JP2012024451A - Gradient magnetic field coil device and magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gradient magnetic field coil device that enlarges gradient magnetic fields without enlarging leakage magnetic field.SOLUTION: The gradient magnetic field coil device includes: spiral-shaped first coils 12a to 12d forming magnetic field distributions, wherein intensity inclines linearly, in a photographing region of a magnetic resonance imaging apparatus; and spiral-shaped second coils 16a to 16d arranged at the opposing side of the photographing region with the first coils 12a to 12d therebetween, and suppressing the leakage magnetic fields produced by the 12a to 12d on their opposing side. Electric current Iflowing through the second coils 16a to 16d is less than the electric current Iflowing through the first coils 12a to 12d (I>I). The number of turns Tof the second coils 16a to 16d is greater than the number of turns Tof the first coils 12a to 12d (T<T). The conductor width Wof the second coils 16a to 16d is narrower than the conductor width Wof first coils 12a to 12d (W>W).

Description

  The present invention relates to a gradient coil apparatus used in a magnetic resonance imaging (hereinafter referred to as MRI) apparatus.

  An MRI apparatus is a device that obtains a cross-sectional image showing physical and chemical properties on a subject using a nuclear magnetic resonance phenomenon that occurs when a subject placed in a uniform static magnetic field is irradiated with a high frequency pulse. In particular, it is used for medical purposes.

  The MRI apparatus generates a magnetic apparatus that generates a uniform static magnetic field in an imaging region into which a subject is inserted, and a gradient magnetic field that is spatially gradient in intensity to give position information to the imaging region. It is composed of a gradient magnetic field coil device, an RF coil that irradiates a subject with a high frequency pulse, a receiving coil that receives a magnetic resonance signal from the subject, and a computer system that processes the received signal and displays an image. Yes.

  As means for improving the performance of the MRI apparatus, first, there is an improvement in the strength of the static magnetic field generated by the magnet apparatus. Since the higher the static magnetic field strength, the clearer images and the various cross-sectional images can be obtained, the MRI apparatus has been continuously developed with a higher static magnetic field strength. As other means for improving the performance, there are an improvement in the strength of the gradient magnetic field and a high-speed drive for generating the gradient magnetic field in a pulsed manner at a high speed. These contribute to shortening of imaging time and improvement of image quality, and are frequently used in high-speed imaging methods that have been actively used in recent years. In particular, due to the improved performance of the drive power supply device for the gradient magnetic field coil device, high-speed switching and large current application are possible.

  In order to generate a strong gradient magnetic field, there are methods such as increasing the current flowing through the gradient coil device or increasing the number of turns of the gradient coil device. However, when the number of turns of the gradient magnetic field coil device is increased, the induced voltage generated at the end of the gradient magnetic field coil device is increased, and a space for insulation is required. Therefore, it is difficult to reduce the size of the gradient magnetic field coil device. Further, when the number of turns of the gradient magnetic field coil device is increased, the space between the lines is increased, the space factor of the conductor in the gradient magnetic field coil device is lowered, and the current density is increased, so that the temperature rises easily. On the other hand, in order to increase the current flowing through the gradient coil device, a wide conductor corresponding to energization of a large current is required. However, in the curved portion of the wide conductor, the current density distribution in the width direction of the conductor is reduced. There is a case where a bias occurs and the gradient magnetic field intended by the design is not generated.

  Since a pulsed current flows through the gradient coil device, the pulsed magnetic field (leakage magnetic field) generates an eddy current in the metal container portion of the magnet device, and the magnetic field due to the eddy current affects the image. For this reason, the gradient coil device that switches a large current at high speed and energizes the main coil that generates a gradient magnetic field in the imaging region, and the pulsed magnetic field (gradient magnetic field and leakage magnetic field) are unnecessary portions other than the imaging region. In many cases, it has a shield coil to prevent leakage.

  As a means for flowing a large current with a high-speed pulse waveform, as disclosed in Patent Document 1, one coil is composed of a plurality of parallel conductors, and the circuit configuration is such that the respective inductances are the same. Proposed. In addition, as a means for reducing the magnetic field of eddy current, an MRI apparatus that employs a superconducting coil in a magnet apparatus has been proposed in which a variable magnetic field due to vibration of the magnet apparatus is reduced by a low resistivity conductor plate. .

British Patent GB 2331808

  The MRI apparatus is required to obtain a clear image at high speed. For this reason, the gradient magnetic field coil apparatus is required to generate a gradient magnetic field having as high a magnetic flux density as possible as quickly as possible. Therefore, a high voltage or a large current is applied to the gradient magnetic field coil device, and the potential difference between the conductors increases by the application of the high voltage, and the heat generation of the gradient magnetic field coil device increases by the application of the large current. There is a tendency. Furthermore, since a large gradient magnetic field is generated, the leakage magnetic field also tends to increase. This leakage magnetic field generates eddy currents in the metal container of the magnet device, and the magnetic field generated by the eddy currents affects the image. It is done.

  Therefore, an object of the present invention is to provide a gradient coil apparatus that can increase the gradient magnetic field without increasing the leakage magnetic field, and further to provide an MRI apparatus equipped with the gradient coil apparatus. Is to provide.

In order to achieve the above object, the present invention provides:
A spiral-shaped first coil that creates a magnetic field distribution whose intensity is linearly inclined in the imaging region of the magnetic resonance imaging apparatus;
A spiral-shaped second coil that is disposed on the opposite side of the imaging region across the first coil and suppresses a leakage magnetic field created by the first coil on the opposite side;
The current flowing through the second coil is less than the current flowing through the first coil,
The number of turns of the second coil is greater than the number of turns of the first coil,
The conductor width of the second coil is a gradient coil device that is narrower than the conductor width of the first coil.

  In addition, the present invention is an MRI apparatus including the gradient magnetic field coil apparatus according to the present invention and a magnet apparatus that creates a static magnetic field in the imaging region.

  ADVANTAGE OF THE INVENTION According to this invention, the gradient magnetic field coil apparatus which can enlarge a gradient magnetic field, without enlarging a leakage magnetic field can be provided, Furthermore, the MRI apparatus by which the gradient magnetic field coil apparatus is mounted is provided. be able to.

It is an expanded view of the circumferential direction of the y main coil (1st coil) and y shield coil (2nd coil) of the gradient magnetic field coil apparatus which concerns on the 1st Embodiment of this invention. 1 is a perspective view of a magnetic resonance imaging apparatus according to a first embodiment of the present invention. It is sectional drawing which cut | disconnected the magnetic resonance imaging apparatus which concerns on the 1st Embodiment of this invention by the plane containing a y-axis and az axis. It is an equivalent circuit diagram of the y main coil (first coil) and the y shield coil (second coil) of the gradient magnetic field coil apparatus according to the first embodiment of the present invention. It is an equivalent circuit diagram of the y main coil (first coil) and the y shield coil (second coil) of the gradient magnetic field coil apparatus according to the second embodiment of the present invention. It is the equivalent circuit schematic of the y main coil (1st coil) and y shield coil (2nd coil) of the gradient magnetic field coil apparatus which concerns on the modification of the 2nd Embodiment of this invention. It is an equivalent circuit diagram of the y main coil (1st coil) and y shield coil (2nd coil) of the gradient magnetic field coil apparatus which concerns on the 3rd Embodiment of this invention. It is the equivalent circuit schematic of the y main coil (1st coil) and y shield coil (2nd coil) of the gradient magnetic field coil apparatus which concerns on the modification of the 3rd Embodiment of this invention. It is a quarter (1st quadrant) of the expansion | deployment figure of the circumferential direction of y main coil (1st coil) of the gradient coil apparatus which concerns on the 4th Embodiment of this invention. It is a quarter (1st quadrant) of the expanded view of the circumferential direction of the y shield coil (2nd coil) of the gradient magnetic field coil apparatus which concerns on the 4th Embodiment of this invention. It is a perspective view of the magnetic resonance imaging apparatus (vertical magnetic field type) which concerns on the 5th Embodiment of this invention. It is a half (1-2 quadrant) of the top view of the z-axis negative side (z <0) of the y main coil (first coil) of the gradient coil apparatus according to the fifth embodiment of the present invention. . It is a half (1-2 quadrant) of the top view of the z-axis negative side (z <0) of the y shield coil (second coil) of the gradient magnetic field coil apparatus according to the fifth embodiment of the present invention. .

  Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. In each figure, common portions are denoted by the same reference numerals, and redundant description is omitted.

(First embodiment)
FIG. 2 is a perspective view of the magnetic resonance imaging (MRI) apparatus 100 according to the first embodiment of the present invention. The MRI apparatus 100 has a substantially triple cylindrical shape. A cylindrical vacuum vessel 1 constituting the magnet device 30 is provided outside the triple. A cylindrical gradient magnetic field coil device 5 is provided inside the vacuum vessel 1. A cylindrical RF coil 11 is provided inside the gradient magnetic field coil device 5. The subject (patient) 7 is inserted inside the RF coil 11 while being twisted on the bed 25, and a tomographic image is taken. The central axes of the triple cylindrical shape of the vacuum vessel 1, the gradient magnetic field coil device 5, and the RF coil 11 are substantially coincident with each other. In order to facilitate the following description, the z-axis is set so as to coincide with the central axis. The y axis is set upward in the vertical direction. The x axis is set in the horizontal direction. The coordinate origin is set at the approximate center of the triple cylindrical shape of the vacuum vessel 1, the gradient magnetic field coil device 5, and the RF coil 11. The θ direction is set as the circumferential direction of the triple cylindrical shape of the vacuum vessel 1, the gradient magnetic field coil device 5, and the RF coil 11. In the θ direction, the x-axis direction is zero (0) [rad], and the counterclockwise rotation direction in FIG. 2 is set to the positive direction of the θ direction.

  FIG. 3 shows a cross-sectional view of the MRI apparatus 100 according to the first embodiment of the present invention cut along a plane (yz plane) including the y-axis and the z-axis. The peripheral area around the coordinate origin is the imaging area 9. The MRI apparatus 100 is a horizontal magnetic field type MRI apparatus in which the direction of the static magnetic field 6 formed in the imaging region 9 is the horizontal direction (z-axis direction). A subject (patient) 7 is moved to the imaging region 9 and a tomographic image is captured. The magnet device 30 generates a uniform static magnetic field 6 in the imaging region 9. The gradient magnetic field coil device 5 generates a gradient magnetic field 10 having a spatially gradient magnetic field intensity in a pulse shape in order to give position information to the imaging region 9. The RF coil 11 irradiates the subject 7 with a high frequency pulse. A receiving coil (not shown) receives a magnetic resonance signal from the subject 7. A computer system (not shown) processes the received magnetic resonance signal and displays the tomographic image. According to the MRI apparatus 100, the physical and chemical properties of the subject 7 are determined using the nuclear magnetic resonance phenomenon that occurs when the subject 7 placed in a uniform static magnetic field is irradiated with a high-frequency pulse. A tomographic image can be obtained, and the tomographic image is used particularly for medical purposes.

  The magnet apparatus 30 includes a static magnetic field main coil (superconducting coil) so as to form a pair on the left and right (parts where z <0 and z> 0) with respect to the z = 0 plane (a plane including the y axis and the x axis). ) 3, a static magnetic field shield coil (superconducting coil) 4 that suppresses leakage to the periphery of the static magnetic field 6 (leakage magnetic field), and a magnetic body that is not illustrated. Each of the coils 3 and 4 has an annular shape with the z axis as a common central axis. The inner diameter of the static magnetic field shield coil 4 is larger than the outer diameter of the static magnetic field main coil 3. In addition, a superconducting coil is often used for the coils 3 and 4, and in that case, the coils 3 and 4 are housed in a container having a three-layer structure. The coils 3 and 4 are accommodated in the liquid helium container 8 together with the liquid helium (He) as a refrigerant. The liquid helium container 8 is contained in a radiation shield 2 that blocks heat radiation to the inside. The vacuum container 1 that is a hollow cylindrical container holds the liquid helium container 8 and the radiation shield 2 while keeping the inside in a vacuum. Even if the vacuum container 1 is arranged in a room at a normal room temperature, the inside of the vacuum container 1 is in a vacuum, so that the heat in the room is not transmitted to the liquid helium container 8 by conduction or convection. Moreover, the radiation shield 2 suppresses that the heat in the room is transmitted from the vacuum vessel 1 to the liquid helium vessel 8 by radiation. For this reason, the coils 3 and 4 can be stably set to the cryogenic temperature which is the temperature of liquid helium, and can function as a superconducting electromagnet. The liquid helium vessel 8, the radiation shield 2, and the vacuum vessel 1 are made of a nonmagnetic member so as not to generate an unnecessary magnetic field, and further, a nonmagnetic metal is used because it is easy to maintain a vacuum. . For this reason, the eddy current is likely to be generated in the liquid helium container 8, the radiation shield 2, and particularly in the vacuum container 1 arranged on the outermost periphery.

  The gradient magnetic field coil device 5 generates a gradient magnetic field in the imaging region 9. The gradient magnetic field is a magnetic field in which the magnetic flux density of the magnetic field in the same direction as the static magnetic field 6 is linearly inclined independently of each other in the three directions of the x axis, the y axis, and the z axis. The time to be generated for each direction is shared, and the gradient magnetic fields in the three directions of the x-axis, the y-axis, and the z-axis are repeatedly generated in the form of pulses. Specifically, FIG. 3 shows a gradient magnetic field 10 whose intensity is linearly inclined in the y-axis direction.

  Between the magnet device 30 and the gradient magnetic field coil device 5, a plurality of small pieces of magnetic material called shims (not shown) are placed. According to the shim, the magnetic field strength of the static magnetic field 6 generated in the imaging region 9 can be partially adjusted, and the imaging region 9 in which the magnetic field strength of the static magnetic field 6 is uniform can be provided. .

FIG. 1 shows circumferential directions (θ direction) of y main coils (first coils) 12 a to 12 d and y shield coils (second coils) 16 a to 16 d of the gradient coil device 5 according to the first embodiment of the present invention. ). The gradient coil device 5 has a layered structure in the radial direction, and spiral y main coils (first coils) 12a to 12d arranged on the imaging region 9 side, and a spiral shape arranged on the vacuum vessel 1 side. Y shield coils (second coils) 16a to 16d are provided. The y main coils (first coils) 12a to 12d generate a gradient magnetic field 10 whose intensity linearly inclines in the y-axis direction in the imaging region 9, but also in an external space such as the vacuum vessel 1 that is a hollow cylindrical vessel. The so-called leakage magnetic field is generated. In order to suppress the leakage magnetic field to the vacuum vessel 1 which is this hollow cylindrical vessel, the y shield coil (second coil) 16a to 16d has a current I _M flowing through the y main coil (first coil) 12a to 12d. so that the current I _S in the opposite direction is made to flow to the. The y shield coils (second coils) 16a to 16d are arranged on the opposite side of the imaging region 9 with the y main coils (first coils) 12a to 12d interposed therebetween.

  The y main coil (first coil) 12a and the y shield coil (second coil) 16a are disposed in the positive region (z> 0) of the z-axis coordinate and are positive in the θ coordinate (circumferential coordinate). The y shield coil (second coil) 16a is disposed so as to overlap and cover the y main coil (first coil) 12a.

  The y main coil (first coil) 12b and the y shield coil (second coil) 16b are arranged in a positive region (z> 0) of the z-axis coordinate and are negative in the θ coordinate (circumferential coordinate). The y shield coil (second coil) 16b is arranged so as to overlap and cover the y main coil (first coil) 12b on the inner side (−π <θ <0).

  The y main coil (first coil) 12c and the y shield coil (second coil) 16c are arranged in the negative region (z <0) of the z-axis coordinate and are negative in the θ coordinate (circumferential coordinate). The y shield coil (second coil) 16c is disposed so as to overlap and cover the y main coil (first coil) 12c.

  The y main coil (first coil) 12d and the y shield coil (second coil) 16d are arranged in the negative region (z <0) of the z-axis coordinate, and are positive in the θ coordinate (circumferential coordinate). The y shield coil (second coil) 16d is disposed so as to overlap and cover the y main coil (first coil) 12d.

  In addition to the y main coils (first coils) 12 a to 12 d and the y shield coils (second coils) 16 a to 16 d, the gradient magnetic field coil device 5 linearly inclines in the imaging region 9 in the x-axis direction. X main coil (first coil, not shown) for generating the gradient magnetic field 10 to be generated, and x shield coil (second coil, not shown) for suppressing the leakage magnetic field generated when the x main coil generates the gradient magnetic field 10 have. The x main coil and the x shield coil are arranged in the same direction with the shape of the y main coil (first coil) 12a to 12d and the y shield coil (second coil) 16a to 16d being π / 2 [ rad].

  The gradient magnetic field coil device 5 includes a z main coil (first coil, not shown) that generates a gradient magnetic field 10 whose intensity is linearly inclined in the z-axis direction in the imaging region 9, and the z main coil generates the gradient magnetic field 10. It has a z shield coil (second coil, not shown) that suppresses the leakage magnetic field generated when it is generated.

  In the first embodiment and the embodiments described later, the present invention will be described as an example in which the present invention is applied to y main coils (first coils) 12a to 12d and y shield coils (second coils) 16a to 16d. However, the present invention can be similarly applied to the x main coil and the x shield coil, and can be similarly applied to the z main coil and the z shield coil.

The y main coils (first coils) 12a to 12d that generate the gradient magnetic field generate a stronger magnetic field (gradient magnetic field) in the imaging region 9 than the y shield coils (second coils) 16a to 16d. The current densities i _M of the coils (first coils) 12a to 12d are larger than the current densities i _S of the y shield coils (second coils) 16a to 16d (i _M > i _S ). The current density is a value proportional to the strength of the magnetic field to be generated and proportional to the magnitude of the current and the length of the electric wire through which the current flows. Accordingly, in reverse, the current density i _S of the y shield coils (second coils) 16a to 16d is limited to be small. Under this limit, to be minimized leakage magnetic field leaking to the outside, so as to eliminate a gap leakage magnetic field leaking densely electric current I _S, y shield coil (second coil) 16 a to 16 d are disposed Reduce variations in current distribution in the region. Specifically, the number of turns _{T S} of y shield coil (second coil) 16 a to 16 d, is greater than the number of turns _{T M} of y main coil (first _{coil) 12a~12d (T M <T S} ). Since the area | region which arrange | positions y shield coil (2nd coil) 16a-16d and y main coil (1st coil) 12a-12d is limited, if an arrangement area becomes fixed at the upper limit, the number of turns will increase, The conductor width for each turn becomes narrow. Therefore, the conductor width _{W S} of the y shield coil (second coil) 16 a to 16 d, is smaller than the conductor width _{W M} of y main coil (first _{coil) 12a~12d (W M> W S} ).

a current _{I M} flowing through the y main coil (first coil) 12 a to 12 d, the product of the number of turns _{T M} is proportional to the current density _{i M (i M αI M ×} T M). and current _{I S} flowing through the y shield coil (second coil) 16 a to 16 d, the product of the number of turns _{T S} is proportional to the current density _{i S (i S αI S ×} T S). From the relational expression of (i _M > i _S ) and (T _M <T _S ), the current I _S flowing through the y shield coils (second coils) 16a to 16d is expressed as y main coil (first coil) 12a. Becomes smaller than the current I _M flowing through ˜12d (I _M > I _S ) and does not match (I _M ≠ I _S ). The ratio I _M / I _S is larger than the ratio i _M / i _S by the ratio T _M / T _S (I _M / I _S = (i _M / i _S ) / (T _M / T _S )). Therefore, in practice, when the current densities i _M and i _S are determined, the number of turns T _M and T _S are determined, and the current I _M is determined, the current I _S is naturally determined.

The current I _M is generated by the drive power supply device 14 and flows as a pulsed current. The y main coils (first coils) 12 a, 12 b, 12 c, and 12 d are connected in series, and those connected in series are connected to the drive power supply device 14. Therefore, the y main coil (first coil) 12a and 12b and 12c and 12d, the current _{I M} in synchronism with the same magnitude flows.

The current _IS is generated in the drive power supply device 15 and flows as a pulsed current. The y shield coils (second coils) 16a, 16b, 16c, and 16d are connected in series, and the series connected in series is connected to the drive power supply device 15. Accordingly, y in the shield coil (second coil) 16a and 16b and 16c and 16d, synchronized with the current _{I S} flows in the same size.

The synchronization control unit 17, a pulse-shaped current I _M that drives the power supply 14 generates a pulse-shaped current I _S that drive power supply 15 to generate is controlled to be synchronized, the magnetic field is generated at the same time, whereas The so-called leakage magnetic field, which is the first magnetic field, is suppressed by the other magnetic field. Thus, according to the first embodiment, the leakage magnetic field can be reliably reduced even when the gradient magnetic field is increased.

FIG. 4 shows an equivalent circuit diagram relating to the y main coils (first coils) 12a to 12d and the y shield coils (second coils) 16a to 16d of the gradient coil device 5 according to the first embodiment of the present invention. The y main coils (first coils) 12a, 12b, 12c, and 12d are connected in series, and a current I _M flows. y shield coil (second coil) 16a and 16b and 16c and 16d are connected in series, the current flows _{I S.}

y main coil (first coil) 12 a to 12 d, respectively, equivalently, a resistor _{R M,} can be expressed as a series connection of an inductance _{L M.} Similarly, each of the y shield coils (second coils) 16a to 16d can be equivalently expressed as a series connection of a resistance R _S and an inductance L _S. As described above, y number of turns _{T S} of the shield coil (second coil) 16 a to 16 d are, y main coil more than the number of turns _{T M} (first _{coil) 12a~12d (T M <T S} ), y Since the conductor width W _S of the shield coils (second coils) 16a to 16d is smaller than the conductor width W _M of the y main coils (first coils) 12a to 12d (W _M > W _S ), the resistance R _S is a resistance. It is larger than R _M (R _M <R _S ), and the inductance L _S is larger than the inductance L _M (L _M <L _S ). The synchronization control unit 17 outputs the current I _M output by the drive power supply device 14 and the drive power supply device 15 based on the magnitude relationship between the resistance R _S and the resistance R _{M and} the magnitude relationship of the inductance L _M of the inductance L _S. Current _IS to be synchronized.

(Second Embodiment)
FIG. 5A shows an equivalent circuit diagram regarding the y main coils (first coils) 12a to 12d and the y shield coils (second coils) 16a to 16d of the gradient magnetic field coil apparatus 5 according to the second embodiment of the present invention. The second embodiment is different from the first embodiment in that y main coils (first coils) 12a to 12d and y shield coils (second coils) 16a to 16d are connected in series. The four y shield coils (second coils) 16a to 16d have two parallel circuits connected in parallel two by two, and the two parallel circuits are connected in series. Due to these differences, in the second embodiment, the drive power supply device 14 can be integrated into one. For this reason, the synchronization control part 17 which synchronizes between drive power supply devices can be omitted.

Further, the by the two parallel circuits, the current I _S flowing through the y shield coil (second coil) 16 a to 16 d, can be half of the current I by the shunt current I outputted from the drive power supply 14 ( 2I _S = I). Since the current I _M flowing through the y main coil (first coil) 12a to 12d is equal to the current I output from the drive power supply device 14 (I _M = I), the y shield coil (second coil) 16a to 16d The flowing current I _S can be half of the current I _M flowing through the y main coil (first coil) 12a to 12d (I _M = 2I _S ), and can be smaller than the current I _M (I _M > I _S ). , They do not match (I _M ≠ I _S ). As a result, the magnetic field intensity generated from the y shield coils (second coils) 16a to 16d is smaller than the magnetic field intensity generated from the y main coils (first coils) 12a to 12d. (Second coil) It is necessary to increase the magnetic field intensity generated from 16a to 16d. hitting the increase in the magnetic field intensity generated from the y shield coil (second coil) 16 a to 16 d, y shield coil number of turns _{T S} (second coil) 16 a to 16 d, y main coil (first coil) 12a to More than the number of turns T _{M of} 12d (T _M <T _S ), preferably doubled (2T _M = T _S ). The y shield coil conductor width _{W S} of the (second coil) 16 a to 16 d, and narrower than the conductor width _{W M} of y main coil (first _{coil) 12a~12d (W M> W S} ), preferably in half (W _M = 2W _S ). Also in the second embodiment, in the y shield coils (second coils) 16a to 16d, it is possible to eliminate the gap where the leakage magnetic field leaks, and to reduce the leakage magnetic field leaking outside as much as possible.

(Modification of the second embodiment)
FIG. 5B is an equivalent circuit diagram relating to y main coils (first coils) 12a to 12d and y shield coils (second coils) 16a to 16d of the gradient magnetic field coil apparatus 5 according to a modification of the second embodiment of the present invention. Indicates. The modification of the second embodiment differs from the second embodiment in that four y shield coils (second coils) 16a to 16d constitute a parallel circuit connected in parallel to each other. .

This parallel circuit, the current _{I S} flowing through the y shield coil (second coil) 16 a to 16 d, it is possible to a quarter of the current I by the shunt current I outputted from the drive power supply 14 (4I _S = I). Since the current I _M flowing through the y main coil (first coil) 12a to 12d is equal to the current I output from the drive power supply device 14 (I _M = I), the y shield coil (second coil) 16a to 16d The flowing current I _S can be reduced to a quarter of the current I _M flowing through the y main coils (first coils) 12a to 12d (I _M = 4I _S ), and can be smaller than the current I _M (I _M > I _S ), not coincident (I _M ≠ I _S ). As a result, the magnetic field intensity generated from the y shield coils (second coils) 16a to 16d is smaller than the magnetic field intensity generated from the y main coils (first coils) 12a to 12d. (Second coil) It is necessary to increase the magnetic field intensity generated from 16a to 16d. hitting the increase in the magnetic field intensity generated from the y shield coil (second coil) 16 a to 16 d, y shield coil number of turns _{T S} (second coil) 16 a to 16 d, y main coil (first coil) 12a to More than the number of turns T _{M of} 12d (T _M <T _S ), preferably 4 times (4 T _M = T _S ). The y shield coil conductor width _{W S} of the (second coil) 16 a to 16 d, and narrower than the conductor width _{W M} of y main coil (first _{coil) 12a~12d (W M> W S} ), preferably 4 minutes Set to 1 (W _M = 4W _S ). Also according to the modification of the second embodiment, in the y shield coils (second coils) 16a to 16d, it is possible to eliminate the gap where the leakage magnetic field leaks, and to reduce the leakage magnetic field leaking outside as much as possible.

(Third embodiment)
FIG. 6A shows an equivalent circuit diagram relating to the y main coils (first coils) 12a to 12d and the y shield coils (second coils) 16a to 16d of the gradient magnetic field coil apparatus 5 according to the third embodiment of the present invention. The third embodiment differs from the first embodiment in that a y-coil (first coil) 12a to 12d and a y-shield coil (second coil) 16a to 16d are connected in parallel. It is the point which has. Due to this difference, in the third embodiment, the drive power supply devices 14 can be integrated into one, and the synchronization control unit 17 that synchronizes the drive power supply devices can be omitted.

As in the first embodiment, y number of turns _{T S} of the shield coil (second coil) 16 a to 16 d are, y main coil more than the number of turns _{T M} (first coil) 12 a to 12 d _{(T M} <T _S ), the conductor width W _S of the y shield coil (second coil) 16a to 16d is narrower than the conductor width W _M of the y main coil (first coil) 12a to 12d (W _M > W _S ). The resistance R _S is larger than the resistance R _M (R _M <R _S ), and the inductance L _S is larger than the inductance L _M (L _M <L _S ). Therefore, in the parallel circuit, the current I output from the drive power supply device 14 flows more to the y main coil (first coil) 12a to 12d side than to the y shield coil (second coil) 16a to 16d side. Try to include. The current I _S flowing through the y shield coils (second coils) 16a to 16d is shunted so as to be smaller than the current I _M flowing through the y main coils (first coils) 12a to 12d (I _M > I _S ). In order to obtain a desired current ratio (I _M / I _S ), an adjustment mechanism 22 that adjusts the variable resistor 18 and / or the inductance (reactor) 19 may be provided as shown in FIG. 6A. Also according to the third embodiment, in the y shield coils (second coils) 16a to 16d, it is possible to eliminate the gap where the leakage magnetic field leaks, and to reduce the leakage magnetic field leaking outside as much as possible.

(Modification of the third embodiment)
FIG. 6B is an equivalent circuit diagram relating to y main coils (first coils) 12a to 12d and y shield coils (second coils) 16a to 16d of the gradient magnetic field coil apparatus 5 according to a modification of the third embodiment of the present invention. Indicates. The modification of the third embodiment is different from the third embodiment in that it includes a parallel circuit in which four y main coils (first coils) 12a to 12d are connected in parallel. Due to the parallel circuit of the four y main coils (first coils) 12a to 12d, the current I output from the drive power supply device 14 is further increased from the third embodiment by the y main coils (first coils) 12a to 12d. Try to flow a lot into the side. y shield coil current _{I S} flowing through the (second coil) 16 a to 16 d becomes even smaller than the current _{I M} flowing through the y main coil (first coil) 12 a to 12 d _{(I M> I} S). On the other hand, as in the first embodiment, y number of turns _{T S} of the shield coil (second coil) 16 a to 16 d are, y main coil more than the number of turns _{T M} (first coil) 12 a to 12 d _{(T M} <T _S ), the conductor width W _S of the y shield coil (second coil) 16a to 16d is smaller than the conductor width W _M of the y main coil (first coil) 12a to 12d (W _M > W _S Therefore, according to the modification of the third embodiment, in the y shield coils (second coils) 16a to 16d, the gap where the leakage magnetic field leaks can be eliminated, and the leakage magnetic field leaking to the outside can be minimized. .

(Fourth embodiment)
FIG. 7A shows a developed view in the circumferential direction (θ direction) of the y main coil (first coil) 12a of the gradient magnetic field coil apparatus 5 according to the fourth embodiment of the present invention, and FIG. 7B shows a y shield coil ( The development view of the peripheral direction (theta direction) of the 2nd coil) 16a is shown. The fourth embodiment is different from the first embodiment in that the y shield coil (second coil) 16a is divided at the spiral inner part 20 and outer part 21. Thereby, the current I _S2 flowing through the inner side portion 20 can be made smaller than the current I _S1 flowing through the outer side portion 21 (I _S1 > I _S2 ). The current I _S1 flowing through the outer portion 21 is set equal to the current I _M flowing through the y main coil (first coil) 12a (I _M = I _S1 ). As a result, similarly to the first embodiment, only two drive power supply devices 14 and 15 are required.

In the inner portion 20, even with a small current I _S2 (I _S1 > I _S2 ), in order to increase the magnetic field to the same level as that of the outer portion 21, the number of turns T _S2 is increased (the pitch per turn is reduced), and the conductor width W _S2 needs to be narrower than the conductor width W _S1 (W _S1 > W _S2 ). Similarly to the first embodiment, the conductor width W _S2 needs to be narrower than the conductor width W _M (W _M > W _S2 ).

In the fourth embodiment, the inner part 20 and the outer part 21 of the spiral shape of the y shield coil (second coil) 16a are divided, but the spiral inner part of the y main coil (first coil) 12a. And may be divided at the outer side. In this case, the current flowing through the inner portion may be made smaller than the current flowing through the outer portion, and the current flowing through the inner portion may be equal to the current flowing through the y shield coil (second coil) 16a.
In the first to fourth embodiments, the MRI apparatus 100, the vacuum vessel 1 and the gradient magnetic field coil apparatus 5 are cylindrical. However, the present invention is not limited to this, and may be cylindrical. It's fine.

(Fifth embodiment)
FIG. 8 is a perspective view of a magnetic resonance imaging (MRI) apparatus (vertical magnetic field type) 100 according to the fifth embodiment of the present invention. The fifth embodiment differs from the first embodiment in that the MRI apparatus 100 is changed from a horizontal magnetic field type to a vertical magnetic field type. In the fifth embodiment, the upper and lower disk-shaped vacuum containers 1 (magnetic poles) are connected to each other by the connecting pillar 24, and a vertical static magnetic field 6 is generated in the imaging region 9 between the vacuum containers 1 (magnetic poles). Moreover, in 5th Embodiment, the gradient magnetic field coil apparatus 5 and the RF coil 11 are formed in disk shape. The y main coil (first coil) 12 and the y shield coil (second coil) 16 formed in the gradient magnetic field coil device 5 are also formed in a disk shape.

  FIG. 9A shows a half of a plan view of the z-axis negative side (z <0) of the y main coil (first coil) 12 of the gradient magnetic field coil apparatus 5 according to the fifth embodiment of the present invention (first <1>). -2 quadrant), and FIG. 9B shows a half (1-2 quadrant) of the plan view of the z-axis negative side (z <0) of the y shield coil (second coil) 16. Since both the y main coil (first coil) 12 and the y shield coil (second coil) 16 are shown in half, they are substantially semicircular.

Similar to the first embodiment, the y main coil (first coil) 12 that creates a gradient magnetic field generates a stronger magnetic field (gradient magnetic field) in the imaging region 9 than the y shield coil (second coil) 16. For this reason, the current density i _M of the y main coil (first coil) 12 is larger than the current density i _S of the y shield coil (second coil) 16 (i _M > i _S ). Accordingly, in reverse, the current density i _S of the y shield coil (second coil) 16 is limited to be small. In order to reduce the leakage magnetic field leaking outside as much as possible under this restriction, the current _IS is flowed densely so as to eliminate the gap through which the leakage magnetic field leaks, and in the region where the y shield coil (second coil) 16 is disposed. The variation of current distribution is reduced. Specifically, y the number of turns _{T S} of the shield coil (second coil) 16, to more than the number of turns _{T M} of y main coil (first coil) 12 _(T M _<T S). Since the area where the y shield coil (second coil) 16 and the y main coil (first coil) 12 are arranged is limited, if the arrangement area is constant at the upper limit, the number of turns increases as the number of turns increases. The conductor width becomes narrower. Therefore, the conductor width W _S of the y shield coil (second coil) 16 is made smaller than the conductor width W _M of the y main coil (first coil) 12 (W _M > W _S ). Thus, according to the fifth embodiment, in the y shield coil (second coil) 16, it is possible to eliminate the gap where the leakage magnetic field leaks, and to reduce the leakage magnetic field leaking outside as much as possible.

1 Vacuum container 5 Gradient magnetic field coil device 6 Direction of static magnetic field 7 Subject (patient)
9 Imaging region 10 Gradient magnetic field whose intensity is linearly inclined in the y-axis direction 12, 12a to 12d y main coil (first coil) of gradient magnetic field coil apparatus
16, 16a-16d y shield coil (second coil) of gradient magnetic field coil apparatus
17 Synchronous control unit 18 Variable resistance 19 Reactor 20 y Inside portion of y shield coil (second coil) thinner than main coil (first coil) 21 Y shield coil (second coil) thicker than main coil (first coil) Outside part 30 Magnet apparatus 100 Magnetic resonance imaging apparatus

Claims (7)

A spiral-shaped first coil that creates a magnetic field distribution whose intensity is linearly inclined in the imaging region of the magnetic resonance imaging apparatus;
A spiral-shaped second coil that is disposed on the opposite side of the imaging region across the first coil and suppresses a leakage magnetic field created by the first coil on the opposite side;
The current flowing through the second coil is less than the current flowing through the first coil,
The number of turns of the second coil is greater than the number of turns of the first coil,
The gradient magnetic field coil device, wherein the conductor width of the second coil is narrower than the conductor width of the first coil. A first power source for feeding power to the first coil;
A second power source for feeding power to the second coil;
The gradient magnetic field coil apparatus according to claim 1, wherein the first power source and the second power source output pulsed currents synchronized with each other. A parallel circuit in which the first coil and the second coil are connected in parallel;
2. The gradient magnetic field coil device according to claim 1, wherein in the parallel circuit, a current flowing through the second coil is shunted so as to be smaller than a current flowing through the first coil.   The gradient magnetic field coil apparatus according to claim 3, wherein the parallel circuit includes an adjustment mechanism that adjusts resistance and / or inductance in order to change a current flowing through the first coil and the second coil. A plurality of the second coils having a parallel circuit connected in parallel;
The first coil is connected in series to the parallel circuit;
2. The gradient magnetic field coil device according to claim 1, wherein in the parallel circuit, a current flowing through the second coil is shunted so as to be smaller than a current flowing through the first coil. The current flowing through the spiral-shaped inner part of the first coil or the second coil is smaller than the current flowing through the spiral-shaped outer part located outside the inner part,
The gradient magnetic field coil apparatus according to claim 1, wherein a conductor width of the inner portion is smaller than a conductor width of the outer portion.   A magnetic resonance imaging apparatus comprising: the gradient magnetic field coil apparatus according to any one of claims 1 to 6; and a magnet apparatus that generates a static magnetic field in the imaging region.

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