JPH0237769B2 - - Google Patents

Description

[Detailed Description of the Invention] "Industrial Application Field" This invention applies high-frequency electromagnetic waves to a test specimen placed in a static magnetic field as a component of a nuclear magnetic resonance apparatus, particularly a nuclear magnetic resonance imaging (image forming) apparatus. The present invention relates to a high-frequency coil for nuclear magnetic resonance used to excite a nuclear magnetic resonance phenomenon or to detect a signal generated by the nuclear magnetic resonance phenomenon.

``Prior Art'' A saddle-shaped coil is known as this type of high-frequency coil used in nuclear magnetic resonance equipment (P.
Mansfield et al. Nuclear Magnetic Resondence in
Biomedicine 263−271 Academic Press 1982).
This saddle-shaped coil has conductive wire wound around it to form two saddle-shaped shapes surrounding the sample to be inspected, and can generate a magnetic field parallel to the direction in which the two saddle-shaped shapes are arranged. This is a coil that makes it possible to apply high-frequency electromagnetic waves in a direction perpendicular to a test specimen whose longitudinal direction is parallel to a static magnetic field, and is useful for imaging test specimens such as the human body. It is often used.

Since there is a large distributed capacitance between the test specimen surrounded by the saddle-shaped coil and the conductor of the coil itself, the dielectric loss is proportional to the cube of the frequency of the high-frequency electromagnetic wave applied to the sample. (DI. Hoult et al. Journal of Magnetic
Resonance, Vol. 34, pp. 425-433, 1979), and due to this dielectric loss, the Q value of the saddle-shaped coil decreases significantly, and in other words, the resonance performance decreases (DGGardian et al.
Journal of Magnetic Resonance Volume 34 449-455
p. 1979).

In order to reduce such dielectric loss, the University of Utah's D.
Alderman et al. conducted chemical analysis using high-frequency electromagnetic waves with a frequency of 300 MHz, using a pair of conductive rings having the same axis and symmetrically arranged with respect to a plane perpendicular to this axis. A high-frequency coil that electrically shields a sample, has a pair of linear conductors arranged parallel and symmetrically with respect to the axis, and has both ends of each linear conductor capacitively coupled to the pair of conductor rings. (Journal of Magnetic Resonance Vol. 36, 447-
451 pages 1979). DWAlderman et al.'s high-frequency coil is a resonator in the form of a cylinder with a pair of slots parallel to its axis (S.Kan et al.
Review of Scientific Instruments Volume 44 1725−
(1973, p. 1733), and this slot is symmetrical about the axis, and when one slot is installed at an angle of about 100° around the axis, it emits spatially relatively uniform high-frequency electromagnetic waves. (HJSchneider et al.
Review of Scientific Instruments Volume 48 68-73
p. 1977).

``Problems to be solved by the invention'' However, the high frequency coil of DWAlderman et al.
As disclosed in the literature by H.J. Schneider et al., and also according to the experimental confirmation by the present inventors, the spatial uniformity of the electromagnetic waves was not necessarily sufficient.

The object of the present invention is to provide a high-frequency coil used for excitation or detection of nuclear magnetic resonance phenomena, which has a high Q value with respect to high-frequency electromagnetic waves, that is, has excellent resonance properties, and has excellent spatial uniformity. An object of the present invention is to provide a high-frequency coil for nuclear magnetic resonance that can generate.

"Means for Solving the Problem" According to the present invention, a pair of annular conductors having the same axis and arranged symmetrically with respect to a plane perpendicular to the axis are provided, and the annular conductors are arranged parallel and symmetrically with respect to the axis. A first pair of conductive wires and a second pair of conductive wires arranged have wing tips along the annular conductor at both ends of each of the conductor wires, and the both ends are arranged so that the pair of annular conductors and the dielectric layer are connected to each other. capacitively coupled via the In order to obtain uniform electromagnetic waves, the first pair of conductive wires is rotated by about 40° to 70°, preferably 50° to 60°, about the axis with respect to the second pair of conductive wires.
A high-frequency feeding point is defined as a point between one of the two conductive wires of the first conductive wire pair and the second conductive wire pair that are close to each other and one of the adjacent annular conductors. moreover,
The blade tips located between adjacent conductive wires are connected by a coupling only by a capacitance formed by the blade tip and the annular conductor via the dielectric layer or by a conductor,
Wing tips other than those mentioned above are connected by coupling only by capacitance formed by the wing tips and the annular conductor via the dielectric layer.

The pair of annular conductors are desirably circular, rectangular, or elliptical because spatial symmetry is easily obtained when the test sample is a human body.

The larger the average radius of the pair of annular conductors is than the average radius of the sample to be inspected, the higher the spatial uniformity of the electromagnetic waves will be with respect to the sample to be inspected. It is necessary to send more power from In order to obtain spatially uniform electromagnetic waves, the average radius of the annular conductor should be 20 to 50% larger than the average radius of the test sample, preferably
A 30-40% increase is practical.

The longer the length of the conductor pair is with respect to the average radius of the annular conductor, the higher the spatial uniformity of the electromagnetic waves in the axial center. It is necessary to send electricity. The length of the conductor pair is
It is practical that the diameter is 0.5 to 4 times, preferably 0.7 to 2.5 times, the average diameter of the annular conductor. When this multiple is around 1.0 times, the spatial uniformity of the electromagnetic waves is slightly impaired, but on the other hand, the power sent to the high-frequency coil to obtain the desired intensity of electromagnetic waves may be small; on the other hand, when the multiple is around 2.0 times. In this case, although the electric power needs to be large, it is possible to obtain a high-frequency coil with excellent spatial uniformity of the electromagnetic waves.

"Operation" The two pairs of conductors capacitively coupled to the pair of annular conductors have a low inductance and therefore can conduct high currents with a low potential difference across them. A capacitive coupling connecting the pair of annular conductors and the two pairs of conductive wires is connected with the inductance to form a resonant circuit. In addition, the pair of annular conductors itself forms a current path and at the same time acts as an electrical shield for the test sample located inside, and has the function of reducing the dielectric loss mentioned above. High resonance is maintained even when the sample to be inspected is placed inside the conducting wire. There are two pairs of conductors, and the first pair of conductors is rotated by 40° to 70°, preferably 50° to 60°, about the aforementioned axis with respect to the second pair of conductors. There is. A high frequency signal is supplied between two conductive wires of the first conductive wire pair and the second conductive wire pair that are close to each other and one of the annular conductors that are close to these conductive wires. At this time, in one half cycle of the high frequency signal, the current is distributed between the two conductors and flows toward the other annular conductor, capacitively coupled to the other annular conductor, and connected to the first conductor pair and the second conductor. Flowing through the other conductor of each two pairs towards said one annular conductor. In the other half cycle of the high frequency signal, current flows in the opposite direction. In this kind of relationship, first,
A high-frequency current flows through the second pair of conductive wires, and the lines of magnetic force generated based on the current flowing through these pairs of conductive wires move along the axis in the space formed by these four conductive wires, that is, the internal space of this high-frequency coil. The direction is perpendicular to the direction in which the conductive wires are arranged close to each other and parallel to the direction in which the conductive wires are arranged close to each other.

Since the angle formed by the first and second pairs of conducting wires around the axes is selected as described above, the high frequency electromagnetic waves excited in the internal space have excellent spatial uniformity. For example, based on a constant current flowing through each of the two pairs of conductive wires, the magnetic field strength generated at a given position in the space around the conductive wires is expressed by Biot-Savart's law. , is defined as the angle at which the sum of the magnetic field strengths has the minimum dependence on position, that is, the difference depending on position is the minimum, especially when the conductor has a relatively thin shape. When , this angle is approximately 60°.

"Example" This invention will be explained in more detail using the drawings.
FIG. 1 shows an embodiment of a high frequency coil according to the present invention. Both the annular conductor 1 and the annular conductor 2 are circular, have the same axis CL, and are arranged symmetrically with respect to a plane perpendicular to the axis CL. The conducting wires 3, 4, 5, and 6 are straight conductors parallel to the axis CL, and each end is covered with a dielectric layer 7 having a small dielectric loss, such as a fluororesin, with respect to the annular conductors 1, 2. 8. In the figure, the conductive wires 3, 4, 5, 6 are arranged close to the outside of the annular conductors 1, 2, respectively. Further, in particular, the capacitive coupling is strong and distributed at the ends of each conductor, for example, the conductor 3 is connected to the annular conductor 1,
Wing tips 3a, 3 are provided on both sides along the shape of 2, respectively.
b, 3c, 3d are integrally formed, and other conductive wires 4,
5 and 6 are cases in which blade tips are similarly formed. The conductors 3 and 4 are a first pair of conductors arranged symmetrically with respect to the axis CL, and the conductors 5 and 6 are a similar second pair of conductors. As shown in the vertical cross-section of the conductors 4 to 6 in FIG. Rotated and positioned.

The wing tips of the conducting wire 3 and the conducting wire 5 that are close to each other can be connected with a conductor, and the wing tips of the conducting wire 4 and the conducting wire 6 that are close to each other can be connected with a conductive material. be. For example, in Figure 1, the conductor 4
The wing tips of the conductive wire 6 and the conductive wire 6 which are close to each other at the ends capacitively coupled to the annular conductor 1 are coupled by a conductor, and a terminal 9 is provided on the conductor.
A terminal 10 is provided on the annular conductor 1 in close proximity to the terminal 9. Terminal 9 and terminal 10 are for sending high frequency power from the outside.

When the terminal 9 becomes positive with respect to the terminal 10 due to the high frequency signal input just now, the current from the terminal 9 is divided into two into the conductors 4 and 6, and flows through the conductors 4 and 6 toward the annular conductor 2 as shown by the arrow. The current that has reached the annular conductor 2 side flows through the annular conductor 2 toward the conducting wires 5 and 3 due to capacitive coupling, and further flows through the conducting wires 5 and 3 toward the annular conductor 1 side as shown by the arrows due to capacitive coupling. These currents flow through the ring conductor 1 due to capacitive coupling.
The terminal 10 is reached through the terminal 10. At this time, the magnetic field line distribution generated based on the current flowing through these conductors 3, 4, 5, and 6 is mostly as shown in Fig. 2 in the internal space 11 surrounded by the conductors 3, 4, 5, and 6. The lines of magnetic force are perpendicular to the axis CL, parallel to the direction in which the conducting wires 4 and 6 are arranged, and are uniformly distributed. terminal 10
When becomes positive with respect to terminal 9, the current flows in the opposite direction to that described above, and the direction of the magnetic lines of force also becomes opposite to that in FIG. 2.

To give an example of the dimensions of each part, the annular conductors 1 and 2 have a diameter of about 600 mm, are made of copper plates about 90 mm wide and about 0.3 mm thick, and the dielectric layers 7 and 8 are made of fluororesin about 4 mm thick. , conductors 3, 4, 5, and 6 each have a length of approximately
Consists of a copper plate measuring 580 mm, width approx. 90 mm, and thickness approx. 0.3 mm.
Regarding the dimensions of the blade tips, for example, the length of the corresponding portions of the blade tips 3a and 3b along the annular conductor 1 is about 40 cm. In the example of the coil shown in FIG. 1, it was observed that the Q value exceeded 100 at approximately 40 MHz both when nothing was placed inside as a test sample and when a human body was placed inside. Furthermore, it has been confirmed through experiments that the distribution of magnetic lines of force in a plane perpendicular to the axis CL between the annular conductors 1 and 2 is spatially extremely uniform as shown in FIG. FIG. 3 shows the high-frequency coil shown in FIG.
Annular conductors 14 and 15 are provided through the respective conductors. The annular conductors 14 and 15 can reduce the influence of distributed capacitance on other conductive objects that may exist outside the high frequency coil.

"Effects of the Invention" As described above, the present invention provides a high-frequency coil that generates high-frequency electromagnetic waves with excellent resonance regardless of the presence or absence of a sample to be inspected, and with excellent spatial uniformity. can get.

[Brief explanation of drawings]

Fig. 1 is a perspective view showing an embodiment of the high-frequency coil for nuclear magnetic resonance according to the present invention, and Fig. 2 schematically shows the actual measurement of the distribution of magnetic lines of force created by the high-frequency coil shown in Fig. 1. 3 are perspective views showing other embodiments of the high frequency coil according to the present invention. 1, 2, 14, 15: ring conductor, 3, 4, 5,
6: Conductive wire, 3a, 3b, 3c, 3d: Wing tip of conductive wire 3, 7, 8, 12, 13: Dielectric layer, 9, 10:
Terminal, 11: Internal space, CL: Axis line.

Claims (1)

[Claims] 1. A high-frequency coil used for excitation or detection of the magnetic resonance phenomenon, comprising: a pair of annular conductors having the same axis and arranged symmetrically with respect to a plane perpendicular to the axis; Wing tips along the annular conductor are formed at both ends thereof, so that the conductor wire is capacitively coupled to the pair of annular conductors via a dielectric layer in parallel and symmetrically with respect to the axis. The conducting wire pairs are a first conducting wire pair and a second conducting wire pair, and the first conducting wire pair is arranged around the axis line with respect to the second conducting wire pair. The blade tips that are rotated by 40° to 70° and located between adjacent conductor wires are coupled only by a capacitance formed by the blade tips and the annular conductor via the dielectric layer, or by a conductor. and the wing tips other than those mentioned above are connected by coupling only by capacitance formed by the wing tips and the annular conductor via the dielectric layer. coil.