A SPACED PLATE WAVEGUIDE PROBE AND METHOD FOR DIELECTRIC MEASUREMENTS

Technical Field This invention relates to a spaced-plate waveguide probe for dielectric measurements and a method of determination of dielectric measurements, including dielectric contrasts; and refers particularly, though not exclusively, to such a probe and method where the spaced-plate probe has two substantially parallel plates.

Background

Each and every material has its own set of electrical characteristics related to its dielectric properties. Knowing these properties enables the use of appropriate materials for intended applications. Information on the dielectric properties is critical to many electrical and electronic applications. For example, the impedance of a substrate, or the electrical loss in an insulated cable, depends strongly on the dielectric properties. Recently, applications in the area of biomedical diagnostics, such as the detection of cancer, have been found to require data of dielectric properties of different tissues and organs, and the contrasts between the dielectric properties of different organs.

Apart from measurement of material characterization, at times it is necessary to determine the constitutive parameters of building materials such as glass window panes, wood, concrete, brick, and so forth. This is becoming increasingly important in modern wireless communications due to the high demands of indoor communication.

Different measurement techniques have been developed to measure dielectric properties. Many factors have to be considered when choosing the appropriate technique to obtain the desired information on the dielectric properties. Some of these factors include: the frequency range, required measurement accuracy, sample size, surface topology, state of the material (liquid, solid, powder, and so forth), destructive or non-destructive tests, and contacting or non-contacting tests.

Breast cancer is the second leading cause of cancer deaths today, and is the second most common form of cancer found among women, following skin cancer. The widely accepted method of breast cancer detection, mammography, requires medical expertise to accurately diagnose the presence of tumour from the measured data and the resultant images generated from the measured data, as the number of cancers found with mammography alone is very much less than that found with both mammography and physical examination. Mammography based on density differences between normal tissues and lesions. However, limitations of mammography include the false negative rates of up to 34% and average false positive rate about 11%. The resultant negative effect of the false positive rate includes the increased healthcare cost, particularly when after a costly and invasive biopsy the suspicious lump may turn out to be a benign lesion. Another consequence is the high emotional distress and anxiety of women who receive false positive mammograms. Other important concerns also include the discomfort suffered by the female due to breast compression during the mammography, and health-related issues such as ionizing radiation exposure. Therefore there is a need for new diagnostic technique for breast cancer.

At present, although there have been intensive studies on breast cancer detection using various microwave techniques, many are still in the stage of conceptualization. Only a few of the proposed techniques have been used in clinical trials. Furthermore, commercial products are still unavailable.

Skin cancer is the most common form of cancer. The number of malignant skin cancers has been on the rise. Currently, the most precise diagnosis of skin cancer is by a biopsy then diagnosis. The patient must undergo an invasive procedure, and will suffer pain. In some circumstances, it might take several samples before it is certain whether the abnormal growth is malignant. There exists a potential risk of the cancerous cells spreading. Total removal of the lesion is necessary. Hence, apart from breast cancer detection, the absence of an accurate, sensitive detection method for skin cancer leads to a demand for a new promising diagnostic technique.

In several areas of interest where it is not permissible to destroy any part of the material under test, in particular the case of biomedical microwave diagnostics, non-destructive dielectric measurement can use only a free-space transmission, an open-ended coaxial line, or an open-ended waveguide.

The free-space transmission method uses two spot-focusing antennas operating in the far-field region with the material under test ("MUT") placed between the antennas. Measurements of the transmitted and reflected signals are performed. This allows calculation of the complex permittivity and permeability to be made.

An open-ended coaxial line has a truncated section of transmission line, opening onto a ground plane. Measurements are made by placing the probe in electrical contact with the MUT. By measuring the admittance, or the reflected signal, the complex permittivity of the MUT can be calculated.

An open-ended waveguide is similar to an open-ended coaxial line. Measurement of the reflected signal is performed by touching the aperture of the waveguide to the surface of the MUT. Complex permittivity is then calculated from the amplitude and phase of the reflected signal.

However, waveguides present frequency limitations. Practical waveguides normally operate at frequencies of more than IGHz. In general, the useful frequency range or bandwidth is limited to two times the lowest cut-off frequency. For example, this may be from 1 GHz to 2 GHz, 2 GHz to 4 GHz, and so forth. Furthermore, there exist errors due to diffraction effects at the sample edges and multiple residual reflections between the antennas.

The most widely used measurement technique in dielectric spectroscopy is the open- ended coaxial line due to its simplicity and accuracy in performing broadband, non- destructive measurements. Currently, the use of this technique has gained significant attention in the measurement of biological tissues. However, it has a number of problems:

1. The coaxial line is recommended for lossy MUT measurements. Low loss MUT measurements give rise to limited complex permittivity accuracy; and

2. The coaxial line is recommended for measuring liquids or semi-solids. The MUT is assumed to have single, smooth and flat surfaces. No air gap should exist between the probe and the MUT.

However, such surface flatness and uniformity is difficult to achieve. Moreover it is very difficult to have good electrical contact. Hence the coaxial line is not suitable for measuring the complex permittivity of solids.

Also the probe is not recommended for use in complex permittivity measurements where the complex permittivity of the MUT (ε' _r ) are: i. for ε ' _r <5, minimum loss tangent must be greater than 0.05 (tan δ > 0.05); and ii. for ε' _r > 5, low loss materials are not recommended, i.e. loss tangent must be greater than 0.5 (tan δ > 0.5)

Summary

According to an exemplary aspect there is provided a spaced-plate waveguide probe for dielectric measurement of a material under test. The probe has a first plate and a second plate spaced from the first plate. There is a first strip of a first dielectric material extending between the first plate and the second plate at first side of the first plate and the second plate. A second strip of a second dielectric material extends between the first plate and the second plate at a second side of the first plate and the second plate. A third strip of a third dielectric material extends between the first plate and the second plate between the first strip and the second strip.

Preferably, the three dielectric materials are to be low loss or lossless materials. More preferably, the loss tangent is less than 1 (tan δ «1) for low loss materials; and is a zero loss tangent (tan δ =0) for lossless materials. The first dielectric material and the second dielectric material may be the same; and the first strip and the second strip may be identical. The third strip may have a real part of the complex permittivity greater than the real part of the complex permittivity of the first strip and the second strip. The third strip may be of a width less than one-half of a guide wavelength of a signal to be

processed by the spaced-plate waveguide probe. There may be a short circuit plate joining the first plate and the second plate at a first end of the first plate and the second plate. The first strip, second strip and third strip may all extend from the short circuit plate to a second end of both the first plate and the second plate, the second end being opposite the first end. The third strip may be a hollow space configured to be filled with a material with known properties, preferably those having similar properties as a material under test. The similar properties may include complex permittivity.

The first plate and the second plate may be parallel. The first plate and the second plate may both be substantially flat, and may be of the same size and shape.

The first plate may have a first flange extending outwardly therefrom at the second end of the first plate. The second plate may have a second flange extending outwardly therefrom at the second end of the second plate. The first flange may be perpendicular to the first plate, and the second flange may be perpendicular to the second plate. The second flange may extend oppositely to the first flange.

The spaced-plate waveguide probe may have a source of microwave radiation. The short circuit plate may be a quarter-guide wavelength from the source. The source may comprise a coaxial probe configured to be an excitation for the spaced-plate waveguide probe.

The spaced-plate waveguide probe may have a first spacer on the first flange and a second spacer on the second flange to space the spaced-plate waveguide probe from the material under test. The material under test may be one of: tissue, human tissue, breast tissue, human breast tissue, a layered media, a layered media backed by air, a layered media backed by a conducting sheet, a layered media backed by an absorber, a single layer dielectric material backed by air, single layer dielectric material backed by conducting sheet and single layer dielectric material backed by an absorber.

The third strip may comprise a plurality of strips of different dielectric constants; and at least one of the first strip and the second strip may comprise a plurality of strips of different dielectric constants.

According to another exemplary aspect there is provided a method for obtaining data relating to a size and position of a tumour in tissue. The method includes placing a spaced-plate waveguide probe against a surface of the tissue and transmitting a signal into the tissue using the spaced-plate waveguide probe. A resonant backscattered signal from the tumour is received at the spaced-plate waveguide probe. A frequency of the resonant backscattered signal is used to provide data of the size of the tumour. A phase shift in the resonant backscattered signal is used to determine a depth of the tumour in the tissue

Preferably, a frequency scan of the resonant backscattered signal amplitudes is used to provide the data of the size of the tumour. More preferably, the frequency scan of the resonant backscattered signal amplitudes is used to provide data of the location of the tumour. Furthermore, the data of the size and location of the tumour is provided after accounting for backscattered signals from the tissue. Advantageously, a frequency scan of the resonant backscattered signal zero crossings in the phase is used to provide additional data of the location of the tumour in the tissue. More advantageously, this is after accounting for the backscattered signals from the tissue.

Zero crossing frequencies of the phase of the resonant backscattered signal may be used to determine a location of the tumour in the tissue. Frequencies of a resonant scattered signal may also be used to provide the data of the size of the tumour. The resonant backscattered signal and the resonant scattered signal and their magnitudes may be used to differentiate between the tumour and clutter.

The spaced-plate waveguide probe may be at least partially in a phantom material with known properties. Preferably the known properties resemble those of the tissue. The tissue may be immersed in the phantom material. The tissue may be human breast tissue.

The spaced-plate waveguide probe may be for the transmitting, and for receiving the resonant backscattered signal; and at least one second spaced-plate waveguide probe may be used for receiving the resonant scattered signal. The at least one second spaced- plate waveguide probe may be at least one element of an array of spaced-plate waveguide probes. The at least one array may be used for enhancing the accuracy in the location of the tumour by using triangulation techniques.

A vector network analyzer may be used to provide the data of the size of the tumour and the data of the location of the tumour; and to determine the data to differentiate between the presence of the tumour or clutter. The resonant backscattered signal may be received at an aperture plane of the transmitting spaced-plate waveguide probe. The resonant scattered signal may be received at an aperture plane of the at least one second spaced- plate waveguide probe. The resonant backscattered and scattered signals may be received in the form of a reflection coefficient and transmission coefficient respectively. The reflection and transmission coefficients may vary between the transmitting spaced- plate waveguide probe and the at least one second spaced-plate waveguide probe in accordance with the presence or absence of a tumour, the size of the tumour and the location of the tumour.

A plurality of measurements may be made at different positions of the tissue.

Differences in the reflection coefficients and/or the transmission coefficients obtained by the plurality of measurements data may be used to determine the presence, size and location of the tumour. The transmitted signal may be over a range of frequencies. The range of frequencies may be IGHz to 10GHz. The spaced-plate waveguide probe may be as described above.

According to a further exemplary aspect there is provided a method for determining the complex permittivity of a material under test by use of the spaced-plate waveguide probe described above. The method includes spacing the spaced-plate waveguide probe from the material under test by a first distance, and taking a measurement of the complex permittivity of the material under test. The spaced-plate waveguide probe is spaced from the material under test by a second distance different to the first distance,

and a measurement of the complex permittivity of the material under test is taken. The spaced-plate waveguide probe is placed on the material under test, and a measurement of the complex permittivity of the material under test is taken.

Spacing may be by use of spacers on the spaced-plate waveguide probe. The spacers may be on first and second flanges of the spaced-plate waveguide probe.

Brief Description of the Drawings

In order that the invention can be fully understood and readily put into practical effect, these shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings.

In the drawings: Figure 1 is a top perspective view from the front of an exemplary embodiment;

Figure 2 is a top view of the embodiment of Figure 1 ;

Figure 3 is a front view of the embodiment of Figure 1;

Figure 4 is a side view of the embodiment of Figure 1;

Figure 5 is a front view of the embodiment of Figure 1 with spacers for air gap calibration;

Figure 6 is a side view corresponding to Figure 5;

Figure 7 is a graph of the electric field distribution across the three dielectric strips of the embodiment of Figure 1;

Figure 8 is a vertical cross- section of a portion of the embodiment of Figure 1 radiating directly into a layered media;

Figure 9 is a view corresponding to Figure 6 where the layered media terminates into infinite space;

Figure 10 is a view corresponding to Figure 6 where the layered media terminates into a highly conductive sheet; Figure 11 is a view corresponding to Figure 6 where the layered media terminates into an absorber;

Figure 12 is a vertical cross-sectional view of the exemplary embodiment of Figure 1 radiating into breast tissue illuminating a small tumour;

Figure 13 is a vertical cross-sectional view of one exemplary embodiment of Figure 1 radiating into breast tissue illuminating a small tumour with a second exemplary embodiment of Figure 1 receiving the backscattered signal;

Figure 14 is a is a horizontal cross-sectional view corresponding to Figure 12;

Figure 15 is a view corresponding to Figure 14 with the exemplary embodiment in various positions;

Figure 16 are two graphs of the magnitude and phase of the differences in the reflection coefficients detected at positions 1 and 2 of Figure 15;

Figure 17 are two graphs of the magnitude and phase of the differences in the reflection coefficients detected at positions 1 and 3 of Figure 15;

Figure 18 are two graphs of the magnitude and phase of the differences in the reflection coefficients detected at positions 2 and 3 of Figure 15; Figure 19 is a graph showing the magnitude of the differences in reflection coefficients at positions 1 and 2 of Figure 15, in the presence of a tumour at a depth of 1.5cm, for five different tumour sizes;

Figure 20 is a graph showing the phase of the differences in reflection coefficients at positions 1 and 2 of Figure 15, in the presence of a tumour at a depth of 1.5cm, for five different tumour sizes;

Figure 21 is a graph corresponding to Figure 19 where the tumour is at a depth of 2cm;

Figure 22 is a graph corresponding to Figure 20 where the tumour is at a depth of 2cm;

Figure 23 is a graph of the skin depth vs frequency in breast tissue;

Figure 24 is an illustration of two arrays of the exemplary embodiment of Figure 1 one of which radiates into breast tissue, illuminating a small tumour;

Figure 25 is two graphs of the magnitude and phase of the scattered signal from the tumour detected by a receiving probe in the array of Figure 24;

Figure 26 is two graphs of the magnitude and phase of the scattered signal from clutter detected by a receiving probe in the array of Figure 24; Figure 27 is a side view illustrating a variation of the exemplary embodiment of Figure

1;

Figure 28 is a side view of the embodiment of Figure 1 with excitation using a coaxial line, or a pair of conductors or microstrip configuration, in which one conductor is connected to the first plate and the other conductor is connected to the second plate of the spaced-plate waveguide probe; and Figure 29 is an illustration corresponding to Figure 1 with a centre slot.

Detailed Description of the Exemplary Embodiments

The spaced-plate waveguide probe 10 illustrated in Figures 1 to 6 has two parallel plates - a top plate 12 and bottom plate 14, As shown in Figures 1 to 6 the two plates 12 and 14 are generally parallel. However, as shown in Figure 27, there may be a small and gradual change (increasing, as shown) in spacing from their inner ends 16 to their outer ends 20 due to the included angle a at the inner ends 16.

The spacing between the plates 12, 14 is preferably such that only the dominant mode can propagate in the spaced-plate waveguide probe 10. In general, such a device has many cut-off frequencies. The dominant mode means the operating frequency above the lowest cut-off frequency but below the next cut-off frequency. The plates 12, 14 need not be flat as long as the dominant mode propagates in the spaced-plate waveguide probe 10. The two plates 12, 14 are generally identical and may be of any suitable size and shape, including rectangular (as shown). At (including adjacent) their inner ends 16 the two plates 12, 14 are joined by a short circuit plate 18 extending for the full width of the plates 12, 14.

At the outer end 20 each plate 12, 14 has a flange 22, 24 respectively extending for the full width of the plates 12, 14 and extending generally perpendicularly to the plates 12, 14. Flange 22 extends generally upwardly relative to plate 12, and flange 24 extends generally downwardly relative to the plate 14. The flanges 22, 24 extend in opposite directions from their respective plates 12, 14. The flanges 22, 24 enhance and shape the radiation into the MUT.

The spaced-plate waveguide probe 10 is therefore similar to a cut-off section of a transmission line terminated by the flanges 22, 24, with dominant mode waves

propagating. The plates 12, 14 as well as the flanges 22, 24 and short-circuit plate 16, are preferably of an electrically-conductive metal.

In order to reduce radiation leakage and confine the fields to a smaller region, two longitudinally-extending dielectric strips 26 are embedded between and join the two plates 12, 14, preferably for their complete length, and leaving a central, hollow space 28 between them. The strips 26 are preferably rectangular in all cross-sections (i.e. are cuboid). More preferably, they are at (including adjacent) the sides of the plates 12, 14. Preferably, the three dielectric materials are to be low loss or lossless materials, i.e. they have a loss tangent much less than 1 (tan δ «1) for a low loss material; and a zero loss tangent for a lossless material. The hollow space 28 can be filled with any low loss or lossless dielectric material with complex permittivity εc as long as the two side strips 26 have a real part of the complex permittivity εs that is much less than that of the central dielectric material 30. As such, the third strip 30 can be of any dielectric material, preferably a low loss or lossless material; as long as the real part of the complex permittivity is very much larger than that of the first and second strips 26 to maintain the energy confinement to centre third strip 30. Absorbers can also be selected for the two dielectric strips 26 to resemble a spaced-plate waveguide of infinite extent. When absorbers are used as the side strips 26, the centre strip 30 can take any dielectric value, and any dielectric material can be selected to fill the centre hollow space 28, e.g. Teflon, air field, and so forth.

Also, the centre strip 30 may be of a material with a graded dielectric constant comprising of a plurality of adjacent layers; with the layers being arranged in the order of decreasing (or increasing) dielectric constants from the centre layer to the layers adjacent to the first and second strips 26. For example, if the dielectric constant in the centre layer of the graded centre strip 30 has a value of 10, the dielectric constant of the adjacent layers to the centre layer may be 9.5 and may gradually decrease to a dielectric constant of 8 in the layers adjacent to the first and second strips 26. This may also apply to each of the side strips 26.

8 000475

12

The electric field at the outer end 20 aperture 32 will be confined to the area of the central dielectric material 30 (Figure 7) if the two side strips 26 have a real part of the complex (or real) permittivity εs that is much less than the real part of the complex permittivity εc of the central dielectric material 30. Depending on the properties of the MUT, a suitable dielectric material can be selected for the two side strips 26. In general, the smaller the value of real part of εs compared to that of εc, the better the confinement of energy in the central dielectric. The two side strips 26 are preferably identical, although they may be different.

The width of the central hollow space 28 is generally preferred to be less than one-half wavelength in the guide, λg/2. For a given value of the spacing between the plates 12, 14, the width is such that it avoids exciting the undesirable higher order modes, e.g. TEn, TE2 ₀ , and so forth. If necessary, a centre slot or plurality of slots 33 similar to those used in slotted transmission lines may be used to attenuate these higher order modes without significantly affecting the dominant mode (Figure 29). A coaxial probe 31 is used as the excitation (and as receiver to measure the backscattered or scattered signal) for the spaced-plate probe 10. The short circuit plate 16 is at a distance from the coaxial probe 31 that is approximately a quarter-guide wavelength λg/4. As shown in Figure 29 any other matching network can be used, particularly one that is not restricted to a quarter-wavelength away from the coaxial probe. In Figure 29 two screw waveguide tuners 2950 are used. Single or multiple screw waveguide tuners may be used for matching the spaced-plate probe over the range of frequencies. Another embodiment of the excitation mechanism (Figure 28) uses a coaxial line or a pair of conductors or microstrip configuration, in which one conductor is connected to the top plate 12 and the other conductor is connected to the bottom ρlatel4 of the spaced-plate waveguide probe 10.

The two dielectric strips 26 are of the same complex permittivity εs. Preferably the real part of the complex permittivity of the central material 30 ε _c is of a factor higher. For example, εc may be 10, with εs being 1. This is to maintain the condition that the side strips 26 have a real part of the complex permittivity εs much less than the real part for the complex permittivity εc of the central dielectric strip 30; or conversely, the central

dielectric strip 30 has a real part of the complex permittivity εc that is much greater than the real part of the complex permittivity εs of the side strips 26.

Dielectric measurements are performed by placing the aperture 32 onto the MUT. The received backscattering signal by the transmitting probe is Sl 1. The received scattered signal by the other receiving probe(s) at the other port(s) is S21. The backscattered signal Sl 1 is measured by a vector network analyzer 40. In measuring the backscattered signal Sl 1, a coaxial cable 42 is connected from a port 44 of the vector network analyzer 40 to the coaxial probe 31. However, any apparatus that can measure the scattering parameters SI l and S21 can be used. Any port of the vector network analyzer 40 can be used for both transmitting and receiving. For example, in Figure 13 port 44 (with transmitting probe connected) is used for transmitting signal to the MUT and receiving SI l from the MUT, port 46 (with receiving probe connected) is used for receiving S21 from the MUT .

An example of a vector network analyzer 40 is the Agilent E8362B 10MHZ-20GHZ Vector Network Analyzer ("VNA").

The complex permittivity of the MUT can be derived using known methodologies. The MUT is assumed to be linear and isotropic, with known magnetic permeability μ.

Therefore, the unknown complex permittivity of the MUT, which has both the real and imaginary components, can be determined using the measured magnitude and phase of the backscattered signal SI l and/or scattered signal S21. Any errors due to random noise in the system may be minimised by averaging over a number of measurements.

The vector network analyzer 40 is used to measure the magnitudes and phases of the reflected/backscattered signals and transmitted/scattered signals respectively. To measure the backscattered signal SI l at the aperture 32 of the transmitting spaced-plate waveguide probe 10, the port 44 of the VNA 40 is connected to the coaxial probe 31 via a coaxial cable 42. Similarly, to measure the scattered signal S21 received by the receiver probe 31, the port 46 of the VNA 40 is connected to the coaxial probe 31 of the receiving probe 31 (Figure 13).

An exemplary form of the spaced-plate dielectric probe 10 has been found to be able to:

1. measure complex permittivity for. a solid material with variations in surface roughness. With surface finish lapped to only ±lOOOμ inches (approximately ±2.5x10 ^'2 mm), similar accuracy to known devices has been achieved with a less stringent requirement for surface roughness; and

2. measure low loss dielectrics with ε _t < 5 without compromising accuracy.

Measuring a multilayered structure may be possible using the spaced-plate dielectric probe 10. The fields emanating from the spaced-plate dielectric probe 10 may be able to penetrate a relatively longer distance compared to that of a coaxial probe. In the case of the spaced-plate waveguide probe 10, the energy radiates away from the aperture while in the case of the coaxial probe, the energy is mostly stored in the vicinity of the aperture. This makes the spaced-plate dielectric probe 10 useful for measuring multilayered structures with a substantial thickness in each layer. The materials of each layer are assumed to be linear and isotropic, with known magnetic permeability μ. To determine the complex permittivity of the material of each layer, the thicknesses of the individual layers are assumed to be known. There is no limitation to the number of layers. Assume a MUT of N layers, there would be 2N unknowns which can be solved by using N measured SIl at N adjacent frequency points; on the assumption that both the ε and ε of the complex permittivities of the materials do not vary significantly over the N adjacent frequency points.

Furthermore, air gap variations between the aperture and the MUT (replace εi with εo) can be used as a form of calibration, and may provide a more precise reading. For example, two different depths of the air gap may be used for each measurement. By this, measurements of the backscattered signal Sl 1 are taken for air gaps of lmm and 5mm respectively. As shown in Figures 5 and 6, when measuring the complex permittivity of solids, spacers 34 of small relative thickness such as, for example, lmm and 5mm can be successively attached to the flanges 22, 24 to ensure air gaps of lmm and 5mm between the aperture 32 and the MUT. Following the calibration described above, the complex permittivity of the MUT can be obtained by having the probe 10 on the MUT.

In addition, at each operating frequency, two or more measurements may be taken for better accuracy of the complex permittivity. As shown in Figure 8, the MUT is a layered media 800 having N layers 802, 804 .... 8ON. In Figure 9, the MUT 900 terminates into infinite half space, i.e. the sample material is backed by air 910 with a permittivity of εo. In Figure 10, the MUT 1000 is terminated into a perfect or good conducting sheet 1010 where σ = ∞ or σ/coε»l respectively. And in Figure 11, the MUT 1100 is terminated into an absorber 1110. In general, the more terminations used means that more information is obtained. This may give more accurate results. The additional information is used to minimise errors when determining the complex permittivity of the MUT.

However, in general, any standard load can be used to serve as a termination for the MUT.

In Figure 12 is shown the obtaining of data for the detection of a tumour 1202 in surrounding tissue 1200. It is possible to detect the dielectric contrast between the tumour 1202 and its surrounding tissue 1200 as the tumour 1202 will exhibit a dielectric constant different to that of the healthy, surrounding tissue 1200. By measuring the complex admittance, or the backscattered signal SIl, and the scattered signal S21 using the vector network analyzer 40, the data indicating the presence and location of the tumour 1202 can be reliably and accurately provided.

Measurements are performed at different microwave frequencies by placing the aperture 32 onto the tissue 1200. The backscattered signal Sl 1 for each frequency is received in the form of a reflection coefficient, F, at the aperture plane 32. This reflection coefficient, a complex number, is compared with that for the case of when only healthy tissue is present. The presence of a small tumour 1202 will alter the reflection coefficient; the larger the contrast in the dielectric properties between a healthy tissue and a tumour, the larger is the contrast in the reflection coefficient measured.

Frequency response is used to determine the presence, location and size of the tumour 1202. The dielectric properties of the tumour 1202 and tissue 1200 are assumed to take

the values given in the Debye Parameters Table. With this knowledge, the presence, location and size of the tumour 1202 can be determined.

This technique is based on the Mie scattering of a dielectric body. The spaced-plate probe 10 acts as the source of excitation. The small scatterer, in this case the tumour 1202, will scatter the incident field 1208. The backscattered field Sl 1 is used to determine the reflection coefficient T at the aperture plane 32. A tumour 1202 at different locations will result in different values of the reflection coefficient. Thus knowledge of the position of the tumour 1202 can be derived from calibration measurements of known geometries of such tumours 1202 or other scatterers.

The waveguide probe 10 is preferably in a phantom material 1210. Preferably, the phantom material 1210 has known properties. More preferably, those known properties resemble the tissue 1200 to minimize scattering which can arise due to dielectric contrast in different mediums. For the particular application in human breast cancer detection, the entire human breast is preferable to be immersed in the phantom material 1210. The phantom material may be, for example, Speag Body Simulating Liquid MSL 2450MHz.

Measurements at different positions of the tissue 1200 are made and the backscattered signals Sl 1 are compared systematically. In the presence of a tumour 1202, the backscattered signals SIl will have a resonating behaviour due to the Mie scattering from the tumour 1202 at resonant conditions, whereas the scattered signal S21 from a healthy tissue will not have the resonating behaviour. For example, in an array of N spaced-plate waveguide probes 10, there can be ^N C ₂ pairs of differences in backscattered signals SI l measured by the N probes. The difference in each pair δF _V is the backscattered signal from the tumour 1202 (if any). Therefore if there is no resonating response in the difference, then there is no tumour at the positions where the two probes 10 are placed. These two positions will be marked as no tumour present. If there is a resonating response, then one of the signals measured by the probes contains the backscattered signal Sl 1 from the tumour 1202. By eliminating the positions with

no tumour 1202, the final position of the tumour 1202 can thus be found. The above measurement and calibration procedures may also be done for scattered signals S21.

Figure 13 shows where two probes 10 are used - one probe lOt being for transmitting, and the other probe 1Or being for receiving. The transmitted signal 1308 is scattered by the tumour 1302. This causes scattered signal S21 to be received by the receiving probe 1Or. The transmitting probe 1Ot is connected to the first port 44 of the vector network analyzer 40 by a coaxial cable 42t whereas the receiving probe 1 Or is connected to the second port 46 of the vector network analyzer 40 by a second coaxial cable 42r. By analyzing the scattered signal 1306 as received by receiving probe 1Or, the size and location of the rumour 1302 can be determined. The accuracy for determining the size and location of the tumour can be enhanced by correlating the Sl 1 and S21 measurements.

In Figure 14 there is shown the use of the probe 10 for the providing of data to enable the detection of a tumour 1402 in a breast 1400. The spaced-plate probe 10 provides clear markers for any malignant tissue present in the breast 1400 by reference to tumour signatures in terms of the resonant behaviour in the frequency responses. In the presence of a tumour 1402, the backscattered signal SI l and scattered signal S21 will have a resonating behaviour due to the scattering from the tumour 1402, whereas the backscattered signal SIl and scattered signal S21 from a healthy tissue 1400 will not have the resonating behaviour. This may provide data that will enable reduced errors in diagnosis.

A major contribution of error using the radar-based method is the distortion in the scattered pulse received. Since the tissue medium is frequency dispersive, the pulse sent and received will differ significantly. Performing phase correlations to determine the presence and position of the tumour is can give rise to errors. The spaced-plate probe 10 may send a continuous wave over the range of swept frequencies. Analysis of the frequency response is then performed. This improves accuracy.

The patient need not to suffer from pain when undergoing the test performed using the spaced-plate probe 10, unlike the discomfort the patient has to endure when the breast are compressed when taking mammograms.

A simulation of a proposed methodology is illustrated in Figure 15. There is a skin layer 1520 having a thickness of 2mm, and a spherical tumour 1502 embedded in the breast tissue 1500 at a depth of 2cm from the skin layer. The breast 1500 and/or the probe 10 are in the phantom material 1510. Any reflections or scattering from the interface of the phantom material 1510 and skin 1520; as well as the interface of the skin 1520 and the breast 1500, are able to be ignored as a known subtraction method can be applied to remove the reflected waves from the interfaces, therefore the effects of these interfaces are ignored in the simulation. The system has time gating capabilities. The first reflections or scattering from the skin-breast interfaces can thus be removed. The dielectric properties of the average normal breast tissue and malignant tumour are given in the following table where a first order Debye dispersion equation is employed to model the frequency dependence of the dielectric properties.

∑(ω) ε _δ - ε _∞ σ _δ

Debye Equation: ε _r (ω) -J ε _σ + ωεo 1 +j(£>τ ωε ₀

Debye Parameters Table:-

Clutter sources 1522, representing the tissue heterogeneity, with dielectric properties of variation of +30% that of the normal tissue are included in the breast model used.

A number of measurements are taken using the probes 10. By comparing the differences in the reflection coefficients and transmission coefficients obtained in the various measurements, the presence of any tumour can be detected. As an illustration, assuming three backscattered signals are received, given as:-

51- Reflection coefficient detected at position 1 (with 7mm diameter tumour 1502);

52- Reflection coefficient detected at position 2 (normal tissue 1500);

53- Reflection coefficient detected at position 3 (with 7mm diameter clutter 1522).

The tumour 1502 is in the vicinity of the probe 10(1) at position 1, whereas the probes 10(2) and 10(3) in the other positions 2 and 3 are near normal breast tissue 1500 and clutter 1522 respectively.

Three readings available will result in three ( ³ C ₂ ) combinations of differences: between Sl and S2; between S2 and S3 and between Sl -S3. Three δF _V are plotted as shown in Figures 16 to 18.

The observations from Figures 16 to 18 can be summarized as follows:

In Figure 16, the plots of the magnitude of the differences in the reflection coefficients Sl and S2 display two resonances at the frequencies 3.9GHz and 6GHz. Similar observations are achieved from Figure 17, which shows the differences Sl -S3. In both cases, Sl, the backscattered signal containing the information of the tumour 1502, is involved. However, in Figure 18 where none of the backscattered signals S2 and S3

contain backscattered field from the tumour 1502, it is noted that the amplitude of δr _v at resonance is found to be about 4.5 times lower than the amplitude of δr _v with the tumour in Figure 16. In terms of power, the difference in the magnitude of δr _v at resonance gives a factor of about 20 times. This factor is useful for distinguishing the presence of a tumour or clutter of the same size. In general, larger scatterers (tumour or clutter) results in lower resonant frequencies, f _r , and in larger scattered power. Furthermore, the larger the dielectric contrast between the scatterer (tumour or clutter) and the surrounding healthy tissue, the larger is the scattered power. Since tumours have much higher dielectric contrast than clutter, this can be used as an additional tool to differentiate between the two.

Tumours 1502 of different sizes will resonate at different frequencies. Therefore the size of the tumour 1502 can be determined from the resonant frequency read from the plot of the magnitude of the difference in the reflection coefficients, one of which may contain the backscattered signal from the tumour 1502. Once the size of the tumour is known, the depth of location of the tumour can be determined from the amplitudes of δF _V at resonant frequencies as the total signal path loss from the spaced-plate probe to the tumour and backscattered to the spaced-plate probe 10 for Sl 1 depends on the tumour's location. The same method applies to the scattered signal S21 for the case where two or more spaced-plate probes 10 are used in a triangulation procedure; In addition, the frequencies at which the phase of δF _V undergoes zero crossing provides further information on the location of the tumour 1502 embedded in the breast 1500.

This concept is illustrated by the following simulation example. Tumour diameters vary from 3mm to 7mm. In Figures 19 and 20, the tumour is embedded at a depth of 1.5cm; whereas in Figures 21 and 22, the tumour is embedded at a depth of 2cm. In each case the graphs cover tumours 1502 in the range 3mm to 7mm in diameter.

It is clear from Figures 19 and 21 that the resonant frequency is determined by the size of the tumour: the smaller the tumour, the higher the resonant frequency. The location of the tumour can be determined from the amplitude of δF _V at resonant frequencies: the larger the depth of the tumour of the same size, the smaller the amplitude of δF _V at

resonant frequencies. From Figure 20, the zero crossings of the phase of δF _V occur at around 3GHz and 6GHz but in Figure 22, these occur at around 2.2GHz and 4.4GHz. This demonstrates that the frequencies at which the zero crossings occur provides additional information to determine the location of the tumour.

Figure 23 shows the skin depth in the breast tissue. The skin depth is in the order of a few centimetres in the frequency range of IGHz to lOGHz. This is sufficient for the signal to penetrate the skin 1520 and tissue 1500 to reach the tumour 1502 and be scattered back to the probes 10. The skin depth is also small enough to prevent unwanted scattered signals from bones and internal organs because such signals are strongly attenuated due to increased path lengths in terms of the skin depth. Hence the spaced-plate waveguide probe 10 has a spacing between the plates 12, 14 and the two side dielectrics 26, 28 to allow the dominant mode waves (in the frequency range of 1 to 10 GHz) to propagate in the waveguide probe 10.

As shown in Figure 24, the detection principle can be extended to having two or more probes 10 in one or more arrays 50. Preferably, all the probes 1Ot and 1Or are identical. Apart from backscattered signal SI l, additional information of the scattered signal S21 can be obtained to further enhance the accuracy in determining the presence and location of the tumour 2402. Each time, preferably only one probe 1 Ot transmits, while the rest of the probes 1Or in the arrays 50 receive. For example, if there are N probes in the array, there will be only one backscattered signal SI l and (N-I) scattered signals S21 received. The use of arrays 50 enables the use of triangulation techniques to enhance the accuracy in the location of the tumour.

In Figure 24 is shown the scattered signal 3001 detected by a receiving probe 1Or at some distance from the transmitting probe 1Ot. The frequency responses in Figure 25 show that the scattered signal S21 by the tumour resonates at approximately 3.9GHz and 6GHz, with zero crossings at around 2.9GHz and 6.5GHz. However in Figure 26, the scattered signal S21 by clutter 2422 does not show any clearly defined resonant behaviour. Also the magnitude of the signal scattered by clutter 2422 is much smaller

than that from a tumour 2402. Combining both sets of data, SIl and S21 provides more accurate information about the presence, size and location of a tumour 2402.

Alternatively, the frequency response profiles can be obtained using a swept frequency technique. The organ 2400 is excited over a wide range of frequencies. The amplitude and phase of the received signal as a function of frequency is measured. The inverse Fourier transform then yields the time domain representation of the response. According to Fourier Transform, an RF pulse can be approximated by a summation of N frequency components. In this swept frequency technique, instead of transmitting a pulse, N frequency components are transmitted. Assuming the medium is linear and time- invariant, the response would be the same.

As shown in Figure 28 it is possible to connect the coaxial probe 31 to the inner end 16 of the spaced-plate waveguide 10. This may be, for example, by connecting the centre signal conductor of the coaxial probe 31 to the top plate 12, and the ground of the coaxial probe 31 to the second plate 14. This type of excitation is similar to that used in microstrip line. In this case the short circuit plate 18 may be replaced by a mounting for the coaxial probe 31.

The flanges 22, 24 may be curved to approximate the shape of the breast, other tissue, or other material under test. When in the array 50, one probe 10 may be used to transmit progressively throughout the arrays so that all probes 10 in their turn will transmit. The transmit cycle could be computer controlled so that the probes 10 are used for transmission in a logical sequence. The probes 10 in an array 50 may be held by a common housing. The common housing may be shaped to fit a breast, other tissue, or other material under test.

In this way an apparatus may be applied to the material under test, connected and switched on. AU data is automatically collected. This may be for a breast, other tissue, or other material under test.

Whilst there has been described in the forgoing description exemplary embodiments, it will be understood by those skilled in the technology that variations in the apparatus, method and system may take place without departing from the invention defined in the following claims: