JP2012016524A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus which permits the deviation of the center of gravity of electric current in a spiral electric conductor 32 to be corrected.SOLUTION: The magnetic resonance imaging apparatus having the spiral electric conductor 32 has slits S, S, and Sprovided along the longitudinal direction in the electric conductor 32 and separating electric current conducting routes L, L, Land Lformed in the electric conductor 32 into multi-sections. Among the separated electric current conducting routes L, L, Land L, those located nearer to the outside of the spiral shape are made wider (wn>w3>w2>w1). The slits S, S, and Sare formed in the locations where the radius rof curvature of the electric conductor 32 is locally reduced in the longitudinal direction of the electric conductor 32.

Description

  The present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI; Magnetic Resonance Imaging) apparatus having a spiral electric conductor.

  An MRI apparatus captures a tomographic image representing the physical and chemical properties of a subject using a nuclear magnetic resonance (hereinafter referred to as NMR; Nuclear Magnetic Resonance) phenomenon, and is widely used particularly for medical purposes. ing.

  In the MRI apparatus, a gradient magnetic field generator is used in order to give position information to a measured NMR signal. The gradient magnetic field generator has a plurality of spiral electric conductors (coils) that generate a gradient magnetic field whose magnetic field intensity linearly changes along three directions orthogonal to each other in the imaging region. By this gradient magnetic field, the MRI apparatus obtains three-dimensional position information of the NMR signal. For this reason, the accuracy of the magnetic field distribution of the gradient magnetic field is important because it affects the accuracy of the positional information of the NMR signal to be measured, and thus the spatial resolution of the image.

  The gradient magnetic field generator needs to generate a large gradient magnetic field in order to improve the spatial resolution of the image and increase the accuracy of the magnetic field distribution of the gradient magnetic field. For this reason, the electrical resistance of the gradient magnetic field generator is reduced in order to increase the current flowing through the gradient magnetic field generator and reduce the burden on the power supply device. That is, the width and thickness of the plurality of spiral electric conductors constituting the gradient magnetic field generator are increased. Here, the thickness in the normal direction of the surface on which the coil pattern of the spiral electric conductor is formed is the thickness of the spiral electric conductor, and the spiral shape in the direction perpendicular to the current direction and the normal direction is used. The dimension of the electrical conductor is defined as the width. The gradient magnetic field generator is manufactured by processing a flat plate-shaped electric conductor having a certain thickness into a spiral coil pattern that generates a desired magnetic field.

  The current flowing in the spiral (coil pattern) electric conductor has a distribution in a direction perpendicular to the current direction (the width direction). This is because the current tends to flow along the shortest path closer to the inner side of the spiral so that the electrical resistance becomes smaller at the portion where the electric conductor is curved (not linear). In particular, when the width of the electric conductor is larger than the radius of curvature inside the curved portion of the electric conductor, the current center of gravity of the current distribution is biased inward from the center of the electric conductor in the conductor width direction. Here, consider a circle in contact with the center line in the width direction of the spiral electric conductor (a line along the center in the width direction), with the end of the conductor located at the center of the circle being the inside and the other end being the outside. Define.

  When the bias of the current center of gravity becomes too large to be ignored with respect to the (design) dimension of the coil pattern of the spiral electric conductor of the gradient magnetic field generator, the gradient magnetic field actually generated by the gradient magnetic field generator and the designed gradient magnetic field It was thought that the error of the error was increased, the measurement accuracy of the position information of the NMR signal was deteriorated, and the spatial resolution of the image was lowered.

  As a conventional technique for correcting the bias of the current center of gravity, a slit is provided in the width direction in a high-frequency (hereinafter referred to as RF) irradiation coil of an electric conductor having a curved portion that generates a high-frequency magnetic field during imaging by an MRI apparatus. A method of realizing an appropriate current distribution by shifting a current path has been proposed (see, for example, Patent Document 1). Further, a method has been proposed in which a slit is provided inside a conductor to suppress the flow of eddy current (for example, see Patent Document 2).

JP-A-8-332177 JP 7-31490 A

  However, in the prior art (Patent Document 1), since the slit is provided in a direction perpendicular to the current direction, the electrical resistance of the entire RF irradiation coil is increased as compared with before the slit is provided, and the load applied to the power supply device is increased. It was thought that the calorific value of the RF irradiation coil increased.

  This is a problem that may occur not only in the RF irradiation coil but also in other spiral electric conductors mounted on the MRI apparatus. For example, in a plurality of spiral electric conductors (coils) included in the gradient magnetic field generator. It is thought that a similar problem will occur. The MRI apparatus includes a gradient magnetic field generator, an RF irradiation coil, an RF receiving coil, a shim coil, and the like as spiral electric conductors.

  Therefore, the problem to be solved by the present invention is to provide an MRI (magnetic resonance imaging) apparatus capable of correcting the bias of the current center of gravity in a spiral electric conductor.

In order to solve the above problems, the present invention provides:
In an MRI (magnetic resonance imaging) apparatus having a spiral-shaped electrical conductor,
Provided in the electrical conductor along the longitudinal direction of the electrical conductor, and having a slit for dividing a current conduction path formed in the electrical conductor into a plurality of parts,
The divided current conducting paths are characterized in that the width of the current conducting paths in the direction of the outside of the spiral shape is wider.

  ADVANTAGE OF THE INVENTION According to this invention, the MRI (magnetic resonance imaging) apparatus which can correct | amend the deviation of the current gravity center in a spiral-shaped electrical conductor can be provided.

1 is a perspective view of a vertical magnetic field type magnetic resonance imaging (MRI) apparatus according to a first embodiment of the present invention. 1 is a longitudinal sectional view of a vertical magnetic field type MRI apparatus according to a first embodiment of the present invention. 1 is an exploded perspective view of a gradient magnetic field generator of a vertical magnetic field type MRI apparatus according to a first embodiment of the present invention. It is a top view of the spiral electric conductor which comprises a part of main coil (y) of a gradient magnetic field generator. It is an enlarged view around the 1st curved part of the spiral-shaped electric conductor which comprises a part of main coil (y) of a gradient magnetic field generator. It is explanatory drawing of the micro area | region used for the calculation of the surroundings of the 1st curved part of a spiral-shaped electric conductor. It is sectional drawing of the 1st curved part of the spiral-shaped electric conductor which comprises a part of main coil (y) of a gradient magnetic field generator. A part of the main coil (y) of the gradient magnetic field generator of the vertical magnetic field type MRI apparatus according to Example 1 (when one slit is provided in the width direction) of the first embodiment of the present invention. It is sectional drawing of the 1st curved part of the spiral-shaped electric conductor which comprises. It is the graph which showed the relationship between the gravity center deviation | shift rate and the distance from an inner side to a slit for every conductor width. It is the graph which showed the relationship between the distance from an inner side to a slit, and a conductor width. A part of the main coil (y) of the gradient magnetic field generator of the vertical magnetic field type MRI apparatus according to Example 2 of the first embodiment of the present invention (when two slits are provided in the width direction) It is sectional drawing of the 1st curved part of the spiral-shaped electric conductor which comprises. It is the graph which showed the relationship between the distance from an inner side to each slit, and a conductor width. It is a top view of the shim coil (vortex-shaped electric conductor (curved part)) of the perpendicular magnetic field type MRI apparatus which concerns on the 2nd Embodiment of this invention. It is a perspective view of a horizontal magnetic field type MRI apparatus according to a third embodiment of the present invention. It is a longitudinal cross-sectional view of the horizontal magnetic field type MRI apparatus which concerns on the 3rd Embodiment of this invention. It is a perspective view of the main coil (y) (spiral-shaped electric conductor) of the gradient magnetic field generator of the horizontal magnetic field type MRI apparatus according to the third 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. 1 is a perspective view of a vertical magnetic field type magnetic resonance imaging (MRI) apparatus according to a first embodiment of the present invention. The MRI apparatus 1 of the first embodiment is a vertical magnetic field type (open type) MRI apparatus in which the direction 7 of the static magnetic field is the vertical direction (z direction). In the MRI apparatus 1, a pair of upper and lower electromagnet apparatuses 2 are arranged so as to sandwich the imaging area 8, so that even if the subject 5 is placed on the imaging area 8 while lying on the bed 6, it is accessed by the examiner. To ensure a sufficient gantry gap. An RF receiving coil 22 is provided so as to surround the periphery of the subject 5. The connecting column 17 is provided between the pair of upper and lower electromagnet devices 2 and is connected to each of the pair of upper and lower electromagnet devices 2 to hold the pair of upper and lower electromagnet devices 2 apart from each other. The pair of upper and lower electromagnet devices 2 generates a static magnetic field in the imaging region 8 with the direction 7 being the vertical direction (z direction) and the magnetic field strength being uniform.

  Further, the gradient magnetic field generator 3 is disposed on the imaging region 8 side of the electromagnet device 2. The gradient magnetic field generator 3 generates a gradient magnetic field in which the magnetic field intensity linearly changes along the three directions orthogonal to each other, the x direction, the y direction, and the z direction in the imaging region 8, and generates a measured NMR signal. It has position information. The MRI apparatus 1 creates an examination image of the subject 5 based on this position information.

  The RF irradiation coil 4 is also arranged on the imaging region 8 side of the electromagnet device 2. The RF irradiation coil 4 irradiates a subject 5 lying on the imaging region 8 with a high-frequency electromagnetic wave and generates an NMR signal from the subject 5.

  In the MRI apparatus 1, an xyz coordinate system having an origin is set in the imaging region 8 in order to facilitate understanding of the structure. The z-axis direction is defined as a direction that matches the direction 7 of the static magnetic field, the y-axis direction is defined as a direction that matches the supine direction of the subject 5, and the x-axis direction is orthogonal to the y-axis and the z-axis. The direction is defined.

  FIG. 2 is a longitudinal sectional view of the vertical magnetic field type MRI apparatus 1 according to the first embodiment of the present invention. The pair of upper and lower electromagnet devices 2 includes a pair of main coils (superconducting coils) 2a disposed opposite to each other above and below the imaging region 8, and a pair of upper and lower shield coils disposed outside the pair of main coils 2a in the z-axis direction. (Superconducting coil) 2b.

  The cooling container 2e can cool a pair of upper and lower main coils 2a and a pair of upper and lower shield coils 2b together with a refrigerant such as liquid helium (He). The cooled main coil 2a and shield coil 2b can function as a superconducting coil. The vacuum container 2c can accommodate the cooling container 2e, and can hold | maintain the space between the vacuum container 2c and the cooling container 2e in a vacuum. The cooling container 2e can be insulated from the outside air (external), and the cooling container 2e can be kept at a low temperature. The radiation shield 2d is provided in a vacuum space between the cooling vessel 2e and the vacuum vessel 2c, can reduce radiant heat from the vacuum vessel 2c to the cooling vessel 2e, and can keep the cooling vessel 2e at a low temperature.

  In addition, a pair of upper and lower shim coils 21 are disposed so as to be opposed to each other in the upper and lower sides with the imaging region 8 interposed inside the pair of upper and lower electromagnet devices 2 in the z-axis direction. The pair of upper and lower shim coils 21 are disposed in the vicinity of the pair of upper and lower gradient magnetic field generators 3. By adjusting the magnetomotive force of each shim coil 21, the magnetic field uniformity of the static magnetic field generated by the electromagnet device 2 in the imaging region 8 can be improved.

  In addition, a pair of upper and lower gradient magnetic field generators 3 are disposed vertically opposite to each other with the imaging region 8 interposed inside the pair of upper and lower electromagnet devices 2 in the z-axis direction. For example, as shown in FIG. 2, the pair of upper and lower gradient magnetic field generators 3 generate a gradient magnetic field 9 whose magnetic field strength is inclined in the y-axis direction.

  A pair of upper and lower RF irradiation coils 4 are disposed so as to face each other up and down across the imaging region 8 inside the pair of upper and lower electromagnet devices 2 in the z-axis direction. The pair of upper and lower RF irradiation coils 4 are disposed in the vicinity of the pair of upper and lower gradient magnetic field generators 3.

  FIG. 3 is an exploded perspective view of the gradient magnetic field generator 3 of the vertical magnetic field type MRI apparatus 1 according to the first embodiment of the present invention. The gradient magnetic field generator 3 generates three main coils (x) 31, a main coil (y) 32, and a main coil (z) in order to generate independent gradient magnetic fields in the three directions of the x axis, the y axis, and the z axis. 33. In addition, the gradient magnetic field generator 3 has three shield coils (3) corresponding to each of the three main coils (x) 31, the main coil (y) 32, and the main coil (z) 33 in order to shield the leakage magnetic fields. x) 35, a shield coil (y) 36, and a shield coil (z) 34. In the vertical magnetic field type MRI apparatus 1, a pair of gradient magnetic field generators 3 are arranged on the top and bottom, and each of the gradient magnetic field generators 3 is provided with a total of six coils 31 to 36. The stacking order of the six coils 31 to 36 shown in FIG. 3 is the upper one of the pair of upper and lower gradient magnetic field generation devices 3, and the stacking order is reversed in the lower gradient magnetic field generation device 3. .

  Each of the main coil (x) 31 and the shield coil (x) 35 has a line-symmetrical spiral-shaped electric conductor having two spiral centers 3a arranged in the x-axis direction. Each of the main coil (y) 32 and the shield coil (y) 36 has a line-symmetrical and spiral electric conductor having two spiral centers 3a arranged in the y-axis direction. Each of the main coil (z) 33 and the shield coil (z) 34 has a spiral electric conductor having one spiral center 3a in the center.

FIG. 4 is a plan view of a spiral electric conductor constituting a part of the main coil (y) 32 of the gradient magnetic field generator 3. FIG. 4 shows a coil pattern of a spiral-shaped electric conductor corresponding to one of the two spiral centers 3a constituting the main coil (y) 32 as a specific example of the spiral-shaped electric conductor having the spiral center 3a. ing. A part of the spiral-shaped electric conductor of the main coil (y) 32 shown in FIG. 4 is a line-symmetric half half of the main coil (y) 32 shown in FIG. The spiral-shaped electric conductor such as the main coil (y) 32 includes a first bending portion 11a having a locally small curvature radius and a second bending portion having a larger curvature radius than the first bending portion 11a and a gentle curve. 11b and a straight portion 12 that is not curved. The first bending portion 11a is provided with slits S ₁ , S ₂ and S ₃ along the longitudinal direction (spiral direction) of the electric conductor. In one slit _{S 1,} or divided into a first curved portion 11a into two, with the two slits _S 1, _{S 2,} or divided into a first curved portion 11a into three, the three slits _{S 1, S} 2, in S _3, it is or divided into four first bending portion 11a. A spiral electric conductor such as the main coil (y) 32 is manufactured by cutting a plate of a good conductor such as copper (Cu) or aluminum (Al). As a method for forming the spiral electric conductor, etching, water jet, cutting by punching, or the like can be used.

FIG. 5 shows an enlarged view around the first curved portion 11a of the spiral electric conductor constituting a part of the main coil (y) 32 of the gradient magnetic field generator 3. FIG. In FIG. 5, three or more slits S ₁ , S ₂ , S ₃ are provided in the first bending portion 11a of the main coil (y) 32 (electrical conductor) along the longitudinal direction of the first bending portion 11a. Yes. The three or more slits S ₁ , S ₂ , S ₃ form a current conduction path formed in the first bending portion 11a of the main coil (y) 32 (electrical conductor) and include four or more (plural) currents. The conductive paths are divided into L ₁ , L ₂ , L ₃ ,... L _n . In the divided current conducting paths L ₁ , L ₂ , L ₃ ,... L _n , the width (conductor width) is wider as the current conducting paths are in the direction of the spiral shape (w ₁ < w ₂ <w ₃ <w _n , where w ₁ is the conductor width of the current conduction path L ₁ , w ₂ is the conductor width of the current conduction path L ₂ , w ₃ is the conductor width of the current conduction path L ₃ , w _n conductor width of the current conduction path _{L n} is). In the first bending portion 11a, the width direction coincides with the radial direction of the curvature radius of the bending of the first bending portion 11a. Regarding the radial direction of the radius of curvature, the side of the first curved portion 11a where the radius of curvature is large can be defined as the outside of the first curved portion 11a, and the side of the first curved portion 11a where the radius of curvature is small is defined as the first curved portion 11a. Can be defined as inside. The conductor width of the outer current conduction path of the first bending portion 11a is wider than the conductor width of the inner current conduction path (w _i <w _{i + 1} , where i is 1, 2, 3, etc. Natural number).

Both ends in the longitudinal direction of the slits S ₁ , S ₂ , S _{3 form} a branching section S ₀ that branches the current conducting paths L ₁ , L ₂ , L ₃ , L _n . Bifurcation S _0, in the longitudinal direction of the main coil (y) 32 (electrical conductors), the radius of curvature of the electrical conductors is provided at a position which changes to a smaller value from the larger value. In FIG. 5, the branch portion S ₀ is provided in a region where the electric conductor is switched from the straight shape in which the curvature radius is considered to be infinite to the first bending portion 11 a having the curvature radius r ₀ .

Current conducting path _L 1 shown _{above, L 2, L 3, ···} L n each conductor width _{w 1, w 2, w 3} , ··· w n , the current path designer desires (coil According to the pattern), it is determined according to the following principle.

ここで、ρは導体の電気抵抗率、ｌは導体の長さ、Ｓは導体の断面積である。ある曲率中心Ｏのもと湾曲した電気導体（第１湾曲部１１ａ）において、導体幅と導体厚さが一定の場合、曲率半径の小さい内側と、大きい外側では、長手方向の導体長さが異なる。このため、距離が長い第１湾曲部１１ａの外側では電気抵抗Ｒが高く、距離の短い内側では電気抵抗Ｒが低くなる。 First, the relationship between the electric resistance R and the dimension of the electric conductor is expressed by the following equation (Equation 1).

Here, ρ is the electrical resistivity of the conductor, l is the length of the conductor, and S is the cross-sectional area of the conductor. When the conductor width and the conductor thickness are constant in the electric conductor curved at the center of curvature O (first curved portion 11a), the conductor length in the longitudinal direction is different between the inner side having a small radius of curvature and the outer side having a large radius of curvature. . For this reason, the electrical resistance R is high outside the first curved portion 11a with a long distance, and the electrical resistance R is low inside the short distance.

As shown in FIG. 6, the electric resistance R of the area of the minute width dr at position (radius of curvature) r of the curvature radius direction of the current conducting path L ₁ is a very small angle dθ of the curvature center O around the angle theta, the electrical conductor ( When expressed by the conductor thickness t of the first bending portion 11a), the length l of the conductor is expressed by the product of the position r and the minute angle dθ (l = rdθ), and the cross-sectional area S of the conductor is the conductor thickness. Since it is represented by the product of t and a minute width dr (S = tdr), the following equation 2 is obtained.

Using equation 2 relationship, the calculation method of the current barycentric position rc in the electric conductors (the first curved portion 11a), a current conducting path L ₁ as an example will be described with reference to FIG. First, the sum of the currents (values) I flowing in the electric conductor (current conduction path L ₁ ) between the inner curvature radius r 0 and the outer curvature radius r ₁ is the longitudinal direction of the electric conductor (current conduction path L ₁ ). When the potential difference between both ends is set to V, the following Equation 3 is obtained.

The position rc of the current center of gravity in the electric conductor (current conduction path L ₁ ) is weighted (r · I) by the position (curvature radius) r in the radius of curvature direction with the current value I, and the sum is obtained. It is obtained by dividing by the sum of I. First, the weighted sum of the current value I at the position r is expressed by the following equation (4).

Therefore, the current center-of-gravity position rc is expressed by the following equation 5 obtained by dividing the weighted sum of the current values I of Equation 4 by the sum of the current values I of Equation 3.

Next, the electric conductor (the first bending portion 11a) has a plurality of current conduction paths L ₁ , L ₂ , L ₃ ,... In the width direction (curvature radius direction) by the slits S ₁ , S ₂ , S _3. Consider the current center-of-gravity position rc when it is divided into L _n . This corresponds to that the current conducting path L ₁ shown in FIG. 6, are arranged a plurality in the width direction, as shown in FIG. 7, the electrical conductors (first curved portion 11a) is, n pieces in the width direction current conducting path _L 1 _of, L _2, L 3, is divided into · · · _{L n,} conducting their current paths _{L 1,} L _2, L 3, that · · · _{L n} are arranged in the width direction It corresponds. The current center-of-gravity position rc when such a plurality of current conduction paths L ₁ , L ₂ , L ₃ ,... L _n are viewed as one electric conductor (first bending portion 11a) is obtained by Equation 5. from the current gravity center position rc of current conducting paths _{L 1} and, each current conducting paths _L 1 that _analogy, L _2, L 3, the current centroid position rc of · · · _{L n} (current conducting path _{L i + 1),} each current conductor It can be calculated by weighting with the current value I _i flowing through the passages L ₁ , L ₂ , L ₃ ,... L _n (current conduction path L _{i + 1} ). Therefore, first, a current value I _i flowing through each current conduction path L ₁ , L ₂ , L ₃ ,... L _n (current conduction path L _{i + 1} ) is obtained. Adjacent current conduction paths L ₁ , L ₂ , L ₃ ,... L _n are branched from slits S ₁ , S ₂ , S ₃ , and the starting and ending points (branching portion S ₀ ) are the first curved portion of the electric conductor. Since this is the place where the straight portion 12 (see FIG. 4 or the second curved portion 11b having a gentle curve) is formed from 11a, the current conduction path L between the slits S _i and S _{i + 1} in the minute length dl of the branch portion S ₀ The electrical resistance Ri in the longitudinal direction of _{i + 1} can be calculated as the following formula 6.

Therefore, the current value _{I i} flowing through the current conduction path _{L i + 1} between slits _{S i} and _{S i + 1,} the current conduction path _{L 1, L 2, L 3} , none of the voltages of both ends in the longitudinal direction of the · · · _{L n} If the voltage V is set equal, the following equation 7 is obtained.

The current centroid position rci current conduction path _{L i + 1} between slits _{S i} and _{S i + 1} is by analogy than the number 5, so that the next few 8.

From Equations 7 and 8, the weighted current barycenter position (Iirci) is as shown in Equation 9 below.

On the other hand, the sum of the current values I _i flowing through the n-divided current conduction paths L ₁ , L ₂ , L ₃ ,... L _n is the sum of the current values I _i of Equation 7, become that way.

Finally, the sum of the weighted current barycentric positions (Iirci) in Equation 9 is divided by the sum of the current values I _i in Equation 10 to obtain a plurality of current conduction paths L ₁ , L ₂ , L ₃ ,. current centroid position rc of L _n in the divided electrical conductors (first curved portion 11a) is can be calculated by Equation 11.

In addition, the geometric center of the width direction of the electrical conductor (first curved portion 11a) is expressed as the following Expression 12.

From the above, in the electrical conductor (first curved portion 11a) provided with the inner curvature radius r0 and the outer curvature radius rn, the difference between the current center of gravity position rc of Equation 11 and the geometric center of Equation 12 is minimized. as the slit _{S 1, S 2, S 3} (S i, S i + 1) of curvature of the radius _{r1, r2, r3 (r i} , r i + 1) ( i.e., the slits _{S 1, S 2, S 3} (S _i , S _{i + 1} )) can be obtained, so that the positions of the slits S ₁ , S ₂ , S ₃ (S _i , S _{i + 1} ) having the smallest deviation of the current center-of-gravity position rc can be obtained. That is, the solution to the minimization problem of the following Equation 13 is the optimal arrangement position of the slits S ₁ , S ₂ , S ₃ (S _i , S _{i + 1} ). The current conducting paths _{L 1, L 2, L 3} , the conductor width of _{··· L n w 1, w 2} , w 3, it is possible to determine · · · _{w n.}

(Example 1 of the first embodiment)
Next, the arrangement position (curvature radius) r ₁ will be described by taking as an example the case where there is one slit S _{1 in} the electrical conductor (first curved portion 11 a).

FIG. 8 shows, as Example 1 of the first embodiment, a cross section of the first curved portion 11a of the spiral electric conductor (main coil (y)) 32 provided with _one slit S1 in the width direction. The figure is shown. The first bending portion 11a is divided into two in the width direction by _one slit S1, and current conduction paths L ₁ and L ₂ are formed. Thus, in the divided current conduction paths L ₁ and L ₂ , the conductor width w ₁ of the current conduction path L ₁ in the spiral inner direction is the electric conductor (main coil in the place where the slit S ₁ is provided. (Y)) More than 0.3 times and less than 0.5 times the conductor width w of the first bending portion 11a constituting a part of 32. Conductor width _{w 1} Because the radius of curvature r1 of the slit _{S 1} (corresponding to the arrangement position of the slit _{S 1)} is equal to the value obtained by subtracting the radius of curvature r0 inside of the first bending portion 11a _(w 1 = r1-r0) , arrangement position of the slit _{S 1} (the radius of curvature r1) has a relation of the following equation 14.

Next, the reason for the relationship of Equation 14 will be described. First, the number 13, the slits S ₁ is there one in the width direction, write down the case n is 2. Equation 13 can be written as Equation 15 below. In addition, r2 is a curvature radius of the outer side of the 1st curved part 11a.

According to Equation 15, for an arbitrary dimension of the first bending portion 11a, that is, an arbitrarily determined ratio (r0 / r2) of the inner curvature radius r0 and the outer curvature radius r2 of the first bending portion 11a. Alternatively, for the ratio of the inner radius of curvature r0 and conductor width w of the first bending portion 11a which is arbitrarily determined (w / r0), the arrangement position of the slit S ₁ to minimize the number 15 (curvature radius) r1 is Can be calculated (determined). 9 shows the arrangement position of the slit _{S 1} a (radius of curvature) r1, the dimensions of the first bending portion 11a, i.e., every 10, 20, 30, 40 of the ratio (w / r0), a first curved The ratio of the calculated value of Formula 15 changed from the inner radius of curvature r0 to the outer radius of curvature r2 of the portion 11a to the conductor width w (center of gravity deviation rate: vertical axis in FIG. 9) is shown. The horizontal axis of FIG. 9, the distance from the inside of the first bending portion 11a to the slit S ₁ a (r1-r0), is normalized by dividing the inside of the radius of curvature r0. This may be a first curved portion 11a of any shape, if the conductor width w ratio (w / r0) is equal shape similar to the inner radius of curvature r0, the optimal position of the slit S ₁ ( This is because the curvature radius r1) is at the same ratio (r1-r0) / r0 in the width direction. The smaller the gravity center deviation rate is close to zero, the more current can flow as desired, and the desired high-precision gradient magnetic field can be formed.

9, for example, the ratio in (w / r0) is 10 (w / r0 = 10) , the arrangement position of the slit _{S 1} a (radius of curvature) r1, from the inner radius of curvature r0 of the first bending part 11a When changing to the outer radius of curvature r2 (r1 = r0 → r1 = r2), the graph of the center-of-gravity deviation rate decreases, takes one minimum value (minimum value), and then increases. It becomes a convex curve. Distance from the inside of the first bending portion 11a to the slit _{S 1} normalized by dividing inside radius of curvature r0 (r1-r0) / r0 ( horizontal axis), when 4.3 ((r1-r0) /R0=4.3), it can be seen that the center of gravity deviation rate (vertical axis) takes the minimum value. Then, a first value range (r1-r0) is to be obtained from the inside to the slit S ₁ of the curved portion 11a when taking the minimum value.

Similarly, when the ratio (w / r0) is 20 (w / r0 = 20), the distance (r1-r0) / r0 (horizontal axis) is 8 ((r1-r0) / r0 = 8). The center-of-gravity deviation rate (vertical axis) takes a minimum value. When the ratio (w / r0) is 30 (w / r0 = 30) and the distance (r1-r0) / r0 (horizontal axis) is 11.7 ((r1-r0) /r0=11.7) ), The center of gravity deviation rate (vertical axis) takes the minimum value. When the ratio (w / r0) is 40 (w / r0 = 40), the distance (r1-r0) / r0 (horizontal axis) is 15.2 ((r1-r0) /r0=15.2. ), The center of gravity deviation rate (vertical axis) takes the minimum value. When the ratio (w / r0) is 50 (w / r0 = 50) and the distance (r1-r0) / r0 (horizontal axis) is 18.5 ((r1-r0) /r0=18.5) ), The center of gravity deviation rate (vertical axis) takes the minimum value. Then, a value distance (r1-r0) is to be obtained up to the slit S ₁ from the inside of the first bending portion 11a when taking these minimum values.

10, the distance from the inside of the first bending portion 11a normalized by dividing by the conductor width w to the slit _{S 1 (r1-r0) /} w ( vertical axis), the curvature of the inner side of the first bending portion 11a radius The relationship with the conductor width w / r0 (horizontal axis) normalized by dividing by r0 is shown.

  From the ratio (w / r0) of 10 in FIG. 9 (w / r0 = 10), the distance (r1−r0) / r0 (horizontal axis) that minimizes the center of gravity bias rate (vertical axis) is 4.3 (( r1-r0) /r0=4.3), and the distance (r1-r0) / w on the vertical axis in FIG. 10 is the expression (r1-r0) / w = {(r1-r0) / r0} / (w / r0) = 4.3 / 10 = 0.43, so that 0.43 ((r1-r0) /w=0.43). The conductor width w / r0 on the horizontal axis in FIG. 10 is 10 (w / r0 = 10) from (w / r0 = 10) at 10 in the ratio (w / r0) in FIG.

  Similarly, from the ratio (w / r0) of 20 in FIG. 9 (w / r0 = 20), from the distance (r1-r0) / r0 (horizontal axis) of 8 that minimizes the center-of-gravity deviation rate (vertical axis) ( (R1-r0) / r0 = 8), and the distance (r1-r0) / w on the vertical axis in FIG. 10 is the expression (r1-r0) / w = {(r1-r0) / r0} / (w / r0 ) = 8/20 = 0.4, which is 0.4 ((r1-r0) /w=0.4). The conductor width w / r0 on the horizontal axis in FIG. 10 is 20 (w / r0 = 20) from (w / r0 = 20) in 20 of the ratio (w / r0) in FIG.

  From the ratio (w / r0) of 30 in FIG. 9 (w / r0 = 30), from the distance (r1−r0) / r0 (horizontal axis) 11.7 that minimizes the centroid bias rate (vertical axis) (( r1-r0) /r0=11.7), and the distance (r1-r0) / w on the vertical axis in FIG. 10 is the expression (r1-r0) / w = {(r1-r0) / r0} / (w / r0) = 11.7 / 30 = 0.39, resulting in 0.39 ((r1-r0) /w=0.39). The conductor width w / r0 on the horizontal axis in FIG. 10 is 30 (w / r0 = 30) from (w / r0 = 30) in the ratio (w / r0) 30 in FIG.

  From the ratio (w / r0) of 40 in FIG. 9 (w / r0 = 40), the distance (r1−r0) / r0 (horizontal axis) that minimizes the center of gravity deviation rate (vertical axis) is 15.2 (( r1-r0) /r0=15.2), and the distance (r1-r0) / w on the vertical axis in FIG. 10 is expressed by the equation (r1-r0) / w = {(r1-r0) / r0} / (w / r0) = 15.2 / 40 = 0.38, resulting in 0.38 ((r1-r0) /w=0.38). The conductor width w / r0 on the horizontal axis in FIG. 10 is 40 (w / r0 = 40) from (w / r0 = 40) in the ratio (w / r0) 40 in FIG.

  From the ratio (w / r0) of 50 in FIG. 9 (w / r0 = 50), from the distance (r1−r0) / r0 (horizontal axis) 18.5 that minimizes the center of gravity deviation rate (vertical axis) (( r1-r0) /r0=18.5), and the distance (r1-r0) / w on the vertical axis in FIG. 10 is the expression (r1-r0) / w = {(r1-r0) / r0} / (w / r0) = 18.5 / 50 = 0.37, resulting in 0.37 ((r1-r0) /w=0.37). The conductor width w / r0 on the horizontal axis in FIG. 10 is 50 (w / r0 = 50) from (w / r0 = 50) at 50 in the ratio (w / r0) in FIG.

Hereinafter, similarly, the conductor width w / r0 in Figures 9 and 10, more than 50, when changing to 1000, from the inside of the first bending portion 11a which is normalized by the conductor width w to the slit _{S 1} The distance (r1-r0) / w (vertical axis) was calculated to complete the graph of FIG. From Figure 10, the distance from the inside of the first bending portion 11a which is normalized by the conductor width w to the slit _{S 1 (r1-r0) /} w ( vertical axis), even any shape, the center of gravity deviation rate In order to reduce and further minimize the value, it has been found that it should be set within the range of more than 0.3 and less than 0.5. Specifically, the distance (r1-r0) / w on the vertical axis in FIG. 10 is 0 (zero), indicating the inner end of the first bending portion 11a, and 1 being the outer end of the first bending portion 11a. it indicates, that the length of the longitudinal axis (r1-r0) / w is in the range of less than 0.5 more than 0.3, the slits _{S 1} is the center in the width direction of the first bending part 11a Therefore, it is set closer to the inside. That is, as shown in FIG. 8, the conductor width w ₁ of the current conduction path L ₁ is set narrower than the conductor width w ₂ of the outer current conduction path L ₂ (w ₂ > w ₁ ). Although the optimum value by (horizontal axis in FIG. 10) the ratio w / r0 conductor width w and the radius of curvature r0 inside of the first bending portion 11a are different, by providing the slit S ₁ in the range shown in Equation 14, the current The bias of the center of gravity position rc can be improved efficiently and a conductor structure that can be corrected can be provided.

  In Example 1 of the first embodiment, in order to explain the expression expansion of Equation 13 and the like, the curvature center O of the inner curvature radius r0 and the curvature center O of the outer curvature radius r2 of the first bending portion 11a. However, although the case where it corresponds is taken up, this invention is not limited to this. When the curvature center O of the radius of curvature is different between the inner side and the outer side, the width direction is divided at a ratio shown in Formula 14 for each minute length in the current direction (longitudinal direction). 1 can be obtained.

(Example 2 of the first embodiment)
Next, the arrangement positions (curvature radii) r ₁ and r ₂ will be described by taking, as an example, the case where there are _two slits S ₁ and S _{2 in} the electric conductor (first bending portion 11 a).

In FIG. 11, as Example 2 of the first embodiment, a first curved portion of a spiral electric conductor (main coil (y)) 32 provided with _two slits S ₁ and S ₂ in the width direction. Sectional drawing of 11a is shown. By two slits _{S 1} of the _{S 2,} the first curved portion 11a is divided into three in the width direction, conducting three current paths _{L 1,} L 2, _{L 3} are formed. Then, the optimum slit position was calculated in the same manner as in Example 1. Specifically, the number 13, the slits are present two slits S ₁ and S ₂ in the width direction, n is written down for the case of 3. In Example 2, r3 becomes the curvature radius outside the first bending portion 11a. And the result shown in FIG. 12 was obtained.

12, a first distance from the inside of the curved portion 11a to the slit _{S 1 (r1-r0) /} w and the slit _S distance to ₂ (r2-r0) normalized by the conductor width w / w (vertical axis) And the conductor width w / r0 (horizontal axis) normalized by the radius of curvature r0 inside the first bending portion 11a is shown. From this, the arrangement position of the slit S ₁ (the radius of curvature r1) was found to have a relationship of the following Expression 16. The arrangement position of the slit S ₂ (the radius of curvature r2) was found to have a relationship of the following Expression 17.

Thus, in the divided current conduction paths L ₁ , L ₂ , and L ₃ , the conductor width w ₁ of the current conduction path L _{1 in} the direction of the spiral shape is the conductor width at the place where the slit S ₁ is provided. The conductor width w ₂ of the current conduction path L ₂ that is approximately 0.15 times greater than w and less than approximately 0.33 times and intermediate in the direction of the spiral shape is the conductor width at the location where the slit S ₁ is provided. becomes substantially 0.33 times the w, conductor width w ₃ of the current conducting paths L ₃ in the direction of the outside of the spiral shape substantially exceed 0.33 times substantially conductor width w at a position that a slit S ₂ It was found to be less than 0.5 times. In the second embodiment, as in the first embodiment, the optimum value varies depending on the ratio w / r0 (horizontal axis in FIG. 12) of the conductor width w and the radius of curvature r0 inside the first bending portion 11a. By providing the arrangement position (curvature radius) r ₁ of the slit S ₁ so as to satisfy the range, and by providing the arrangement position (curvature radius) r ₂ of the slit S ₂ so as to satisfy the range shown in Expression 17, the current center of gravity It is possible to efficiently improve the bias of the position rc and provide a correctable conductor structure.

(Example 3 of the first embodiment)
Next, when there are four or more slits S ₁ , S ₂ , S ₃ (S _i , S _{i + 1} ) in the electric conductor (first bending portion 11a), their arrangement positions (curvature radii) r1, r2, r3 _{(r i, r i + 1} ) will be described. In the third embodiment, the optimum slit position can be calculated in the same manner as in the first embodiment. Specifically, the number of slits S ₁ , S ₂ , S ₃ (S _i , S _{i + 1} ) in the width direction What is necessary is just to write down about the case where n is 5 or more which exists 4 or more. As the number of slits S ₁ , S ₂ , S ₃ (S _i , S _{i + 1} ) increases, the width of each slit becomes narrower, and thus the bias of the current center-of-gravity position rci within each slit becomes smaller. For this reason, when the number of slits increases, the bias of the entire current center-of-gravity position rc can be further reduced. In addition, when the width | variety of each slit becomes narrow, the tendency to narrow a width | variety inside an electric conductor (1st curved part 11a) and to widen outside will also become small. In addition, since the increase in the number of slits is accompanied by an increase in the number of processing steps, an optimal number of slits is selected from the viewpoint of correcting the bias of the current center of gravity position rc and increasing the processing cost.

(Second Embodiment)
FIG. 13 is a plan view of a shim coil (a spiral electric conductor (curved portion)) 21 of the vertical magnetic field type MRI apparatus according to the second embodiment of the present invention. As the vertical magnetic field type MRI apparatus of the second embodiment, the vertical magnetic field type MRI apparatus 1 described in FIGS. 1 and 2 of the first embodiment can be used, and a gradient magnetic field generator mounted on the vertical magnetic field type MRI apparatus can be used. As 3, the gradient magnetic field generator 3 described in the first embodiment may be used, or a conventional gradient magnetic field generator 3 may be used. And in 2nd Embodiment, as shown in FIG. 13, the case where slit S1 is provided and applied to the shim coil 21 which performs the fine adjustment which improves the uniformity in the imaging region 8 of the static magnetic field 7 is demonstrated. .

The spiral-shaped electric conductor of the shim coil 21 has a first curved portion 11a having a locally smaller radius of curvature near the spiral center 3a, and a first curved portion 11a having a larger radius of curvature than the first curved portion 11a on the outer peripheral side and a gentle curve. 2 curved portions 11b. Then, the first bending portion 11a, along the longitudinal direction of the electrical conductor (spiral direction), the slits S ₁ is provided. In the slit _{S 1,} divided first bending portion 11a into two, respectively, has become current conducting path _L 1, _{L 2.} In the divided current conducting paths L ₁ and L ₂ , the width w ₂ of the current conducting path L _{2 in} the direction outside the spiral shape is larger than the width of the current conducting path L _{1 in} the direction inside the spiral shape. It is wider than w ₁ (w ₁ <w ₂ ). Longitudinal end of the slit _{S 1} is adapted to branch portion _{S 0} for branching current conducting path _L 1, _{L 2.} Bifurcation S _0, in the longitudinal direction of the shim coil 21 (electrical conductors), the radius of curvature of the electrical conductors is provided in place at a predetermined value.

  When the current flowing through the shim coil 21 is small, the cross-sectional area of the cross section in the plane perpendicular to the longitudinal direction is small, and current bias is less likely to occur. However, in order to increase the amount of adjustment of the magnetic field by the shim coil 21, when the current passed through the shim coil 21 is large, it is necessary to increase the cross-sectional area and decrease the electrical resistance in order to increase the current. At this time, if the cross-sectional area is increased and the width of the electric conductor is increased, current bias may occur, and thus the present invention is applicable.

(Third embodiment)
FIG. 14 is a perspective view of a horizontal magnetic field type MRI apparatus 1 according to the third embodiment of the present invention, and FIG. 15 is a longitudinal sectional view thereof. The MRI apparatus 1 of the third embodiment is different from the MRI apparatus 1 of the first embodiment in that the direction 7 of the static magnetic field is in the horizontal direction. Accordingly, the electromagnet device 2 has a cylindrical shape whose central axis is the horizontal direction (z-axis direction), and the gradient magnetic field generating device 3, the RF irradiation coil 4, and the shim coil 21 are also cylindrical.

  FIG. 16 is a perspective view of the main coil (y) (a spiral electric conductor) 32 of the gradient magnetic field generator 3 of the horizontal magnetic field type MRI apparatus 1 according to the third embodiment of the present invention. The gradient magnetic field generator 3 generates three main coils (x), a main coil (y) 32, and a main coil (z) in order to generate independent gradient magnetic fields in the three directions of the x axis, the y axis, and the z axis. In order to shield the leakage magnetic field from the three main coils (x) 31, the main coil (y) 32, and the main coil (z) 33, the three shield coils (x) and the shield coils (y) corresponding to each of them are shielded. And the shield coil (z), FIG. 16 shows the main coil (y) 32 as an example. The main coil (y) 32 has four spiral electric conductors having a spiral center 3a.

The spiral-shaped electric conductor of the main coil (y) 32 includes a first curved portion 11a (11) having a locally small radius of curvature, and a second curved portion having a larger radius of curvature than the first curved portion 11a and a gentle curve. It has a curved portion 11b (11) and a straight portion 12 that is not curved. Then, the first bending portion 11a, along the longitudinal direction of the electrical conductor (spiral direction), the slits S ₁ is provided. Then, the first bending portion 11a, along the longitudinal direction of the electrical conductor (spiral direction), the slits S ₁ is provided. In the slit _{S 1,} divided first bending portion 11a into two, respectively, has become current conducting path _L 1, _{L 2.} In the divided current conducting paths L ₁ and L ₂ , the width w ₂ of the current conducting path L _{2 in} the direction outside the spiral shape is larger than the width of the current conducting path L _{1 in} the direction inside the spiral shape. It is wider than w ₁ (w ₁ <w ₂ ). Longitudinal end of the slit _{S 1} is adapted to branch portion _{S 0} for branching current conducting path _L 1, _{L 2.} Bifurcation S _0, in the longitudinal direction of the main coil (y) 32 (electrical conductors), the radius of curvature of the electrical conductors is provided at a position which changes to a smaller value from the larger value. For example, the branch portion S ₀ is provided in a region where the electric conductor is switched from a linear shape in which the curvature radius is considered to be infinite to the first curved portion 11 a having the curvature radius r ₀ . According to the third embodiment, similarly to the first embodiment, it is possible to efficiently improve the bias of the current center-of-gravity position rc and provide a correctable conductor structure.

1 MRI system (magnetic resonance imaging system)
DESCRIPTION OF SYMBOLS 3 Gradient magnetic field generator 4 RF irradiation coil 11 Bending part 11a 1st bending part 11b 2nd bending part 21 Shim coil (spiral-shaped electric conductor)
31 Main coil (x) (Electric conductor with spiral shape)
32 Main coil (y) (a spiral electric conductor)
33 Main coil (z) (Spiral electric conductor)
34 Shield coil (z) (spiral shaped electrical conductor)
35 Shield Coil (x) (Electric conductor with spiral shape)
36 Shield Coil (y) (Electric conductor with spiral shape)
S _{1, S} 2, _{S 3} slits

Claims (4)

In a magnetic resonance imaging apparatus having a spiral electric conductor,
Provided in the electrical conductor along the longitudinal direction of the electrical conductor, and having a slit for dividing a current conduction path formed in the electrical conductor into a plurality of parts,
The magnetic resonance imaging apparatus characterized in that the current conduction paths separated by the slits are wider in width toward the current conduction paths in the direction outside the spiral shape.   The magnetic resonance imaging apparatus according to claim 1, wherein the slit is formed in a region where a radius of curvature of the electric conductor is locally small in the longitudinal direction. The electrical conductor is provided with one slit in the width direction along the longitudinal direction,
One slit splits the current conduction path into two,
The width of the current conduction path in the direction of the inside of the spiral shape between the divided current conduction paths is more than 0.3 times and 0.5 times the conductor width of the electric conductor at the place where the slit is provided. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is less than or equal to 3. The electrical conductor is provided with two slits in the width direction along the longitudinal direction,
The two slits divide the current conduction path into three,
In the divided current conducting paths, the width of the innermost current conducting path in the inner direction of the spiral shape exceeds 0.15 times the conductor width of the electric conductor at the place where the slit is provided. Less than 0.33 times,
The width of the current conduction path in the middle of the spiral shape between the divided current conduction paths is approximately 0.33 times the conductor width of the electric conductor at the location where the slit is provided,
In the divided current conducting paths, the width of the current conducting path located on the outermost side in the outer direction of the spiral shape exceeds 0.33 times the conductor width of the electric conductor at the place where the slit is provided. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is less than 0.5 times.

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