JP5127308B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a nuclear magnetic resonance imaging apparatus that measures nuclear magnetic resonance signals from hydrogen, phosphorus, etc. in a subject and visualizes nuclear density distribution, relaxation time distribution, and the like.

  In a magnetic resonance imaging apparatus (MRI apparatus), when a subject to be imaged is larger than the FOV or when a part of the subject is near the outside of the FOV, aliasing artifacts are generated in the image. The aliasing artifact is generated depending on the imaging conditions such as FOV and phase encoding direction regardless of the imaging target region.

  For example, when an image of the subject's chest is taken centering on the heart, the left shoulder is within the FOV, and the right shoulder is folded over the left shoulder when the right shoulder is outside the FOV. There is. As described above, as a case where folding artifacts are particularly likely to occur, there is cardiac imaging that frequently uses double obliques.

  In cardiac examinations using an MRI apparatus, cardiac function analysis, viability examination, coronary angiography, etc. are performed. Various oblique sections such as a section along the coronary artery are set. Furthermore, even in the case of the same short-axis image, there are very large individual differences depending on the body shape of the subject, such as a SAG cross section or a COR cross section, and the imaging cross section cannot be determined uniquely.

  Therefore, it is necessary to set an imaging slice for each examination in consideration of a state in which folding does not occur as much as possible or a phase encoding direction that does not cause a problem in diagnosis even if it occurs. In particular, since coronary angiography is performed by 3D imaging with a volume of several centimeters, it takes several minutes even if the imaging time is short. Cost.

  Patent Document 1 describes a technique for obtaining a good image in which aliasing artifacts in the phase encoding direction are suppressed without extending the imaging time.

  In other words, the technique described in Patent Document 1 makes it possible to vary the step width and the number of steps in the phase encoding direction, and variably set the measurement visual field in the phase encoding direction, thereby widening the phase distinguishable range of nuclear spins. ing. This suppresses the occurrence of folding artifacts.

Japanese Patent No. 2805405

  However, even with the technique described in Patent Document 1, the occurrence of folding artifacts cannot be completely removed. In particular, in cardiac imaging that frequently uses complicated oblique sections, the possibility that aliasing artifacts occur in an image remains unsolved.

  If the occurrence of the folding artifact is found after the photographing is completed, it takes time for the re-imaging, leading to an extension of the inspection time.

  An object of the present invention is to realize a magnetic resonance imaging apparatus capable of avoiding a situation in which re-shooting has to be performed after the end of shooting due to occurrence of folding artifacts.

  In order to solve the above problems, the present invention is configured as follows.

The magnetic resonance imaging apparatus of the present invention displays on the display means a display means for displaying an imaging cross section of the subject, and displays a positioning image of the imaging cross section of the subject on the basis of a set measurement visual field in the positioning image. And a control unit that estimates a folding artifact generated in the actual captured image and causes the display unit to display the estimated folding artifact .

  According to the present invention, since the occurrence position of the folding artifact is estimated and displayed before the main shooting, the operator can estimate that the image cannot be used for the diagnosis before the main shooting. Therefore, it is possible to avoid re-shooting in advance, and it is possible to avoid unnecessary extension of the diagnosis time.

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

  FIG. 1 is a schematic configuration diagram of a typical MRI apparatus to which the present invention is applied. In FIG. 1, the MRI apparatus includes a magnet 402 that generates a static magnetic field around a subject 401, a gradient magnetic field coil 403 that generates a gradient magnetic field in a space where the subject 401 is disposed, and a high-frequency magnetic field in this region. And an RF probe 405 that detects an MR signal generated by the subject 401.

  The gradient magnetic field coil 403 is configured by gradient magnetic field coils in three directions of X, Y, and Z, and each generates a gradient magnetic field in accordance with a signal from the gradient magnetic field power supply 409. The RF coil 404 generates a high-frequency magnetic field according to the signal from the RF transmitter 410. The signal of the RF probe 405 is detected by the signal detection unit 406, processed by the signal processing unit 407, and converted into an image signal by calculation. The image is displayed on the display unit 408.

  The gradient magnetic field power supply 409, the RF transmission unit 410, and the signal detection unit 406 are controlled by the control unit 411, and the control time chart is generally called a pulse sequence. The bed 412 is for the subject to lie down.

  Next, a photographing method will be described. Different phase encodings are given depending on the gradient magnetic field, and echo signals obtained by the respective phase encodings are detected. As the number of phase encodings, values such as 128, 256, and 512 are usually selected per image.

  Each echo signal is usually obtained as a time-series signal composed of 128, 256, 512, and 1024 sampling data. These data are two-dimensionally Fourier transformed to create one MR image.

  Although not shown, the MRI apparatus is provided with a keyboard and a mouse for changing or adding a sequence, instructing display on the display unit 408, or the like.

  Next, a first embodiment of the present invention will be described. FIG. 2 sets the cross section (measurement field of view (FOV)) 202 of the main imaging on the positioning image 201 (FIG. 2A) of Ax (axial surface) imaged around the heart portion of the subject. It is explanatory drawing. In addition, after acquiring the positioning image with a wide FOV, it is a generally performed technique to increase the spatial resolution by narrowing the FOV during actual photographing.

  Here, on the positioning image 201 displayed on the display unit 408 or the like, the estimated image 210 ((B) in FIG. 2) of the main imaging result is displayed on the control unit 411 at the stage where the FOV 202 is set with the mouse or the like described above. Estimate and display. The estimated image 210 is obtained by cutting out and displaying a part of the positioning image 201, and displays the folding artifact 203 (the arm portion in the positioning image 201) that appears when the main photographing is executed.

  In addition, the phase encoding direction when estimating the result image can be switched and displayed, or the estimated image can be displayed in another direction at the same time, and the estimated images 210 ((B) in FIG. 2) and 220 ((C in FIG. 2) It is possible to examine which direction is suitable by comparing in different phase encoding directions (RL direction, AP direction) as in ()). 2A, 2B, and 2C may be displayed on one screen, or may be displayed on separate screens.

  Here, FIG. 3 shows a method of estimating and displaying the occurrence position of folding. In FIG. 3, it is assumed that a slice position 302 for shooting is set on the positioning image 301. In the example shown in FIG. 3, since the phase encoding direction is the RL direction, the arm portions 310 and 311 outside the slice position 302 become folding artifacts.

  FIG. 9 shows an operation flowchart according to the first embodiment of the present invention. In FIG. 9, when the operation is started (step 1001), a positioning image is first taken (step 1002) and displayed on the UI (step 1003, corresponding to 201 in FIG. 2). Then, the slice position of the main photographing is displayed on the displayed positioning image (step 1004, corresponding to 202 in FIG. 2), and the photographing position is adjusted (step 1005).

  After adjusting the photographing position, an image in which the aliasing artifact occurrence position is estimated is displayed (step 1006, corresponding to 210 in FIG. 2). This image is confirmed (step 1007), and if there is no problem, actual photographing is executed (step 1008). If it is necessary to adjust the slice position again in step 1007, the process returns to step 1005 to repeat the adjustment.

  Here, in step 1006, a method of estimating the folding occurrence position will be described with reference to FIG. The phase rotation amount (R) by the phase encoding gradient magnetic field is proportional to the position (y) as shown in the following equation (1), and rotates by 2π within the region of the slice position 302 (FOV in the phase encoding direction) (FIG. 3). 322).

R (y) = (2π / FOV) × y ――― (1)
Since the phase rotation amount (R) is rotated by 2π, the arms 310 and 311 existing in the region outside the slice position 302 have phase rotation amounts corresponding to reference numerals 320 and 321, respectively.

  Since the reference numerals 320 and 321 are equivalent to the portions 330 and 331 in the reference numeral 322, respectively, the arm 310 is folded back at the position of the reference numeral 340 and the arm 311 is returned at the position of the reference numeral 341.

  As described above, if the position (y) on the positioning image is known, the aliasing artifact generation position can be estimated.

  As described above, the FOV is set so that no artifact is generated by estimating the aliasing artifact occurrence position before the actual photographing and displaying the image for the set FOV. The FOV can be set so that the artifacts do not overlap with the necessary parts.

  Therefore, according to the first embodiment of the present invention, it is possible to prevent a situation in which re-shooting has to be performed after the end of shooting due to occurrence of folding artifacts, and to avoid an unnecessary increase in imaging time. Can be realized.

  Here, the first embodiment described above is an example in the case where the positioning image and the slice plane of the main photographing are in the same plane, but the present invention can also be applied when there is an oblique.

  In the second embodiment of the present invention, the aliasing artifact that occurs is estimated and displayed on the oblique surface. FIG. 4 is an explanatory diagram of the second embodiment of the present invention. In FIG. 4, in the case of an oblique slice plane 502, the region 503 is turned back and enters the main photographing slice. Therefore, the area 530 in which the area 503 appears as the aliasing artifact is displayed on the aliasing artifact confirmation image 520.

  Depending on the slice position 502, the aliasing artifact confirmation image 520 can be adjusted to a 2D single slice if the slice position 502 is a 2D single slice, or a 3D slab if it is a 3D slab.

  In addition, when the aliasing artifact confirmation image 502 is a multi-slice or a 3D slab, it is possible to display only one central slice.

  The difference from the above-described first embodiment of the present invention is that there is no image of the area to be turned back, so that the color of the area corresponding to the area 530 is changed and the like is shown to the operator (user). In the second embodiment, a positioning image is also displayed on the main photographing slice.

  In the example shown in FIG. 4, the positioning image is three slices (510, 511, 512), and these positions are displayed on the aliasing artifact confirmation image 520. Furthermore, as shown in FIG. 5, it is also possible to mark a part on the UI that will interfere with the diagnosis when overlapping artifacts overlap.

  For example, when the heart portion is surrounded by a dotted line 601, the corresponding position is also displayed at the same position on the positioning image line (510, 511, 512) shown in the aliasing artifact confirmation image 520. The example shown in FIGS. 4 and 5 shows an example of the three slices of the Ax positioning image, but it is also possible to more accurately confirm the relationship between the folding artifact occurrence position and the imaging target part by increasing the positioning image.

  Even if 5 orthogonal slices of Ax, COR, and SAG are taken for a total of 15 slices, if a high-speed sequence of about 1 sec per slice is used as in the SSFP sequence, the positioning image acquisition time is about 15 sec. And for a short time. Furthermore, it is possible to capture a positioning image that covers the entire heart with a reduced spatial resolution, and to determine the actual imaging slice on the image.

  For example, if a 30 cm area centering on the heart is imaged at a slice thickness of 8 mm, 38 slices are required. However, if the matrix size is 64 × 64 and TR is 4 ms, the imaging time per slice is 256 ms. Even if 38 slices are taken, it takes about 10 seconds.

  In the case of such positioning image acquisition, since it is possible to cut out in an arbitrary direction, the aliasing artifact confirmation image becomes an image 540 shown in FIGS. 4 and 5, and the aliasing artifact generation region 541 and the imaging target part ( (Such as the heart) becomes clearer.

  The region 601 that interferes with the diagnosis when the aliasing artifact is applied as shown in FIG. 5 is clearly shown as a region 602 in which the shade level or color is changed so that it can be distinguished from other portions. With such a mechanism, it is possible to confirm whether or not the aliasing artifact occurrence position and whether or not the aliasing artifact is applied to the imaging target part before imaging.

  As described above, according to the second embodiment of the present invention, it is possible to estimate the aliasing artifact that occurs and display the image even on the oblique surface in the set FOV. It is possible to realize a magnetic resonance imaging apparatus that can prevent a situation in which re-shooting is required later and avoid an unnecessary increase in imaging time.

  In cardiac imaging, a rectangular field of view that narrows the FOV in the phase encoding direction to the minimum necessary is often used in order to shorten the imaging time. When the rectangular field of view is used, the area where the folding artifact is generated becomes wider. As shown in FIG. 6, when the main photographing slice 702 in the positioning image 701 is a rectangular field of view, the regions 710 and 711 can be folded artifacts.

  In such a case, there is a possibility that the wrapping will come to the very vicinity of the imaging target area 703 or a partial wrapping may be applied to 703. Even if the folding area is set so as not to reach 703, a slight body motion or the like causes a flow artifact in the phase encoding direction, resulting in image quality degradation in the imaging target area.

  In the third embodiment of the present invention, when a rectangular field of view that narrows the FOV in the phase encoding direction to the minimum necessary is used, if a region that may be a folding artifact is estimated, a pre-saturation pulse is automatically applied to the region. It is an example.

  By applying the pre-saturation pulse to the areas 710 and 711 shown in FIG. 6, the signals in the same area are almost suppressed. Therefore, even if some aliasing artifacts are applied to the imaging target area 703, image quality degradation is minimal. It becomes the limit. In addition, parallel imaging is often used in cardiac imaging. However, if parallel imaging is performed in a state in which there is a fold in the FOV, there may be a case where a development failure occurs at the time of parallel reconstruction in the folded portion. Also for the purpose of preventing such image defects, it is effective to suppress the signal of the part that is folded in advance as in the third embodiment.

  As described above, according to the third embodiment of the present invention, even when a rectangular field of view that narrows the FOV to the minimum necessary is used, an area where aliasing artifacts are generated is estimated and displayed before the actual photographing. Because it is configured to apply pre-saturation pulses to the area, magnetic resonance imaging that prevents the need to re-shoot after the end of shooting due to the occurrence of artifacts and avoids unnecessary extension of imaging time An apparatus can be realized.

  Note that the third embodiment is applicable to both cases where there is an oblique and what is not.

  Further, in the third embodiment described above, the pre-saturation pulse is automatically applied to the region where the folding region is generated. However, the operator or the like can select whether or not to apply the pre-saturation pulse. In this way, the selection button can be displayed on the display unit and can be configured to follow the selection.

  By the way, not only in the case of using a rectangular field of view, the FOV is set too small, and as shown in FIG. Also in the aliasing artifact confirmation image shown in FIG. 7, it can be seen that aliasing artifact generation areas 810 and 811 are applied to the diagnosis target area 803.

  In such a case, the fourth embodiment of the present invention presents an example of how far the FOV can be expanded to suppress the influence on the diagnosis target area due to the occurrence of the folding artifact.

  As shown in FIG. 7, when the value obtained by subtracting the diagnosis area 822 from the FOV 821 is smaller than the sum of the areas 820 and 823 outside the FOV 821, the aliasing artifact is always applied to the diagnosis target area 803.

  At this time, the minimum FOV 821 value satisfying “821” − “822” ≧ “820” + “823” is displayed on the UI. For example, as shown in FIG. 7, it is displayed in the minimum FOV display area 840, or the FOV is automatically changed on the slice 802 (reference numeral 830 in FIG. 7 indicates the state after the change). In addition, it is possible to adjust so that the diagnosis target region 803 is at the center of the FOV. In this case, the aliasing artifact confirmation image is as shown at 903 in FIG.

  As described above, according to the fourth embodiment of the present invention, the region where the aliasing artifact is generated is estimated and displayed before the main imaging, and the influence of the occurrence of the aliasing artifact on the diagnosis target region is extended to what extent. Since it is configured to display whether or not the image can be suppressed, it is possible to prevent a situation in which the image must be re-taken after the end of the image capture due to the occurrence of an artifact, and an unnecessary increase in the image capture time can be avoided Can be realized.

  Next, a fifth embodiment of the present invention will be described. The fifth embodiment of the present invention is an example showing a region where folding by parallel imaging may occur.

  Usually, in parallel imaging, data acquisition is performed by thinning out phase encoding, so that an image reconstructed in a folded state is developed into an unfolded image by a coil sensitivity map. In this case, if the acquisition state of the sensitivity map is different from the state at the time of shooting, there is a possibility that the image remains folded at the time of development.

  The position where such an image defect (residual aliasing) occurs can be estimated depending on how much data is thinned out at the time of measurement, that is, depending on the number of times of parallel imaging.

  For this reason, in the fifth embodiment of the present invention, an area in which folding may occur is displayed according to the number of times of parallel imaging. For example, as shown in FIG. 10A, when the magnification of parallel imaging is 1.5, the areas where the aliasing may occur are 25% areas 1102 and 1103. Further, as shown in FIG. 10B, when the magnification of parallel imaging is 1.75 times, the regions where the aliasing may occur are 37.5% regions 1104 and 1105.

  In the case of FIG. 10A, the folding artifact is not superimposed on the diagnosis target area, but in the case of FIG. 10B, the folding artifact is partially superimposed on the diagnosis target area.

  In this way, by estimating and displaying the image where the aliasing occurs according to the parallel imaging multiple speed, there is no problem even if the aliasing artifact does not overlap the diagnosis target area or even if it partially overlaps. The double speed number can be selected.

  According to the fifth embodiment of the present invention, when using parallel imaging, it is configured to estimate and display a region where aliasing artifacts occur according to the number of times of parallel imaging before the main imaging. To realize a magnetic resonance imaging apparatus that can appropriately select the parallel imaging multiple speed, prevent the situation that re-shooting must be performed after the end of shooting due to the occurrence of artifacts, and avoid unnecessary extension of imaging time Can do.

It is a schematic block diagram of the typical MRI apparatus with which this invention is applied. It is image display explanatory drawing in the 1st Embodiment of this invention. It is a figure explaining the estimation display of the return | turnback artifact generation | occurrence | production position in this invention. It is image display explanatory drawing in the 2nd Embodiment of this invention. It is image display explanatory drawing in the 2nd Embodiment of this invention. It is a figure explaining the 2nd Embodiment of this invention. It is a figure explaining the 4th Embodiment of this invention. It is a figure explaining the 4th Embodiment of this invention. It is an operation | movement flowchart in the 1st Embodiment of this invention. It is a figure explaining the 5th Embodiment of this invention.

Explanation of symbols

  201, 301, 512, 701, 801 ... positioning image, 203 ... folding artifact, 210,220 ... estimated image of the actual imaging result, 401 ... subject, 402 ... static magnetic field magnet, 403 ... Gradient magnetic field coil, 404 ... RF coil, 405 ... RF probe, 406 ... Signal detection unit, 407 ... Signal processing unit, 408 ... Display unit, 409 ... Inclination Magnetic field power supply, 410 ... RF transmitter, 411 ... control unit, 412 ... bed

Claims (7)

Display means for displaying a positioning image for setting an imaging section of the subject ;
Means for setting a measurement visual field on the positioning image;
Control means for estimating a folding artifact generated in the actual captured image based on the measurement field of view set on the positioning image, and displaying the estimated folding artifact on the display means;
A magnetic resonance imaging apparatus comprising,
The control means estimates a position where a folding artifact occurs in a main imaging slice that is oblique with respect to two or more positioning images, and causes the display means to display the estimated folding artifact. Equipment . Display means for displaying a positioning image for setting an imaging section of the subject;
Means for setting a measurement visual field on the positioning image;
Control means for estimating a folding artifact generated in the actual captured image based on the measurement field of view set on the positioning image, and displaying the estimated folding artifact on the display means;
A magnetic resonance imaging apparatus comprising:
The magnetic resonance imaging apparatus characterized in that the control means causes the display means to display the minimum value of the measurement visual field required to remove the folding artifact from the designated area of the positioning image . The magnetic resonance imaging apparatus according to claim 1 or 2 ,
Upper Symbol control means estimates the aliasing artifacts occur when changing the phase encode direction, the magnetic resonance imaging apparatus characterized by displaying the aliasing artifacts that the estimated on the display means. The magnetic resonance imaging apparatus according to any one of claims 1 to 3,
A magnetic resonance imaging apparatus characterized in that the setting of the measurement visual field can be changed . The magnetic resonance imaging apparatus according to any one of claims 1 to 4,
The magnetic resonance imaging apparatus characterized in that the control means applies a pre-saturation pulse to the estimated generation source of the folding artifact. The magnetic resonance imaging apparatus according to any one of claims 1 to 5 ,
The magnetic resonance imaging apparatus characterized in that the control means sets a measurement visual field in which a folding artifact does not overlap with a designated region of the positioning image . The magnetic resonance imaging apparatus according to claim 1 or 2 ,
The control means estimates a folding artifact occurring in the actual captured image based on a set multiple speed of parallel imaging, and causes the display means to display the estimated folding artifact. apparatus.

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