FIELD

The present disclosure relates to a method for removing shield current along a RF (radio frequency) cable for MRI RF coil and further relates to a transmission line for MRI RF coil.

BACKGROUND

Shield current or common mode current in MRI RF coil cables has negative impact to image quality or even generates serious RF burn to patient. Cable trap, or say balun, is usually used to minimize shield current in cables. With the development of coil array technology, it is common to group multiple coaxial cables into a RF cable which constitutes a transmission line together with the baluns. A floating balun, which is a variation on the traditional Bazooka balun is commonly used for this phase array coil. This balun consists of two coaxial metallic cylinders, where one end is shorted together and the other end is connected with capacitors and tuned to resonance. In another form, capacitors are evenly placed at both sides of the cylinder. This balun is floating, requiring no connections or welding to the coaxial cables and allows for more flexibility to place the balun at desired position. Also this balun is completely splitable, and the two halves are not connected, so it is easy to be replaced when damaged. Performance of a floating balun usually depends on its length and ratio of out diameter vs. inner diameter. In many cases, baluns have to be placed near the coil elements and thus their sizes are usually limited to keep the RF coil thin and flexible for better customer experience. Figure la and lb show conventional transmission lines using a compact stripline balun and a short bazooka balun respectively, both of which are floating baluns. The stripline balun is 8cm in length, the outer diameter is 1.5cm and the inner diameter is 1.0cm. The short bazooka balun in Fgiure lb has short length around 1.5cm, the inner diameter is 1.0cm and the outer diameter of 2.4cm. However, the performance of removing the shield current using these conventional transmission lines is low, the common mode blocking (CMB) coefficient S21 through the cable is measured lower than -6dB at SUMMARY

In one aspect of the present disclosure, a transmission line for MRI RF coil is provided, comprising:

one or more floating baluns with a tubular shape; and

a RF cable comprising one or more coaxial cables which is wound around each of the one or more floating baluns along the axial direction of the each balun and passed through the each balun at least twice.

By using this transmission line, the length of the RF cable in the hollow space of the baluns can be increased, so the performance of removing shield current can be greatly improved.

Optionally, the RF cable wound around the one or more floating baluns is configured as that the distribution of the RF cable on the cross section perpendicular to the axial direction of the one or more floating baluns is axisymmetric. The reason for this is that such a distribution contributes to a highly confined magnetic filed, thereby the balun will be RF invisible or magnetic transparent to the MRI system including such baluns.

Optionally, the RF cable wound around the one or more floating baluns is configured with a shape of the Arabic number '8' and passed through the one or more floating baluns three times, which also contributes to a highly confined magnetic filed.

Optionally, if there are two or more floating baluns, at least two of these floating baluns are arranged in parallel to form a floating balun array. The coupling between these baluns slightly degrades the performance, but result in a wider bandwidth.

Optionally, the RF cable is wound around the floating balun array along the inner walls of the at least two floating baluns, because more parts of the RF cable can be in the hollow space of the baluns in this manner.

In such a case, the floating balun array comprises a first floating balun and a second floating balun which are arranged in parallel, and the floating balun array has a first end and a second end, wherein the RF cable firstly passes through the first floating balun from the first end to the second end according to a first pattern and secondly passes through the second floating balun from the second end to the first end according to a second pattern, and passes through the first and second floating baluns according to the first and second patterns at least once more.

In some embodiments, at least one of the one or more floating baluns is a stripline balun. In some embodiments, at least one of the one or more floating baluns is a bazooka balun.

In another aspect of the present disclosure, a method for removing shield current along a RF cable for MRI RF coil is provided, comprising:

winding the RF cable around each of one or more floating baluns with a tubular shape along the axial direction of the each floating balun so that the RF cable passes through the each floating balun at least twice.

Optionally, the method further comprises: configuring the RF cable wound around the one or more floating baluns as that the distribution of the RF cable on the cross section perpendicular to the axial direction of the one or more floating baluns is

axisymmetric.

Optionally, the method further comprises: if there are two or more floating baluns, arranging at least two of these floating baluns in parallel to form a balun array.

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

DESCRIPTION OF THE FIGURES

In the following drawings:

Figure la illustrates a schematic stereogram of a conventional transmission line using a stripline balun;

Figure lb illustrates a schematic stereogram of a conventional transmission line using a bazooka balun;

Figure 2 illustrates a schematic stereogram of an example transmission line according to an embodiment of the present disclosure;

Figure 3 illustrates an example axisymmetrical distribution of a RF cable on the cross section perpendicular to the axial direction of its corresponding balun.

Figure 4 illustrates a schematic stereogram of another example transmission line according to an embodiment of the present disclosure; and

Figure 5 illustrates a schematic stereogram of another example transmission line according to an embodiment of the present disclosure ₀ DETAILED DESCRIPTION

Figure 2 illustrates a schematic stereogram of an example transmission line 200 according to an embodiment of the present disclosure. The transmission line 200 comprises one or more floating baluns 202 which have a tubular shape; and a RF cable 201 comprising one or more coaxial cables, which is wound around each of the one or more floating baluns along the axial direction of the each balun and passed through the each balun at least twice.

In figure 2, only one floating balun is shown and the RF cable comprises for example four coaxial cables. In this example, the floating balun is a stripline balun which is the same as the balun in figure la and the transmission line 200 is made by winding the RF cable around the floating balun along its axial direction so that the RF cable passes through the floating balun three times. As shown, the transmission line is configured with a shape of the Arabic number '8'.

Compared to the conventional transmission line shown in la, the performance of removing the shield current along the RF cable by using the transmission line 200 is greatly improved, because the length of the RF cable in the hollow space of the balun is significantly increased. The measured S21 at 1.5T is about -25dB.

It is noted that the shape of the Arabic number '8' is just a preferred option, in that this could achieve a highly confined magnetic field, which is very advantageous in practice. Also, other shapes are contemplated. It should be understand that the RF cable could be wound around the floating balun more times for a better performance.

Preferably, the RF cable is wound around the floating balun as that the distribution of the RF cable on the cross section perpendicular to the axial direction of the balun (i.e. the distribution of the projection of the RF cable on the cross section perpendicular to the axial direction of the balun) is axisymmetric(see figure 3). The reason for this is that such a distribution contributes to a highly confined magnetic field, thereby the floating balun will be RF invisible or magnetic transparent to the MRI system including the balun.

Figure 3 illustrates an example axisymmetrical distribution of a RF cable on the cross section perpendicular to the axial direction of a floating balun 302, in which the RF cable is wound around the floating balun along its axial direction and passed through the floating balun six times. As can be seen, the left parts 301a, 301b, 301c and the right parts 301d, 301e, 301f of the RF cable are axisymmetric.

Figure 4 illustrates a schematic stereogram of another example transmission line 400 according to an embodiment of the present disclosure. The transmission line 400 comprises one or more floating baluns 402 which have a tubular shape; and a RF cable 401 comprising one or more coaxial cables, which is wound around each of the one or more floating baluns along the axial direction of the each balun and passed through the each balun at least twice.

In figure 4, only one floating balun is shown and the RF cable comprises for example four coaxial cables. In this example, the floating balun is a bazooka balun.

Similar to the transmission line of figure 2, the performance of removing the shield current by using such a transmission line could be greatly improved, but these two transmission lines may be used in different scenarios according to their shapes.

As an example, when the floating balun has an outside diameter(OD) of 2.4cm, an inside diameter(ID) of 1.0cm and a length of 1.5cm, and is wound and passed through by the RF cable three times, the measured S21 at 1.5T could reach -23dB. As another example, when the floating balun has an outside diameter(OD) of 3cm, an inside diameter(ID) of 1.5cm and a length of 1.5cm, and is wound and passed through by the RF cable six times, the measured S21 at 1.5T could reach -35dB.

It is noted that, the selection of the size of the floating balun is not restrictive, but dependent on the need for housing the RF cable. Preferably, the RF cable is wound around the floating balun such that the distribution of the RF cable on the cross section perpendicular to the axial direction of the balun is axisymmetric, because such a distribution contributes to a highly confined magnetic filed, thereby the balun will be RF invisible or magnetic transparent to the MRI system including the balun. Moreover, the number of times the RF cable passed through the balun is also optional.

Figure 5 illustrates a schematic stereogram of another example transmission line 500 according to an embodiment of the present disclosure. The transmission line 500 comprises two or more floating baluns which are arranged in parallel to form a floating balun array; and a RF cable 501 comprising one or more coaxial cables which is wound around each of the two or more floating baluns along the axial direction of the each balun and passed through the each balun at least twice.

In this example, the transmission line 500 comprises two floating baluns 502,503, both of which are the same as the balun in figure la, and the RF cable 501 comprises four coaxial cables. These two floating baluns 502, 503 are arranged in parallel to form a floating balun array, and the RF cable 501 is wound around the floating balun array along the inner walls of the two floating baluns. As shown, the floating balun array has a first end and a second end, wherein the RF cable 501 firstly passes through the floating balun 502 from the first end to the second end according to a first pattern and secondly passes through the floating balun 503 from the second end to the first end according to a second pattern, and passes through the floating baluns 502 and 503 according to the first and second patterns once more.

In this case, the measured S21 at 1.5T is -20dB. Apparently, this performance is slightly lower than the performance of the transmission line in figure 2. Namely, the coupling between the two floating baluns degrades the performance of removing the shield current. But, due to the presence of two baluns, this approach also brings obviously advantageous effect, i.e. it could result in a wider bandwidth, which makes the resonant frequency of the floating balun array not to be accurately limited in 63.86MHz.

It is noted that these two stripline baluns are also exemplary and not restrictive. For example, one or both of the floating baluns may be bazooka baluns. Similarly, a balun array with three or more floating baluns is also feasible. Furthermore, winding the RF cable along the inner walls of the baluns is to make more parts of the RF cable locate in the hollow space of the baluns, thereby improving the performance. Other ways of winding the RF cable are also contemplated.

Although the present disclosure has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present disclosure is limited only by the accompanying claims. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate. In addition, in the claims, the word "comprise" or "include" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way.