NL2012685B1

NL2012685B1 - Waveguide antenna device for generating or receiving a radiofrequency signal within an MRI-bore of an MRI-system, and an MRI-system provided with a waveguide antenna device.

Info

Publication number: NL2012685B1
Application number: NL2012685A
Authority: NL
Inventors: Kroeze Hugo; Jacob Van Der Werf Abe; Wilhelmus Johannes Klomp Dennis; Jan Eberhard Raaijmakers Alexander
Original assignee: Machnet B V
Priority date: 2014-04-24
Filing date: 2014-04-24
Publication date: 2016-07-08
Also published as: NL2012685A

Abstract

The present invention relates to a waveguide antenna device for generating and/or receiving a radiofrequency signal within an MRI-bore of an MRI-system. The waveguide antenna device comprising a waveguide having an axial direction and comprising a dielectric material, the waveguide being configured to direct the radiofrequency signal towards an area of interest in front of the waveguide. The waveguide antenna device further comprises an antenna, which antenna is configured to generate a RF signal within the waveguide. The waveguide and antenna are disposed to emit a single mode radiofrequency signal in the axial direction of the waveguide.

Description

Waveguide antenna device for generating or receiving a radiofrequency signal within an MRI-bore of an MRI-system, and an MRI-system provided with a waveguide antenna device

The present invention relates to a waveguide antenna device for generating or receiving a radiofrequency signal within an MRI-bore of an MRI-system. The present invention further relates to an MRI-system provided with a waveguide antenna device according to the invention.

An MRI-system is a medical imaging device that can depict both anatomical structures and functional information of tissues within the human body. An MRI-system uses a strong magnetic field. As a result, the energy levels of specific atomic nuclei are split. Transitions between these specific energy levels by the nuclei can be induced by RF fields, after which the nuclei will emit RF fields with the same frequency, which RF-fields have to be detected. From the detected signals, anatomical images and other information can be derived, if the magnetic field is spatially encoded with so-called gradient fields.

Since the introduction of the first MRI-systems, the strength of the main magnetic field (Bo) has increased gradually over the years. Currently, regular clinical scanners have a main magnetic field strength of 1.5 or 3.0 Tesla. In research facilities, scanners with 7 Tesla or more are being used. RF components of MRI-systems are known with some varieties. Clinical scanners of 1.5 and 3.0 Tesla can use a body coil to generate a transmit field, which excites the atomic nuclei into their higher energy state. The body coil has the advantage that it can provide a homogeneous transmit field. The same body coil can be used for receiving as well, but due to the larger area of reception, the noise levels are relatively high. To detect the RF signal emitted from the excited nuclei, most often local receiving coils are used.

These coils have a limited area of reception, which is large enough to depict the volume of interest, but to avoid picking up unnecessarily high noise levels. Examples of such coils are simple loop coils, birdcage coils and receiving arrays, comprising multiple array elements, such as loop coils or microstrips .

An alternative approach, used to depict non-hydrogen nuclei or for ultra-high field strengths, is to use local transmitting coils to excite the atomic nuclei. In this case, the function of transmitting and receiving of the local coils may be merged and the coils might act as a transmitter and as a receiver. Local transmit coils may be combined with separated receiving coil arrays.

One single loop coil can only generate linear polarization. Existing approaches to arrive at circular polarization are quadrature fed birdcage coils, orthogonal loop coil pairs or coil arrays. All these approaches achieve circular polarization by current carrying conductors, aligned in the transverse plane, and appropriately sequenced activation of these conductors.

It is an object of the invention to provide a waveguide antenna device for providing an improved radiofrequency signal at deep-lying parts of a human body.

To this end, the waveguide antenna device according to the invention comprises a waveguide having an axial direction and comprising a dielectric material, the waveguide being configured to direct the radiofrequency signal towards an area of interest in front of the waveguide. The waveguide antenna device further comprising an antenna, which antenna is configured to generate a RF signal within the waveguide. The waveguide and antenna are disposed to emit a single mode radiofrequency signal in the axial direction of the waveguide.

Deep-lying parts or organs of a human body, such as a part of a brain or a prostate, are difficult to depict on MRI-images due to their position. The RF signals emitted by the known RF components of MRI-systems generate an electromagnetic field, also called a Bi-field, which is not sufficiently strong for generating images with much detail. An advantage of the waveguide antenna device according to the invention is the possibility to direct the RF signal in the axial direction of the waveguide, thereby generating an improved Bi-field within the area of interest near a distal end of the waveguide, i.e. in front of the waveguide. The waveguide antenna device can be placed against, e.g., the cranial end of a head or the perineum of a person. In this way, the RF signal is directed in the axial direction to a deep-lying part of the head or the perineum, thereby generating an improved Βι-field in the area of interest. The improved Bi-field may lead to improved images of the area of interest.

It is noted that directing a RF signal in axial direction to an area of interest in front of the waveguide, which is against or near a distal end of the waveguide, is also called ‘end-firing'.

It may further be possible to use the waveguide antenna device in an endocavity of a body of a person. The RF signal generated by the antenna has a certain frequency, which may relate to the field strength of the used MRI-system, for example 1.5T, 3T or 7T. For a fixed frequency, the wavelength of the RF signal changes in dependency of electromagnetic properties of a medium, in particular the relative dielectric permittivity thereof, through which the RF signal travels. In case of a medium with a high relative dielectric permittivity, the wavelength of the RF signal within the waveguide will be smaller than is the case by a medium with a low relative dielectric permittivity. When the waveguide is a circular waveguide, the diameter, in particular the minimal diameter, of the waveguide of the waveguide antenna device may depend on the relative dielectric permittivity of dielectric material and/or the frequency of the RF signal. Therefore, due to the dielectric material the dimensions of a waveguide antenna device used for, e.g. endocavity purposes, may be adjusted depending on the endocavity the antenna is to be inserted. It is therefore possible to make the waveguide antenna device small enough to be used in an endocavity, in order to minimize the inconvenience for the person being scanned. In case the waveguide is positioned against the perineum of a person, it is easier to position the antenna when it has smaller dimensions .

It may further be possible to move the waveguide antenna away from or towards a person lying in an MRI-bore of an MRI-system without moving the person, for instance when the waveguide antenna device is placed against a cranial end of the head of a person. The person, therefore, may maintain a same position during scanning and/or positioning the waveguide antenna device. In case the waveguide antenna device is placed at the cranial end of the head of the person, a field of view of the person may remain free, at least during scanning. A free field of view during scanning may be advantageous for instance for fMRI and/or interventional MRI.

The waveguide and the antenna may be configured to generate a circular polarized radiofrequency signal. In relation to the physics of MRI, the required RF field for reliable excitation of the atomic nuclei is circular polarized, with the axis of polarization along the Bo field. By providing a waveguide antenna device with a waveguide comprising a dielectric material, which waveguide antenna device is able to emit a circular polarized RF signal, the sizes, e.g. a diameter of the waveguide, of the waveguide antenna device can be reduced and reliable excitation of the atomic nuclei still may be achieved. Since the presently provided waveguide antenna device is useable in a single mode, in this case for end-firing a circular polarized RF signal in front of the waveguide, it is desired that a RF signal with the lowest desired frequency fits into the waveguide without damping. This lowest frequency is called the cut-off frequency. By increasing the relative dielectric permittivity of the dielectric material, the sizes of the waveguide may decrease, and the waveguide antenna device still may be able to emit a circular polarized field.

The antenna may be attached to the proximal end of the waveguide and/or may extend into the waveguide, wherein the antenna is disposed to emit a signal into the waveguide. Since a signal generated by the antenna is emitted into the waveguide, signal loss due to, e.g., air is prevented.

Further, by emitting the signal directly into the waveguide, substantially the whole signal may be directed to the area of interest. As a result, the Bi-field within the area of interest is improved.

It is preferred that the antenna in radial direction is equal to or smaller than the sizes of the waveguide in radial direction with respect to the axial direction.

In a preferred embodiment, the waveguide comprises two or more parts, wherein each of the parts comprises a dielectric material. In a more specific embodiment, the parts are concentrically positioned with respect to each other. It might be possible that the relative dielectric permittivity of the parts comprising a dielectric material differs, for example by producing the parts from different materials.

In case a helical antenna is used, this antenna may extend into the waveguide and/or may be positioned between the two parts, in particular forming a concentric, cylindrical geometry. The helical antenna then surrounds a first one of the parts, and the second one of the parts surrounds the helical antenna. As a result, the properties of the Bi-field can be influenced, such that, e.g., substantially only a Bi+-field is generated by the waveguide antenna device. In this case the Bi+-field may have a higher strength in comparison to when a Bi+-field and a Bit-field are generated.

The waveguide, or, in case of a waveguide antenna device comprising two or more parts, at least one of the parts may be solid. Due to the waveguide being at least partly solid, it is possible to reduce the sizes, at least the sizes in radial direction with respect to the axial direction of the waveguide. Further, when the waveguide is at least partly solid, the generated RF signal may be better coupled into a part of the human body in contact with a distal end of the waveguide of the waveguide antenna device.

In particular when the relative dielectric permittivity of the dielectric material of the waveguide and the relative dielectric permittivity of the area of interest are substantially the same or within a predetermined range, the RF signal may be coupled from the waveguide antenna device into the area of interest with minimal losses. Differences of relative dielectric permittivity may cause reflections of the RF signal at the contact surface of the waveguide and the human tissue. A reflector may be provided at a proximal end of the waveguide, opposite to a distal end in front of which the RF signal is emitted. The reflector prevents that a part of the RF-signal leaves the wave waveguide antenna device at the proximal end thereof, e.g. due to internal reflections within the waveguide or any other causes. As a result, the RF signal within the area of interest may be improved.

In a preferred embodiment, a shielding may be provided around at least part of the waveguide. The shielding may be attached to the waveguide and it may extend from the proximal end to the distal end, or from the proximal end to a certain distance from the distal end. The shielding prevents the RF signal from leaving the waveguide at a side thereof, such that substantially no losses of RF signal occur at the side of the waveguide and the RF signal may be directed to the area of interest.

The shielding may comprise a metal foil, and/or the shielding may be attached to the waveguide. A metal foil is sufficient to prevent the RF-signal from leaving the waveguide at a side thereof. Further, a metal foil is usually thin, or even very thin, and, therefore, the sizes of the waveguide stays substantially the same, in particular in radial direction of the waveguide with respect to the axial direction thereof, even with the metal foil attached to the waveguide.

The dielectric material may have a relative dielectric permittivity of 10, 15, 30, 35, 50, or more. Such relative dielectric permittivities may also be found in parts of the human body. For example, the head of a person has a relative dielectric permittivity of approximately 50 and the pelvis of a person has an relative dielectric permittivity of approximately 36. By choosing a dielectric material with approximately the same relative dielectric permittivity, reflections of the RF signal when the RF signal leaves the waveguide antenna device and enters a human body may be prevented or at least reduced. This is called dielectric matching.

The dielectric material may be selected from a group comprising: oils, oil mixtures, alcohol, water, heavy water, ethylene glycol, ceramic, and/or a combination thereof. It is noted that by mixing materials with different relative dielectric permittivities, it is possible to obtain a mixture with a desired relative dielectric permittivity.

The antenna may be selected from a group comprising: a helix antenna, a patch antenna, a dipole antenna, a stub antenna and/or a combination thereof. The waveguide may be rectangular-shaped, circular-shaped or elliptical shaped.

The waveguide antenna device may be coated, such that the waveguide antenna device might be used internally in a human body. Further, an evanescent field around the waveguide antenna device may be reduced such that the amount of disorders within the eventually obtained images may be reduced.

Another aspect of the invention relates to an MRI-system provided with a waveguide antenna device according to the invention.

Hereinafter, exemplary embodiments of the invention will be described in further detail. It should be appreciated, however, that these embodiments may not be construed as limiting the scope of protection for the present invention.

Aspects of the invention will be explained in greater detail by reference to exemplary embodiments of the invention shown in the drawings, in which:

Fig. 1 illustrates a schematic overview of a first embodiment of the waveguide antenna device;

Fig. 2 illustrates a schematic overview of a second embodiment of the waveguide antenna device;

Figs. 3-4 illustrate simulation results of a simulation with the first embodiment of the waveguide antenna device;

Figs. 5-6 illustrate simulation results of a simulation with the second embodiment of the waveguide antenna device;

Figs. 7-8 illustrate simulation results of a simulation with the third embodiment of the waveguide antenna device;

Fig. 9 illustrates a schematic overview of a fourth embodiment of the waveguide antenna device;

Fig. 10 illustrates a schematic overview of a fourth embodiment of the waveguide antenna device;

Fig. 11 illustrates a schematic overview of a fifth embodiment of the waveguide antenna device;

Fig. 12 illustrates a schematic overview of a sixth embodiment of the waveguide antenna device;

Figs. 13-14 illustrate simulation results of a simulation with the fourth embodiment of the waveguide antenna device;

Figs. 15-16 illustrate simulation results of a simulation with the fifth embodiment of the waveguide antenna device; and

Figs 17-18 illustrate simulation results of a simulation with the sixth embodiment of the waveguide antenna device .

It is noted that the drawings are schematic, not necessarily to scale and that details that are not required for understanding the present invention may have been omitted. The terms "upward", "downward", "below", "above", and the like relate to the embodiments as oriented in the drawings, unless otherwise specified. Further, elements that are at least substantially identical or that perform an at least substantially identical function are denoted by the same numeral. A first embodiment of a waveguide antenna device 1 is illustrated in Fig. 1. The waveguide antenna device 1 comprises a waveguide 2, which waveguide 2 is solid and circular-shaped. The waveguide 2 has a proximal end 8 and a distal end 7. In use, it is intended that a Bi-field is generated in front of the distal end 7 in an area of interest indicated with A. The Bi-field is intended to be directed in the axial direction of the waveguide 2, as is indicated with arrow Z. This is also called 'end-firing'. Therefore, the waveguide antenna device 1 according to the invention may be called an 'end-firing' waveguide antenna device.

The waveguide antenna device 1 is provided with a reflector 4, wherein the reflector 4 is located at the proximal end 8 of the waveguide 2. When the antenna 3 is emitting a RF signal, it is intended to be directed towards the distal end 7 of the waveguide 2 and the area A of interest. However, it might occur that a part of the RF signal is emitted towards the proximal end 8 of the waveguide 2. The reflector 4 reflects the part of the RF signal emitted towards the proximal end 8 of the waveguide 2, such that this part of the RF signal is added to a remaining part of the RF signal directed towards the distal end 7 of the waveguide 2 and the area A of interest.

The waveguide 2 comprises a dielectric material and in this embodiment, the waveguide 2 has a relative dielectric relative dielectric permittivity of 36. However, the waveguide 2 may have a relative dielectric permittivity of , 15, 30, 35, 50, or more. Every value between the mentioned values is considered to be possible too. The waveguide 2 may be made of a material selected from a group comprising oils, oil mixtures, alcohol, water, heavy water, ethylene glycol, ceramic, and/or a combination thereof. Any other suitable material may be used for the waveguide 2.

In this embodiment, the antenna is a helical antenna 3 which is provided around the waveguide 2. In order to be able to generate a RF signal properly, a length of one helical winding equals a wavelength of the RF signal. A spacing between the windings is approximately one quarter of a wavelength and a full length of the waveguide antenna device 1 preferably includes at least three windings. The windings of the antenna 3 generate circularly polarized fields, both B and E fields, which move along the antenna 3. This results in a circularly polarized propagating wave that is emitted along the axial direction of the waveguide. The dielectric material reduces the signal wavelength, such that dimensions of the waveguide 2 and/or the waveguide antenna device 1 may be reduced. The dimensions of the waveguide antenna device 1 also depend on the Bo field strength of the MRI-system in which the waveguide antenna device 1 is to be used.

It is noted that the antenna 3 may be selected from a group comprising a patch antenna, a dipole antenna, a stub antenna and/or a combination thereof. A simulation has been carried out with the waveguide antenna device 1 as shown in Fig. 1. The results of the simulation are shown in Figs. 3 and 4, wherein Fig. 3 illustrates the simulation result with respect to a Bi+-field and Fig. 4 illustrates the simulation result with respect to a Bi~-field, both generated by the waveguide antenna device 1 as shown in Fig. 1. The waveguide antenna device 1 is placed against a phantom (not shown). The phantom has a relative dielectric permittivity of 36.

Since the distal end 7 of the waveguide antenna device 1 is placed against the phantom (not shown), the area A of interest is substantially located inside the phantom. Further, due to the phantom and the waveguide 2 having substantially the same relative dielectric permittivity, substantially no reflection of the RF signal occurs at the boundary between the distal end 7 of the waveguide antenna device 1 and the phantom. Thus, the major part of the generated RF signal is directed to the area A of interest.

As can be seen in Figs. 3 and 4, the Bi+-field and the Bi_-field have substantially the same field distribution. Small differences are derivable from the figures.

In Fig. 2, a second embodiment of the waveguide antenna device 1 is illustrated. In this embodiment, the waveguide comprises two parts 2, 5 and the helical antenna 3 is provided between these two parts 2, 5, thereby forming a concentric, circular geometry. As can be seen in Fig. 2, one of the two parts 2, 5 is placed around the second one of the two parts 2, 5. Thus, the two parts 2, 5 are concentrically positioned with respect to each other. The two parts 2, 5 may have the same or different relative dielectric permittivities.

In Fig. 2, it is shown that the waveguide comprises two different parts 2, 5. It is also possible that the waveguide is formed from a single part and that the antenna, in particular the helical antenna 3, is inserted into the single part. The antenna 3 is in both cases surrounded by a dielectric material. The antenna 3 being placed around a dielectric material and being surrounded by a dielectric material leads to an improved Bi+-field.

Figs. 5 and 6 illustrate results of a simulation which has been carried out with the waveguide antenna device 1 as shown in Fig. 2. Fig. 5 illustrates the simulation result with respect to a Bi+-field and Fig. 6 illustrates the simulation result with respect to a Bi~-field, both generated by the waveguide antenna device 1 as shown in Fig. 2.

For this simulation, the waveguide antenna device 1 is also placed with the distal end 7 thereof against a phantom (not shown). The parts 2, 5 of the waveguide may have substantially the same relative dielectric permittivity or may have different relative dielectric permittivities. In this case, both parts 2, 5 have substantially the same relative dielectric permittivity. The relative dielectric permittivity of the phantom (not shown) is the same as the relative dielectric permittivity of the parts 2, 5 of the waveguide of the waveguide antenna device 1.

As can be seen in Fig. 5, the Bi+-field is enlarged and increased in comparison with the simulation results shown in Fig. 3. The Bi_-field is decreased and reduced in comparison with the Bi_-field as shown in Fig. 4. Thus, the antenna 3 being surrounded by a dielectric materials leads to an improved Bi+-field and a reduced Bi_-field. As a result, exciting of the atomic nuclei might be improved, which might lead to improved images.

The waveguide antenna device 1 may be provided with a shielding 6, which is provided around the waveguide 2. In the embodiment shown in Fig. 9, the shielding 6 extends from the proximal end 8 to a certain distance from the distal end 7, such that a gap is present between the shielding 6 and tissue of a human body (not shown) at a location where the waveguide antenna device 1 is placed against a part of a human body, such as a cranial end of a head. The gap between the tissue of the human body (not shown) may provide an isolation between the shielding 6 of the waveguide antenna device 1 and the tissue of the human body (not shown).

It is also possible that the shield extends over the complete side of the waveguide 2. This means that the shielding 6 extends from the proximal end 8 to the distal end 7 without leaving a gap between the tissue of a human body, against which the waveguide antenna device 1 is placed, and the shielding 6 around the waveguide 2. The waveguide 2, 5 comprises two parts 2, 5, also called an inner substrate 2 and an outer substrate 5. A helical antenna 3 is placed between the inner substrate 2 and the outer substrate 5, as already mentioned in relation to Fig. 2.

In another embodiment, the waveguide 2, 5 may be made of a single piece and the helical antenna 3 may be inserted into the single piece.

In case the waveguide 2 only comprises dielectric material, it may occur that a part of the RF signal leaks to the side of the waveguide antenna device 1. The shielding 6 placed around at least part of the waveguide 2, 5 may prevent that at least a part of the RF signal leaks to the side of the waveguide antenna device 1. Further, the shielding 6 may reflect the RF signal, in particular towards the distal end 7 of the waveguide 2, 5 and the area A of interest.

Figs. 7 and 8 illustrate simulations results of a simulation which has been carried out with the waveguide antenna device 1 as shown in Fig. 2. Fig. 7 illustrates the simulation result with respect to a Bi+-field and Fig. 8 illustrates the simulation result with respect to a Bi~-field, both generated by the waveguide antenna device 1 as shown in Fig. 2.

For this simulation, the waveguide antenna device 1 is also placed with the distal end 7 thereof against a phantom (not shown). The parts 2, 5 of the waveguide may have substantially the same relative dielectric permittivity or may have different relative dielectric permittivities. In this case, both parts 2, 5 have substantially the same relative dielectric permittivity. The relative dielectric permittivity of the phantom (not shown) is substantially the same as the relative dielectric permittivity of both parts 2, 5 of the waveguide of the waveguide antenna device 1.

As can be seen in Fig. 7, leakage of the RF signal at the side of the waveguide antenna device 1 is reduced and emission of the RF signal is more concentrated at the distal end 7 of the waveguide 2, 5 and into the area A of interest.

In comparison, the waveguide antenna device 11 as shown in Fig. 5 has much more leakage of the RF signal to the side of the waveguide antenna device 1. A fourth embodiment of a waveguide antenna device 1 is illustrated in Fig. 10. The waveguide antenna device 1 comprises a waveguide 2 comprising a dielectric material. Optionally, the waveguide 2 is substantially solid and made of dielectric material. A shielding 6 is provided around the waveguide 2, which shielding 6 extends from the proximal end 8 of the waveguide to substantially the distal end 7 of the waveguide 2.

In this embodiment, the antenna waveguide device 1 comprises a patch antenna 3. The patch antenna may be made of a circular, conductive disc 3 and may be placed in front of a reflector (not shown), i.e. between the reflector and the waveguide. The patch antenna 3 may comprise two driving ports 10 at different positions of the patch antenna 3 in order to be able to emit a circular polarized RF signal, at least in combination with the waveguide 2. A simulation has been carried out with the waveguide antenna device 1 as shown in Fig. 10. The results of the simulation are shown in Figs. 13 and 14. Fig. 13 illustrates the simulation result with respect to a Bi+-field and Fig. 14 illustrates the simulation result with respect to a Bi_-field.

For this simulation, the waveguide antenna device 1 is also placed with the distal end 7 thereof against a phantom (not shown). The relative dielectric permittivity of the phantom (not shown) is substantially the same as the relative dielectric permittivity the waveguide 2 of the waveguide antenna device 1. The simulation has been carried out as if the waveguide antenna device 1 is used within a MRI-system with a Bo-field of 3T (128MHz).

As can be seen in Fig. 13, the Bi+-field generated by the waveguide antenna device 1 of Fig. 10 is larger and stronger than the Bi_-field of the waveguide antenna device 1, as can be seen in Fig. 14. The Bi+-field therefore may be concentrated within the area of interest, which leads to improved MRI-images depicting the area of interest. A fifth embodiment of a waveguide antenna device 1 is shown in Fig. 11. The waveguide antenna device 1 comprises a waveguide 2 comprising a dielectric material. Optionally, the waveguide 2 is solid and made of a dielectric material. The waveguide antenna device 1 comprises two dipoles antennas 3, which may be provided at or near the proximal end 8 of the waveguide 2. Optionally, the waveguide antenna device 1 comprises a reflector (not shown) provided at the proximal end 8 of the waveguide 2 in order to at least reduce leakage of the RF signal emitted by the dipole antennas 3 at the proximal end 8 of the waveguide 2.

In an embodiment, the dipole antennas 3 are placed substantially transversal to each other and on top of each other along the axial direction of the waveguide 2. A simulation has been carried out with the waveguide antenna device 1 as shown in Fig. 13. The results of the simulation are shown in Figs. 15 and 16. Fig. 15 illustrates the simulation result with respect to a Bi+-field and Fig. 16 illustrates the simulation result with respect to a Bi_-field.

As can be seen in Fig. 15, the fifth embodiment of the waveguide antenna device 1 may effectuate a Bi+-field which is larger and stronger than the Bi_-field as shown in Fig. 16, which both are generated with the waveguide antenna device 1 as shown in Fig. 11. A sixth embodiment of a waveguide antenna device 1 is shown in Fig. 12. The waveguide antenna device 1 comprises a waveguide 2 comprising a dielectric material. Optionally, the waveguide 2 is solid and substantially made of a dielectric material. In this embodiment, the waveguide 2 is rectangular and comprises two dipole antennas 3. The dipole antennas 3 may be placed at or near the proximal end 8 of the waveguide 2. Optionally, the waveguide antenna device 1 comprises a reflector (not shown) provided at the proximal end 8 of the waveguide 2 in order to at least reduce leakage of the RF signal emitted by the dipole antennas 3 at the proximal end 8 of the waveguide 2. The waveguide 2 may be surrounded by a shielding 6 (not shown) in order to reduce leakage of the RF signal at the side(s) of the waveguide.

In an embodiment, the dipole antennas 3 are placed substantially transversal to each other and on top of each other along the axial direction of the waveguide 2. A simulation has been carried out with the waveguide antenna device 1 as shown in Fig. 12. The results of the simulation are shown in Figs. 17 and 18. Fig. 17 illustrates the simulation result with respect to a Bi+-field and Fig. 28 illustrates the simulation result with respect to a Bi_-field.

For this simulation, the waveguide antenna device 1 is also placed with the distal end 7 thereof against a phantom (not shown). The relative dielectric permittivity of the phantom (not shown) is substantially the same as the relative dielectric permittivity the waveguide 2 of the waveguide antenna device 1. The simulation has been carried out as if the waveguide antenna device 1 is used within a MRI-system with a Bo-field of 7T (300MHz).

As can be seen in Fig. 17, the sixth embodiment of the waveguide antenna device 1 may effectuate a Bi+-field which is larger and stronger than the Bi~-field as shown in Fig. 18, which both are generated with the waveguide antenna device 1 as shown in Fig. 14.

It is noted that the above analyses show embodiments that have optimized Bi+ performance, i.e. for transmit purposes. Similarly, by choosing the opposite winding direction (helix antenna) or opposite phase difference between the ports, for instance in case of a patch antenna or dipole antennas, the Bi- performance is optimized. This is desirable if the antenna is meant to receive a RF signal. A waveguide antenna with a patch antenna or a pair of dipole antennas can be operated for transmitting and receiving using a quadrature hybrid, which ensures +90 degrees phase difference in transmit and -90 degrees phase difference in receive.

It is noted that the waveguide may be rectangularshaped, circular-shaped or elliptical shaped.

The invention is not restricted to the above-described embodiments, which can be varied in a number of ways within the scope of the claims. It is, for example possible that the waveguide antenna device is used in combination with a conventional RF coil, such as a birdcage coil, to improve a certain part of a Bi-field generated by the birdcage coil.

It is further possible that, in case a dipole antenna or a stub antenna is used for generating the RF signal, more than one dipole antenna or stub antenna is used for generating the RF signal. Moreover, it is possible that a waveguide of a waveguide antenna device comprises a combination of a cylindrical body filled with a fluid.

Claims

A waveguide antenna device for generating and / or receiving a radio frequency signal in an MRI aperture of an MRI system, the waveguide antenna device comprising: a waveguide having an axial direction and comprising a dielectric material, the waveguide being configured to direct the radio frequency signal toward an area of interest for the waveguide; and an antenna, which antenna is configured to generate an RF signal within the waveguide, the waveguide and the antenna being positioned to emit a single-mode, radio frequency signal in the axial direction of the waveguide.

The waveguide antenna device of claim 1, wherein the waveguide and the antenna are configured to generate a circularly polarized radio frequency signal.

A waveguide antenna device according to claim 1 or 2, wherein the antenna is disposed at the proximal end of the waveguide and / or extends into the waveguide, the waveguide being positioned to emit a signal in the waveguide.

4. A waveguide antenna device according to any one of the preceding claims, wherein the waveguide comprises two or more parts, each of the parts comprising a dielectric material.

The waveguide antenna device of claim 4, wherein the two parts are positioned concentrically with respect to each other.

A waveguide antenna device according to any one of the preceding claims, wherein the waveguide or, in the case of a waveguide antenna device according to claim 4 or 5, at least one of the two parts is solid.

7. Waveguide antenna arrangement according to any of the preceding claims, wherein a reflector is provided at a proximal end of the waveguide.

A waveguide antenna device according to any one of the preceding claims, further comprising a shield provided around at least a portion of the waveguide.

The waveguide antenna device according to claim 8, wherein the shielding comprises a metal foil, and / or wherein the shielding is attached to the waveguide.

10. A waveguide antenna device as claimed in any preceding claim, wherein the dielectric material has a relative dielectric permittivity of 10, 15, 30, 35, 50 or more.

A waveguide antenna device according to any preceding claim, wherein the dielectric material is selected from a group comprising: oils, oil blends, alcohol, water, heavy water, ethylene glycol, ceramics, and / or a combination thereof.

A waveguide antenna device according to any one of the preceding claims, wherein the antenna is selected from a group comprising: a helical antenna, a patch antenna, a dipole antenna, a stub antenna, and / or a combination thereof.

A waveguide antenna device according to any one of the preceding claims, wherein the waveguide is rectangular, circular or elliptical.

A waveguide antenna device as claimed in any one of the preceding claims, wherein the waveguide antenna device is covered so that the waveguide antenna device can be used internally in a human body.

MRI system provided with a waveguide antenna device according to one of the preceding claims.