Radio frequency shield

Abstract

A radio frequency (RF) shield (60) includes two concentric cylindrical tubes (61, 62), each having conductive layers spaced apart by a dielectric (63). One conductive layer is divided into a set of two or more longitudinal strips (64). The other conductive layer is divided into a set of two or more circumferential loops (65). The sets of longitudinal strips (64) and circumferential loops (65) intersect to form an overlapping region.

Description

[0001] This application is a divisional application of the invention patent application with application number 202280018629.1, application date February 25, 2022, and invention name "RF coil and RF shielding with T-shaped connectors between electrical conductors". Technical Field

[0002] The present invention relates to a radio frequency (RF) screen for surrounding an RF coil. Background Art

[0003] From the international application WO2006067746 a radio frequency coil is known, which is used to transmit or receive signals at a magnetic resonance frequency and comprises an arrangement formed by substantially parallel crosspieces and one or more substantially annular strip-shaped end rings, which are arranged substantially transverse to the parallel crosspieces and connected to the crosspieces. Each substantially annular strip-shaped end ring comprises at least two conductor layers separated by a dielectric layer. A radio frequency shield substantially surrounds the arrangement formed by substantially parallel crosspieces. At least one conductor layer of each end ring is connected to the radio frequency shield. Summary of the invention

[0004] The present invention relates to a radio frequency (RF) screen for surrounding an RF coil. The RF screen of the present invention is defined in claim 1. The RF screen of the present invention is based on groups of overlapping axial and circumferential conductive segments (which are electrically insulated from each other) and a distributed capacitive coupling formed by the overlapping area of ​​the group of conductive segments. The RF screen counteracts the RF field to extend radially to the outside of the RF coil. The impedance of the distributed capacitive coupling is adjusted to make the RF screen substantially transparent to gradient magnetic field pulses. Therefore, the RF screen of the present invention can be radially positioned between the gradient coil and the RF coil in a magnetic resonance examination system so that the RF coil can be positioned relatively close to the examination area. In this way, the RF coil can operate effectively in both transmit mode and receive mode. The RF screen can be used in combination with the RF coil of the present invention or in combination with a conventional RF coil.

[0005] These and other aspects of the invention will be explained with reference to the embodiments described below and the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 The PCB layout of the inner side of the RF screen of the present invention is shown;

[0007] Figure 2 The PCB layout of the outer side of the RF screen of the present invention is shown;

[0008] Figure 3 The PCB stacking structure of the RF screen of the present invention is shown;

[0009] Figure 4It is a graphical representation of the coronal magnetic field of an ideal RF screen;

[0010] Figure 5 is a graphical representation of the coronal magnetic field of the RF screen of the present invention;

[0011] Figure 6 It is a graphical representation of the coronal electric field of an ideal RF screen;

[0012] Figure 7 is a graphical representation of the coronal electric field of the RF screen of the present invention; and

[0013] Figure 8 Flow calculation results showing the 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. Note the significant reduction in absorbed power, especially for the X gradient. DETAILED DESCRIPTION

[0014] The present invention relates to a new RF screen 60, for example for use with the QBC described in international application WO2022 / 184564.

[0015] An RF screen (also called a shield) is an integral part of a modern quadrature body coil that shields the electromagnetic fields inside the QBC from components outside the QBC. In the absence of an RF screen, this coupling to the outside world degrades the symmetry, uniformity, and quality and predictability of the SNR of the QBC. The disadvantage of an RF screen is that its presence close to the field-generating rungs and loops of the birdcage structure reduces their efficiency.

[0016] In its simplest form, an RF screen can consist of a layer of copper or aluminum foil wrapped around a birdcage structure. However, the high conductivity of such materials can lead to high eddy currents being generated by the switching gradient fields during imaging. To avoid this problem, QBC's RF screens are designed to have high impedance in the kHz range of the gradient fields. A common way to achieve this is to use a material with low conductivity, such as phosphor bronze or titanium, with sufficient thickness to maintain proper RF shielding. The same effect can be achieved using a fine mesh made of steel or bronze.

[0017] Another approach is to divide the RF screen into smaller parts. RF conductivity is achieved between the parts by connecting them through capacitors. These capacitors can be implemented as lumped elements that are brazed between the parts, or by overlapping the parts in a multilayer panel to form integrated capacitors. The effect of eddy currents can be further reduced by extending the path that the eddy currents must follow to surround a certain area of ​​conductive material. This is achieved by adding even more (local) slits to the remaining segments without separating them further. The current flowing through the surface generates a magnetic field on its own, which cancels out the original current, severely increasing the resistance near the center of the sheet. This forces the current to flow mostly near the edges of the sheet. This is called the proximity effect.

[0018] The magnitude of the electromotive force is proportional to the size of the conducting surface, while the magnitude of the resulting current is determined by the resistance of the path around that surface. Therefore, for the same area, reducing the overall perimeter will reduce eddy currents. Thus, segmentation (e.g., in the form of laminations in a transformer core) and slitting reduce eddy currents.

[0019] The segmentation of RF screens is limited based on the understanding that the free flow of RF currents requires large capacitance between the segments. As a result, single layer RF screens have a large number of surface mount capacitors installed, which increases cost, production time and reduces quality. In addition, the lumped capacitors act as choke points for the RF currents, thus changing their natural path and thus reducing the effectiveness of the RF screen. In the case of multi-layer screens with integrated capacitors, the size of the overlap between the segment panels determines the total capacitance between them, and given that the required size of the capacitors requires a large overlap, the number of total segments is severely limited.

[0020] Aspects of the present invention apply an aggressive level of segmentation to the multi-layer RF screen 60, creating a large mesh of smaller coupling capacitors. Using modern computational electromagnetics software, it was determined that the large network of smaller capacitors allows for sufficiently unrestricted flow of RF current to enable the screen to function adequately as an RF screen. It actually functions better than a conventional slit screen compared to it, due to a more uniform impedance distribution across the screen.

[0021] Thus, this aspect of the invention significantly reduces eddy current levels and hence gradient coil induced heat generation compared to older designs, while providing better overall RF performance.This allows the application of higher gradient fields than were feasible with previously known body coil designs.

[0022] Figure 1 The PCB layout of the inner side of the RF screen of the present invention is shown;

[0023] Figure 2The PCB layout of the outer side of the RF screen of the present invention is shown;

[0024] Figure 3 The PCB stacking structure of the RF screen of the present invention is shown;

[0025] This embodiment of the invention consists of a laminated PCB cylinder 60, which can be optionally divided into multiple smaller sub-panels, having at least two conductive layers 64, 65 radially spaced a certain distance apart, so that in combination with the choice of dielectric material 65, there is a specific capacitance per unit area between the conductive layers 64, 65.

[0026] One conductor layer is divided into segments 64 of equal width along the circumference of the cylinder, the other dimension of the segments extending along the entire length of the cylinder, for example as strips.

[0027] The other conductor layer is divided into segments 65 of equal width along the length of the cylinder, the other dimension of the segments extending along the entire circumference of the cylinder, also called annulus.

[0028] The distance between these segments, ie the removed parts of the conductor, is kept to a minimum.

[0029] In this way, a checkerboard pattern 66 of overlapping slots 64, 65 and loops is formed, thereby forming a large cylindrical network of layered capacitors.

[0030] A continuous loop 65 along the entire perimeter of the screen can capture switching gradient fields perpendicular to the transverse plane of the scanner's aperture, such as the field produced by the z-gradient. Therefore, the loop should be cut at least once more to break the loop. Preferably, this cut is made in the middle of the strip, so that two smaller capacitors are formed and a continuous RF path is maintained along the entire loop.

[0031] It was determined by numerical analysis that the width of the strips and loops should not be greater than 20 mm and the gaps 64, 65 between the strips and loops should not be greater than 1 mm. The resulting design had 32 loops and 96 strips for a total of 3072 capacitors.

[0032] The previous assumption that smaller capacitors would not allow RF currents to flow sufficiently freely and efficiently has proven to be unfounded. It has been found that although the capacitance of the individual elements is relatively small, the network of inductively and capacitively coupled segments 66 produces an impedance at RF frequencies comparable to that found in previous screen designs.

[0033] In summary, a minimal list of features of the invention is as follows:

[0034] Two concentric cylindrical tubes of conductive material, separated by a dielectric, surround the RF coil.

[0035] One conductive layer is divided into at least two strips 64 .

[0036] The other conductive layer is divided into a minimum of two loops 65 .

[0037] The minimum dimension of the surface of the segment should not be greater than 20 mm.

[0038] The strips and loops overlap in a checkerboard pattern, forming a network of coupling capacitors.

[0039] Note: The value of the width of the segments depends entirely on the level of eddy current suppression required. If the minimum width requirement is not taken into account, the present invention is very close to the previous generation of layered screens, which are divided into strips rather than rings.

[0040] The PCB consists of two layers 67 of copper with a thickness of 18 μm, separated by a 64 μm ISOLA 370HR dielectric material 63. The innermost copper layer, i.e. the layer facing the inside of the hole, is divided into 24 parallel copper strips, 20 mm wide and 700 mm long, separated by 1 mm wide gaps (slits 152). The outermost copper layer, i.e. the layer facing the gradient coil, is divided into 32 parallel copper pieces, which are connected at the edge of the PCB to form a loop around the hole. These loop segments are 20 mm wide and 510 mm long, separated by 1 mm wide gaps (slits 152). To avoid the formation of large continuous loops, each loop segment itself is divided into two segments, in which a 1 mm wide slit is located alternately at a distance of 117 mm from the left and right sides of the board.

[0041] RF model results from ANSYS HFSS show the normalized magnetic and electric field distributions in the coronal plane of a perfect electrical conductor screen (left) and a prototype screen (right). Note the similarity of the two field distributions, especially the lack of significant electric fields extending from the sides of the screen.

[0042] The four panels are wound onto a body coil carrier containing a birdcage coil antenna, and the loop segments are electrically connected.

[0043] RF simulation and measurement results show the proper functionality of the novel RF screen.

[0044] Figure 4 A graphical representation of the coronal magnetic field of an ideal RF screen.

[0045] Figure 5 is a graphical representation of the coronal magnetic field of the RF screen of the present invention.

[0046] Figure 6 is a graphical representation of the coronal electric field of an ideal RF screen.

[0047] Figure 7is a graphical representation of the coronal electric field of the RF screen of the present invention. That is, Figures 4 to 7 The following are the results of electromagnetic model calculations, which show the distribution of normalized magnetic and electric fields on the coronal plane of the body coil for a perfect screen and a novel screen design. The calculations show that there are no significant differences in field strength and distribution between the novel screen design and the perfect screen. Of particular note is the lack of a significant electric field extending from the middle of the screen, indicating that the system interaction is unlikely to be significantly greater than that of a perfect screen. To confirm this excellent system isolation, the QBC was inserted into a gradient coil with a continuous copper screen and a novel RF screen. The difference in the main mode frequency shift due to GC system interaction between the perfect screen and the novel RF screen is negligible (369kHz vs 332kHz), as is the reduction in the body coil quality factor (-20vs-21).

[0048] Figure 8 Figure 2 shows the absorbed power density [kW / m2] of a conventional RF screen (top) and the RF screen of the present invention due to gradient-induced eddy currents. 3 Note the significant reduction in absorbed power, especially for the X gradient.

[0049] The Stream calculation algorithm was used to calculate the eddy current power density induced by the gradient coils on the RF screen panels of the novel RF screen and the RF screen 3TM2. The results (shown in the figure) show an expected reduction in absorbed power of a factor of 3.3 for the X and Y gradients, a factor of 3.9 for the Z gradient, and an expected reduction in peak power of a factor of 42.1.

[0050] Thermal probe measurements of the novel RF screen confirmed that the panel remained well within the temperature requirements for maximum system performance (<60°C steady state, Grms = 31.25 mT / m, frequency = 1.6 kHz + 2 x 16 kW RF power).

[0051] An alternative embodiment of the present invention may be to apply a checkerboard network of laminated capacitors to the screen of other RF coils, such as the head coil, rear coil or front coil. In addition, the checkerboard pattern may also be applied to other sensitive subsystems that require shielding.

[0052] The main applications of the present invention are:

[0053] Reduces eddy currents induced by gradient fields in the RF screen and thus:

[0054] The heating induced by the gradient field of the RF screen is mitigated, thereby mitigating the inner hole heating.

[0055] Reduces eddy current induced field errors and associated anomalies such as EPI ghosting.

[0056] Therefore, since inner bore heating would limit the applied gradient power, the present invention expands the possibility of applying stronger gradient fields and shorter switching times.

[0057] This concept achieves its purpose without the need for additional lumped capacitors and the associated increased cost and reduced reliability.

[0058] The uniform and symmetrical layout of the screen makes its embodiments broadly applicable to most body coil designs, as well as a wide range of other applications requiring RF shielding with low gradient sensitivity.

Claims

1. A radio frequency (RF) shield (60), comprising: two concentric cylindrical tubes (61, 62), each having a conductive layer separated by a dielectric (63), A conductive layer is divided into a set of two or more longitudinal strips (64), another conductive layer is divided into a set of two or more circumferential rings (65), and Groups of longitudinal strips (64) and circumferential loops (65) intersect to form overlapping areas.

2. The radio frequency (RF) shield (60) of claim 1, wherein: The width of the strip (64) and the ring (65) is no greater than 20 mm, and the slit (64, 65) therebetween is no greater than 1 mm.

3. A radio frequency assembly comprising a radio frequency (RF) shield (60) according to claim 1 and an RF coil, the RF coil comprising two concentric cylindrical tubes formed of a conductive material separated by a dielectric so as to surround the RF coil, wherein The conductive layer disposed on one of the cylindrical tubes is divided into at least two strips (64), and Another conductive layer disposed on another of the cylindrical tubes is divided into at least two loops (65), and wherein The smallest dimension of the surface of the segment is no greater than 20 mm, and The strips and loops overlap in a checkerboard pattern, forming a coupling capacitor network.

4. The radio frequency (RF) shield (60) of claim 1, wherein: The two concentric cylindrical tubes are formed into a laminated PCB cylinder, whose innermost copper layer, i.e. the radially inward layer, is divided into 24 parallel copper strips, 20mm wide and 700mm long, separated by 1mm wide gaps (slits 152), and whose outermost copper layer, i.e. the radially outward layer, is divided into 32 parallel copper pieces, which form rings around the holes when connected at the edge of the PCB, these ring segments are 20mm wide and 510mm long, separated by 1mm wide gaps (slits 152), and each ring segment itself is divided into two segments, in which a 1mm wide slit is positioned alternately at a distance of 117mm from the left and right sides of the board.

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