JP3007383B2 - Magnetic resonance imaging equipment - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-frequency receiving coil for a magnetic resonance imaging apparatus for imaging a desired portion of a subject using magnetic resonance.

[Conventional technology]

In a magnetic resonance imaging apparatus (hereinafter, referred to as an MRI apparatus), an nucleus is irradiated with high frequency to excite it, and a high frequency signal emitted from the resonated nucleus (this is called an NMR signal) is detected. A coil constituting a resonance circuit is usually used for radiating and detecting a high-frequency signal, and the detection sensitivity of the receiving coil is an important factor in determining an S / N ratio of an image to be obtained. Various coils modified from them have been considered.

In general, a receiving coil having a smaller coil diameter and a smaller sensitivity area has higher sensitivity. Using this feature
For the MRI apparatus, a local receiving coil for imaging a local site of a subject with high sensitivity has been developed.

In addition, since such a receiving coil has a narrow sensitivity area,
It must be used in close contact with the target site.

[Problems to be solved by the invention]

Many human bodies have two identical organs or two organs at symmetrical positions, such as an orbit, a temporomandibular joint, and a limb joint. In addition, since these can obtain a high-quality image by using the local receiving coil, they are also high-frequency imaging target sites in the MRI apparatus. When there are lesions at these sites, there is a clinical need to take images of two symmetrical sites and compare them with the normal side. However, in the local receiving coil according to the related art, since the sensitivity region exists only in a narrow range, it is difficult to image two locations at the same time. There is a problem that once comes.

In order to solve this problem, it is conceivable to simultaneously image a symmetrical part using two local receiving coils. In this case, the S / N ratio is about 30 compared to a case where only one local receiving coil is used. % Decrease. This will be described in detail.

FIG. 3 is a circuit diagram of the receiving coil 1. A capacitor 3 for tuning to a signal frequency is connected to a coil 2 which is a conductor loop close to the subject. A preamplifier is connected to the output of the receiving coil 1, and its input resistance is indicated by a resistor 35. In the resonance state, the receiving coil circuit looks like a pure resistance from the viewpoint of the preamplifier. However, the resistance value is obtained by adding a loss caused by the mounting to the subject to the resistance component of the receiving coil element in the resonance state. It is known that the noise component generated by the receiving coil is proportional to the square root of the resistance value.

Here, suppose that simultaneous imaging is performed using two receiving coils shown in FIG. As shown in FIG. 4, the connection includes a parallel connection (A) and a series connection (B).
In (A), the resistances 36a and b at resonance (assuming that both have the same value)
Is halved, so the noise voltage is Becomes On the other hand, the signal voltage is reduced by half because the signal source resistance is reduced by half, and as a result, the S / N ratio is reduced. That is, it is reduced by about 30%. Similarly, in the case of (B), the result is reduced by about 30%.

An object of the present invention is to provide a high-frequency receiving coil for an MRI apparatus that solves the above-mentioned problems of the local receiving coil according to the related art.

[Means for solving the problem]

The above problem can be solved by switching and using the outputs from the two local receiving coils.

In an MRI apparatus, when imaging two or more cross sections simultaneously,
The signal measurement is not performed at the same time, but is performed sequentially at different timings, which is called multi-slice imaging. For example, when imaging a pair of right and left organs at the same time, first receive a signal from the left organ, receive a signal from the right organ after a certain time, and further receive a signal from the left organ again after a certain time. In this way, image data is collected and imaged at the same time. Assuming that the local receiving coil is attached to each of the left and right organs, only one of the local receiving coils is actually receiving a signal at a certain point during signal measurement,
The other receiving coil is only a noise source. Therefore,
If the outputs from the two local receiving coils are switched and used in synchronization with the timing of data collection, the above-described problem is solved, and an image having a good SN ratio equivalent to that of a single local receiving coil is obtained. It becomes possible.

At this time, the problem is that the receiving coil that is not performing signal reception is a resonance system, which affects the receiving coil during reception and becomes a load. Therefore, if the output of the receiving coil is switched as it is, the reduction of the SN ratio cannot be improved.

To solve this problem, a variable capacitance element that can be controlled from the outside is connected to each local receiving coil, and this is controlled in synchronization with the switching of the receiving coil, thereby controlling the tuning state and receiving signals. The receiving coil that is not used is shifted the resonance point greatly. As a result,
The other receiving coil does not become a load, and the problem of lowering the SN ratio can be solved.

[Action]

According to the invention, two local receiving coils according to the prior art are used.
In multi-slice imaging using a single receiver coil, the signal-to-noise ratio was reduced by combining output signals, and the problem of reduced sensitivity caused by coil interference was resolved. And a high quality image can be obtained.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 5 is a configuration diagram showing an example of the overall configuration of the MRI apparatus according to the present invention. This MRI apparatus obtains a tomographic image of the subject 6 by utilizing a nuclear magnetic resonance (NMR) phenomenon, and includes a static magnetic field generating magnet 10, a central processing unit (hereinafter referred to as a CPU) 11, a sequencer 12, System 13, magnetic field gradient generating system 14, receiving system
15 and a signal processing system 16. The static magnetic field generating magnet 10
Is for generating a strong and uniform static magnetic field around the subject 6, and a permanent magnet type, a normal conduction type, or a superconducting type magnetic field generating means is disposed in a certain space around the subject 6. I have. The sequencer 12 has a CPU 11
And sends various commands necessary for data collection of tomographic images of the subject 6 to the transmission system 13, the magnetic field gradient generation system 14, and the reception system 15. The transmission system 13 includes a high-frequency generator 17, a modulator 18, a high-frequency amplifier 19, and a high-frequency coil 20 on the transmission side, and transmits a high-frequency pulse output from the high-frequency transmitter 17 to the modulator in accordance with a command of the sequencer 12. The amplitude is modulated by 18 and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 19 and then supplied to the high-frequency coil 20 arranged close to the subject 6 so that the subject is irradiated with the electromagnetic wave. It has become. The above magnetic field gradient generation system
14 is a gradient magnetic field coil 21 wound in three directions of X, Y, Z
And a gradient magnetic field power supply 22 for driving each coil. By driving the gradient magnetic field power supply 22 for each coil in accordance with an instruction from the sequencer 12, X, Y, Z
The gradient magnetic fields Gx, Gy, Gz in the three axial directions are applied to the subject 6. The slice plane for the subject 6 can be set by the method of applying the gradient magnetic field. The receiving system 15 includes a receiving coil 1, a preamplifier 23, a quadrature phase detector 24, and an A / D converter 25. NMR signal) is detected by the receiving coil 1 arranged close to the subject 6, and the preamplifier 23 and the quadrature phase detector
The data is input to an A / D converter 25 via a digital converter 24, is converted into a digital value, and is further collected by a quadrature phase detector 24 at a timing according to a command from the sequencer 12. It is sent to the processing system 16. The signal processing system 16 includes a CPU 11, a recording device such as a magnetic disk 26 and a magnetic tape 27, and a display 28 such as a CRT. A distribution obtained by performing signal intensity analysis of an arbitrary cross section or performing an appropriate operation on a plurality of signals is imaged and displayed on the display 28. In the figure, the high-frequency coil 20 on the transmitting side, the receiving coil 1 and the gradient coil 21
Is a static magnetic field generating magnet arranged in the space around the subject 6
It is located in 10 magnetic field spaces.

Here, the receiving coil 1 according to the present invention comprises local receiving coils 1a and 1b as shown in FIG. 1, and comprises a conductor loop 2, a capacitor 3 and a variable capacitance diode 4, respectively. Each of the local receiving coils 1a and 1b is connected to a preamplifier 23. The preamplifier 23 is composed of a switching switch 5 and a high frequency amplifier 7, and has a signal output 30, a switch control input 31,
A ground terminal 33, a tuning voltage input 32 for applying a tuning voltage to the variable capacitance diode 4, and a high resistance 8 for preventing the circuit from affecting received signals are provided.

It is necessary to optimize the size and shape of the conductor loop 2 of the local receiving coils 1a and 1b according to the site or organ to be imaged. As an example, imaging of a temporomandibular joint will be described. The temporomandibular joint is a pair of right and left joints at the front of the ear canal, as shown in FIGS. 37a and 37b.
The usefulness of MRI is recognized, and it is a site with many imaging requirements.
At the time of imaging of the temporomandibular joint, the local receiving coils 1a and 1b are set on the head 6 of the subject as shown in FIG. As the local receiving coils 1a and 1b, a solenoid coil having a diameter of about 10 cm can be considered. With such a small-diameter receiving coil, high sensitivity can be expected instead of narrowing the sensitivity distribution. When the right and left temporomandibular joints are imaged simultaneously by multi-slice imaging, data measurement is performed not at the same time but at different timings. The data of the site of interest 37a is collected, and after a certain period of time, the data of the site of interest 37b is collected. At this time, only the local receiving coil 1a is involved in the data measurement of the attention site 37a, and the local receiving coil 1b is completely unnecessary. Therefore, the switching switch 5 in FIG. 1 switches this output. Therefore,
The switching switch 5 must switch to the output of the required local receiving coil 1a or 1b in synchronization with the data measurement.
The control signal output from the sequencer 12 in FIG. 5 is connected to a switch control input 31 to control the switching timing. The switching switch 5 needs to switch a weak high-frequency signal at a high speed of several milliseconds. Alternatively, two preamplifiers 7 may be used, and thereafter, the switching switch 5 may be inserted.

The local receiving coils 1a and 1b are tuned to the NMR signal frequency by the resonance capacitance obtained by adding the inductance of the conductor loop 2 and the capacitance of the capacitor 3 to the capacitance of the variable capacitance diode 4. If they are arranged in such a manner, they interfere with each other and become a load. For this reason, simply switching the output is not sufficient, and means for removing the tuning of the unused receiving coil so as not to become a load is required. For this, a variable capacitance diode 4 having a variable capacitance sufficiently larger than the capacitance of the capacitor 3 is used, and the tuning voltage from the tuning voltage input 32 is switched by the switch 5 as shown in FIG. By switching at the same time, tuning and non-tuning are controlled.

FIG. 2 shows an example of the appearance of the local receiving coils 1a and 1b for the temporomandibular joint. Preamplifier containing parts such as switching switch 5
The reference numeral 23 is provided at a place sufficiently away from the imaging space by the high-frequency cable 9 so as not to be affected by a strong static magnetic field and not to reduce the uniformity of the static magnetic field. The preamplifier 23 is connected to an external device via a multi-pin connector 34.

The local receiving coils 1a and 1b according to the present invention can be applied not only to the temporomandibular joint but also to other parts in a similar manner. For example, there are an orbit (see FIG. 7), a shoulder joint (see FIG. 8), a knee joint (see FIG. 9), an internal organ such as a kidney (see FIG. 10), and a breast (see FIG. 11). By using the local receiving coils 1a and 1b having the optimum shapes for these attention parts 37a and 37b, two parts can be simultaneously imaged with a high SN ratio.

〔The invention's effect〕

As described above, the present invention, in the local imaging of a human body symmetric organ by the MRI apparatus, without lowering the SN ratio,
Since it is possible to image two sites at the same time,
There is an effect that high quality imaging or reduction of imaging time can be achieved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing the configuration of a receiving coil according to the present invention,
FIG. 2 is an external view showing a configuration example of the receiving coil, FIG. 3 is an explanatory diagram showing a basic circuit of the receiving coil, FIG. 4 is an explanatory diagram showing a method of combining the receiving coil, and FIG. FIG. 6 is a view showing the arrangement of receiving coils when imaging the temporomandibular joint, FIG. 7 is a view showing the arrangement of receiving coils during imaging of the orbital joint, and FIG. 8 is a view showing the arrangement of receiving coils when imaging the shoulder joint. FIG. 9, FIG. 9 is a layout diagram of the receiving coil at the time of knee joint imaging, FIG. 10 is a layout diagram of the receiving coil at the time of kidney imaging, and FIG. 11 is a layout diagram of the receiving coil at the time of breast imaging. 1a, b ... local receiving coil, 2 ... conductor loop, 3 ...
Capacitor, 4 variable capacitance diode, 5 switching switch, 6 subject, 7 high frequency amplifier, 8
... High resistance, 9 ... High frequency cable, 23 ... Preamplifier,
30 ... Signal output, 31 ... Switch control input, 32 ... Tuning voltage input, 33 ... Ground terminal, 34 ... Connector, 35 ... Input resistance, 36a, b ... Resonance at resonance, 37a, b ... … Attention site.

Claims (2)

(57) [Claims] A magnetic field generating means for irradiating an electromagnetic wave to a test object; a tuning circuit for tuning a frequency of a magnetic resonance signal from the test object; High-frequency coil to be detected, image reconstruction means for obtaining an image representing the physical properties of the target object using the detection signal, and generation of each magnetic field, irradiation of electromagnetic waves,
In a magnetic resonance imaging apparatus including a control unit for controlling timing for detecting a magnetic resonance signal or the like and an image reconstructing unit, a high-frequency coil for detecting the magnetic resonance signal may be used as an object to be inspected at the same site existing at a symmetric position. And means for simultaneously switching the output from each of the high-frequency coils and the voltage input to the tuning circuit, wherein the control means alternately detects magnetic resonance signals from the two test objects and simultaneously obtains an image. A magnetic resonance imaging apparatus, which controls the switching means and switches the switching means at a magnetic resonance signal detection timing. 2. The two high-frequency coils each constitute a resonance circuit using a variable capacitance element that can be controlled externally, and the control means varies the capacitance of the variable capacitance element at a magnetic resonance signal detection timing. A magnetic resonance imaging apparatus comprising: