JPH0243495B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to magnetic resonance (hereinafter referred to as "MR")
The present invention relates to a magnetic resonance imaging apparatus that uses phenomena to obtain images that reflect the spin density, relaxation time constant, etc. of a specific atomic nucleus present in a living subject.

[Technical background of the invention] In this type of magnetic resonance imaging apparatus,
The part that detects the signal induced from the subject by the MR phenomenon (this is called an MR signal) consists of a saddle-shaped coil installed around the subject and a capacitor that together forms a resonant circuit. . Since the MR signal is very weak, in order to detect the signal efficiently, Q (Quality Actor) is required.
requires a very large resonant circuit. Therefore, the resonance characteristics of the above-mentioned resonance circuit become sharp. Therefore,
A slight change in the capacitance component of the resonant circuit significantly changes the amplitude of the detected signal, that is, the sensitivity of the detector. On the other hand, there is stray capacitance between the test object and the detection coil, and this varies depending on the test object, so the capacitance of the capacitor is changed each time the test object changes, and the capacitance is precisely tuned to the resonance point. It is necessary to do so.

[Object of the Invention] An object of the present invention is to provide a magnetic resonance imaging apparatus that makes it possible to automatically tune a detection unit when collecting NMR signals.

[Summary of the Invention] The present invention comprises a resonant circuit including a receiving coil and a receiving tuning means including a variable capacitance element whose capacitance value is set by a control voltage applied from the outside, and in which the capacitance is set to the variable capacitance element. control voltage generation means for supplying a voltage for control; continuous wave application means for applying a continuous wave at a resonant frequency to the reception tuning means as a pseudo reception signal; and an envelope for envelope detection of the output of the reception tuning means. The present invention is characterized in that it comprises a detection means, and a means into which the output of the envelope detection means is input/output and which controls the control voltage generation means based on the signal input and performs tuning control of the reception tuning means.

[Embodiment of the Invention] FIG. 1 shows the configuration of the entire system in an embodiment of the present invention.

In FIG. 1, 1 is a transmitting probe head consisting of a transmitting coil, and 2 is a receiving probe head consisting of a receiving coil. In these transmitting and receiving probe heads 1 and 2, saddle-shaped transmitting and receiving coils are orthogonal to each other as shown in the figure. It constitutes a cross-coil type probe head arranged in the direction.

Although not shown, there are static magnetic field magnets and gradient magnetic field coils.
linear gradient magnetic field for imparting signal position information;
By applying high-frequency excitation pulses (high-frequency magnetic fields) from the probe heads 1 and 2 to the subject, an MR phenomenon occurs in a specific region.

As is well known, each of the magnetic field generating elements described above is an element generally included in a magnetic resonance apparatus for medical diagnosis, that is, this type of magnetic resonance imaging apparatus. The transmission tuning section 3 tunes to a high frequency of a specific frequency, and in response to the output of the transmitting section 4 applies a high frequency excitation pulse that tunes to a specific atomic nucleus in the subject through the transmission probe head 1 as an electromagnetic wave to the subject. do.

In addition, a gradient magnetic field coil and a probe head 1,
2 is driven according to an imaging procedure called a pulse sequence. This pulse sequence is installed in a computer 13, which will be described later. The transmitter 4 includes an oscillator 41 that generates a continuous wave at the specific frequency, that is, a resonance frequency, and a modulator 42 that generates an excitation pulse based on the output of the oscillator 41.
and a power amplifier 43 that amplifies the output of this modulator 42. The MR signal from the subject is transmitted to the reception tuning section 5 via the reception probe head 2.
The signal is received by a preamplifier 6, amplified by a preamplifier 6, and applied to two phase detectors 8A and 8B via a switch 7. These phase detectors 8A and 8B include an oscillator 41 of the transmitter 4.
Based on the signal generated by phase shifter 9, 90° phase shifter 1
Two types of reference waves generated at 0 and having the same frequency as the MR signal and having phases different from each other by 90° are provided. Phase detectors 8A and 8B phase-detect the received MR signal using the reference wave, and the detected outputs are amplified separately by amplifiers 11A and 11B, and converted into A/D (analog-digital) signal via low-pass filters 12A and 12B, respectively. ) The data is digitized by converters 13A and 13B and input to the computer 14. calculator 1
4, the two digitized signals are subjected to a predetermined phase correction process to obtain NMR echo signal data. A D/A (digital-analog) converter 15 outputs a control voltage v _c according to the output of the computer 14.
This constitutes a control voltage generator that provides the reception tuning section 5 with the following. The resistor 16 constitutes a circuit that derives a continuous wave of the resonant frequency output from the oscillator 41 of the transmitting section 4 via the switch 17 and supplies it to the reception tuning section 5. The amplifier 18 is connected to the preamplifier 6 via the switch 7.
receives the signal from and amplifies it. The signal amplified by the amplifier 18 undergoes envelope detection by an envelope detector 19, is digitized by an A/D converter 20, and is input to the computer 14. Switches 7 and 17 are controlled by computer 14 to switch the operating mode of this configuration. During tuning control, switch 7 gives the output of preamplifier 6 to amplifier 18, switch 17 is turned on, and MR During signal acquisition, the preamplifier 6
The output of is given to phase detectors 8A and 8B, and
is off. It should be noted that the oscillator 41 and modulator 42 of the transmitting section are also controlled by the computer 14 so as to operate as required.

FIG. 2 shows in detail the reception tuning section 5 and its surrounding parts in the above-described configuration.

In FIG. 2, a series circuit formed by a variable capacitance diode 51 whose capacitance changes depending on a voltage applied in the reverse direction and a large capacitance capacitor 52 provided in series on the cathode side of this variable capacitance diode 51 is shown. The probe is connected in parallel to the receiving probe head (coil) 2 to form an LC parallel resonant circuit. Here, the capacitance of the capacitor 52 is set to be sufficiently larger than that of the variable capacitance diode 51, so that the series combined capacitance of both is almost determined by the variable capacitance diode 51. In this case, the series circuit of the variable capacitance diode 51 and the capacitor 52 was connected with the anode side of the variable capacitance diode 51 as the ground side, but this series circuit could also be connected in the opposite direction to the illustration, with the capacitor 52 side as the ground side. good. The parallel circuit is further provided with an anti-parallel diode (sometimes referred to as a "crossing diode") 53 consisting of a pair of diodes connected in anti-parallel to each other. Further, a connection point between the variable capacitance diode 51 and the capacitor 52 is connected to the output side of the D/A converter 14 via a resistor 54, and a control voltage v _c from the D/A converter 14 is applied thereto. As the resistor 54, a resistor with a high resistance value is used to prevent high frequency received MR signals etc. from flowing into the D/A converter 6 side. In addition, in the case shown in the figure, the signal line is connected to the D/A converter 14 by the capacitor 52.
and is cut off by direct current. In addition, the anti-parallel diode 53 prevents damage to the input section of the preamplifier 6 due to leakage of high-power high-frequency excitation pulses applied to the subject from the transmitting side to the receiving side, and prevents distortion caused by the variable capacitance diode 51. It is something. The variable capacitance diode 51, the capacitor 52, the anti-parallel diode 53, and the resistor 54 constitute the reception tuning section 5. The control voltage v _c applied to the variable capacitance diode 51 is given from the D/A converter 14,
This voltage adjustment setting is performed as follows.

First, with the subject placed in the transmitting/receiving probe heads 1 and 2, the switching devices 7 and 17 are set to the tuning control mode, that is, connected as shown. In this state, the receiving probe head 2 is connected to the oscillator 41 of the transmitting section 4, and a low voltage continuous wave at a resonant frequency before being amplified by the power amplifier 43 is applied to the receiving probe head (coil) 2. Ru.
Here, assuming that the resistance value of the resistor 16 is sufficiently large, the resonant circuit of the detection section (the series circuit of the variable capacitance diode 51 and the capacitor 52 is defined as the variable capacitance CV) as shown in the equivalent circuit shown in FIGS. 3a and 3b. ),
This is equivalent to connecting a current source IS at the resonant frequency. The voltage generated between the terminals of the resonant circuit is applied to the amplifier 18 by the switch 7, and the amplified voltage is applied to the envelope detector 19. This detector 1
Reference numeral 9 is constructed using an envelope detector using a diode, etc., and outputs a DC voltage proportional to the envelope component of the input signal at the resonant frequency, that is, the amplitude of the input signal. This DC voltage is converted into a digital quantity by the A/D converter 20 and input to the computer 14. Initially, the output voltage v _c of the D/A converter 15 is the minimum value of the predetermined control range.
Set it to V ₀ . The voltage generated in the resonant circuit of the reception tuning section 6 is amplified and detected, and then sent to the A/D converter 13.
It is input to the computer 14 through A and 13B. This voltage value is set as P ₀ and is recorded in the storage device within the computer 14. This voltage P ₀ corresponds to the amplitude of the voltage generated in the resonant circuit when the control voltage v _c of the variable capacitance diode 51 is set to V ₀ . Next, the output control voltage v _c of the D/A converter 15 is set as V ₁ =V ₀ +ΔV(V←
V ₀ +ΔV), record the detected output voltage in the same manner as described above, and set the value as P ₁ . Further sequentially V ₂
=V ₁ +ΔV…V _o =V _o-1 The control voltage is increased as +ΔV, and the detected output voltage at that time is P ₂ …P _o .

Here, if the first control voltage v _c =V ₀ is sufficiently small, this is the control voltage (to be obtained control voltage) v _c = V _R
By increasing the control voltage v _c , the resonance condition is approached, and the detection output increases monotonically until reaching the resonance point. When the control voltage v _c = V _n is applied and the detection output P _n is obtained, this is compared with the previous detection output P _n-1, and as long as P _n > P _n-1, the control voltage is
Repeating the operation of increasing v _c by ΔV until P _k <
Continue until P _k-1 . At this time, the control voltage v _c =V _k-1 corresponding to the detected output P _k-1 is a value that provides the resonance condition of the tuning section, and is fixed.

Next, by executing a pulse sequence for imaging, the switchers 7 and 17 are switched to MR signal acquisition mode in order to acquire signals for obtaining an MR tomographic image. That is, the switch 17 is opened, and the output of the preamplifier 6 is connected to the receiving system circuits below the phase detectors 8A and 8B. At this time, the control voltage determined by the above operation is applied to the variable capacitance diode 51. In other words, the control voltage v _c is fixed. In this state, a high frequency excitation pulse is applied to the transmitting probe head 1 to collect NMR signals.
By performing the above operation every time the subject changes, the tuning of the reception tuning section 5 is always maintained.

In addition, in the pulse sequence for imaging, the transmission system, the gradient magnetic field coil, and its control system are activated. As mentioned above, during tuning performed prior to imaging, only the transmitting system is activated and without applying an excitation pulse to the subject, a pseudo reception signal is simply generated and this is applied to the reception tuning section 5. I take it as a thing. Moreover, by continuously applying the pseudo reception signal as a continuous wave to the reception tuning unit 5 at one time while changing the control voltage, it is possible to find the control voltage v _c that provides the tuning condition in a single time. can.

When done in this way, the
Rather than performing control using NMR signals, a continuous wave is applied to the receiving probe head (coil) 2 to adjust and set the tuning conditions, so optimal tuning conditions can be obtained in a short time.

Note that the present invention is not limited to the embodiments described above and shown in the drawings, but can be implemented with various modifications without changing the gist thereof.

For example, in the above embodiment, a continuous wave at the resonant frequency was applied to the receiving probe head 2, but in the case of the cross coil method as in the above embodiment, the continuous wave at the resonant frequency is applied as shown in FIG. Even if the voltage is applied to the transmitting probe head via the switch 21 without passing through the modulator 42 and power amplifier 43, and the minute leakage voltage induced in the receiving probe head is amplified and detected, the result is almost the same as described above. You can synchronize by doing this.

Further, instead of the capacitor 52 shown in FIG. 2, another variable capacitance diode may be connected in series with the cathodes connected to each other to enable fine adjustment of the tuning.Of course, as described above, The series circuit of the variable capacitance diode 51 and the capacitor 52 in FIG. 2 may be arranged in the opposite direction from that shown.

Furthermore, although the above-mentioned embodiment shows the case where a saddle-shaped receiving coil as shown in FIG. Is possible.

Furthermore, in the same embodiment, a cross-coil method was used in which the transmitting coil and the receiving coil were perpendicular to each other, but as shown in FIG. Almost the same implementation as described above is also possible in a single coil system in which 22 is configured. In FIG. 4, 23 is a power amplifier group for transmission, 24 is an anti-parallel diode for preventing malfunction, 25 is a variable capacitor for tuning, and 26 is an auxiliary coil.

[Effects of the Invention] According to the present invention, it is possible to provide a magnetic resonance imaging apparatus that can automatically and quickly synchronize a detection unit when collecting MR signals.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the overall configuration of an embodiment of the present invention, Fig. 2 is a circuit diagram showing the main structure of the embodiment, and Figs. 3 a and b explain the operation of the embodiment. FIGS. 4 and 5 are circuit diagrams and block diagrams showing the configurations of other different embodiments of the present invention, respectively. DESCRIPTION OF SYMBOLS 1...Transmission probe head, 2...Reception probe head, 3...Transmission tuning section, 4...Transmission section, 5
... Reception tuning section, 6 ... Preamplifier, 7, 17,
21...Switcher, 8A, 8B...Phase detector, 9
...Phase shifter, 10...90° phase shifter, 11A, 11
B, 18...Amplifier, 12A, 12B...Low pass filter, 13A, 13B, 20...A/D converter, 14...Computer, 15...D/A converter,
16...Resistor, 19...Envelope detector, 22...
Transmission/reception probe head, 23, 43... Power amplifier, 24, 53... Anti-parallel diode (crossing diode), 25... Variable capacitor, 26... Auxiliary coil, 41... Oscillator, 42... Modulator, 5
1... Variable capacitance diode, 52... Capacitor, 54... Resistor.

Claims (1)

[Claims] 1. In a magnetic resonance imaging apparatus that uses magnetic resonance signals induced from a subject in which a magnetic resonance phenomenon is occurring to image an image in which at least one of the spin density and relaxation time constant of a specific atomic nucleus in the subject is reflected. , a transmission system that applies a high-frequency excitation pulse to the subject, a reception coil that receives the magnetic resonance signal, and a resonant circuit that forms a resonant circuit together with the reception coil. a receiving tuning means including a variable capacitance element in which a capacitance value is set; an imaging processing means for generating an image using a magnetic resonance signal obtained via the receiving coil and the receiving tuning means; continuous wave applying means for inputting a continuous wave having a resonance frequency of a specific atomic nucleus generated from a transmission system to the reception tuning means as a pseudo reception signal; and a predetermined initial value.
Voltage V that starts from V ₀ and adds an increase ΔV over time ←
control voltage generation means for supplying V ₀ +ΔV as a control voltage to the reception tuning means; an envelope detection means for envelope-detecting the output of the reception tuning means to obtain a DC voltage; a storage means for storing a DC voltage value obtained by the envelope detection means; and controlling the control voltage generation means. inputting the pseudo reception signal to the reception tuning means while changing the control voltage V by the controller, determining the maximum value of the DC voltage value based on each DC voltage value stored in the storage unit; The control voltage value V when
A magnetic resonance imaging apparatus comprising: a control means for immobilizing the magnetic resonance imaging apparatus.