WO2010023586A2

WO2010023586A2 - Rf antenna arrangements for a mit apparatus

Info

Publication number: WO2010023586A2
Application number: PCT/IB2009/053589
Authority: WO
Inventors: Peter Vernickel; Peter Mazurkewitz; Christoph Leussler; Christian Findeklee; Daniel Wirtz; Ulrich Katscher
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2008-08-26
Filing date: 2009-08-14
Publication date: 2010-03-04
Also published as: WO2010023586A3

Abstract

Several RF antenna arrangements are disclosed, particularly for use as RF transmitter and/or receiver antenna arrangements in a MIT apparatus, with which an increased signal-to-noise ratio, an improved MIT imaging performance, an increased spatial and temporal resolution of the generated images, a reduced acquisition time and greater measurement reliability can be obtained.

Description

DESCRIPTION

RF ANTENNA ARRANGEMENTS FOR A MIT APPARATUS

FIELD OF THE INVENTION

The invention relates to RF antenna arrangements, particularly for use as RF transmitter and/or receiver antenna arrangements in a MIT apparatus.

BACKGROUND OF THE INVENTION

Magnetic induction tomography is a non-invasive imaging technique with applications in industry and medicine. MIT is based on couplings between RF transmitters and RF receivers placed around the object of interest to be imaged. More in detail, a time- varying (primary) electromagnetic field is applied by means of RF transmitter antenna arrangements (or generator or excitation coils) to the object to be imaged. Due to at least one of the three passive electromagnetic properties of the material to be imaged, namely, electric conductivity, permittivity and magnetic permeability, the primary electromagnetic field is disturbed, so that a secondary electromagnetic field results which is detected by RF receiver antenna arrangements (or measurement or detection coils) for generating an image of the object under examination.

MIT bears the potential of being a comparably inexpensive and fast imaging technique for stroke detection, validation and therapy monitoring.

SUMMARY OF THE INVENTION

For examining human tissue, the primary electromagnetic field is applied with a radio frequency (RF) of the order of between about 100 kHz and about 10 MHz. For transmitting the primary electromagnetic field and for receiving the secondary electromagnetic field, RF antenna arrangements or coils with different shapes are known. One object of the invention is to provide an RF antenna arrangement particularly for a MIT apparatus, such that an increased signal-to-noise ratio or an improved MIT imaging performance can be obtained.

Another object of the invention is to provide an RF antenna arrangement particularly for a

MIT apparatus, such that an increased spatial or temporal resolution of the generated images, a reduced acquisition time or greater measurement reliability can be obtained.

As defined in claim 1, the object is solved by an RF antenna arrangement for transmitting and/or receiving RF signals onto/from an object to be imaged in a MIT apparatus, the arrangement comprising at least a first and a second RF antenna element or coil for generating and/or receiving at least a first and a second RF electromagnetic field, respectively, which are linearly independent of each other with respect to at least one of their orientation, alignment, shape and field pattern profile.

The dependent claims disclose advantageous embodiments of the invention.

Claim 2 defines several RF antenna elements or coils which can be advantageously used for constituting the RF antenna arrangement according to the invention.

Claims 3 to 11 define preferred embodiments of the RF antenna elements or coils as defined in claim 2.

Claim 12 defines a preferred embodiment of the conductors of the RF antenna elements or coils with which an improved signal-to-noise ratio can be obtained.

Claims 13 and 14 define preferred shapes or forms of the RF antenna elements or coils so that they can be positioned very close to an object of interest.

It will be appreciated that features of the invention can be realized in any combination without departing from the scope of the invention as defined by the appended claims. Further details, features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, which are given by way of example and with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figs. l(A) and l(B) are schematic views of an RF antenna element according to a first embodiment of the invention in the form of a dipole antenna;

Figs. 2(A) and 2(B) are schematic views of an RF antenna element according to a second embodiment of the invention in the form of a TEM element;

Fig. 3(A) is a schematic view of an RF antenna element according to a third embodiment of the invention in the form of a combined loop antenna / TEM element; Fig. 3(B) shows a schematic distribution of a magnetic field generated by a loop antenna mode of the RF antenna element shown in Figure 3(A); Fig. 3(C) shows a schematic distribution of a magnetic field generated by a TEM mode of the

RF antenna element shown in Figure 3(A);

Fig. 4 is a schematic view of an RF antenna element shown in Fig. 3(A) in a first variant; Fig. 5 is a schematic view of an RF antenna element shown in Fig. 3(A) in a second variant; Fig. 6 is a schematic view of an RF antenna element according to a fourth embodiment of the invention in the form of a shielded loop antenna; Fig. 7 shows a diagram of the resistances of different conductors for the RF antenna elements in dependence on the RF frequency;

Fig. 8 is a schematic view of an RF antenna element according to a fifth embodiment of the invention in the form of a composite RF antenna element;

Fig. 9 is a schematic view of a first sub-coil of the RF antenna element shown in Figure 8; Fig. 10 is a schematic view of a second sub-coil of the RF antenna element shown in Figure

8;

Fig. 11 is a schematic view of a third sub-coil of the RF antenna element shown in Figure 8; and

Fig. 12 is a schematic view of the combination of the RF antenna element shown in Figure

8. DESCRIPTION OF EMBODIMENTS

Generally, the above-mentioned objects are achieved by at least one of two main aspects of the invention, namely, by the selection of the RF antenna elements or coils, which constitute the RF antenna arrangement, with respect to their type or shape or alignment or orientation, and by the type or form of the conductors by means of which the RF antenna elements or coils are formed.

According to the first aspect, said selection of the RF antenna elements or coils allows at least two and particularly a plurality of electromagnetic fields to be generated, which are linearly independent of each other particularly due to their different shapes or pattern profiles or orientations or alignments, so that a plurality of such RF antenna elements or coils can be operated in parallel (which are distributed along or around the object of interest), and accordingly more independent measurements can be made in order to increase the spatial and temporal resolution of the generated images.

According to the second aspect, use of litz wires instead of solid conductors or rods or conducting strips for forming the RF antenna elements or coils reduces the noise caused by resistive components in the case of solid conductors, thus improving the signal-to-noise ratio of the RF antenna arrangement. This also reduces the acquisition time during the MIT image generation (because fewer average steps are necessary) and/or improves the reliability of the MIT measurements.

The above first aspect is realized by providing different shapes, types and/or alignments of the

RF antenna elements or coils in the form of e.g. dipole antennas, TEM (Transversal Electromagnetic ) resonators and combinations of at least one TEM resonator with at least one loop antenna element, as well as by combinations of these RF antenna elements with known RF antenna elements, and/or by providing quadrature coil arrangements, particularly in an overlapping or overlying arrangement. The above shapes, types and/or alignments of the

RF antenna elements or coils can be provided in a three-dimensionally bent form or in a plain or flat (i.e. two-dimensional) form. Providing an RF antenna arrangement either with a dipole antenna element in combination with a TEM resonator and/or the above combined RF antenna element in the form of a TEM resonator with a loop antenna element and/or the above overlapping or overlying loop antenna arrangement results in the advantage that the RF antenna elements or coils are inherently decoupled (particularly if they are symmetrically designed) so that one RF antenna element or coil is prevented from exciting a current in the other RF antenna element or coil to a considerable extent. This prevents RF antenna elements or coils from generating an electromagnetic field when driving one of the RF antenna elements or coils. Particularly in the case of strong coupling, these two fields would be linearly dependent or at least very similar, so that particularly the spatial and temporal resolution of the generated images would deteriorate considerably.

Also the placement of the RF antenna elements or coils around an object to be imaged is usually crucial for obtaining a variety of linearly independent fields. The RF antenna elements or coils according to the invention have the advantage that they can be placed with an arbitrary direction of their conductor elements and with no preferred direction in relation to the object, because linearly independent (orthogonal) field components can be generated by means of the above RF antenna elements or coils.

Figures l(A) and l(B) show, by way of example, a first embodiment of an RF antenna element in the form of a dipole antenna element 1 which is positioned in front of an RF screen 2.

More in detail, Figure l(A) is a side view of this dipole antenna element 1 and the RF screen

2 together with an object of interest OI which is to be imaged. Figure l(B) is a three- dimensional view onto such an RF antenna element in which the dipole antenna element 1 and the RF screen 2 are again indicated. The RF screen 2 is preferably a cylindrical screen which partly or fully encloses the object of interest OL

Generally, one or more of these dipole antenna elements 1 can be mounted with an arbitrary direction of their conductor elements at the inner side (i.e. facing the object OI) of a preferably cylindrical RF screen 2. If no couplings of the dipole element to other external RF fields are to be expected, it is also possible to use a cylindrical RF screen 2 which is only partly closed, or to omit the RF screen 2.

As can be seen in Figure l(A), the dipole antenna element 1 is mounted with its center at the

RF screen 2 and is connected with an RF chain CH for operating the dipole antenna element 1 for transmitting and/or receiving RF signals.

Additionally, lumped elements such as a capacitor can be provided between the two conductors of the RF dipole element 1 in order to tune or match the RF dipole element 1 appropriately to the impedance of the input/output of the RF chain CH. Furthermore, lumped or distributed reactance elements can be provided at the RF dipole element 1 in order to form baluns or to compensate for mutual couplings.

It is also possible to realize only one half of the RF dipole element, resulting in a monopole

(λ/4) RF antenna element.

Figures 2(A) and 2(B) show, by way of example, a second embodiment of an RF antenna element in the form of a TEM element 3, 4. Again, Figure 2(A) is a side view of the TEM element 3, 4 together with an object of interest OI to be imaged. Figure 2(B) is a three- dimensional view onto the TEM element 3, 4.

More in detail, the TEM element comprises a conductor element 3 and an RF screen 4 in front of which the conductor element 3 is mounted. The RF screen 4 is again preferably a cylindrical screen which, in accordance with the explanations above, either partly or fully encloses the object of interest OL However, in this embodiment, the RF screen 4 is to be considered as a part of the TEM element 3, 4 so that it cannot usually be omitted.

One or more of such conductor elements 3 can generally be mounted with an arbitrary direction at the inner side (i.e. facing the object OI) of a preferably cylindrical RF screen 4.

Each conductor element 3 is coupled at an arbitrary position, preferably at one of its ends, to the input/output of an RF chain CH for operating the TEM antenna element 3, 4 for transmitting and/or receiving RF signals.

Lumped elements such as capacitors can be provided between each of the two ends of the conductor element 3 and the RF screen 4 in order to tune or match the TEM element 3, 4 appropriately to the impedance of the input/output of the RF chain CH.

Due to the fact that the RF antenna elements shown in Figures 1 and 2 generate/receive different shapes or patterns of electromagnetic fields (which are linearly independent of each other), they can be advantageously positioned in combination around an object to be imaged.

Figure 3(A) shows a third embodiment of an RF antenna element in the form of a combination of a loop antenna element with a TEM antenna element. This combined RF antenna element comprises a conductor loop 5 and a preferably cylindrical RF screen 6, wherein the conductor loop 5 is again mounted at the inner side of the RF screen 6 (i.e. facing the object OI) which partly or fully encloses the object of interest OI in accordance with the explanations above.

The loop antenna element is provided by the conductor loop 5, into which the output/input of a first RF chain (schematically indicated by a first voltage source Vl) is connected, so that the RF signals to be transmitted and/or received, respectively, travel around the (circumference of the) conductor loop 5, i.e. in the case of Figure 3(A) in opposite directions in each of the two oppositely positioned parallel branches of the loop 5.

The TEM antenna element is provided by the conductor loop 5 which is connected at one side with one terminal of the output/input of a second RF chain (schematically indicated by a second voltage source V2), while the other terminal of the output/input of the second RF chain is connected with the RF screen 6. Furthermore, the opposite other side of the conductor loop 5 is provided with two conductive connections 51, 52 to the RF screen 6, so that the RF signals to be transmitted and/or received, respectively, travel from one terminal of the output/input of the second RF chain via the conductor loop 5 (i.e. in a parallel direction in both opposite branches of the loop 5) to the RF screen 6 and in the RF screen 6 back to the other terminal of the output/input of the RF chain. According to the embodiment shown by way of example in Figure 3(A), the conductor loop 5 has an at least substantially rectangular shape comprising a first and a second branch as well as a third and a fourth branch which are arranged opposite and parallel to each other. The first and the second branch each have a length 1 which exceeds the length w of both the third and the fourth branch.

According to Figure 3(A), the first voltage source Vl is connected into the third branch, both ends of which are connected with the RF screen 6. The second voltage source V2 is connected between the opposite fourth branch and the RF screen 6.

Such a combined RF antenna element, which is provided by the combination of a loop antenna element with a TEM antenna element, can be used for the transmission and/or reception of two linearly independent field patterns (also termed two "modes").

Again, one or more of these combined RF antenna elements can be mounted with an arbitrary direction of the conductor elements at the inner side of a preferably cylindrical RF screen 6.

This combined RF antenna element has been realized as an example for a human body (chest) sized object of interest with a length 1 of 40 cm of the first and the second branch, a length w of 10 cm of the third and the fourth branch, a distance d of 3.5 cm of the conductor loop 5 from the RF screen 6 and a diameter of 680 cm of the cylindrical RF screen 6.

Figures 3(B) and 3(C) show the resulting shapes of the magnetic fields Bi in a central transversal plane (z = 0) of the conductor loop 5, perpendicularly to the first and the second branch of the conductor loop.

Figure 3(B) shows the magnetic field which is generated by the RF loop antenna element, in which the currents flow around the conductor loop 5. Figure 3(C) shows the magnetic field which is generated by the TEM antenna element, in which the currents flow in the same direction in both the first and the second branch of the conductor loop 5. Due to reciprocity, Figures 3(B) and 3(C) also show the resulting sensitivity of the RF loop antenna element and the TEM antenna element, respectively, to magnetic fields in the case of reception of RF signals if inputs of the related RF chains are connected at the positions indicated in Figure 3(A) by the first and the second voltage source Vl, V2, respectively.

It is to be noted that the conductor loop 5 must not necessarily have a rectangular shape as shown in Figure 3(A). Figure 4 shows, by way of example, a triangular shape of a conductor loop 5, and Figure 5 shows, also by way of example, an oval or elliptic shape of a conductor loop 5, both comprising the first and the second voltage source Vl, V2 and the conductive connections 51, 52 to the RF screen 6 (which is not indicated) as explained above with reference to Figure 3(A). Other shapes and forms can be realized as well.

Figure 6 shows an RF antenna element according to a fourth embodiment of the invention in the form of a shielded RF loop antenna. It comprises a conductor loop 7 which is positioned at a distance d from an RF screen 8. In an area enclosed by the conductor loop 7, the RF screen 8 comprises a cut-out 81 in the form of an opening so that an open RF screen 8 is provided.

Furthermore, Figure 6 schematically shows an input/output V of an RF chain which is connected into the conductor loop 7 in order to generate an RF current within the loop which is to be transmitted as an RF signal, and/or to receive an RF signal in the form of an RF current which is induced within the conductor loop 7.

Such an RF current flowing around the conductor loop 7 induces a current in the RF screen 8 in a counter-rotating direction. Electromagnetic fields which are present at the reverse side of the RF screen 8 (i.e. at the side opposite to that at which the conductor loop 7 is mounted) are thus attenuated. The same applies accordingly if the RF antenna element is used for receiving RF signals, so that external fields which are present at the reverse side of the RF screen 8 are attenuated accordingly as well.

Such a loop antenna with an open RF screen can be realized not only in a rectangular shape as shown in Figure 6, but also in other shapes such as circular, elliptic, oval, or triangular or other arbitrary closed shapes of the conductor loop 7 and/or the open area of the RF screen 8 enclosed by the conductor loop 7. Furthermore, the open RF screen 8 can also be bent into a three-dimensional form as shown in Figures l(B) and 2(B) and explained in the related description above.

Apart form the advantages above, such an open antenna design provides better accessibility of an object, particularly a patient to be imaged, and it offers improved comfort for a patient.

As mentioned above in connection with the second main aspect of the invention, the electromagnetic properties of the above RF antenna elements, particularly with respect to their signal-to-noise ratio can be improved by using litz wires instead of solid conductors for constituting the conductors of RF antenna elements or coils. The diagram of Figure 7 shows the resistances of several conductors formed by a plurality of litz wires in comparison with a conductor formed of one solid wire, in dependence on the frequency of the current flowing within the conductors.

In Figure 7, all examples of conductors have a total diameter of 1 mm, and the conductors formed by a plurality of litz wires have a filling factor of 0.5. Curve (a) shows the resistance of a conductor formed by litz wires with a diameter of 50 μm each; curve (b) shows the resistance of a conductor formed by litz wires with a diameter of 30 μm each; curve (c) shows the resistance of a conductor formed by litz wires with a diameter of 20 μm each; curve (d) shows the resistance of a conductor formed by litz wires with a diameter of 15 mm each; and curve (e) shows the resistance of a conductor formed by one solid wire with a diameter of 1 mm.

This diagram shows that, up to a frequency of about 10⁷ Hz, litz wires have a smaller resistance in comparison with that of a conductor with one solid wire. Furthermore, Figure 7 shows that the smaller the diameter of the litz wire, the smaller the resistance of the related conductor at a certain frequency.

Another possibility of improving the electromagnetic properties of the RF antenna elements or coils according to the invention is to place these elements as close as possible to the object to be imaged. In order to optimize this, the RF antenna elements or coils can be adapted to the shape of the object by forming or bending the RF screens as described above and/or by bending the conductors of the RF antenna elements accordingly. An RF antenna arrangement comprising one or more of such RF antenna elements or coils can be realized in dependence on the proposed use of the RF antenna arrangement for different types of objects, e.g. in the form of a head coil, a neck coil, a combined head-neck coil, a heart coil, a lung coil, or other types of surface coils with different shapes and dimensions for fitting to the body region of interest. The RF antenna elements or coils are preferably made from flexible materials in order to ensure an optimal adaptation of their shape or form to a shape or form of the surface of an object of interest.

The RF antenna elements or coils described above with reference to Figures 1, 2, 3(A), 4, 5 and 6 are usually connected for operation with their own RF chain each. When a large number of such RF antenna elements or coils around an object to be imaged are used, the required number of RF chains (i.e. RF transmitter and/or receiver channels) increases accordingly.

In order to limit this increase and to improve flexibility, Figure 8 shows a fifth embodiment. It shows an example of an RF antenna element which comprises at least two but preferably three (or even more) overlying or overlapping loop or coil antenna elements (hereinafter referred to as "sub-coils") with different shapes or alignments which are operated via a common input/output I/O to be connected with an RF chain. The RF antenna element shown in Figure 8 comprises three sub-coils which are separately shown in Figures 9 to 11.

A related RF screen on which these coils can be mounted is not indicated in Figures 8 to 11.

Figure 9 shows a first sub-coil 9 having the shape of a circle for generating and/or receiving an electromagnetic field in the z-direction along the z-axis (wherein the origin of the coordinates x, y, z is positioned in the center of the RF antenna element).

A first terminal (a) of this first sub-coil 9 is connected with a first terminal of an input/output

VO of a related RF chain, and a second terminal (b) of the first sub-coil 9 is connected with a first terminal (c) of a second sub-coil 10 shown in Figure 10. This second sub-coil 10 is provided for generating and/or receiving electromagnetic fields in the x-direction. Its first terminal (c) is connected with a first part 101 which extends along the diameter of the second sub-coil 10, a second part 102 which describes a first half circle to a first side, a third part 103 which again extends along the diameter of the second sub-coil 10, parallel to the first part 101, and a fourth part 104 which describes a second half circle to a second side, opposite in relation to the first half circle of the second part 102, where the second sub-coil ends 10 with its second terminal (d).

The second terminal (d) is connected with a first terminal (e) of a third sub-coil 11 shown in

Figure 11, preferably via a conductor (not indicated) which is selected in such a way that, unlike a coaxial cable, it does not generate any substantial electromagnetic field.

The third sub-coil 11 is provided for generating and/or receiving electromagnetic fields in the y-direction. Its first terminal (e) is connected with a first part 111 which extends along the diameter of the third sub-coil 11, turned 90° in relation to the first and the third part 101, 103 of the second sub-coil 10. A subsequent second part 112 of the third sub-coil 11 describes a first half circle to a first side. Then a third part 113 follows, which again extends along the diameter of the third sub-coil 11, parallel to the first part 111. Finally, a fourth part 114 extends along a second half circle to a second side, opposite in relation to the first half circle of the second part 112, where the third sub-coil 11 ends with its second terminal (f).

This second terminal (f) of the third sub-coil is connected with the second terminal of the input/output VO of the RF chain, preferably via another conductor (not indicated) which is selected in such a way that, unlike a coaxial cable, it does not generate any substantial electromagnetic field.

Alternatively, the three sub-coils 9, 10, 11 described above can be connected with their own RF chain each in order to allow independent control of combinations of the three electromagnetic field patterns which are generated/received by the three sub-coils for transmitting and/or receiving RF signals, respectively. Another alternative is shown in Figure 12, in which a first, a second and a third RF switch SwI, Sw2, Sw3 are connected in parallel with each first, second and third sub-coil 9, 10, 11, respectively, (i.e. between each first and second terminal of the first to third sub-coil). By independently closing one or two of these switches SwI, Sw2, Sw3, each one or two of the three sub-coils 9, 10, 11 can be short-circuited or bypassed so that no RF field is generated or received by the related sub-coil(s).

Each sub-coil 9, 10, 11 can thus be used alone (for example, all of the three single sub-coils one after the other), or two of the three sub-coils can be used in different combinations, or all of the three sub-coils can be activated.

This embodiment has the advantage that independent MIT measurements can be made with an increased flexibility by choosing suitable field patterns by simply switching the above switches SwI, Sw2, Sw3 as desired.

While the RF antenna element shown in Figures 8 to 11 has been described for loop antenna elements, the overlapping placement of sub-coils can be principally realized with other RF antenna elements or coils as well, particularly those which do not have a circular shape but an elliptic, oval, rectangular, triangular or another closed loop shape or form.

Finally, this RF antenna element can be provided in such a way that it can be bent in a three- dimensional shape in order to better match the outer shape or form of an object to be imaged as described above.

While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustrations and the description are to be considered illustrative or as examples and are not limiting; the invention is not limited to the disclosed embodiments. Variations of embodiments of the invention described in the foregoing are possible without departing from the scope of the invention as defined by the appended claims.

Variations of the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, use of the verb "comprise" and its conjugations does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

Claims:

1. An RF antenna arrangement for transmitting and/or receiving RF signals onto/from an object to be imaged in a MIT apparatus, said arrangement comprising at least a first and a second RF antenna element or coil (1; 3, 4; 5, 6; 7, 8; 9, 10, 11) for generating and/or receiving at least a first and a second RF electromagnetic field, respectively, which are linearly independent of each other with respect to at least one of their orientation, alignment, shape and field pattern profile.

2. An RF antenna arrangement according to claim 1, wherein the RF antenna elements or coils are provided in the form of at least one of an RF monopole or dipole element (1), a TEM antenna element (3, 4), a loop or coil antenna element (7, 8; 9, 10, 11) and a combination of a loop antenna element with a TEM antenna element (5, 6).

3. An RF antenna arrangement according to claim 2, wherein the RF monopole dipole or element (1) is mounted on an RF screen (2).

4. An RF antenna arrangement according to claim 2, wherein the TEM antenna element (3, 4) comprises a conductor element (3) which is positioned on an RF screen (4).

5. An RF antenna arrangement according to claim 2, wherein the combination of a loop antenna element with a TEM antenna element (5, 6) comprises a conductor loop (5) and an output/input of a first RF chain (Vl) which is connected into the conductor loop (5), for driving and/or receiving a first current flowing around the conductor loop (5).

6. An RF antenna arrangement according to claim 2, wherein the combination of a loop antenna element with a TEM antenna element (5, 6) comprises a conductor loop (5) which is connected at one side with an RF screen (6),

1 and wherein the output/input of a second RF chain (V2) is connected between the opposite, other side of the conductor loop (5) and the RF screen (6), for driving and/or receiving a second current flowing from one terminal of the output/input of the second RF chain (V2) via the conductor loop (5) into the RF screen (6).

7. An RF antenna arrangement according to claim 2, wherein the loop or coil antenna element (7, 8) is provided by a conductor loop (7) and an RF screen (8) at which the conductor loop (7) is positioned with a distance from the RF screen (8), the RF screen (8) comprising an open area in the form of a cut-out which is surrounded by the conductor loop (7).

8. An RF antenna arrangement according to claim 2, wherein the loop or coil antenna element (9, 10, 11) is provided by a combination of at least two overlapping or overlying loop antenna elements or coils for generating and/or receiving RF fields each in a different direction of the Cartesian coordinate system.

9. An RF antenna arrangement according to claim 8, wherein the at least two loop antenna elements or coils (9, 10, 11) are connected in series so that the loop or coil antenna element (9, 10, 11) can be operated by means of one RF chain.

10. An RF antenna arrangement according to claim 8, wherein each switch (SwI, Sw2, Sw3) is connected in parallel with each loop or coil antenna element (9, 10, 11) for short-circuiting the same.

11. An RF antenna arrangement according to claim 2, wherein the loop or coil antenna elements (5; 7; 9, 10, 11) have at least one of a rectangular, a triangular, a circular, an oval, an elliptic or another closed loop shape.

12. An RF antenna arrangement according to claim 2,

2 wherein at least one of the RF antenna elements or coils is formed by conductors made of litz wires.

13. An RF antenna arrangement according to claim 1, wherein the RF antenna elements or coils (1; 3, 4; 5, 6; 7, 8; 9, 10, 11) are manufactured from a flexible material so that they can be bent in a three-dimensional or in a plain or flat (i.e. two-dimensional) shape or form in accordance with a shape or form of the surface of an object of interest.

14. An RF antenna arrangement according to claim 1, which is provided in the form of a head coil, a neck coil, a combined head-neck coil, a heart coil, a lung coil.

15. A MIT apparatus comprising an RF antenna arrangement according to any one of the preceding claims.