JP2007267774A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus which removes distortions if there are inhomogeneities of a static magnetic field, non-linearities of a gradient magnetic field, or the like and synthesizes excellent images. <P>SOLUTION: A spatial select filter corresponding to respective characteristics of a static magnetic field distribution or the like is created by acquiring information on the static magnetic field distribution of the MRI apparatus, linearities of the gradient magnetic field, and a sensitivity distribution of an RF coil (Steps 304, 305). Data of the created spatial select filter are stored, multi-station imaging is performed, and whether spatial select film processing is performed or not is determined (Steps 306-308). A captured image is masked by using the selected spatial select filter after determining which of the spatial select filters is used (Steps 309, 310). Next, the image after the filter processing is displayed, and image synthesis processing is started (Steps 311, 312). Then, a result of synthesis processing is displayed (Step 313). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging apparatus, and more particularly to an apparatus for imaging a whole body by moving a subject.

  A technique is known in which a bed on which a subject is arranged is moved and a whole body of the subject is imaged by a magnetic resonance imaging apparatus (MRI apparatus) (for example, Patent Document 1).

  The whole body MRI apparatus for imaging the whole body of the subject includes a method of moving the bed stepwise and a method of continuously moving the bed. In the method of moving the bed stepwise, for example, when the imaging surface is parallel to the movement direction, first, the movement of the subject is stopped and imaged, and a first image having a length T in the movement direction is obtained.

  Next, after moving by the distance T, the image is stopped and imaged again to obtain a second image having a length T in the moving direction. This is repeated n times, and the obtained n images from the first to the n-th are stitched together to obtain an image of length nT (multi-station imaging).

  When images obtained by multi-station imaging are combined and synthesized, a good synthesized image cannot be obtained due to image distortion caused by non-uniformity of static magnetic field of the device or nonlinearity of GC (gradient magnetic field coil) There is.

  Therefore, Patent Document 1 discloses that image distortion is minimized by discarding image data in the vicinity of a boundary portion of images to be joined.

JP 2004-97826 A

  However, the above-mentioned Patent Document 1 only describes that image data in the vicinity of the boundary portion of the images to be joined is discarded, and does not describe a specific technique for obtaining a good image. For this reason, even if the image distortion can be minimized by discarding the image data in the vicinity of the boundary portion, even the necessary image data may be discarded.

  In order to remove the image distortion at the time of composition, it is conceivable to take an image with a narrow field of view (FOV), but by narrowing the FOV, the composition can be performed in the same manner as the technique described in Patent Document 1 above. Even if it goes, there is a possibility that even an unquestionable part will be removed.

  Further, if the FOV is narrowed, it becomes difficult to capture an image with a field of view (for example, a lateral width during abdominal imaging) that is desired to be included in one image.

  An object of the present invention is to realize a magnetic resonance imaging apparatus capable of removing a distortion and synthesizing a good image even when static magnetic field inhomogeneity or gradient magnetic field non-linearity exists.

  The magnetic resonance imaging apparatus of the present invention includes a spatial selection filter that removes an image distortion portion of an image reconstructed by the image reconstruction means.

  The space selection filter is set to a shape that removes the image distortion portion at the edge portion of the reconstructed image and leaves other necessary portions.

  The entire image of the subject is synthesized by synthesizing a plurality of images in which the image distortion portion at the edge portion is removed and other necessary portions are left.

  Even when static magnetic field inhomogeneity or gradient magnetic field non-linearity exists, a magnetic resonance imaging apparatus capable of synthesizing a good image by removing image distortion without sacrificing FOV can be realized. .

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

FIG. 1 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied.
In FIG. 1, the MRI apparatus generates a high-frequency magnetic field in a subject placement region, a magnet 102 that generates a static magnetic field in the placement region of the subject 101, a gradient magnetic field coil 103 that creates a gradient magnetic field in the subject placement region. An RF coil 104 and an RF coil 105 for detecting a nuclear magnetic resonance signal (NNMR signal) generated by the subject 101 are provided.

  The gradient magnetic field coil 103 is composed of gradient magnetic field coils in three directions of X, Y, and Z, and generates a gradient magnetic field in response to a signal from the gradient magnetic field power supply 109. Further, the RF coil 104 generates a high frequency magnetic field in accordance with a signal from the RF transmission unit 110. The NNMR signal detected by the RF coil 105 is detected by a signal detection unit 106, signal processed by a signal processing unit (image reconstruction means) 107, and converted into an image signal by calculation. The image signal converted by the signal processing unit 107 is displayed on the display unit 108.

  The gradient magnetic field power supply 109, the RF transmission unit 110, and the signal detection unit 106 are controlled by the control unit 111. This control time chart is generally called a pulse sequence. The bed 112 is for a subject to lie down, and the movement of the bed 112 is controlled by the control unit 111 via the bed driving unit 113.

FIG. 2 is an overall schematic operation flowchart in one embodiment of the present invention.
In FIG. 2, multi-station imaging is performed (step 202), and a spatial selection filter process described later is performed on the reconstructed image (step 203). Next, synthesis processing is started on the image subjected to the spatial selection filter processing (step 204). Then, the synthesis result is displayed (step 205).

  Here, the spatial selection filter processing in step 203 is processing for removing a portion with large distortion from the obtained image and selecting a portion with little or no distortion. This spatial selection filter processing may be performed at the user's discretion after displaying the captured image and confirming the distortion, and the large distortion portion is uniformly removed without depending on the user's judgment. It is also possible to automatically synthesize images.

  The spatial selection filter determined in advance can also be set from the specifications of the MRI apparatus. In addition, the user can perform pre-imaging before the main imaging and draw a circle or a rectangle on the result image to set the shape and size of the spatial filter.

  FIG. 3 is an explanatory diagram in the case where the user performs pre-imaging before the main imaging and sets the shape and size of the space selection filter. FIG. 3A shows an image obtained by imaging using a lattice phantom. A filter shape 114 for removing a portion having a large image distortion is set for the obtained image ((B) of FIG. 3). Then, the shape of the space selection filter 115 is set ((C) in FIG. 3).

Next, the case of setting the space selection filter based on the specifications of the MRI apparatus will be described with reference to FIG.
(A) of FIG. 4 is explanatory drawing in the case of setting a spatial filter from static magnetic field uniformity. For each MRI apparatus, for example, the range within ± 1 ppm from the center of the static magnetic field is 50 cm DSV, and the range within ± 0.3 ppm is 40 cm DSV. Therefore, based on the specifications of the static magnetic field strength 301 determined for each MRI apparatus, the space selection filter can be set to a circle with a diameter of 50 cm or a circle with a diameter of 40 cm.

  (B) of FIG. 4 is explanatory drawing in the case of setting a spatial filter from the linearity of a gradient magnetic field coil (GC). For example, when the definition that the position shift due to non-linearity of GC is up to 5% of the FOV is made, 5 of the FOV (length of one side of the imaging field) is obtained from the GC linearity data 302 which is the specification data of the MRI apparatus. It can be calculated that FOV “M” mm is within the linearity error of%. Thereby, the spatial filter can be set as a circular filter having a diameter of Mmm.

  FIG. 4C is an explanatory diagram when the sensitivity distribution 303 of the RF coil for detecting the NMR signal is used. However, the sensitivity distribution information of the RF coil is not for the purpose of removing the distortion of the image position itself, but for the purpose of removing the luminance decrease in the vicinity of the FOV that causes the image quality deterioration after the image synthesis in addition to the distortion of the magnetic field. use.

  From the sensitivity distribution specification of each coil, if the filter processing range obtained based on the magnetic field information includes a region where the sensitivity is reduced by a predetermined value (%), that region of the image before synthesis Then, brightness correction is performed.

  As described above, instead of performing luminance correction, the moving distance of the bed can be adjusted so that the subject can be imaged at the most appropriate position according to the sensitivity distribution of the coil.

  Moreover, although the example mentioned above is an example in the case of using the sensitivity distribution specification of each coil, at the time of pre-scan, data is acquired for sensitivity distribution calculation of the coil, and the sensitivity distribution obtained using the acquired data is obtained. It is also possible to use it.

  FIG. 5 is an operation flowchart in the case where the spatial selection filter is set and the image is synthesized using the specification data of the MRI apparatus.

  In step 304 of FIG. 5, information on the static magnetic field distribution of the MRI apparatus, the linearity of the gradient magnetic field, and the sensitivity distribution of the RF coil is obtained. Next, in step 305, a spatial selection filter corresponding to each characteristic such as a static magnetic field distribution is created. In step 306, the created spatial selection filter data is stored, and in step 307, multi-station imaging is performed.

  Subsequently, in step 308, a determination process is performed as to whether or not to perform a space selection filter process. This step 308 may be a process in which the user makes the determination, or a process in which a determination criterion such as using a GrE sequence that may cause a large amount of distortion from the information of the imaging sequence is automatically determined. Good.

  Next, if it is determined in step 309 that the spatial selection filter processing is to be performed, it is determined which of the spatial selection filters stored in step 306 (such as those due to static magnetic field inhomogeneity or gradient magnetic field linearity) to be used. . This spatial selection filter may be determined by displaying a captured image processed by each stored spatial selection filter and selecting it for the user, or from the information of the sequence used for imaging, a GrE sequence can be referred to as this spatial selection filter. A table (described later) may be determined in advance and automatically selected.

  In step 310, the captured image is masked using the selected space selection filter. Next, in step 311, the image after filter processing is displayed, and in step 312, image synthesis processing is started. In step 313, the result of the synthesis process is displayed.

Next, a case where a spatial selection filter is determined from shimming information and imaging sequence information will be described.
Shimming is a process of correcting the magnetic field by applying an image to measure non-uniformity of the magnetic field before imaging, and using the result to pass a current through the shim coil to increase the uniformity of the magnetic field. By using the information obtained by the shimming, it is possible to grasp the magnetic field uniformity more accurately than the specification level information of the MRI apparatus. Furthermore, an approximate image distortion amount can be calculated by using the parameters of the imaging sequence (protocol).

  For example, since the shift amount of the frequency at the position x is known from the shimming data, it is possible to calculate how many pixels the position x is shifted using the bandwidth of the imaging parameter and the information on the number of samples. The spatial selection filter is determined using the deviation amount as a threshold value.

  FIG. 6 is an operation flowchart for determining a spatial selection filter from shimming information and imaging sequence information. In step 401 of FIG. 6, as described above, imaging for acquiring shimming data is performed, and in step 402, an imaging protocol is set.

  Next, in step 403, a distortion amount of the image is calculated by calculation from shimming data and imaging parameters, and a spatial selection filter that can remove the distortion is determined. Subsequently, multi-station imaging is performed in step 404, and in step 405, the captured image is filtered by the space selection filter determined in step 403.

  In step 406, the obtained image is displayed and confirmed. The image composition process (step 407) may be started as it is without being displayed. In step 408, the composite image is displayed.

  By using shimming data, more detailed distortion information can be obtained. Rather than applying the data only to the spatial selection filter, the portion with the distortion amount of X% is removed by the spatial selection filter, and the portion with the small distortion amount is passed to the synthesis processing algorithm for distortion correction. It can also be used as information for

  Here, FIG. 7 shows an example of a correspondence table for selecting an optimum one for the imaging sequence among the various spatial selection filters described above. In FIG. 7, in the case of a GrE sequence that is easily affected by the static magnetic field inhomogeneity, the spatial selection filter 1 calculated by combining the static magnetic field inhomogeneous information and the GC linearity information is selected.

  In the case of an SE / FSE sequence that is less susceptible to static magnetic field inhomogeneity than the GrE sequence, the spatial selection filter 2 calculated from only static magnetic field inhomogeneous information is selected.

  Furthermore, in the case of an EPI sequence that is more susceptible to static magnetic field inhomogeneity, the spatial selection filter 3 calculated from shimming data obtained immediately before imaging is selected.

  An example of the display screen 108 when the image processed using the space selection filter as described above is combined is shown in FIG.

  FIG. 8A is a screen display as to which of the space selection filters 1 to 3 is to be selected. The selection buttons of the space selection filters 1, 2, and 3, and the captured image before processing by the space selection filter (There is a high-luminance portion 500 due to distortion at the edge of the image). If the space selection filter 3 is selected on this selection screen, as shown in FIG. 8B, a captured image before processing by the space selection filter 3 and an image (high) after processing by the space selection filter 3 are displayed. The luminance portion 500 is removed) and a result image obtained by synthesizing the filtered image is displayed on the display screen. As shown in FIG. 8B, it is possible to obtain a good composite image in which only the high luminance part due to the distortion of the edge part is removed and the necessary part remains.

Here, the execution range of the space selection filter will be described with reference to FIG.
Since the inhomogeneity of the magnetic field is measured with respect to the center position of the magnetic field, the spatial selection filter 115 is not the center of the FOV (W is the width of the FOV), but as shown in FIG. Call to match.

  As shown in FIG. 9B, a spatial selection filter is applied to a position shifted from the image center with respect to an image captured by the off-center FOV.

  FIG. 10 is a diagram illustrating an example of a synthesized image without performing processing by the spatial selection filter according to the present invention. FIG. 10A shows a whole body positioning screen 501. Since an overlap portion is required between stations for multi-station image composition, positioning is performed while viewing the relationship between stations on the whole body positioning screen.

  When the start button 502 is pressed after the positioning is completed, imaging is started. After the imaging is completed, a captured image 503 is displayed as shown in FIG. Reference numeral 503 denotes an example of an image in which a high-luminance portion 500 exists due to distortion in the peripheral portion of the image.

  As described above, when the high brightness portion 500 due to the distortion of the edge of the image exists, if the synthesis process is performed as it is, the high brightness portion 500 remains on the composite image 504, and the image quality is deteriorated.

  On the other hand, when image processing is performed using the spatial selection filter according to the present invention, as shown in FIG. 8B, the necessary image portion is not removed and the FOV is not narrowed. Thus, it is possible to obtain a good composite image from which the high luminance portion 500 due to the distortion of the image edge portion is removed.

  In addition, although the example mentioned above is an example at the time of applying this invention to the whole body captured image which synthesize | combines a some image, this invention is applicable even when it is a case where only a single image is obtained.

1 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied. It is a whole schematic operation | movement flowchart in one Embodiment of this invention. It is explanatory drawing in case a user performs pre-imaging before main imaging and sets the shape and size of a space selection filter. In this invention, it is explanatory drawing in the case of setting a space selection filter from the specification of an MRI apparatus. In the present invention, it is an operation flowchart when a space selection filter is set using specification data of an MRI apparatus and an image is synthesized. In the present invention, it is an operation flowchart in the case of determining a spatial selection filter from shimming information and imaging sequence information. It is a figure which shows an example of the correspondence table for selecting the optimal thing for an imaging sequence among space selection filters. It is a figure which shows an example of the display screen in the case of carrying out a synthetic | combination process of the image processed using the space selection filter. It is a figure explaining the execution range of a space selection filter. It is a figure which shows the example of the image synthesize | combined without performing the process by the space selection filter by this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 Subject 102 Static magnetic field magnet 103 Gradient magnetic field coil 104 RF coil 105 RF probe 106 Signal detection part 107 Signal processing part 108 Display part 109 Gradient magnetic field power supply 110 RF transmission part 111 Control part 112 Bed 113 Bed drive part 115 Space selection filter

Claims (10)

A static magnetic field generating means, a gradient magnetic field generating means, a high frequency signal transmitting / receiving means, a bed in which the subject is arranged, and an image is reconstructed based on a nuclear magnetic resonance signal from the subject received by the high frequency transmitting / receiving means. Image reconstruction means, display means for displaying the reconstructed image, static magnetic field generation means, gradient magnetic field generation means, bed, high frequency magnetic field pulse transmission means, high frequency reception means for receiving the nuclear magnetic resonance signal, image In a magnetic resonance imaging apparatus comprising control means for controlling the operation of reconstruction means and display means,
A magnetic resonance imaging apparatus comprising: a spatial selection filter that removes an image distortion portion of an image reconstructed by the image reconstruction means.   2. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit controls movement of the bed so that images of a plurality of consecutive parts of the subject are reconstructed by the image reconstruction unit, The magnetic resonance imaging apparatus is characterized in that the distorted portion of the image is removed by the spatial selection filter, and a plurality of images from which the distorted portion has been removed are combined and displayed on the display means.   3. The magnetic resonance imaging apparatus according to claim 1, wherein the spatial selection filter is set in a shape for removing an image distortion portion at a peripheral portion of the image reconstructed by the image reconstruction unit. A magnetic resonance imaging apparatus.   3. The magnetic resonance imaging apparatus according to claim 1, wherein the space selection filter has a circular shape, and a diameter thereof is set based on a static magnetic field uniformity of the static magnetic field generating means.   3. The magnetic resonance imaging apparatus according to claim 1, wherein the spatial selection filter has a circular shape, and a diameter thereof is set based on gradient magnetic field linearity of the gradient magnetic field generating means.   6. The magnetic resonance imaging apparatus according to claim 4, wherein the image reconstruction unit corrects the luminance of the image based on the sensitivity distribution of the high-frequency receiving unit.   5. The magnetic resonance imaging apparatus according to claim 4, further comprising shimming means for correcting the uniformity of the static magnetic field, wherein the diameter of the spatial selection filter is set from the uniformity of the static magnetic field corrected by the shimming means. A magnetic resonance imaging apparatus.   3. The magnetic resonance imaging apparatus according to claim 1, wherein the spatial selection filter has a circular shape, and a first spatial selection filter whose diameter is set based on a static magnetic field homogeneity based on specification data of the static magnetic field generation unit; The second spatial selection filter whose diameter is set from the gradient magnetic field linearity of the gradient magnetic field generating means, and the static magnetic field uniformity of the static magnetic field generating means are measured, and the diameter is set from the measured data And a third spatial selection filter, wherein one of the first, second, and third spatial selection filters or a combination of the first and second spatial selection filters is selected. Resonance imaging device.   9. The magnetic resonance imaging apparatus according to claim 8, wherein an operator uses one of the first, second, and third spatial selection filters or a combination of the first and second spatial selection filters as the display means. A magnetic resonance imaging apparatus, wherein a screen for selection is displayed.   9. The magnetic resonance imaging apparatus according to claim 8, wherein the control means is a combination of any one of the first, second and third spatial selection filters or first and second spatial selection filters in accordance with an imaging sequence. A magnetic resonance imaging apparatus.

Images