JP3748657B2 - Magnetic resonance inspection equipment - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance inspection apparatus (hereinafter referred to as an MRI apparatus) having a shim coil for correcting the uniformity of a static magnetic field, and in particular, in order to obtain a highly accurate inspection result, the apparatus state is changed according to the inspection method. The present invention relates to an MRI apparatus that can be switched and used properly.
[0002]
[Prior art]
An MRI apparatus uses a nuclear magnetic resonance (hereinafter referred to as NMR) phenomenon to noninvasively measure the inside of a human body or the like to obtain an image or spectrum for use in medical diagnosis, and uses X-rays or ultrasound. Since useful medical diagnostic information that cannot be obtained with conventional inspection apparatuses can be obtained, it is widely used in medical facilities.
[0003]
Such an MRI apparatus is provided with a high-frequency magnetic field coil for causing an NMR phenomenon to occur in a nuclear spin constituting a subject tissue placed in a static magnetic field, and for measuring an NMR signal. In order to identify which part of the human body is obtained, a gradient magnetic field coil is combined with a static magnetic field magnet. The gradient coil uses three gradient magnetic field coils that generate gradient magnetic fields whose magnetic field strengths change along the x, y, and z axes, and position information (phase encoding or frequency encoding) is given by these gradient magnetic fields. An image of the cross section of the subject can be obtained by reconstructing an image of the NMR signal by a two-dimensional Fourier transform method.
[0004]
As described above, in the two-dimensional Fourier transform method, a gradient magnetic field is applied in a pulsed manner, but this change in magnetic flux induces an eddy current in an electric conductor (magnetic circuit and its container) existing around the gradient magnetic field coil. (Journal of Magnetic Resonance 66, 573-576, “Active Magnetic Screening of Gradient Coils in NMR Imaging”). In particular, when a superconducting magnet is used as the static magnetic field magnet, a non-negligible eddy current is generated in the container and the heat shield cylinder (copper or aluminum) incorporated in the container.
[0005]
A magnetic field in which such an eddy current is generated becomes an error in phase encoding or frequency encoding, and lowers the resolution of an image after Fourier transformation. In order to solve this problem, a shield type gradient magnetic field coil that cancels magnetic flux leaking outside the gradient magnetic field coil has been developed (US Pat. No. 4,737,716).
[0006]
On the other hand, in MRI equipment, high-speed imaging method (echo planar imaging: EPI) that measures NMR signals for one image with one high-frequency magnetic field irradiation, NMR spectrum measurement, and spectroscopic imaging that images chemical shifts Since high uniformity of the static magnetic field is required, a shim coil for correcting the static magnetic field uniformity is combined with the static magnetic field magnet. A method for correcting the difference in magnetic susceptibility for each subject has also been proposed (Japanese Patent Laid-Open No. 60-161552).
[0007]
[Problems to be solved by the invention]
The problem of interference between the gradient coil and the magnet can be prevented by the conventional technique (US Pat. No. 4,737,716), and the resolution of the image can be improved. The interference with the shim coil arranged in the nearest place was not considered.
[0008]
Accordingly, an object of the present invention is to provide an MRI apparatus capable of controlling interference between a gradient magnetic field coil and a shim coil in accordance with the purpose of an inspection method. We also provide an MRI system that can realize both an inspection method that captures high-resolution images with minimal eddy currents, an image with low distortion by increasing the uniformity of the static magnetic field, and an inspection method that measures spectra. The purpose is to do.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, an MRI apparatus of the present invention includes a means for generating a static magnetic field in a space in which a subject is arranged, a magnetic field uniformity correcting means for improving the uniformity of the static magnetic field, and a position in the space. Gradient magnetic field generating means for generating gradient magnetic fields having different magnetic field strengths, means for generating a high-frequency magnetic field for causing a nuclear magnetic resonance phenomenon in a subject, means for detecting a nuclear magnetic resonance phenomenon, and detected nuclear magnetic resonance Means for processing the signal and displaying the processing result, and further comprising means for controlling the interference between the magnetic field uniformity correcting means and the gradient magnetic field generating means.
[0010]
The means for controlling interference includes switch means for switching connection between the magnetic field uniformity correction means and a power source for driving the magnetic field uniformity correction means, and control means for controlling the operation of the switch means in accordance with an inspection mode.
[0011]
The means for controlling the interference means that the switch means is connected when the inspection mode requires high uniformity of the static magnetic field, and the switch means when the inspection mode is hardly affected by the uniformity of the static magnetic field. Is disconnected.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of the present invention will be described below with reference to the drawings. In addition, the numerical value shown by the following example is a mere illustration, Comprising: This invention is not limited to those numerical values.
[0013]
FIG. 2 is a schematic configuration diagram showing an embodiment of an MRI apparatus to which the present invention is applied. In FIG. 2, a magnet 1 generates a static magnetic field of an MRI apparatus, and a permanent magnet, a normal conducting magnet or a superconducting magnet is used. In the example shown in the figure, a superconducting wire is wound like a solenoid and a container containing liquid helium is contained. A superconducting magnet housed inside is used. The typical dimensions of this magnet container are a bore diameter of 1 meter and a length of 1.5 meters so that the human body can inspect it. The magnetic field strength generated at the center of the bore is designed and manufactured so that, for example, a value of 5 ppm can be achieved in a spherical space of 40 centimeters in the center of the bore. However, since the magnetic field uniformity depends on the environment around the magnet, in order to adjust the magnetic field uniformity according to the installation environment of the magnet, a small piece 2 of magnetic material is attached to the bore inner wall of the magnet 1.
[0014]
Further, a shim coil 3 for correcting the uniformity of the static magnetic field is incorporated inside the magnet 1. The shim coil 3 is composed of, for example, a plurality of shim coils composed of nine types of patterns, and achieves the magnetic field uniformity required for the examination by adjusting the current (shim current) that flows through the shim coil for each examination site of the subject. To do. A shim power source 4 for flowing shim current is connected to the shim coil 3 via a switch circuit (Sw) 5. The switch circuit 5 switches on / off the shim coil 3 according to a command from the computer 14 described later.
[0015]
An x, y, z gradient magnetic field coil 6 is incorporated inside the shim coil 3. These gradient magnetic field coils 6 are connected to x, y, and z gradient magnetic field (GC) power supplies 7, 8, and 9, respectively. As a result, the steady-state current for correcting the magnetic field inhomogeneity component of the x, y, and z terms of the static magnetic field in the examination space and the magnetic field strength are changed along the x, y, and z axes in the examination site space. Thus, a pulse current for giving position information to the NMR signal flows.
[0016]
Therefore, in this embodiment, the shim coil 3 and the gradient magnetic field coil 6 function as static magnetic field uniformity correction means.
[0017]
A high frequency coil 10 is incorporated inside the gradient coil 6. The high-frequency coil 10 generates a high-frequency magnetic field having a frequency for exciting the nuclear spin at the examination site, and detects precession of the excited nuclear spin as an NMR signal. A high frequency circuit 11 is connected to the high frequency coil 10 to supply high frequency power for excitation and amplify the NMR signal. As the high-frequency coil 10, separate high-frequency coils may be used for excitation and detection of a high-frequency magnetic field.
[0018]
The subject 12 lies on the patient table 13 for transportation, and the examination site is arranged at the center of the magnet 1.
[0019]
The computer 14 controls the operation state of each of the above-described components and performs calculation processing on the detected NMR signal. The result of the arithmetic processing is displayed on the monitor 15 connected to the computer 14. An operation console 16 of an input device is connected to the computer 14, and inspection conditions, image processing conditions, and the like can be set using this console.
[0020]
FIG. 1 is a diagram illustrating in detail the gradient magnetic field coil 6, the shim coil 3, the gradient magnetic field power supplies 7 to 9 and the shim power supply 4 of FIG. In the figure, the gradient coil 6 cancels the magnetic field generated outside the inner coil 61 and the inner coil 61 that generates a gradient magnetic field in the x, y, and z directions inside the subject 12. And an outer coil 62 for the purpose. The x, y, and z coils of the inner coil 61 and the outer coil 62 are connected in series and are connected to independent gradient magnetic field power supplies 7, 8, and 9, respectively. The number of turns and dimensions of the inner coil 61 and the outer coil 62 are designed so that the magnetic field is zero at the position where the magnet 1 is present. With this configuration of the gradient coil, eddy currents induced in the container of the superconducting magnet 1 and the heat shield cylinder (copper or aluminum) incorporated in the container can be suppressed.
[0021]
Shim coil is disposed between the magnet 1 and the outer coil 62, the embodiment in nine shim coils 31 to 39 are shown being wound. The nine kinds of shim coils are composed of correction coils for each term of, for example, X ² , Y ² , Z ² , X ³ , Z ³ , Y ³ , XY, ZY, and XZ. The output sides of the switch circuits (Sw) 51 to 59 are connected to the shim coils 31 to 39, respectively. Shim power supplies 41 to 49 are connected to the input sides of the switch circuits 51 to 59 via inductance L. The inductance L increases the impedance of a closed loop including a shim coil, a shim power source, and an inductance to make it difficult for a transient current to flow through the shim circuit. For example, the inductance L is used.
[0022]
The switch circuits 51 to 59 close or open the shim circuit according to a control signal from the computer 14, and a known switch circuit such as a combination of a diode and a transistor can be used. The interference between the shim coil and the gradient coil when the switch circuit is on and off will be described with reference to FIG. Here, for simplicity, it will be described x ² shim coils is the same in other shim coils.
[0023]
In a state where the switch circuit connection signal is output from the computer 14, as shown in FIG. 3A, the shim coil 31 forms a closed loop with the inductance L, the shim power source 41, and a current flows. By setting this current value to an appropriate value, the x ² term of the static magnetic field is corrected and a uniform static magnetic field can be achieved. On the other hand, since the x-channel coil of the outer coil 62 of the gradient magnetic field coil is arranged in the immediate vicinity of the shim coil 31, even-numbered higher-order distortions (x ² , x ⁴ ,...) Of the x channel interfere with the x ² shim coil 31. To do. Here, when the gradient magnetic field is pulse-driven, an electromotive force is generated in the shim coil 31 by the inductive coupling coefficient M and the turn ratio of the coils. The current due to this electromotive force flows in the closed loop and becomes an error magnetic field of the gradient magnetic field. The generation of this error magnetic field is suppressed as much as possible by increasing the impedance of the closed loop by the inductance L and making it difficult for the transient current to flow. .
[0024]
On the other hand, in the state where the shut-off signal is output from the computer 14, the shim coil 31 is in an open loop as shown in FIG. In this case, even if an electromotive force is generated by pulse driving of the gradient magnetic field, a current that becomes an error magnetic field does not flow, so that a highly accurate gradient magnetic field can be achieved.
[0025]
Such a switch circuit is turned on / off when the computer 14 sends a control signal so that a state suitable for the inspection method (on or off) is automatically selected when the inspection method is selected by the console 16. 16 is provided with means for selecting on / off of the shim coil, so that the operator can select a state suitable for the inspection method, and the computer 14 can send a control signal based on this selection operation. It is also possible to individually control a plurality of shim coils.
[0026]
For example, when an inspection method that requires high uniformity of the static magnetic field, for example, measurement by the EPI method or MR spectrum measurement is executed, a connection signal for the switch circuit is output from the computer 14. In addition, inspection methods that are not easily affected by static magnetic field uniformity and inspection methods that require gradient magnetic field accuracy compared to static magnetic field uniformity, such as high-speed spin echo method and imaging methods that obtain high spatial resolution images with a small field of view. Is executed, the computer 14 outputs a cutoff signal.
[0027]
Next, an embodiment in which a series of inspections are performed using the MRI apparatus having the above configuration will be described with reference to FIGS. In this embodiment, first, the cross section of the subject 12 is imaged by the fast spin echo (FSE) method, and then the NMR spectrum measurement inspection of the local region is performed based on the image imaged by the FSE method.
[0028]
Similar to the spin echo method, the FSE method measures the phase of a spin that rapidly disperses with an apparent transverse relaxation time (T2 *) by an inhomogeneous magnetic field again after a certain time and measures it as an echo signal. Although it is an imaging method that is not easily affected by uniformity, n echo signals are generated by one excitation of a nuclear spin, and a different phase encoding divided by n is given to each echo signal. Accuracy is required.
[0029]
Therefore, when executing the sequence of the FSE method, as shown in FIG. 4, before the start-up, the computer 14 generates a cut-off signal 71 and opens all the switch circuits 51 to 59 so that the shim coils are in an open loop state. . The uniformity of the magnetic field is such that the values required for the FSE method are achieved by first-order shimming of x, y, and z by the gradient coil.
[0030]
In this state, a normal FSE method sequence is executed. That is, the multi-echo signal 77 is generated by the high-frequency pulse train 73 and the z gradient magnetic field pulse 74. The echo signal 77 is not particularly limited, but the case of 4 echoes is shown in the figure. The y gradient magnetic field pulse 75 for phase encoding is applied before and after the echo signal measurement so that each echo signal has a different phase encoding. That is, in the first echo signal, the integral value of the pulses 751 and 752 of the y gradient magnetic field becomes the phase encode amount, and in the second echo, the integral value of the pulses 751, 752, 753, and 754 of the y gradient magnetic field becomes the phase encode amount. . Similarly, for the third and subsequent echoes, the sum of the integral values of the gradient magnetic field pulses applied up to the generation of the echoes becomes phase encoding. Each echo signal to which the different phase encoding is applied is measured while applying the x gradient magnetic field pulse 76. The echo signal measured by repeating the sequence of FIG. 4 while changing the y gradient magnetic field pulse 751 (256/4 = 64 steps in the figure) until the amount of phase encoding (for example, 256) required for one image is reached. To reconstruct an image.
[0031]
Next, based on the image photographed above, an inspection of NMR spectrum measurement in a local region is performed. In this case, the magnetic field uniformity of the part including the region needs to be as high as about 0.1 ppm in order to separate the spectrum. Therefore, as shown in FIG. 5, a connection signal 81 is output from the computer 14, all the switch circuits are closed, and the shim coil circuit is closed loop.
[0032]
Prior to the spectrum measurement, an auto shimming sequence 82 for determining the current flowing through the shim coil is activated. The auto shimming sequence is a method for obtaining a shim current for correcting the disturbance of the static magnetic field due to the difference in magnetic susceptibility of various tissues at the examination site. For example, a method described in Japanese Patent Application Laid-Open No. 60-161552 can be adopted.
[0033]
That is, a slice selection gradient magnetic field for slice selection and a 90 ° pulse are applied to excite nuclear spins constituting the subject tissue, and a slice selection gradient magnetic field and a 180 ° pulse are applied τ hours after the 90 ° pulse application. The echo signal is measured while applying the readout gradient magnetic field after (τ + Δτ) time from the 180 ° pulse application. When the time τ elapses after the 180 ° pulse application, the spin phase change caused by the static magnetic field inhomogeneity immediately after the 90 ° pulse application is completely cancelled. Only the phase information generated by is included. Therefore, such a sequence is repeated while changing the phase encoding amount, and the static magnetic field inhomogeneous distribution can be obtained from the phase information of the measured echo signal.
[0034]
The value of the current flowing in the shim coil can be obtained by calculation from the static magnetic field inhomogeneous distribution obtained as described above and shim characteristics of the shim coil (generated magnetic field intensity per unit current), and the shim power source is driven based on this current value. To do.
[0035]
By executing the auto shimming sequence 82, the spectrum measurement sequence 83 is started in a state in which the uniformity of the static magnetic field becomes a target value, for example, 0.1 ppm or less. The spectrum measurement sequence 83 executes a sequence for measuring an NMR signal in an arbitrary region (usually 1 cm ³ ) from a three-dimensional space such as an ISIS (Image Selected In vivo Spectroscopy) method. In the ISIS method illustrated in FIG. 5, the selective excitation high-frequency pulse 841 is first applied with the x gradient magnetic field pulse 85 applied. Next, the selective excitation high frequency pulse 842 is applied in a state where the y gradient magnetic field pulse 86 is applied. Further, the selective excitation high frequency pulse 843 is applied in a state where the z gradient magnetic field pulse 87 is applied. Subsequently, a high frequency pulse 844 for signal detection is applied to detect the NMR signal 88.
[0036]
In order to select a three-dimensional region, the phases of the three selective excitation pulses 841 to 843 in the sequence 83 of FIG. 5 are changed to +/−, and combinations of gradient magnetic fields of x, y, and z, 8 (2 ³ ) Repeat the sequence. Thereby, the NMR signals of the target region of interest are always added for each sequence, and the NMR signals from the peripheral region are canceled out. An NMR spectrum of the region of interest is obtained by Fourier transforming the obtained NMR signal.
[0037]
As described above, according to the MRI apparatus of the present invention, when performing a series of measurements, it is possible to carry out a plurality of intended examinations under optimum conditions while the subject is placed in the apparatus.
[0038]
In the above embodiments, the high-speed spin echo method and the NMR spectrum measurement have been described as the inspection methods, but the present invention is not limited to these inspection methods. In the above embodiment, the case where all the shim coils are turned on or off has been described. However, only some of the shim coils may be turned on depending on the purpose of the inspection. For example, turn on only the most important shim coil to obtain uniformity according to the static magnetic field distribution and minimize the magnetic field error of the gradient coil by minimizing the interference between the gradient coil and shim coil, or high gradient It is possible to turn off only the shim coil in the immediate vicinity of the gradient coil that requires magnetic field accuracy to minimize the magnetic field error for the gradient coil.
[0039]
In the illustrated embodiment, the shim coil is disposed outside the gradient magnetic field coil. However, the shim coil is not limited to this arrangement. For example, local static magnetic field correction is performed inside the gradient magnetic field coil. Even a shim coil provided for the purpose can be applied.
[0040]
【The invention's effect】
As described above, according to the present invention, it is possible to set the optimum conditions according to the inspection purpose by providing the means for correcting the uniformity of the static magnetic field and the means for controlling the interference between the gradient magnetic field generating means. Thus, it is possible to obtain a highly accurate inspection result by minimizing the generation of eddy currents, and to obtain a highly accurate inspection result by optimizing the uniformity of the static magnetic field.
[Brief description of the drawings]
FIG. 1 is a diagram showing details of gradient magnetic field coils and shim coils in one embodiment of the MRI apparatus of the present invention. FIG. 2 is a block diagram of an MRI apparatus showing one embodiment of the present invention. 4A and 4B are diagrams illustrating the interference of the shim coil, in which FIG. 5A shows an on state of the shim coil, and FIG.
FIG. 4 is a diagram showing an example of an inspection pulse sequence (fast spin echo method) performed by the MRI apparatus of the present invention. FIG. 5 is an example of an inspection pulse sequence (spectrum measurement) performed by the MRI apparatus of the present invention. Figure [Explanation of symbols]
1. Magnets (means for generating a static magnetic field)
2 .... Small magnetic material (Static magnetic field uniformity correction means)
3, 31-39... Shim coil (static magnetic field uniformity correction means)
4, 41-49 ..... shim power supply (static magnetic field uniformity correction means)
5, 51-59... Switch circuit (means for controlling interference)
6, 61, 62 ... Gradient magnetic field coils 7 to 9 ... Gradient magnetic field power supply 10 ... High frequency coil 11 ... High frequency circuit 12 ...・ Subject 13 ・ ・ ・ ・ ・ ・ Patient table 14 ・ ・ ・ ・ ・ ・ Computer (means for controlling interference)
15 ・ ・ ・ ・ ・ ・ Monitor 16 ・ ・ ・ ・ ・ ・ Operation Console

Claims (4)

Means for generating a static magnetic field in a space in which the subject is placed, magnetic field uniformity correction means for improving the uniformity of the static magnetic field, and generation of a gradient magnetic field for generating a gradient magnetic field having a different magnetic field strength depending on the position in the space Means, a means for generating a high-frequency magnetic field to cause a nuclear magnetic resonance phenomenon in the subject, a means for detecting the nuclear magnetic resonance phenomenon, an arithmetic processing of the detected nuclear magnetic resonance signal, and processing results In a magnetic resonance inspection apparatus comprising means for displaying,
The magnetic field homogeneity correcting means and the gradient magnetic field generating means have a means for controlling interference, and the means for controlling the interference is a switch means for switching the connection between the magnetic field homogeneity correcting means and the power source for driving the magnetic field homogeneity correcting means. And a control means for controlling the operation of the switch means according to the examination mode. The means for controlling the interference is an inspection mode in which, when the inspection mode requires a high degree of uniformity of the static magnetic field, the switch unit is brought into a connected state and is hardly influenced by the uniformity of the static magnetic field. The magnetic resonance inspection apparatus according to claim 1, wherein the switch means is disconnected in some cases. In the inspection mode in which measurement by the EPI method or MR spectrum measurement is performed, the means for controlling the interference causes the switch unit to be in a connected state, and in the inspection mode in which measurement by the spin echo method or the fast spin echo method is performed, 2. The magnetic resonance examination apparatus according to claim 1, wherein the switch means is disconnected. The means for controlling the interference executes the first inspection mode with the switch means in a disconnected state when performing inspection in the second inspection mode based on an image taken in the first inspection mode. 2. The magnetic resonance inspection apparatus according to claim 1, wherein the second inspection mode is executed with the switch means in a connected state.

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