JPH0243494B2 - - 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 the "MR signal") consists of a saddle-shaped coil installed around the subject and a capacitor that together forms a resonant circuit. be done. Since the MR signal is very weak, a resonant circuit with a very large Q (quality factor) is required to detect the signal efficiently.

Therefore, the resonance characteristics of the above-mentioned resonance circuit become sharp.
Therefore, with a slight change in the capacitance component of the resonant circuit,
The amplitude of the detected signal, and thus the sensitivity of the detector, changes significantly. On the other hand, there is a stray capacitance between the test object and the detection coil, and since this varies depending on the test object, the capacitance of the capacitor is changed each time the test object changes to precisely tune to the resonance point. There is a need.

[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 MR signals.

[Summary of the Invention] The present invention constitutes a resonant circuit together with a receiving coil, and has a receiving tuning means including a variable capacitance element whose capacitance value is set according to a control voltage value, and when tuning, transmitting Start only the system,
The control voltage value when the peak value of the magnetic resonance signal at that time is the maximum is fixed.

[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.These transmitting/receiving probe heads 1 and 2 have saddle-shaped transmitting and receiving coils that 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, a static magnetic field magnet and a gradient magnetic field coil are provided.
A linear gradient magnetic field for imparting positional information of MR signals and a high-frequency excitation pulse (high-frequency magnetic field) from the probe heads 1 and 2 are applied to the subject, thereby causing an MR phenomenon in a specific region. .

As is well known, each of the above-mentioned magnetic field generating elements 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 MR signal from the object is received by the reception tuning unit 5 via the reception probe head 2 and sent to the preamplifier 6.
The signal is amplified and applied to two phase detectors 7A and 7B. These phase detectors 7A and 7B include a transmitter 4
Phase shifter 8, 90° phase shifter 9
Two types of reference waves, which are generated by the MR signal and have the same frequency as the MR signal and whose phases differ by 90 degrees from each other, are provided.
Phase detectors 7A and 7B phase-detect the received MR signal using the reference wave, and the detected outputs are amplified separately by amplifiers 10A and 10B, and are converted into A/D (analog-digital) signals via low-pass filters 11A and 11B, respectively. ) The data is digitized by converters 12A and 12B and input to the computer 13. In the computer 13, a predetermined phase correction process is performed using the two digitized signals to obtain MR echo signal data. D/A (digital-analog) converter 1
Reference numeral 4 constitutes a control voltage generator that provides a control voltage vc corresponding to the output of the computer 13 to the reception tuning section 5.

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. It is provided in parallel with the receiving probe head 2 (coil) to form a parallel resonant circuit of the LC. 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,
Although the series circuit of the variable capacitance diode 51 and the capacitor 52 is connected with the anode side of the variable capacitance diode 51 being connected to the ground side, this series circuit may be connected in the opposite direction to that shown, so that the capacitor 52 side is connected to the ground side. 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. Also, the connection point between the variable capacitance diode 51 and the capacitor 52 is connected to a resistor 54.
connected to the output side of the D/A converter 14 via
A control voltage vc from the D/A converter 14 is applied. As the resistor 54, a resistor with a high resistance value is used to prevent the high frequency received MR signal from flowing into the D/A converter 6 side. Further, in the illustrated case, the signal line is directly disconnected from the D/A converter 14 by the capacitor 52. The anti-parallel diode 53 also 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 the variable capacitance diode 5
This is to prevent distortion caused by 1. 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 vc applied to the variable capacitance diode 51 is given from the D/A converter 14,
Setting of this voltage is performed as follows.

First, with the subject placed inside the transmitting/receiving probe heads 1 and 2, a high-frequency excitation pulse (90° pulse and 180° pulses) are applied to the transmitting probe head 1 (coil), and an MR signal is obtained at the receiving probe head 1. At this time, the output control voltage vc of the D/A converter 14 is initially set to the minimum value _V0 as shown in FIG. 4c. The MR signal induced in the receiving probe head 1 is amplified and detected by a preamplifier 6, phase detectors 7A, 7B, etc., and then sent to an A/D converter 12.
It is input to the computer 13 through A and 12B. The peak _value of the echo signal sampled here as shown in FIG. Next, the output control voltage vc of the D/A converter 14 is changed to V ₁ =V ₀ +ΔV (V
←V ₀ +ΔV), echo signals are collected in the same manner as described above, and the peak value thereof is set as P ₁ . Further sequentially V ₂
P ₂ is the peak value of the echo signal when the control voltage is increased as =V ₁ +ΔV,...,V _o =V _o-1 +ΔV,
..., P _o .

Here, if the first control voltage vc=V ₀ is sufficiently small, this is the control voltage (to be obtained control voltage) vc=
By successively increasing the control voltage VC, which is smaller than V _R , the resonance condition is approached, and the peak value of the echo signal monotonically increases until it reaches the resonance point. Apply the control voltage vc = V _n to calculate the peak value of the echo signal.
When P _n is obtained, this value is compared with the previous peak value P _n-1 , and as long as P _n > P _n-1 , the control voltage vc is set to ΔV
The operation of increasing is repeated until P _k <P _k-1 . The control voltage vc=V _k-1 corresponding to the peak value P _k-1 at this time is a value that provides the resonance condition of the tuning section, and is fixed. A flowchart of the above processing is shown in FIG.

Then, while collecting signals for obtaining a tomographic image by executing a pulse sequence for imaging,
The control voltage vc determined by this operation is applied to the variable capacitance diode 51 of the reception tuning section 5. In other words, the control voltage VC is fixed. Each time the subject changes, the tuning of the receiving section is always maintained by performing the above operations prior to the original signal collection. In addition, in the pulse sequence for imaging, the transmission system, the gradient magnetic field coil, and its control system are activated. As described above, in the pulse sequence for tuning, the gradient magnetic field coil and its control system are not activated, only the transmission system is activated, and 90° pulses and 180° pulses are transmitted to the subject.

In this case, since control is performed directly using the MR signal obtained from the subject, there is no need to prepare a special signal supply system for tuning control, and the MR signal can always be obtained under optimal conditions. Can be done.
Note that the present invention is not limited to the embodiments described above and shown in the drawings, but can be modified in various ways without changing the gist thereof.

For example, 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. Alternatively, the series circuit of the variable capacitance diode 51 and the capacitor 52 shown in FIG. 2 may be arranged in a direction opposite to 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 case was shown in which a cross-coil method was used in which the transmitting coil and the receiving coil were orthogonal 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 the probe head 15 is constructed.
In FIG. 3, 16 is a transmission power amplification section, 17
18 is an antiparallel diode for preventing malfunction, 18 is a variable capacitor for tuning, and 19 is an auxiliary coil.

[Effects of the Invention] According to the present invention, it is possible to provide a magnetic resonance imaging apparatus that enables automatic tuning of the 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 configuration diagram showing the main part configuration of the same embodiment, and FIG. 3 is a main part structure of another embodiment of the present invention. FIG. 4 is a timing chart of tuning control for explaining the operation of the above embodiment of the present invention, and FIG. 5 is a flowchart of tuning control. DESCRIPTION OF SYMBOLS 1...Transmission probe head, 2...Reception probe head, 3...Transmission tuning section, 4...Transmission section, 5
...Reception tuning section, 6...Preamplifier, 7A, 7B
...Phase detector, 8...Phase shifter, 9...90° phase shifter, 10A, 10B...Amplifier, 11A, 11B
...Low pass filter, 12A, 12B...A/
D converter, 13... Computer, 14... D/A converter, 15... Transmission/reception probe head, 16... Transmission power amplifier section, 17, 53... Antiparallel diode (crossing diode), 18... variable capacitor,
19... Auxiliary coil, 51... 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 receiving coil that receives the magnetic resonance signal, and a variable capacitance element that forms a resonant circuit together with the receiving coil and whose capacitance value is set according to a control voltage value applied from the outside. a tuning means, a quadrature phase detection means for quadrature phase detecting the magnetic resonance signals obtained through the reception coil and the reception tuning means, and a high frequency excitation for the subject for tuning prior to execution of the imaging pulse sequence. sequence execution means for executing a tuning pulse sequence that operates only a transmission system that transmits pulses, and a Fourier transform for the output of the quadrature phase detection means when the sequence execution means is started and the tuning pulse sequence is executed. a signal processing means that obtains the peak value of the magnetic resonance signal by performing processing; and a voltage V←V ₀ + which starts from a predetermined initial value V ₀ and adds an increase ΔV over time.
control voltage generating means for supplying ΔV as a control voltage to the reception tuning means; and control voltage generation means for supplying ΔV as a control voltage to the reception tuning means; a storage means for storing the peak values; and the tuning pulse sequence is repeatedly executed while changing the control voltage V, and the maximum value of the peak values is determined based on each peak value stored in the storage means. A magnetic resonance imaging apparatus comprising: a control means for fixing the control voltage value V at the time of the determination.