JPS6365851A - Nuclear magnetic resonance imaging apparatus - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a nuclear magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and in particular, to minimizing thermal noise etc. from sources other than the selective excitation plane of a subject. The present invention relates to a high-frequency coil suitable for use as a receiver.

[Conventional technology]

In order to improve the performance of the high-frequency coil, which is the most important factor determining the image quality of M-photography, development has proceeded in the direction of bringing it into close contact with the subject. As a result, surface coils and abdominal close-contact coils, which have a narrow field of view but high spatial resolution, have been developed.

However, there is a lower limit to the size of the coil in order to ensure a constant field of view, and the coil also plays the role of picking up thermal noise from the subject in addition to signals. Conventionally, no consideration has been given to improving this point.

[Problem that the invention seeks to solve]

The above conventional technology takes into consideration that the high-frequency receiving coil has a spread like the magnetic flux 30 shown in FIG. 4 in order to secure a constant field of view. Since the coil running near the selective excitation surface 29 is far away from the selective excitation surface 29, considering the sensitivity distribution of the coil, it will contribute to signal reception from the same surface. It is thought that the person who contributes to the picking up has a larger contribution. Therefore, it is conceivable to contract the coil 32 in the Y-axis direction to narrow the spread of the magnetic flux 30.

However, considering the selective excitation surface 31, if the coil 32 is shortened in the 9Y-axis direction, the reception sensitivity will be deteriorated by moving the coil 32 away from the signal from the selective excitation i31.

As described above, increasing the 2/N ratio of the high-frequency receiving coil and securing the field of view are contradictory to each other.

Therefore, the purpose of the present invention is to develop a method of receiving a high-frequency receiver coil of the same size as the conventional one using a partial cutout that is largely involved in signal reception from the selective excitation surface, and not picking up unnecessary thermal noise. The objective is to establish this and further improve the SIN ratio of images.

[Means for solving problems]

Means of the present invention for solving the above problems includes a static magnetic field generating magnet that generates a static magnetic field in a direction perpendicular to the body axis direction of the subject (vertical direction), and is arranged close to the subject. A solenoid-type LC resonant circuit is formed independently for each turn, and has a high-frequency coil that detects the electromagnetic waves irradiated to the subject or the electromagnetic waves emitted from the subject, and each has a variable capacitance. This is done by incorporating and being able to independently control the tuning of each turn.

[Effect]

A solenoid-type high-frequency receiving coil composed of an LC resonant circuit that can be independently controlled for each turn can be tuned for each arbitrary turn or in combination with each other. For example, in the case of a cross-sectional image, ofMl is to tune only the coil running near the selected excitation area, leave the others untuned, and set the reception state only to the coil near one selected excitation plane.

Thereby, the high frequency receiving coil only receives signals from the selective excitation plane? reception and unnecessary thermal noise from other parts of the subject can be minimized.

By changing the position of the coil having the same St in accordance with the selective excitation plane, unnecessary thermal noise can be minimized regardless of the position of the first selective excitation plane, and reception can be performed at the position closest to the signal. Therefore, by changing the position of the coil that tunes to the pulse sequence at any time during imaging, it can be applied to multi-slice (simultaneous multi-sectional imaging).

〔Example〕

Hereinafter, the present invention will be described in detail based on embodiments and the accompanying drawings.

FIG. 3 is a block diagram showing the overall configuration of a nuclear magnetic resonance imaging apparatus according to the present invention. This nuclear magnetic resonance imaging apparatus uses a nuclear magnetic resonance (NMR) phenomenon to obtain a tomographic image of a subject, and includes a static magnetic field generating magnet 10,
It consists of a central processing unit (CPU) 11, a sequencer 12, a transmission system 13, a magnetic field gradient generation system 14, a reception system 15, and a signal processing system 16. The static magnetic field generating magnet 10 generates a strong and uniform static magnetic field around the circumference υ of the subject 1 in a direction perpendicular to the body axis direction (vertical direction), and the magnet 10 generates a strong and uniform static magnetic field in the direction (vertical direction) perpendicular to the body axis direction of the subject 1. A permanent magnet type, normal conduction type, or superconducting type magnetic field generating means 4 is installed in the space.
Q: It is located. The sequencer 12 is a CPU
The transmission system 13 and the magnetic field gradient generation system 14 operate under the control of
It is also sent to the receiving system 15. The transmission system 13 includes a high-frequency oscillator 17, a modulator 18, a high-frequency amplifier 19, and a high-frequency coil 20a on the transmitting side.
The amplitude is modulated by the modulator 18 in accordance with the command of the magnetic wave, and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 19 and then supplied to the high-frequency coil 20a placed close to the subject 1. The above-mentioned subject 1 is irradiated with the light. The magnetic field gradient generation system 14 includes X, Y
.

It consists of gradient magnetic field coils 21 wound in the triaxial directions of 2 and a gradient magnetic field power supply 22 that drives each coil,
By driving the oblique magnetic field source 22 around 1 of each coil according to the command from the sequencer 12,
, z in the three axial directions of the gradient magnetic fields Gx, Gy, Qz of the object 1
It is designed to be applied to Depending on how this gradient magnetic field is applied, a slice plane for the subject 1 can be set.

The receiving system 15 includes a solenoid-type high-frequency coil 20b on the receiving side, an amplifier 23, a quadrature phase detector 24, and A/1).
the transmitting side high frequency coil 20 consisting of a converter 25;
The electromagnetic waves (NMR signals) in response to the electromagnetic waves irradiated from the object 1 are detected by the high frequency coil 20b placed close to the object 1, and are A/D converted via the amplifier 23 and the quadrature phase detector 241. The signals are input to the signal processing system 1 and converted into digital quantities, which are then sampled by the quadrature phase detector 24 at the timing according to the commands from the sequencer 12, resulting in two series of collected data.
It is set to be sent to 6.

This signal processing system 16 includes a CPU II and a magnetic disk 2.
6, a recording device such as a magnetic tape 27, and a display 28 such as a CRT.The CPU 1 performs processing such as Fourier transform, correction coefficient calculation, and image reconstruction, and displays the signal intensity distribution of an arbitrary cross section or multiple signals. The distribution obtained by performing appropriate calculations is converted into an image and displayed on the display 28. In FIG. 1, the high-frequency coils 20a, 20b and the gradient magnetic field coil 21 on the transmitting side and the receiving side are arranged in the magnetic field space of the static magnetic field generating magnet 10 arranged in the circumferential space of the subject 1. There is.

Here, in the present invention, the high frequency coil 20b on the receiving side
As shown in FIG. 1, an LC resonant circuit is formed independently for each turn. This figure shows an embodiment in which the present invention is applied to a receiving coil for the head. The coils are independent for each turn, but all have the same configuration as the valley turns.

Now, attention is paid to the coil 33 located at the top of the head of the subject l. The coil is composed of a coil conductor portion made of a non-magnetic metal having low magnetic resistance, such as a copper bud, and a tuning circuit 34. In addition, in consideration of the coupling between the coil and the high-frequency irradiation coil 20a, an insulating layer made of a material with low high-frequency loss is provided on the next surface so that the subject does not come into direct contact with the coil conductor.

One implementation of tuning circuit 34 is shown in FIG. An inductor 43, which is determined by the coil conductor, and a capacitor for fine tuning, such as a Larmor frequency (resonant frequency determined by the static magnetic field strength and the nuclide) tuning capacitor and a 37% varicap diode, which are connected in series with the inductor 43. Impedance matching capacitor 3 connected in parallel to /S 39 and inductor 43
8, and a feeding point 42.

Next, the control method and operating mechanism will be described.

Now, assume that the vicinity of the top of the head of the subject 1 is selectively excited and a horizontal 'ffr image is carved. As described above, first, only the coil 33 closest to the selective excitation plane is tuned. Tuning capacitor 3
The constants of 7 and the impedance matching capacitor 38 are determined in advance so that the zero impedance of the coil with the subject 1 inserted is, for example, a pure resistance of approximately 50Ω. However, due to individual differences in the shape and size of the test site of the subject 1, the state of synchronization may deviate. Therefore, the four variable DC sources 41 in the sequencer 112 supply the fine tuning capacitor 39 with the optimum l pressure that allows for just tuning. 1 resistance 40 is a sufficiently large value for 50Ω, for example, IO
By setting the voltage to KΩ and increasing the voltage division ratio, noise is prevented from entering the coil side.

As a method for determining the optimum voltage to be supplied to the fine tuning capacitor 39, there may be a method using the S/N ratio of the signal obtained from the supply α point 42 as a standard. In order to minimize the coupling between the coil 33 and the coils of other turns, the coils of the turns are not adjusted by the fine tuning adjustment capacitor 39, but are intentionally kept out of tune.

The signal obtained from the feed point 42 is amplified by the preamplifier 35. Nine signals obtained from sources other than the coil 33 are similarly amplified by each preamplifier. Therefore, a switch 36 is provided to select only the signal from the coil 33, and only the signal from the preamplifier 35 is connected and sent to the quadrature phase detector 24.

The above-mentioned determination of the optimum voltage of the fine tuning capacitor 39 and control of switching of the switch 36 are all performed by the sequencer 12. By selecting the position of the coil for tuning depending on the position of the imaging plane, it is possible to image the horizontal 'ljT plane at any position in exactly the same way as described above.

The sequencer 12 for each coil before starting imaging.
If the optimum voltage to be supplied to the fine tuning capacitor 39 is stored in advance in the memory within the sequencer 12, it becomes possible to switch the coils in m f& for each main in synchronization with the pulse sequence.

This allows application to multi-slices (simultaneous multi-sectional imaging of one image).

As described above, the present invention is particularly effective for cross-sectional images, but considering the gain and phase of the amplification No. 1J from each preamplifier, and adding the output signals, the sagittal plane, coronal plane It can also be applied to cross-sectional imaging.

In this example, in order to maximize the S/N ratio, each coil is provided with an independent preamplifier, but with only one preamplifier,
Other implementation methods are also conceivable, such as a method of switching the coils, or a method of connecting each coil and selecting a desired coil using a regulating capacitor 39. The intention of book #5 is to select the coil with the maximum S/N ratio on the imaging surface, and it is intended that other embodiments that meet this intention are also included in the fourth aspect of the present invention. [Effects of the Invention] As explained above, the present invention separates the solenoid-type high-frequency coil for each turn to detect the electromagnetic force emitted from the subject l, and incorporates a controllable variable capacitor in each turn. Because of this configuration, it is possible to tune at any turn of the coil) or only at the turns near the selective excitation surface.

As a result, the high-frequency receiving coil can receive the signal at the coil closest to the selective excitation surface, which has the effect of minimizing reception of unnecessary thermal noise from the other surface.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an embodiment of the high-frequency coil according to the present invention, FIG. 2 is an explanatory diagram showing a tuning circuit of the high-frequency coil, and FIG. 3 is 4i: Overall configuration of the MRI apparatus according to the invention. FIG. 4 is an explanatory diagram showing problems with solenoid-type high-frequency coils in conventional MRI apparatuses. 1... Subject, 10... Static magnetic field generating magnet, 11...
・Central processing unit (CPU), 12.../-kensa, 1
3... Transmission system, 14... Magnetic field gradient generation system, 15...
- Receiving system, 16... Signal control system, 20a... High frequency coil on the transmitting side, 20b... High frequency coil on the receiving side,
29.31...Selective excitation surface, 30...Magnetic flux, 32.
33... Coil, 34... Tuning circuit, 35... Preamplifier, 36... Switch, 37... Tuning capacitor, 38... Impedance matching capacitor,
39... Capacitor for fine tuning, 40... Resistor, 4
1...DC variable power supply, 42...1 Dr point, 43...
・Inductor.

Claims (1)

[Claims] 1. It has a static magnetic field generating magnet that generates a static magnetic field in a direction perpendicular to the body axis of the subject (vertical direction) and is placed close to the subject to irradiate the subject with electromagnetic waves, or In a nuclear magnetic resonance imaging system that has a high-frequency coil that detects electromagnetic waves emitted from A nuclear magnetic resonance imaging device characterized by independently controlling capacitance.