JP2001212107A - MRI equipment - Google Patents

Abstract

An object of the present invention is to keep a pattern length of an ASGC shield coil in an axial direction as short as possible and to suppress noise generated due to a magnetic field leaking from an axial end. An MRI apparatus includes a gantry having a gradient magnetic field coil disposed in a bore of a static magnetic field magnet. The gradient magnetic field coil is an ASG having a shield coil 12 _shield.
C. The pattern length of the shield coil 12 _shield with respect to the axial length of the static magnetic field magnet is such that an eddy current generated at the axial end of the bore surface of the static magnetic field magnet due to the magnetic field leaked from the axial end is generated at other portions. It is determined to be equal to or less than the eddy current. The pattern length of the winding part CR of the shield coil 12 _shield extends, for example, about 20 mm outward in the bore axis direction from the side end surface of the static magnetic field magnet, and the winding position is located outside the winding part CR in the bore axis direction. Are arranged closer to the center in the bore axis direction than the position where was theoretically analyzed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a medical MRI.
More particularly, the present invention relates to an MRI apparatus having a structure provided with a gradient coil provided near a static magnetic field magnet and having a shield coil for shielding a leakage magnetic field from the outside.

[0002]

2. Description of the Related Art A medical MRI apparatus is a system for taking a tomographic image or measuring an NMR spectrum using a magnetic resonance phenomenon of nuclear spin in a subject. Such an MRI apparatus includes a gantry for inserting and arranging a subject, for example, having a substantially cylindrical diagnostic opening (worm bore). The gantry is a static magnetic field magnet that forms a diagnostic opening and generates a static magnetic field, a gradient magnetic field coil that generates a gradient magnetic field pulse superimposed on the static magnetic field,
And RF pulse signal (including MR signal)
And an RF coil for transmitting and receiving signals.

As the gradient magnetic field coil, a shield type coil assembly having a shield coil for suppressing a leakage magnetic field to the outside is frequently used. As an example of this, an active (self) shield type gradient magnetic field coil (ASGC: Active)
ely ShieldedGradient Coi
1) are known (eg, US Pat. No. 4,733,1).
No. 89, 4,737,716).
ASGC includes a coil assembly for generating a magnetic field for each of the X, Y, and Z channels of the MRI apparatus, and each coil assembly has a main coil and a shield coil. This provides a shield structure in which the gradient magnetic field is hardly leaked to the outside for each channel.

[0004]

In a conventional shield type gradient magnetic field coil, for example, an MRI apparatus equipped with an ASGC, a diagnostic opening (worm bore) which covers a static magnetic field magnet is formed. The magnetic flux leaked from the ASGC is magnetically coupled to the metal cover inside the bore, thereby generating an eddy current on the cover, which causes noise. In other words, Lorentz force generated due to the generation of eddy current hits the cover of the static magnetic field magnet,
It generates noise.

For example, in the case of ASGC, considering the shielding performance, it is necessary to wind the shield coil with an infinitely long pattern length ideally (analytically). "Pattern length" means that when a copper wire is wound around a bobbin in a predetermined pattern,
This is the length in the bobbin axial direction (hereinafter, referred to as the axial direction), and may be simply referred to as the axial length.

However, as a practical matter, there is a limit to the pattern length of the shield coil, and it is particularly desirable to limit the pattern length of the product to as short a pattern length as possible. On the other hand, there is a situation in which the leakage magnetic field from the axial end of the shield coil does not affect the imaging region spatially formed in the diagnostic opening. Therefore, taking both into consideration, only the winding pattern of the copper wire at the axial end of the shield coil is packed as close to the axial center as possible, thereby shortening the pattern length of the entire coil.

[0007] In such a shield coil, regardless of whether the pattern length is reduced or not, a leak magnetic field naturally exists from the axial center of the coil pattern in each channel of the ASGC. There is noise. The leakage magnetic field from the ASGC when the pattern length of the shield coil is not reduced at the end in the axial direction is largest at the center of the pattern. However, if the pattern length is reduced as described above, the leakage magnetic field from the axial end of the shield coil also increases. In other words, when the pattern length is reduced, the leakage magnetic field increases at the axial end,
The noise generated at the end of the static magnetic field magnet in the axial direction also increased, and the overall noise increased.

Conventionally, noise caused by a leakage magnetic field at an axial end which does not directly affect the magnetic characteristics that the imaging region only needs to have a uniform magnetic field has priority on shortening the axial length of the shield coil. I was too close to my eyes.

In view of the above, the present invention has been made in view of such circumstances, and in an MRI apparatus provided with an active (self) shield type gradient magnetic field coil (ASGC), the axial pattern length of the shield coil is reduced as much as possible. To provide an MRI apparatus capable of maintaining a short value and suppressing noise generated due to a magnetic field leaking from an axial end,
Aim.

[0010]

According to the present invention, the problem of noise generated when the gradient magnetic field coil is driven and the shielding performance of the gradient magnetic field coil, that is, the leakage magnetic field are associated and dealt with basically.

To achieve the above object, the present invention specifically provides an MRI (Magnetic Resonance Imaging) apparatus provided with a gantry in which an active shield type gradient magnetic field coil is disposed in a bore of a static magnetic field magnet. The magnetic field coil is an active shield type gradient magnetic field coil having a shield coil,
The pattern length of the shield coil with respect to the axial length of the static magnetic field magnet is determined by the eddy current generated at the axial end of the bore surface of the static magnetic field magnet due to the magnetic field leaking from the axial end of the gradient magnetic field coil. It is characterized in that it is determined to be equal to or less than an eddy current generated at a portion other than the axial end of the bore surface.

Preferably, the pattern length of the winding portion of the shield coil extends outward in the bore axis direction from the side end surface of the static magnetic field magnet, and the pattern length extends in the bore axis direction of the winding portion. This is to adopt a configuration in which windings are arranged closer to the center in the bore axis direction than the positions where the winding positions are theoretically analyzed. For example, the extension of the pattern length from the side end surface of the static magnetic field magnet is at least 20 [mm].

As another preferred example, the shape of the axial end portion of the bore formed on the inner peripheral side of the static magnetic field magnet has an opening shape whose diameter increases as it advances outward in the axial direction. You may.

As still another example, among the respective channels of the gradient magnetic field coil, both return lines of the main coil and the shield coil of at least one channel are arranged so as to cancel magnetic fields generated by energization. is there. In this case, preferably, both return lines of the main coil and the shield coil are juxtaposed in a state of being substantially adjacent to each other at the same radial position in the assembly of the main coil.

[0015]

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

(First Embodiment) An MRI (magnetic resonance imaging) apparatus according to a first embodiment will be described with reference to FIGS.

This magnetic resonance imaging apparatus uses an active (self) shield type gradient coil (ASGC: Active).
ely Shielded Gradient Coi
1) An apparatus of the type provided with

FIG. 1 shows a schematic cross section along the axial direction of a gantry 1 in this MRI apparatus. This gantry 1
Is formed in a cylindrical shape, and an inner worm bore functions as a diagnostic space OP, so that a subject P can be inserted into the bore at the time of diagnosis. Gantry 1
An XYZ orthogonal coordinate system is set with the axis direction as the Z axis.

The gantry 1 has a magnet 11 for a static magnetic field which is formed in a substantially cylindrical shape and substantially forms the above-mentioned bore, and a substantially cylindrical gradient magnetic field coil 1 disposed in the bore of the magnet 11.
2, a shim coil 13 attached to, for example, an outer peripheral surface of the gradient magnetic field coil 12, and an RF coil 14 disposed in a bore of the gradient magnetic field coil 12. The subject P is placed on a bed (not shown) with the RF coil 14 arranged around the subject P, and placed in a bore (diagnostic space).

The static magnetic field magnet 11 is formed of a superconducting magnet. That is, a plurality of heat radiation shield containers and a single liquid helium container are housed in the outer vacuum container, and a superconducting coil is wound and installed inside the liquid helium container. The outer peripheral surface of the outer vacuum vessel is a metal cover 1
Covered with 1A.

The ASGC 12 is formed here as an active shield type. This coil 12
Has a coil assembly for each of the X, Y, and Z channels, and the coil assembly has a shield structure in which a magnetic field is hardly leaked to the outside for each channel. In this shield state, a pulse-like gradient magnetic field is generated in each of the X-axis, Y-axis, and Z-axis directions.

More specifically, as shown in FIG. 2, the ASGC 12 has an X coil assembly 12 of X, Y, and Z channels.
X, Y coil assembly 12Y, Z coil assembly 1
2Z is laminated in an insulated state for each coil layer, and has a substantially cylindrical shape as a whole. Each of the X coil assembly 12X, the Y coil assembly 12Y, and the Z coil assembly 12Z includes a main coil having a plurality of winding units that generate gradient magnetic fields in the X, Y, and Z axis directions, and a winding unit of the main coil. And a shield coil having a plurality of so-called shield windings for suppressing or reducing leakage of the generated gradient magnetic field (pulse) to the outside world. Each of the coil assemblies 12X, 12Y, 12Z is connected to an independent gradient magnetic field power supply for each channel.

FIG. 3 illustrates the positional relationship between the main coil and shield coil of the X coil assembly 12X or the Y coil assembly 12Y and the side end surface (side end) 11Aa of the static magnetic field magnet 11.

The Y coil assembly 12Y has four saddle-type winding portions (coil patterns) wound around the bobbin B for both the main coil and the shield coil. That is, in each coil, two sets of saddle-shaped winding portions that are juxtaposed and connected in series in the Z-axis direction are opposed to each other in the Y-axis direction. A total of eight winding portions of the main coil and the shield coil are electrically connected in series, for example, to a common gradient magnetic field power supply. At this time, an energization path is created so that the currents flowing in the main coil and the shield coil are opposite to each other. This makes it possible to generate a linear gradient magnetic field in the Y-axis direction while having a shielding function.

FIG. 4 shows a coil pattern drawn by one of the four saddle-type winding portions of the shield coil.
Although this winding part CR is actually wound on a bobbin in a saddle shape, it is shown in a flat form here. Each winding part CR is formed by winding a plate-shaped conductor along a substantially spiral pattern as described above. The winding position of the conductor is determined analytically under a predetermined magnetic flux distribution condition.

The X coil assembly 12X is similarly arranged with the Y coil assembly 12Y rotated 90 degrees about the Z axis.

On the other hand, each of the main coil and the shield coil of the Z coil assembly 12Z is formed by spirally winding a flat conductor on a bobbin along a winding position determined analytically. Therefore, the Z coil assembly 12Z
Has a spiral coil pattern. Therefore, each of the main coil and the shield coil is formed by electrically connecting two winding portions wound on both left and right sides in the Z-axis direction of the bobbin in series with each other, and flows in the respective coils in opposite directions. A linear Z-channel gradient magnetic field can be generated while having a shielding function by an electric current.

The coil assemblies 12X, 12Y, 12 of the active shield type gradient magnetic field coil 12 having such a structure.
In each Z, the winding part CR of the shield coil 12 _shield is wound based on the winding method according to the present invention. That is, the shielding performance of the gradient magnetic field coil 12, that is, the degree of the leakage magnetic field is determined in relation to the noise.

Specifically, as shown in FIGS. 4 and 5, in the winding portion CR, the windings L1 and L2 wound around the ends in the Z-axis direction are closer to the center in the Z-axis direction than the winding positions determined in the analysis. And roll it. The positions of the windings L1 and L2 obtained analytically are shown in FIG.
As shown by the imaginary line in the figure, the pattern length of the winding part CR (see FIG. 4) is shortened by packing it closer to the center, and if necessary, the necessary axial length of the bobbin B is reduced. Can be shorter. At this time, the desired magnetic field characteristics of the imaging region formed in the diagnostic opening are maintained.

When the pattern length is reduced in this way, the magnetic field leaking from the end of the shield coil 12 _shield and reaching the metal cover (outer end surface) 11A outside the static magnetic field magnet 11 is increased as compared with the case where the pattern length is not reduced. Thereby, the eddy current generated on the metal cover 11A due to the leakage magnetic field increases, and the noise due to the magnetic coupling increases.

Therefore, in the present embodiment, the pattern length of the shield coil 12 _shield is set so as to reduce the amount of noise caused by the leakage magnetic flux from the end while maintaining the pattern length of the shield coil 12 _{shield at} a short value.

FIGS. 6 (a) to 6 (d) show the simulation results for this setting. In these figures, the length D, which compares the substantial pattern length of the shield coil 12 _shield in the Z-axis direction with the position of the side end face of the static magnetic field magnet 12, is used as a parameter, and this length D is changed to various values. The graph shows changes in the leakage magnetic field Bz and the eddy current NI when this is performed.

Specifically, in each of (a) to (d), in the graph on the left column, the horizontal axis indicates the shield coil 12.
The position of the _{shield in} the Z-axis direction is taken, and the amount of the leakage magnetic field is taken on the vertical axis. On the other hand, in the graph in the right column, the horizontal axis also indicates the position in the Z-axis direction, and the vertical axis indicates the magnitude of the eddy current. In both graphs, the horizontal axis is the shield coil 12
_This indicates the axial position of one side (for example, the positive side) from the center of the _{shield in} the Z-axis direction. Further, both the leakage magnetic field and the eddy current show values simulated at predetermined positions on the surface of the metal cover of the static magnetic field magnet 12.

The "substantial pattern length" when setting the length D is defined as the shield coil 12
_It means the position up to the innermost winding L1 of the packed windings L1 and L2 forming a _shield .

FIG. 6A shows the shield coil 12.
_{In the case where} the substantial pattern length of the _shield is equal to the axial length of the static magnetic field magnet 11 (that is, D = 0 shown in FIG. 5), FIG. 10 [cm] longer (that is, D = 10 shown in FIG. 5)
FIG. 5C shows the case where the substantial pattern length is 20 [cm] longer than the side end face (that is, D =
20 (cm)) and FIG. 5 (d) when the substantial pattern length is 30 [cm] longer than the side end face (ie, FIG. 5).
(D = 30 cm) shown in FIG.

As can be seen from these figures, the magnitude of the magnetic field leaking in the central region in the Z-axis direction (near Z = 30 to 40 cm) and the magnitude of the eddy current generated by this are substantially equal to the pattern length and the static magnetic field. Even if the positional relationship parameter D with the end face position changes, it hardly changes. On the other hand, it can be seen that their amounts in the axial end region (around Z = 70 to 80 [cm]) greatly change depending on the positional relationship parameter D.

Specifically, in the case of FIG. 6C (D = 20
[Cm], the magnitude of the eddy current generated in the axial end region (near Z = 70 to 80 [cm]) of the static magnetic field magnet with respect to the eddy current due to the leakage magnetic field is changed in the axial central region (Z = 30-4
(In the vicinity of 0 [cm]), which tends to be equal to or less than the magnitude of the eddy current generated at about 0 [cm]. That is, this D = 2
In the case of 0 [cm], the noise generated from the end of the static magnetic field magnet is equal to or less than the noise generated from other parts of the magnet. Therefore, from the viewpoint of suppressing noise, D = 2
The structure of 0 [cm] is optimal.

D = 0 or 10 [c] in FIGS.
m], the magnitude of the eddy current generated in the axial end region of the magnet is still larger than that in the central region. Conversely, when D = 30 [cm] in FIG. Although the magnitude of the eddy current generated in the axial end region of the central region is smaller than that in the central region, the eddy current in the central region is considered to be a fixed amount determined by other requirements. Making the magnitude of the eddy current generated in the region lower than that in the central region is meaningless from the viewpoint of noise suppression.

Therefore, the shield pattern 12 shield _ld of each channel of the _ASGC 12 has its substantial pattern length set to the distance D from the side end face of the static magnetic field magnet 11.
= A short value that is only about 20 cm outward in the axial direction, and it is possible to reduce the noise due to the leakage magnetic flux from the end to the same amount as that at the center. As a result, the overall noise can be reduced as compared with the conventional method while maintaining the axial length of the shield coil of the ASGC 12 (that is, the axial length of the gantry 1) as short as possible to eliminate the size increase.

(Second Embodiment) A second embodiment of the present invention will be described with reference to FIG. Here, the same reference numerals are used for the same or equivalent components as those of the first embodiment described above,
The description is simplified or omitted.

The gantry 1 according to the second embodiment includes:
The shape of the side end surface of the static magnetic field magnet 11 is improved from the leakage magnetic field from the axial end region of the shield coil, that is, the noise caused by the eddy current.

As shown in FIG. 7, the side end surface (side end portion) 11Aa of the magnet 11 has a "wide opening shape" in which it becomes rounder and expands as it advances outward from the inner peripheral surface side in the axial direction. . This feature can be clearly understood when compared with the “cylindrical shape” shown in FIG.

Thus, the magnet side end surface (side end portion) 11A
In (a), the radial distance of the magnet reaching the metal cover increases as it advances in the axial direction, so that the amount of eddy current generated decreases as it advances in the axial direction, and the amount of noise decreases accordingly. Therefore, the extent of this roundness is determined based on simulations and experiments, taking into account factors such as the uniformity of the magnetic field in the imaging field formed in the static magnetic field and the suppression of noise. The amount of noise generated from
It can be set to be equal to or less than the noise amount due to the leakage magnetic field from other parts in the axial direction. In this case, the pattern length of the shield coil 12 _shield may be the same as the conventional one.

As described above, by forming the side end face (side end) 11Aa of the static magnetic field magnet 11 in a wide opening shape, noise can be suppressed, but the ASGC has a shorter axial length than that of the static magnetic field magnet. Thus, the gantry can be made compact in the axial direction.

(Third Embodiment) A third embodiment of the present invention will be described with reference to FIG.

The gantry 1 according to the third embodiment has an AS
X, Y, Z coil assembly 12X, 12 of GC12
In each of Y and 12Z, an arrangement position of a return line (also referred to as a crossover line) for returning a wiring from a far winding part CR (coil pattern part) in the bore radial direction to the power supply terminal side is determined from the viewpoint of noise suppression. Configuration. This configuration may be performed alone, or may be performed in combination with the characteristic configuration of the first or second embodiment.

In general, the positions of the main coil and the shield coil of each channel of the ASGC in the radial direction of the bore where the shield coil is wound are different from each other, so that the main coil has its own return line, and the shield coil has its own return line. A line will be placed. In other words, since the main coil and the shield coil have different positions of the return line in the bore radial direction, the magnetic field cannot be completely canceled even if the currents flowing through the return line are opposite to each other. For this reason, a magnetic field which cannot be completely canceled from the return line of the main coil and the shield coil leaks to the outside, which contributes to noise.

Therefore, in the present embodiment, as shown in FIG. 8, in the coil assembly of each channel, the return line (crossover line) R of the shield coil 12 _shield is set.
_{The shield} is arranged and wired at the same radial position as the return line (crossover line) R _main of the main coil 12 _main . Moreover, the wiring is performed on the bobbin on the main coil 12 _main side. The return line R _shield on the shield coil side may be inserted together when the main coil 12 _main is made.

The wiring to the shield coil 12 _shield is connected after rising from the power supply terminal T by the rising line L _up in the bore radial direction. Also, the shield coil 12
wiring from _{shi the eld} is lowered in the bore radially falling edge line _{L down} in the axial direction end portion on the opposite side,
Connected to the return line R _main described above, another power supply terminal T
Is returned to. In FIG. 8, the shape of the winding portion is represented as a Z channel, but this return line structure can be similarly applied to the X channel and the Y channel.

Therefore, both return lines R _main and R
_Since the currents flowing through the _shields are opposite to each other, and are located at the same positions in the bore radial direction and are almost aligned, almost all magnetic fields generated from their return lines are canceled out. Therefore, noise due to the current flowing through the return line is also reliably reduced. In addition, since the return line R _shield on the _shield coil side is disposed on the main coil side, it is further away from the static magnetic field magnet by that amount, and this point is also effective for noise reduction. Furthermore, in the present embodiment, more that the pattern length is shorter than the shield coil, the use of the main coil 12 _main side with spatially relatively margin, return lines of the shield coil 12 _{Shiel d} R
Space for wiring the _shield can be secured.
Since there is no need to secure such a space on the shield side, it is possible to maintain a short axis of the ASGC.

[0051]

As described above, the MR according to the present invention is used.
According to the I apparatus, in an MRI apparatus having an active (self) shield type gradient magnetic field coil (ASGC), the pattern length in the axial direction of the shield coil of the ASGC is determined in relation to noise suppression, so that the shield coil The pattern length in the axial direction is kept as short as possible, which keeps the gantry compact and suppresses the noise generated by the magnetic field leaking from the axial end, improving the comfort for patients. An MRI apparatus can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a gantry of a magnetic resonance imaging apparatus according to an embodiment of the present invention.

FIG. 2 shows a plane orthogonal to the Z-axis direction of ASGC (I in FIG. 1).
FIG. 2 is a schematic cross-sectional view taken along a line I-II).

FIG. 3 is a view for explaining a positional relationship between a shield coil of the ASGC according to the first embodiment and a side end of a static magnetic field magnet.

FIG. 4 is a diagram for explaining a winding and a pattern length of a winding portion of a shield coil.

FIG. 5 is a view for explaining a positional relationship between an ASGC shield coil and a side end of a static magnetic field magnet.

FIG. 6 is a graph showing a simulation result of a leakage magnetic field and an eddy current using a distance difference D between a substantial pattern length of a shield coil and an axial length of a magnet as a parameter.

FIG. 7 is a view for explaining a positional relationship between a shield coil of an ASGC according to a second embodiment and a side end of a static magnetic field magnet.

FIG. 8 is a schematic explanatory diagram illustrating a return line structure of a main coil and a shield coil in each channel of an ASGC according to a third embodiment.

[Explanation of symbols]

Reference Signs List 1 Gantry 11 Static magnetic field magnet 11Aa Side end face (side end) 12 Active shield type gradient coil (ASGC) 12X X-channel gradient coil assembly 12Y Y-channel gradient coil assembly 12Z Z-channel gradient coil assembly 12 _main Main coil (primary coil) 12 _shield shield coil (secondary coil) CR winding part R _main , R _shield return line (crossover line)

Claims (6)

[Claims] 1. An MRI (Magnetic Resonance Imaging) apparatus having a gantry having a gradient magnetic field coil disposed in a bore of a static magnetic field magnet, wherein the gradient magnetic field coil is an active shield type gradient magnetic field coil having a shield coil. The pattern length of the shield coil with respect to the axial length of the static magnetic field magnet is determined by the eddy current generated at the axial end of the bore surface of the static magnetic field magnet due to the magnetic field leaking from the axial end of the gradient magnetic field coil. An MRI apparatus characterized in that it is determined to be equal to or less than an eddy current generated in a portion other than an axial end of a bore surface. 2. The MRI apparatus according to claim 1, wherein a pattern length of a winding portion of the shield coil extends outward in a bore axis direction from a side end face of the static magnetic field magnet, and the winding portion has a pattern length. An MRI apparatus in which a winding packed closer to the center in the bore axis direction than the position where the winding position is theoretically analyzed is disposed on the outside in the bore axis direction. 3. The MRI apparatus according to claim 2, wherein an extension of the pattern length from a side end face of the static magnetic field magnet is at least 20 [mm]. 4. The M according to claim 1, wherein
An MRI apparatus according to the RI apparatus, wherein an axial end of a bore formed on an inner peripheral side of the static magnetic field magnet has an opening shape whose diameter increases as it goes outward in the axial direction. 5. The M according to claim 1, wherein
In the RI apparatus, the MRI apparatus is configured such that, in each of the channels of the gradient coil, both return lines of the main coil and the shield coil of at least one channel cancel each other out of the magnetic field generated by energization. 6. The MRI apparatus according to claim 5, wherein both return lines of the main coil and the shield coil are juxtaposed with each other at substantially the same position in the radial direction of the assembly of the main coil. MRI
apparatus.