High frequency coil with T-shaped connector between conductors

Abstract

The radio frequency (RF) coil has a conductor layout with several axial rungs 11 and several circumferential rings 12, at least one of the rungs 11 being coupled to at least one of the rings by a T-shaped connector that has a distributed capacitive coupling between the ring 12 and the rungs 11.

Description

[Technical field]

[0001] The present invention relates to a radio frequency (RF) coil having a conductor layout with multiple axial rungs and multiple circumferential rings joined by a T-shaped connector. [Background technology]

[0002] Such an RF coil is known from US 2009 / 0219026. Summary of the Invention [Problem to be solved by the invention]

[0003] The known RF coil is a birdcage type multi-channel transmit / receive RF coil, in which a set of coil units form a cylindrical shape having a circumference and opposite ends. The coil units are connected in series at respective opposite ends of the cylindrical shape along the circumferential direction of the cylindrical shape. Each coil unit has at least one capacitor. The capacitance value of the capacitor in each coil unit is selected primarily to achieve a primary mode coil RF resonance at a desired frequency. A plurality of capacitors are connected within the coil unit at the edge of one end face of the cylindrical shape, respectively.

[0004] It is an object of the invention, inter alia, to provide an RF coil for a magnetic resonance examination system having improved electromagnetic properties. [Means for solving the problem]

[0005] This object is achieved by an RF coil according to claim 1. The insight of the invention is that the distributed capacitive coupling at the T-shaped connector where the ring and rung meet each other forms a parallel circuit capacitor array. This achieves multiple independent current paths of similar impedance that distribute the current in a way that reduces the proximity effect, reduces local RF heating and suppresses eddy current generation due to gradient switching in the magnetic resonance examination system. The distributed capacitive coupling configuration further has a large impedance transition between the capacitor plates that is mainly radially concentrated, resulting in a reduced electric field fanning out towards the examination region of the magnetic resonance examination system (i.e. towards the examined patient). Furthermore, the RF coil of the invention is simpler and cheaper to manufacture, in particular since it does not require the installation of a large number of lumped capacitors. The invention achieves improved electromagnetic properties, in particular in that the RF coil has improved gradient field decoupling and RF performance. The invention can be used in a magnetic resonance examination system having an RF transmit antenna for generating an RF field in the examination zone and a gradient magnetic field system for generating gradient magnetic field pulses. The RF field and the gradient magnetic fields manipulate the nuclear spins so that the generated magnetic resonance signals are encoded for a particular contrast and spatial resolution. When used in a magnetic resonance examination system, the present invention provides T-sections with low gradient coupling, especially for laminated RF coils.

[0006] In the framework of this application, the term T-shaped encompasses any geometric arrangement in which conductors meet in different orientations, often at right angles, although it should be noted that other lateral orientations may also be used. Thus, the T-shape may be linear, diagonal, or may or may not be part of a more elaborate geometric combination of conductors, such as the shapes of a + sign or an X sign, which can be considered as a combination of multiple (two) strict T-shapes.

[0007] The insight of the present invention is that to achieve better capacitive coupling without the use of discrete capacitive elements, a large conductive overlap is required, but on the other hand, a large conductive overlap area increases susceptibility to eddy currents. In accordance with the present invention, the structured, separate, galvanically isolated conductor overlap areas of both the ring and the rungs achieve both improved capacitive coupling and reduced eddy current susceptibility, thus reducing heating.

[0008] The present invention includes an RF receiving coil for receiving (RF) magnetic resonance signals, and an RF (B1 + ) a RF transmit coil for generating a field, and an RF transmit / receive coil having an RF transmit / receive coil that can operate in transmit and receive modes.

[0009] These and other aspects of the invention will be further elaborated with reference to the embodiments defined in the dependent claims.

[0010] In a preferred embodiment of the RF coil of the present invention, the rungs and rings have respective sets of overlapping axial and circumferential conductive segments forming a T-shaped conductor and galvanically isolated from one another, and the distributed capacitive coupling is formed by the overlapping areas of the sets of conductive segments. The axial and circumferential conductive segments are separated in the radial direction, for example by a layer of dielectric material between them. In this embodiment, the distributed capacitive coupling in the T-connectors forming the parallel circuit capacitor array is achieved in a simple manner, for example by overlapping conductive segments separated by a dielectric layer. The axial direction is along the longitudinal axis of the RF coil. The circumferential direction is the circumferential direction around the longitudinal axis. The longitudinal and circumferential directions correspond to the RF coil cylindrical geometry constituting the RF coil volumetric resonator.

[0011] In another example, the axial and circumferential sets can have multiple elongated conductors oriented circumferentially and axially, respectively, This configuration has good radio frequency characteristics while suppressing eddy currents, e.g., due to gradient switching.

[0012] In another example, the T-connector comprises a printed circuit board, with the axial and circumferential conductive segments disposed on opposite surfaces of the printed circuit board. By using well-controlled PCB technology, simple, accurate and inexpensive manufacture of the RF coil of the present invention is achieved.

[0013] In another example, the axial and circumferential conductive segments are sets of circumferentially and axially oriented flat elongated conductor strips, each slit. These slit strips effectively suppress the generation of eddy currents. For examples where optimal results are achieved using segments with a minimum linear dimension not exceeding 20 mm and segments spaced apart by no more than 1 mm, good results are achieved both in terms of suppressing eddy currents and avoiding disturbance of the RF field pattern. However, the relevant technical effect is not limited to this practical example.

[0014] The sets of axial and circumferential conductive segments may be integral with the rings and rungs, respectively, thereby forming T-shaped connectors connecting the rings and rungs, which allows for easy and accurate construction of the RF coil of the present invention.

[0015] In a practical embodiment, the RF coil of the present invention has several, e.g., two, three or more, axially spaced circumferential rings oriented perpendicular to the longitudinal axis of the coil and joined by a plurality of six, twelve or more axially extending rungs, each pair of rings and rungs being joined by a T-shaped connector that includes distributed capacitive coupling between the rings and rungs.

[0016] The invention further relates to a radio frequency (RF) screen surrounding an RF coil. The RF screen of the invention is defined in claim 8. The RF screen of the invention is based on a set of overlapping axial and circumferential conductive segments forming a T-shaped conductor and galvanically insulated from one another, and a distributed capacitive coupling formed by the overlapping areas of the sets of conductive segments. The RF screen cancels the RF fields extending outside (radially) the RF coil. The impedance of the distributed capacitive coupling is adjusted so that the RF screen remains substantially transparent to the gradient magnetic field pulses. The RF screen of the invention can therefore be placed radially in a magnetic resonance examination system between the gradient coil and the RF coil, so that the RF coil can be placed relatively close to the examination zone. The RF coil can therefore operate efficiently in both transmit and receive modes. The RF screen can be used in combination with the RF coil of the invention or in combination with a conventional RF coil.

[0017] These and other aspects of the invention will be explained with reference to the embodiments described hereinafter and with reference to the accompanying drawings. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram of an MRI system showing the positions of the gradient coils and the body coil. [Diagram 2] FIG. 2 shows a T-section of the 4-rung birdcage body coil of the present invention. [Diagram 3] FIG. 2 is a top view of a prior art single T-section with a lumped capacitor. [Figure 4] FIG. 13 is a top view of a single T-section with a stacked capacitor, where the copper on the other side of the dielectric is depicted with a white pattern and there is a large continuous copper area in the center of the T-section. [Diagram 5]A top view of a new implementation of a T-section, where the copper on the opposite side of the dielectric is depicted in white, the extension of one section entirely overlaps the other, and the slit extends entirely along the conductive section. [Figure 6] FIG. 2 shows the inner PCB layout of the RF screen of the present invention. [Figure 7] FIG. 2 shows the exterior of the PCB layout of the RF screen of the present invention. [Figure 8] FIG. 2 shows the PCB stackup of the RF screen of the present invention. [Figure 9] Graphic diagram showing the coronal magnetic field for an ideal RF screen. [Figure 10] FIG. 2 is a graphic representation of the coronal magnetic field of the RF screen of the present invention. [Figure 11] Graphical representation of the coronal electric field of an ideal RF screen. [Figure 12] 1 is a graphic representation of the coronary field of the RF screen of the present invention; [Figure 13] FIG. 13 shows a stream calculation of absorbed power density [kW / m3] due to gradient-induced eddy currents for a conventional RF screen (top) and an RF screen of the present invention (bottom), showing a significant reduction in absorbed power, especially for the X-gradient. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] FIG. 1 shows a diagram of an MRI system 1 showing the locations of the gradient coils 20 and the RF body coil.

[0020] The MRI scanner in this example has three cylindrical concentric subsystems: a static magnet, a gradient coil 20, and a quadrature coil 10 (FIG. 1) surrounding a patient bore. The gradient coils generate gradients in the static magnetic field, which are used for localization of image signals pulsed at frequencies in the kHz range. Between the gradient coils and the patient bore is a quadrature body coil, which transmits and receives RF field pulses in the MHz range used for image acquisition.

[0021] A typical body coil design is the birdcage, which has multiple conductive rods, also known as rungs 11, connected to each other at both ends by two large rings 12 along the central axis of the patient bore. To make the birdcage coil resonate and deliver the RF field to the patient at the optimal frequency, capacitors usually separate sections of the rings 12, sections of the rungs 11, or both sections. Modern implementations of body coils typically have the conductive rings and rungs 11 printed out on a large PCB. This PCB is folded into a cylindrical shape and connected at the edges to form the birdcage.

[0022] Capacitors used in birdcage coils have traditionally been discrete electronic components soldered to the body coil, but more recent designs have integrated the capacitor function into a larger multi-layer PCB layout. The advantage of an integrated capacitor is that it eliminates the need for soldering, with associated advantages in terms of manufacturing and reliability, but at the expense of size and capacitance range.

[0023] 2 shows a T-section 13 on a four rung birdcage body coil 11 of the present invention. The birdcage configuration has axially spaced rings 12 connected by a number of axially extending rungs 11, of which four are shown in the figure by way of example, with each rung 11, 11 abutting a respective ring 12 via a T-shaped section 13.

[0024] 3 shows a top view of a prior art single T-section 13 with a lumped capacitor 16. In the central region there is a large continuous copper area 17.

[0025] 4 shows a top view of one example of a single T-shaped section 13 of the present invention having a stacked capacitor 14. The copper on the other side of the dielectric is depicted with a hatched pattern 151. Note the large continuous conductive metal, e.g., copper, area in the center of the T-section 13. The copper on the other side of the dielectric is depicted with a hatched pattern. Note the large continuous copper area in the center of the T.

[0026] FIG. 5 shows a top view of a new implementation of T-section 13. The metal, such as copper, on the other side of the dielectric is depicted with a hatched pattern. Note that the extension 142 of one section generally overlaps with the other section 143, and that the slits 141, 152 extend generally along the conductive sections 11, 12. The copper on the other side of the dielectric is depicted with hatched patterns. Note that one section extends to generally overlap with the other section, and that the slits 152 extend generally along the conductive sections.

[0027] A time-varying magnetic flux through a closed conductive loop results in a current flowing through the loop. These so-called eddy currents generate heat in the conductive medium, which generates its own magnetic field that opposes the original field. The gradient coil generates such a time-varying magnetic field, and any conductive medium inside the gradient coil, such as the rungs 11 and rings 12 of the body coil, experiences these eddy currents. The resulting heat can pose risks to both the patient and the components, and the disruption of the magnetic field reduces the quality of the image. This effect is well understood, and design heuristics preclude the presence of large conductive surfaces and loops in the gradient field. The method used is to split large conductors into smaller parts by slits 152 and, if necessary, bridge the gaps by capacitors. Due to their electromagnetic properties, these capacitors act as gaps at the lower frequencies of typical gradient field variations, but act as connections at the much higher operating frequencies of the body coil.

[0028] Typically, this is how eddy currents are handled in the body coil ring 12 and rungs 11. However, this leaves the T-section 13 where the rungs 11 meet the ring 12 unslit since the change in RF current direction would make proper placement of the capacitor difficult. Partially slit T-sections 13 of body coils with lumped and stacked capacitors are shown in Figures 3 and 4, respectively. Attempts have been made to change the size and shape of the T-section 13, some including the addition of discrete capacitors, with varying degrees of success. In the latest generation body coils, eddy currents in the body coil T-section 13 remain the primary cause of high inner bore temperatures.

[0029] Using advanced computational analysis methods, the T-section 13 layout of the present invention effectively or completely separates the ring 12 and rung 11 sections, splitting the sections themselves into multiple smaller strips, and minimizing the width of all conductive elements. The rung 11 and ring 12 sections are physically separated by a dielectric layer and electrically connected only through stacked capacitors. Because both the ring 12 and rung 11 sections are completely slit along their lengths, the overlapping sections form a large parallel circuit capacitor array. Figure 5 shows an implementation of this new T-section 13 concept.

[0030] The present invention combines both tuning and bridging capabilities into this single layer capacitor array without significantly impeding the inherent RF current flow. In effect, the new layout provides multiple independent paths for the current to have similar impedances, spreading the current throughout the conductor, limiting proximity effects and reducing eddy current heating as well as reducing localized RF heating. Also, the large impedance transitions are concentrated in the vertical plane and between the capacitor plates, resulting in less electric field extending toward the patient.

[0031] The present invention has particular application to quadrature body coils (QBC) that use a printed circuit board (PCB) for the internal components, referred to herein as the antenna, that are directed towards the patient.

[0032] The minor dimension of the conductive surface of the QBC antenna PCB should be less than 20 mm. The antenna ring 12 sections should have no conductive connection with the rung 11 sections (i.e., T-sections 13) to which they abut. The antenna rung 11 sections on one layer of the PCB extend across the ring 12 sections on the other layer of the PCB, thus forming a network of parallel plate capacitors integrated within the PCB structure. (See Figure 5).

[0033] The invention described is meant to apply to a particular geometric feature of the antenna conductor called the T-section 13 and to address a particular problem encountered in this T-section 13, namely overheating of the T-section 13 due to eddy currents induced by the gradient coil. However, as shown herein, the positive effects go far beyond this single problem. Thus, the application of some of the features can be and apply to many other PCB structures used within the bore of an MRI scanner, including QBC antennas and screens as well as other MRI coils. A unique aspect of the invention is that it combines both the segmentation of feature 2 and the overlapping pattern of feature 4 to generate a complex pattern of distributed coupling capacitors. This allows a significant reduction in the size of the largest conductive surface, in this case the T-section 13 of the antenna.

[0034] Since this problem solves a problem specific to QBCs that use laminated PCB structures for antennas, an alternative solution of the present invention uses an alternative structure that provides a small continuous surface parallel to the surface of the gradient coil, such as a QBC with copper rod structure. In the case of PCB antennas, an alternative method uses a narrowing of the width of the ring 12 at the T-section 13 to reduce the presented surface area.

[0035] The main applications of the present invention are: Eddy currents in the T-section 13 of the antenna induced by the gradient magnetic field are reduced. Gradient magnetic field induced heating of the antenna T-section 13 is reduced, resulting in reduced heating of the inner bore. Eddy current induced magnetic field errors and related anomalies such as EPI ghosts are further reduced. Consequently, since inner bore heating can limit the applied gradient power, the present invention expands the possibilities for application of stronger gradient fields and shorter switching times. The capacitor array connections between the rings 12 and rungs 11 effectively distribute the RF current over a larger surface and reduce the proximity effect. This reduces the peak surface currents, lowering power losses and increasing coil efficiency. As a result, heating induced in the T-section 13 by local RF currents is also reduced by implementation of this concept. An overlapping rung 11-ring 12 transition portion more effectively shields the patient from electric fields across the transition than a more horizontally oriented transition. This reduces undesirable system interactions between the body coil and the patient, such as body coil detuning and electric field induced patient SAR. This concept uses standard laminate PCB body coil concepts and can be applied to redesign most low pass and band pass implementations of existing laminate PCB body coil designs.

[0036] This concept achieves its objectives without the need for additional lumped capacitors and the associated increased cost and reduced reliability.

[0037] Another embodiment of the invention to be described relates to a novel RF screen 60 for use with, for example, the QBC of the present invention.

[0038] RF screens (also called shields) are an integral part of today's quadrature body coils that shield components outside the QBC from the electromagnetic fields inside the QBC. Without an RF screen, this coupling with the outside world would degrade the quality and predictability of the QBC's symmetry, uniformity, and SNR. A drawback of the RF screen 60 is that its presence close to the birdcage's magnetic field generating rungs 11 and rings 12 reduces their efficiency.

[0039] In its simplest form, an RF screen can comprise a layer of copper or aluminum foil wrapped around a birdcage. However, during imaging, the high conductivity of this type of material results in high eddy currents from the switching gradient fields. To avoid this problem, QBCRF screens are designed to have high impedance in the kHz range of the gradient fields. One common way to achieve this is by using a material with low conductivity, such as phosphor bronze or titanium, thick enough to maintain adequate RF shielding. The same effect can be achieved by using a fine mesh made from steel or bronze.

[0040] Another method is to segment the RF screen into smaller sections. RF conductivity is achieved between the sections by connecting them through capacitors. These capacitors can be implemented as lumped elements soldered between the sections, or by stacking the sections in a multi-layer panel to form an integrated capacitor. The effects of eddy currents can be further reduced by lengthening the path the eddy currents take to encircle a certain area of ​​conductive material. This is achieved by further (partially) slitting the remaining segments without further dividing them. The current flowing on the surface creates its own magnetic field, cancelling the original current and greatly increasing the resistance near the center of the sheet. This causes the current to flow mainly near the edges of the sheet. This is called the proximity effect.

[0041] The magnitude of the electromotive force is proportional to the size of the conductive surface, and the magnitude of the resulting current is determined by the path resistance around the conductive surface. Therefore, for the same area, reducing the circumference reduces the eddy currents. Therefore, segmentation (lamination of transformer cores) and slitting reduce eddy currents.

[0042] Segmentation of RF screens was limited by the understanding that a large capacitance was required between the segments in order for RF current to flow freely. This resulted in single layer RF screens being populated with a large number of surface mounted capacitors, which increased cost and manufacturing time, and reduced quality. Furthermore, lumped capacitors act as choke points for the RF current, altering its natural flow, thus reducing the effectiveness of the RF screen. For multi-layer screens with built-in capacitors, it was determined that the amount of overlap between segment panels determined the total capacitance between the panels, and the size of the capacitors required would require so much overlap that the total number of segments would be severely limited.

[0043] The proposed embodiment of the present invention applies a large level of segmentation to the multi-RF screen 60, generating a large mesh of smaller coupling capacitors. Through the use of modern computational electromagnetic software, it has been determined that a large network of smaller capacitances allows for the uninhibited flow of enough RF current for the screen to function properly as an RF screen. It actually performs better compared to a conventional slitted screen due to the uniform distribution of impedance across the screen.

[0044] This aspect of the invention therefore significantly reduces the levels of eddy currents, and therefore gradient coil induced heating, compared to previous designs, while providing better overall RF performance, thereby allowing the application of higher gradient fields than were possible with previously known body coil designs.

[0045] FIG. 6 shows the inner PCB layout of the RF screen of the present invention.

[0046] FIG. 7 shows the outer PCB layout of the RF screen of the present invention.

[0047] FIG. 8 shows the PCB stackup of the RF screen of the present invention.

[0048] This implementation of the present invention has a laminated PCB cylinder 60, optionally divided into several smaller sub-panels, with at least two conductive layers 64, 65 radially spaced apart a distance such that, in combination with the selection of dielectric material 65, a certain capacitance per unit area exists between the conductive layers 64, 65.

[0049] One conductive layer is divided into segments 64 of equal width around the circumference of the cylinder, the other dimension of the segment extending along the entire length of the cylinder.

[0050] The other conductive layer is divided into segments 65 of equal width along the length of the cylinder, with the other dimension of the segment extending completely around the circumference of the cylinder.

[0051] The distance between these segments, and therefore the portion of the conductor removed, is kept to a minimum.

[0052] In this manner, a checkerboard pattern 66 is formed of overlapping slits 64, 65 and hoops forming a large cylindrical network of stacked capacitors.

[0053] A continuous hoop 65 along the entire circumference of the screen can capture the fields generated by switching gradient fields perpendicular to the cross-section of the scanner bore, e.g. z-gradients. The hoop should therefore be slit at least one more time to break the hoop. It is preferable to apply this slit in the middle of the strip, thus forming two smaller capacitors and leaving a continuous RF path along the entire hoop.

[0054] It was determined by numerical analysis that the width of the strips and hoops should be no more than 20 mm, and the slits 64, 65 between the strips and hoops should be no more than 1 mm. The resulting design has 32 hoops and 96 strips for a total of 3072 capacitors.

[0055] The assumption that smaller capacitors would not allow a sufficiently free and efficient flow of RF current has proven not to be warranted. Although the capacitance of a single element is small, the network of inductively and capacitively coupled segments 66 has been found to provide impedance at RF frequencies comparable to previous screen designs.

[0056] In summary, the minimal list of features of the present invention is as follows:

[0057] Two concentric cylindrical tubes of conductive material surrounding an RF coil with a dielectric between them.

[0058] One conductive layer is segmented into a minimum of two strips 64 .

[0059] The other conductive layer is segmented into a minimum of two hoops 65 .

[0060] The smallest dimension of the surface of a segment must not exceed 20 mm.

[0061] The strips and hoops overlap in a checkerboard pattern to form a network of coupled capacitors.

[0062] NB: The value of the segment width depends entirely on the level of eddy current suppression required. If we leave out the minimum width requirement, the present invention is much closer to previous generation laminated screens that were divided into strips instead of hoops.

[0063] The PCB has two copper layers 67 of 18 μm thickness separated by 64 μm of ISOLA 370 HR dielectric 63. The innermost copper layer, i.e. the layer facing the inside of the bore, is divided into 24 parallel copper strips of 20 mm width and 700 mm length, the copper strips being separated by a gap (slit 152) of 1 mm width. The outermost copper layer, i.e. the layer facing the gradient coil, is segmented into 32 parallel copper pieces which, when connected at the edge of the PCB, form a hoop surrounding the bore. These hoop segments are 20 mm wide, 510 mm long and separated by a gap (slit 152) of 1 mm width. To avoid continuous large hoops, each hoop segment is itself divided into two segments by 1 mm wide slits located 117 mm from the left and right sides of the plate, alternating.

[0064] Results from the RF model ANSYS HFSS show the normalized magnetic and electric field distributions in the coronal plane for a perfect conducting screen (left) and the prototype screen (right). Note the similarity in both field distributions, especially the absence of significant electric fields extending out from the sides of the screen.

[0065] The four panels are wrapped around a body coil carrier that contains the birdcage coil antenna, and the hoop segments are galvanically connected.

[0066] RF simulations and measurements demonstrate proper functioning of the new RF screen.

[0067] FIG. 9 is a graphical representation of the coronal magnetic field for an ideal RF screen.

[0068] FIG. 10 is a graphical representation of the coronal magnetic field of the RF screen of the present invention.

[0069] FIG. 11 is a graphical representation of the coronal field of an ideal RF screen.

[0070] FIG. 12 is a graphical representation of the coronal electric field of the RF screen, which is the RF screen of the present invention. ie, FIG. 9-FIG. 12 are electromagnetic model calculations showing the distribution of normalized magnetic and electric fields in the coronal plane of the body coil for the complete screen design and the new screen design. The calculations showed no significant difference in magnetic field strength and distribution between the new screen design and the complete screen design. Of particular note is the absence of significant electric fields extending from the center of the screen, indicating that system interactions are less likely to be significantly greater than with the complete screen. To confirm this superior system isolation, a QBC was inserted into the gradient coil with the continuous screen and the new RF screen. The difference in main mode frequency shift due to GC system interactions was negligibly small between the complete screen and the new RF screen (369kHz vs. 332kHz), as was the degradation of the body coil quality factor (-20 vs. -21).

[0071] Figure 13 shows the stream calculation of absorbed power density [kW / m3] due to gradient-induced eddy currents for the conventional RF screen (top) and the RF screen of the present invention. Note that the absorbed power is significantly reduced, especially for the X-gradient.

[0072] A stream computation algorithm was used to calculate the gradient coil induced eddy current power density on the RF screen panel for both the new RF screen and the RF screen 3T M2. The results shown in the figures show a predicted reduction in absorbed power of 3.3 times for x and y gradients and 3.9 times for z gradients, as well as a predicted peak power reduction of 42.1 times.

[0073] Thermal probe measurements of the new RF screen confirm that the panel stays well within the temperature requirements for maximum system performance (steady state below 60°C for Grms=31.25mT / m, freq=1.6kHz+2×16kW RF power).

[0074] As an alternative embodiment of the present invention, the checkerboard network of laminated capacitors can be applied to the screens of other RF coils such as head coils, rear coils, front coils, etc. Additionally, the checkerboard pattern can be applied to other sensitive subsystems requiring screening.

[0075] The main applications of the present invention are: The gradient magnetic field induced eddy currents in the RF screen are reduced. Thus, RF screen gradient field induced heating is reduced and therefore inner bore heating is reduced. Magnetic field errors due to eddy currents are reduced, reducing anomalies such as EPI ghosts. As a result, the present invention expands the possibilities for application of stronger gradient fields and shorter switching times, since inner bore heating can limit the applied gradient power. This concept achieves this objective without additional lumped capacitors and the associated increased cost and reduced reliability. The uniform and symmetric layout of the screen makes its implementation universally applicable to most body coil designs, as well as a wide range of other applications requiring low gradient sensitive RF screening.

Claims

1. 1. A radio frequency (RF) coil having a conductor layout with axial rungs and circumferential rings, at least one of the rungs being coupled to at least one of the rings by a T-shaped connector, the T-shaped connector having a distributed capacitive coupling between the ring and the rungs, the rungs and the rings having sets of overlapping axial and circumferential conductive segments, the overlapping conductive segments forming the T-shaped conductor and galvanically isolated from one another, the distributed capacitive coupling being formed by overlapping areas of the sets of conductive segments.

2. The RF coil of claim 1 , wherein the set of axial conductive segments and the set of circumferential conductive segments each include a plurality of elongated conductors oriented in the axial and circumferential directions.

3. 2. The RF coil of claim 1, wherein said T-shaped connector comprises a printed circuit board, said axial and circumferential conductive segments being disposed on opposing surfaces of said printed circuit board.

4. 4. An RF coil according to claim 1, wherein the axial and circumferential conductive segments are sets of individual flat elongated conductor strips oriented and slit in the circumferential and axial directions, respectively.

5. 5. An RF coil as claimed in claim 1, wherein the sets of axial and circumferential conductive segments are integral with the ring and rungs, respectively, to form T-shaped connectors.

6. 6. An RF coil as claimed in claim 1, comprising a plurality of circumferential rings and a plurality of axial rungs, pairs of the rungs and rings having respective sets of overlapping axial and circumferential conductive segments, the overlapping conductive segments forming a T-shaped conductor coupling the pairs of rings and rungs and being galvanically isolated from one another, and the distributed capacitive coupling being formed by overlapping areas of the sets of conductive segments.

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