JP2013146472A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To facilitate an adjustment of amplitude of a gradient magnetic field pulse in an MRI apparatus.SOLUTION: A magnetic resonance imaging apparatus includes: a static magnetic field generating means that applies a static magnetic field to a subject placed in an imaging space; a gradient magnetic field generating means that applies a gradient magnetic field pulse to the subject; and a control means that transmits, to the gradient magnetic field generating means, a control signal for generating the gradient magnetic field pulse in the imaging space. The magnetic resonance imaging apparatus further includes an adjustment means that adjusts a gain for generating the gradient magnetic field pulse. The adjustment means adjusts the gain by measuring the dimension of a spherical phantom and comparing it with a case of a reference dimension.

Description

  The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and more particularly to a technique for adjusting the gain of a gradient magnetic field pulse waveform.

MRI equipment measures NMR signals generated by spins of the nuclei that make up the subject, especially human tissue, and visualizes the shape and function of the head, abdomen, limbs, etc. in two or three dimensions It is a device to do. In imaging, a slice selection gradient magnetic field pulse and a high-frequency magnetic field pulse are applied in order to select and excite a specific region after placing a subject in a static magnetic field. Also, frequency and phase encoding gradient magnetic field pulses are applied for frequency or phase encoding.
Patent Document 1 compensates by measuring a magnetic field that is induced by an eddy current generated by a gradient magnetic field pulse (mainly generated in a cryostat arranged on the back of the gradient coil) and changes in space and time. Techniques to do this are disclosed.
On the other hand, there is a conventional technique described in Non-Patent Document 1 as a method of actually measuring the waveform of a gradient magnetic field pulse. This is a sequence in which a slice gradient magnetic field for measuring the waveform is used as a test gradient magnetic field, a predetermined thin slice is excited, a test gradient magnetic field is applied according to imaging parameters, and a signal is acquired, and a signal is acquired without applying a test gradient magnetic field. This is a method of actually measuring a gradient magnetic field output waveform of a test gradient magnetic field (slice gradient magnetic field) by calculation between signals obtained by these two sequences.

Japanese Patent Laid-Open No. 10-272120

P. Latta et al / Magnetic Resonance Imaging 25 (2007) 1272-1276

  However, as a result of studying the above prior art, the present inventor has noticed the following. That is, as described in Patent Document 1, in magnetic resonance imaging using gradient magnetic field pulses, it is also important to accurately measure and compensate for the generated eddy currents, but in the first place, the gain of the gradient magnetic field pulses is accurately adjusted. Otherwise, the sequencer's intended frequency encoding or phase encoding, slice selection and slice encoding cannot be performed. In the MRI system, the gain of the gradient magnetic field pulse is a parameter that needs to be adjusted for accurate slice selection, frequency or phase encoding, etc. The obtained image becomes a large size or a small size.

  Conventionally, when adjusting the gradient magnetic field pulse, thin acrylic rods are arranged at the apexes of the four rhombuses of a thin cylindrical phantom (with a spacing of 100 mm between the opposing acrylic rods) The gain of the gradient magnetic field pulse in the X direction, the gradient magnetic field pulse in the Y-axis direction, and the gradient magnetic field pulse in the Z-axis direction was measured while changing the direction. In such a method, it is necessary to change the direction in which the thin cylindrical phantom is placed when measuring the gain of the three gradient magnetic field pulses. It was.

In order to solve the above problems, the present invention provides a static magnetic field generating means for applying a static magnetic field to a subject placed in an imaging space, a gradient magnetic field generating means for applying a gradient magnetic field pulse to the subject, In an MRI apparatus comprising a control means for transmitting a control signal for generating the gradient magnetic field pulse to the imaging space to the gradient magnetic field generation means,
An adjustment unit that adjusts a gain for generating the gradient magnetic field pulse, the adjustment unit measures the size of a spherical phantom, and compares it with a reference size in the MRI apparatus, adjust.

  A static magnetic field generating means for applying a static magnetic field to a subject arranged in the imaging space; a gradient magnetic field generating means for applying a gradient magnetic field pulse to the subject; and for generating the gradient magnetic field pulse in the imaging space. And an adjustment means for adjusting a gain for generating the gradient magnetic field pulse, the adjustment means comprising: an adjustment means for adjusting the gain of the gradient magnetic field pulse; The waveform is adjusted by actually measuring the waveform obtained by calculating between the signals obtained when the waveform of the gradient magnetic field pulse is applied and when not applying it, and comparing it with the set value of the output of the gradient magnetic field pulse.

  According to the present invention, it is possible to easily adjust the amplitude of the gradient magnetic field pulse in the MRI apparatus.

Overview of an example of an MRI apparatus according to the present invention (a) A template image and (b) an image of a spherical phantom imaged in a hospital. Flowchart showing the operation flow of the first embodiment (a) Distorted image of the template image and (b) Distorted image of the image of the spherical phantom imaged at the hospital Diagram of spherical phantom placed at a position different from the center of the static magnetic field Diagram showing data flow from input waveform to output waveform of gradient magnetic field pulse The flowchart which shows the flow of operation | movement of Example 2. Diagram showing gradient pulse waveform

Hereinafter, embodiments of the present invention will be described in detail.
First, an overall outline of an example of an MRI apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing an overall configuration of an embodiment of an MRI apparatus according to the present invention. This MRI apparatus obtains a tomographic image of a subject using an NMR phenomenon. As shown in FIG. 1, the MRI apparatus 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, and a sequencer 4.

  The static magnetic field generation system 2 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 1 if the vertical magnetic field method is used, and in the direction of the body axis if the horizontal magnetic field method is used. Thus, a permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the subject 1.

  The gradient magnetic field generation system 3 is a gradient magnetic field coil 9 that applies gradient magnetic field pulses in the three-axis directions of X, Y, and Z, which is the coordinate system (stationary coordinate system) of the MRI apparatus, and a gradient that drives each gradient magnetic field coil. The magnetic field power supply 10 is comprised, and the gradient magnetic field power supply 10 of each coil is driven in accordance with a command from the sequencer 4 to be described later, whereby gradient magnetic fields gxin, gyin, and gzin are applied in the X, Y, and Z axis directions. At the time of imaging, a slice direction gradient magnetic field pulse is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and in the remaining two directions orthogonal to the slice plane and orthogonal to each other A phase encoding direction gradient magnetic field pulse and a frequency encoding direction gradient magnetic field pulse are applied, and position information in each direction is encoded in the echo signal.

  The sequencer 4 is a control means that repeatedly applies a high-frequency magnetic field pulse and a gradient magnetic field pulse in a predetermined pulse sequence, operates under the control of the digital signal processing device 8, and performs various operations necessary for collecting tomographic image data of the subject 1. The command is sent to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6.

  The transmission system 5 irradiates the subject 1 with a high-frequency magnetic field pulse in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the biological tissue of the subject 1, and the high-frequency transmitter 11 and the modulator 12 It comprises a high frequency amplifier 13 and a high frequency coil (transmission coil) 14a on the transmission side. The high-frequency magnetic field pulse output from the high-frequency transmitter 11 is amplitude-modulated by the modulator 12 at the timing according to the command from the sequencer 4, and the amplitude-modulated high-frequency magnetic field pulse is amplified by the high-frequency amplifier 13 and then close to the subject 1. Thus, the subject 1 is irradiated with a high-frequency magnetic field pulse.

  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, and receives a high-frequency coil (receiving coil) 14b on the receiving side and a signal amplifier 15 And a quadrature phase detector 16 and an A / D converter 17. After the NMR signal of the response of the subject 1 induced by the electromagnetic wave irradiated from the high frequency coil 14a on the transmission side is detected by the high frequency coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15, The quadrature phase detector 16 divides the signal into two orthogonal signals at the timing according to the command from the sequencer 4, and each signal is converted into a digital quantity by the A / D converter 17 and sent to the signal processing system 7.

  The signal processing system 7 performs various data processing and display and storage of processing results, and has an external storage device such as an optical disk 19 and a magnetic disk 18 and a display 20 composed of a CRT, etc. Are input to the digital signal processing device 8, the digital signal processing device 8 executes processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 1 as a result on the display 20. Recording is performed on the magnetic disk 18 or the like of the external storage device.

  The operation unit 25 inputs various control information of the MRI apparatus and control information of processing performed in 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 an 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 coil 14a and the gradient magnetic field coil 9 on the transmission side are opposed to the subject 1 in the static magnetic field space of the static magnetic field generation system 2 into which the subject 1 is inserted. If the horizontal magnetic field method is used, it is installed so as to surround the subject 1. The high-frequency coil 14b on the receiving side is installed so as to face or surround the subject.

  At present, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as being widely used clinically. By imaging information on the spatial distribution of proton density and the spatial distribution of relaxation time in the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.

  Next, Example 1 of the present invention will be described with reference to FIGS. However, the outline of Example 1 is as follows. That is, first, a template image (FIG. 2 (a)) is obtained using a spherical phantom in the first machine or the like in which the gain of the gradient magnetic field pulse is accurately adjusted. After that, each product of MRI equipment (after Unit 2 etc.) is shipped to the hospital. The spherical phantom is imaged under the same imaging conditions as when the template image was obtained with the first machine at the factory. Finally, the size of the spherical phantom on the obtained image (FIG. 2 (b)) is compared with the template image, and the gain of the gradient magnetic field pulse in each direction of XYZ is measured.

  Next, FIG. 3 is a flowchart illustrating the operation flow of the first embodiment. However, the flowchart of FIG. 3 is a flowchart showing a work procedure after the MRI apparatus is shipped to the hospital. Each step of the flowchart of FIG. 3 will be described in order.

(Step 301)
An operator places a spherical phantom at the center of the static magnetic field of the MRI apparatus. This arrangement is performed manually. The spherical phantom used here is the same as that used when the template image was obtained in the factory.

(Step 302)
Next, the operator images the spherical phantom installed in step 301 using the MRI apparatus. However, the imaging conditions and the like here are the same as the imaging conditions when the template image is acquired in the factory. For example, the spin echo method is used as the imaging sequence. In addition, although the installation accuracy when installing the phantom in step 301 is increased, if the imaging slice thickness in this step is further increased, the longest diameter of the spherical phantom will be included in the captured image. .

(Step 303)
Next, the image obtained in step 302 is compared with the size of the phantom appearing on the template image. Here, more specifically, there are the following two methods for comparing the size of the phantom appearing on the image obtained in step 302 and the phantom appearing on the template image.

  The first is a method of comparing the phantom profile appearing on the image obtained in step 302 with the phantom profile appearing on the template image. More specifically, the size of the phantom that appears on the image obtained in step 302 is compared with the size of the phantom that appears on the template image based on the skirt width of the profile of the phantom image.

  Second, the phantom image that appears on the image obtained in step 302 and the phantom image that appears on the template image are binarized and extracted, and two circular phantom images are superimposed and either A method of comparing the difference in the size of two phantom images (which is several times larger) by changing the enlargement / reduction ratio and finding the enlargement / reduction ratio at which the area of the difference image becomes an inflection It is.

By these methods, when the image enlargement / reduction ratio of the template image based on the image actually obtained by imaging the phantom in step 202 is R, the optimum gain value (G ₀ ) is the value at the time of imaging. Any value may be used as long as the gain value (G) is divided by R.

  When comparing the difference in the size of two phantom images using these two methods, it is easily affected by the edge portion of the two phantom images. In particular, in a vertical magnetic field type permanent magnet type MRI apparatus, it is difficult to keep the uniformity of the magnetic field constant, and therefore there is a high possibility that distortion (FIG. 4) will occur in the obtained image. However, since the obtained template image also includes distortion information due to similar magnetic field inhomogeneity, there is no problem in obtaining high gain with high accuracy.

(Step 304)
Judge whether all three axes, X-axis direction, Y-axis direction, and Z-axis direction have been adjusted. If all the axes have been adjusted, the process ends. If not, the process proceeds to step 202.

  According to the first embodiment, when the MRI apparatus is moved from the factory to the hospital, if the gain of the gradient magnetic field pulse changes, the gain is changed to the gradient magnetic field pulse in the X-axis direction and the gradient magnetic field pulse in the Y-axis direction. Each of the gradient magnetic field pulses in the Z-axis direction can be measured without changing the installation direction of the phantom. However, in the above embodiment, the position of the spherical phantom is the center of the static magnetic field, but the position is different from the center of the static magnetic field, such as Pos.2 to Pos.5 in FIG. Needless to say, the gain of the gradient magnetic field pulse may be obtained.

  That is, according to the present embodiment, a static magnetic field generating means for applying a static magnetic field to a subject arranged in an imaging space, a gradient magnetic field generating means for applying a gradient magnetic field pulse to the subject, and the imaging space In an MRI apparatus comprising a control means for transmitting a control signal for generating a gradient magnetic field pulse to the gradient magnetic field generation means, an adjustment means for adjusting a gain for generating the gradient magnetic field pulse is provided, the adjustment means Adjust by measuring the size of the spherical phantom and comparing it with the reference size.

  The reference size in the MRI apparatus is the size of the phantom in the case of the MRI apparatus (for example, the first machine) in which the gain is adjusted by the above-described adjustment method using an opposing acrylic rod. That is, the reference size is the size of the phantom in the image measured by the reference MRI apparatus.

  Next, a second embodiment of the present invention will be described with reference to FIGS. However, in Example 2, as described in Non-Patent Document 1, the following method is used. That is, the slice gradient magnetic field for measuring the waveform is set as the test gradient magnetic field. After exciting a given thin slice, a test gradient magnetic field was applied according to the imaging parameters and a signal was acquired, and a reference sequence was acquired without applying the test gradient magnetic field. This is a method of actually measuring a gradient magnetic field output waveform of a test gradient magnetic field (slice gradient magnetic field) by calculation between signals.

  Then, the intensity (amplitude) of a predetermined input waveform (A in FIG. 6) and the output waveform actually measured by applying the gradient magnetic field pulse to the phantom measured by the technique described in Non-Patent Document 1 (FIG. 6). The optimum gain of the gradient magnetic field output is calculated by comparing the intensity (amplitude) of B).

  Next, FIG. 7 is a flowchart showing the flow of operation of the second embodiment. However, the flowchart of FIG. 7 is a flowchart showing a work procedure after the MRI apparatus is shipped to the hospital. Each step of the flowchart of FIG. 7 will be described in order.

(Step 701)
An operator places the phantom in the center of the static magnetic field. This arrangement is performed manually. The phantom used here is the same as that used when the template image was obtained in the factory, but it does not have to be spherical.

(Step 702)
Using the conventional technique described in Non-Patent Document 1, the temporal change in the intensity (amplitude) of the output waveform actually applied to the phantom by the gradient magnetic field pulse is measured. However, various corrections are applied to the waveform actually output from the gradient magnetic field power supply 10. For example, according to the technique described in Patent Document 1, a technique for measuring and compensating for a spatially and temporally changing magnetic field induced by an eddy current is disclosed. More specifically, “compensate for a spatially and temporally changing magnetic field induced by eddy current” means that the input waveform from the sequencer is as shown in FIG. The correction is to output the gradient magnetic field power supply 10 with a sharpened waveform as shown in FIG. However, the measurement of the temporal change in the intensity (amplitude) of the output waveform actually applied to the phantom by the gradient magnetic field pulse in this embodiment is the waveform of the pulse actually applied to the phantom (or subject). FIG. 78c) shows the result after the actual influence of eddy current and the like.

(Step 703)
Intensity (amplitude) of a given input waveform; and amp_in, intensity of the measured output waveform in step 602 (amplitude); Amp_out comparing, determining the optimum gain G _0. More specifically, G ₀ is calculated by multiplying G by (amp_in / amp_out).

  As described above, since the output gradient magnetic field waveform is the waveform of the gradient magnetic field pulse actually applied to the phantom, when various corrections are not functioning, a rectangular waveform is obtained due to the influence of distortion or overshoot. I can't. Originally, it is desirable to make adjustments while various corrections are functioning, but even if the correction is not functioning, use the averaging process and fitting process to obtain the actual intensity (amplitude). It is thought that it is good to ask for.

(Step 704)
Judge whether all three axes, X-axis direction, Y-axis direction, and Z-axis direction have been adjusted. If all the axes have been adjusted, the process ends. If not, the process proceeds to step 702.

  According to the above-described embodiment, gain characteristics depending on these can be obtained by applying gradient magnetic field pulses having various gradient magnetic field strengths, durations, and rise times.

According to the present embodiment, a static magnetic field generating means for applying a static magnetic field to a subject arranged in an imaging space, a gradient magnetic field generating means for applying a gradient magnetic field pulse to the subject, and the gradient magnetic field in the imaging space In an MRI apparatus comprising a control means for transmitting a control signal for generating a pulse to the gradient magnetic field generating means,
Adjusting means for adjusting a gain for generating the gradient magnetic field pulse, the adjusting means between the signals obtained when the waveform of the gradient magnetic field pulse is applied and when the waveform of the gradient magnetic field pulse is not applied; There is provided an MRI apparatus characterized in that an adjustment is performed by actually measuring by the above calculation and comparing it with the set value of the output of the gradient magnetic field pulse.

  The present invention can be used for an MRI apparatus.

  4 sequencers, 10 gradient power supplies, 9 gradient coils

Claims (3)

Static magnetic field generating means for applying a static magnetic field to a subject arranged in the imaging space, gradient magnetic field generating means for applying a gradient magnetic field pulse to the subject, and control for generating the gradient magnetic field pulse in the imaging space A magnetic resonance imaging apparatus comprising: a control unit that transmits a signal to the gradient magnetic field generation unit;
Adjusting means for adjusting a gain for generation of the gradient magnetic field pulse, the adjusting means measuring the size of a spherical phantom and adjusting it by comparing with a case of a reference size; A magnetic resonance imaging apparatus.   The magnetic resonance imaging apparatus according to claim 1, wherein the reference size is a size of a phantom in an image measured by a reference magnetic resonance imaging apparatus. Static magnetic field generating means for applying a static magnetic field to a subject arranged in the imaging space, gradient magnetic field generating means for applying a gradient magnetic field pulse to the subject, and control for generating the gradient magnetic field pulse in the imaging space A magnetic resonance imaging apparatus comprising: a control unit that transmits a signal to the gradient magnetic field generation unit;
Adjusting means for adjusting a gain for generating the gradient magnetic field pulse, the adjusting means between the signals obtained when the waveform of the gradient magnetic field pulse is applied and when the waveform of the gradient magnetic field pulse is not applied; A magnetic resonance imaging apparatus characterized in that an adjustment is carried out by actually measuring by the calculation and comparing it with a set value of a gradient magnetic field pulse output.

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