JP2008212553A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To generate an adequate image at any time by accommodating itself to a variation in movement of a subject. <P>SOLUTION: A gradient magnetic field power source 3, a transmission part 7, a reception part 9, a data collecting part 10b, and the like acquire magnetic resonance signals from the subject 200. A main control part 10g detects the movements of the subject 200. The main control part 10g controls the power source 3, the transmission part 7, the reception part 9, the data collecting part 10b, and the like in a manner to continuously acquire the magnetic resonance signal for each line in a k-space corresponding to an imaging cross section in an acquisition order determined from the detected movements. A reconstruction part 10c reconstructs an image from the latest magnetic resonance signal acquired for each line in the k-space under the control of the main control part 10g. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging apparatus having a fluoroscopy function.

  In fluoroscopy in a magnetic resonance imaging apparatus, the structure of a subject is rendered almost in real time by repeatedly performing image reconstruction in synchronization with this collection operation while repeatedly collecting magnetic resonance signals for k-space. .

In this fluoroscopy, the order of magnetic resonance signal acquisition (encoding order) for each line in k-space is determined at the imaging planning stage and is not changed while fluoroscopy is continued.
JP 2004-201886 A

  The basic encoding order is determined so as to make the acquisition frequency of magnetic resonance signals uniform for all lines in k-space. In such an encoding order, the information of all frequency components can be obtained uniformly, so that the image quality can be improved. On the other hand, the body movement of the subject, the movement of the organ, or the movement of the body fluid such as the flow of body fluid can be used. There is a risk of not being able to follow. Therefore, in the encoding order in which the acquisition frequency of the magnetic resonance signal for each line is made non-uniform so that it can follow a specific movement, the time resolution related to the specific movement can be improved, but it relates to a movement different from the above specific movement. There is a possibility that the temporal resolution is lowered and the image quality is deteriorated.

  The present invention has been made in consideration of such circumstances, and an object of the present invention is to provide a magnetic resonance imaging apparatus that can always generate an appropriate image in response to a change in movement of a subject. There is to do.

  The magnetic resonance imaging apparatus according to the first aspect of the present invention is determined based on an acquisition unit that acquires a magnetic resonance signal from a subject, a detection unit that detects the movement of the subject, and a detection result by the detection unit. First acquisition means for controlling the acquisition means so as to continuously acquire magnetic resonance signals for each line in the k space corresponding to the imaging section in the acquisition order; and for each line in the k space Reconstructing means for reconstructing an image based on the latest magnetic resonance signal obtained by the obtaining means.

  According to the present invention, it is possible to always generate an appropriate image while adapting to changes in the movement of the subject.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram showing a configuration of a magnetic resonance imaging apparatus (MRI apparatus) 100 according to the present embodiment. The MRI apparatus 100 includes a static magnetic field magnet unit 1, a gradient magnetic field coil 2, a gradient magnetic field power supply 3, a bed 4, a bed control unit 5, a transmission RF coil 6, a transmission unit 7, a reception RF coil 8, a reception unit 9, and A computer system 10 is provided.

  The static magnetic field magnet unit 1 has a hollow cylindrical shape and generates a uniform static magnetic field in an internal space. The static magnetic field magnet unit 1 includes a static magnetic field magnet 11 and a correction coil 12. As the static magnetic field magnet 11, for example, a permanent magnet or a superconducting magnet is used. The correction coil 12 is a combination of a plurality of coils. The correction coil 12 generates a correction magnetic field for correcting the uniformity of the static magnetic field generated by the static magnetic field magnet 11.

  The gradient magnetic field coil 2 has a hollow cylindrical shape and is disposed inside the static magnetic field magnet unit 1. The gradient coil 2 is a combination of three coils corresponding to the X, Y, and Z axes orthogonal to each other. The gradient magnetic field coil 2 generates a gradient magnetic field in which the above three coils are individually supplied with electric current from the gradient magnetic field power supply 3 and the magnetic field intensity changes along the X, Y, and Z axes. The Z-axis direction is, for example, the same direction as the static magnetic field. The gradient magnetic fields of the X, Y, and Z axes are arbitrarily used as, for example, a slice selection gradient magnetic field Gs, a phase encoding gradient magnetic field Ge, and a readout gradient magnetic field Gr. The slice selection gradient magnetic field Gs is used to arbitrarily determine an imaging section. The phase encoding gradient magnetic field Ge is used to change the phase of the magnetic resonance signal in accordance with the spatial position. The readout gradient magnetic field Gr is used for changing the frequency of the magnetic resonance signal in accordance with the spatial position.

  The subject 200 is inserted into the cavity (imaging port) of the gradient magnetic field coil 2 while being placed on the top plate 41 of the bed 4. The top plate 41 is driven by the bed control unit 5 and moves in the longitudinal direction and the vertical direction. Usually, the bed 4 is installed such that the longitudinal direction is parallel to the central axis of the static magnetic field magnet unit 1.

  The transmission RF coil 6 is disposed inside the gradient magnetic field coil 2. The transmission RF coil 6 is supplied with a high frequency pulse from the transmission unit 7 and generates a high frequency magnetic field.

  The transmission unit 7 transmits a high-frequency pulse corresponding to the Larmor frequency to the transmission RF coil 6.

  The receiving RF coil 8 is disposed inside the gradient magnetic field coil 2. The receiving RF coil 8 receives a magnetic resonance signal radiated from the subject due to the influence of the high-frequency magnetic field. An output signal from the receiving RF coil 8 is input to the receiving unit 9.

  The receiving unit 9 generates magnetic resonance signal data based on the output signal from the receiving RF coil 8.

  The computer system 10 includes an interface unit 10a, a data collection unit 10b, a reconstruction unit 10c, a storage unit 10d, a display unit 10e, an input unit 10f, and a main control unit 10g.

  The interface unit 10a is connected to the gradient magnetic field power source 3, the bed control unit 5, the transmission unit 7, the reception RF coil 8, the reception unit 9, and the like. The interface unit 10a inputs and outputs signals exchanged between these connected units and the computer system 10.

  The data collection unit 10b collects digital signals output from the reception unit 9 via the interface unit 10a. The data collection unit 10b stores the collected digital signal, that is, magnetic resonance signal data, in the storage unit 10d. When data for calculating the correction amount of the static magnetic field is to be collected, the data collection unit 10b collects magnetic resonance signal data regarding the inside of the ROI under the control of the main control unit 10g. Thus, the data collection unit 10b constitutes a collection unit together with the main control unit 10g.

  The reconstruction unit 10c performs post-processing, that is, reconstruction such as Fourier transform, on the magnetic resonance signal data stored in the storage unit 10d, and obtains spectrum data or image data of the desired nuclear spin in the subject 200. Ask.

The storage unit 10d stores magnetic resonance signal data and spectrum data or image data for each patient. In addition, the storage unit 10d stores cut-out cross-section information representing cut-out cross-sections regarding cross-sectional images generated by past cross-section conversion processing. Display below. As the display unit 10e, a display device such as a liquid crystal display can be used.

  The input unit 10f inputs various commands and information according to the operation of the operator. The input unit 10f appropriately includes a pointing device such as a mouse or a trackball, a selection device such as a mode change switch, or an input device such as a keyboard.

  The main control unit 10g includes a CPU, a memory, and the like (not shown), and comprehensively controls the above-described units of the MRI apparatus 100. Further, the main control unit 10g has the following functions specific to this embodiment in addition to the function of controlling each unit so that the MRI apparatus 100 realizes a known operation of the known MRI apparatus. . One of the functions described above is that the gradient magnetic field power source 3, the bed control unit 5, the transmission unit 7, the reception unit 9, and data so as to collect magnetic resonance signals for fluoroscopy (hereinafter referred to as fluoroscopy scanning). The collection unit 10b and the like are controlled. One of the above functions is that the gradient magnetic field power source 3, the bed control unit 5, and the transmission unit are configured to collect magnetic resonance signals for motion detection (hereinafter referred to as motion detection scans) more frequently than fluoroscopy scanning. 7. Control the receiving unit 9 and the data collecting unit 10b. One of the functions is to detect the movement of the subject 200 as a change in the magnetic resonance signal based on the magnetic resonance signal collected by the movement detection scan. The function of controlling the fluoroscopy scan includes a function of changing the encoding order in the fluoroscopy scan according to the movement of the subject 200.

  Next, the operation of the MRI apparatus 100 configured as described above will be described.

  FIG. 2 is a diagram showing the relationship between the execution timings of fluoroscopy scan (abbreviated as fluoroscan in FIG. 2), motion detection scan, image reconstruction, and encode order setting process when executing fluoroscopy.

  As shown in FIG. 2, the main control unit 10g mainly performs the fluoroscopy scan, and causes the motion detection scan to be performed at a lower frequency than the fluoroscopy scan. In the example of FIG. 2, every time a fluoroscopy scan is performed for four lines, a motion detection scan is performed for one line. In FIG. 2, the numerical values shown for the fluoroscopic scan and the motion detection scan indicate the phase encoding step numbers. However, here, in the fluoroscopic scan, as shown in FIG. 3, magnetic resonance signals for 16 lines in k space are collected by 16-step phase encoding from “−8” to “7”, and in the motion detection scan, FIG. As shown in Fig. 8, magnetic resonance signals for eight lines in k-space are obtained by eight-step phase encoding of "-8", "-6", "-4", "-2", "0", "2", "4", and "6". We are going to collect.

  Thus, the motion detection scan is performed by a coarser phase encoding step than the fluoroscopic scan. Appropriate pulse sequences for fluoroscopy scanning and motion detection scanning are generally different from each other. For example, the pulse sequence for fluoroscopy scan is FSE (fast spine echo) or FFE (fast field echo), and the pulse sequence for motion detection scan is FE (field echo). The main control unit 10g controls each unit so that the fluoroscopy scan and the motion detection scan are performed according to a predetermined pulse sequence. The cross section for performing the motion detection scan may be the same as or different from the cross section positioned for the fluoroscopic scan. The cross section for performing the motion detection scan is different from the cross section positioned for the fluoroscopic scan when, for example, contrast imaging is performed. In this case, the cross section where the motion detection scan is performed is a cross section where the motion of the contrast agent is large. Note that the motion detection scan can be performed on the same cross section by a pulse sequence for fluoroscopy scanning. In this case, the magnetic resonance signal acquired by the fluoroscopic scan can be used as it is for motion detection.

  The main control unit 10g sets the fluoroscopy scan encoding order to the standard encoding order in the initial state. The standard encoding order is, for example, a sequential order. In the period Pa in FIG. 2, the fluoroscopic scan is performed according to the standard encoding order.

  The main control unit 10g executes the encoding order setting process as shown in FIG. 2 every time the motion detection scan for one line is completed. FIG. 5 shows the procedure of the encoding order setting process.

  In step Sa1, the main control unit 10g acquires k-space information after updating the corresponding line with the magnetic resonance signal acquired by the motion detection scan before the start of the current encoding order setting process, that is, acquired for each line at this time. The low frequency, medium frequency and high frequency components are separated from the latest magnetic resonance signal.

  In step Sa2, the main control unit 10g based on the current frequency components separated in step Sa1 of the current encoding order setting process and the past frequency components separated in step Sa1 of the previous encoding order setting process. The amount of change of each frequency component is calculated. The amount of change may be calculated as a difference between the current frequency component and the past frequency component, or may be calculated as an integral value of the difference thus obtained over a certain period.

  In steps Sa3 to Sa6, the main control unit 10g determines whether or not a large change has occurred based on the amount of change of each component calculated in step Sa2, and if a large change has occurred, the main subject of the change is which frequency. Determine if it is a component. This determination can be made, for example, by analyzing which change amount for each frequency component exceeds a predetermined threshold value.

  If there is a large change mainly in all frequency components or low frequency components, the main control unit 10g proceeds from step Sa3 or step Sa4 to step Sa7. In step Sa7, the main control unit 10g sets the encode order for increasing the collection density of the central portion of the k space, that is, the low frequency portion, as the encode order of the fluoroscopic scan.

  If a large change has occurred mainly due to the medium frequency component, the main control unit 10g proceeds from step Sa5 to step Sa8. In step Sa8, the main control unit 10g sets an encode order for increasing the collection density of the medium frequency part of the k space as an encode order for the fluoroscopic scan.

  If a large change occurs mainly due to the high frequency component, the main control unit 10g proceeds from step Sa6 to step Sa9. In step Sa9, the main control unit 10g sets the encode order for increasing the collection density of the high-frequency portion of the k space as the encode order for the fluoroscopic scan.

  If no significant change has occurred in any of the frequency components, the main control unit 10g proceeds from step Sa6 to step Sa10. In step Sa10, the main control unit 10g sets the standard encode order as the fluoroscopy scan encode order.

  When the main control unit 10g finishes setting the encoder order in any of steps Sa7 to Sa10, the main control unit 10g ends the encoder order setting process.

  Note that the encoding orders set in steps Sa7 to Sa10 may be defaults of the MRI apparatus 100, respectively, or may be specified in advance by the user.

  FIG. 2 shows an example in which an encode order that increases the collection density of the central portion in response to the detection of a change in the main component of the low frequency component as a result of the encode order setting process started at time T1 is shown. . That is, in the example shown in FIG. 2, as a result of the encoding order setting process started at time T1, the line of step “0”, that is, the line in the center of the k space at a rate of once every four times in the subsequent period Pb The acquisition of the magnetic resonance signal is performed.

  Now, as shown in FIG. 2, the main control unit 10g controls the reconstruction unit 10c to perform image reconstruction every time acquisition of the magnetic resonance signal for the line in the center of the k space is completed. Thus, the interval TB at which image reconstruction is performed in the period Pb is shorter than the interval TA at which image reconstruction is performed in the period Pa, and the time resolution related to the low frequency portion is improved in the period Pb than in the period Pa. The main control unit 10g performs image reconstruction every time acquisition of the magnetic resonance signal for the phase encoding step with higher acquisition frequency is completed in order to improve the time resolution related to the intermediate frequency portion or the high frequency portion. I will let you.

  Thus, according to the present embodiment, since the collection density of the frequency portion that has changed with the movement of the subject is increased, the time resolution of each frequency portion can be adaptively changed following the movement of the subject. It is possible to render an image that follows the movement of the subject while minimizing the degradation of the image quality.

  In addition, according to the present embodiment, when any change in both frequency components is small, the standard encode order for acquiring the magnetic resonance signal at an equal ratio is set for each phase encode step, so that the subject can be imaged with high image quality. Can be drawn.

  This embodiment can be variously modified as follows.

  When a large change occurs mainly with all frequency components, the standard encoding order may be set.

  When the magnetic resonance signal is collected for each segment by dividing the k space into a plurality of continuous lines, the collection density for each segment may be changed. Alternatively, the number of segment divisions (the number of lines included in each segment) may be changed. To change the number of segment divisions, for example, increase the number of segment divisions in the case of changes mainly in low frequency components, and decrease the number of segment divisions in the case of changes mainly in medium frequency components or high frequency components. In the case where there is no major change in the component main component or in each frequency component, it can be considered to return the segment division number to the standard number. Note that default values may be applied to the standard number of divisions, the number of divisions after increase, or the number of divisions after decrease, respectively, or values specified by the user may be used. Increasing the number of segment divisions stabilizes the contrast, but decreases the time resolution. Whether to change the collection density of the segments or the number of segment divisions may be one of the defaults or may be in accordance with user designation.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment.

1 is a diagram showing a configuration of a magnetic resonance imaging apparatus 100 according to an embodiment of the present invention. The figure which shows the relationship of the execution timing of the fluoroscopy scan at the time of performing a fluoroscopy, a motion detection scan, an image reconstruction, and an encoding order setting process. The figure which shows an example of arrangement | positioning of the line in the fluoroscopy scan in k space. The figure which shows an example of arrangement | positioning of the line in the motion detection scan in k space. The figure which shows the procedure of an encoding order setting process.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Static magnetic field magnet unit, 2 ... Gradient magnetic field coil, 3 ... Gradient magnetic field power supply, 4 ... Bed, 5 ... Bed control part, 6 ... Transmission RF coil, 7 ... Transmission part, 8 ... Reception RF coil, 9 ... Reception part DESCRIPTION OF SYMBOLS 10 ... Computer system, 10a ... Interface part, 10b ... Data collection part, 10c ... Reconstruction part, 10d ... Storage part, 10e ... Display part, 10f ... Input part, 10g ... Main control part, 100 ... Magnetic resonance imaging apparatus (MRI apparatus).

Claims (11)

An acquisition means for acquiring a magnetic resonance signal from the subject;
Detecting means for detecting movement of the subject;
First control means for controlling the acquisition means so as to continuously acquire magnetic resonance signals for each line of k-space corresponding to the imaging section in an acquisition order determined based on a detection result by the detection means; ,
A magnetic resonance imaging apparatus comprising: reconstruction means for reconstructing an image based on the latest magnetic resonance signal acquired by the acquisition means for each line in the k space.   The magnetic resonance imaging apparatus according to claim 1, wherein the detection unit detects a movement of the subject based on a magnetic resonance signal acquired by the acquisition unit for the same cross section as the imaging cross section.   The detection means diverts a part of the magnetic resonance signal acquired by the acquisition means under the control of the first control means as a magnetic resonance signal for motion detection, and this magnetic resonance for motion detection The magnetic resonance imaging apparatus according to claim 2, wherein the movement of the subject is detected based on a signal. The apparatus further comprises second control means for controlling the acquisition means so as to acquire the magnetic resonance signal for motion detection at a lower frequency than acquisition of the magnetic resonance signal under the control of the first control means. ,
The magnetic resonance imaging apparatus according to claim 2, wherein the detection unit detects a motion of the subject based on the magnetic resonance signal for motion detection. The acquisition unit is configured to acquire a magnetic resonance signal for motion detection on a motion detection cross section different from the imaging cross section at a frequency lower than the acquisition of the magnetic resonance signal under the control of the first control unit. Further comprising second control means for controlling
The magnetic resonance imaging apparatus according to claim 1, wherein the detection unit detects a movement of the subject based on the magnetic resonance signal for motion detection.   The second control unit controls the acquisition unit to acquire the magnetic resonance signal for motion detection by a sequence different from the acquisition of the magnetic resonance signal under the control of the first control unit. The magnetic resonance imaging apparatus according to claim 4, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus.   The first control unit controls the acquisition unit to divide each line in the k-space into a plurality of segments and continuously acquire the magnetic resonance signals for lines belonging to one segment. The magnetic resonance imaging apparatus according to claim 1, wherein the number of divisions of the segment is changed based on a detection result by the detection unit. The detection means detects the movement of the subject as a degree of change with respect to each of a plurality of frequency components included in the magnetic resonance signal for movement detection,
6. The magnetic resonance imaging according to claim 3, wherein the first control unit increases the number of divisions of the segment as the degree of change with respect to a lower frequency component increases. apparatus. The detection means detects the movement of the subject as a degree of change with respect to each of a plurality of frequency components included in the magnetic resonance signal for movement detection,
The said 1st control means sets the said acquisition order so that the collection density of the information regarding the frequency component with a large degree of the said change may be improved, The any one of Claim 3 thru | or 5 characterized by the above-mentioned. The magnetic resonance imaging apparatus described.   The magnetic resonance imaging apparatus according to claim 1, wherein the first control unit sets the acquisition order according to a default rule.   The magnetic resonance imaging apparatus according to claim 1, wherein the first control unit sets the acquisition order according to a rule specified by a user.

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