JPS63270036A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To reduce the disturbance of a transmitting signal wave form, by providing a switching diode circuit in a transmitting signal transmission path showing high impedance at the time of transmission. CONSTITUTION:At the time of the transmission of a selection exciting pulse, a timing part 50 show low impedance and the low impedance at a point A is converted to high impedance by a 1/4 wavelength cable 41. Further, switching diode circuits 42, 49 are in an ON-state; when the diode circuit 42 is in an ON-state, a series resonance circuit is short-circuited and a matching circuit 35 is functioned only by variable condensers 46, 48 and an auxiliary coil 47. In this state, impedance matching is taken on the high impedance side of the 1/4lambda cable 41 and on the output side of a power amplifier 18. Since both of the diode circuits 42, 49 are provided to the high impedance part in a transmitting signal transmission path, the disturbance of the wave form of a transmitting signal is reduced in the degree corresponding to high transmitting signal voltage and a slice characteristic becomes well.

Description

[Detailed Description of the Invention] [Object of the Invention] (Field of Industrial Application) The present invention relates to magnetic resonance (HR)
The present invention relates to a magnetic resonance imaging apparatus that obtains an MR image of a subject using the MR (nance) phenomenon.

(Prior art) Magnetic resonance imaging device (hereinafter referred to as MRI device)
The method applies a uniform static magnetic field to a desired part of the subject, and uses a transmitting RF coil to form an RFta field perpendicular to this static magnetic field, producing a magnetic field resonance phenomenon only in a specific slice part from which a tomographic image is obtained. Roughly speaking, the magnetic resonance phenomenon (hereinafter referred to as MR multiplied signal) generated from the atomic nucleus after the RFfa field is released is detected by the receiving RF coil. Roughly speaking, a composite MR multiplied signal is obtained by applying a linear magnetic field gradient having a linear slope to the static magnetic field (a coordinate system rotated by θ° from the X axis in the X' axis direction), and By converting the X' axis of the slice part to
CTfl= is formed by rotating within the Y plane and obtaining projection information in each direction within the XY plane.

FIG. 5 shows a transmission/reception system of such an MRI apparatus. As shown in the figure, a switching diode circuit (also called a cross diode circuit) is included in a tuning section 50 that includes a transmitter/receiver coil 4, a transmitter/receiver coil resistor 30, and a tuning capacitor 32 to form a series resonant circuit. )
33 and the power amplifier 1 via the matching box 34
8 transmission signals are supplied. Further, the MR multiplied signal detected by the tuning section 50 is taken into the preamplifier 19 via the auxiliary coil 36 and the auxiliary coil resistor 37. A switching diode circuit 38 and a tuning capacitor 39 are provided between the input end of the preamplifier 19 and the installation line.

6 and 7 show equivalent circuits of the transmitter/receiver system shown in FIG. 5 during transmission and reception, respectively.

Due to series resonance, only the transmitting/receiving coil resistance 30 is present in the tuning section 50, and the switching diode circuits 33 and 38 are in an on state during transmission and an off state during reception.

By the way, the switching diode circuit 33 shown in FIG. 5 basically consists of two switching diodes connected in antiparallel, and its voltage-current characteristics are as shown in FIG. Due to this characteristic, the switching diode circuit 33
It functions to prevent noise components from flowing out from the power amplifier 18. The transmission signal output from the power amplifier 18 usually has a waveform in which the carrier frequency is modulated by a sink function, as shown in FIG. will be supplied.

(Problem to be Solved by the Invention) However, since the switching diode 33 has the characteristics shown in FIG. 8, the waveform of the transmission signal supplied to the tuning circuit 50 is discontinuous as shown in FIG. Become. In particular, in the circuit configuration shown in Figure 5,
As is clear from the equivalent circuit during transmission (FIG. 6), the series resonance of the tuning circuit 50 results in low impedance, so the signal voltage in this part becomes low, so the transmission signal waveform by the switching diode circuit 33 The current situation is that the MR image is severely disturbed (this deteriorates the slice characteristics and deviates from the rectangular shape), which has an adverse effect on the MR image.

SUMMARY OF THE INVENTION The present invention aims to eliminate the above-mentioned drawbacks and to provide an MRI apparatus in which disturbances in transmitted signal waveforms caused by switching diode circuits are reduced.

[Structure of the Invention] (Means for Solving the Problems) The present invention includes a tuning section formed by connecting a transmitting/receiving coil and a tuning capacitor in series, and a transmitting signal is supplied to the tuning section via a switching diode circuit. In a magnetic resonance imaging apparatus that forms a magnetic resonance image of a subject based on a magnetic resonance phenomenon of the subject, the impedance conversion means converts the impedance of the tuning unit during transmission into a high impedance. is provided in the transmission signal transmission path, and a matching circuit is provided between the high impedance side of the impedance conversion means during transmission and the output side of the power amplifier for impedance matching between the two, and a matching circuit is provided in the transmission signal transmission path. The switching diode circuit is arranged in a high impedance section.

(Function) The voltage of the transmission signal is high in the transmission signal transmission path that has been converted to a high impedance by the impedance conversion means, so by providing the switching diode circuit in this portion, the waveform of the transmission signal by the switching diode circuit can be changed. Disturbance can be reduced.

(Example) FIG. 1(a) is a block diagram of one embodiment of the present invention.

The magnet assembly 1 includes a static magnetic field coil 2 that applies a main magnetic field of constant strength to a subject inserted therein, and a gradient magnetic field coil 3 that applies gradient magnetic fields in the X direction, y direction, and two directions to the subject. , a transmitting/receiving coil 4 for applying a high frequency pulse (RF pulse) for exciting the spin of an atomic nucleus, and for detecting an MR low signal from within the subject.

The data processing computer 11 is connected to a display device 12 and a controller 13 serving as control means. Controller 13
is connected to the gradient magnetic field control circuit 14 and the gate circuit 17. The gradient magnetic field control circuit 15 is connected to the gradient magnetic field coil 3. High frequency oscillator 16 is connected to gate circuit 17 . Gate circuit 17 is connected to power amplifier 18 . Power amplifier 18 is connected to transmitter/receiver coil 4 via duplexer circuit 5 . Further, the transmitting/receiving coil 4 is connected to a preamplifier 19 via a duplexer circuit 5. Preamplifier 19 is connected to phase detection circuit 20 . Phase detection circuit 20 is connected to waveform memory 21 . Waveform memory 21 is connected to data processing computer 11 .

The controller 13 generates a timing signal for collecting observation data of magnetic resonance signals, and controls the operation of the gradient magnetic field drive circuit 14 and the gate circuit 17. Thereby, the controller 13 controls the gradient magnetic fields GZ, G"i/, G'
;1 and the generation sequence of high frequency pulse RF.

The gradient magnetic field control circuit 14 controls the current of the gradient magnetic field coil 3 and applies a gradient magnetic field to the subject.

The static vii field control circuit 15 controls the current supplied to the static magnetic field coil 2 and applies a static magnetic field Ha to the subject.

The high frequency oscillator 16 generates a high frequency signal whose frequency is controlled by the controller 13. The gate circuit 17 modulates the high frequency signal output from the high frequency oscillator 16 using the timing signal from the controller 13 to generate high frequency pulses. The power amplifier 18 amplifies the power of the high frequency pulse output from the gate circuit 17, and this amplified output is supplied to the transmitting/receiving coil 4 via the duplexer 5.

The preamplifier 19 is for amplifying the MR low signal from the transmitting/receiving coil 4, and the phase detection circuit 20 is for phase detecting the amplified MR low signal. The waveform memory 21 stores phase-detected waveforms and signals.

The data processing computer 11 controls the operation of the controller 13, receives time information from the controller 13, reads data from the waveform memory 21, and processes signals based on observed magnetic resonance. Furthermore, data processing computer 1
1 can also display operation instructions to the operator on the display device 12.

In the above configuration, the power amplifier 18. Preamplifier 19.
A transmitting/receiving system 51 having a duplexer 5 and an RF coil 4
is formed. Next, details of this transmitting/receiving system 51 will be explained based on FIG. 1(b).

As shown in the figure, this transmission/reception system 51 includes a 1/4 wavelength cable 41 connected to a series connection point between the tuning capacitor 32 and the auxiliary coil 36 in the tuning circuit 50, and It has a matching circuit 35 and a switching diode 49 arranged between the other end and the power amplifier 18.

The matching circuit 35 performs impedance matching, and includes variable capacitors 43. A switching diode 4 is connected to a series resonant circuit of an auxiliary coil 44 and an auxiliary coil resistor 45.
2 are connected in parallel, a variable capacitor 46 is connected in series to this parallel circuit, and one end of the variable capacitor 43 is connected to the other end of the 1/4 wavelength cable 41 and one end of the auxiliary coil 47. A variable capacitor 48 is connected between the end and the ground line. The other end of the auxiliary coil 47 in the matching circuit 51 is connected to the output end of the power amplifier 18 via a switching diode circuit 49.

The configuration of the receiving system is the same as that shown in FIG.

Next, the operation of the embodiment device having the above configuration will be explained.

First, in order to obtain a tomographic image at a specific position of the subject,
A current is applied to the static magnetic field coil 2 via the static magnetic field control circuit 15 to provide a uniform static magnetic field Ha in the direction of the arrow Z axis in FIG. 1(a). This causes the magnetization to move toward the Z-axis direction.

Next, a desired slice plane is selectively excited according to a normal pulse sequence, and MR multiplied signals from the slice plane are collected. Here, MR signals of the slice plane are collected using the pulse echo method. That is, a selective excitation pulse (π/
2 pulses).

A desired slice plane is specified by applying a gradient rin field GZ for slicing, and a gradient magnetic field Gy for phase encoding is applied.
By applying the reading gradient magnetic field GX and the π pulse, one line of echo data h<'+'4 on the slice plane is generated.Then, while changing the intensity of the phase encoding gradient magnetic field Gy, that is, the necessary data is acquired by phase encoding. is collected, and an MR image can be reconstructed based on this.Echo data detection is performed by the transmitting/receiving coil 5, and the detected data is taken into the phase detection circuit 20 via the preamplifier 19, where the spectrum is An analysis is performed, and an image is reconstructed by the computer 11 based on the analysis results.

Next, details of the operation of the transmitting/receiving system 51 will be explained.

The transmission of the selective excitation pulse is represented by the equivalent circuit shown in FIG. Since the tuning section 50 is tuned by series resonance, only the transmitting/receiving coil resistance 30 remains, resulting in low impedance. The resistance value of this coil resistor 30 is r3
0, the impedance at one end of the 1/4 wavelength cable 41, that is, at point A, is equal to r3), but 1/4
The impedance at the other end of the 4-wavelength cable 41 is Zo2
/r3 (converted to 1. Here, Zo is the 1/4 wavelength cable 41
is the characteristic impedance of In other words, the low impedance at point A is converted to high impedance by this 1/4 wavelength cable 41. In this sense, the 1/4 wavelength cable 41 is referred to as impedance conversion means.

Also, during transmission, the switching diode circuit 4
2 and 49 are in the on state. When the switching diode 42 is in the on state, the variable capacitor 43. A series resonant circuit consisting of an auxiliary coil 44 and an auxiliary coil resistor 45 is connected as a matching circuit 35.
Equivalently variable capacitor 46, 48 and auxiliary coil 47
Only.

In this state, impedance matching is achieved between the high impedance side of the 1/4λ cable 41 and the output side of the power amplifier 18. However, in the device of this embodiment, since both the switching diode circuits 42 and 49 are provided in a high impedance part of the transmission signal transmission path, in other words, in a part where the transmission signal voltage is high, the influence on the transmission signal waveform is small. This is not a particular problem.

Next, when receiving the MR multiplied signal, it is represented by an equivalent circuit shown in FIG. During reception, the switching diode circuits 42 and 49 are turned off, and due to the series resonance of the variable capacitors 43 and 46 and the auxiliary coil 44, the impedance of this resonant circuit is determined by the coil resistor 45 having a low resistance. Therefore, the impedance seen from the coil resistance 45 from point A is converted to high impedance by the 1/4 wavelength cable 41, and as a result, the equivalent circuit in FIG. 3 can be replaced with the equivalent circuit in FIG. /4 wavelength cable 41
and the coil resistance 45 can be ignored. As is clear from this equivalent circuit, the transmitting system is equivalently separated during reception in the apparatus of this embodiment, and no noise flows into the receiving system from the transmitting system.

In this way, in the device of this embodiment, the switching diode circuit 49 is provided in the transmission signal transmission path that becomes high impedance during transmission, and unlike the conventional device (Fig. Compared to the case where the diode 33 is provided, the disturbance in the waveform of the transmission signal is smaller due to the higher transmission signal voltage, and therefore the slicing characteristics are improved.

Although one embodiment of the device of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications can be made. For example, in the above embodiment, the 1/4 wavelength cable 41 was used as the impedance conversion means, but in addition to this 1/4 wavelength cable,
(2n+1>/4 wavelength cable (n is a positive integer)
can be used.

[Effects of the Invention] As detailed above, according to the present invention, the M
An RI device can be provided.

[Brief explanation of drawings]

FIG. 1(a) is a block diagram of an apparatus according to an embodiment of the present invention.
Figure (b) is a circuit diagram showing details of the main parts of Figure (a), Figure 2 is an equivalent circuit diagram at the time of transmission of Figure 1 (b), Figures 3 and 4 are Figure 1 ( b) is an equivalent circuit diagram at the time of reception, Figure 5 is a circuit diagram of the transmitting and receiving system in the conventional device, Figures 6 and 7 are equivalent circuit diagrams at the time of transmission and reception of Figure 5, respectively, and Figure 8 is the equivalent circuit diagram at the time of transmission and reception of Figure 5. Characteristic diagrams of the switching diode circuit, FIGS. 9 and 10, are waveform diagrams for explaining waveform disturbances of transmission signals. 4... Transmission/reception coil, 18... Power amplifier, 19.
...Preamplifier, 32... Tuning capacitor, 35.
...Matching circuit, 49...Switching diode circuit, 50...Tuning section, 41...1/4 wavelength cable (impedance conversion means). Agent Patent Attorney Nori Chika Ken Shu
Takeshi Kondo 51 (b) Figure 1 φ4 Figure 6

Claims (1)

[Claims] It has a tuning section formed by connecting a transmitter/receiver coil and a tuning capacitor in series, and a power amplifier that supplies a transmission signal to the tuning section via a switching diode circuit. In a magnetic resonance imaging apparatus that forms a magnetic resonance image of a specimen, impedance conversion means for converting the impedance of the tuning section during transmission into high impedance is provided in the transmission signal transmission path, and the impedance conversion means converts the impedance during transmission into high impedance. Magnetic resonance imaging characterized in that a matching circuit is provided between the output side of the power amplifier and the output side of the power amplifier for impedance matching between the two, and the switching diode circuit is arranged in a high impedance section in the transmission signal transmission path. Device.