WO2014195277A1

WO2014195277A1 - Rf screen for mri systems

Info

Publication number: WO2014195277A1
Application number: PCT/EP2014/061418
Authority: WO
Inventors: Peter Forthmann; Jan Hendrik Wuelbern; Peter Vernickel; Christian Findeklee
Original assignee: Philips GmbH; Koninklijke Philips NV
Current assignee: Philips GmbH; Koninklijke Philips NV
Priority date: 2013-06-06
Filing date: 2014-06-03
Publication date: 2014-12-11
Anticipated expiration: 2015-12-06

Abstract

The invention relates to a magnet assembly for an MRI system with an improved RF screen. The RF screen is located between an RF coil assembly and a gradient coil assembly and comprises multiple layers of a screening foil. The screening foil comprises a dielectric foil and a plurality of conductive patches which are applied to a first side of the dielectric foil. By wrapping up the screening foil several times an RF screen comprising a plurality of capacitors is created, which effectively shields RF radiation from the gradient coil assembly. Further the shape and arrangement of the conductive patches is chosen such that the flow of eddy currents in the conductive patches caused by a gradient magnetic field is reduced.

Description

RF Screen for MRI Systems

TECHNICAL FIELD OF THE INVENTION

The invention relates to a magnet assembly for a Magnetic Resonance Imaging (MRI) system with an improved RF screen and a method for screening RF radiation in an MRI system.

BACKGROUND AND RELATED ART

MRI systems usually employ three types of magnetic fields for imaging: a static magnetic field produced by permanent magnets to align the magnetization of the atomic nuclei in the object to be imaged, one or more gradient magnetic fields produced by a gradient coil assembly to obtain spatial information and a magnetic field that is being switched on and off with radio frequency (RF) to alter the magnetization of the atomic nuclei of the object to be imaged thereby producing a measurement signal.

Inside an MRI system the RF coil assembly and the gradient coil assembly are usually placed in close proximity. As a result a part of the RF radiation may couple into the gradient coil assembly thereby causing losses in the RF signal and reducing the Q-value of the RF coil assembly. Thus it is necessary to screen the RF radiation from the gradient coil assembly thereby decoupling the gradient coils and the RF coils.

In a common MRI system the RF coil assembly is placed inside the gradient coil assembly. Thus the gradient magnetic fields produced by the gradient coil assembly have to pass through the RF coil assembly. As a result an RF screen put between the RF coil assembly and the gradient coil assembly has to be transparent to gradient magnetic fields.

SUMMARY OF THE TNVENTION

The present invention relates to a magnet assembly for an MRI system comprising a magnet, a gradient coil assembly, an RF coil assembly and an RF screen. The RF screen is located between the RF coil assembly and the gradient coil assembly and comprises multiple layers of a screening foil.

The screening foil comprises a dielectric foil and a plurality of conductive patches adhering to a first side of the dielectric foil. The dielectric foil may be made of any material suitable to be used as a carrier foil like for example polyethylene or other polymers. Further the dielectric foil has to exhibit a permittivity ε or relative permittivity e_r suitable for use as a dielectric in a capacitor. The conductive patches may be made of any conductive material suitable to be applied onto a carrier foil. For example the conductive patches may be made of copper.

To form an RF screen the multiple layers of screening foil are arranged such that the first side of a first layer of foil adheres to a second side of a second layer of foil. Further, the multiple layers of screening foil are arranged such, that conductive patches of different layers overlap thereby forming a plurality of capacitors consisting of the

overlapping patches and the dielectric foil between the patches.

The capacitors thus will exhibit a capacity proportional to the permittivity ε or the relative permittivity ε_Γ of the dielectric foil and the area A of overlap between the patches and inversely proportional to the distance d between the overlapping patches that is given by the thickness of the foil. Since the dielectric foil can be made very thin the capacity of the capacitors formed by the overlapping patches and the dielectric foil may be designed to be very high.

Since overlapping patches usually are part of at least two capacitors and since the impedance of a capacitor for AC-signals is in inverse proportion to the capacity of the capacitors, the RF screen described above provides RF current paths with low impedance for mirror currents produced by an electromagnetic field produced by the RF coil assembly. Thus the RF screen provides approximately a short circuit for RF signals.

Further, the size and arrangement of the conductive patches is chosen such that the RF screen has high impedance at audio frequencies thereby reducing the flow of eddy currents within the conductive patches. For example the conductive patches may be disconnected from one another and the shape and size of the individual patches may be chosen very small, such that no closed conducting loops sufficient for the flow of eddy currents exist.

Eddy currents are usually caused by the magnetic field produced by the gradient coil assembly. As a result the RF screen formed by multiple layers of screening foil is almost transparent to the gradient magnetic fields produced by the gradient coil assembly.

The MRI system as described before may have the advantage that RF radiation provided by the RF coil assembly is effectively screened from the gradient coil assembly by the RF screen thereby reducing losses in RF-signals and maintaining the Q-value of the RF- circuitry. At the same time the flow of eddy currents within the conductive patches of the screen is reduced because of the size, shape and arrangements of the conductive patches.

A further advantage may be that the screen is made up of screening foil by simply wrapping up multiple layers of the screening foil and applying the multiple layers onto the RF coil assembly or the gradient coil assembly. Thus it may be very easy to apply the RF screen described above to an MRI system.

The screening foil itself is made up of a dielectric foil and a plurality of conductive patches adhering to one side of the foil. Thus production of such a screening foil may be well feasible which may allow for low-cost production of the foil.

Thus, the RF screen used in the MRI system described above may be inexpensive and easy to apply.

In some embodiments the conductive patches are arranged in a regular pattern. To provide efficient screening of RF radiation it is necessary to create ample overlap between conductive patches by wrapping several layers of screening foil on top of each other. Thus, arranging the conductive patches in a regular pattern may facilitate mounting of the screening foil since it may be easy to arrange the layers of screening foil such that full coverage of the object to be screened with conductive material is achieved. In the case of a random

arrangement of conductive patches even an RF screen made of a huge number of layers of screening foil might still exhibit areas where no coverage with conductive material is provided.

In some embodiments the conductive patches may be of rectangular shape wherein at least one edge length of the conductive patches is less than 4 cm. This may have the advantage that the flow of eddy currents within the conductive patches is reduced. It has been found that for an efficient reduction of eddy currents in rectangular-shaped conductive patches it is advantageous to have at least one edge of the conductive patch that has a length of less than 4 cm.

In some embodiments the conductive patches have the form of strips. The length of the strips may be chosen such that it corresponds to the length of the RF coil assembly. Further, the width of the strips should not exceed 4 cm to provide for efficient reduction of the flow of eddy currents, as pointed out before. The strips are further separated by a gap such that wrapping up the screening foil produces the series of capacitors as described before. The gap may for example have a width of 1 mm.

Using conductive patches designed as strips may have the advantage that the mounting process is further simplified. Already two layers of screening foil may provide full coverage of the RF coil assembly with conductive material, thereby effectively screening the RF radiation. Due to the ratio of gap and strip of approximately 1 mm to 40 mm, the requirements on the mounting tolerances are very low.

In some embodiments the conductive patches have a maximum area of about 20 cm² each. This may have the advantage that eddy currents induced by the gradient fields are further reduced. Limiting the maximum area of the conductive patches to 20 cm² may be especially advantageous if the conductive patches have a geometric shape other than a rectangular one. For example the RF screen may employ conductive patches having the shape of triangles or other polygons. Further, the arrangement of the conductive patches may resemble the structure of a honeycomb. Even circular patches or patches with an arbitrary shape may be used. However, for efficient reduction of eddy currents, the area of the patches should not exceed 20 cm².

In some embodiments the conductive patches may form a checkerboard pattern. This may have the advantage that mounting the RF screen may be further simplified since the mounting tolerances are further reduced. In an ideal checkerboard pattern full coverage with patches may be obtained already with two layers of screening foil.

In some embodiments the conductive patches are arranged such that the structure formed by multiple layers of the screening foil mimics the structure of the RF coil without forming closed ohmic loops. This may have the advantage that the mirror current can flow with a lower number of capacitive gaps which reduces the effective surface impedance inside the RF screen. A further advantage may be that even though the RF coil assembly is not fully covered with conductive material, the RF radiation is still effectively screened since the RF coil itself is completely covered with conductive material. Thus, less conductive material is needed.

In some embodiments the dielectric foil of the screening foil has a thickness between 10 μιη and 100 μιη. This may have the advantage that the impedance of the capacitors formed by the conductive patches and the dielectric foil is very low, thus forming a circuit with very low impedance for RF screen currents. However, a certain level of mechanical stability and electrical isolation given by the thickness of the dielectric foil has to be maintained.

In some embodiments the dielectric foil material is chosen such that exposing the dielectric foil to heat causes the dielectric foil to shrink. This may have the advantage that applying the dielectric foil to the surface of an RF coil assembly is further simplified since the screening foil adapts itself to the shape of the RF coil assembly like this is used in shrink sleeving. However, one has to make sure that even a shrinking of the foil does not produce galvanic contacts between the conductive patches thereby forming closed conductive loops within the RF screen. Thus, the distance between the conductive patches as well as the layer thickness have to be chosen appropriately when using a heat shrinking foil as dielectric foil.

In some embodiments the screening foil may be produced as a continuous tube. Especially if the dielectric foil is chosen such that it shrinks upon application of heat this may facilitate mounting of the RF screen. To apply the RF screen onto an RF coil assembly one would simply have to slip a number of screening tubes over the assembly and heat them up.

In some embodiments the dielectric foil is self-adhesive. This may have the advantage that mounting is further simplified since no additional glue or other fixation means has to be applied.

Besides coating a dielectric foil with conductive patches to form the screening foil, conductive particles may also be directly immersed in the base material for the dielectric foil. If dense enough, the conductive particles may cause a partially conductive behavior. Modern extruding methods allow a homogeneous as well as inhomogeneous distribution of the metal particles. This allows structuring the foil as described above.

In another aspect the invention relates to a method for screening RF radiation in an MRI system, wherein the method comprises:

Providing a gradient coil assembly,

Providing an RF coil assembly,

Applying multiple layers of screening foil to an outer surface of the RF coil assembly, the screening foil comprising a dielectric foil and a plurality of conductive patches, wherein the plurality of conductive patches adhere to a first side of the dielectric foil and wherein the multiple layers of screening foil are arranged such that first side of a first layer of foil adheres to a second side of a second layer of foil and such that conductive patches of different layers overlap, thereby forming a plurality of capacitors in between the conductive patches, such that RF current paths with low impedance for mirror currents produced by an electromagnetic field produced by the RF coil exist, wherein the arrangement of the conductive patches further causes a high impedance at audio frequencies, thereby reducing the flow of eddy currents within the conductive patches, the eddy currents being caused by the magnetic fields produced by the gradient coil,

Inserting the RF coil assembly and the applied screening foil into the gradient coil assembly, In some embodiments the method the multiple layers of screening foil are applied to an inner surface of the gradient coil assembly instead of or in addition to applying the multiple layers of screening foil to the outer surface of the RF coil assembly.

It is understood that one or more of the aforementioned embodiments of the invention may be combined as long as the combined embodiments are not mutually exclusive.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, preferred embodiments of the invention are described in greater detail by way of example only, making reference to the drawings in which:

Fig. 1 is a schematic of a part of an MRI system comprising a gradient coil assembly, an RF coil assembly and an RF screen,

Fig. 2a is a schematic of an RF screen formed by two layers of screening foil, Fig. 2b is a schematic of the equivalent circuit of the RF screen shown in Fig.

2a,

Fig. 3a is a schematic of a screening foil employing conductive strips,

Fig. 3b is a schematic of an RF screen formed by coiling the screening foil shown in Fig. 3a,

Fig. 4a is a schematic of a screening foil employing square conductive patches arranged in a checkerboard pattern,

Fig. 4b is a schematic of an RF screen formed by stacking three layers of the screening foil shown in Fig. 4a.

In the following, similar elements are denoted by the same reference numerals.

DETAIL DESCRIPTION OF THE EMBODIMENTS

Fig. 1 is a schematic of a part of an MRI system 100 as referenced above. The magnetic resonance imaging system 100 comprises a magnet 102. The magnet 102 is a superconducting cylindrical type magnet 102 with a bore 126 through it. The use of different types of magnets is also possible for instance it is also possible to use both a split cylindrical magnet and a so called open magnet. A split cylindrical magnet is similar to a standard cylindrical magnet, except that the cryostat has been split into two sections to allow access to the iso-plane of the magnet, such magnets may for instance be used in conjunction with charged particle beam therapy. An open magnet has two magnet sections, one above the other with a space in-between that is large enough to receive a subject: the arrangement of the two sections area similar to that of a Helmholtz coil. Open magnets are popular, because the patient space is less confined. Inside the cryostat of the cylindrical magnet there is a collection of superconducting coils. Within the bore 126 of the cylindrical magnet 102 there is an imaging zone 128 where the magnetic field is strong and uniform enough to perform magnetic resonance imaging.

Within the bore 126 of the magnet there is also a gradient coil assembly 104 which is used for acquisition of magnetic resonance data to spatially encode magnetic spins within the imaging zone 128 of the magnet 102. The gradient coil assembly 104 is intended to be representative. Typically gradient coil assemblies 104 contain three separate sets of coils for spatially encoding in three orthogonal spatial directions.

Adjacent to the imaging zone 128 is a radio-frequency (RF) coil assembly 106 for manipulating the orientations of magnetic spins within the imaging zone 128 and/or for receiving radio transmissions from spins also within the imaging zone 128. The radio frequency antenna may contain multiple coil elements.

An RF screen 108 is located between the gradient coil assembly 104 and the RF coil assembly 106. A patient 110 lying on a patient table 112 may reside within the MRI system 100.

When operating the MRI system 100 for imaging certain body parts of the patient 110 the RF screen 108 prevents RF radiation provided by the RF coil assembly 106 from coupling to the gradient coil assembly 104 while the gradient fields provided by the gradient coil assembly 104 pass through the RF screen 108 without significant losses.

Fig. 2a is a schematic of an RF screen 108 formed by two layers of screening foil 114. One layer of screening foil 114 comprises a dielectric foil 116 and a plurality of conductive patches 118. The conductive patches 118 all adhere to a first side of the dielectric foil 116. In the example depicted here, the first side of the dielectric foil is the upper side. Laminating at least two layers of screening foil 114 onto each other such that a first side of a first layer adheres to a second side of the second layer, creates a structure comprising a plurality of capacitors 120. The capacitors 120 are formed by overlapping conductive patches 118 and the dielectric foil 116 between them.

By choosing an appropriate material and thickness for the dielectric foil 116 the capacity of the capacitors 120 can be tailored within vast degrees of freedom. Preferably the thickness and material of the dielectric foil 116 is chosen such, that the capacity of the capacitors 120 is sufficiently high for forming a circuit with very low impedance for RF screen currents. Since the capacity of the capacitors 120 is approximately proportional to the inverse of the distance between the patches 118, given by the thickness of the dielectric foil 116, the dielectric foil 116 itself should be as thin as possible. However the thickness of the dielectric foil 116 should not be too low, since a too low thickness might increase the risk of the screening foil 114 ripping upon mounting.

The conductive patches 118 may be made of any suitable conductive material that can be applied onto a foil as a thin film. For example the conductive patches may be made of copper and may be applied to the foil 116 by metallization of the foil with a predetermined pattern.

Fig. 2b is a schematic of an equivalent circuit for the RF screen 108 shown in Fig. 2a. As described before, the overlapping conductive patches 118 and the dielectric foil 116 between the conductive patches 118 form capacitors 120. In the example depicted in Fig. 2a, each conductive patch 118 is a part of at least two capacitors 120. Thus, the resulting electric circuit corresponds to a series of capacitors 120. If the capacity of those capacitors 120 is sufficiently high, the circuit shown in Fig. 2b corresponds to a short circuit for RF currents, since the impedance of a capacitor is in inverse proportion to its capacity and to the frequency of the RF signal. Thus the RF screen shown in Fig. 2a may efficiently screen RF radiation.

Fig. 3a is a schematic of an RF screening foil 114, wherein the conductive patches 118 are designed as strips. The conductive patches 118 are further separated by gaps 122 to reduce the flow of eddy currents induced by the gradient magnetic fields of the gradient coil assembly 104. The conductive patches 118 adhere to a first side of the dielectric foil 116. It has to be noted that the dimensions of the strips and the gaps 122 are merely illustrative.

According to some embodiments the gap 122 between two conductive patches 118 may be in the order of 1mm, while the width of the conductive patch 118 in x-direction should not exceed 40 mm. The length of the foil 114 and the conductive patches 118 in y- direction may be given by the length of the cylinder to be covered. The length of the foil 114 in x-direction is a function of the circumference of the cylinder to be covered. Since the screening foil 114 has to be wrapped at least twice around the cylinder to be covered, the length of the screening foil 114 in x-direction has to be equal or greater than two times the circumference of the cylinder to be covered.

Fig. 3b is a schematic of an RF screen 108 formed by coiling at least two layers of the screening foil 114 shown in Fig. 3a. The distance between adjacent layers of screening foil 114 in Fig. 3b does not represent actual dimensions, but is extremely increased. Normally the distance between two layers is given by the thickness of the foil 114. A first (inner) layer starts at conductive strip 200 and ends with conductive strip 202. Thus, the first layer ends just one half strip before the circumference of the cylinder is fully covered. Immediately a second layer starts with strip 204 covering the missing part of the first layer and one half of the first strip 200 of the first layer already in place.

Continuously all gaps between the strips of the first layer are covered by strips of the second layer until the last gap between the strips of the first layer and the second layer is covered by conductive strip 206.

Thus, it is possible to provide full coverage of an MRI body coil with conductive material using only two layers of screening foil 114. The overlap between the strips of the first layer and the second layer with the dielectric foil material between them provides the structure of an RF screen 108 that has already been shown in Figs. 2a and 2b. Since the width of the conductive strips in x-direction is chosen to be less than 40mm, the flow of eddy currents within the RF screen shown in Fig. 3 is significantly reduced.

Fig. 4a is a schematic of a screening foil 114, wherein the conductive patches 118 have a square shape and are arranged in a checkerboard pattern. As described before, the conductive patches 118 adhere to a first side of a dielectric foil 116. The conductive patches 118 preferably have an edge length of less than 4cm, thus reducing the flow of eddy currents within the screening foil. Further, at least some of the conductive patches 118 have to be disconnected from each other such that no ohmic loops are formed by the junctions between the conductive patches 118.

Fig. 4b is a schematic of an RF screen 108 formed by multiple layers of the screening foil 114 shown in Fig. 4a. As shown in Fig. 4b the plurality of overlapping conductive patches 118 together with the dielectric foil 116 between the patches 118 form a plurality of capacitors 120. Thus, an electric circuit is formed comparable to the electric circuit shown in Fig. 2b. The remaining areas 124 which are not covered by conductive material could be covered by applying a fourth layer of screening foil 114 on top of the RF screen 108 shown in Fig. 4b. List of Reference Numerals

100 MRI system

102 permanent magnet

104 gradient coil assembly

106 RF coil assembly

108 RF screen

110 patient

112 patient table

114 screening foil

116 dielectric foil

118 conductive patch

120 capacitor

122 gap

124 area

126 bore

128 imaging zone

200 conductive strip

202 conductive strip

204 conductive strip

206 conductive strip

Claims

CLAIMS:

1. A magnet assembly for an MRI system (100), comprising

a magnet (102) adapted to provide a magnetic field suitable for aligning magnetic spins of atomic nuclei within an imaging zone (126),

a gradient coil assembly (104) adapted to provide gradient magnetic fields suitable for spatially encoding the magnetic spins within the imaging zone,

an RF coil assembly (106) adapted to provide RF magnetic fields suitable for manipulating the orientations of the magnetic spins within the imaging zone, and

an RF screen (108),

the RF screen being located between the RF coil assembly and the gradient coil assembly, the RF screen comprising multiple layers of a screening foil (114), the screening foil comprising a dielectric foil (116) and a plurality of conductive patches (118), wherein the plurality of conductive patches adhere to a first side of the dielectric foil and wherein the multiple layers of screening foil are arranged such that the first side of a first layer of foil adheres to a second side of a second layer of foil and such that conductive patches of different layers overlap, thereby forming a plurality of capacitors (120), the capacitors being formed by overlapping patches and dielectric foil, such that RF current paths with low impedance for mirror currents produced by an electromagnetic field produced by the RF coil assembly exist, wherein the size and arrangement of the conductive patches further causes a high impedance at audio frequencies, thereby reducing the flow of eddy currents within the conductive patches, the eddy currents being caused by the magnetic fields produced by the gradient coil assembly.

2. The magnet assembly for an MRI system of claim 1, wherein the conductive patches are arranged in a regular pattern.

3. The magnet assembly for an MRI system of claim 1 or 2, wherein the conductive patches are of rectangular shape and wherein at least one edge length of the conductive patches is less than 4 cm.

4. The magnet assembly for an MRI system of any of the preceding claims, wherein the conductive patches are strips, wherein the length of the strips corresponds to the length of the RF coil assembly, wherein the width of the strips does not exceed 4 cm and wherein the strips are separated by a gap.

5. The magnet assembly for an MRI system of claim 1 or 2, wherein the conductive patches have a maximum area of 20 cm² each.

6. The magnet assembly for an MRI system of any of the preceding claims, wherein the conductive patches form a checkerboard pattern.

7. The magnet assembly for an MRI system of any of the preceding claims, wherein the conductive patches cover more than 50% of the surface area of the first side of the dielectric foil.

8. The magnet assembly for an MRI system of any of the preceding claims, wherein the conductive patches are arranged such that the structure formed by multiple layers of the screening foil mimics the structure of the RF coil without forming closed ohmic loops.

9. The magnet assembly for an MRI system of any of the preceding claims, wherein the dielectric foil has a thickness between 10 μιη and 100 μιη.

10. The magnet assembly for an MRI system of any of the preceding claims, wherein the dielectric foil material is chosen such that exposing the dielectric foil to heat causes the dielectric foil to shrink.

11. The magnet assembly for an MRI system of any of the preceding claims, wherein the screening foil is formed as a continuous tube.

12. The magnet assembly for an MRI system of any of the preceding claims, wherein the dielectric foil is self-adhesive.

13. A method for screening RF radiation in a magnet assembly for an MRI system, the method comprising: providing a gradient coil assembly adapted to provide gradient magnetic fields suitable for spatially encoding magnetic spins within an imaging zone,

providing an RF coil assembly adapted to provide RF magnetic fields suitable for manipulating orientations of the magnetic spins within the imaging zone,

- applying multiple layers of screening foil to an outer surface of the RF coil assembly,

the screening foil comprising a dielectric foil and a plurality of conductive patches, wherein the plurality of conductive patches adhere to a first side of the dielectric foil and wherein the multiple layers of screening foil are arranged such that first side of a first layer of foil adheres to a second side of a second layer of foil and such that conductive patches of different layers overlap, thereby forming a plurality of capacitor, the capacitors being formed by overlapping patches and dielectric foil, such that RF current paths with low impedance for mirror currents produced by an electromagnetic field produced by the RF coil assembly exist, wherein the size and arrangement of the conductive patches further causes a high impedance at audio frequencies, thereby reducing the flow of eddy currents within the conductive patches, the eddy currents being caused by the magnetic fields produced by the gradient coil assembly,

inserting the RF coil assembly and the applied screening foil into the gradient coil assembly, and

- operating the MRI system to generate an image.

14. The method of claim 13, wherein the multiple layers of screening foil are applied to an inner surface of the gradient coil assembly instead of or in addition to applying the multiple layers of screening foil to the outer surface of the RF coil assembly.