JP2009284958A - Magnetic resonance imaging system and phantom - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize a single phantom adjustable to a plurality of kinds without being accompanied by the alteration of an arranging position in an MRI system. <P>SOLUTION: The phantom 26 has a cubic base and all of the side parts of the regular hexahedron of the cube-shaped base are formed into a curved face shape while all of the corner parts of the regular hexahedron are also formed into the curved face. The signal values of the sharpened parts (corner parts) being the shape of the regular hexahedron become high but, if the corner parts are formed into the curved surface shape like the phantom 26, the signal values of them can be prevented from becoming high and the curving of the contour of an imaged can be suppressed. Since the distance between mutually parallel sides are kept constant if the concentration of the signal values is not considered at the time of processing of the image of the phantom 26 and the image is normal, the image can be processed easily and rapidly. Since the center slice images in X-, Y- and Z-axis directions are mutually equal in the phantom 26, image processing can be performed by uniform processing without altering the installation direction of the phantom 26 at every axis. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging apparatus and a phantom for adjusting a gradient magnetic field.

  A magnetic resonance imaging apparatus (MRI apparatus) applies a high-frequency magnetic field to a subject placed in a uniform static magnetic field, thereby causing a nuclear magnetic resonance phenomenon to occur in nuclei (usually protons) present in an arbitrary region of the subject. A tomographic image of the region is obtained from a nuclear magnetic resonance signal (echo signal) generated thereby. At this time, it is necessary to accurately adjust characteristic values such as the gradient magnetic field gain, slew rate, and eddy current correction.

  Since each adjustment value in the MRI apparatus is different for each apparatus, it is necessary to set an accurate adjustment value after the installation is completed. For this reason, adjustment is performed using a phantom as described in Patent Document 1.

JP-A-4-26411

  In the prior art, the phantom and the receiving coil required for each adjustment of the gradient magnetic field gain, slew rate, etc. require different postures and different ones. It is necessary to replace the phantom and the receiving coil.

  For example, in the gradient magnetic field gain adjustment, adjustment is performed using a dedicated phantom, but the adjustment must be performed while changing the installation direction of the phantom for each of the X, Y, and Z axes. In addition, when adjusting the gradient magnetic field slew rate, it is necessary to use a phantom other than the phantom used for adjusting the gradient magnetic field gain.

  When adjusting the gain of the gradient magnetic field, the operator must change the installation direction of the phantom or change to another phantom for each adjustment, which is inefficient.

  An object of the present invention is to realize a single phantom capable of adjusting a plurality of types of characteristic values without changing the arrangement position, and a magnetic resonance imaging apparatus capable of adjusting a gradient magnetic field using the phantom. That is.

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

  In the magnetic resonance imaging apparatus, a sliced image of a phantom having a regular hexahedron with curved corners and sides is taken, and based on the phantom dimensional information obtained from the taken slice image and the actual phantom dimensional information, The characteristic value of the gradient magnetic field generating means is adjusted.

  In addition, in a magnetic resonance imaging apparatus, a slice image obtained when a diffusion gradient magnetic field pulse of a phantom whose corners and sides of a regular hexahedron are curved and a slice image obtained when no diffusion gradient magnetic field pulse is applied are taken. The characteristic value of the gradient magnetic field generating means is adjusted based on the result of comparing the phantom dimensional information obtained from the two captured slice images.

  A single phantom capable of adjusting a plurality of types of characteristic values without changing the arrangement position and a magnetic resonance imaging apparatus capable of adjusting a gradient magnetic field using the phantom 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, a magnetic resonance imaging apparatus obtains a tomographic image of a subject using a nuclear magnetic resonance (NMR) phenomenon, and includes a static magnetic field generation system 2, a gradient magnetic field generation system 3, a transmission system 5, A reception system 6, a signal processing system 7, a sequencer 4, and a central processing unit (CPU) 8 are provided.

  The static magnetic field generation system 2 generates a uniform static magnetic field in a body axis direction or a direction perpendicular to the body axis in a space around the subject 1, and a permanent magnet system or a normal conduction system around the subject 1. Alternatively, a superconducting magnetic field generating means is arranged. The gradient magnetic field generation system 3 includes a gradient magnetic field coil 9 wound in three axial directions of X, Y, and Z, and a gradient magnetic field power source 10 that drives each gradient magnetic field coil. The gradient magnetic field power supply 10 applies the gradient magnetic fields Gs, Gp, and Gf in the three-axis directions of X, Y, and Z to the subject 1 by driving each coil in accordance with a command from the sequencer 4 described later.

  More specifically, a slice direction gradient magnetic field pulse (Gs) is applied in one of X, Y, and Z to set a slice plane for the subject 1, and the phase encode direction gradient magnetic field is applied in the remaining two directions. A pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied, and position information in each direction is encoded in the echo signal.

  The sequencer 4 is a control unit that repeatedly applies a high-frequency magnetic field pulse (RF pulse) and a gradient magnetic field pulse in a predetermined pulse sequence. The sequencer 4 operates under the control of the CP 8 and sends various commands necessary for collecting tomographic image data of the subject 1 to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6.

  The transmission system 5 irradiates an RF pulse to cause nuclear magnetic resonance to the atomic nuclear spin constituting the biological tissue of the subject 1, and includes a high-frequency oscillator 11, a modulator 12, a high-frequency amplifier 13, A high-frequency coil 14a on the transmission side.

  The high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at a timing according to a command from the sequencer 4, and after the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13, the high-frequency pulse is arranged close to the subject 1. Is supplied to the high frequency coil 14a. Thereby, the subject 1 is irradiated with electromagnetic waves (RF pulses).

  The receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the biological tissue of the subject 1. The reception system 6 includes a reception-side high frequency coil 14 b, an amplifier 15, a quadrature phase detector 16, and an A / D converter 17. The response electromagnetic wave (NMR signal) of the subject 1 induced by the electromagnetic wave irradiated from the transmission-side high frequency coil 14 a is detected by the high frequency coil 14 b disposed in the vicinity of the subject 1. Then, after being amplified by the amplifier 15, it is divided into two orthogonal signals by the quadrature phase detector 16 at a timing according to a command from the sequencer 4, and each is converted into a digital quantity by the A / D converter 17, It is sent to the signal processing system 7.

  The signal processing system 7 includes an external storage device such as an optical disk 19 and a magnetic disk 18 and a display 20 made up of a CRT or the like. When the data from the reception system 6 is input to the CPU 8, the signal processing system 7 performs processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 1 as a result of the display 20. And recorded on the magnetic disk 18 or the like of the external storage device.

  The operation unit 25 is used to input various control information of the MRI apparatus and control information of processing performed by the signal processing system 7 and includes a trackball or mouse 23 and a keyboard 24. The operation unit 25 is disposed in the vicinity of the display 20, and the operator controls various processes of the MRI apparatus interactively through the operation unit 25 while looking at the display 20.

  In FIG. 1, the high-frequency coils 14 a and 14 b and the gradient magnetic field coil 9 on the transmission side and the reception side are installed in the static magnetic field space of the static magnetic field generation system 2 arranged in the space around the subject 1. .

  Next, the phantom which is the 1st Embodiment of this invention is demonstrated.

  FIG. 2 is an overview of the phantom 26 according to the first embodiment of the present invention. As shown in FIG. 2, the phantom 26 has a cubic base shape. That is, all the sides of the regular hexahedron are curved, and all the corners of the regular hexahedron are curved.

  FIG. 3 is a diagram showing an image of the center slice of the phantom 26. As shown in FIG. 3, the central slice image of the phantom 26 has a shape in which square corners are rounded, and is an image having sides parallel to each other. Moreover, if it is a center slice, the same image is obtained in any cross section.

  If the shape is a regular hexahedron, the signal value of the pointed part (corner part of the regular hexahedron) becomes high, but the corner part is rounded like the phantom 26 (the corner part has a curved surface shape). As a result, the signal value can be prevented from becoming high and the contour of the image can be prevented from being curved.

  As a result, when the image processing of the phantom 26 is performed, the concentration of signal values is not taken into consideration and the distance between the sides parallel to each other is kept constant if the image is a normal image. Further, since the central slice images in the X, Y, and Z axis directions of the phantom 26 are the same, it is possible to perform image processing with uniform processing without changing the installation direction of the phantom for each axis. is there.

  Next, an operation example in which the gradient magnetic field gain is automatically adjusted using the phantom 26 in the first embodiment of the present invention will be described.

The optimum gradient magnetic field gain (G _optimal ) includes the gradient magnetic field gain (G _scan ) used at the time of imaging, the actual size of the phantom (S _actual ), and the size of the phantom on the image output by image reconstruction (S _image). ) And is calculated by the following equation (1).

G _optimal = (S _actual / S _image ) · G _scan (1)
Using the principle expressed by the above formula (1), the phantom image is analyzed, and the size S _{image of the} phantom 26 is calculated to automatically calculate the optimum gradient magnetic field gain. That is, the actual size of the phantom 26 is divided by the size of the phantom 26 obtained from the slice image, and the optimum gradient magnetic field gain is calculated by multiplying the actual value of the phantom 26 and the gradient magnetic field gain used at the time of photographing. Adjust the characteristic value of the means.

  Next, an example of an image analysis method will be described. As shown in FIG. 4, a profile line 31, a profile line 32, and a profile line 33 that are parallel to each other are drawn at the center of the image of the phantom 26, and the signal values on the profile lines 31, 32, and 33 are used as shown in FIG. As shown in FIG. 4, the noise area 34 and the phantom area 35 are divided.

  By the above processing, the end point coordinates of the phantom area on the three profile lines 31, 32, 33 are obtained, and the data of the triangle (A) 36 and the triangle (B) 37 shown in FIG. 6 are extracted from the coordinates. The triangle 36 is orthogonal to the profile lines 31, 32, 33, and extends from the intersection of the profile line 33 and the upper surface of the phantom 26 to the profile line 31, and from the intersection of the upper surface of the phantom 26 and the profile line 33, the phantom 26. Are formed by a straight line extending to the intersection of the upper surface of the profile line 31 and the profile line 31 and a straight line between the two intersections where each of the two straight lines intersects the profile line 31.

  The triangle 37 includes a straight line extending from the center O of the image to the intersection of the lower surface of the phantom 26 and the profile line 32, a straight line orthogonal to the lower surface of the phantom 26 and extending from the center O of the image to the lower surface of the phantom 26, Each of the two straight lines is formed by a straight line between two intersections intersecting the lower surface of the phantom 26.

Using the fact that the triangles 36 and 37 are similar to each other, the size (S _image ) of the phantom 26 is calculated by the following equation (2), and the optimum gradient magnetic field gain is obtained by substituting it into the equation (1). That is, the distance 2a between the profile lines 31 and 33, which is a dimension of a straight line of the triangle 36, is known, and the distance b of the other straight line can be calculated from the obtained image, and the triangle 36 is a right triangle. Therefore, the length of the remaining sides can be calculated from the length of the two sides. Then, the distance (length) from the lower surface of the phantom 26 to the center O is calculated from the image, and by substituting these into the following equation (2), the size (S _image ) of the phantom 26 can be calculated.

S _image = (4a / √ ((2a) ² + b ² )) · length (2)
In the example described above, the size (S _image ) of the phantom 26 with respect to the vertical direction of the _image is calculated, but the horizontal size (S _image ) of the phantom 26 can also be calculated in the same manner. Note that, by the above processing, the inclination when the phantom 26 is installed can be allowed to an angle at which the parallel sides (upper surface, lower surface) of the phantom 26 and the profile line 31 and the profile line 33 can intersect. For example, when the interval between the profile line 31, the profile line 32, and the profile line 33 is 25 mm and the length of the phantom 26 parallel to each other is 100 mm, the inclination when the phantom 26 is installed is set to the three profile lines 31. , 32, 33 can be allowed up to about 16 degrees.

  Note that the arithmetic processing of the above formulas (1) and (2) is executed by the CPU 8.

  Next, the gradient magnetic field gain adjustment operation in the first embodiment of the present invention will be described with reference to the flowchart of FIG. This adjustment operation is executed by a control command of the CPU 8.

  First, the operator instructs which axis to adjust, and instructs the start of adjustment (step 1). Next, an image measurement condition is determined from the X, Y, and Z axes to be adjusted, and the phantom 26 is imaged (step 2). The end point coordinates of the phantom area on the three profile lines 31, 32, 33 described above are acquired from the acquired imaging data, and the size of the phantom 26 on the image is calculated (step 3). An optimum gradient magnetic field gain is calculated from the calculated size of the phantom 26 on the image (step 4).

  It is determined whether the previous adjustment value is within the range of values that can be taken by the adjustment value (step 5). If not, the process moves to step 6 to display an error and the adjustment operation is terminated. In this case, the phantom 26 may be inclined too much.

  In step 5, if the adjustment value is within the range, the process moves to step 7. Then, the adjustment value is displayed to the operator (step 7), and the adjustment value is set in the MRI apparatus (step 8). It is judged from the instruction in Step 1 whether or not other axes need to be adjusted. If necessary, the process moves to Step 2. If it is not necessary, the adjustment is terminated (step 9).

  As described above, according to the first embodiment of the present invention, since the phantom 26 has a shape in which the corners of the regular hexahedron are curved, a plurality of types of phantoms can be used without changing the arrangement position of the phantom. A single phantom capable of adjusting the characteristic value and a magnetic resonance imaging apparatus capable of adjusting the characteristic value of the gradient magnetic field using the phantom can be realized.

  Here, in the prior art, the work for adjusting the gradient magnetic field required about 30 minutes, but according to the first embodiment of the present application, the adjustment work is finished in about 4 minutes. Is possible.

  The material of the phantom 26 can be an acrylic resin. The contents of the phantom 26 can be nickel chloride.

  Next, a second embodiment of the present invention will be described. The second embodiment is an example in which an image distortion correction value when an MPG pulse (diffusion gradient magnetic field pulse) is applied is automatically calculated using the phantom 26.

  In the technology prior to the present invention, the distortion correction value of the image at the time of applying the MPG pulse is based on an image in which a conventionally used bottle phantom is imaged and the MPG pulse is not applied. This is a correction value for adjusting the size and distortion of the image. Conventionally, it is determined by an operator taking an image while changing the adjustment value and evaluating the image. In this case, the position and type of the bottle-type phantom must be changed for each axis, and a long time is required for adjustment.

  For this reason, the correction value adjustment can be processed in a short time by using the phantom 26 of the present invention for calculating the distortion correction value of the image when the MPG pulse is applied.

  In other words, by using the phantom 26 of the present invention, the same profile line analysis as in the first embodiment is performed on the vertical and horizontal axes on the image, so that the size of the phantom 26 on the image can be reduced. calculate.

  A size error is calculated by comparing the size of the image of the phantom 26 to which the MPG pulse is not applied and the size of the image to which the MPG pulse is applied. The above operation is repeated for an image captured while changing the distortion correction value, and the correction value with the smallest size error is adopted as the optimum distortion correction value.

  Next, in the second embodiment of the present invention, the MPG pulse distortion correction value adjustment operation will be described with reference to the flowchart of FIG.

  First, the operator instructs the range of distortion correction values to be changed and the number of times of imaging, and instructs the start of adjustment (step 11). The CPU 8 of the MRI apparatus sets a distortion correction value to be used for imaging from the range and number of instructed distortion correction values, and images the phantom 26. (Steps 12, 13).

  From the acquired imaging data, the size of the phantom area on the profile line with respect to the vertical and horizontal axes of the image to which the MPG pulse is not applied and the applied image is acquired, and the error is acquired (step 14). Next, in step 15, it is determined whether or not the number of times of imaging is less than the specified number of times, that is, whether or not imaging is performed by changing an adjustment value such as a gain of the gradient magnetic field generating means. If the image is to be changed, the process moves to step 12. If it is determined in step 15 that the number of times of imaging satisfies the specified number and there is no need to change the adjustment value, the process moves to step 16.

  A distortion correction value with the smallest error between the size of the image to which the MPG pulse is not applied and the size of the image to which the MPG pulse is applied is determined as an adjustment value (step 16). Then, it is determined whether or not the determined adjustment value is valid (step 17). If it is not valid, an error is displayed (step 18), and the process ends.

  If it is determined in step 17 that the adjustment value is within a reasonable range, the operator is notified of the adjustment result (step 19), the adjustment value is applied to the MRI apparatus (step 20), and the adjustment is terminated.

  Since other configurations are the same as those of the first embodiment of the present invention, illustration and description thereof are omitted.

  According to the second embodiment of the present invention, the image distortion correction value calculation process at the time of applying the MPG pulse can be processed in a short time.

  FIG. 9 shows another example of the shape of the phantom of the present invention. The phantom 27 shown in FIG. 9 forms three sets of two surfaces parallel to each other on the surface of the sphere, a straight line passing through the center of the sphere perpendicular to the two surfaces of one set, and the other A straight line that is orthogonal to the two surfaces of one set and passes through the center of the sphere is orthogonal to each other.

  Even with the phantom 27 having such a shape, the same effect as the phantom 26 can be obtained. Note that the phantom 27 has a smaller volume than the phantom 26 compared to the phantom 26 when the distance from the center to the plane portion is the same.

1 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied. It is an external view of an example of the phantom in the embodiment of the present invention. It is a figure which shows the example of a captured image of the center slice of the phantom shown in FIG. In the 1st Embodiment of this invention, it is explanatory drawing of image data analysis. In the 1st Embodiment of this invention, it is a graph which shows an example of the analysis result of the signal value on a profile line. In the 1st Embodiment of this invention, when the phantom inclines, it is explanatory drawing of the method of calculating the size of the phantom on an image. 5 is a flowchart of a gradient magnetic field gain adjustment operation in the first embodiment of the present invention. 9 is an operation flowchart for adjusting a distortion correction value of an MPG pulse in the second embodiment of the present invention. It is an external view of the other example of the phantom in embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Subject, 2 ... Static magnetic field generation system, 3 ... Gradient magnetic field generation system, 4 ... Sequencer, 5 ... Transmission system, 6 ... Reception system, 7 ... Signal Processing system, 8 ... Central processing unit (CPU), 9 ... Gradient magnetic field coil, 10 ... Gradient magnetic field power supply, l1 ... High frequency oscillator, 12 ... Modulator, 13 ... High frequency amplifier 14a, 14b ... high frequency coil, 15 ... amplifier, 16 ... quadrature phase detector, 17 ... A / D converter, 18 ... magnetic disk, 19 ... optical disc, 20. ..Display, 23 ... Track ball (mouse), 24 ... Keyboard, 26, 27 ... Phantom, 31-33 ... Profile line

Claims (7)

Static magnetic field generation means, gradient magnetic field generation means, high frequency transmission / reception means, signal processing display means for reconstructing and displaying an image of a subject based on a nuclear magnetic resonance signal received by the high frequency transmission / reception means, and the static In a magnetic resonance imaging apparatus having a magnetic field generating means, a gradient magnetic field generating means, a high frequency transmitting / receiving means, and a control means for controlling a signal processing display means,
The control means controls the static magnetic field generation means, the gradient magnetic field generation means, the high frequency transmission / reception means, and the signal processing display means to pick up a slice image of a phantom having a regular hexahedron with corners and sides curved, A magnetic resonance imaging apparatus, wherein a characteristic value of the gradient magnetic field generating means is adjusted based on dimensional information of the phantom obtained from a slice image and actual dimensional information of the phantom.   2. The magnetic resonance imaging apparatus according to claim 1, wherein the characteristic value is a gradient magnetic field gain of the gradient magnetic field generating means.   3. The magnetic resonance imaging apparatus according to claim 2, wherein the control means divides the actual dimension of the phantom by the dimension calculated from the slice image, and the obtained value is a gradient magnetic field gain obtained by photographing the slice image. The magnetic resonance imaging apparatus is characterized in that the obtained gain is used as an optimum gradient magnetic field gain of the gradient magnetic field generating means.   4. The magnetic resonance imaging apparatus according to claim 3, wherein the control means is dimensional information of the phantom obtained from the slice image based on an intersection position of the contour of the slice image and three profile lines parallel to each other. The magnetic resonance imaging apparatus characterized by calculating. Static magnetic field generation means, gradient magnetic field generation means, high frequency transmission / reception means, signal processing display means for reconstructing and displaying an image of a subject based on a nuclear magnetic resonance signal received by the high frequency transmission / reception means, and the static In a magnetic resonance imaging apparatus having a magnetic field generating means, a gradient magnetic field generating means, a high frequency transmitting / receiving means, and a control means for controlling a signal processing display means,
The control means controls the static magnetic field generation means, the gradient magnetic field generation means, the high frequency transmission / reception means, and the signal processing display means, and applies a diffusion gradient magnetic field pulse of a phantom whose corners and sides of the regular hexahedron are curved. Based on the result of comparing the phantom dimensional information obtained from the two slice images obtained by capturing the slice image and the slice image when the diffusion gradient magnetic field pulse is not applied, the gradient magnetic field A magnetic resonance imaging apparatus characterized by adjusting a characteristic value of a generating means. In the phantom where the slice image is taken in order to adjust the characteristic value of the gradient magnetic field generating means of the magnetic resonance imaging apparatus,
A phantom characterized in that corners and sides of a regular hexahedron have a curved shape. In the phantom where the slice image is taken in order to adjust the characteristic value of the gradient magnetic field generating means of the magnetic resonance imaging apparatus,
Three sets of two surfaces parallel to each other are formed on the surface of the sphere, orthogonal to the two surfaces of one set, perpendicular to the center of the sphere, and orthogonal to the two surfaces of the other set The phantom is characterized in that the straight lines passing through the center of the sphere are orthogonal to each other.

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