JP2008067781A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an MRI apparatus capable of acquiring a sufficient number of slice images in a short imaging time and having a function of detecting and correcting the body movement of a subject. <P>SOLUTION: A target site 35 is positioned in slices 25-28 at a time (tn). When executing a prescribed pulse sequence, magnetic spins of slices 21-24 and 29-32 are excited by 180°; the magnetic spins of the slices 25-28 are excited by 90°; the magnetic spins of the slices 21-24 and 29-32 rotate 180° and the magnetic spins of the slices 25-28 rotate 90°. At a time (tn+1), the target site 35 is positioned in the slices 26-29; the magnetic spins of the slices 21-24 and 29-32 are excited by 180°; the magnetic spins of the slices 25-28 are excited by 90°, and the magnetic spin of the slice 29 rotates 270°. This apparatus can calculate that the target site 35 moves by the slice width by the magnetic spin detection in the slice 29. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging apparatus (MRI apparatus), and more particularly to an MRI apparatus having a function of detecting body movement of a subject.

  In the MRI apparatus, a gate method is known that reduces image quality deterioration due to respiratory motion of a subject. As described in Patent Document 1, the imaging using the gate method is to move the imaging section in accordance with the movement of the diaphragm using a signal obtained by exciting a part of the diaphragm as a trigger.

  That is, the movement in the body axis direction is canceled by moving the imaging section in the body axis direction of the subject, and the subject is relatively stationary.

JP 2004-57226 A

  However, in the conventional technique, in order to detect the movement of the diaphragm of the subject, a navigation echo signal must be acquired, and thus the number of slice images acquired from the subject must be reduced. There was a point. In addition, there is a problem that the photographing time is extended due to generation and acquisition of the navigation echo signal.

  Further, in order to use the signal of the diaphragm of the subject as a trigger, in addition to the normal imaging pulse sequence, a second pulse sequence for selecting the diaphragm and generating the trigger signal is added. Was complicated.

  An object of the present invention is to realize an MRI apparatus having a function of detecting and correcting body movement of a subject capable of acquiring a sufficient number of slice images in a short imaging time.

  The magnetic resonance imaging apparatus of the present invention includes a static magnetic field generating means, a gradient magnetic field generating means, a high frequency signal transmitting means, a high frequency signal receiving means, and an image of a subject based on an echo signal received by the high frequency signal receiving means. Includes signal processing display means for reconfiguring and displaying the control means and control means. Then, the control means images a plurality of slice images of the subject in a multi-slice direction set in the body movement direction of the subject, and is positioned in at least one end region of the plurality of slice images. Based on the image signal of the slice image, body motion information of the subject is detected, and the position correction of the plurality of slice images is performed using the detected body motion information.

  It is possible to realize an MRI apparatus having a function of detecting and correcting body movement of a subject, which can acquire a sufficient number of slice images in a short imaging time.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 15 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied. In FIG. 15, the MRI apparatus includes a superconducting coil 1 for generating a uniform magnetic field space and 3 for generating a gradient magnetic field whose magnetic field strength changes linearly along the three axial directions of x, y, and z. A set of gradient magnetic field generating coils 2, a high frequency magnetic field generating coil 3 for inducing magnetic resonance of the subject 10, a receiving coil 4 for detecting a magnetic resonance signal of the subject, a gradient magnetic field power source 5, and a high frequency A magnetic field power source 6.

  Further, the MRI apparatus includes a bed 7 for transporting the subject into the static magnetic field generating coil, a control unit 8 for controlling the operation of the gradient magnetic field generating coil 2, the high frequency magnetic field generating coil 3, and the receiving coil 4, and a control command. A console 9 for performing image reconstruction and image display. Here, the central axis direction of the superconducting coil 1 is defined as the z direction.

  FIG. 1 is a schematic diagram of multi-slice imaging for explaining an embodiment of the present invention. FIG. 1 shows a state in which slices having a thickness d are continuously selected from slice 21 to slice 32 along the z direction in the coordinate system 34 based on the MRI apparatus shown in FIG. Further, it is assumed that a coordinate system 33 with respect to the subject is defined, and the coordinates of the coordinate system 34 and the coordinate system 33 coincide with each other at 0 hours after the start of measurement.

  Here, the multi-slice direction is set in the body movement direction of the subject, the range from the slice 21 to the slice 24 is the region 1 (end region), the range from the slice 25 to the slice 28 is the region 2, and the slice 29 to the slice 32 is Separated from region 3 (edge region). Area 2 is an area that is actually desired to be photographed. In the present invention, the area from which slices are cut out may be divided into three, and the number of slices in each area is not limited to the number described in FIG.

  FIG. 2 is a projection view in the y direction shown in FIG. 1, and reference numeral 35 denotes a target portion of the subject. The target portion 35 has a length equal to the region 2 in the z direction and can reciprocate in the z direction. Here, the motion of the target part 35 is assumed. 2A to 2D show the positions of the target sites 35 at time tn, time tn + 1, time tn + 2, and time tn + 3.

  Focusing on the fixed point 36 on the target site 35, the position of the fixed point 36 in the slice 27 at time tn is 0, and the position of the fixed point 36 at time tn + 1, time tn + 2, and time tn + 3 is + d, 0, -2d. Here, d is the slice width shown in FIG. A graph showing the change of the fixed point 36 is as shown in FIG.

  However, FIG. 3A shows a change in the fixed point 36 as seen in the coordinate system 34 based on the MRI apparatus as described above. Since the fixed point 36 is stationary in the coordinate system 33 with the target site 35 as a reference, it is clear that it is shown as (B) in FIG.

  In one embodiment of the present invention, the fixed point 36 of the target portion 35 that moves as shown in FIG. 3A is made stationary as shown in FIG. 3B, and image quality deterioration due to movement is reduced.

  Next, a procedure for obtaining an image by the pulse sequence shown in FIG. 4 will be described. In the following description, operation control of each unit, rearrangement of image data described later, and the like are performed by the control unit 8.

  The pulse sequence in FIG. 4 shows the application of a high-frequency magnetic field and a gradient magnetic field in the x, y, and z directions and the timing of echo capture. The pulse sequence shown in FIG. 4 is known as a gradient echo method.

  The region of interest is selectively excited by the high-frequency magnetic field 71 and the slice gradient magnetic field 72, and position information 75 is given to the echo by the phase encode gradient magnetic field 73 and the read gradient magnetic field 74. This echo is stored in the memory of a computer built in the console 9, and this pulse sequence is repeated at intervals of TR shown in FIG. 4 to acquire the number of echoes necessary for image reconstruction, and 1 by image reconstruction processing. Individual two-dimensional images are obtained.

  In order to perform the multi-slice measurement on the above-described region 1, region 2, and region 3 in FIG. 1, as shown in FIG. 5, the pulse sequence of FIG. 4 is repeated during TR for the number of multi-slices. The In FIG. 5, three multi-slice measurements are shown for simplicity. As the center frequency of the high-frequency magnetic field, a different center frequency is selected according to the slice position, and the slice 21 to the slice 32 are distinguished from each other.

  Here, the pulse sequence of FIG. 5 is a combination of two types of pulse sequences. The slices of region 1 and region 3 are measured by a pulse sequence without applying 73 phase encoding gradient magnetic fields, as indicated by 81 and 83. On the other hand, the slice of region 2 is measured by a pulse sequence in which 73 phase encode gradient magnetic fields are applied as indicated by 82. Furthermore, the slices of region 1 and region 3 are excited by a high-frequency magnetic field 711 having an excitation angle of 180 °, as indicated by 81 and 83, and the slice of region 2 is excited by a high-frequency magnetic field having an excitation angle of α °, as indicated by 82. Excited by 71. Note that α represents an arbitrary angle, and α = 90 ° here.

  As described above, in the multi-slice measurement shown in FIG. 5, two types of pulse sequences having different combinations of excitation angles of the phase encoding gradient magnetic field and the high-frequency magnetic field are connected.

  Next, the behavior of the magnetic spin of each slice will be explained by executing the pulse sequence of FIG. 5, and the mechanism for calculating the displacement of the target site according to the present invention will be clarified.

  6 to 9 show the position of the target portion 35 at time tn, time tn + 1, time tn + 2, and time tn + 3 shown in FIG. 2, and the direction of the magnetic spin in each slice at that time. Show. All magnetic spins in each slice are directed in the + z direction, which is the same as the static magnetic field direction, before starting measurement, and the y-direction component is zero. In the MRI apparatus, only the magnetic spin of the y direction component can be detected.

  Here, the time tn is the time at which the nth repetition of the pulse sequence is executed, and the difference from the adjacent time is TR. Further, N is the number of echoes necessary for the image reconstruction, and n <N.

  As shown in FIG. 6A, the target site 35 is located at the position of the slice 25 to the slice 28 at time tn. When the pulse sequence shown in FIG. 5 is executed, the magnetic spins from slice 21 to slice 24 and slice 29 to slice 32 are excited by 180 °, and the magnetic spins from slice 25 to slice 28 are excited by 90 °.

  Therefore, as shown in FIG. 6B, the magnetic spin directions in the slices at time tn are rotated by 180 ° from the slice 21 to the slice 24 and from the slice 29 to the slice 32, and from the slice 25 to the slice 28. The magnetic spin is rotated 90 °. At this time, since the slice 25 to the slice 28 have the y-direction component of the magnetic spin, signals from the slice 25 to the slice 28 are detected.

  Next, at time tn + 1, as shown in FIG. 7A, the target site 35 is located at the position from the slice 26 to the slice 29. When the pulse sequence of FIG. 5 is executed, the magnetic spins from slice 21 to slice 24 and slice 29 to slice 32 are excited by 180 °, and the magnetic spins from slice 25 to slice 28 are excited by 90 °.

  Therefore, as shown in FIG. 7B, the magnetic spin directions in the slices at time tn + 1 are rotated by 180 ° from the slice 21 to the slice 24 and from the slice 30 to the slice 32. The magnetic spin of slice 28 is rotated by 90 °, and the magnetic spin of slice 29 is rotated by 270 °. At this time, since the slice 25 to the slice 29 have the y-direction component of the magnetic spin, signals from the slice 25 to the slice 29 are detected.

  Here, the magnetic spin of slice 29 is obtained by exciting the remaining component of the magnetic spin of slice 28 at time tn by 180 °. By detecting magnetic spin in the slice 29, it can be calculated that 35 target sites have moved by + d equal to the slice width. That is, it can be seen that the detection of the magnetic spin in the region 3 serves as an index for calculating the displacement amount of the target site.

  Next, at time tn + 2, as shown in FIG. 8A, the target site 35 is located at the position of the slice 25 to the slice 28. Since the target site 35 is at the same position as that at time tn, the state of magnetic spin is the same as in FIG. 6B, as shown in FIG.

  Finally, at time tn + 3, as shown in FIG. 9A, the target site 35 is at the position from the slice 23 to the slice 26. When the pulse sequence of FIG. 5 is executed, the magnetic spins from slice 21 to slice 24 and slice 29 to slice 32 are excited by 180 °, and the magnetic spins from slice 25 to slice 28 are excited by 90 °.

  Therefore, as shown in FIG. 9B, the magnetic spin directions in slices at time tn + 3 are 180 ° rotated from slice 21 to slice 22 and slice 27 to slice 32, and slice 25 Thus, the magnetic spin of slice 28 is rotated 90 °, and the magnetic spin of slice 23 and slice 24 is rotated 270 °.

  At this time, since the slice 23 to the slice 28 have the y-direction component of the magnetic spin, signals from the slice 23 to the slice 28 are detected. Here, the magnetic spins of the slice 23 and the slice 24 are obtained by exciting the remaining components of the magnetic spins of the slice 25 and the slice 26 at time tn + 2 by 180 °.

  As described above, when the target site moving in the z direction is measured using the pulse sequence of FIG. 5, the displacement of the target site can be calculated from the magnetic spins detected in the regions 1 and 3.

  The operation of the pulse sequence of FIG. 5 has been described above. Next, a procedure for calculating the displacement of the target portion 35 will be described. The measured signal is stored in the image data memory in the console 9 in accordance with the measurement order for each slice. Here, a procedure for calculating the displacement of the target portion 35 in FIG. 7 will be described.

  FIG. 10 shows the arrangement of the echoes of the slice 29 on the memory from time tn to time tn + 3. As described above, the total number of echoes per slice is N. As shown in FIG. 10A, the echo 7 is time tn, and as shown in FIG. 10B, the echo 58 is time tn +. 1. As shown in FIG. 10C, the echo 59 is measured at time tn + 2, and as shown in FIG. 10D, the echo 60 is measured at time tn + 3.

  Similarly, FIG. 11 shows the arrangement of the echoes of the slice 30 on the memory. As shown in FIG. 11A, the echo 61 is at time tn, as shown in FIG. 11B, the echo 62 is at time tn + 1, and as shown in FIG. At time tn + 2, as shown in FIG. 11D, the echo 64 is an echo measured at time tn + 3.

  First, focusing on the echo of the slice 29, if the echoes 57 to 60 are individually reconstructed and the signal intensity is plotted on the graph along the time, the graph shown in FIG. 12A is obtained. As shown in FIG. 7B, since the signal of the slice 29 is detected at time tn + 1, the signal intensity becomes maximum at time tn + 1.

  Here, the signal intensity at time tn + 1 is decomposed into a net signal component 81 and a noise component 82. The noise component is a y-direction component of magnetic spin generated from other than the target site due to imperfection of 180 ° excitation. Similarly, the graph shown in FIG. 12B is obtained from the echo of the slice 30.

  As shown in FIG. 7B, since the signal of the slice 30 is not detected, only the noise component is plotted in FIG.

  Therefore, comparing (A) of FIG. 12 and (B) of FIG. 12, only the slice 29 has increased in signal at time tn + 1. Therefore, the displacement of the target site at this time is the width of one slice, that is, It is calculated to be equal to d.

  In this way, if the echoes at the respective times are compared in time series with respect to all the slices in the regions 1 and 3, it is possible to determine how much the target site has been displaced at what time.

  As described above, the procedure for calculating the displacement of the target part from the signals of the slices of the region 1 and the region 3 is shown. Next, the signal processing procedure of the slice of the region 2 will be described. Here, the echo information is rearranged between slices in the region 2 using the displacement information of the target part, and converted into an echo string of a slice in which the movement of the target part is canceled. That is, the image data memory for storing a plurality of slice image data is provided, and the control means is based on the plurality of slice images before the body movement of the subject, and the image data memory corresponding to each of the imaging regions of the subject. A plurality of slice images captured after body movement of the subject are rearranged so as to be stored in the storage area of the image data memory corresponding to the imaging region of the subject. .

  FIG. 13 shows the arrangement of the echoes of the slice 25, slice 26, slice 27, and slice 28 on the memory immediately after the measurement by the pulse sequence shown in FIG. 5 is completed.

  As described above, the total number of echoes per slice is N. In slice 25, echo 41 is at time tn, echo 42 is at time tn + 1, echo 43 is at time tn + 2, and echo 44 is at time tn + 3. This is a measured echo.

  In the slice 26, the echo 45 is measured at time tn, the echo 46 is measured at time tn + 1, the echo 47 is measured at time tn + 2, and the echo 48 is measured at time tn + 3.

  In slice 27, echo 49 is measured at time tn, echo 50 is measured at time tn + 1, echo 51 is measured at time tn + 2, and echo 52 is measured at time tn + 3.

  In slice 28, echo 53 is measured at time tn, echo 54 is measured at time tn + 1, echo 55 is measured at time tn + 2, and echo 56 is measured at time tn + 3.

  In FIG. 6B, FIG. 7B, FIG. 8B, and FIG. 9B, the echo arrangement of FIG. As shown in FIG. 14, since the target portion 35 is shifted to the right by one slice, the echo at time tn + 1 is shifted to the left by one slice in each slice in FIG. That is, the echo 42 at time tn + 1 in FIG. 14A is the echo 46, the echo 46 at time tn + 1 in FIG. 14B is the echo 50, and the time tn + in FIG. 14C. 1 echo 50 is rewritten to echo 54.

  In this way, in order to cancel the displacement of the target portion 35, the echo of each slice may be overwritten on the slice in the direction opposite to the displacement. However, it should be noted that the rearrangement of the echoes is established only between the slices in the region 2, and the exchange of the echoes in the slices in the region 1 or the region 3 and the echoes in the slices in the region 2 is not established.

  Accordingly, the echo 54 at time tn + 1 of the slice 28 in FIG. 14D is a missing region without being overwritten. Similarly, when the echoes at time tn + 2 and time tn + 3 are rearranged between slices, the echo shown in FIG. 13 is rearranged like the echo shown in FIG. Eventually, the echo arrangement of FIG. 14 is the final arrangement, and an image with reduced influence of motion is obtained by image reconstruction.

  As a result of the echo rearrangement, when all the echoes necessary for image reconstruction are not prepared, such as slice 25, slice 26, and slice 28, slice 25, slice 26, and slice 28 are discarded or missing. Appropriate data interpolation such as re-imaging of the area may be performed.

  For example, the above-described problem of echo loss can be alleviated by using a technique such as parallel imaging that can restore an image without acquiring all the necessary number of echoes for image reconstruction. Normally, when data that lacks the number of echoes necessary for image reconstruction is reconstructed, the image is folded due to the effect of narrowing the field of view. The pixel value of such an image is an overlap of the original pixel value and the folded pixel value.

  In parallel imaging, echoes are measured by arranging a plurality of receiving coils in order to solve the problem of image folding. In the image reconstruction, if the sensitivity distribution of each receiving coil measured in advance is used, the original pixel value and the folded pixel value can be separated, so that the folded image is restored to an unfolded image. Can do.

  Parallel imaging is a method of shortening the imaging time by reducing the number of echoes to be measured. However, in the present invention, echo loss due to echo rearrangement can be similarly restored by image reconstruction of parallel imaging.

  In actual photographing, multi-slice photographing is performed in a number equal to or greater than that of the above-described embodiment of the present invention. There is no practical problem even if it is discarded.

  In the above description, the gradient echo method is applied to the imaging of the region 2, but the same effect can be obtained by applying another pulse sequence such as the spin echo method.

  Moreover, although the example mentioned above is an example which calculates the displacement of the target site | part 35 from the slice signal of the area | regions 1 and 3, and rearranges the data of the echo signal between slices, Even if the displacement of 35 is calculated and the slice position is corrected in accordance with the calculated displacement, the body movement of the subject that can acquire a sufficient number of slice images in a short imaging time can be obtained. An MRI apparatus having a function of detecting and correcting can be realized. For example, it is also possible to calculate the displacement of the target part 35, calculate the period for returning to the same position, and perform gating at the timing when the target part 35 is at the same position in accordance with the period.

  In the case of the above-described example (for example, abdominal axial imaging), body motion is detected using slice signals in both areas 1 and 3, but either one of areas 1 or 3 is detected. It is also possible to detect body movement using a slice signal. For example, in the case of abdominal coronal imaging, since the body movement of the subject in the direction of the bed is limited, it is only necessary to monitor the body movement in the direction of the opposite side of the bed. One slice signal may be used.

  Furthermore, in the present invention, the slice images need only be rearranged only when the body movement is detected. Therefore, if no body movement is detected, the data of only the region 2 may be used. In this case, the data in the areas 1 and 3 can be discarded.

It is a schematic diagram of multi-slice imaging for explaining an embodiment of the present invention. FIG. 2 is a projection view in the y direction shown in FIG. 1. It is a graph which shows the change of the fixed point shown in FIG. It is a figure which shows the pulse sequence which showed the application of a high frequency magnetic field and a gradient magnetic field, and the timing of echo acquisition. It is a figure which shows the pulse sequence for carrying out multi-slice measurement of the area | region 1, the area | region 2, and the area | region 3. FIG. It is a state schematic diagram of a magnetic spin in one embodiment of the present invention. It is a state schematic diagram of a magnetic spin in one embodiment of the present invention. It is a state schematic diagram of a magnetic spin in one embodiment of the present invention. It is a state schematic diagram of a magnetic spin in one embodiment of the present invention. It is a schematic diagram of the data arrangement | sequence in one Embodiment of this invention. It is a figure explaining arrangement | positioning on the memory of the echo signal in one Embodiment of this invention. It is a graph which shows the time change of echo signal strength. It is a figure which shows the data arrangement | sequence on the memory immediately after the pulse sequence in one Embodiment of this invention. It is a figure which shows the state after rearranging the data arrangement | sequence in one Embodiment of this invention. 1 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Superconducting coil, 2 ... Gradient magnetic field generation coil, 3 ... High frequency magnetic field generation coil, 4 ... Reception coil, 5 ... Gradient magnetic field power supply, 6 ... High frequency magnetic field power supply, 7 ··· Bed, 8 ··· Control unit, 9 ··· console, 21 to 32 ··· slice, 33 ··· coordinate system based on subject, 34 ··· based on MRI apparatus Coordinate system, 35 ... target part

Claims (6)

Static magnetic field generating means, gradient magnetic field generating means, high-frequency signal transmitting means, high-frequency signal receiving means, and signal processing for reconstructing and displaying an image of a subject based on echo signals received by the high-frequency signal receiving means In a magnetic resonance imaging apparatus comprising: display means; and control means for controlling the static magnetic field generation means, gradient magnetic field generation means, high frequency signal transmission means, high frequency signal reception means, and signal processing display means.
The control means includes
Based on an image signal of a slice image located in at least one end region of the plurality of slice images in a multi-slice direction set in the body movement direction of the subject. A magnetic resonance imaging apparatus characterized by detecting body motion information of a subject and correcting the positions of the plurality of slice images using the detected body motion information.   2. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit detects body motion information of the subject based on image signals of a plurality of slice images located in both side end regions of the plurality of slice images, A magnetic resonance imaging apparatus characterized by performing position correction on the plurality of slice images using detected body motion information.   2. The magnetic resonance imaging apparatus according to claim 1, further comprising an image data memory for storing the plurality of slice image data, wherein the control unit is configured to image the subject based on the plurality of slice images before the body motion of the subject. A storage area of the image data memory corresponding to each part is defined, and a plurality of slice images captured after body movement of the subject according to the body movement information are the image data memory corresponding to the imaging part of the subject The magnetic resonance imaging apparatus is rearranged so as to be stored in the storage area.   2. The magnetic resonance imaging apparatus according to claim 1, wherein the control means calculates a timing at which the imaging region of the subject returns to the same position according to the body movement information of the subject, and images the subject at that timing. A magnetic resonance imaging apparatus.   The magnetic resonance imaging apparatus according to claim 3, wherein when the missing slice image is generated by rearranging the plurality of slice images captured after the body movement of the subject, the control unit detects the missing slice image. An image corresponding to the above is taken and stored in the image data memory.   4. The magnetic resonance imaging apparatus according to claim 3, wherein the control means picks up a slice image of the subject by parallel imaging picked up using a plurality of high-frequency signal receiving means, and is picked up after the body movement of the subject. When a missing slice image occurs due to the rearrangement of a plurality of slice images, the image corresponding to the missing slice image is stored in the image data memory using the image obtained by the parallel imaging. A magnetic resonance imaging apparatus.

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