WO2018086706A1

WO2018086706A1 - A magnetic resonance imaging antenna

Info

Publication number: WO2018086706A1
Application number: PCT/EP2016/077473
Authority: WO
Inventors: Juan Diego SANCHEZ HEREDIA; Buon Kiong Lau
Original assignee: Sanchez Heredia Juan Diego; Buon Kiong Lau
Priority date: 2016-11-11
Filing date: 2016-11-11
Publication date: 2018-05-17

Abstract

The disclosure relates to a magnetic resonance imaging antenna configured to radiate electromagnetic radiation having a fixed wavelength substantially in a first direction, the antenna comprising: a dielectric block comprising a dielectric material; a radiating element arranged on or within the dielectric block, the radiating element having an electrical length of λ/2, λ being an RF wavelength within the dielectric material; one or more reflectors comprising an electrically conducting material and arranged on and/or within the dielectric block distanced from the radiating element, the reflector having an electrical length being longer than the electrical length of the radiating element; and one or more directors comprising an electrically conducting material and being arranged on and/or within the dielectric block distanced from the radiating element in the first direction with respect to the radiating element.

Description

A MAGNETIC RESONANCE IMAGING ANTENNA

Field of the invention

The present invention relates to a magnetic resonance imaging antenna and a method for emitting electromagnetic radiation from a magnetic resonance imaging antenna into a target.

Background art

Magnetic resonance imaging (MRI) is an imaging technique used for example within radiology to image the anatomy and the physiological processes of a body. An MRI apparatus use strong magnetic fields, radio waves, and field gradients in order to form images of the body. MRI is based upon nuclear magnetic resonance (NMR). Some atomic nuclei may absorb and emit radio frequency energy when exposed to an external magnetic field. In MRI, hydrogen atoms are most-often used to generate a radio-frequency signal that is received by antennas in close proximity to the anatomy being examined. Hydrogen atoms exist naturally in humans and other biological organisms forming a part of for example water and fat. For this reason, most MRI scans essentially map the location of water and fat in the body. Pulses of radio waves excite a nuclear spin energy transition, and magnetic field gradients localize the signal in space. By varying the parameters of the pulse sequence, different contrasts can be generated between tissues based on the relaxation properties of the hydrogen atoms therein.

Ultrahigh field magnetic resonance imaging (UHF MRI) is developed for imaging deep organs. High magnetic field strengths may have a high potential for magnetic resonance imaging and spectroscopy. For example, the signal-to-noise ratio increases and the sensitivity to detect small local changes in the magnetic properties of tissue is expected to become higher.

UHF MRI is, however, complicated for biological and medical applications due to the effects arising from the short radiofrequency (RF) wavelength in biological tissue. This results in higher in-body field attenuation than conventional MRI, which causes inhomogeneity of the field as well as reduced signal-to-noise (SNR) at deep locations of the body due to lack of penetration depth. The RF power deposition is also increased due to a higher frequency, and Specific Absorption Ratio (SAR) levels become an important concern from a radiation exposure safety point of view.

Magnetic resonance imaging antennas such as radiative RF coils are nowadays the most promising technique to overcome the low penetration depth of RF fields in UHF MRI. The field homogeneity problem may be addressed through the use of coil arrays controlling the phase difference between elements. However, a problem with current magnetic resonance imaging antennas available for UHF MRI is to obtain a good performance while at the same time keeping the dimensions of the antenna small enough to allow high density arrays.

Summary

It is an object to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solve at least the above mentioned problem.

According to a first aspect, these and other problems are solved in full, or at least in part, by a magnetic resonance imaging antenna configured to radiate electromagnetic radiation having a fixed wavelength substantially in a first direction, the antenna comprising: a dielectric block comprising a dielectric material; a radiating element arranged on or within the dielectric block, the radiating element having an electrical length of λ/2, λ being an RF (radio frequency) wavelength within the dielectric material; one or more reflectors comprising an electrically conducting material, the one or more reflectors being arranged on and/or within the dielectric block and distanced from the radiating element, the one or more reflectors having an electrical length being longer than the electrical length of the radiating element; and one or more directors comprising an electrically conducting material, the one or more directors being arranged on and/or within the dielectric block and distanced from the radiating element in the first direction with respect to the radiating element. The proposed design exploits the use of parasitic scatterers to modify some characteristics of the antenna radiation patterns. The proposed design further utilizes parasitic scatterers to reduce the electric nearfields, and hence the local SAR. The parasitic scatterers are the additional (parasitic) structures near the radiating element of the antenna, i.e. the one or more reflectors and the one or more directors, which, when combined, provide some degrees of freedom to control the field distributions generated around the radiating element. When the resonance frequency of the scatterer is equal to or lower than the incident wave generated by the radiative element, the scatterer reflects most of the incident energy (the scatterer acts as a reflector). On the other hand, if the scatterer has higher resonant frequency than the radiating element, then it contributes to the radiation, directing the propagated wave along the direction defined by the radiating element and the scatterer (i.e. the scatterer acts as a director).

The design is advantageous as it allows improving the radiation directivity of the antenna and at the same time greatly reducing the local SAR at the interface between antenna and sample by efficiently suppressing the electric nearfield while transmitting the magnetic field. In other words, the design allows for an improved efficiency in emitting magnetic field into a target, while at the same time decreasing the emission of an electric field by effectively containing a larger part of the electric field within the dielectric block. Moreover, the short RF wavelength of the propagated wave within the dielectric material allows for a reduced physical size of the antenna.

Thus the magnetic resonance imaging antenna may have improved radiative properties and at the same time being possible to be made physically smaller. Furthermore, the aforementioned advantages may also reduce the mutual coupling between the fields generated by two or more magnetic resonance imaging antennas of the disclosure. Therefore, the proposed antenna design is particularly suitable to be used when a number of magnetic resonance imaging antennas are combined in an array

configuration, for example in a configuration where a plurality of antennas is disposed such that each dielectric block faces an adjacent dielectric block at their longer side. Such an array configuration may allow even higher ratio between the magnetic field strength to the electric field strength at deep locations in a target.

It is understood that the RF wavelength depends on the relative permittivity of the dielectric material within the dielectric block. This implies that the RF wavelength λ may be constant, or at least approximately constant, as function of position within the dielectric block when the dielectric block is made from a single material and thus has a constant relative permittivity. It further implies that the RF wavelength λ may vary as function of position within the dielectric block if the dielectric block comprises more than one dielectric material and thus have a varying relative permittivity. In such a case, the RF wavelength, λ, should be interpreted as an effective wavelength. For example, a distance expressed in terms of an electrical length, such as for example the electrical length of λ/2, between a first point in a first dielectric material within the dielectric block and a second point in a second dielectric material within the dielectric block may be unambiguously determined by integrating over a spatial distribution of RF wavelengths over the distance.

The one or more reflectors may be arranged in a reflector plane, the radiating element may be arranged in a radiator plane and the one or more directors may be arranged in one or more director planes, wherein each plane of the reflector plane, the radiator plane and the one or more director planes has a surface normal being parallel to the first direction.

It is understood that the one or more directors being distanced from the radiating element in the first direction with respect to the radiating element should be interpreted as the distance between the one or more directors and the target being shorter than the distance between the radiating element and the target. This may allow for the one or more directors to influence the electromagnetic radiation emitted by the radiating element in the first direction, or essentially in the first direction, as said electromagnetic radiation passes the one or more directors on its way towards the target.

The dielectric block may be box-shaped such as a right parallelepiped.

However, the block may alternatively be of an arbitrary shape such as for example a cylindrical shape, an oblate shape or a prolate shape. The dielectric block may alternatively be of an irregular shape. The dielectric material needs to be of high permittivity, in order to obtain a short RF wavelength of the propagated wave inside the dielectric, and thus keep the size of the radiating element small while still being power efficient. Moreover, a high permittivity dielectric material also provides more efficient transfer of electromagnetic fields into the target, which, for biological and medical applications, also comprise high-permittivity dielectric material(s). The dielectric material of the antenna may have a relative permittivity higher than 20. Examples of such high permittivity dielectric materials are Glycerol (C3H8O3), Barium Hexaferrite (BaBio.2Fen .8O19) and Barium Titanate

(BaTiOs). The dielectric material may be a solid material, but may alternatively be a liquid or gaseous material. The dielectric material may be pure water (H2O). An advantage using water is that it has high relative permittivity and is inexpensive.

The dielectric block may comprise a plurality of layers. Each of the plurality of layers may comprise a specific type of dielectric material with a specific relative permittivity. The plurality of layers may comprise different types of dielectric materials. Alternatively, one or more of the plurality of layers may comprise the same type of dielectric material. Combining a plurality of layers may improve the directivity of the radiation. By for example using a dielectric material having one relative permittivity as an inner core of the dielectric block and surround the inner core with a surface layer

comprising another material having a relative permittivity higher than that of the inner core may aid in confining the electric field inside the magnetic resonance imaging antenna. The use of a plurality of layers may make it possible to design magnetic resonance antennas with a smaller size. This may be beneficial from a practical point of view. Furthermore, in the case of a plurality of magnetic resonance antennas being used in an array formation, the use of a plurality of layers may reduce the mutual coupling between the magnetic resonance imaging antennas within the array.

To obtain an efficient radiating element, the dimension of the radiating element is defined by its electrical length and the wavelength of the dielectric material in contact with, or surrounding, the element. For example, when the element is embedded in a dielectric material, the electrical length of a half- wavelength dipole antenna is λ/2 where λ depends on the permittivity of the material.

The radiating element may be of different type having different characteristics and different shapes. For example, the radiating element may be a dipole antenna, a folded dipole or a bowtie antenna. The radiating element is made from an electrically conducting material. One such material is for example copper.

The radiating element may be located on the dielectric block. However, the radiating element may also be located within the dielectric block. The radiating element may be enclosed within the dielectric block.

The one or more reflectors may comprise an electrically conducting material. The one or more reflectors may have an electrical length being longer than the electrical length of the radiating element.

The one or more reflectors may be beneficial as they may prevent back propagation (i.e. a propagation of radiation in the direction opposed to the first direction). This may allow the wave scattered by the one or more reflectors to cancel the back propagation through destructive interference. Making the reflectors wider may improve their effectiveness. In an embodiment, the reflectors have a physical width such that the ratio between the physical width and the physical length is at least 0.1 .

The distance between the radiating element and the reflector that is closest to the radiating element may be λ/4±10%. Within said distance the one or more reflectors may scatter a wave that cancels the back propagated radiation from the radiating element through destructive interference.

However, the distance between the radiating element and the reflector that is closest to the radiating element may be less or equal to λ/4±10%.

Allowing the distance between the radiating element and the reflector that is closest to the radiating element being less than λ/4±10% may be beneficial to reduce the overall size of the magnetic resonance imaging antenna.

In an embodiment where the one or more reflectors are arranged in a reflector plane and the radiating element is arranged in a radiator plane, wherein each plane of the reflector plane and the radiator plane has a surface normal being parallel to the first direction, a distance between the radiator plane and the reflector plane may be λ/4±10%. Alternatively, the distance between the radiator plane and the reflector plane may be less or equal to λ/4±10%.

The one or more directors may comprise an electrically conducting material. The one or more directors may be beneficial as they may reduce the electric field at the surface between antenna and target (where the electric field may otherwise be very high) without reducing the magnetic field propagating into the target. Thus the one or more directors effectively increases the ratio between the magnetic field strength to the electric field strength in the radiation propagating into the target. This may be desirable for an MRI application where a strong magnetic field may be beneficial from a detection quality point of view but where a strong electric field may pose a problem as it may increase the local SAR.

The one or more directors may be shorter than the radiating element. This may be beneficial for some embodiments as it may increase their ability to direct the radiation. Alternatively, the one or more directors may be longer than, or of the same length as, the radiating element, which may be beneficial in some cases to reduce the local electric field maxima generated at the interface between the antenna and the target.

The director that is closest to the radiating element may be arranged at a distance from the radiating element being larger than or equal to λ/4±10%.

In an embodiment where the one or more directors are arranged in one or more director planes and the radiating element is arranged in a radiator plane, wherein each plane of the one or more director planes and the radiator plane has a surface normal being parallel to the first direction, the director plane of the one or more director planes being closest to the radiating element may be arranged at a distance from the radiator plane being larger than or equal to λ/4±10%.

The one or more directors may be arranged in a single director plane having a surface normal parallel to the first direction. In such a case, the effectiveness of the one or more directors is higher when they are placed at a distance slightly higher than λ/4 from the radiator plane. The one or more directors may be a plurality of directors, wherein the plurality of directors is arranged in two or more director planes, wherein each director plane of said two or more director planes having a surface normal parallel to the first direction. In such a case, some of the plurality of directors may be placed closer than λ/4 from the radiating element, in order to increase the directivity of the radiation while keeping the overall antenna small.

At least two directors may share a specific director plane of the one or more director planes. In an embodiment where at least two directors share a specific director plane, the distance between adjacent directors of the specific director plane is λ/8±10%. Keeping the distance within this interval may be beneficial to prevent the formation of local electric field maxima and therefore deposited power in the target.

According to a second aspect a magnetic resonance imaging apparatus is provided, the magnetic resonance imaging apparatus comprising a magnetic resonance imaging antenna according to the first aspect.

According to a third aspect there is provided a method for emitting electromagnetic radiation into a target using a magnetic resonance imaging antenna according to the first aspect, the method comprising the steps of: electrically connecting the magnetic resonance imaging antenna to a frequency generator unit, directing the magnetic resonance imaging antenna with its first direction directed towards a target, activating the frequency generator unit, thus allowing radiation to be emitted from the magnetic resonance imaging antenna into the target.

The second and third aspect may generally have the same features and advantages as the first aspect.

A further scope of applicability of the present invention will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. Hence, it is to be understood that this invention is not limited to the particular component parts of the device described or steps of the methods described as such device and method may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It must be noted that, as used in the specification and the appended claim, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements unless the context clearly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps.

Brief descriptions of the drawings

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing

embodiments of the invention. The figures are provided to illustrate the general structures of embodiments of the present invention.

Figure 1 a shows a perspective view of a magnetic resonance imaging antenna 100 according to an embodiment of the present disclosure.

Figure 1 b shows a cross-sectional view of the magnetic resonance imaging antenna 100 shown in Fig. 1 a.

Figure 2a shows a perspective of a magnetic resonance imaging antenna 200 according to an embodiment of the present disclosure.

Figure 2b shows a cross-sectional view of the magnetic resonance imaging antenna 200 shown in Fig. 2a.

Detailed description

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

Fig. 1 a and b shows a magnetic resonance imaging antenna 100 according to an embodiment. The magnetic resonance imaging antenna 100 is configured to radiate electromagnetic radiation having a fixed wavelength substantially in a first direction 101 . The magnetic resonance imaging antenna 100 is typically aligned such that the first direction is directed towards a target. The target may be a body of a patient but may also be a biological sample or the like.

The magnetic resonance imaging antenna 100 comprises a dielectric block 102 comprising a dielectric material. In the embodiment, the dielectric block 102 is a right parallelepiped. However, the block may be of arbitrary shape, such as for example a cylindrical shape, an oblate shape or a prolate shape. The dielectric material may have a relative permittivity higher than 20. The dielectric material needs to be of high permittivity, in order to keep the size of the radiating element small and still being power efficient. Moreover, a high permittivity dielectric material also provides more efficient transfer of electromagnetic fields into the body, which is also a high permittivity dielectric material. Examples of such high permittivity dielectric materials are for example Glycerol (C3H8O3), Barium Hexaferrite (BaBio.2Fen .8O19) and Barium Titanate (BaTiOs). In an embodiment, the dielectric material is pure water (H2O). In the embodiment, and other embodiments wherein a liquid and/or gaseous dielectric material is used, the dielectric block 102 thus must be contained within an enclosing structure, such as a vessel or the like, the inner surface of the enclosing structure forming the outer boundaries of the dielectric block 102.

The magnetic resonance imaging antenna 100 further comprises a radiating element 104 arranged on or within the dielectric block 102. In the embodiment, the radiating element is arranged in a radiator plane 1 14. The radiator plane is aligned such that it has a surface normal being parallel to the first direction. The radiating element 104 has an electrical length of λ/2 where λ is the RF wavelength within the dielectric material. The radiating element 104 may have different radiating properties and be of different shape. For example, the radiating element 104 may be a dipole antenna, a folded dipole or a bowtie antenna. The radiating element 104 is made from an electrically conducting material. In the embodiment, the radiating element 104 is made from copper.

The RF wavelength will depend on the relative permittivity of the dielectric material within the dielectric block. Thus, for the embodiment in Fig. 1 a and b, the RF wavelength λ will be constant, or at least approximately constant, as function of position within the dielectric block, as a result from the dielectric block being made from a single material and thus having a constant relative permittivity.

The magnetic resonance imaging antenna 100 further comprises one or more reflectors 106 arranged on the dielectric block 102 distanced from the radiating element 104. In the embodiment shown in Fig. 1 a-b, there are two reflectors 104. In the embodiment, the reflectors are arranged in a reflector plane 1 12. The reflector 1 12 plane is aligned such that it has a surface normal being parallel to the first direction. The one or more reflectors 106 comprises an electrically conducting material. The one or more reflectors 106 have an electrical length being longer than the electrical length of the radiating element 104. The one or more reflectors 106 are there to prevent back propagation (i.e. a propagation of radiation in the direction opposed to the first direction 101 ). Thus, the one or more reflectors 106 have to be longer than the radiating element 104. This allows the wave scattered by the one or more reflectors 106 to cancel back propagation through destructive interference. Making the reflectors wider may improve their effectiveness. In an

embodiment the reflectors 106 have a physical width such that the ratio between the physical width and the physical length is at least 0.1 .

A reflector will, theoretically, cancel back propagated waves totally when the distance between the reflector and radiating element is λ/4. Thus it may be desirable that the distance between the radiating element 104 and the reflector 106 that is closest to the radiating element 104 is less or equal to λ/4±10%. However, to reduce the overall size of the antenna, the one or more reflectors 106 may also be placed closer than λ/4 to reduce the overall size of the antenna, but this may decrease their ability to cancel back propagation. Furthermore, the high impedance mismatch caused by the interface between the high permittivity material and the surrounding air (of low permittivity) will effectively act as a reflector. Thus, the use of a high permittivity dielectric material may help to further reduce back propagation.

The magnetic resonance imaging antenna 100 further comprises one or more directors 108 being arranged on and/or within the dielectric block 102 distanced from the radiating element 104 in the first direction 101 with respect to the radiating element 104. In the embodiment, there are five directors 106. In the embodiment, the directors are arranged in a director plane 1 10. The director plane 1 10 is aligned such that it has a surface normal being parallel to the first direction. A role of the one or more directors 108 is to reduce the electric field at the surface between antenna and target (where the electric field may otherwise be very high) without reducing the magnetic field propagated into the target. Thus the one or more directors 108 effectively increases the ratio between the magnetic field strength to the electric field strength in the propagated radiation. This is desirable for an MRI application where a strong magnetic field is beneficial from a detection quality point of view but where a strong electric field may pose a problem as it increases the local SAR value in the target. The one or more directors 108 comprises an electrically conducting material. In the embodiment, the one or more directors 108 are longer than the radiating element 104. Alternatively, the one or more directors 108 may be shorter than, or of the same length as, the radiating element 104. In the embodiment, the one or more directors are arranged in a director plane 1 10 having a surface normal parallel to the forward direction 101 . The director plane 1 10 is arranged at a distance from the radiator plane 1 14 being larger than or equal to λ/4±10%. The distance between adjacent directors is λ/8±10%.

The geometrical arrangement of the components of the magnetic resonance imaging antenna may be different. An alternative embodiment is shown in Fig. 2a-b. In the embodiment, the one or more reflectors 106 are arranged in a reflector plane 212 and the radiating element 204 is arranged in a radiator plane 214. The reflector plane 212 and the radiator plane 214 each has a surface normal being parallel to the first direction 201 . Furthermore, the one or more directors are a plurality of directors 108, wherein the plurality of directors 108 is arranged in two or more director planes, wherein each director plane of said two or more director planes having a surface normal parallel to the first direction 201 . In the embodiment, the plurality of directors 208 are arranged in two director planes 210a, 210b.

The use of more than one layer of directors, may provide an improved directivity of the radiation. The director plane that is closest to the radiating element 204 (i.e. the director plane 210a in Fig. 2b) may be arranged at a distance from the radiating element 204 being larger than or equal to λ/4±10%. However, in embodiments comprising directors arranged in more than one plane, some of the directors may, alternatively, be located in a director plane being closer than λ/4 from the radiator plane in order to increase the directivity of the radiation while keeping the overall antenna small.

Furthermore, the dielectric block 202 comprises a plurality of layers

230a,230b. In the embodiment, there are two layers 230a,230b. Each of the plurality of layers 230a,230b may comprise a specific type of dielectric material with a specific relative permittivity. The plurality of layers 230a,230b may comprise different types of dielectric materials. Alternatively, one or more of the plurality of layers 230a,230b may comprise the same type of dielectric material. Combining a plurality of layers 230 may improve the directivity of the radiation. In the embodiment, the dielectric block 202 comprises an inner core 230b of a dielectric material having one relative permittivity, the inner core 230b being surrounded by an outer layer 230a comprising a dielectric material with another relative permittivity. The relative permittivity of the inner core 230b is lower than the relative permittivity of the surrounding layer 230a. This aids in confining the electric field inside the antenna 200. Thus, the use of a plurality of layers can make it possible to design magnetic resonance antennas with a reduced size. Furthermore, in the case of a plurality of magnetic resonance antennas being used in an array formation, the use of a plurality of layers may reduce the mutual coupling between the magnetic resonance imaging antennas within the array. In the embodiment shown in Fig. 2a-b, the dielectric block comprises more than one dielectric material and thus have a varying relative permittivity as function of position within the dielectric block. Thus, the RF wavelength λ will not be a constant as function of position within the dielectric block. The RF wavelength, λ, must then be interpreted as an effective wavelength. For example, a distance expressed herein in terms of an electrical length, such as for example the electrical length of λ/2, between a first point in a first dielectric material within the dielectric block and a second point in a second dielectric material within the dielectric block may be unambiguously determined by integrating over a spatial distribution of RF wavelengths over the distance.

In an embodiment of the present inventive concept is also provided a magnetic resonance imaging apparatus comprising a magnetic resonance imaging antenna. Such a magnetic resonance imaging antenna could be for example one from the embodiments described in detail hereinabove.

However, it may equally well be another embodiment of a magnetic resonance imaging antenna within the scope of the claims.

One embodiment of the magnetic resonance imaging apparatus comprises a plurality of magnetic resonance imaging antennas disposed in one or more array formations.

There will now be described a method for emitting electromagnetic radiation into a target using a magnetic resonance imaging antenna. The method comprises the first step of electrically connecting the magnetic resonance imaging antenna to a frequency generator unit. In a next step, the magnetic resonance imaging antenna is directed with its first direction directed towards a target. In a next step, the frequency generator unit is activated, thus allowing radiation to be emitted from the magnetic resonance imaging antenna into the target.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

For example, the dielectric block may be shaped differently. For example, the dielectric block may be cylindrically shaped such that its planar surfaces have their normal parallel with the first direction. As another example, the one or more reflectors and/or the one or more directors may be shaped differently from the rectangular and/or linear structure of the embodiments disclosed herein. For example, the one or more reflectors and/or the one or more directors may be one or more from the list of: shaped nonlinearly in a plane orthogonal to the first direction, have varying cross section as function of position along their length, be positioned within one or more planes having one or more normal forming one or more angles with the first direction.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

Claims

1 . A magnetic resonance imaging antenna configured to radiate electromagnetic radiation having a fixed wavelength substantially in a first direction, the antenna comprising:

a dielectric block comprising a dielectric material ;

a radiating element arranged on or within the dielectric block, the radiating element having an electrical length of λ/2, λ being a RF wavelength within the dielectric material ;

one or more reflectors comprising an electrically conducting material, the one or more reflectors being arranged on and/or within the dielectric block and distanced from the radiating element, the one or more reflectors having an electrical length being longer than the electrical length of the radiating element; and

one or more directors comprising an electrically conducting material, the one or more directors being arranged on and/or within the dielectric block and distanced from the radiating element in the first direction with respect to the radiating element.

2. The magnetic resonance imaging antenna according to claim 1 , wherein the one or more reflectors are arranged in a reflector plane, the radiating element is arranged in a radiator plane and the one or more directors are arranged in one or more director planes,

wherein each plane of the reflector plane, the radiator plane and the one or more director planes has a surface normal being parallel to the first direction.

3. The magnetic resonance imaging antenna according to claim 2, wherein a distance between the radiator plane and the reflector plane is less or equal to λ/4±10%.

4. The magnetic resonance imaging antenna according to claim 2, wherein the distance between the radiator plane and the reflector plane is λ/4±10%.

5. The magnetic resonance imaging antenna according to any one of the preceding claims, wherein the dielectric material has a relative permittivity higher than 20.

6. The magnetic resonance imaging antenna according to any one of claim 2-5, wherein the director plane of the one or more director planes being closest to the radiating element is arranged at a distance from the radiator plane being larger than or equal to λ/4±10%.

7. The magnetic resonance imaging antenna according to claim 2-6, wherein at least two directors share a specific director plane of the one or more director planes and wherein the distance between adjacent directors of the specific director plane is λ/8±10%.

8. The magnetic resonance imaging antenna according to any one of claim 2-7, wherein the one or more directors are a plurality of directors arranged in two or more director planes.

9. The magnetic resonance imaging antenna according to any one of the preceding claims, wherein the dielectric block comprises water.

10. The magnetic resonance imaging antenna according to any one of the preceding claims, wherein the dielectric block comprises a plurality of layers.

1 1 . The magnetic resonance imaging antenna according to any one of the preceding claims, wherein the one or more reflectors have a physical width such that the ratio between the physical width and the physical length is at least 0.1 .

12. A magnetic resonance imaging apparatus comprising a magnetic resonance imaging antenna according to any of claims 1 -1 1 .

13. Method for emitting electromagnetic radiation into a target using a magnetic resonance imaging antenna according to any one of claim 1 -1 1 , the method comprising the steps of:

electrically connecting the magnetic resonance imaging antenna to a frequency generator unit,

directing the magnetic resonance imaging antenna with its first direction directed towards a target,

activating the frequency generator unit, thus allowing radiation to be emitted from the magnetic resonance imaging antenna into the target.