The field of the invention.

The present invention relates to a Large Aperture Hydrophone (LAH), and to construction of such, for measurement of traveling longitudinal acoustic pressure fields. Further the invention relates to methods for measuring, characterising, reconstructing and predicting, forward and backward propagation of, such acoustic fields.

Background of the invention.

Many methods have been developed for the measurement of the acoustic properties of materials in a broad number of application areas for imaging and metrology including but not limited to: medical imaging and the measurement of acoustic properties of biological tissues; non-destructive testing (NOT) of materials and imaging; SONAR e.g. in the fisheries industry and for military application; and seismology. An appropriate characterisation of the acoustic fields used in these applications is essential, indeed, it is a regulatory requirement that the fields produced by medical ultrasound scanners are to be characterised and documented to comply with safety regulatory standards (Acoustic Output Measurement Standard for

Diagnostic Ultrasound Equipment, AIUM/NEMA 2004).

There are several methods used to measure and/or image and/or characterise the acoustic field including: point hydrophone technologies; Schlieren optical methods; radiation force balances; ca!orimetry; transducer face movement, and wire calibration targets for example, among others. Point hydrophone and Schlieren methods are able to measure the so-called radiofrequency (rf) pressure field. The most common method is via point-to-point measurement with a point hydrophone (a small microphone that measures the field pressure at a nominal point in a liquid). The fields produced can be complicated in shape (due to diffraction effects and the ubiquitous use of short pulsed fields) and measurement of the entire field (a time recording of the field at all spatial locations) presents a substantial body of data. In practice a restricted and possibly over simplified measurement set is used, usually in 'axial pulse - beam profile' format. Use of theoretical formalisms (such as the Angular Spectrum method) can be employed to predict what the field looks like at other positions not measured.

There are a number of issues with the use of point hydrophone methods to characterise acoustic fields. The ideal point hydrophone device measures the field at a point of zero spatial extent. However, all realisable devices have a finite nonzero aperture, in order to measure the field without distortion due to spatial averaging across the aperture of the hydrophone, an aperture dimension of less than one half wavelength is required. The majority of commerciaiiy available probes have an aperture in the order of 0.5 mm, which are adequate for fields below 3MHz. The smallest aperture devices are in the order of 0.04 mm, but have a reduced sensitivity as the active area is smaller, and also exhibit a distinct frequency dependent response.

Using the LAH of the invention, a complete measurement and characterisation of the acoustic field is possible, avoiding the issues of the finite nonzero aperture of the state of the art hydrophones such as the point hydrophone, and greatly simplifying the measurement procedure, instead of scanning the entire 3D space with a point hydrophone device, measurements are performed by angulation of the LAH of the invention over ail angles. This can be performed by angulations of the LAH around a single convenient spatial location (a pivot point) in the field. The Directivity Spectrum Theory (int. J. imag. Sys. Technol., vol. 8 #1 , 45-51 , 1997), developed specifically for use with the LAH, can then be used to produce a complete knowledge of the field at all spatial locations, as would be measured by an ideal point hydrophone at all locations in the field.

Measurements are performed in water in which the canonical loss-less wave equation is assumed to hold (although the theory can be readily extended to lossy media), namely

C denotes the speed of ultrasound in water, t denotes time and f is the position vector. A four dimensional function of the independent variables and t is defined in terms of the four independent conjugate variables, and ω,

Restricting to satisfy the canonical wave equation (1) restricts the integral to be taken

over the energy shell,

We select the forward travelling wave branch ώ = +Ck by inserting δ(ώ - Ck) into equation (2). The integration yields, The Directivity Spectrum D(k) is seen from equation (4) to be the three-dimensional Fourier Transformation of the pressure field at one time instant, for convenience to be defined at

The Directivity Spectrum can be measured directly by integrating p(f, t) over an infinite spatial plane. Consider the two-dimensional (spatial) projection onto the (z, t) plane,

The integration can be performed by prudent use of representations of the delta function, yielding,

Where τ— Ct.

The data set required is produced by angulation of the hydrophone over the Θ and φ, polar and azimuthal angles. The (one dimensional voltage-time signal) LAH output at a specific angulation is Fourier Transformed. The values of the Fourier Transformed signal relate directly to the values of the Directivity Spectrum along a line in /e-space, intercepting the origin and in the (θ, φ) direction specified as the normal to the integrating plane of the LAH. A complete measurement of the Directivity Spectrum is then obtained by angulation of the LAH over ail relevant polar and azimuthal angles and 'sweeps out' a measurement of the Directivity Spectrum.

P(z— T) is a one dimensional spatially invariant function and as such the measured waveform does not suffer from diffraction effects. Thus the physical realisation of the integrating plane (the Large Aperture Hydrophone) can measure aspects of the acoustic field devoid of diffraction effects. This makes the device suitable for measuring the acoustic properties of materials without the need for a diffraction correction: examples are attenuation, dispersion and non-linear propagation parameters.

The ideal large aperture hydrophone would perform a uniform integration of the acoustic field across its aperture in an effectively infinite two-dimensional spatial plane. In practice the plane of integration is just required to be large enough to intercept the extent of the entire acoustic field. This is defined here as greater than 98% of the acoustic energy in the Fourier component of the ultrasound field orthogonal to the LAH aperture, impinging upon the aperture of the LAH, more preferably greater than 99.9% and ideally 100%. Increasing the integration area of the LAH aperture beyond this limit changes the output measured signal by < 2%, < 0.1% and 0% respectively. Thus, as long as the integration plane encompasses the entire extent of the acoustic field, the signal is equivalent to integration in an infinite plane. In this respect it is possible to more closely realise an 'ideal' large aperture hydrophone, than to realise the 'ideal' point hydrophone (with an active area approaching a zero spatial dimension aperture).

The applicant has identified a device and method of measuring acoustic fields using a large aperture hydrophone and features for its implementation. The device realises an "idea!" large aperture hydrophone if the aperture is large enough to intercept the extent of the entire acoustic field (defined above). The approach greatly simplifies the measurement and has the potential to produce more accurate results. Evanescent waves that appear in the Angular Spectrum method are not present in the Directivity Spectrum formalism. Evanescent waves make back propagation problematic as they have exponential growth, amplifying noise content in the measurement. The LAH also enables accurate measurements to be performed in areas where point hydrophone measurements are problematic. High frequency fields (e.g. 20+ MHz used for example in medical ultrasound dermatology, intravascular imaging and ophthalmology, among others) can be difficult to measure accurately with point hydrophones. This is because the point device must have a small active area compared to the wavelength of the field being measured, and this is fundamentally limited by the physics of the problem (higher frequencies require smaller point hydrophones). There are technical challenges in the construction of hydrophones with smaller active areas, and there is also an associated loss in sensitivity of the hydrophone as the active measurement area becomes smaller. The Large aperture hydrophone approach does not suffer this limitation. Optical Schlieren methods have been demonstrated, but noise levels are relatively high, and quantitative rf pressure field determination with low noise is extremely challenging.

Summary of the invention

Thus a first aspect the invention provides a device being a Large Aperture Hydrophone (LAH) comprising, i) A large aperture planar active area which is uniformly sensitive and comprises a piezoelectric material;

ii) Uniform electroding over surfaces of the piezoelectric material;

iii) Additional external electrodes that encompass the entire planar active area; iv) A mechanical frame to maintain a planar configuration of the piezoelectric material.

Detailed description of the preferred embodiments of the invention

The large aperture hydrophone of the invention should provide a uniform integration over the plane, and the planar active area of the LAH is large enough to intercept the entire acoustic field that is to be characterised. This can be achieved by means of a flat and uniformly sensitive piezoelectric material acting as the receiver. In order to provide a relatively flat frequency response in an appropriate frequency range a thin membrane, also called a film, is desirable wherein the thickness of the film is considerably less than the acoustic wavelength of the field being characterised. Accordingly, the shape of the planar active area is preferably circular and should be large enough to intercept the extent of the entire acoustic field (defined above). However, as the thickness decreases the voltage output response to an acoustic field will generally decline. The planar active area has a thickness in the range of 5 to 50 microns. For use of the LAH in measurements of diagnostic medical ultrasound fields a thickness between 5 to 50 microns is preferable. For diagnostic ultrasound fields in the 1-10 MHz range 10 to 30 micron thickness is the preferred range. For fields below 1 MHz 50 micron plus thickness is the preferred range. For higher frequency fields (above 10 MHz) a thickness below 30 microns is preferred. For fields above 20 MHz a thickness below 15 microns is preferred. For fields above 30 MHz a thickness below 10 microns is preferred. Any piezoelectric material will be suitable. An example is a polyvinylidene fluoride (PVDF) membrane (or copolymer) with electroding on the surface (for example gold on chrome sputter deposited electrodes).

Additional electrodes (external electrodes) are placed in contact with, or otherwise coupled to, the two surfaces of the planar active area to provide electrical contact to record the voltage across this. These electrodes preferably encompass the entire active area of the membrane to ensure uniform response to the field.

Brief description of the drawings

Figure 1 depicts an expanded schematic of the parts of the Large Aperture Hydrophone construction. Figure 2 depicts the LAH device after the holder pieces 4 and 5 have been clamped together.

Figure 3 shows a photograph of a constructed large aperture hydrophone.

Figure 4 shows a photograph of a large aperture hydrophone in a water bath and a single element ultrasound transducer being characterised. Figure 5 shows acoustic field reconstructions from a pianar single element transducer.

Figure 6 shows acoustic field reconstructions from a focused single element transducer. in a preferred embodiment the device comprises external electrodes which are contact electrodes, held in physical and electrical contact with the planar active area, i.e. with the piezoelectric material, via mechanical force. This simple method of contact eiectroding avoids the need for soldering. The electrically conductive electrodes, preferably two, one for each side of the piezoelectric material, are mechanically pressed onto the surfaces of the pianar active area and held in position with pressure onto the surface of the piezoelectric material by means of the frame. For uniform sensitivity across the aperture, the electrodes enclose the entire active area of the LAH. Mechanical pressure is used to maintain electrical contact of the electrodes to the electrode surfaces of the piezoelectric film. In one embodiment a conductive fluid, gel, cement or other material may also be applied to improve the electrical contact. An example of the external electrodes is two annular electrodes. The annular electrodes may be fiat in their thickness profile, or other profile for example circular such as a conducting O-ring. One of the electrodes is of larger dimension than the other, for example two G-rings of slightly differing diameter, see Figure 1. When the O-rings are pressed onto the surface (and clamped by the holder) of a flexible piezoelectric membrane such as PVDF the membrane is automatically mechanically tautened and stretched to form a pianar membrane. This method is best performed at the ambient temperature at which the LAH is to be used, for example at 37 degrees centigrade for a hydrophone to be used in a 37 degree centigrade water bath. Accordingly, in a second aspect the invention provides a method of making the Large Aperture Hydrophone comprising assembling the device as described above.

In a third aspect the invention provides a method of using a Large Aperture Hydrophone (LAH), such as the LAH of the invention for measurement and characterisation of acoustic fields. It has been found that using the LAH of the invention greatly simplifies the

measurement of acoustic fields and further provides more accurate results than prior methods using known devices.

In one embodiment the method includes measurement of the Directivity Spectrum. The Directivity Spectrum is established by direct measurement. In another embodiment of the method the invention provides a method of determining the pressure field, p(f, t) of an acoustic field, at any point in space and time, by using the LAH to measure the Directivity Spectrum. This provides a fast and computationally efficient method of field calculation from the Directivity Spectrum measured by the LAH via the use of Equation (4). For example, the spatial pressure field at time t = 0, p(r, 0), can be calculated by direct three-dimensional inverse Fourier Transformation of the Directivity Spectrum, using efficient Fast Fourier Transformation algorithms. The three-dimensional pressure field at any other time instant can also be calculated by applying a simple phase factor, βχρ(-ίωί) into the right hand side of Equation (4), i.e. (8)

In a further embodiment the invention provides a method of using the LAH in the area of therapy (drug and gene delivery) with medical ultrasound. There are a number of technologies being developed for ultrasound activated drug and gene delivery systems, A number of technologies exist where micro or nano particles (bubbles and/or nano/micro particles) are triggered to release their drug and/or gene payload under ultrasound activation. The activation mechanisms are ultrasound exposure related. The pressures used, frequency, number of cycles in the pulse and the duty cycle, are all potentially important parameters in triggering release of compounds. Use of ultrasound also affords other bio- effect mechanisms to enhance and/or target delivery and uptake of the compounds after release. For these applications an accurate knowledge of the field and tailoring optimisations are of critical importance. It is important to note that a significant proportion of the beam forming employed in phased array medical ultrasound transducers is implemented by 'focusing in reception'. The output field from the imaging transducer is not necessarily represented by the pulse-echo impulse response of the imaging system. Accurate characterisation and optimisation of these fieids is crucial for optimising their use in these applications. Accordingly, the embodiment covers the use of the LAH in a method of characterising the acoustic field in a process of ultrasound activated drug and gene delivery. The characterising may take place before, during or after drug delivery. The large aperture hydrophone approach provides a simple and direct method of visualising and predicting such output fields, and has great potential in the tailoring and optimisation for these applications.

In a still further embodiment the method of use of the LAH includes the determination of the acoustic axis and centre frequency of an arbitrary pulsed acoustic field. Defining the acoustic axis and centre frequency for pulsed ultrasound fields can be problematic for non- symmetric fieids and sources. For symmetric fields it is often natural to choose an axis of symmetry. For a non-symmetric field the determination of the acoustic axis is more problematic. This is the case for many fieids used, for example, in medicai imaging that are formed by arrays of elements that incorporate beam forming (such as beam steering two- dimensional arrays). The centre frequency is usually defined as the first moment of the power spectrum from a time domain measurement of the field at a single spatial location (point). The question arises as to which point in space this measurement should be made, providing an unsatisfactory arbitrary aspect to the measurement. The LAH measurement of the Directivity Spectrum allows for the computation of the acoustic axis direction of the field based on the first moment of the square of the modulus of the Directivity Spectrum (in k~ space), the so called Power Spectrum of the field via,

n then defines the direction of the acoustic axis.

Equation (10) determines the direction of the acoustic axis. In order to define the acoustic axis line in the spatial domain a point on the line also needs to be determined, This can

!ii ^ \ f \ ² f

be calculated as the first moment of

The centre frequency of the field, K _c , is defined using,

The spatial frequency K _c can be converted into a temporal frequency via a> _c — CK _C . This approach is general in that it applies to arbitrary asymmetric pulsed fields, and removes the ambiguity associated with the definitions of centre frequency and acoustic axis direction. Thus the direction of the acoustic axis can be determined from the values of the Directivity Spectrum measured with the LAH invention of the disclosure, and lengthy and involved methods to search for the acoustic axis performed with point hydrophone based

characterisation of the field can be avoided. Note that the Directivity Spectrum, is time independent. For measurements of the

Directivity Spectrum at different pivot point locations of the Large Aperture Hydrophone only a phase change results. Thus the definitions of centre frequency and acoustic axis direction (defined by equations (10) and (1 1) , based on and thus independent of the phase of the Directivity Spectrum) are independent of the pivot point used in the Large Aperture Hydrophone measurement of the field (for propagation according to the canonical loss-iess wave equation). in still a further embodiment the method of use of the LAH includes the determination of directivity parameters of an arbitrary pulsed acoustic field. The directivity pattern of the acoustic field (power as a function of frequency and angular distribution), D _s , is directly encoded (with the inclusion of an appropriate normalisation factor) in the square of the modulus of the Directivity Spectrum (hence the nomenclature),

Equation (12) is expressed using spherical co-ordinates. The directivity pattern, D _p , can obtained by integration of D _s oyer k, (with provision of an appropriate scaling factor) via,

in still a further embodiment the method of use of the LAH includes the calculation of the total acoustic power of an arbitrary acoustic field. The total acoustic power of the field can be calculated by integration of the appropriately normalised version of D _s , via,

Further, in another embodiment the method of use of the LAH includes the determination of the power density in the focal plane of high intensity acoustic fields. The total power output of the acoustic field can be calculated from the Directivity Spectrum via equation (14). Note that the measurement of the field with the LAH can be performed at any convenient spatial location in the field, even in the near field. This has great advantage when measuring high power acoustic fields as the measurement can be performed in the near field avoiding the high power density at the focal plane which can damage the field measurement device.

Example applications are the measurement of high intensity focused ultrasound (HiFU) fields used in medical ultrasound thermal ablation applications, and Lithotripsy fields. The propagation assuming applicability of the canonical loss-less wave equation (equation (1)) does not include non-linear effects which will be present in high power fields. However, linear propagation of the pulse allows calculation of the focal area. Division of the total power of the acoustic field, calculated from equation (14), by the focal area allows an upper bound to be placed on the power density of the field at the focus, even for non-linear propagation. Thus the approach allows accurate determination of an upper bound for the power density of highly focused HIFU and Lithotripsy fields via the Directivity Spectrum.

Alternatively the LAH data obtained from using the LAH can be used to measure a data set to incorporate into other non-linear propagation techniques such as those based on the KZK family of techniques for example the method described in (Acoust Phys. 2010 January 1 ; 56(3): 354-363).

A further embodiment provides a method of the use of the LAH in the efficient and accurate determination of acoustic field parameters specified in the medical ultrasound regulatory standards. The Directivity Spectrum contains a complete knowledge of the acoustic field propagating according to the canonical loss-less wave equation (equation (1)). For typical medical ultrasound applications using diagnostic ultrasound transducers and phased arrays, the field can be characterised by using the LAH to measure the Directivity Spectrum, Once the Directivity Spectrum is established by direct measurement, the acoustic parameters required by the regulatory bodies can be calculated in siiico, rather than via laborious and time-consuming measurement of the field via point hydrophone devices. This allows a much more complete determination of these parameters via in siiico methods much more efficiently than direct measurement with point hydrophone devices. This offers a substantial saving of resource. Alternatively the in siiico results can be used to indicate a much reduced spatial area of the field to be measured with a point hydrophone. For example the identification of locations of highest acoustic intensity in the field.

In yet another embodiment of the method of use of the LAH provides a more accurate determination of de-rating of medical ultrasound acoustic fields. Medical ultrasound field characterisation is performed in a water bath (temperature controlled water tank). A derating is applied to predict the field parameters when propagating in-situ in biological tissue. A standard de-rating (constant attenuation) is often applied of 0.3dB/crn/MHz (0.0345 Np/cm/MHz). This is often applied to the maximum point hydrophone measurement measured in the water bath. However, the maximum in-situ often does not correspond to the maximum location measured in the water bath. In addition the spectral content of the imaging pulse is based on the spectrum of the point hydrophone measurement at the maximum and does not take into account the three-dimensional nature of the pulse. The LAH and Directivity Spectrum approach allows a potentially more accurate application of the de-rating. The approach is as follows. The l_AH measures the Directivity Spectrum and the pulse is propagated back to the source. Attenuation is then applied utilising the complex k formalism when propagating the pulse forward. The attenuation is applied in k-space (0.3 dB/cm/MHz) to the three-dimensional Fourier Transform of the spatial field, and the spatial field reconstructed at any desired time instant after that. Searching for field maxima and other field parameters may then be applied in siiico over the whole space and time duration of the field. This method allows more accurate application of the de-rating and more complete calculation of field parameters efficiently. Examples

A LAH of the invention is schematicaiiy shown in Figure 1. The planar piezoelectric material 1 has external electrodes 2 and 3 in electrical contact with the two surfaces of 1 , labelled 1 a and 1 b. Electrical contacts 6 and 7 are attached to the external electrodes 2 and 3.

Mechanical holder pieces 4 and 5 are clamped together to form a rigid mechanical framework. There may be groves, 8, in 4 and/or 5 to accommodate for the external electrodes 6 and 7. Figure 2 shows the LAH after the holder pieces 4 and 5 of the mechanical frame have been damped together, holding together the piezoelectric material 1 with the electrical contacts 6 and 7. Measurement of the acoustic field is performed with a LAH of the invention (prototype shown in Figure 3) in a wafer bath (temperature controlled water tank). Consider the unit vector f in spherical coordinates, (r, θ, φ), normal to the plane of the LAH detector. The

measurements are performed by angulation of the LAH over polar and azimuthai angles, φ and Θ, with ail planes of angulation intersecting a common point (pivot point). Figure 3 shows an example construction of large aperture hydrophone. 8. In this example the LAH was constructed using a 27-micron thick PVDF membrane with gold on chrome electroding. Two nitrogen filled seamless gold plated O-ring electrodes are used as the contact electrodes. When the holder pieces are clamped together the electrical contacts to the film surface electrodes are made and maintained by mechanical pressure. The film is also tautened to form and maintain a flat surface by the geometry of the contact electrodes.

Figure 4 shows the LAH 9 mounted in an angulation holder in a water tank, measuring the acoustic output from the ultrasound transducer 10.

Some measurement example results are shown in Figure 5. Quantitative pressure fieid images in a plane generated from large aperture hydrophone measurements for a pulsed field. Top left Figure 5 is a legend indicating locations of the pulse (from the transducer face): 1 1 : 0cm; 12: 1.5cm; 13: 4.5cm; 14: 1 1.5cm from transducer face. The pulses are travelling from bottom to top of the image. Top right Figure 5 is the spatial pressure field in a cross sectional two-dimensional plane. Bottom left Figure 5 is the envelope of the rf pressure field in a cross sectional two-dimensional plane. Bottom right Figure 5 shows 3D contours of the intensity fieid. The axis units in Figure 5 are cm. These are based on the reconstruction of the spatial pressure fieid calculated from measurement (over polar and azimuthai angulations) at a single location with the LAH,

Experimental results from a short focused fieid, typical of those used in medical imaging applications, is shown in Figure 6. Quantitative pressure field images in a plane generated from iarge aperture hydrophone measurements for a short pulsed field. The images on the left hand side of Figure 6 are of a contour of the field intensity and the right image is a greysca!e of rf pressure field in a plane intercepting the acoustic axis. The top set of images in Figure 6 is of the field at the transducer face. The middle images of Figure 6 are of the field at the nominal focal point of the transducer. The bottom images of Figure 6 are in the far field. Axis dimensions are in cm. These types of quantitative image are natural output of the Directivity Spectrum and can easily be obtained using the LAH device of the invention.