JPH03109041A - Receiving coil for nuclear magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To uniformize a sensitivity distribution by arranging a high frequency receiving coil to comprise three coils disposed perpendicular to one another so that information established is obtained simultaneously from each thereof. CONSTITUTION:Solenoid type coils 3 are combined above orthogonal surface coils 2a and 2b in such a manner as to compensate a low sensitivity part 33. In such arrangement, the three coils have sensitivity at about 90 deg. to one another to be orthogonal so that independent signals can be detected simultaneously from the three coils. Then, outputs of surface coils 2a and 2b are connected to input terminals (a) and (b) of a receiving circuit 23 and when outputs of the coils 3 are connected to an input terminal (c), output levels of the surface coils 2a and 2b give almost equal values. As a phase difference becomes about 90 deg.. A phase shifter 30a is set to 0 deg. and the one 30b is set to about +90 deg. or about -90 deg.. An attenuators 31a and 31b are both set to -0dB. In contrast, the outputs of the coils 3 are adjusted to such a phase correction value as to make an image pickup part the best according thereto automatically or manually. Attenuation value of the attenuator 31c is adjusted to meet optimum adjusting conditions according to a sensitivity difference between the coils 3. This enables a higher sensitivity with the uniformization of a sensitivity distribution of the orthogonal coils.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a high-frequency receiving coil for a nuclear magnetic resonance imaging apparatus that images a desired location of a subject using nuclear magnetic resonance.

[Conventional technology]

A nuclear magnetic resonance imaging device excites atomic nuclei by irradiating them with high frequency waves, and detects high frequency signals (called NMR signals) emitted from the resonated atomic nuclei. Coils are usually used to irradiate and detect high-frequency signals; saddle-type,
Solenoid type coils and various modified coils have been considered.

Since the irradiation and detection are performed in different time periods, both are
A method is also known in which one coil is used for both purposes.

However, nuclear magnetic resonance imaging equipment that targets a single human body uses a relatively large irradiation coil to achieve spatially wide and varied irradiation and high sensitivity (detection with a good signal-to-noise ratio).
Relatively small detection coils placed close to the human body are often used. If the irradiation coil is placed in the same direction as the detection coil, the two will be coupled at high frequencies and the detection sensitivity will be reduced, so they are usually placed on orthogonal axes.

Incidentally, since the sensitivity of the detection coil directly affects the S/N ratio of a reconstructed image, many studies and improvements have been made to the detection coil. The excited spins.

Because small magnetic pole pieces behave as if they are rotating on the same plane, an orthogonal coil system has been proposed that uses two orthogonal coil systems to detect this problem. J53-324
-327.1983 C, N, CIIEN et al.)
In principle, the amount of signal is doubled, and since it is random noise, the amount of noise is 47 times greater, and as a result, it is said that the S/N ratio increases to 5 times. However, this is only possible if the sensitivities of the two coils to be combined are equal; if the sensitivities are different, the amount of improvement in the S/N ratio will be less than 4Σ times, and there is an addition condition for obtaining the maximum improvement. In order to satisfy this addition condition, the phase difference and level difference between the two coil outputs must be corrected and added (Japanese Patent Application Laid-Open No. 64-1999-
17636).

[Problem to be solved by the invention]

In the orthogonal coils according to the prior art, it is desirable to use two coils of the same shape from the viewpoint of the amount of improvement in the S/N ratio. However, in this case, since the sensitivity distributions of the two coils are equal, if the two coils are used as orthogonal coils, the high sensitivity areas will be higher and the low sensitivity areas will be relatively lower, resulting in a decrease in the uniformity of the sensitivity distribution. Put it away.

This problem occurs more prominently in a surface coil whose sensitivity is partially increased in order to perform local imaging of a subject.

SUMMARY OF THE INVENTION An object of the present invention is to provide a high-frequency receiving coil for a nuclear magnetic resonance imaging apparatus, which solves the problem of the prior art that the uniformity of the sensitivity distribution of the orthogonal coil decreases.

[Means to solve the problem]

The above-mentioned problem is caused by the fact that there is a correlation between the sensitivity distributions of the two coils constituting the conventional orthogonal coil, and the non-uniformity of the sensitivity is emphasized. Therefore, the present invention combines a conventional orthogonal coil with a receiving coil having a different sensitivity distribution, and arranges the coil so that the sensitivity of this coil compensates for the low sensitivity part of the orthogonal coil, thereby maintaining uniformity of sensitivity over a wide range. It was configured so that

At this time, the receiving coil that is added to the conventional orthogonal coil must be arranged orthogonal to the two coils that make up the orthogonal coil in order to simultaneously receive independent signals. If the orthogonality is not achieved, it will couple with the orthogonal coil and reduce its sensitivity. Therefore, if the two sensitivity directions of the orthogonal coils are the X-axis and Y-axis, the sensitivity direction of the receiving coil to be added must be the Z-axis.

A principle diagram of the improved orthogonal coil according to the present invention is shown in FIG. If the direction of the static magnetic field is the Z-axis direction, the plane of rotation of the spin is
-Y plane, and the conventional orthogonal coil is the receiving coil la (
This is a combination of a receiving coil lb (sensitivity direction X-axis) and a receiving coil lb (sensitivity direction Y-axis). A receiving coil IC having sensitivity in a direction perpendicular to these (sensitivity direction Z-axis) is arranged as shown in the figure.

The receiving circuit 23 includes preamplifiers 5a, 5b, and 5c connected to the respective coils, and a combining circuit 6, and outputs a combined signal to an output terminal 7.

This is because the plane of rotation of spin is the X-Y plane.

The receiving coil 1c having sensitivity in the Z-axis direction has a sensitivity distribution different from that of a normal solenoid coil.

Although the center of the coil has no sensitivity, it is possible to capture flux from spins near the coil, and as a result, the sensitivity distribution of this receiving coil becomes torus-shaped. In the orthogonal coil made up of the receiving coil 1a and the receiving coil 1b, the part where the coils intersect (upper and lower in the figure) has the highest sensitivity, and the center and the vicinity of the coil have the next highest sensitivity. Therefore, the sensitivity decreases relatively at the four locations far from the coil conductor on the X-Y plane at the center of the coil.
This results in an uneven sensitivity distribution. Therefore, by arranging the third orthogonal receiving coil 1c as shown in the figure, it is possible to compensate for the sensitivity of this part.

The number of received signals obtained independently from the three receiving coils is 1.
The signals received by the coils 1a and 1b, which are conventional orthogonal coils, are combined by the combining circuit 6, but at this time, the signals received by the coils 1a and 1b, which are conventional orthogonal coils, are detected at the same level with a phase difference of about 90 degrees.
This can be corrected by a phase shifter and added, but the received signal from the coil IC does not have a fixed phase difference, and the detection level has a different sensitivity distribution, so it is different from the other two coils. It will be different. Therefore, as mentioned above,
There is an optimal addition condition depending on the imaged region, and the synthesis circuit 6 must be able to adjust to an arbitrary phase difference correction amount and synthesis ratio in order to realize this.

In addition, each receiving coil must be tuned so that it resonates with the number of cycles of the nuclear magnetic resonance signal, and for this adjustment, the combining circuit 6 is configured so that the receiving output from each coil can be output independently. functions are also necessary.

(Operation) According to the present invention, it is possible to improve the problem of non-uniform sensitivity distribution of orthogonal coils in conventional nuclear magnetic resonance imaging apparatuses and improve the S/N ratio, thereby producing high-quality images. Obtainable.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 5 is a configuration diagram showing an example of the overall configuration of a nuclear magnetic resonance imaging apparatus according to the present invention. This nuclear magnetic resonance imaging apparatus obtains a tomographic image of the subject 9 using the nuclear magnetic resonance (NMR) phenomenon.

It consists of a static magnetic field generating magnet 10, a central processing unit (hereinafter referred to as CPU) 11, a sequencer 12, a transmitting system 13, a magnetic field gradient generating system 14, a receiving 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 subject 9. The magnet 10 generates a strong and uniform static magnetic field around the subject 9, and applies a permanent magnet type, normal conductivity type, or superconducting type to a certain expanse of space around the subject 9. A magnetic field generating means is arranged. The sequencer 12 operates under the control of the CPU II, and sends various commands necessary for data collection of tomographic images of the subject 9 to the transmitting system 13, the magnetic field gradient generating system 14, and the receiving system 1.
5. 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 transmitting side. The subject 9 is irradiated with electromagnetic waves by amplifying the high-frequency pulses in a high-frequency amplifier 19 and then supplying the high-frequency pulses to a high-frequency coil 20 placed close to the subject 9 . The magnetic field gradient generation system 14 is composed of gradient magnetic field coils 21 wound in the three axes directions of x, y, and z, and a gradient magnetic field type 122 that drives each coil.
By driving the gradient magnetic field power supply 22 of each coil in accordance with the command from 2, gradient magnetic fields Gx, Gy, and Qz in three axial directions of X, Y, and Z are applied to the subject 9. A slice plane for the subject 9 can be set by applying this gradient magnetic field. Above receiving system 1
5 consists of a high-frequency coil 1 on the receiving side, a receiving circuit 23, a quadrature phase detector 24, and an A/D converter 25, and the electromagnetic wave of the response of the subject 9 due to the electromagnetic wave irradiated from the high-frequency coil 20 on the transmitting side. (NMR signal) is detected by the high frequency coil 1 placed close to the subject 9, and is detected by the receiving circuit 23.
and is inputted to the A/D converter 25 via the quadrature phase detector 24 and converted into a digital quantity, and further made into two sets of collected data sampled by the quadrature phase detector 24 according to a timing instruction from the sequencer 12. , the signal is sent to a signal processing system 16. This signal processing system 16 includes a CPU], 1, and a data memory 8. It consists of a recording device such as a magnetic disk 26 and a magnetic tape 27, and a display 28 such as a CRT.
Processing such as Fourier transformation, correction coefficient calculation 1 image reconstruction, etc. is performed using PUII, and the signal intensity distribution of an arbitrary cross section or the distribution obtained by performing appropriate calculations on multiple signals is converted into an image and displayed on the display 28. It has become. In addition, in FIG.

The high-frequency coils 20 and 1 and the gradient magnetic field coil 21 on the transmitting side and the receiving side are arranged in a magnetic field space of a static magnetic field generating magnet 10 arranged in a space around the subject 9.

Here, one embodiment of the high-frequency coil 1 according to the present invention will be described using a surface coil in a vertical magnetic field type nuclear magnetic resonance imaging apparatus as an example.

In the vertical magnetic field method, as shown in FIG. 6, static magnetic field generating magnets 10 are arranged in the vertical direction of the subject 9, and a static magnetic field is applied in the direction. Then, as shown in Figure 7, the direction of the static magnetic field (
Since the spin S rotates on the XY plane with respect to the Z-axis direction, the nuclear magnetic resonance signal is detected in the direction B or C in FIG.

Usually, a solenoid type coil with good performance is used as the receiving coil 1 to detect the signal in the direction B. Subject 9
When imaging the head or abdomen of a subject, there is no problem with this type of receiving coil, but there is a demand for high-quality imaging of parts of the subject's back, such as the vertebrae, and for this purpose, it is necessary to locally increase the sensitivity. surface coils have been developed. FIG. 8 is a diagram showing the principle of a surface coil 2 for a vertical magnetic field type nuclear magnetic resonance imaging apparatus described in Japanese Patent Application Laid-Open No. 63-153055.

By winding the coil in this shape, it can be attached to the back of the subject to capture the flux from the spin S and detect the signal. The subject lies supine on this coil and images are taken.

As shown in the figure below, the sensitivity distribution of this coil is highest at point a at the center, and the sensitivity is zero at point a at the center of the opposing conductor. This zero point of sensitivity exists in a plane perpendicular to the longitudinal direction of the coil.

FIG. 9 shows a case where the circular solenoid coil 3 is similarly used as a surface coil for vertical magnetic field.

The sensitivity direction of the coil as a whole is vertical and coincides with the direction of the static magnetic field, so there is almost no sensitivity in a wide range at the center of the coil. However, in the vicinity of the coil conductor, flux from some spins can be captured, resulting in a toroidal sensitivity distribution along the coil conductor as shown below.

FIG. 10 shows the vertical magnetic field surface coil 2a shown in FIG.
and 2b in an orthogonal state (described in Utility Model Application No. 01-49500). In the diagram, the two coils are separated for clarity, but
Actually, these two coils are in close contact with each other.

By configuring the surface coil as an orthogonal coil in this way, the sensitivity at the center can be improved, but the zero-sensitivity portion (point b) shown in FIG. 8 overlaps, and the coil Low-sensitivity areas 33 are generated at four locations around the area. For this reason, this orthogonal coil has a markedly non-uniform sensitivity, and although a good image can be obtained in the central part where the coil conductor is located, the peripheral part is influenced by the low-sensitivity part 33, resulting in a non-uniform image. Therefore, a problem arises in that the installation on the subject 9 must be optimally positioned depending on the region to be imaged.

It is ideal to have high sensitivity over a wide range, and the improved perpendicular magnetic field orthogonal surface coil 1 of the present invention solves this problem.
The principle diagram of this is shown in Fig. 2. Conventional orthogonal surface coil 2a,
2b is combined with the solenoid type coil 3 shown in FIG. 9 in a shape that compensates for the low sensitivity part 33 (3
two coils are in close contact). The sensitivity directions of these three coils are at about 90 degrees to each other and can be orthogonal. As a result, it becomes possible to simultaneously detect independent signals from the three coils.

The detection output of the improved orthogonal surface coil 1 is connected to a receiving circuit 23 shown in FIG. The receiving circuit 23 is the preamplifier 5
a, 5b, 5c, and a phase shifter 3 for correcting the phase difference.
0a, 30b, 30c and.

It consists of attenuators 31a, 31b, 31c for adjusting the output level difference to the above-mentioned optimal addition conditions, and an adder 32 for adding these three outputs, and an output terminal 7.
Output to.

When the outputs of surface coils 2a and 2b are connected to input terminals a and b, and the output of solenoid type coil 3 is connected to input terminal C, since surface coils 2a and 2b have the same shape, the sensitivity is equal and the output level is The values are almost equal. Also, since the phase difference is about 90 degrees, the phase shifter 30a is set to 0 degrees, and the phase shifter 30b is set to about +90 degrees or -90 degrees.
By setting both the attenuators 31a and 31b to 10 dB, the optimum addition condition is achieved. On the other hand, since the output of the solenoid coil 3 does not have a specific phase difference compared to the other two coils, the phase correction amount of the phase shifter 30c cannot be set to a constant value. For this reason, depending on the imaged region, the amount of phase correction is automatically or manually adjusted using software so that the amount of phase correction is optimal for that region. Further, the amount of attenuation of the attenuator 31c is adjusted according to the sensitivity difference between the solenoid coils 3 so as to obtain the optimum addition condition.

In this way, it is possible to control the sensitivity distribution of the surface coil depending on the imaging region so that the sensitivity of that region is the highest. However, it is not possible to make the sensitivity uniform over a wide range. Therefore, as shown in FIG. 4, the receiving circuit 23 is separated into three systems (5a + 5b, 5c), the quadrature phase detector 24 and the A/D converter 25 are separated, and the data memory inside the signal processing system 16 is separated. By storing the detected data in 8 and composing them on the image during image reconstruction, there is no problem with the phase difference or level difference of the received signals, and it is possible to synthesize signals that make the most effective use of the amount of information. . In this case, the solenoid-type coil 3 compensates for the four low-sensitivity portions 33 that exist in the conventional orthogonal surface coil, so it is possible to obtain a surface coil with excellent uniformity of sensitivity.

〔Effect of the invention〕

As described above, the present invention equalizes the sensitivity distribution of the conventional orthogonal coil in a nuclear magnetic resonance imaging system, improves the sensitivity, and also makes it possible to change the sensitivity region. It is possible to solve problems caused by non-uniform sensitivity distribution, and there is an effect that a high quality image with a high SN ratio can be obtained.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram showing the principle of an improved orthogonal surface coil according to the present invention, Fig. 2 is an explanatory diagram showing an embodiment of the orthogonal surface coil according to the present invention, and Fig. 3 is a configuration of a receiving circuit accompanying the orthogonal surface coil. Fig. 4 is a block diagram of a separate receiving system with orthogonal surface coils, Fig. 5 is a block diagram showing the overall structure of the nuclear magnetic resonance imaging apparatus according to the present invention, and Fig. 6 is a block diagram of a reception system using a vertical magnetic field method. FIG. 7 is an explanatory diagram showing the spin rotation surface. FIG. 8 is an explanatory diagram showing a surface coil for vertical magnetic field. FIG. 9 is an explanatory diagram showing a solenoid type surface coil for vertical magnetic field. FIG. 11 is an explanatory diagram showing a conventional orthogonal surface coil, and FIG. 11 is an explanatory diagram showing a low sensitivity portion in the conventional orthogonal surface coil. 1... Receiving coil, 2... Surface coil for vertical magnetic field,
3... Solenoid type coil, 5... Preamplifier, 6
. . . Synthesis circuit, 7. Output terminal, 8. Data memory, 9. Test object, 10. Static magnetic field generating magnet, 23.
... Receiving circuit, 24... Quadrature phase detector, 25...
・A/D converter. 30... Phase shifter, 31... Attenuator, 32... Adder, 33... Low sensitivity part. Brown ■ Figure 3 #2 Figure 3 Figure 3 Tea 4 Figure Shi-Y= Wame Sakai Sword Figure 10 [D Certain 11 Trouble 3 Figure 9

Claims (1)

[Claims] 1. Magnetic field generating means for a static magnetic field and a gradient magnetic field, a high-frequency receiving coil for irradiating electromagnetic waves onto an object to be examined and detecting a nuclear magnetic resonance signal from the object to be examined, and using the detection signal In the nuclear magnetic resonance imaging apparatus, the high-frequency coil for detecting the nuclear magnetic resonance signal is composed of three coils arranged orthogonally. , a receiving coil for a nuclear magnetic resonance imaging apparatus, characterized in that it is configured to simultaneously obtain information established from each of them. 2. The receiving coil for a nuclear magnetic resonance imaging apparatus according to claim 1, wherein each of the three nuclear magnetic resonance signal receiving coils can be used independently by switching a switch. 3. The receiving coil for a nuclear magnetic resonance imaging apparatus according to claim 1, further comprising means for correcting the phase difference and level difference of the outputs from the three nuclear magnetic resonance signal receiving coils and adding them together. 4. Nuclear magnetism according to claims 1 and 3, wherein when adding the outputs from the three nuclear magnetic resonance signal receiving coils, the optimum addition conditions can be set depending on the imaging region. Receiving coil for resonance imaging equipment. 5. Detecting the outputs from the three nuclear magnetic resonance signal receiving coils with independent receiving systems and storing the data;
2. A receiving coil for a nuclear magnetic resonance imaging apparatus according to claim 1, further comprising means for synthesizing data in an image reconstruction step.