MICROWAVE IMAGING SYSTEM

Abstract

A microwave imaging system and method are disclosed for generating a 3-D map of a body. The system comprises a source of coherent microwave radiation for irradiating the body, at least one microwave detector for detecting at a plurality of locations around the body the amplitude and phase of radiation that has passed through, or has been reflected by, the body, an analyser connected to receive signals from the or each detector and from the source and operative to produce a holographic image indicative at each detection location the phase of the received radiation relative to the phase of radiation received directly from the source at the same location, and a processor for processing the holographic image to calculate in three dimensions the positions of localized physical parameters within the body.

Claims

1. A method of generating a 3-D map of a body which comprises: irradiating the body with coherent microwave radiation from a source, detecting at a plurality of locations around the body the amplitude and phase of radiation that has passed through, or has been reflected by, the body, determining at each location the phase of the received radiation relative to a reference signal indicative of the phase of the transmitted radiation to produce a holographic image of the body, and processing the holographic image to calculate the positions in three dimensions of localized physical parameters within the body.

2. The method according to claim 1, wherein multiple holographic images are produced using microwave radiation transmitted with different frequencies.

3. The method according to claim 2, wherein the different frequencies are transmitted and received sequentially in time.

4. The method according to claim 1, wherein multiple holographic images are produced using microwave radiation transmitted and received with different polarisations.

5. A microwave imaging system for generating a 3-D map of a body, comprising a source of coherent microwave radiation for irradiating the body, at least one microwave detector for detecting at a plurality of locations around the body the amplitude and phase of radiation that has passed through, or has been reflected by, the body, an analyser connected to receive signals from the or each detector and from the source and operative to produce a holographic image indicative at each detection location the phase of the received radiation relative to the phase of radiation received directly from the source at the same location, and a processor for processing the holographic image to calculate in three dimensions the positions of localized physical parameters within the body.

6. The microwave imaging system according to claim 5, wherein the source of radiation is operative to transmit radiation at several different frequencies.

7. The microwave imaging system according to claim 8, wherein the bandwidth of the transmitted radiation is equal to at least twice the centre frequency to the radiation.

8. The microwave imaging system according to claim 6, wherein frequency of the radiation transmitted by the source is swept.

9. The microwave imaging system according to claim 5, wherein the source and the at least one detector include transmitting and receiving elements, respectively, having different polarisation planes.

10. The microwave imaging system according to claim 5, wherein the source and the at least one detector comprise arrays of transmitting and/or receiving elements located on opposite sides of the body to be imaged.

11. The microwave imaging system according to claim 10, wherein the propagation paths radiation between the transmitting and receiving elements are not all parallel to one another.

12. The microwave imaging system according to claim 10, in which the elements in the arrays are non-equally spaced.

13. The microwave imaging system according to claim 10, wherein the arrays of transmitting and receiving elements are supported on a substrate in the form of a cylinder or tube of regular cross section.

14. The microwave imaging system according to claim 10, wherein the transmitting and receiving elements of the arrays are connected to a master frequency-controlled oscillator and transmission line network.

15. The microwave imaging system according to claim 10, wherein the transmit and receive antenna arrays are movable or switchable between transmit and receiver function.

16. The microwave imaging system according to claim 5, further comprising optical or acoustic devices to enable the surface profile of the body to be measured contemporaneously with the electromagnetic measurements.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The invention will now be described further, by way of example, with reference to the accompanying drawings, in which:

[0033] FIG. 1 shows a multi-slice multi-polarisation capture system configuration,

[0034] FIG. 2 shows a tube carrying antenna arrays disposed around a human body in the process of being imaged,

[0035] FIGS. 3a and 3b are views from above and below, respectively, a tripolar antenna element, and

[0036] FIG. 4 is a schematic diagram of a microwave imaging system of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

System Overview

[0037] The drawings show an imaging system for an object, such as human or animal tissue, providing localized material information in three dimensions which simultaneously illuminates the object with an interrogating electromagnetic wave by digitally recording a radio frequency hologram with multi-polarisation information on features within the addressed volume of interest. The system consists of transmit and receive arrays to provide optimum spatial resolution with a minimum number of elements in the transmit and receiver arrays.

[0038] The transmit array launches a coherent electromagnetic wave which is used to provide the interrogation signal of the object and the receive array is used as a coherent receiver to provide near diffraction limited imaging capability with the aid of suitable digital signal processing. To simultaneously and coherently detect by the receive array the received signal scattered by the target object, a frequency and phase reference signal is taken from the master transmit oscillator and is combined with the received signal at each receiver element to provide amplitude, phase and polarisation information at each receive element location with due regard to the relative transmit element position.

[0039] The transmit and receive elements are positioned in such a way that the planes of the wave vectors occupied by the transmitted and received waves can be configured to record slices that are not parallel. The array configuration is able to provide more complete information on the scattering objects within the interrogated volume from a number of view angles. In this way, the system records a series of radio holograms of the object of interest which is polarisation sensitive and contains multiple view and slice information which is not possible using other imaging modalities.

[0040] Referring first to FIG. 4, the illustrated microwave imaging system comprises a transmit array 10 that is excited by a stable oscillator 11 to which the individual elements of the antenna are connected by way of a signal splitter 14. In this way, the transmit array 10 produces a stable fixed frequency wave that is directed at the body 21 being imaged. The oscillator can be adjusted to work at different frequencies but for clarity, and in any one data gathering operation, only a single frequency is considered.

[0041] The signal splitter 14 additionally takes off a portion of the output of the oscillator 12 to be used as a reference signal that is applied via a transmission line 16 to all the elements of a receiving antenna array 18.

[0042] The output of each of the receiving antenna elements 18 is connected to a respective mixer 20 that combines the signals received by the antenna elements with the reference signal of the oscillator received 12 via the transmission line 16. The output of each mixer 20 is in turn connected to a quadrature splitter 22 to produce signals that represent the real and imaginary components of the scattered signal at each receiver antenna element 18.

[0043] The outputs from the quadrature splitters 22 are next fed to mixers 24 that are connected to an encoder 26. The encoder 26 in one embodiment acts as a multiplexer to enable the signals from the antenna elements 18 to share a common communications bus 28. The processor 30 in such an implementation would be a de-multiplexing processor that performs the conjugate process of the decoder 26 and separates the signals for presentation to a holographic processor 32.

[0044] As an alternative to being designed as a multiplexer, the encoder 26 may be implemented as a spread spectrum processor where the sampled signals are multiplied by a family of pseudo random binary sequences such that all the signals can be transmitted along the same wires and occupy the same frequency band.

[0045] In this case, the processor 30 would be a correlator processor that takes the known spreading sequences and performs a multiplication operation simultaneously on the signal on the common communications bus. This operation is termed code division multiplexing (CDM). The output of each multiplication process is then filtered and the output of each are the recovered signals from 18.

[0046] The holographic processor 32 generates signals indicative of the spatial distribution of the phase and amplitude at the receive site. Following this, a processor 34 processes this data using Fresnel integral based back propagation techniques (16-24) to generate an image of the object at user defined planes within the object which can be non-orthogonal.

[0047] The transmitting and receiving antenna elements may be arranged on a tube 40 surrounding the body in the manner shown in FIG. 2, wherein the position of each antenna element is marked with an X. It will be noted that the antenna element spacing is intentionally non-uniform. By suitably positioning and orienting the tube 40 in relation to the body being imaged it is possible to produce images in the various slice capture planes shown in FIG. 1, the image produced for each slice being derived from radiation polarised along three axes represented by arrows in FIG. 1.

[0048] An embodiment of a three-axis antenna is shown in FIG. 3. The antenna element 50 comprises a dielectric substrate 52 with printed conductors on its opposite sides. On the side shown in FIG. 3a, the printed conductors form two dipoles 54 and 56 that have vertical and horizontal polarisation axes, respectively, in the plane of the drawings. A third antenna having a polarisation axis normal to the plane of the drawing is formed by two loops 58 and 60 disposed on the opposite sides of the substrate 52, the connection to the loop 60 on the underside being established via a plated through hole 62.

[0049] It should be mentioned that it is alternatively possible to use biconical antennae. Furthermore, though three-axis polarisation is preferred, two-axis polarisation may be employed.

[0050] In some embodiments, each individual antenna element on the tube 40 may be switched so that it may selectively serve as a transmitting and a receiving antenna. Alternatively, the tube 40 may be rotatable about the body being imaged.

[0051] In parallel with the processing described above, the system comprises optical or acoustic transmitters and receivers mounted on the tube 10 and connected to a conventional processor to generate a 3D map of the surface of the body being imaged. This allows the processing of the microwave radiation to take into account the distance travelled by the EM waves through air before passing through the body.

[0052] Using a system as described above, it is possible to produce images at multiple frequencies, multiple planes and multiple polarisation states that are combined under user control using image rendering algorithms, taking into account frequency dependent electromagnetic properties of particular tissue types, to give a 3D mapped representation (a hologram) of tissue properties within the imaged volume (12).

[0053] Further processing delivers the ability to render a tomographic presentation which can be rotated and manipulated by the user in the manner of conventional CT and MRI images but with further information on the vector electromagnetic properties of the tissue.

[0054] The spatial resolution is ultimately limited by the highest transmit frequency used in the hologram generation and the wavelength of that frequency within the medium of the object. Ultimately this upper frequency is determined by the overall frequency response of the antenna elements and the other radio components in the system. This limit can be determined at the design stage by the choice of suitable components.

[0055] Radio frequency electronics generally increases in cost as the frequency increases, but there is economic gain to be had by using those particular frequencies which are used in high volume (consumer, automotive and military) applications and widely available and ideally low cost products. The invention therefore sets out to utilize these types of electronics and components where possible in any new device and combined with the attractiveness of very low cost computing power that is available in such products as GPU cards and gaming PCs.

[0056] A further consumer growth area has been in broadcast television and other relatively high frequency communications and radar devices. This has generated a growth in the availability of higher frequency components working in the microwave and millimeter wave range.

[0057] It should be noted that the wavelengths of microwave frequencies (in the range of 6 to 10 GHz) and millimeter wave frequencies within biological tissue is much less than in free space, so the achievable spatial resolution is very much smaller than in free space.

[0058] A draw back reported in the literature has been the absorption of radio waves by tissue. While this is true for a number of frequencies, in coherent systems offer very large dynamic ranges and current available equipment can cope well with dynamic ranges of up to a 100 dBs or so (and beyond). Therefore, coherent approaches offer the ability to work with much lower signal levels, and modern signal processing which can include correlation algorithms known from radio astronomy, offer the prospect of working with extremely lossy materials and poor signal to noise ratios.

[0059] The problem of signal processing can be reduced greatly by using known communications techniques to encode the received signal at each received point so that this reduces the possibility of unwanted signals coupling into the recorded data. One can also reduce the processing costs by down converting the signals coherently to a lower frequency base band.

[0060] Images at multiple frequencies, multiple planes and multiple polarisation states are combined under user control using image rendering algorithms, taking into account frequency dependent electromagnetic properties of particular tissue types, to give a 3D mapped representation (a hologram) of tissue properties within the imaged volume (12).

[0061] Further processing delivers the ability to render a tomographic presentation which can be rotated and manipulated by the user in the manner of conventional CT and MRI images but with further information on the vector electromagnetic properties of the tissue.

[0062] Additional imaging information can be obtained by rotating or changing the disposition of the transmit and receive array either mechanically or electronically.

[0063] The spatial resolution is ultimately limited by the highest transmit frequency used in the hologram generation and the wavelength of that frequency within the medium of the object. Ultimately this upper frequency is determined by the overall frequency response of the antenna elements and the other radio components in the system. This limit can be determined at the design stage by the choice of suitable components.

[0064] The computational methods that need to be employed in processing of the received signals are not described herein but are known in the art and well documented in the published articles and books listed below, all of which are hereby incorporated herein by reference.

[0065] Although various specific implementations have been described, the skilled person will appreciate that the invention may be embodied in many other forms.

[0066] Approximately as employed herein means?10%.

[0067] In the context of this specification comprising is to be interpreted as including.

[0068] Aspects of the invention comprising certain elements are also intended to extend to alternative embodiments consisting or consisting essentially of the relevant elements.

[0069] Where technically appropriate, embodiments of the invention may be combined.

[0070] Embodiments are described herein as comprising certain features/elements. The disclosure also extends to separate embodiments consisting or consisting essentially of said features/elements.

[0071] Technical references such as patents and applications are incorporated herein by reference.

[0072] Any embodiments specifically and explicitly recited herein may form the basis of a disclaimer either alone or in combination with one or more further embodiments.

REFERENCES

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