CN1258683C - C-type open MRI panel radio-frequency coil - Google Patents

Abstract

The present invention discloses a flat plate type radio frequency coil for magnetic resonance imaging, which is used for the establishment of a radio frequency field between two magnetic pole surfaces of a magnet of a C-shaped open type magnetic resonance system. The present invention comprises two flat plate type coils which are respectively fixed on an upper magnetic pole surface and a lower magnetic pole surface, each coil is allocated with a conduction body group composed of a plurality of coil conduction body elements which are arranged in a parallel way, both ends of the conduction body group are provided with public connection conduction bodies, the coil conduction body element extends from the public connection conduction body arranged at one side of the flat plate coil to the public connection conduction body arranged at the other side of the flat plate coil, and a coil flat surface is formed by the coil conduction body elements and the public connection conduction bodies. Additionally, the public connection conduction bodies arranged on the upper flat plate coil and the lower flat plate coil are connected with another two public connection conduction bodies which are arranged at both outer sides of the flat plate coils and perpendicular to the flat plate coils, the public connection conduction bodies arranged at any side are connected with a radio frequency interface, and therefore, an independent RF coil is formed by the upper flat plate coil and the lower flat plate coil.

Description

The flat radio-frequency coil of a kind of C type open MRI

Technical field

The present invention relates to magnetic resonance imaging (MRI) system, relate in particular to the flat radio-frequency coil of a kind of C type open MRI.

Background technology

During magnetic resonance imaging system work, human body is placed in the strong static magnetic field, by the atomic nucleus in human body parts zone being excited to human body emission radio-frequency pulse.After radio-frequency field was removed, these nuclear radiations that are excited went out radiofrequency signal, are received by antenna.Because in this course, added gradient magnetic, therefore can obtain the space distribution information of human body, thereby reconstruct the two dimension or the 3-D view of human body by radiofrequency signal.

What be used for setting up radio-frequency field and received RF signal is radio-frequency coil, and single radio-frequency coil both can be used for emission, can be used for again receiving, such as head coil commonly used.Equally, radio-frequency coil also can be used to only emission, or receives only, and is used for received signal exactly such as widely used surface coils.

The open type magnetic resonance imaging (MRI) system of existing employing C type structure magnet forms static magnetic field by two pole surfaces up and down, and two pole surfaces connect the formation magnetic circuit by the vertical yoke (upright yoke) of a side.The radio-frequency coil of this magnet will be made open, and to make things convenient for patient's turnover, usual way is to place a plurality of conductor groups in the position near pole surface, to produce the radio-frequency field perpendicular to main field.Simultaneously, the current return of this coil is arranged very important, because coil-conductor itself does not form the loop naturally, usual way is as the loop with screen layer.United States Patent (USP) 5467017 disclosed a kind of designs, as shown in figure 10, up and down two planar coil conductors 101 respectively by two concentric cable 102 with the power input, be provided with electric capacity 103 respectively at each coil interface place and 104 pairs of two coils are regulated respectively.The same RF power divider of the other end of two cables 102 105 connects, and screen layer 106,107 is respectively as the current return of two coils.In the sort circuit, need distribute the electric current of two coil flat boards, and will carry out tuning respectively two coils at the rf input interface place, complex structure, two coils have certain working alone property up and down, and influence each other when adjusting, and make adjustment become and are not easy.

As current paths, solved the current return problem with screen layer, still, the electric current of screen layer has certain negative function to radio-frequency field, has reduced the efficient of coil.

Summary of the invention

The invention discloses a kind of flat radio-frequency sending coil of the C of being used for type open MRI systems, be used between magnet two pole surfaces, setting up radio-frequency field.Coil comprises: two flat coils that are separately fixed at up and down on two pole surfaces, the conductor group that each coil configuration is made up of a plurality of coil-conductor elements, the two ends of conductor group are provided with public bonding conductor, the coil-conductor element extends to public bonding conductor, coil-conductor element and the interior coil plane of public bonding conductor formation plate coil of opposite side along the public bonding conductor of plate coil one side.For making two coils be linked to be an integral body, with placing the vertical public bonding conductor of outer two other of plate coil and plate coil to be connected, the close magnet of this vertical public bonding conductor founds yoke between the public conductor in two plate coils; The public bonding conductor that any side and plate coil are vertical is connected with radio frequency interface.Ideally, radio-frequency coil should be created in uniform radio-frequency field perpendicular to static magnetic field in the imaging area.In Cartesian coordinates, establishing the static magnetic field direction is the Z direction, and the radio-frequency field direction is a directions X, and the another one direction is the Y direction.Be located at static magnetic field center Z ₀There is the sheet of current of uniform infinitely great Y direction in equidistant up and down place, and direction is opposite, and current density is λ.Field between two sheet of current is:

B _x=μ 0 _λFor Z＞Z ₀

B _x=-μ ₀λ is for Z＜Z ₀

B _x=2 μ ₀λ is for-Z ₀＜Z＜Z ₀

In fact, because coil is big or small restricted, therefore, coil design all is to make radio-frequency field even as far as possible in certain imaging scope.

Usually, for obtaining desired uniform field, normally laying each one group of conductor up and down near the place of pole-face, every group of conductor is made up of the equidistant coil-conductor element that several extend along the Y direction.The current opposite in direction of two conductor groups up and down, the electric current of every group of conductor is identical, by increasing or reducing number of conductors, can obtain in want space than uniform radio-frequency field.

Parallel conductor group discussed above does not also constitute closed circuit, so extra conductor element need be provided, removes to connect these two conductor groups conductor element separately.Many selective possibilities are applicable to the path of finishing circuit, and so do optimal means and will depend on the specific imaging purposes and the design of system.If the current element of path closure is arranged on away from the imaging district, then the field in the imaging district will be the field of coil-conductor element basically.This will be described below.

If conductor length is L, be applied to from y=-L/2 to y=L/2, extend and be set at x=x along the y direction cut down your (Biot-Savart) law than a Sa difficult to understand.And z=z.The strip current element at place,

$B_x=\frac{{\mathrm{\μ}}_0}{4\mathrm{\π}}\frac{z-z_0}{{(x-x_0)}^2+{(z-z_0)}^2}[\frac{L/2-y}{\sqrt{{(L/2-y)}^2+{(x-x_0)}^2+{(z-z_0)}^2}}+$

$\frac{L/2+y}{\sqrt{{(L/2+y)}^2+{(x-x_0)}^2+{(z-z_0)}^2}}$

Can derive to point on the scene (x, y z) locate the following formula of magnetic-field component:

Complete coil will be to containing N at x=x ₀(n) and z=z ₀The strip conductor at place and N at x=x ₀(n) and z=-z ₀The extra conductor at place, wherein n=1 extends to n=N, and the total radio-frequency field that is produced by two row strip conductors is:

$B_z=\frac{{\mathrm{\μ}}_0}{4\mathrm{\π}}\frac{x-x_0}{{(x-x_0)}^2+{(z-z_0)}^2}[\frac{L/2-y}{\sqrt{{(L/2-y)}^2+{(x-x_0)}^2+{(z-z_0)}^2}}+$

$\frac{L/2+y}{\sqrt{{(L/2+y)}^2+{(x-x_0)}^2+{(z-z_0)}^2}}$

B _y＝0

$B_x=\underset{n=1}{\overset{N}{\mathrm{\Σ}}}{B}_x(x_0\left(n\right),z_0)+\underset{n=1}{\overset{N}{\mathrm{\Σ}}}{B}_x(x_0\left(n\right),-z_0)$

B _y＝0

$B_z=\underset{n=1}{\overset{N}{\mathrm{\Σ}}}{B}_z(x_0\left(n\right),z_0)+\underset{n=1}{\overset{N}{\mathrm{\Σ}}}{B}_z(x_0\left(n\right),-z_0)$

For single conductor, if the length L of conductor and quantity (x-x ₀) and (z-z ₀) compare and become very long, then:

$B_z\→\frac{{\mathrm{\μ}}_0}{2\mathrm{\π}}\frac{x-x_0}{{(x-x_0)}^2+{(z-z_0)}^2}$

B _y＝0

$B_x\→\frac{{\mathrm{\μ}}_0}{2\mathrm{\π}}\frac{z-z_0}{{(x-x_0)}^2+{(z-z_0)}^2}$

At imaging volume center place (z)=(0,0,0), the central field of single conductor is for x, y:

$B_x=-\frac{{\mathrm{\μ}}_0}{4\mathrm{\π}}\frac{{\mathrm{Lz}}_0}{(x_0^2+z_0^2){(\frac{L^2}{4}+x_0^2+z_0^2)}^{1/2}}$

B _y＝0

$B_z=-\frac{{\mathrm{\μ}}_0}{4\mathrm{\π}}\frac{{\mathrm{Lx}}_0}{(x_0^2+z_0^2){(\frac{L^2}{4}+x_0^2+z_0^2)}^{1/2}}$

If N is an even number, central field is:

$B_x=-\frac{{\mathrm{\μ}}_0}{\mathrm{\π}}\frac{{\mathrm{Lz}}_0}{(x_0^2+z_0^2){(\frac{L^2}{4}+x_0^2+z_0^2)}^{1/2}}$

B _y＝0

B _z＝0

If N is an odd number, central field is:

$B_x=-\frac{{\mathrm{\μ}}_0}{2\mathrm{\π}}\frac{{\mathrm{Lz}}_0}{z_0^2{(\frac{L^2}{4}+z_0^2)}^{1/2}}$

B _y＝0

B _z＝0

Therefore, this coil in field that imaging region produces mainly along directions X.

The objective of the invention is to two such plate coils are connected into an integral body, thereby make loop construction simple, regulate easily, save power.

Further, the present invention connects together two common plate coils by two of the side perpendicular public bonding conductors, makes two plate coils constitute an independently coil.Each plate coil all has a series of coil-conductor group that stretches along the Y direction, therefore has public bonding conductor that these coil-conductors are connected together respectively at the two ends of coil-conductor group, and the public bonding conductor in the plane is to not contribution of radio-frequency field.

Each public bonding conductor stretches out coil plane along directions X to the upright bound conjugate of magnet, so that vertical bonding conductor is complementary with the shell of magnet.The public bonding conductor of two plate coil phase the same sides links together by a public bonding conductor vertical with plate coil up and down, the public bonding conductor of an other side is linked together by another vertical public bonding conductor, like this, up and down two independently plate coil just link together, constitute an independently coil.

On the common conductor of any side, offer a rf input interface, and be provided with corresponding capacitor, so that whole winding is regulated.Because vertical public bonding conductor is not far from imaging space, current density is big, and therefore, it has certain contribution to the imaging area radio-frequency field, can influence the distribution of radio-frequency field simultaneously.

Second purpose of the present invention is that the capacitor of different sizes is installed on each conductor element in the conductor group, the current density of each conductor just can change by the size of control capacittance like this, because radio-frequency field is to be formed by all conductor actings in conjunction, the change of each conductor current density will change the distribution of radio-frequency field, regulate the shape of radio-frequency field, thereby make the radio-frequency field of imaging area can keep required homogeneity.

The 3rd purpose of the present invention is by the spacing that changes coil-conductor, coil-conductor is adjusted radio-frequency field at the position on the coil plane, the coil-conductor distribution shape on coil plane shape.For compensating of the influence of vertical public bonding conductor to radio-frequency field, the center of the conductor group on each flat board can be not in the geometric center of flat board, simultaneously, the distance of each conductor element in every group of conductor group can be unequal, such as by conductor group integral body is moved to the magnet opening direction, or will move to the magnet opening direction near the conductor of magnet opening part, can change the distribution of radio-frequency field, the geometric center of the homogeneity range of radio-frequency field is withdrawn into the geometric center of imaging area.

Obviously, its current density difference of the conductor of different in width, and the variation of conductor width has changed the distribution of current element on the coil flat board that conductor forms in fact, thereby influence the distribution of radio-frequency field, the present invention is by changing the width adjustment radio-frequency field distribution of coil-conductor, with the uniformity coefficient and the geometric position of realizing desired radio-frequency field.

Further, the connected mode that is positioned at dull and stereotyped public bonding conductor and coil-conductor can change the distribution of radio-frequency field.Each conductor element in the coil-conductor group can link to each other with public bonding conductor separately, and each coil-conductor just has a tie point with public bonding conductor like this; Also can earlier one group of conductor element be connected with a transition common conductor, this transition common conductor links to each other with public bonding conductor by one or more again.Because electric current is through the path difference of public bonding conductor, its impedance is also different, and the electric current that therefore flows through each coil-conductor is also different, like this, by above-mentioned mode, can change the current density of each conductor element in the coil-conductor group, thereby change the distribution of radio-frequency field.

Because vertical public bonding conductor has contribution to the radio-frequency field of imaging area among the present invention, therefore, the position that changes two vertical public bonding conductors obviously can influence the distribution of radio-frequency field.Usually, vertical public bonding conductor apart from the distance of the upright yoke of magnet by magnet and configuration design decision thereof, only can be by changing the distribution that distance between the vertical public bonding conductor changes radio-frequency field.In addition, because vertical public bonding conductor generally can be very near from the upright yoke of magnet, therefore, between vertical public bonding conductor and the upright yoke of magnet, add a radio shielding layer, and this screen layer is also with the screen layer connection in the plate coil outside up and down, make radio-frequency coil be in the electromagnetic environment of a sealing, avoided its influence of outer bound pair effectively.

Description of drawings

Fig. 1 is the schematic diagram of the flat radio-frequency coil of C type of the present invention;

Fig. 2 is that the present invention changes the embodiment that the coil-conductor spacing is adjusted radio-frequency field;

Fig. 3 is that the present invention changes the embodiment that the coil-conductor width is adjusted radio-frequency field;

Fig. 4 is that the present invention changes the embodiment that the coil-conductor shape is adjusted radio-frequency field;

Fig. 5 is that the present invention changes another embodiment that the coil-conductor spacing is adjusted radio-frequency field;

Fig. 6 is that the present invention changes the embodiment that vertical public bonding conductor spacing is adjusted radio-frequency field;

Fig. 7 is the structure that coil-conductor directly links to each other with public bonding conductor, and three tie points are arranged among the figure;

An embodiment of Fig. 8 structure that to be coil-conductor link to each other with the public bonding conductor of transition has the conductor of two weak points to be connected between the public bonding conductor of transition and the public bonding conductor among the figure;

Fig. 9 is provided with the embodiment that capacitor is adjusted radio-frequency field between coil-conductor of the present invention;

Figure 10 is the structure principle chart of the flat coil of prior art;

Figure 11 is an embodiment of screen layer of the present invention.

Embodiment

As shown in Figure 1, plate coil 13 respectively has one group of coil-conductor group 110 up and down, the two ends of coil-conductor group 110 link to each other with two in the plate coil public bonding conductors 11, wherein, public bonding conductor 11 stretches out plate coil 13 to the upright bound conjugate of magnet, and be positioned at the public bonding conductor 12 of coil vertical direction and be connected with two, thereby constitute the loop of a closure.Obviously, coil-conductor is a directions X in the direction of the field that imaging area produces, and two coil-conductors 110 are identical in the direction of the field that imaging area produces up and down.Be provided with capacitor 14 in a vertical public bonding conductor, be used for the resonance frequency of regulating winding, and link to each other with rf input interface thus.Can see that two dull and stereotyped radio frequency line astragals up and down of this moment have constituted a unified whole coil, so coil is easy to adjust, and are simple in structure, working stability.The public bonding conductor 11 at conductor group 110 two ends is opposite at the field direction that imaging area produces, to not contribution of radio-frequency field.Because two vertical public bonding conductors 12 have identical component in the field that imaging area produces at directions X, therefore radio-frequency field are had contribution, the efficient of this coil is higher.

Because 12 pairs of imaging area radio-frequency fields of vertical public bonding conductor have contribution, therefore, when coil-conductor 110 is positioned at dull and stereotyped center, the distribution of imaging area radio-frequency field will depart from imaging center, homogeneity also changes, for proofreading and correct this variation, Fig. 2 has provided a solution, the position of each conductor in the coil-conductor group 210 is moved, distance between the conductor is not being equated, because the imaging area radio-frequency field is by all coils conductor 210 and vertical public bonding conductor 12 coefficient results, and the contribution maximum of coil-conductor 210, therefore, change the position of each conductor in the coil-conductor, to the distribution of imaging area radio-frequency field be exerted an influence,, can make the distribution of imaging area radio-frequency field get back to uniform state by the position arrangement of suitable coil-conductor.We find in an experiment, the conductor group that three conductor is constituted place dull and stereotyped in the middle of, the shimming district center will depart from the about 5cm of magnet imaging center, adopt this embodiment way after, radio frequency shimming district center can be withdrawn into the imaging area center, uniformity coefficient also brings up to 11% by about 15%.

Fig. 3 has showed an alternative embodiment of the invention, the width of each conductor in the coil-conductor group 310 is set to difference, like this, the current density difference of each conductor, and, the variation of conductor width will make the position distribution of current element in plate coil 13 that it carries change, and finally change the distribution of radio-frequency field, and correct design can make the radio-frequency field distribution of imaging area even.

Fig. 4 and Fig. 5 show another one embodiment of the present invention, and the shape that can change each conductor in the coil-conductor 410 changes the distribution of radio-frequency field, finally make radio-frequency field be tending towards even distribution.Fig. 6 is the another one method, changes the spacing of vertical public bonding conductor 12, also can reach same purpose.

Fig. 7 shows the influence of different connected modes to coil-conductor 110 distribution of current with Fig. 8.Coil-conductor 110 directly links to each other with public bonding conductor 11 among Fig. 7, forms three tie point 70a, 70b, 70c, and owing to the impedance difference in different paths, the current density in each conductor is inequality, what the current density in the outside will be less than the inboard.Fig. 8 is another connected mode, coil-conductor 110 is connected by a public bonding conductor 810 of transition earlier, cross-over connection conductor 811a by two weak points between the public bonding conductor 810 of transition and the public bonding conductor 11 is connected with 811b, like this, because variation has taken place in path of current, the current density that finally flows through each coil-conductor 110 is also compared with Fig. 7 and is changed, thereby influences the distribution of radio-frequency field, and suitable electric current distributes can make being evenly distributed of radio-frequency field.

Fig. 9 is another embodiment of the present invention.Each conductor in the coil-conductor group 610 is provided with capacitor 90a, 90b, 90c, and the size of regulating each condenser capacitance can change the current density in each coil-conductor, therefore can adjust the distribution of radio-frequency field and makes it even.

Figure 11 is an embodiment of screen layer of the present invention.Because radio-frequency coil is near magnet, it can be subjected to the influence of magnet structure material.For eliminating this influence, be provided with the screen layer of making by good conductor 115 near the magnet pole place in the outside of two dull and stereotyped radio-frequency coils 13 up and down.Because the present invention has vertical public bonding conductor in the side near upright yoke place, therefore near upright yoke place a screen layer 116 of being made by good conductor is set also in this vertical outside that connects common conductor, and this screen layer 116 is electrically connected with 115, therefore constituted an incorporate screen layer, reduced or eliminated of the influence of magnet structure material radio-frequency coil.

One up and down coil-span be that experiment on the radio-frequency coil of 40cm shows, the uniformity coefficient of radio-frequency field can be improved about 5-8% from Fig. 3 to several method shown in Figure 9, improve the uniformity coefficient of radio-frequency field.

Claims (5)

1, a kind of C of being used for type open type magnetic resonance system sets up the radio-frequency coil of radio-frequency field between magnet two pole surfaces, mainly form by two flat coils that are separately fixed at up and down on two pole surfaces, the conductor group that each coil configuration is made up of a plurality of coil-conductor elements, the two ends that it is characterized in that the conductor group that the coil-conductor element is formed are provided with public bonding conductor, the coil-conductor element extends to the public bonding conductor of opposite side along the public bonding conductor of a dull and stereotyped side, up and down between the public bonding conductor at the coil-conductor group two ends in two plate coils respectively with placing near the outer upright yoke of magnet two other of being positioned at of plate coil to be connected with the vertical public bonding conductor of plate coil, constitute an integral body, any one is connected with radio frequency interface with the vertical public bonding conductor of plate coil, is placed with capacitor on coil-conductor element and the public bonding conductor vertical with coil.

2, radio-frequency coil as claimed in claim 1, it is characterized in that by changing width, the spacing of coil-conductor element, coil-conductor in the position of coil plane, the shape that distributes on coil plane and vertical public bonding conductor spacing can adjust the distribution of radio-frequency field.

3, radio-frequency coil as claimed in claim 1 or 2, it is characterized in that the coil-conductor element can connect with public bonding conductor separately, also can earlier one group of coil-conductor element group be linked to each other with a public bonding conductor of transition, the public bonding conductor of this transition is connected with public bonding conductor again; Adjust the distribution of current of coil-conductor element by changing the coil-conductor element with the connected mode of public bonding conductor, make radio-frequency field distribution even.

4, radio-frequency coil as claimed in claim 1 or 2 is characterized in that being provided with screen layer between the outside of perpendicular public bonding conductor and the upright yoke of magnet, and is connected as a single entity with the outer screen layer of coil.

5, radio-frequency coil as claimed in claim 3 is characterized in that being provided with screen layer between the outside of perpendicular public bonding conductor and the upright yoke of magnet, and is connected as a single entity with the outer screen layer of coil.

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