JP2006255091A - Magnetic resonance imaging system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable proper DC correction by a DC value with slight error. <P>SOLUTION: A control part 107 corrects each data stored in a memory part 104 so as to reduce its direct current component. The control part 107 performs correction based on a DC value which is estimated according to all data included in a K space to which each data belongs. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a magnetic resonance imaging (MRI) apparatus that generates an image using a magnetic resonance phenomenon.

  In the MRI apparatus, DC correction for reducing a direct current (DC) component included in the collected signal is performed.

This is performed by estimating an average value of values indicated by data corresponding to a fixed ratio of a part of the four corners in the K space as a DC value, and subtracting the DC value from all the data in the K space.
JP-A-1-256944

  However, the estimation of the DC value as described above may have a large error depending on the state of the collected signal. If DC correction is performed using a DC value having a large error, there is a risk of DC artifacts.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a magnetic resonance imaging apparatus capable of performing appropriate DC correction using a DC value with a small error.

  In order to achieve the above object, the present invention comprises correction means for correcting each data included in the K space so as to reduce a direct current component based on all the data included in the K space, and magnetic resonance. An imaging device was constructed.

  According to the present invention, it is possible to perform appropriate DC correction using a DC value with a small error.

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 (hereinafter referred to as an MRI apparatus) according to the present embodiment. The MRI apparatus shown in FIG. 1 includes a static magnetic field magnet 1, a gradient magnetic field coil 2, a gradient magnetic field power supply 3, a bed 4, a bed control unit 5, an RF coil unit 6, a transmission unit 7, a reception unit 8, a hybrid circuit 9, and a computer. A system 10 is provided.

  The static magnetic field magnet 1 has a hollow cylindrical shape and generates a uniform static magnetic field in an internal space. As the static magnetic field magnet 1, for example, a permanent magnet, a superconducting magnet or the like is used.

  The gradient magnetic field coil 2 has a hollow cylindrical shape and is disposed inside the static magnetic field magnet 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 strength is inclined 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 correspond to, for example, the slice selection gradient magnetic field Gs, the phase encoding gradient magnetic field Ge, and the readout gradient magnetic field Gr, respectively. 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 encode the phase of the magnetic resonance signal in accordance with the spatial position. The readout gradient magnetic field Gr is used to encode the frequency of the magnetic resonance signal in accordance with the spatial position.

  The subject P is inserted into the cavity (imaging port) of the gradient coil 2 while being placed on the top 41 of the bed 4. The couch 4 is driven by the couch controller 5 and moves the top board 41 in the longitudinal direction (left-right direction in FIG. 1) and up-down direction. Usually, the bed 4 is installed such that the longitudinal direction thereof is parallel to the central axis of the static magnetic field magnet 1.

  The RF coil unit 6 is disposed inside the gradient magnetic field coil 2. The RF coil unit 6 receives a high frequency pulse (RF pulse) from the transmission unit 7 and generates a high frequency magnetic field. The RF coil unit 6 receives a magnetic resonance signal radiated from the subject. The RF coil unit 6 may have a transmission coil and a reception coil individually, or may have a coil for both transmission and reception. Further, only one transmission coil, reception coil, and transmission / reception coil may be provided, or a plurality of types having different transmission / reception ranges and transmission / reception characteristics may be provided in parallel.

  The transmission unit 7 includes an oscillation unit, a phase selection unit, a frequency conversion unit, an amplitude modulation unit, and a high-frequency power amplification unit (all not shown). The oscillation unit generates a high-frequency signal having a resonance frequency unique to the target nucleus in the static magnetic field. The phase selection unit selects the phase of the high-frequency signal. The frequency conversion unit converts the frequency of the high-frequency signal output from the phase selection unit. The amplitude modulation unit modulates the amplitude of the high-frequency signal output from the frequency modulation unit, for example, according to a sync function. The high frequency power amplification unit amplifies the high frequency signal output from the amplitude modulation unit. As a result of the operation of each of these units, the transmission unit 7 transmits an RF pulse corresponding to the Larmor frequency to be supplied to the RF coil unit 6.

  The receiving unit 8 includes a pre-stage amplifier, a phase detector, and an analog / digital converter (all not shown). The preamplifier amplifies the magnetic resonance signal output from the hybrid circuit 9. The phase detector detects the phase of the magnetic resonance signal output from the preamplifier. The analog-digital converter converts the signal output from the phase detector into a digital signal.

  The hybrid circuit 9 supplies the RF coil unit 6 with the high-frequency pulse transmitted from the transmission unit 7 during the transmission period. The hybrid circuit 9 supplies the signal output from the RF coil unit 6 to the reception unit 8 during the reception period. The transmission period and the reception period are instructed from the computer system 10. Further, when a plurality of receiving coils are provided in the RF coil unit 6, the hybrid circuit 9 selectively inputs signals output from these coils in parallel or in a time division manner. It is instructed from the computer system 10, for example, in what form the signal output from which coil is input.

  The computer system 10 includes an interface unit 101, a data collection unit 102, a reconstruction unit 103, a storage unit 104, a display unit 105, an input unit 106, and a control unit 107.

  The interface unit 101 is connected to the gradient magnetic field power supply 3, the bed control unit 5, the transmission unit 7, the reception unit 8, the hybrid circuit 9, and the like. The interface unit 101 inputs and outputs signals exchanged between these connected units and the computer system 10.

  The data collection unit 102 collects digital signals output from the reception unit 8. The data collection unit 102 stores the collected digital signal, that is, magnetic resonance signal data, in the storage unit 104.

  The reconstruction unit 103 performs post-processing, that is, reconstruction such as Fourier transform, on the magnetic resonance signal data stored in the storage unit 104, and obtains spectrum data or image data of the desired nuclear spin in the subject P. Ask.

  The storage unit 104 stores magnetic resonance signal data and spectrum data or image data for each patient.

  The display unit 105 displays various information such as spectrum data or image data under the control of the control unit 107. As the display unit 105, a display device such as a liquid crystal display can be used.

  The input unit 106 receives various commands and information input from the operator. As the input unit 106, 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 can be used as appropriate.

  The control unit 107 includes a CPU, a memory, and the like (not shown), and comprehensively controls the MRI apparatus according to the present embodiment. Further, the control unit 107 serves as means for realizing some functions as described below in addition to means for realizing functions generally provided in the MRI apparatus. One of these functions estimates the DC value contained in the collected signal. One of the above functions performs DC correction based on the estimated DC value.

  Next, the operation of the MRI apparatus configured as described above will be described. Since the operation for obtaining the image of the subject P is the same as the conventional one, the description thereof is omitted here. In the following, processing related to DC correction will be described.

  The control unit 107 performs DC correction on the magnetic resonance signal data collected by the data collection unit 102 and stored in the storage unit 104. The DC correction is performed with data belonging to one K space as one unit. Each of the real part channel and the imaginary part channel is also processed individually.

  FIG. 2 is a flowchart showing a processing procedure of the control unit 107 for DC correction. FIG. 2 shows a process for performing DC correction on data belonging to one K space. Therefore, the same processing is performed on the magnetic resonance signal data belonging to the K space and the slices of the real part channel and the imaginary part channel and the K spaces having different time phases.

  In step Sa1, the control unit 107 confirms whether or not reconstruction is performed using a DWI (Diffusion Weighted Imaging) method. If the MRI apparatus is set to a mode that uses the DWI method, the control unit 107 proceeds from step Sa1 to step Sa5, but otherwise proceeds to step Sa2.

  In step Sa2, the control unit 107 confirms whether or not reconstruction is performed using an EPI (Echo Planar Imaging) method. If the MRI apparatus is set to a mode that uses the EPI method, the control unit 107 proceeds from step Sa2 to step Sa5, but otherwise proceeds to step Sa3.

  In step Sa3, the control unit 107 confirms whether the signal strength is equal to or less than a threshold value. The signal strength is a numerical value representing the strength of the signal represented by the magnetic resonance signal data within the range of the K space to be processed. As the signal intensity, for example, a peak value of signal levels represented by a plurality of data belonging to the K space to be processed, an average value of signal levels represented by all data belonging to the K space, or a real part and an imaginary part Each average value of can be applied. If the signal strength is less than or equal to the threshold value, the control unit 107 proceeds from step Sa3 to step Sa5, but otherwise proceeds to step Sa4.

  In step Sa4, the control unit 107 confirms whether or not the degree of variation is equal to or greater than a threshold value. The degree of variation is a numerical value indicating the magnitude of intensity variation represented by the magnetic resonance signal data within the range of the K space to be processed. As the degree of variation, for example, a deviation of a level other than the peak value among signal levels represented by a plurality of data belonging to the K space to be processed, or a deviation of a level other than the peak value of the real part and the imaginary part is applied. it can. If the degree of variation is greater than or equal to the threshold value, the control unit 107 proceeds from step Sa4 to step Sa5, but otherwise proceeds to step Sa7.

  In step Sa5, the control unit 107 assigns all data within the range of the K space to be processed for estimation of the DC value. After this, the control unit 107 proceeds to step Sa7. The relationship between the estimation data range and the K space at this time is as shown in FIG. In FIG. 3, the square frame indicates the outer edge of the K space, and the hatched area indicates the range of the estimation data.

  On the other hand, in step Sa6, the control unit 107 allocates data corresponding to part of the four corners of the range of the K space to be processed for estimation of the DC value. After this, the control unit 107 proceeds to step Sa7. The relationship between the estimation data range and the K space at this time is as shown in FIG. In FIG. 4, the outer square frame indicates the outer edge of the K space, and the hatched area indicates the range of the estimation data. In the example of FIG. 4, the estimation data range is set to a fixed part of the K space as in the conventional case. The range of the estimation data at this time may be, for example, a range shown by hatching in FIG. 5 or FIG. That is, at least data corresponding to the center of the K space is not assigned to the estimation data. Further, the range of the estimation data may be changed according to the magnitude of the signal intensity and the degree of variation.

  In step Sa7, the control unit 107 estimates the DC value based on the data assigned for estimation.

  In step Sa8, the control unit 107 subtracts the estimated DC value from all data included in the K space to be processed.

  Thus, according to the present embodiment, when the DWI method is used, the DC value is estimated based on all the data in the K space. In the DWI method, the intensity of the collected signal is generally small, and the peak value is not so large as compared with the surrounding intensity, so that the estimation accuracy of the DC value is improved by referring to a large number of data.

  When the EPI method is used, the DC value is estimated based on all data in the K space. In the EPI method, the signal intensity is increased to some extent, but the variation in the signal is increased. Therefore, the estimation accuracy of the DC value is improved by referring to a large number of data.

  Even when the DWI method is not used, if the signal intensity is small enough to be equal to or less than the threshold value, the DC value is estimated based on all data in the K space. Since the signal strength is small, the estimation accuracy of the DC value is improved by referring to a large number of data.

  Even if the EPI method is not used, if the signal variation is so large that the signal variation exceeds the threshold value, the DC value is estimated based on all the data in the K space. Since the signal variation is large, the estimation accuracy of the DC value is improved by referring to a large number of data.

  When none of the above four conditions is met, the data corresponding to the center of the K space is not referred to for estimating the DC value. This makes it possible to accurately estimate the DC value even when the peak value becomes very large.

  As described above, according to the present embodiment, the DC value can always be estimated with a small error. Based on the DC value with a small error in this way, appropriate DC correction can be performed. As a result, the occurrence of DC artifact can be suppressed.

This embodiment can be variously modified as follows.
Arbitrary one to three condition judgments in steps Sa1 to Sa4 may be omitted.
Alternatively, the DC value may be estimated unconditionally based on all data in the K space.

  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 components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment.

The figure which shows the structure of the magnetic resonance imaging apparatus which concerns on one Embodiment of this invention. The flowchart which shows the process sequence of the control part 107 in FIG. 1 for DC correction | amendment. The figure which shows the relationship between the range of the data for estimation allocated in step Sa5 in FIG. 2, and K space. The figure which shows the relationship between the range of the data for estimation allocated in step Sa6 in FIG. 2, and K space. The figure which shows the modification of the relationship between the range of the data for estimation allocated in step Sa6 in FIG. 2, and K space. The figure which shows the modification of the relationship between the range of the data for estimation allocated in step Sa6 in FIG. 2, and K space.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Static magnetic field magnet, 2 ... Gradient magnetic field coil, 3 ... Gradient magnetic field power supply, 4 ... Bed, 5 ... Bed control part, 6 ... RF coil unit, 7 ... Transmission part, 8 ... Reception part, 9 ... Hybrid circuit, 10 DESCRIPTION OF SYMBOLS Computer system 101 ... Interface part 102 ... Data collection part 103 ... Reconstruction part 104 ... Storage part 105 ... Display part 106 ... Input part 107 ... Control part

Claims (7)

  A magnetic resonance imaging apparatus comprising correction means for correcting each data included in the K space so as to reduce a direct current component based on all the data included in the K space. The correction means includes
Means for estimating the magnitude of the DC component based on all data contained in the K space;
2. The magnetic resonance imaging apparatus according to claim 1, further comprising means for subtracting the estimated magnitude of the DC component from each data included in the K space. Further comprising selection means for selecting the first mode or the second mode based on a condition for acquiring data included in the K space or a state of data included in the K space;
The correction means reduces the direct current component of each data included in the K space based on all the data included in the K space when the first mode is selected, and the second mode The magnetic resonance imaging apparatus according to claim 1, wherein when selected, the magnetic resonance imaging apparatus is reduced based on only a part of data included in the K space.   The magnetic resonance imaging according to claim 3, wherein the selection unit selects the first mode or the second mode based on an intensity of a magnetic resonance signal represented by data included in the K space. apparatus.   The magnetic resonance imaging according to claim 3, wherein the selection unit selects the first mode or the second mode based on variation in intensity represented by a plurality of data included in the K space. apparatus.   The magnetic resonance according to claim 3, wherein the selection unit selects the first mode when an EPI (Echo Planar Imaging) method is used to acquire data included in the K space. Imaging device.   4. The magnetic resonance according to claim 3, wherein the selection unit selects the first mode when a DWI (Diffusion Weighted Imaging) method is used to acquire data included in the K space. 5. Imaging device.

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