JP2013017493A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus in which heat generated in decoupling circuit can be prevented in a high-frequency reception system.SOLUTION: The magnetic resonance imaging apparatus includes: a magnetostatic field generating means for generating a static magnetic field in a space where a subject is placed; a gradient magnetic field generating means for generating a gradient magnetic field superimposed on the static magnetic field; a high-frequency magnetic field generating means for irradiating the subject with a high-frequency magnetic field; a detecting means for detecting a magnetic resonance signal generated from the subject; an imaging means for imaging the detected signal; a decoupling means for preventing magnetic coupling between the high-frequency magnetic field generating means and the detecting means; and a biasing means for supplying current to the decoupling means. The imaging apparatus further includes a current detecting means for detecting the value of the current; and an adjusting means for adjusting the value of the current in accordance with the value of the current detected by the current detecting means.

Description

  The present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI) apparatus, and more particularly, to a technique for stabilizing a decoupling circuit in a high-frequency receiving system.

  The MRI device measures NMR signals generated by the spins of the subject, especially the tissues of the human body, and visualizes the form and function of the head, abdomen, limbs, etc. in two or three dimensions Device. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field, frequency-encoded, and measured as time series data. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In order to detect this NMR signal, various high-frequency receiving coils are prepared for the MRI apparatus according to the examination target site and application of the subject, and this high-frequency receiving coil is magnetically coupled with the high-frequency transmitting coil. When this occurs, an unnecessary current is generated in the high-frequency receiving system when the high-frequency magnetic field is irradiated by the high-frequency transmitting coil, and unnecessary heat is generated or the circuit is unnecessarily damaged. In order to prevent this, a decoupling circuit is provided in the high-frequency receiving system so that unnecessary current does not flow.

  Patent Document 1 discloses a technique related to such a decoupling circuit, the decoupling circuit includes a temperature sensor, and a high-frequency transmission coil is used when the temperature detected by the temperature sensor exceeds a threshold value. The irradiation of the high frequency magnetic field is stopped.

JP 2008-212437 JP

  However, in the prior art described in Patent Document 1, no consideration has been given to how to control the current when a current flows from the bias circuit to the diode of the decoupling circuit. If excessive current flows to the diode of the decoupling circuit, unnecessary heat may be generated in the diode.

  An object of the present invention is to suppress heat generated in a decoupling circuit in a high frequency receiving system of an MRI apparatus.

  In order to solve the above problems, according to the present invention, a static magnetic field generating means for generating a static magnetic field in a space where a subject is arranged, and a gradient magnetic field generating means for generating a gradient magnetic field superimposed on the static magnetic field, , A high-frequency magnetic field generating means for irradiating the subject with a high-frequency magnetic field, a detecting means for detecting a magnetic resonance signal generated from the subject, an imaging means for imaging the detected signal, and the high-frequency magnetic field generation Current detecting means for detecting the value of the current in an MRI apparatus comprising decoupling means for preventing magnetic coupling between the means and the detecting means, and bias means for supplying current to the decoupling means, There is provided an MRI apparatus comprising adjusting means for adjusting the current value according to the current value detected by the current detecting means.

  In addition, the adjustment means includes storage means for storing an allowable range of the current value, and the adjustment means adjusts the current value so that the current value falls within the allowable range. An MRI apparatus characterized by the above is provided.

  Further, there is provided an MRI apparatus, wherein the adjustment means includes a current limiting means for limiting a current supplied to the decoupling means between the decoupling means and the current detection means. The

  According to the present invention, heat generated in the decoupling circuit in the high frequency receiving system of the MRI apparatus is suppressed.

The figure for demonstrating the general outline | summary of an example of the MRI apparatus which concerns on this invention. The figure for demonstrating the conventional high frequency transmission coil and a high frequency receiving coil. The figure for demonstrating the structure of the high frequency receiving coil which concerns on the Example of this invention. 4 is a flowchart for explaining the operation of the circuit shown in FIG.

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

  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 the overall configuration of an embodiment of an MRI apparatus according to the present invention. This MRI apparatus irradiates a high frequency magnetic field to the subject, a static magnetic field generating means for generating a static magnetic field in a space in which the subject is arranged, a gradient magnetic field generating means for generating a gradient magnetic field superimposed on the static magnetic field, and the subject A high-frequency magnetic field generating means, a detecting means for detecting a magnetic resonance signal generated from the subject, and an imaging means for imaging the detected signal, and obtaining a tomographic image of the 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, a sequencer 4, and a central processing unit. (CPUI) 8.

  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 includes a gradient magnetic field coil 9 that applies a gradient magnetic field in the three-axis directions of X, Y, and Z, which is a coordinate system (stationary coordinate system) of the MRI apparatus, and a gradient magnetic field that drives each gradient magnetic field coil. Gradient magnetic fields Gx, Gy, and Gz are applied in the X, Y, and Z axis directions by driving the gradient magnetic field power supply 10 of each coil according to a command from the sequencer 4 described later. . At the time of imaging, a slice direction gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and the remaining two orthogonal to the slice plane and orthogonal to each other A phase encoding direction gradient magnetic field pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied in one direction, and position information in each direction is encoded into an echo signal.

  The sequencer 4 is a control means that repeatedly applies a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) and a gradient magnetic field pulse in a predetermined pulse sequence, and operates under the control of the CPU 8 to collect tomographic image data of the subject 1 Various commands necessary for the transmission are 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 RF pulses in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the living tissue of the subject 1, and includes a high frequency oscillator 11, a modulator 12, and a high frequency amplifier. 13 and a high-frequency transmission coil 14a on the transmission side. The RF 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 the amplitude-modulated RF pulse is amplified by the high-frequency amplifier 13 and then placed close to the subject 1. By supplying the high frequency transmission coil 14a, the RF pulse is irradiated to the subject 1.

  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 is in phase with the high-frequency receiving coil 14b and the signal amplifier 15 on the receiving side. It comprises a detector 16 and an A / D converter 17. The NMR signal of the response of the subject 1 induced by the electromagnetic wave irradiated from the high-frequency transmission coil 14a on the transmission side was detected by the high-frequency reception coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15 Thereafter, the signals are divided into two orthogonal signals by the quadrature phase detector 16 at a timing according to a command from the sequencer 4, converted into digital quantities 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 includes an external storage device such as an optical disk 19 and a magnetic disk 18, and a display 20 including a CRT or the like. When data from the reception system 6 is input to the CPUI8, the CPUI8 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, and an external storage device On the magnetic disk 18 or the like.

  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 close to 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 transmission 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 in 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 receiving coil 14b on the receiving side is disposed so as to face or surround the subject 1.

  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, FIG. 2 is a diagram for explaining FIG. 2 for a conventional high-frequency transmitting coil and high-frequency receiving coil. In FIG. 2, 13 is a high-frequency amplifier, 14a is a high-frequency transmission coil for generating a high-frequency magnetic field in the subject, 32 is a high-frequency magnetic field irradiated from the high-frequency transmission coil, and 14b is a high-frequency reception coil for receiving NMR signals. . Further, reference numeral 34 indicates that when the high-frequency magnetic field 32 supplied by the high-frequency amplifier 13 is applied to the subject from the high-frequency transmission coil 14a, the high-frequency reception coil 14b reduces the magnetic coupling generated between the high-frequency transmission coil 14a and the high-frequency transmission coil 14a. The decoupling circuit is composed of an inductance 35, a diode 36 and a capacitor 37. Reference numeral 38 denotes a bias circuit for applying a bias current to the decoupling circuit.

  Next, the operation of the decoupling circuit 34 shown in FIG. 2 will be described. A voltage is applied from the bias circuit 38 to the decoupling circuit 34 at least during a period in which a high frequency magnetic field is applied from the high frequency transmission coil 14a. Then, the diode 36 in the decoupling circuit 34 becomes conductive, and the resistance becomes low. As a result, the decoupling circuit 34 in the high frequency receiving coil 14b resonates due to the high frequency magnetic field 32 applied from the high frequency transmitting coil 14a. As a result, since the impedance at both ends of the capacitor 37 increases, a part of the high frequency receiving coil 14b is equivalent to an open state, and magnetic coupling with the high frequency transmitting coil 14a is reduced. As a result, it is possible to prevent unnecessary current from being generated in the high-frequency receiving system, unnecessary heat from being generated, and unnecessary damage to the receiving circuit.

  However, in the configuration of the conventional decoupling circuit 34 shown in FIG. 2, simultaneously with the application of the high frequency magnetic field 32 by the high frequency transmission coil 14a, a voltage is excited in the high frequency reception system, but the high frequency applied from the high frequency transmission coil 14a As the output of the magnetic field 32 increases, the voltage excited by the decoupling circuit 34 also increases. In this case, the amount of current flowing through the decoupling circuit 34 also increases, and the diode 36, the inductance 35, and the capacitor 37 are likely to generate heat. As a result, heat generated from the diode 36, the inductance 35, and the capacitor 37 may be conducted to the coil surface, which may increase the temperature stimulus to the subject.

  Therefore, in general, in order to reduce the amount of heat generated by the diode 36 having the largest amount of heat generation in the elements constituting the decoupling circuit 34, a device such as adding a heat sink or selecting a diode having a small amount of heat generation is employed. It was made.

  Here, as the magnetic field intensity generated by the MRI apparatus increases as the functionality of the MRI apparatus increases, the required high-frequency magnetic field intensity also increases, so the high-frequency magnetic field 32 applied from the high-frequency transmission coil 14a is increased. Increased. As a result, the amount of heat generated by the decoupling circuit 34 increases, and there is a problem in that temperature stimulation to the subject increases.

Next, an embodiment of the present invention will be described with reference to FIGS.
FIG. 3 is a diagram for explaining the configuration of the high-frequency receiving coil according to the embodiment of the present invention. In the configuration of FIG. 3, the value of the current from the bias circuit to the output coupling circuit is detected between the decoupling circuit 34 serving as a heat source and the bias circuit 38 for supplying current to the decoupling circuit 34. The current detection circuit 39 as the detection means and the current limit circuit 40 as the current limit means for limiting the current supplied to the decoupling means are provided. Specifically, one end of the current limiting circuit 40 is connected to one end of the decoupling circuit 34, one end of the current detection circuit 39 is connected to the other end of the current limiting circuit 40, and a bias circuit is connected to the other end of the current detection circuit 39. Are connected at one end. The current limiting circuit 40, the current detection circuit 39, and the bias circuit 38 are connected to the sequencer 4, and are controlled by the sequencer 4.

Next, FIG. 4 is a flowchart for explaining the operation of the circuit shown in FIG.
(Step 41)
When the sequencer 4 irradiates the high-frequency magnetic field 32 using the high-frequency transmission coil 14a, for example, according to the irradiation amount of the high-frequency magnetic field irradiated from the high-frequency transmission coil 14a, the amount of current flowing from the bias circuit to the decoupling circuit is Determine as Iset. In addition, the allowable error range of the current value data output from the current detection circuit 39 is set such that the minimum is Isetmin and the maximum is Isetmax.

(Step 42)
The sequencer 4 sends a command to apply a current from the bias circuit 38 to the decoupling circuit 34.

(Step 43)
The sequencer 4 controls the current limiting circuit 40 so that excessive current does not flow to the decoupling circuit 34.

(Step 44)
It is determined whether or not the current value data Isense output from the current detection circuit 39 exceeds a value including an error of the value Iset to be passed through the decoupling circuit 34 (Isense> Isetmax). If exceeded, the process proceeds to step 45, and if not, the process proceeds to step 46.

(Step 45)
The current flowing in the decoupling circuit 34 is limited to the current value Ilimit by the current limiting circuit 40, but the current value Ibias output from the bias circuit 38 is reduced by the sequencer 4 to suppress the heat generated in the current limiting circuit 40. Let That is, the sequencer 4 or the bias circuit 38 is provided with adjusting means for adjusting the current value in accordance with the current value detected by the detecting means, and is controlled thereby.

(Step 46)
Further, it is determined whether or not the current value data Isense detected by the current detection circuit 39 is lower than the value Iset to be passed through the decoupling circuit 34 (Isense <Isetmin). If so, the process proceeds to step 47, and if not, the process proceeds to step 48.

(Step 47)
The sequencer 4 controls the bias circuit 38 to increase the current value Ibias output from the bias circuit 34 so that the current value detected from the current detection circuit 39 satisfies Isetmin ≦ Isense ≦ Isetmax. That is, the sequencer 4 or the bias circuit 38 is provided with adjusting means for adjusting the current value in accordance with the current value detected by the detecting means, and is controlled thereby.

(Step 48)
It is determined whether or not to stop the application of current for decoupling. If not, the process proceeds to step 45.

  By controlling the current flowing through the decoupling circuit 34 as described above, a stable value of current is supplied to the decoupling circuit 34, heat generation due to excessive current input is suppressed, and temperature stimulation to the subject is also reduced. Can be realized.

  As mentioned above, although the Example of this invention was described, this invention is not limited to these.

  The present invention can be used in a magnetic resonance imaging apparatus.

  39 Current detection circuit, 40 Current limit circuit

Claims (3)

A static magnetic field generating means for generating a static magnetic field in a space in which the subject is arranged;
A gradient magnetic field generating means for generating a gradient magnetic field superimposed on the static magnetic field;
High-frequency magnetic field generating means for irradiating the subject with a high-frequency magnetic field;
Detecting means for detecting a magnetic resonance signal generated from the subject;
Imaging means for imaging the detected signal;
In a magnetic resonance imaging apparatus comprising decoupling means for preventing magnetic coupling between the high-frequency magnetic field generating means and the detection means, and bias means for supplying a current to the decoupling means,
A magnetic resonance imaging apparatus comprising: current detection means for detecting the current value; and adjustment means for adjusting the current value according to the current value detected by the current detection means.   The adjustment means includes storage means for storing an allowable range of the current value, and the adjustment means adjusts the current value so that the current value falls within the allowable range. The imaging apparatus according to claim 1.   The said adjustment means is provided with the current limiting means which restrict | limits the electric current supplied to the said decoupling means between the said decoupling means and the said current detection means, The Claim 1 or 2 characterized by the above-mentioned. Magnetic resonance imaging equipment.

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