JP4938492B2 - RF coil - Google Patents

Description

The present invention relates to a RF coil (radio frequency coil), in particular, in NMR (nuclear magnetic resonance) apparatus, in which relates to a preferred RF coil in irradiates an alternating magnetic field.

  In the field of NMR, a high-resolution NMR apparatus capable of applying a resonance frequency signal in a highly uniform magnetic field has been developed. In particular, in order to analyze a sample with a small amount of sample and a small signal strength such as protein with high accuracy, the uniformity of the magnetic field strength must be increased.

  In the NMR apparatus, the magnetic field is two of a static magnetic field by a superconducting magnet and an alternating magnetic field by an RF coil that performs transmission and reception, and both must be orthogonal to each other. Hereinafter, the direction of the static magnetic field is the z direction, the irradiation direction of the alternating magnetic field is the x direction, and the direction orthogonal to the x and z directions is the y direction.

  By irradiating an alternating magnetic field, the nuclear magnetization falls to the xy plane and returns to the z-axis direction again. This phenomenon is called FID (free induction decay), and an amount proportional to the component of nuclear magnetization projected onto the xy plane is observed as the induced electromotive force.

  Since the flip angle between the nuclear magnetization and the z-axis is proportional to the alternating magnetic field strength at each position, the flip angle varies due to nonuniformity of the alternating magnetic field strength. Since the received signal is the sum of the FID signals obtained from the nuclear magnetization contained in the sample, the AC magnetic field uniformity is one of the factors affecting the measurement sensitivity. In particular, in the case of a sequence in which irradiation is performed a plurality of times, variations in flip angles are integrated, and the influence becomes significant.

  Here, the AC magnetic field homogeneity is the distribution of the AC magnetic field intensity over the entire region where the sample is present. Therefore, it is necessary to improve the AC magnetic field uniformity in the longitudinal direction of the test tube and in the direction perpendicular thereto.

  As an example of the structure of the RF coil that generates a substantially uniform alternating magnetic field in the sample region, there is a saddle type as in Non-Patent Document 1. In this case, one having a spread angle of 120 degrees from the center axis of the test tube is known as one of the solutions showing the optimal AC magnetic field uniformity in the direction perpendicular to the test tube length. Patents that utilize this include Patent Documents 1 and 2, for example.

  Compared to a saddle type coil that fixes a spiral coil to a cylindrical bobbin, a flat plate coil that is pressed against a flat plate as in Non-Patent Document 2 can obtain a high AC magnetic field uniformity in a direction perpendicular to the longitudinal direction of the test tube. It is known that

JP 2002-341001 JP-A-7-303622 Ginsberg DM et al., Optimum Geometry of Saddle Shaped Coils for Generating a Uniform Magnetic Field.Rev Sci Instrum 1970; 41: 122-123 W. W. Brey et al., NHMFL Reports, 2006, Vol.13, No.2

  In the following, a closed curve formed by four conductors parallel to the longitudinal direction of the test tube 1 and a straight line or a curved conductor in a direction perpendicular thereto is referred to as one “loop”. The test tube longitudinal direction is defined as the loop height, and the distance between the conductors in the direction perpendicular to the test tube longitudinal direction is defined as the loop width.

  When a plurality of windings are mounted by a method of winding in a spiral shape, the conducting wire is arranged in one plane or curved surface. In that case, the width and height are different in each loop, and it is inevitable that a loop with a low height is formed. As a result, the AC magnetic field uniformity in the test tube longitudinal direction is lowered.

  When a plurality of windings are mounted by a conventional spiral winding method, the conducting wires are arranged in one plane or curved surface. In order to realize a multi-turn coil, the conducting wire parallel to the longitudinal direction of the test tube must be wired at a position deviated from the spread angle of 120 degrees. As a result, the AC magnetic field uniformity in the direction perpendicular to the length of the test tube is lower than the optimal solution in the case of one turn.

The present invention has been made in view of the above points, and an object thereof is to provide an RF coil having a high degree of uniformity of an alternating magnetic field.

RF coil of the present invention, in order to achieve the above object, an RF coil for emitting an alternating magnetic field to the sample in the test tube in the NM R equipment, the RF coil is formed by a conductive wire, a plurality and the loop consists of a loop between the connecting portion connecting between the loop and formed in each of the loop different planes or in a curved surface, so that the longitudinal direction between the tubes of all these loops is equal The conductors that are configured and parallel to the longitudinal direction of the test tube of all the loops are arranged on an extension line having a spread angle of 120 degrees from the central axis of the test tube .

In order to achieve the above object, an RF coil of the present invention is an RF coil for irradiating a sample in a test tube in an NMR apparatus with an alternating magnetic field, and as the RF coil, a solenoid coil, a conductive wire is formed by, consists of a loop joined portion for connecting a plurality of loops and the loop, and each loop is formed in different planes or curved surfaces, the longitudinal distance of the tubes of all these loops A flat coil that is configured to be equal and in which the conductors parallel to the longitudinal direction of the test tube of all the loops are arranged on an extension line having a spread angle of 120 degrees from the central axis of the test tube And a central axis of the solenoid coil and a normal line of the flat coil are arranged so as to be orthogonal to each other.

  According to the present invention, the coil unit is composed of two sets of coils that are symmetrical with respect to the test tube. The coil on each side has a plurality of loops, and the loops are connected in series. Therefore, the current path of the coil can be drawn with a single continuous line from the beginning to the end of winding. The connection method between the coils may be either serial or parallel.

  Place all loops in different planes and keep the loop height constant. Thereby, it becomes possible to raise all the loops to the limit of the allowable space, and the AC magnetic field uniformity in the longitudinal direction of the test tube is improved.

By arranging the respective loops in different planes, it is possible to arrange the conductors parallel to the longitudinal direction of the test tube in all the loops on an extension of a spread angle of 120 degrees when viewed from the central axis of the test tube. That is, the value of each loop width of the coil of the present invention is described by equation (1).
Loop width = (distance between test tube center and coil) × 2 tan (120 ° / 2) (1)
By setting the loop width to the value of the formula (1) in all loops, even with multiple winding coils, the AC magnetic field homogeneity is about the same as the optimal solution for one turn in the direction perpendicular to the test tube length. Obtainable.

  In the coil shape of the present invention, it is not necessary to make a loop with a low height even in a multi-turn coil, and it is possible to increase all the loops to the limit of allowable space. Therefore, the uniform region of the alternating magnetic field can be expanded with respect to the longitudinal direction of the test tube.

  In the coil shape of the present invention, it is possible to arrange all the conducting wires parallel to the longitudinal direction of the test tube on the extension of the spread angle of 120 degrees from the center of the test tube. Thereby, a high AC magnetic field uniformity can be realized even in a direction perpendicular to the length of the test tube.

  The best embodiment of the present invention will be described. The coil unit of the present invention comprises two sets of coils that are symmetrical with respect to an NMR test tube. The coil on each side has a plurality of loops, and the loops are connected in series. Therefore, the current path of the coil can be drawn with a single continuous line from the beginning to the end of winding. The connection method between the coils may be either serial or parallel.

  In the RF coil of the present invention, all loops are arranged in different planes, and the loop height is made constant. Thereby, it becomes possible to raise all the loops to the limit of the allowable space, and the AC magnetic field uniformity in the longitudinal direction of the test tube is improved. By arranging the respective loops in different planes, it is possible to arrange the conductive wires parallel to the longitudinal direction of the test tube in all the loops on the extension of the spread angle of 120 degrees when viewed from the central axis of the test tube. That is, the value of each loop width of the coil of the present invention is described by equation (1).

  By setting the loop width to the value of the formula (1) in all loops, even with multiple winding coils, the AC magnetic field homogeneity is about the same as the optimal solution for one turn in the direction perpendicular to the test tube length. Obtainable. Hereinafter, a plurality of embodiments of the present invention will be described.

FIG. 1 shows a schematic diagram of a horizontal NMR apparatus using a split-type magnet. Split type magnet 4 which generates a static magnetic field is formed by the superconductive magnet, the thermal radiation shield container and the liquid helium vessel is contained in the outer vacuum vessel. A superconducting coil is wound and installed inside the liquid helium container. A shim coil (not shown) is provided to correct the non-uniformity of the static magnetic field created by the superconducting magnet. The coil of the present invention can also be implemented in a vertical magnetic field type NMR apparatus. "

A liquid sample to be measured is enclosed in a cylindrical test tube 1. Out of the test tube 1 is performed using the gap of the split type magnet 4 from above. The test tube 1 is rotated to about several tens Hz on its axis by a sample rotating mechanism (not shown) as necessary. This averages the non-uniformity of the static magnetic field that could not be corrected by a shim coil or the like.

  The probe 5 is inserted from the horizontal direction which is the axial direction of the split magnet 4. An RF coil 2 that transmits and receives RF signals, a gradient magnetic field coil (not shown) that generates a gradient magnetic field, and the like are mounted on the tip of the probe 5. The gradient coil is formed in an active shield type. This coil has a shield structure that generates a linear gradient magnetic field and suppresses a leakage magnetic field to the outside.

As shown in FIG. 1, the RF signal generated by the transmitter 10 is supplied to the RF coil 2. At the time of signal acquisition, the switching device 13 separates the transmitter 10 and the receiver 12 and controls the operation of the receiver 12. The signal acquired by the R F coils 2 is sent to the receiver 12 is amplified by the amplifier 11. An NMR spectrum is obtained by Fourier transforming this signal.

Figure 2 shows the structure of R F coils 2 which forms a loop in a plane. Hereinafter, the RF coil 2 of this embodiment is referred to as a “flat coil”. The coil unit is composed of two sets of coils that are symmetrical with respect to the NMR test tube 1 . The flat coil 2 on each side has a plurality of loops, and the loops are connected in series via an inter-loop connecting portion, and become a single line continuous from the start to the end of winding of the coil.

  The flat coil 2 is arranged so that all the loops are arranged in different planes, and the loop height is made equal. Each loop is placed in a different plane. The subject of the present invention is to make all loop heights as high as possible in the RF transmit coil. By making each loop in a different plane, all the loop heights can be made equal and increased to the allowable space limit.

  In order to improve the magnetic field uniformity in the xz plane, it is necessary to adjust the width of each loop. As one of the optimum solutions, there is a structure in which the vertical conductors are arranged on an extension of a spread angle of 120 degrees when viewed from the central axis of the test tube 1 in all the loops. That is, the value of each loop width is described by equation (1). In all loops, by setting the loop width to the value of equation (1), even with multiple windings, the AC magnetic field homogeneity is about the same as the optimal solution for one turn in the direction perpendicular to the test tube length. Can be obtained.

  FIG. 3 shows a layout of the flat coil. The relative positional relationship with a test tube is shown, (A) is an xy plane, (B) is a yz plane, (C) is a projection view of an xz plane. In the figure, the direction of the static magnetic field is the z direction, the irradiation direction of the alternating magnetic field is the x direction, and the direction orthogonal to the x and z directions is the y direction.

FIG. 4 is a mounting diagram of the flat coil as viewed from the top of the NMR apparatus. To the sidewall 7, the same or equal, or attaching the trapezoidal columnar jig 8 for wire fixed with a comparable magnetic susceptibility. If a conducting wire is wound around the trapezoidal columnar jig 8 a predetermined number of times, a flat coil shape can be obtained. In order to improve the manufacturability it is good to use a wire of rectangular. Furthermore, it is preferable to provide a lead wire fixing groove in the trapezoidal columnar jig 8 in advance. In this manner, the lined vertically conductor on an extension line of the spread angle 120 °, the easier fabrication. Static magnetic field homogeneity viewpoint same as the side walls 7 and a trapezoidal columnar jig 8 from, or using the processing plate 9 of a material having a comparable susceptibility, so that the side wall 7 as viewed from the test tube 1 side is flat It is desirable to make it.

  5, 6, and 7 show characteristics of the saddle type coil, the flat plate coil, and the flat type coil of this embodiment. The number of turns of the coil is 5 turns. 5A to 7A, the space where each coil can be installed is set to ± 10 mm in the y direction and ± 5 mm in the x direction.

  The sensitivity distribution in a saddle type coil, a flat plate coil, and a flat type coil is shown in FIG. It is calculated from the Biosavall equation, and shows the sensitivity distribution in the range of Φ4 mm from the y-axis with the coil center at y = 0 mm. Here, the sensitivity in NMR is the electric power induced in the coil by the magnetization in the unit volume, and this value is equal to the magnetic field strength generated at each position when 1 W electric power is applied to the coil from the reciprocity theorem.

The electromotive force Vi induced by the nuclear magnetization contained in the minute region i in the receiving coil can be obtained by the equation (2), and the volume can be calculated over the entire sample space.
Vi = sin (π / 2 × Bi × t / BT90) × Bi × dV (2)
Bi represents the magnetic field at each position, t represents the AC pulse application time, BT90 represents the time required for the nuclear magnetization to fall by 90 degrees, and dV represents the volume of the minute region. The sample space was assumed to be y = ± 8 mm and a Φ4 mm space centered on the y-axis.

  As shown in the figure, the AC magnetic field uniformity in the longitudinal direction (y direction) of the test tube in the case of the flat type coil of this example is improved as compared with the case of the conventional saddle type coil and flat plate coil. In addition, the AC magnetic field uniformity of the flat coil of this example is also improved in the direction perpendicular to the longitudinal direction of the test tube.

  5C shows the relationship between the pulse application time and the induced electromotive force in the case of a saddle type coil, a flat plate coil, and a flat type coil of this embodiment. In general, “810 ° / 90 ° intensity ratio” is used as an index of AC magnetic field uniformity. 5, 6, and 7, the intensity ratio is 70%, 67%, and 94%, and it can be seen that the coil of this example exhibits the highest AC magnetic field uniformity.

Example 2 is an example using a "saddle coil" as R F coils. FIG. 8 shows the arrangement structure of the saddle type coil. Each loop of the saddle type coil 3 is formed on a different surface, and the loops are connected like a connection part between loops, and are formed from one conductor.
In the case of the saddle type coil 3 of the present embodiment, the same devices and parts as those of the first embodiment are required. The mounting procedure is substantially the same, but as shown in FIG. 9, if a curved groove is dug in advance in the trapezoidal columnar jig 8, it is easy to create.

  The third embodiment is an example in which the flat coil 2 and the solenoid coil of the first embodiment are used together. In NMR measurement, nuclides such as 1H, 2H, 13C, and 15N are measured. For example, the resonance frequencies in a static magnetic field of 7 Tesla are 300 MHz, 46 MHz, 75 MHz, and 30 MHz, respectively. In order to handle a plurality of frequencies with a single coil, a tuning circuit may be incorporated. However, if the frequencies of nuclides are close to each other such as 2H and 15N, it becomes difficult to cut off the frequencies. Therefore, a flat type coil and a solenoid coil are used in combination.

  FIG. 10 shows an arrangement of a combination of a flat type coil and a solenoid coil. The flat coil 2 is used for the solenoid coils 6, 13C and 15N for 1H and 2H. The magnetic field generation direction of these coils is orthogonal to the y direction and the x direction, and the mutual inductance can be reduced.

  The dimension of the solenoid coil 6 is set to a sufficiently large coil diameter with respect to the outer wall Φ5 mm of the test tube 1 so that the test tube 1 can be easily inserted and removed. In order to reduce the mutual inductance with the transmission coil, it is desirable that the central axis of the solenoid coil 6 and the normal line of the flat coil 2 have a geometrical arrangement orthogonal to each other.

  As another combination example, a method of using the flat coil 2 for transmission and the solenoid coil 6 for reception can be considered. As described above, by separating the transmission and reception coils, it is possible to satisfy both the AC magnetic field uniformity required for the transmission coil and the high sensitivity required for the reception coil. Even in this case, it is a great advantage that the mutual inductance of both coils is small.

1 is an overall view of a horizontal magnetic field type NMR apparatus using a split type magnet. The block diagram of Example 1 which used the flat type coil for RF coil. The projection figure to the xy plane, yz plane, and xz plane of a flat type coil. The mounting block diagram of the flat coil 2. FIG. Explanatory drawing which shows the characteristic of the conventional saddle type coil. Explanatory drawing which shows the characteristic of the conventional flat coil. FIG. 3 is an explanatory diagram illustrating characteristics of the flat coil according to the first embodiment. The block diagram of Example 2 using the saddle type coil for RF coil. Mounting configuration diagram of a saddle type coil. FIG. 6 is an external perspective view of a combination of a flat coil and a solenoid coil according to a third embodiment.

Explanation of symbols

1 ... tube, 2 ... RF coil (flat coil), 3 ... saddle coil, 4 ... Split Katama Gunetto, 5 ... probe, 6 ... solenoid coil, 7 ... side wall, 8 ... trapezoidal columnar jig, 9 ... Processing plate, 10 ... Transmitter, 11 ... Amplifier, 12 ... Receiver, 13 ... Switching machine.

Claims (5)

A RF coil for emitting an alternating magnetic field to the sample in the test tube in the NM R equipment,
The RF coil is formed by a conductive wire, a plurality of loops consists of a loop between the connecting portion connecting between the loop and each loop is formed in different planes or a curved surface, the test of all these loops The conductors are configured such that the longitudinal distances of the tubes are equal , and the conductors parallel to the longitudinal direction of the test tubes of all the loops are arranged on an extension line having a spread angle of 120 degrees from the central axis of the test tubes. An RF coil which is characterized by being made . The RF coil according to claim 1, wherein
The RF coil is a flat coil in which the loop is formed in a plane . The RF coil according to claim 1, wherein
The RF coil is a saddle type coil in which the loop is formed in a curved surface . The RF coil according to claim 1, wherein
The RF coil , wherein the loop width of all the loops perpendicular to the longitudinal direction of the test tube is formed to be proportional to the distance from the central axis of the test tube . An RF coil for irradiating an alternating magnetic field to a sample in a test tube in an NMR apparatus,
The RF coil is composed of a solenoid coil and a conductive wire, and includes a plurality of loops and an inter-loop connecting portion for connecting the loops, and each loop is formed in a different plane or curved surface. The longitudinal distances of the test tubes of the loop are configured to be equal, and the conductors parallel to the longitudinal direction of the test tubes of all the loops have a spread angle of 120 degrees from the central axis of the test tube. An RF coil , wherein a flat coil arranged on an extension line is combined, and a central axis of the solenoid coil and a normal line of the flat coil are orthogonal to each other .

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