WO2010049523A1

WO2010049523A1 - Antenna arrangement and apparatus for microwave imaging

Info

Publication number: WO2010049523A1
Application number: PCT/EP2009/064383
Authority: WO
Inventors: Karl Andersson; Nils-Fredrik Kaiser; Torleif Martin; Lars Pettersson
Original assignee: ARBEXA INDUSTRIES AB
Current assignee: ARBEXA INDUSTRIES AB
Priority date: 2008-10-30
Filing date: 2009-10-30
Publication date: 2010-05-06
Anticipated expiration: 2011-04-30
Also published as: SE0850061A1; SE532807C2

Abstract

The present invention relates to an antenna arrangement for use with microwave imaging of a biological object. The antenna arrangement comprises: a plurality of antennas, wherein each of said plurality of antennas has a front side intended to face said object. The plurality of antennas are arranged to, when in use, be in a position to allow microwave imaging of said object; and an energy leakage reducing material is provided on a back side of each of said plurality of antennas, said back side being opposite said front side. A corresponding apparatus for microwave imaging is also provided.

Description

ANTENNA ARRANGEMENT AND APPARATUS FOR MICROWAVE IMAGING

TECHNICAL FIELD

The present invention relates to imaging of biological objects using microwaves. In particular, the invention relates to an antenna arrangement, antenna design and an apparatus for coupling the microwave energy from antenna elements into a depicted object.

BACKGROUND

Microwave imaging is a relatively new technology with great potential for use in medical and related industries. Specifically, the technology is useful for non-invasive, internal imaging of the physiologic properties of tissues and organs.

Known microwave imaging apparatuses uses microwave radiation to image objects by detecting the effects the object has on the microwave field after it has been transmitted through the object. The changes effected in the microwave field (e.g. attenuation, reflection, and diffraction) during transmission depend on the dielectric permittivity and conductivity properties of the object under investigation.

The microwave range currently used for microwave imaging of biological objects is between about 0.1 and about 10 GHz, but other ranges of the microwave spectrum can be used as long as the radiation is non-ionizing. In addition, there are biophysical parameters which should be considered when determining a compatible frequency range. One aspect of microwave imaging systems is the interface between the microwave antenna(s) and the object. In case the interface is improper, the microwave energy emitted by the antenna may be reflected on the surface of the object rather than penetrating it. To reduce this phenomenon, a bolus material can be provided. The bolus material has similar dielectric properties as the object to be imaged and thereby reduces reflection of the microwave energy.

Prior art describes microwave imaging techniques essentially of two types, the first type being where transmitting and receiving antennas in close proximity of the object being imaged, and the second type being where transmitting and receiving antennas distant from the object being imaged. In the first type (close proximity), the antennas are typically in direct contact with the object, possibly with a thin (<10mm) bolus layer (solid or liquid, most often a liquid with high viscosity) . A brief description of bolus for use in close proximity of the object is disclosed in EP-O 694 282. In the second type (distantly located antennas), the bolus is a liquid or a gas. Examples of currently used bolus solutions for distantly located antenna setups are disclosed in ES-2007134.

In cases where one apparatus is intended for imaging of similar yet slightly different objects, there is a problem of positioning the antennas. One possible solution is to put the plurality of antennas in physical contact with the object. This method may not require any bolus material at all, but the positions of all antennas need to be accurately registered and the object itself needs to be slightly deformable to allow complete contact of each antenna with the object.

US-2003/0088180 discloses an antenna array device placed adjacent to the breast or other portion of the body to be imaged, using a matching element, such as a liquid filled bag, which conforms to the contour of the breast or other part of the body being imaged to minimize air gaps and unwanted reflections of microwave energy.

In all systems of the prior art, however, an improved signal to noise ratio would improve image data, and therefore the image generation.

SUMMARY OF THE INVENTION

An object of the present invention is to increase a signal to noise ratio compared to the prior art.

A first aspect of the invention is an antenna arrangement for use with microwave imaging of a biological object. The antenna arrangement comprises: a plurality of antennas, wherein each of the plurality of antennas has a front side intended to face the object wherein the plurality of antennas are arranged to, when in use, be in a position to allow microwave imaging of the object; and an energy leakage reducing material is provided on a back side of each of the plurality of antennas, the back side being opposite the front side.

By providing the energy leakage reducing material on the back of the antennas, more of the useful signal is transmitted into the object, improving the signal to noise ratio, whereby image data is improved. Moreover, the improved signal to noise radio also allows for a shorter time of microwave radiation for data acquisition to obtain the same quality of image data. This shorter radiation may improve resolution, especially with a dynamically changing object.

The plurality of antennas may be arranged to, when in use, essentially surround the object.

Microwaves are refracted in a border between two materials, changing the direction of the microwaves. In order to reconstruct the interior structure of the object, it is advantageous to have measurements of how microwave energy is transmitted through the object in as many directions as possible. More combinations of transmitting and receiving antenna positions, as is the case with antennas essentially surrounding the object, thus leads to better data to reconstruct the object. Due to physical constraints, it may sometimes not be possible to have antennas surrounding the object completely. However, even if the antennas do not completely surround, where there are one or more gaps between the antennas which are small in relation to a complete circle around the object (i.e. essentially surrounding antennas ) , better data is obtained compared with the prior art.

Furthermore the increased signal to noise ratio may allow imaging to be performed, e.g. when imaging is performed of a living patient, without requiring any removal of clothing or similar.

The antenna arrangement may further comprise a microwave reflection reducing material provided on the front side. The microwave reducing material can for example be a bolus .

The antenna arrangement may further comprise an antenna support holding the plurality of antennas, and wherein the flexible antenna support is arranged to be placed around the object, while allowing ends of the antenna support to be fixed in relation to each other.

The microwave reflection reducing material may have a thickness of at least 1 cm, as measured from each of the plurality of antennas. By providing such a thick microwave reflection reducing material, the antennas can be more easily fixed. For example, the antenna arrays can be rigid, allowing the position of each antenna to be known, reducing the complexity of calculations.

A thickness of each of the plurality of antennas, as measured from the front side to the back side, may be less than 0.5 mm. Each of the plurality of antennas may be a dipole antenna, a bowtie antenna, a patch antenna or a monopole antenna.

The energy leakage reducing material and the microwave reflection reducing material may be of the same material. Being of the same material can simplify production.

Each of the antennas may be enclosed with the same material.

The energy leakage reducing material and the microwave reflection reducing material may be of different materials . The energy leakage reducing material may be a material adapted to reduce leakage towards air, and the microwave reflection reducing material may be a material with dielectric properties which resemble the object.

The microwave reflection reducing material may comprise a deformable solid material.

The microwave reflection reducing material may be a material selected from the group consisting of: silicone, any other elastomer, soft plastic, gelatine or any combination of these.

The microwave reflection reducing material may comprise a liquid confined in an elastic bag.

The microwave reflection reducing material is provided with a non-sticky surface on a side intended to face the object.

A second aspect of the invention is an apparatus for microwave imaging of a biological object. The apparatus comprises: an antenna arrangement according to the first aspect; a microwave transmitter connected to the antenna arrangement; a microwave receiver connected to the antenna arrangement; and a controller arranged to, using the transmitter and the receiver, generate an image of the object.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Fig la-b are schematic diagrams showing antennas and bolus in a microwave imaging system according to an embodiment of the present invention; and

Fig 2 is a schematic diagram illustrating an antenna support according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain 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 by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

For the purpose of the present application, and for clarity, the term object is defined as an object on which a microwave image is about to be taken. The object can be a complete biological organism, living or dead. The object can further be a part of a living or dead biological organism (e.g. leg, arm, head, breast, penis, testicles, tail, trunk or any other part of a biological organism). The object typically has a volume of 0.1 — 500 litres, preferably 0.5-15 litres, and typically extends less than 3 m, preferably less than 0.5 m. The term bolus here comprises a microwave reflection reducing material (solid or liquid), having dielectrical properties matching the object, whereby the antenna is matched to the object. The effect is similar to immersion oil in optical microscopy.

Referring now to Figs la-b, a set-up of antennas and bolus in a microwave imaging system according to an embodiment of the present invention is shown. In Fig Ia, three arrays 100, 101, 102 of antennas are depicted seen in perspective. The antenna arrays 100, 101, 102 are also known as antenna supports. Each array 100, 101, 102 comprises four individual microwave antennas 110 arranged in a configuration of four antennas in a row. The structure upon which the four antennas are mounted, which can be seen as rectangles in Fig Ia, can optionally be provided with a ground plane. Such a ground plane can also be provided separate from the structure. In other words, an internal or external ground plane can optionally be provided. The antennas 110 are typically flat or thread like, i.e. the antennas 110 have a thickness less than 5 mm, preferably less than 2 mm, or even more preferably less than 0.5 mm. Other configurations are equally possible. Two of the arrays 100, 101 are mounted in a fix manner. The third array 102 is mounted on a sledge 120 so that the position of the third array can be adjusted and clamp the object for which an image is about to be taken. Knowing the position of the sledge 120 allows for the position of each antenna to be known.

A controller 130, such as a general purpose laptop or desktop computer, is arranged to control the process. The controller 130 is connected to a microwave transmitter and receiver 134. Optionally, the transmitter/receiver 134 is divided into separate a separate transmitter and receiver parts. The connection 132 between the controller 130 and the transmitter/receiver 134 can be any appropriate wire- based or wireless connection, for example a USB (universal serial bus), RS232 type serial connection, Centronics type parallel connection, Bluetooth, wireless USB, Ethernet, wireless network, etc.

In Fig Ib, the three arrays 100, 101, 102 are depicted from above, including an object 150 and bolus 160 in the diagram. The antenna arrays 100, 101, 102 are in direct contact with bolus material 160 and energy leakage reducing material 162. The movable array of antennas 102 is pressed against the object 150.

The bolus material 160 has similar dielectric properties as the object and thereby reduces reflection of the microwave energy on the surface of the object. The bolus material 160 is therefore in contact both with the antennas and the object.

The bolus can be either a deformable solid, such as an elastomer like silicone, soft plastic, gelatine, or similar, or an encapsulated liquid such as oil confined within an elastic bag, like a rubber balloon, wherein the part of the bolus contacting the object has a non- sticky, clean surface.

The leakage reducing material 162 has dielectric properties chosen to improve antenna stability. Antenna stability means that resonance problems are reduced, which is more likely to occur when there is air behind the antenna. The antenna then performs more uniformly across the entire operating frequency range. Moreover, with increased stability, the antenna handles different materials better, reducing resonance problems when an object with different dielectric properties is introduced. The improved stability in turn results in a higher signal to noise ratio. Furthermore, the antenna bandwidth (i.e. the increase operating range in frequency terms of the antenna) is increased. The physical size of the antenna can also be reduced. The leakage reducing material 162 normally resembles the bolus material 160 to greater extent than it resembles air. Nevertheless, the leakage reducing material 162 does not need to be identical to the bolus material, even if it is advantageous if they have similar dielectric properties. Hence, the leakage reducing material 162 is selected based on dielectric properties and can for example be ceramic materials, solid or deformable plastic materials including polyethers, polyesters, or a liquid such as alcohols enclosed by a membrane. It is to be noted that many other materials can be used, as long as the material has the appropriate dielectric properties. In case the object 150 is a leg of a human, it will, due to the three antenna arrays in this embodiment, have approximately a triangular cross section and there will be skeletal bones 151 inside the leg. Thus, the outline of the antenna array arrangement shown in Figs la-b is adapted for simple fixation of elongated, essentially triangular objects like human legs in a structure wherein all antenna positions are either fixed or obtained through one reading of the position of the sledge 120. When the sledge 120 is pushed in, the antennas thus surround the object to be imaged. In a situation illustrated in Fig IB, the system can for example be used to detect deep vein thrombosis.

As will be shown below in conjunction with Fig 2, other constructions allowing the antennas, when in use, to surround the object to be imaged are equally possible.

Furthermore, the flexible bolus material is provided with a non-sticky, clean surface in the space between the leg and the arrays. Consequently, the leg, or other object, is enclosed, minimising any air gaps and associated microwave energy leakage.

To obtain the image, the controller 130 commands microwaves to be emitted from one antenna and the transmitted microwave energy is measured in the majority of, or even all, the other antennas. In one embodiment, the transmitted microwave is measured in all other antennas except the antennas directly adjacent to the transmitting antenna on either side. The adjacent antennas can be excluded when there is significant crosstalk from the transmitting antenna to the adjacent antennas. When this is repeated so that at least one, typically a plurality of, or even all antennas have emitted microwaves, sufficient data for image reconstruction is obtained. Optionally the transmission and reception is repeated for a plurality of frequencies to obtain better data for image generation. Image reconstruction can be performed by the controller 130 e.g. by solving Maxwell's equations and iteratively change the material properties of the internal representation of the object until the distribution of dielectrical properties in the object results in a theoretical microwave transmission pattern closely resembling the measured pattern.

Microwave imaging relies on a sufficient number of antenna elements being placed around an object, wherein the elements emit and receive microwaves according to a predefined protocol. As an example, for an object of approximately 1 liter volume, approximately 10 antennas are sufficient. Due to the size of each antenna element (typically about 10 mm to about 50 mm), which in turn depends on the frequency of the microwave radiation, it may be difficult to obtain an array with a sufficient number of antennas per unit area near an object. This means that to achieve a high resolution image of an object, the array of antennas must either be situated distant from the object (so that each antenna element occupies only a fraction of the solid angle from the perspective of the object) or it has to be translated in space as to scan a sequence of nearby images of the object, which in concert can be used for reconstruction of a high resolution image. Furthermore, during the time of emitting or receiving microwaves, the position of the antenna elements relative to the object should be known.

Using the bolus in this embodiment, the circumference for the antenna arrays around the object is increased, whereby a larger number of antennas with fixed positions can be used compared to when the antennas are pressed directly onto the object.

Advantageously, antennas are matched to the surrounding medium in order to function well. In case an antenna is pressed against a biological object and the back side is exposed to air, the large difference in electromagnetic properties of the object and the air may cause the antenna to function sub-optimally and leak a substantial part of the emitted energy into air.

In cases where an antenna is pressed against the object, it may happen that some, or even most, of the emitted microwave energy is transmitted backwards, i.e. away from the object. In accordance with the present invention, a suitable leakage reducing material 162 is provided on the back of the antenna, whereby a larger part of the microwave energy is transmitted to the object to be imaged. This may for example be realised by encapsulating the antenna in bolus material, providing bolus material not only in front of the antenna but also on the back of the antenna. It is to be noted though that the leakage reducing material 162 on the back of the antenna does not need to be the same as in the front. With an appropriate material provided on the back of the antennas, energy leakage is reduced. Microwave radiation into air is normally not dangerous per se, but may interfere with electronic devices and cause failure of various apparatuses. In hospital environments, such failures may have fatal outcome for patients. Moreover, by reducing the leakage, more of the radiation is transmitted to the object to be imaged, increasing the signal to be measured, whereby the signal to noise ratio is increased.

The optimal distance between the antennas and the object depends on several facts. Firstly, the number of antennas required for acceptable imaging of the desired object will dictate the approximate size of the antenna arrangement. The size of the desired object may be much smaller than the antenna arrangement size, leading to a smallest distance between the antennas and the object exceeding 2 cm, or even exceeding 5 cm. In the other end, when the object size is large enough to allow a sufficient number of antennas along the boundary of the object, there is still a need for the fix antennas to have an at least 1 mm, preferably an at least 1 cm and even more preferably at least 2 cm thick layer of bolus. Particularly the thicker bolus layer of lcm or more allows coupling of emitted microwaves when a number of similar objects are imaged, even if the objects have slight differences in size and shape. Furthermore, in order for the antennas surrounded by a known medium, the antenna and bolus arrangement is designed to always have at least 5 cm, preferably at least 1-2 cm, and even more preferably 0.5 cm smallest distance between any antenna and the object. Fig 2 is a schematic diagram showing an antenna arrangement according to an embodiment of the present invention. Here, an antenna support 100, also known as an antenna array, comprises a number of antennas 110. On the inside of the antennas 110, i.e. on the side of the antennas which is intended to face the object to be imaged, a bolus material 160 is provided, which reduces reflections of microwaves in the transition to the object. On the back side, a second material 162 is provided which reduces leakage of microwave radiation. In one embodiment, this second material 162 is the same as the bolus 160. In another embodiment, the second material 162 is selected specifically to reduce microwave leakage and may be a different material compared to the bolus 160.

The antenna support 100 comprises a fastener 170 which allows the antenna support to full encompass, or surround, the object to be imaged. The fastener is optionally adjustable, allowing the circumference of the antenna support to be varied depending on the size of the object to be imaged.

It is to be noted that while the embodiments above disclose the use of a bolus between the antennas and the object, this is not necessary for the invention. It is also possible to place the antennas directly on the surface of the object, e.g. the skin of a human leg.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Claims

1. An antenna arrangement for use with microwave imaging of a biological object, said antenna arrangement comprising: a plurality of antennas, wherein each of said plurality of antennas has a front side intended to face said object c h a r a c t e r i s e d i n t h a t said plurality of antennas are arranged to, when in use, be in a position to allow microwave imaging of object; and an energy leakage reducing material is provided on a back side of each of said plurality of antennas, said back side being opposite said front side.

2. The antenna arrangement according to claim 1, wherein said plurality of antennas are arranged to, when in use, essentially surround said object.

3. The antenna arrangement according to claim 1 or 2 , further comprising a microwave reflection reducing material provided on said front side.

4. The antenna arrangement according to any one of the preceding claims, further comprising an antenna support holding said plurality of antennas, and wherein said flexible antenna support is arranged to be placed around said object, while allowing ends of said antenna support to be fixed in relation to each other.

5. The antenna arrangement according to any one of the preceding claims, wherein said microwave reflection reducing material has a thickness of at least 1 cm, as measured from each of said plurality of antennas.

6. The antenna arrangement according to any one of the preceding claims, wherein a thickness of each of said plurality of antennas, as measured from said front side to said back side, is less than 0.5 mm.

7. The antenna arrangement according to any one of the preceding claims, wherein each of said plurality of antennas is a dipole antenna, a bowtie antenna, a patch antenna or a monopole antenna.

8. The antenna arrangement according to any one of claims 1-7, wherein said energy leakage reducing material and said microwave reflection reducing material is of the same material.

9. The antenna arrangement according to claim 8, wherein each of said antennas are enclosed with said same material.

10. The antenna arrangement according to any one of claims 1-7, wherein said energy leakage reducing material and said microwave reflection reducing material are of different materials.

11. The antenna arrangement according to any one of the preceding claims, wherein said energy leakage reducing material is a material adapted to reduce leakage towards air, and said microwave reflection reducing material is a material with dielectric properties which resemble said object.

12. The antenna arrangement according to any one of the preceding claims, wherein said microwave reflection reducing material comprises a deformable solid material.

13. The antenna arrangement according to claim 12, wherein said microwave reflection reducing material is a material selected from the group consisting of: silicone, any other elastomer, soft plastic, gelatine or any combination of these.

14. The antenna arrangement according to any one of claims 1-11, wherein said microwave reflection reducing material comprises a liquid confined in an elastic bag.

15. The antenna arrangement according to any on of the previous claims, wherein said microwave reflection reducing material is provided with a non-sticky surface on a side intended to face said object.

16. An apparatus for microwave imaging of a biological object, said apparatus comprising: an antenna arrangement according to any one of claims 1 to 15; a microwave transmitter connected to said antenna arrangement; a microwave receiver connected to said antenna arrangement; and a controller arranged to, using said transmitter and said receiver, generate an image of said object.