WO2014006391A1

WO2014006391A1 - Sensing methods and apparatus

Info

Publication number: WO2014006391A1
Application number: PCT/GB2013/051746
Authority: WO
Inventors: Kalyan Sarma; Meng-Han Kuok
Original assignee: Sparq Wireless Solutions Pte Ltd; Martin, Philip
Priority date: 2012-07-04
Filing date: 2013-07-02
Publication date: 2014-01-09
Also published as: GB201211894D0

Abstract

We describe a system for characterising the dielectric properties of the surface of a material, the system comprising: a piezoelectric probe for interrogating a surface of said material; an RF drive unit configured to excite electroacoustic oscillations in said piezoelectric probe; and a signal analysis system, coupled to said piezoelectric probe, to output data characterising a dielectric property of said material responsive to said excited electroacoustic oscillations.

Description

Sensing Methods and Apparatus

FIELD OF THE INVENTION This invention relates to apparatus and methods for characterising the dielectric properties of a material. Applications for the techniques we describe include medical diagnostic techniques.

BACKGROUND TO THE INVENTION

Techniques for measuring the electromagnetic properties of materials include placing the material in between plates of a parallel plate capacitor or in a resonant cavity, and employing an open-ended coaxial probe or transmission line in combination with a network analyser.

We have previously described an antenna for wireless telecommunication comprising a piezoelectric material layer (WO201 1/135356). We have also described electroacoustic antennas in our unpublished PCT application GB2012/050706. We now describe material characterisation techniques employing a piezoelectric probe.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is therefore provided a system for characterising the dielectric properties of the surface of a material, the system comprising: a piezoelectric probe for interrogating a surface of said material; an RF drive unit configured to excite electroacoustic oscillations in said piezoelectric probe; and a signal analysis system, coupled to said piezoelectric probe, to output data characterising a dielectric property of said material responsive to said excited electroacoustic oscillations.

Broadly speaking, in operation a surface of the piezoelectric probe displays a time- varying pattern of charge which interacts with the material to, in effect, probe the surface dielectric properties of the material. Interaction between the excited electroacoustic oscillations in the piezoelectric probe and the dipole or other charges within the material result in modification to the amplitude and/or phase of the oscillations (the latter may give rise to a frequency shift) and this modification may be detected to infer information about the dielectric properties of the probed material. Embodiments of the technique can achieve a factor of 10³ improvement on existing approaches, with sensitivity in the parts per million range; potentially the spatial resolution may also be improved.

In some embodiments the piezoelectric probe is provided with a set of electrodes to allow the probe to be driven directly by the RF (radio frequency) drive unit. Alternatively the electroacoustic oscillations may be excited wirelessly, for example by providing an electrically conductive resonant cavity around at least part of the probe and driving the cavity to excite the probe. Alternatively, because the probe can act as an antenna, the electroacoustic oscillations may be excited by a remote drive from an antenna coupled to the RF drive unit; the probe may then be located in either the near field or the far field of the antenna. Optionally in a wirelessly excited probe the piezoelectric material may be provided with a layer of dielectric material on a (longitudinal) surface of, or substantially covering, the piezoelectric material, for example of polymer such as PMMA, to withdraw charge to the surface of the piezoelectric material and facilitate coupling to electromagnetic radiation.

Preferably the probe comprises a rod of piezoelectric material, preferably with an aspect ratio of at least 5:1 or 10:1 (length to (maximum) lateral dimension). This helps to provide a very high Q, which in turn facilitates high sensitivity measurements. In embodiments the signal analysis system comprises an electrical impedance analyser to determine one or both of a real component and a complex component of an electrical impedance of the probe. Preferably this is done via a wired connection to electrodes on the probe but alternatively this measurement may be performed wirelessly to remotely sense the excited oscillations. This may be accomplished by a complex impedance measurement on an RF drive to a cavity or antenna stimulating the oscillations. Alternatively the signal analysis system may determine a reflection coefficient for an RF drive applied to the piezoelectric probe. This may comprise determining a real and/or imaginary component of this reflection coefficient, more particularly an amplitude and/or phase of a reflected RF signal. In some other approaches the electroacoustic oscillations are detected via their acoustic rather than their electric effect. Thus in embodiments the system includes a remote detector of acoustic waves in the piezoelectric probe, for example a non- contact optical detector such as a laser interferometer. Acoustic waves of various different types may exist within the probe and flexural or longitudinal waves are easier to detect than torsional/shear waves. In embodiments, therefore, it can be preferable to mount the piezoelectric probe so as to encourage the formation of longitudinal waves, by mounting the probe at a longitudinal (standing wave) node. Thus the probe may be mounted at one longitudinal end, but it can be preferable to displace the mount away from a longitudinal end of the probe towards or substantially at a central position, where conservation of momentum provides that relatively larger amplitude longitudinal waves are generated.

When characterising material it is advantageous to measure the dielectric properties of the material at a plurality of different frequencies rather than at a single frequency. This is especially important in medical applications since although a uniform material may have a predictable dielectric constant (relative permittivity) human/animal tissue has a very unpredictable permittivity and thus it is important to obtain spectral information on the permittivity to characterise the material.

Spectral information on the permittivity may be obtained in a number of ways. For example when the piezoelectric probe is driven, especially when driven with a waveform which is not a pure sign wave, multiple different oscillatory modes are generally excited simultaneously (multiple longitudinal modes and/or other modes) resulting in a comb of excitation frequencies. These can be used to interrogate the dielectric response at multiple different frequencies. Optionally the frequency of the RF drive unit may also be varied so as to match multiple different resonant frequencies of the piezoelectric probe. Since there are many modes of oscillation it is possible to achieve relatively good frequency coverage in this manner.

The piezoelectric probe may be driven at harmonics of a fundamental resonant frequency of the probe. However the probe may also be driven at one or more sub- harmonics of the probe. This is facilitated by the high Q and very low sub-harmonics may be achieved, for example at or below i₀, V₂₀, V₅₀, V₇₀ or ¹ /100th of the fundamental. This they in turn used to excite electroacoustic oscillations at (in this context) very low frequencies, for example below 0.1 MHz. Such frequencies are of particular interest, especially for human/animal tissue, as these frequencies a giant dielectric response may be observed. This is related to molecular and/or macromolecular rotation or reconfiguration, for example of polymer chains.

In embodiments the signal analysis system provides data defining a spectrum of the dielectric response of the probed material covering a range of different frequencies. The imaginary component of the permittivity, corresponding to the absorption, is of particular interest, and can be characteristic of the material/material type. In the context of diagnostic apparatus, for example, the spectrum of the imaginary component of the electrical permittivity can be characteristic of the presence of a tumour. Thus in principle embodiments of the system may be employed for detecting the presence and/or locations/extent of a tumour, either in vitro or in vivo. Since the probe is small it may be provided in a hand held form or at the tip of a surgical instrument, for example for use in an operating theatre. Potentially embodiments of the system may be able to distinguish between different types of tumour. More generally embodiments of the system may be employed to distinguish between different types of human/animal tissue. The relationship between the measured parameters, for example the complex electrical impedance of the piezoelectric probe, and the complex permittivity of the probed material are not straightforward for non-uniform material. In principle one may be determined from the other but in some preferred implementations there is no need to calculate the permittivity of the material per se and instead one or more measured parameters at one or more frequencies are employed either directly or indirectly to provide data defining a signature of the probed material. A material may then be identified or characterised into one or more categories by latching a measured signature with one or more stored signatures. The skilled person will appreciate that many different techniques are available for such pattern matching, including Bayesian techniques and/or techniques which decompose a signature into its principle components.

Embodiments of the system, rather than driving at a single frequency, may drive at a plurality of frequencies simultaneously. More particularly, embodiments of the system may employ Fourier transform dielectric spectroscopy by, for example, driving the piezoelectric probe with a pulse train or other multiple-frequency drive signal, then capturing a time-domain response of the dielectric response from the sample. The response may be in any of the previously mentioned formats - real/imaginary impedance component(s); real/imaginary reflection coefficient component(s); or other formats. The signal analysis system may then perform a time-to-frequency domain conversion to determine a spectrum of the dielectric response of the material. The time-frequency domain conversion may be performed using any of the range of techniques, including but not limited to, a Fourier transform-based technique, wavelets, and the like. Such an approach can potentially provide an improvement in measurement speed and signal-to-noise ratio. As previously mentioned the output data may be used as a finger print to match with one or more stored "fingerprints" rather than necessarily being converted into a direct measurement of the electrical permittivity of the probed material. As previously mentioned, embodiments of the above described system are particularly useful in diagnostic apparatus, for characterising human or animal tissue. For this purpose it can be helpful to excite relatively low frequency oscillations, for example less than 500 MHz. Embodiments of the system may also be employed in a scanning probe microscope (SPM) such as an AFM (atomic force microscope). The piezoelectric probe may then be employed in addition to or instead of the scanning probe tip. This facilitates characterisation of the surface properties of a material. Embodiments of the system may also be used in an electron microscope such as a scanning electron microscope. For such an application remote, wireless excitation of the piezoelectric probe can be useful, for example using a cavity to excite resonant modes at microwave frequencies (of order GHz) or using a local antenna to excite the probe.

In embodiments the system may include a stage for moving the surface of the material and the probe relative to one another, for example to scan the probe over the material, to map the dielectric properties of the surface of the material. Preferably the probe, in embodiments one end of the piezoelectric rod, is held as close as practicable to the surface of the material. Thus embodiments of the apparatus may also include a control loop to maintain the probe at or within a target distance of the material surface, for example less than 1 μηι. The distance to the surface may be sensed by any of a range of techniques, for example an optical technique. In a related aspect the invention provides a method of characterising a dielectric property of a material, the method comprising: bringing a piezoelectric probe into proximity with a surface of the material; exciting electroacoustic oscillations in said piezoelectric probe, wherein said electroacoustic oscillations are associated with a changing pattern of charge on said probe; detecting modifications of said changing pattern of charge by said dielectric property of said material to characteristic said dielectric property of said material; wherein said detecting comprises detecting modifications of said electroacoustic oscillations by said material.

The invention also provides apparatus for characterising a dielectric property of a material, the apparatus comprising: a mount carrying a piezoelectric probe, to enable said piezoelectric probe to be brought into proximity with a surface of the material; a driver to excite electroacoustic oscillations in said piezoelectric probe, wherein said electroacoustic oscillations are associated with a changing pattern of charge on said probe; and a system to detect modifications of said changing pattern of charge by said dielectric property of said material to characteristic said dielectric property of said material by detecting modifications of said electroacoustic oscillations by said material.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

Figure 1 shows apparatus for characterising the dielectric properties of the surface of a material, according to an embodiment of the invention;

Figure 2 shows examples of data characterising a dielectric property of a material of the type which may be obtained from the apparatus of Figure 1 ; Figure 3 shows apparatus according to a second embodiment of the invention in which the piezoelectric probe is excited within a resonant cavity and monitored remotely by optical interferometry; Figure 4 shows, schematically, wireless excitation of a piezoelectric probe by an antenna;

Figures 5 and 5b show, schematically, mounting techniques for a piezoelectric probe; and

Figures 6a to 6c show examples of a piezoelectric probe with more than two electrodes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Figure 1 this shows apparatus 100 for characterising the dielectric properties of the surface 1 02 of a material. The apparatus comprises a piezoelectric probe 150 bearing a pair of electrodes 1 52a,b disposed at substantially equal distances to either side of a centre point of the probe which, in the illustrated embodiment, is in the form of a rod with a circular cross section. A stage (not shown) holds the material and probe so that they may be moved in X and Y directions laterally with respect to one another. The probe 1 50 is maintained at a distance c/ from surface 1 02 which may be of order 50μηι. This may be achieved either by providing a simple mechanical control to bring probe 1 50 towards the surface 102 or, for smaller distances, servo control of the distance may be employed.

An RF drive unit 106 provides a drive to electrodes 152 at a frequency which may vary from less than 1 00KHz up to more than 1 GHz, 10GHz or 100GHz (a skilled person will appreciate that different drive units will be employed for different frequency ranges), depending upon the desired frequency of characterisation. An impedance analyser 1 08 is also connected to electrodes 152 to determine the complex impedance between electrodes 152 and output complex impedance data Ζ (ω) = Z_r (ω) + ίΖ_ί (ω) . In use the frequency of RF drive 106 is scanned over a range of frequencies, successively interrogating resonant frequencies of the probe which, in general, define a comb-like response. Figure 2 illustrates example impedance data 200 which may be determined from the apparatus, and corresponding permittivity data. Broadly speaking the electrical impedance relates to the acoustic impedance of the piezoelectric probe, which in turn relates to the complex electrical permittivity 250 of the material, ε (ώ) = ε _r (ώ) + ie_i (ώ) .

Figure 2 illustrates a simple example but for complex, non-uniform material such as body tissue the relationship between the impedance and permittivity is not straight forward and thus, in practice, curves 250 may not be determined. Instead the impedance data may be used as a characteristic fingerprint of the particular material under investigation; optionally this may be matched to stored fingerprints in a computer system (not shown) to identify the material or material class. The imaginary component of the complex impedance is of particular interest and, as illustrated, in practice the apparatus does not determine a complete curve but samples this quantity at a set of discreet points 202 corresponding to resonant frequencies of the probe 150.

Although Figures 1 and 2 show apparatus which analyses the electrical impedance of probe 150 the skilled person will appreciate that the electrical/acoustic response of the probe may be determined in other ways, for example from an electrical signal reflection coefficient, or by direct or indirect interrogation of the mechanical acoustic oscillations in the probe.

Thus Figure 3 illustrates a variant apparatus 300 in which the RF drive is coupled to a conducting, metallic cavity 302 containing the probe. Mechanical oscillations of the probe, typically in the range 1 -10μηι, are monitored by a sensitive laser interferometer 304 which outputs an acoustic data signature. As before this may be used as a fingerprint or further processed, for example using a model of the material under investigation, to determine the electric or permittivity response of the material. Figure 4 illustrates a further variant apparatus 400 in which driver 106 drives an antenna 402 to couple RF energy into probe 150. The driver 106 may also be configured to determine a reflection coefficient of antenna 402 to provide the material characterising data such as complex impedance data. As illustrated in Figure 2, preferably the apparatus determines a spectrum of the electrical response of the material over a range of frequencies, to better characterise the material. Broadly speaking the probe interrogates the surface charge properties of the material relating to energy dissipation through mechanisms such as relaxation, conduction, and thermal loss/scattering, as well as, optionally, the multipole response of the material. It will be appreciated that the apparatus of Figure 1 may be employed to characterise samples either in vivo or in vitro.

Various techniques may be employed to support the piezoelectric probe 150, as illustrated in Figure 5. Thus a support 154 may be provided at one end of the probe or in the centre of the probe or at multiple positions along the probe. Further probes supported at one end, optionally an impedance mismatch layer 156 may be provided to produce a sharper acoustic boundary, for increased efficiency. The skilled person will appreciate that, broadly speaking, the mounting points will define nodes of the acoustic oscillations or at least will dampen oscillations which do not have nodes at the mounting locations. Figure 5 also illustrates an embodiment in which the probe is mounted at multiple locations, which may be defined by the electrodes 152.

The probe 150 may replace the tip of a scanning probe microscope or may be employed as an additional tip for a scanning probe microscope. Alternatively the probe may be employed in an electron microscope, in which case an arrangement of the type illustrated in Figure 3 is preferable. Any of the illustrated arrangements may be employed to characterise body tissue.

Piezoelectric probes

In embodiments the piezoelectric probe 150 comprises a rod or bar of piezoelectric material with a length of at least five or ten times a lateral dimension of a cross-section of the rod/bar. In embodiments the aspect ratio of the piezoelectric material may be at least 20:1 or 50:1 (here "rod" includes, for example, wires and whiskers). In general the cross section of the rod may be of any shape, for example square, rectangular, circular or oval. Electrodes are deposited on the piezoelectric material, which is arranged to have an inherent polarisation in a longitudinal direction along its length. The spacing of the electrodes may be chosen to match to an impedance of the driver, for example approximately 50 ohms or 75 ohms. Optionally three or more electrodes may be employed for improved coupling/efficiency. Because of a phase delay between these electrodes one or more of them may be provided with a phase matching element (for example a delay line) and/or the electrodes may be coupled to different taps of a transformer or balun to facilitate phase matching. Pairs of electrodes may be disposed at, preferably regular, intervals, along the length of the rod; electrodes of opposite polarity may be interleaved to facilitate differential driving. The electrodes may be fabricated using standard photolithographic methods.

A continuous or discontinuous layer of dielectric material, for example a layer of polymer such as PMMA, may be provided on one or more longitudinal surfaces or faces of a piezoelectric rod or bar. This effectively withdraws charge to the surface of the piezoelectric material and thus facilitates efficient coupling of the probe to the probed material. Optionally an additional layer of metallisation may be provided at one or both ends of the bar to produce a sharper (acoustic) impedance transition, again for increased efficiency. The piezoelectric rod is supported at one or more locations along the bar at which acoustic vibrations are substantially at a minimum (nodes). In embodiments the high aspect ratio of the piezoelectric rod or bar naturally enhances the Q of the probe.

At resonance the length of the piezoelectric probe is an integral number of half wavelengths of an acoustic wave travelling (longitudinally) in the probe at the drive frequency. However the drive frequency may be tuned so that it is not necessary to fabricate a probe of a particular length. Depending upon the material, the wavelength for, say, a 100 MHz electromagnetic (EM) wave may be of order 50 μηι, and shorter at higher frequencies. The operating frequency may extend up to tens or hundreds of GHz, so the probe length may be small. However to facilitate fabrication the length of the probe may be n half wavelengths where n is at least 5, 10, 20, 30, 40, 50, 10², 10³ or more. A probe may have micron to mm dimensions for operation in the KHz to GHz frequency range. Example probe lengths are in the ranges 1 -10μηι; 10-1000 μηι; and 1 -10mm. At a resonant frequency of operation the probe may have a Q of at least 10⁴.

Potentially any piezoelectric material may be employed. Examples of suitable materials include, but are not limited to: quartz, aluminium nitride, zinc oxide, diamond and others. In some embodiments the longitudinal axis of the probe defines a direction of substantially maximum inherent polarisability of the piezoelectric material. In others the piezoelectric may be aligned to minimise a frequency-temperature coefficient of the probe; or some balance or combination of these two criteria may be employed. For example, in embodiments the piezoelectric rod comprises AT-cut quartz. The piezoelectric probe rod may be cut from a wafer aligned according to the relevant crystalline lattice.

Figure 1 shows a centre-fed whisker based single dipole piezoelectric probe 150 with electrodes around the centre point of the half wave, where the electrical impedance is low. The electrode gap determines the impedance of the probe; this may be adjusted to couple energy from the driver into longitudinal (or torsional, and at lower frequencies flexural) electroacoustic oscillations of the probe. For lower electrical impedance multiple electrodes may be employed, as shown in Figure 6a. This arrangement has several parallel electrodes with small (<45 degrees) phase differences between them due to the distribution of the excited electroacoustic waves. Their individual phases can be made coherent with a charge coupled device or a suitable network, as illustrated by phase delay elements 156. As shown in Figure 6b, and as previously mentioned, a similar result can be achieved with a matching transformer or balun 158 to transform the impedance.

If desired the spacings of the multiple electrodes can be aligned to match the electroacoustic nodes and anti-nodes of the excited waves, as shown in Figure 6c. This employs a "zebra" electrode pattern in which there is a 180° phase difference between each electrode 152a,b. This means a signal can be tapped every 360° from one group of electrodes in phase, and used with another group of the same spacing, but shifted relative to the first group by 180°. Thus a single phase feed can be used to couple to the structure while maintaining a high Q factor.

Example mounting arrangements to avoid damping of the electroacoustic wave and reducing efficiency are shown in Figure 5, in which the mounting points are "bound" rather than free. The location of the mounting points will influence the spectrum of the excited electroacoustic wave; low damping is desirable to provide a wide spectrum of operation. Thus, for example, the probe may be supported by the electrode wires or supported at one end, which then defines a fixed boundary. Optionally a layer of metal 1 56 or other material may be deposited at the fixed end, to produce a sharper acoustic impedance transition for increased efficiency. Potentially a 2D probe may be employed rather than a rod-like probe. In such an arrangement the piezoelectric material may have a surface at the probe tip (adjacent the probed material) which extends laterally in two dimensions. The probe may then be driven to excite a standing surface wave pattern of charge at the probe tip. Overall the probe may (but need not necessarily) have a plate-like form.

This may be achieved by providing electrodes, for example interdigitated electrode fingers, to excite acoustic waves from one or a pair of (adjacent) lateral edges, to excite acoustic waves in one direction or a pair of (orthogonal) directions to generate a 2D surface pattern of charge. Thus standing waves may be generated in either an X or Y direction in a lateral surface plane of the device by launching a travelling wave in the surface in a particular direction, interference between the standing waves generating "carpet" patterns of charge, in effect, plasma. The surface waves may comprise a surface wave mode of the piezoelectric material. The acoustic wavelength defining the separation of charged regions may be of order 1 0⁵ -10⁵ times smaller than the electromagnetic wave wavelength (from v = fA, comparing to the shear wave velocity in the piezoelectric material). The size of the pattern may be larger or smaller than the acoustic wavelength but in general will be smaller than the electromagnetic wavelength. A range of shapes of the excited pattern may be employed including, but not limited to, a circular, square or rectangular and oval patterns.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

CLAIMS:

1 . A system for characterising the dielectric properties of the surface of a material, the system comprising:

a piezoelectric probe for interrogating a surface of said material;

an RF drive unit configured to excite electroacoustic oscillations in said piezoelectric probe; and

a signal analysis system, coupled to said piezoelectric probe, to output data characterising a dielectric property of said material responsive to said excited electroacoustic oscillations.

2. A system as claimed in claim 1 wherein said piezoelectric probe comprises a rod of piezoelectric material having a length of at least five times a dimension of a lateral cross-section of said rod; and

first and second electrodes on said rod of piezoelectric material, spaced apart along the length of said rod; and

wherein said rod of piezoelectric material has an inherent polarisation in a longitudinal direction along said length of said rod.

3. A system as claimed in claim 1 or 2 wherein said RF drive unit comprises a wireless RF drive unit to wirelessly excite said electroacoustic oscillations in said probe.

4. A system as claimed in claim 3 wherein said wireless RF drive unit comprises a resonant cavity, and wherein said piezoelectric probe is located within said resonant cavity.

5. A system as claimed in any one of claims 1 to 4 wherein said signal analysis system comprises an electrical impedance analyser.

6. A system as claimed in any one of claims 1 to 4 further comprising a remote detector, coupled to said signal analysis system, to remotely sense said excited electroacoustic oscillations in said probe.

7. A system as claimed in claim 6 wherein said remote detector comprises a non- contact optical detector.

8. A system as claimed in any preceding claim wherein said piezoelectric probe is mounted at a node, in particular at a longitudinally substantially central location.

9. A system as claimed in any preceding claim configured to characterise said dielectric property at a plurality of different frequencies of said electroacoustic oscillations.

10. A system as claimed in claim 9 wherein said frequencies comprise sub- harmonies of fundamental resonant frequency of said probe.

1 1 . A system as claimed in claim 9 or 10 wherein said probe comprises a rod of piezoelectric material having an aspect ratio of at least 5:1 , and wherein said frequencies comprise frequencies of a plurality of different longitudinal modes of oscillation of said probe.

12. A system as claimed in claim 9, 10 or 1 1 wherein said dielectric property comprises a spectrum of an imaginary component of an electrical permittivity of said material.

13. A system as claimed in any one of claims 9 to 12 wherein said RF drive unit is configured to drive said piezoelectric probe at a plurality of different frequencies.

14. A system as claimed in claim 13 wherein said RF drive unit is configured to drive said piezoelectric probe with a pulse train, and wherein said signal analysis system comprises a signal processor to convert time domain response data from said excited electroacoustic oscillations to frequency domain data representing a frequency spectrum of said dielectric property.

15. A system as claimed in any preceding claim further comprising a stage for moving said surface of said material and said probe relative to one another, for mapping said dielectric properties of said surface of said material.

16. A scanning probe microscope comprising the system of any one of claims 1 to 15, wherein a tip of said scanning probe microscope comprises said piezoelectric probe.

17. An electron microscope comprising the system of any one of claims 1 to 15.

18. Diagnostic apparatus comprising the system of any one of claims 1 to 15, for characterisation of human or animal tissue.

19. Diagnostic apparatus as claimed in claim 18 wherein said RF drive unit is configured to excite said electroacoustic oscillations at a frequency of less than 500MHz, preferably less than 100MHz.

20. A method of characterising a dielectric property of a material, the method comprising:

bringing a piezoelectric probe into proximity with a surface of the material; exciting electroacoustic oscillations in said piezoelectric probe, wherein said electroacoustic oscillations are associated with a changing pattern of charge on said probe;

detecting modifications of said changing pattern of charge by said dielectric property of said material to characteristic said dielectric property of said material; wherein

said detecting comprises detecting modifications of said electroacoustic oscillations by said material.

21 . A method as claimed in claim 20 wherein said exciting comprises electrically exciting said piezoelectric probe.

22. A method as claimed in claim 21 wherein said detecting comprises detecting one or both of a real component and an imaginary component of an electrical impedance of said probe.

23. A method as claimed in claim 20, 21 or 22 comprising characterising said dielectric property of said material over a range of frequencies of said electroacoustic oscillations.

24. A method of characterising human or animal tissue by characterising a dielectric property of said tissue using the method of any one of claims 20 to 23.

25. Apparatus for characterising a dielectric property of a material, the apparatus comprising:

a mount carrying a piezoelectric probe, to enable said piezoelectric probe to be brought into proximity with a surface of the material;

a driver to excite electroacoustic oscillations in said piezoelectric probe, wherein said electroacoustic oscillations are associated with a changing pattern of charge on said probe; and

a system to detect modifications of said changing pattern of charge by said dielectric property of said material to characteristic said dielectric property of said material by detecting modifications of said electroacoustic oscillations by said material.

26. Medical diagnostic apparatus comprising the apparatus of claim 25.