JP4995650B2 - Magnetic resonance imaging system - Google Patents

Description

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

  A method of shortening the photographing time by using a rectangular visual field that narrows the visual field in the phase encoding direction has been widely applied. Particularly in cardiac imaging, since there are many examples of imaging in a breath-hold state, it is necessary to shorten the imaging time as much as possible, and a rectangular field of view is frequently used.

  As a precaution when applying a rectangular field of view, there is a folding artifact. The aliasing artifact occurs when the object to be photographed protrudes outside the field of view. Therefore, when a rectangular visual field that narrows the visual field in the phase encoding direction is used, it is necessary to prevent image quality degradation due to aliasing artifacts.

  Further, when parallel artifacts are used together with aliasing artifacts, the accuracy of parallel imaging-related processing at the time of image reconstruction is reduced, and image quality may be significantly degraded.

  Thus, when performing imaging using a rectangular field of view, it is important to prevent aliasing artifacts, particularly when parallel imaging is also used. Here, when photographing three basic cross sections of an axial plane, a sagittal plane, and a coronal plane, it is easy to grasp the readout direction and the phase encoding direction, and it is possible to estimate the folding caused by the rectangular field of view.

  On the other hand, in cardiac imaging where imaging is performed on a double oblique surface, it is difficult to grasp the readout direction and phase encoding direction, and if the image is not reconstructed, there is an accurate phase encoding direction or whether or not the phase encoding direction is folded. I can't judge.

  For this reason, for example, Patent Document 1 describes a technique for preventing folding artifacts. The technique described in Patent Document 1 substantially prevents the appearance of aliasing artifacts by setting the measurement field of nuclear magnetic resonance signals (NMR signals) wider than the field of view of the reconstructed image and increasing the number of phase encodings. It is out.

JP-A-5-76518

  However, according to the technique described in Patent Document 1, since an area other than the field of view is imaged, which leads to an extension of the imaging time, it is difficult to meet the demand for shortening the imaging time.

  An object of the present invention is to realize a magnetic resonance imaging apparatus capable of shortening imaging time and preventing occurrence of aliasing artifacts.

  In order to achieve the above object, the present invention is configured as follows.

  Rotate the gradient magnetic field application direction with respect to the slice plane to be imaged set using a predetermined imaging field for the subject, determine the phase encoding direction that minimizes the field in the phase encoding direction, and set the number of phase encodings The rectangular field of view is determined based on the determined phase encoding direction and the number of phase encodings, and the photographing of the slice plane to be imaged is performed using the determined rectangular field of view.

  In addition, the direction in which the imaging target slice plane set using a predetermined imaging field of view for the subject has the thinnest dimension is detected by image processing, the detected direction is determined as the phase encoding direction, and the number of phase encodings is determined. The rectangular field of view is determined based on the determined phase encoding direction and the number of phase encodings, and the photographing of the slice plane to be imaged is performed using the determined rectangular field.

  It is possible to realize a magnetic resonance imaging apparatus that can shorten the imaging time and can prevent the occurrence of aliasing artifacts.

  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 magnet 402 that generates a static magnetic field around a subject 401, a gradient magnetic field coil 403 that generates a gradient magnetic field, an RF coil 404 that generates a high-frequency magnetic field, and a subject 401. And an RF probe 405 for detecting an MR signal.

  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. In addition, the RF coil 404 generates a high-frequency magnetic field in accordance with a signal from the RF transmission unit 410. The signal of the RF probe 405 is detected by the signal detection unit 406, signal 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 operation-controlled by the control unit 411, and the control time chart is generally called a pulse sequence. The bed 412 is for the subject 401 to lie down.

  At present, the imaging object of the MRI apparatus is the main constituent substance of the subject 401, proton, which is widely used clinically. By imaging the spatial distribution of the proton density and the spatial distribution of the relaxation phenomenon in the excited state, the form or function of the human head, abdomen, limbs, etc. is photographed two-dimensionally or three-dimensionally.

  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.

  Next, a first embodiment of the present invention will be described with reference to FIG. 2, FIG. 3, and FIG. The first embodiment will be described taking cardiac short-axis imaging as an example.

  In cardiac imaging, it is a common practice to shorten the imaging time using a rectangular field of view after acquiring a positioning image with a wide field of view. Further, as shown in FIG. 2A, the cardiac short-axis imaging is often imaging such as a double oblique surface 201, and the imaging target is inclined in the plane. 2A shows a positioning image 201 when photographing the short axis of the heart, and FIG. 2B shows an image obtained when the main photographing is performed using a rectangular field by a conventional method. This is an image 203. The imaging method shown in FIG. 2B is a method in which the field of view in the phase encoding direction is narrowed with respect to the positioning image 201 shown in FIG. It is.

  On the other hand, the actual captured image in the first embodiment of the present invention is an image 205 shown in FIG. The first embodiment of the present invention is characterized in that the main photographing is performed with the rectangular field of view narrowed to such an extent that the photographing target 202 does not fold, with the direction in which the thickness of the photographing target 202 is the thinnest as the phase encoding direction. .

  In other words, when shooting the entire subject using a rectangular field of view, if the phase encoding direction is determined so that the field of view in the phase encoding direction is minimized, the number of phase encodings can be minimized while maintaining a predetermined spatial resolution. Is possible. In fact, the width 206 in the method using the first embodiment of the present invention has a larger field of view in the phase encoding direction than the width 204 in the phase encoding direction in the conventional method shown in FIG. The number of phase encodings can be reduced under the condition that the space is narrowed and the spatial resolution is maintained.

  FIG. 3 is a flowchart regarding the phase encoding direction determination in the first embodiment of the present invention. In FIG. 3, first, a positioning image that is a double oblique surface is set (step 701, corresponding to the positioning image 201 in FIG. 2A).

  Next, pre-scanning for adjusting a static magnetic field, a gradient magnetic field, etc. is performed (step 702). After that, a radial scan is performed, a predetermined process is applied to the acquired data, the phase encoding direction is determined so that the field of view in the phase encoding direction is minimized, and the minimum phase is maintained while maintaining a predetermined spatial resolution. The number of encoding is determined (steps 703 to 711).

  However, if the amount of rotation is not π or more in step 709, a predetermined angle (for example, 30 degrees) is added to the rotation angle, and the process returns to step 703. Then, Steps 703 to 709 are executed. In Step 709, when the rotation amount becomes π or more, Steps 710 and 711 are executed.

  After the phase encoding number is determined in step 711, the main photographing is executed using the determined phase encoding direction and the phase encoding number (step 712, corresponding to the image 202 in FIG. 2C).

  Here, in steps 703 to 711, the phase encoding direction is determined from the data acquired by the radial scan so that the field of view in the phase encoding direction is minimized, and the minimum number of phase encodings is maintained while maintaining a predetermined spatial resolution. The process for determining the value will be described with reference to FIGS.

  First, the object is irradiated with a high frequency magnetic field pulse to excite magnetization (step 703), a readout gradient magnetic field is applied in the direction 301 shown in FIG. 4A (step 704), and the generated echo signal is generated. Receive (step 705). The received echo signal is subjected to a one-dimensional Fourier transform to create projection data 302 (step 706), and the width 303 of the object to be imaged with respect to the direction 301 is calculated from the range where the projection data is larger than the threshold value (step 707).

  Next, the projection angle is changed, that is, the readout gradient magnetic field application direction is rotated by the amount of rotation (θ) (step 708), and the processes of steps 703 to 706 are repeated to obtain the result shown in FIG. A width 305 of the photographing target with respect to the direction 304 shown is calculated. The same processing is repeated, and when the rotation amount becomes π or more, the processing from step 703 to step 708 ends.

  FIG. 4B shows the relationship between the rotation amount obtained by the above radial scan and the width of the object to be imaged. As shown in FIG. 4B, the profile 306 is minimum when the rotation amount is 307. That is, the width of the object to be photographed is minimized. By setting the readout gradient magnetic field application direction at the rotation amount 307 to the phase encoding direction, the direction in which the width of the imaging target is minimized can be matched with the phase encoding direction (step 710).

  In addition, the minimum number of phase encodings when shooting with maintaining the spatial resolution can be determined by dividing the width of the imaging target obtained from the projection data in the phase encoding direction by a predetermined spatial resolution (step) 711).

  As described above, according to the first embodiment of the present invention, the phase encoding direction is set to the direction in which the width of the object to be imaged is minimized, the rectangular field of view is set, the number of phase encoding is determined, and the main imaging is performed. Therefore, even when a rectangular field of view is used on a double oblique surface, it is possible to reduce the imaging time and to realize a magnetic resonance imaging apparatus capable of preventing the occurrence of aliasing artifacts. .

  In the above-described first embodiment of the present invention, the case where the shooting target is smaller than the field of view is shown as an example in the positioning image. However, the present embodiment is also applicable to an off-center where a part of the shooting target is outside the rectangular field of view. The invention is applicable.

  The second embodiment of the present invention is an example in which the present invention is applied to the off-center case. FIG. 5 is an explanatory diagram of the second embodiment of the present invention. In FIG. 5, a part (shaded portion) of the object to be photographed protrudes outside the field of view 503 of the positioning image 501. In this case, since the folding artifact 502 occurs in the phase encoding direction, the width of the photographing target cannot be determined from the projection data.

  Therefore, if the field of view and the width of the object to be imaged coincide with each other and it is determined that the field is off-center, the strength of the readout gradient magnetic field applied in step 704 in FIG. To expand the field of view so that That is, with the center of the field of view fixed, the field of view is expanded until the field of view and the width of the object to be imaged do not match.

  When the field of view enlargement is completed, the width 505 of the object to be imaged in an arbitrary direction 506 can be measured by performing steps 703 to 706. Similarly, by setting the field of view sufficiently large and performing radial scanning, the phase encoding direction is determined so that the field of view in the phase encoding direction is minimized, and the number of phase encodings is set while maintaining a predetermined spatial resolution. It is possible to minimize.

  Here, by setting the center of the projection data as the center of the actual captured image, it is possible to obtain an actual captured image in which the imaging targets are evenly arranged in the field of view even when the positioning image is off-center. .

  A method of speeding up imaging using a rectangular field of view is useful for imaging other than the heart, and the present invention can also be applied to kinematic imaging in which knees and elbow joints are imaged at various angles.

  The third embodiment of the present invention is an example when the present invention is applied to kinematic photography. FIG. 6 is an explanatory diagram of the third embodiment of the present invention, and shows an example when the knee is photographed on the sagittal surface. (A), (b), and (c) of FIG. 6 are taken by changing the angle at which the knee is bent.

  In kinematic imaging, imaging is performed in various positions, so that the positioning images are different for each body position (images 601, 603, and 605). Therefore, the number of minimum phase encodings when photographing using a rectangular field of view while maintaining the spatial resolution also differs for each body position.

  In the third embodiment of the present invention, the minimum number of phase encodings is automatically detected to the extent that aliasing artifacts do not occur (the detection method is the same as in the first and second embodiments), and the actual captured image (Images 602, 604, 606) are obtained. The third embodiment of the present invention is different from the first and second embodiments in that the subject to be photographed protrudes in the lead-out direction in the actual photographed image, but there is no aliasing artifact in this direction. The image quality is not affected.

  Thus, as shown in the third embodiment, the present invention can also be applied to an imaging target whose angle can be changed, such as a knee.

  Note that the present invention can automatically determine an appropriate phase encoding direction and the number of phase encodings regardless of the actual imaging sequence.

  In the above-described example, the direction in which the width of the imaging target is the smallest is detected while rotating the gradient magnetic field application direction. For example, the positioning image 201 illustrated in FIG. It is also possible to detect the direction in which the width of the object to be imaged is minimized by performing image processing.

  2A and 2C can also be displayed on the display unit 408. In this case, after automatically determining the direction in which the width of the photographing target is minimized, the keyboard manually connected to the control unit 411 or the like by an operator using an image as shown in FIG. It is also possible to adjust the rectangular field of view using an adjusting means such as a mouse or a mouse.

  As described above, in the present invention, since the photographing is performed with the rectangular field of view narrowed to the extent that the folding is not caused with the thinnest direction of the photographing target as the phase encoding direction, the folding artifact does not occur. Therefore, the imaging time can be shortened without degrading the image quality. The success rate of imaging can be improved even for patients who are not good at breath holding. Further, if parallel imaging is used in combination, the photographing time can be further shortened.

1 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied. It is explanatory drawing of the 1st Embodiment of this invention. It is a flowchart about the phase encoding direction determination of the 1st Embodiment of this invention. It is explanatory drawing of the method of determining the phase encoding number which becomes the minimum, hold | maintaining predetermined | prescribed spatial resolution in the 1st Embodiment of this invention. It is explanatory drawing of the 2nd Embodiment of this invention. It is explanatory drawing of the 3rd Embodiment of this invention.

Explanation of symbols

  201, 501, 601, 603, 605... Positioning image, 202... Imaging object, 205, 602, 604, 606... Actual imaging image, 401. 403 ... Gradient magnetic field coil 404 ... RF coil 405 ... RF probe 406 ... Signal detection unit 407 ... Signal processing unit 408 ... Display unit 409 ... Gradient magnetic field power supply, 410 ... RF transmitter, 411 ... control unit, 412 ... bed

Claims (7)

In a magnetic resonance imaging apparatus comprising a static magnetic field generating means, a gradient magnetic field generating means, a high frequency signal transmitting / receiving means, and a control means for controlling the gradient magnetic field generating means and the high frequency signal transmitting / receiving means, and images a subject.
Having a minimum width direction detecting means for detecting a direction in which the width of the subject is minimized,
The magnetic resonance imaging apparatus, wherein the control means performs imaging of the subject with the minimum width direction as a phase encoding direction. The magnetic resonance imaging apparatus according to claim 1.
The magnetic resonance imaging apparatus characterized in that the control means uses a value obtained by dividing the minimum width by a predetermined resolution as the number of phase encodes. The magnetic resonance imaging apparatus according to claim 1 or 2,
The control means obtains a plurality of projection data of the subject having different projection angles within a predetermined slice plane,
The minimum width direction detecting means selects projection data having a minimum width from the plurality of projection data, and sets the direction of the projection angle from which the projection data having the minimum width is acquired as the minimum width of the subject. A magnetic resonance imaging apparatus, characterized in that The magnetic resonance imaging apparatus according to claim 1.
The control means determines whether or not a part of the subject is out of the predetermined imaging field, and determines that a part of the subject is out of the predetermined imaging field. Enlarge the field of view so that a part of the subject in the field is within the field of view, and rotate the gradient magnetic field application direction relative to the slice plane in the enlarged field of view to minimize the field of view in the phase encoding direction. A magnetic resonance imaging apparatus characterized by determining a phase encoding direction and determining a number of phase encodings. 5. The magnetic resonance imaging apparatus according to claim 1, further comprising an image display means.
The control means causes the image display means to display the imaging target slice plane within the predetermined imaging visual field, and to display the set rectangular visual field and the slice surface of the subject within the rectangular visual field. Magnetic resonance imaging device.   6. The magnetic resonance imaging apparatus according to claim 5, further comprising an adjustment unit that allows an operator to adjust the rectangular field of view displayed on the image display unit, and the control unit is configured to perform the operation based on a command from the preparation unit. A magnetic resonance imaging apparatus characterized by changing a rectangular field of view. In a magnetic resonance imaging apparatus comprising a static magnetic field generating means, a gradient magnetic field generating means, a high frequency signal transmitting / receiving means, and a control means for controlling the gradient magnetic field generating means and the high frequency signal transmitting / receiving means, and images a subject.
The control means includes
The direction in which the imaging target slice plane set for the subject with the predetermined imaging field of view becomes the thinnest dimension is detected by image processing, and the detected direction is determined as the phase encoding direction and the number of phase encodings is determined. A magnetic resonance imaging apparatus characterized in that a rectangular field of view is determined based on the determined phase encoding direction and the number of phase encodings, and main imaging of the imaging target slice surface is performed using the determined rectangular field of view.

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