at which the table 152 is stopped and the multi-station shooting S is started during the moving table shooting C is as follows. While C (i) is being measured while moving the table 152, the table 152 is stopped when the center of the sub-field of the moving table photographing C coincides with the center of the sub-field of the multi-station photographing S, and the multi-station photographing is performed. Start S.

  Note that in the case of an MRI apparatus in which the influence of static magnetic field inhomogeneity and gradient magnetic field nonlinearity is small even at the end of the sub-field, the sub-field fc of the moving table imaging C may be made larger. FIG. 8 is a diagram for explaining the arrangement of measurement data when the sub-field fc of the moving table photographing C is set large. As shown in the figure, when the sub visual field fc of the moving table photographing C is increased, the moving speed of the table 152 can be increased, and the measuring time of the moving table photographing C can be shortened. Here, a case where fc = fs-o is shown as an example. The time required for all the measurements of C (i) is 3 × nx × tc in FIG. 8A and 4 × nx × tc in FIG. 8B. In the example of FIG. 8A, in order to further shorten the shooting time, the measurement area by the moving table shooting C lacks a part of the measurement area covered by the multi-station shooting S. Accordingly, the image quality at both ends of the entire field of view of the moving table photographing C is deteriorated. However, since the edge of the field of view is usually not an important part, there is a high possibility that there is no problem. In either case, the time can be greatly shortened compared to the case where the shooting time is (7 × nx × tc) in the example of FIG.

  As described above, according to the present embodiment, when performing imaging in a range exceeding the uniform region of the static magnetic field of the MRI apparatus, morphological image imaging by the moving table imaging C performed while the table 152 is moving and the table 152 being stopped are performed. Since the functional image capturing performed in the above is repeated alternately, it is possible to capture the morphological image and the functional image, which was difficult with the conventional moving table photographing C alone. In addition, since unnecessary reciprocation of the table 152 is unnecessary, it is possible to perform imaging in a shorter time than when imaging each separately, and to improve inspection throughput.

  In addition, according to the present embodiment, since the functional image and the morphological image of the same part of the imaging target can be acquired in close proximity in time, there is almost no influence of the temporal change of the imaging target due to body movement or the like. In addition, it is possible to take a functional image and a morphological image with almost no displacement. Therefore, when the morphological image and the functional image are merged as shown in FIG. 5, the positions of both can be matched.

  In this embodiment, the case where the second signal is an echo signal that can reconstruct a functional image has been described as an example, but the present invention is not limited to this. That is, the second signal is not particularly limited as long as an image having a resolution or contrast different from that of the image obtained from the first signal can be obtained. For example, an echo signal that reconstructs a high-definition image by the spin echo method or the fast spin echo method, such as an echo signal that is difficult to acquire while moving the table, or a contrast different from the image obtained from the first signal In addition, an echo signal obtained using different imaging parameters in the same pulse sequence as the first signal acquisition may be used.

<< Second Embodiment >>
Next, a second embodiment to which the present invention is applied will be described. Also in this embodiment, the first signal is measured while moving the table 152 as in the first embodiment, and during this time, the operation of the table 152 is performed so that the table 152 is periodically stopped and the second signal is measured. The table movement control unit 150 is controlled. Also in this embodiment, the case where the first signal is an echo signal obtained by a pulse sequence capable of reconstructing a morphological image as in the first embodiment will be described as an example. However, in the present embodiment, the second signal is an echo signal from which the adjustment parameter and the correction parameter can be calculated. The calculated adjustment parameter is used for photographing, and the correction parameter is used for image reconstruction. The basic configuration of the MRI apparatus of this embodiment is the same as that of the first embodiment. For this reason, the following description will focus on the configuration different from the first embodiment. Hereinafter, also in this embodiment, photographing performed while moving the table 152 is referred to as moving table photographing C.

  The photographing control performed by the computer 109 of this embodiment will be described. FIG. 9 is a flowchart of processing performed by the computer 109 from shooting to image display according to the present embodiment. Also in this embodiment, the number of repetitions M is calculated prior to shooting. Details of the calculation will be described later.

  First, when an instruction to start shooting is received, the shooting number i is initialized (step 301). Then, the shooting number i is incremented by 1 (step 302).

  Next, the movement of the table 152 is stopped (step 303), the echo signal is measured, and the adjustment parameter P (i) and the correction parameter F (i) are calculated from the measured echo signal (step 304). Here, the adjustment parameter P (i) is, for example, the resonance frequency f0, the optimum irradiation intensity H1 of the high-frequency magnetic field, the optimum gain of the receiver, and the like. The correction parameter F (i) includes, for example, ΔH0 indicating a non-uniform distribution of a static magnetic field and ΔG indicating a gradient magnetic field nonlinearity distribution.

  The obtained adjustment parameter P (i) is set in the MRI apparatus 100, and the correction parameter F (i) is stored in the storage medium 111 or the like in association with the imaging number i (step 305). Then, the movement of the table 152 is started (step 306), and C (i) is measured (step 307). The above measurement is repeated until i becomes an integer M (steps 302 to 308).

  The measurement result C (i) is corrected for static magnetic field inhomogeneity and gradient magnetic field nonlinearity using the correction parameter F (i), and the image Ic is reconstructed (step 309). Then, the reconstructed image Ic is displayed on the display (step 310).

  Note that the number of repetitions M depends on how much the imaging target changes in the moving direction (z direction) of the table 152 within the field of view fa of the entire imaging range. If the object to be photographed is uniform in the z direction, M is 1. However, generally, when the subject to be imaged is a human, the optimum adjustment parameter P (i) differs depending on the region, and therefore M> 1. In general, when the subject to be imaged is a human, sufficient image quality can be obtained over the entire visual field fa by resetting the adjustment parameter P every 20 to 40 cm. Therefore, for example, when imaging the whole body of a human with a height of 180 cm, if the adjustment parameter P is reset every 30 cm, M becomes 6.

  In the present embodiment, the sub-field fc of the moving table photographing C can be determined in consideration of only the image quality degradation around the sub-field due to non-uniform static magnetic field and gradient magnetic field nonlinearity, and does not depend on M or other parameters. Also, vc is determined according to Equation 3 as in the first embodiment.

  Conventionally, when imaging in a range exceeding the uniform region of the static magnetic field of the MRI apparatus, the adjustment parameter P and the correction parameter F are measured by moving the table 152 in advance. According to this embodiment, since the moving table photographing C performed while the table 152 is moving and the measurement of the adjustment parameter P and the correction parameter F performed while the table 152 is stopped are alternately repeated, these parameters are measured in advance. This eliminates the need to move the table 152. Therefore, imaging can be performed in a shorter time than in the prior art, and the inspection throughput can be improved.

  In addition, according to the present embodiment, the adjustment parameter P and the correction parameter F can be measured and the moving table photographing C can be performed close to each other in time. Therefore, the adjustment parameter P can be set and corrected more accurately.

<< Third Embodiment >>
Next, a third embodiment to which the present invention is applied will be described. In this embodiment, parallel shooting is performed. A probe sensitivity distribution is calculated using a signal received during table movement, and an image is reconstructed from the signal acquired while the table is stopped using the calculated probe sensitivity distribution. Also in this embodiment, the basic configuration of the MRI apparatus is the same as that of the first embodiment. However, the MRI apparatus 100 of this embodiment includes a plurality of probes 107 and realizes parallel imaging. Hereinafter, the description will be given focusing on different portions.

  The elapsed time t during photographing, the change in the position of the table 152 in the z direction, and the arrangement of measurement data will be described with reference to FIGS. 3 and 10, respectively. FIG. 10 is a diagram for explaining the arrangement of measurement data according to the present embodiment. In the present embodiment, the shooting procedure is basically the same as in the first embodiment. That is, the first signal C (i) is measured while moving the table 152, and the table 152 is periodically stopped during this period to measure the second signal S (i). In the present embodiment, the first signal C (i) is used for creating a sensitivity distribution, and the second signal S (i) is used for image reconstruction.

  However, in the present embodiment, as shown in FIG. 10, the first signal C (i) used for creating the sensitivity distribution is acquired only for the low spatial frequency region of the measurement space. A sensitivity distribution image of the probe is reconstructed from C (i). Since the sensitivity distribution of the probe generally changes spatially smoothly, it is only necessary to measure the echo signal only in the low spatial frequency region of the measurement space. Therefore, the measurement of C (i) is less than in normal imaging, and about 32 echo signals near zero encoding are measured.

  Moreover, since parallel imaging is performed in the present embodiment, it is not necessary to measure all echo signals for the measurement of S (i). The echo signal is measured by thinning out according to the speeding up rate of the parallel shooting. For example, in the case of shooting in which 256 echo signals are measured for each Si in normal shooting (shooting an image in which the x direction is 256 pixels), in parallel shooting at 2 × speed (speeding rate is 2), one echo signal is used. Every other S (i), 1/2 echo signal (here, 128 echo signals) is measured. As a result, the shooting time is also reduced to ½, and high-speed shooting is possible.

  In the present embodiment, the sensitivity distribution image Ic and the image Is are reconstructed from the measured C (i) and S (i), respectively. The image reconstruction method is basically the same as in the first embodiment. However, the sensitivity distribution image Ic is an image with low spatial resolution because the number of echoes to be measured is small. Further, since the echo signal is thinned out in the measurement of S (i), the image Is is folded in the x direction. This aliasing is removed using the sensitivity distribution image Ic. The process of removing the aliasing of the image Is using the sensitivity distribution image Ic is called parallel reconstruction. Parallel reconstruction is performed by solving simultaneous equations created from the image Ic and the image Is as usual.

  Next, shooting control performed by the computer 109 of this embodiment will be described. FIG. 11 is a flowchart of processing performed by the computer 109 from shooting to image display according to the present embodiment. Also in the present embodiment, the number of repetitions N is calculated prior to shooting, as in the first embodiment. Details of the calculation of the number of repetitions N are the same as in the first embodiment.

  First, when an instruction to start shooting is received, the shooting number i is initialized (step 501). Next, the shooting number i is incremented by 1 (step 502).

  The movement of the table 152 is started (step 203), and C (i) is measured (step 504). The measurement result is inverse Fourier transformed in the z direction and is arranged in the kx-z space which is a hybrid space. When the measurement of C (i) is completed, the movement of the table 152 is stopped (step 505), and the measurement of S (i) is performed (step 206). The measurement result is inverse Fourier transformed in the z direction and is arranged in the kx-z space which is a hybrid space.

  The measurement of C (i) and the measurement of S (i) are repeated N times. That is, when the measurement of the signal S (i) is completed, it is determined whether or not the measurement number i has reached the number of repetitions N (step 507). If the number of repetitions is equal to or less than N, the process returns to step 502 and the measurement is repeated.

  When the measurement number i reaches N, the table 152 starts moving (step 508), the last C (i) is measured, that is, C (N + 1) is measured (step 509), and the measurement result is z It is inverse Fourier transformed in the direction and arranged in the kx-z space which is a hybrid space.

  Then, using the measurement results C (i) (i = 1 to N + 1) and S (i) (i = 1 to N), the sensitivity distribution image Ic and the image Is in the imaging range 154 are reconstructed, respectively (step). 510, 511). Next, parallel image reconstruction is performed to remove aliasing from the image Is using the sensitivity distribution image Ic (step 512), and the result is displayed on the display 110 (step 513).

  In the present embodiment, the sub visual field fs at the time of execution of the measurement S (i) for creating the image Is is determined from the uniformity of the static magnetic field space as usual. On the other hand, the sub visual field fc at the time of execution of the measurement C (i) for creating the sensitivity distribution image Ic can be arbitrarily determined as in the first embodiment, but the image quality due to static magnetic field inhomogeneity or gradient magnetic field non-linearity. In order to suppress deterioration, it is generally made smaller than fs. FIG. 10 shows a case where fc = fs−o, which is slightly smaller than fs, as an example.

  Further, the moving speed, moving time, and stop time of the table 152 of this embodiment are the same as those of the first embodiment.

  As described above, according to the present embodiment, when performing imaging in a range exceeding the uniform region of the static magnetic field of the MRI apparatus, the sensitivity required to be measured separately from the data to be measured for reconstructing the image. Since the distribution data can be measured while moving the table, the time for measuring the sensitivity distribution data becomes unnecessary, and the imaging time can be shortened.

  According to each of the embodiments described above, when performing imaging in a range exceeding the uniform region of the static magnetic field of the MRI apparatus, by performing signal reception while the table is moving and signal reception while the table is stopped, one table is obtained. Multiple types of information can be obtained by movement. Since reception of both is performed alternately, use of different information acquired at the time of reconstruction, fusion after reconstruction, and the like can be easily performed. Therefore, according to each of the embodiments described above, it is possible to realize shooting with high shooting efficiency by incorporating the features of multi-station shooting without sacrificing the high speed of moving table shooting.

It is a block diagram of the MRI apparatus of 1st embodiment. It is a figure for demonstrating the relationship between the table of 1st embodiment, and a photography visual field. It is a figure for demonstrating the relationship between the time during imaging | photography and the position of a table of 1st embodiment. It is a flowchart of the process from imaging | photography to image display of 1st embodiment. It is a figure which shows the example of the image reconstructed and united in 1st embodiment. It is a figure which shows arrangement | positioning to the hybrid space of the measurement data of 1st embodiment. It is a figure for demonstrating the relationship between the time during imaging | photography when the moving speed of the table of 1st embodiment is not constant, and the position of a table. It is a figure which shows arrangement | positioning to the hybrid space of the measurement data of the other example of 1st embodiment. It is a flowchart of the process from imaging | photography to image display of 2nd embodiment. It is a figure which shows arrangement | positioning to the hybrid space of the measurement data of 3rd embodiment. It is a flowchart of the process from imaging | photography to image display of 3rd embodiment. It is a figure explaining the example of arrangement | positioning to the hybrid space of the measurement data in the conventional parallel imaging | photography.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100: MRI apparatus, 101: Magnet which generates a static magnetic field, 102: Gradient magnetic field coil, 103: Subject, 104: Sequencer, 105: Gradient magnetic field power supply, 106: High frequency magnetic field generator, 107: Probe, 108: Receiver 109: Computer, 110: Display, 111: Storage medium, 150: Table movement control unit, 151: Sub visual field, 152: Table, 153: Table movement direction, 154: Shooting range.

Claims (9)

Transmitting means for transmitting a high-frequency signal to a subject placed in a static magnetic field, receiving means for receiving a nuclear magnetic resonance signal generated from the subject, and placing the subject on the transmitting means A magnetic resonance imaging apparatus comprising: a transport unit that relatively moves; and a control unit that controls operations of the transmission unit and the reception unit according to a predetermined pulse sequence and controls the transport unit,
The pulse sequence includes a first pulse sequence that causes the receiving unit to receive first magnetic resonance signals that are alternately executed, and a second pulse sequence that causes the receiving unit to receive a second magnetic resonance signal. Prepared,
The control means controls the movement means to move while performing control according to the first pulse sequence, and to stop the conveyance means while performing control according to the second pulse sequence. Magnetic resonance imaging device. The magnetic resonance imaging apparatus according to claim 1,
Reconstructing means for reconstructing an image of the subject from a nuclear magnetic resonance signal received by the receiving means;
The magnetic resonance imaging apparatus, wherein the image reconstruction means reconstructs a first image from the first nuclear magnetic resonance signal and a second image from the second nuclear magnetic resonance signal. The magnetic resonance imaging apparatus according to claim 2,
The magnetic resonance imaging apparatus, wherein the first image is a morphological image and the second image is a functional image. The magnetic resonance imaging apparatus according to any one of claims 1 to 3,
A magnetic resonance imaging apparatus characterized in that an imaging range in a direction in which the carrier means moves a signal obtained by one measurement by the first pulse sequence is smaller than the imaging range by the second pulse sequence. The magnetic resonance imaging apparatus according to any one of claims 1 to 4,
The magnetic resonance imaging apparatus, wherein the control means performs control so that the first pulse sequence is executed first and last. The magnetic resonance imaging apparatus according to claim 1,
The control means acquires an adjustment parameter including a resonance frequency or an optimum irradiation intensity of a high frequency magnetic field from the second magnetic resonance signal. The magnetic resonance imaging apparatus according to claim 6,
The acquired adjustment parameter is used for the first pulse sequence executed immediately after acquisition of the imaging parameter. The magnetic resonance imaging apparatus according to claim 1,
Reconstructing means for reconstructing an image of the subject from a nuclear magnetic resonance signal received by the receiving means;
The control means acquires a correction parameter including static magnetic field distribution information or gradient magnetic field nonlinear information of the magnetic resonance imaging apparatus from the second nuclear magnetic resonance signal,
The magnetic resonance imaging apparatus, wherein the correction parameter is used when the reconstruction unit reconstructs an image from the first or second nuclear magnetic resonance signal. The magnetic resonance imaging apparatus according to claim 2,
The receiving means comprises a plurality of receiving coils;
The first image is a sensitivity distribution image representing a sensitivity distribution of each of the plurality of receiving coils.
The magnetic resonance imaging apparatus, wherein the image reconstruction unit uses the sensitivity distribution image when reconstructing the second image.

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