Andrew Hurrell

A Fabry–Pérot fiber-optic ultrasonic hydrophone for the simultaneous measurement of temperature and acoustic pressure

A dual sensing fiber-optic hydrophone that can make simultaneous measurements of acoustic pressure and temperature at the same location has been developed for characterizing ultrasound fields and ultrasound-induced heating. The transduction mechanism is based on the detection of acoustically- and thermally-induced thickness changes in a polymer film Fabry-Perot interferometer deposited at the tip of a single mode optical fiber. The sensor provides a peak noise-equivalent pressure of 15 kPa (at 5 MHz, over a 20 MHz measurement bandwidth), an acoustic bandwidth of 50 MHz, and an optically defined element size of 10 microm. As well as measuring acoustic pressure, temperature changes up to 70 degrees C can be measured, with a resolution of 0.34 degrees C. To evaluate the thermal measurement capability of the sensor, measurements were made at the focus of a high-intensity focused ultrasound (HIFU) field in a tissue mimicking phantom. These showed that the sensor is not susceptible to viscous heating, is able to withstand high intensity fields, and can simultaneously acquire acoustic waveforms while monitoring induced temperature rises. These attributes, along with flexibility, small physical size (OD approximately 150 microm), immunity to Electro-Magnetic Interference (EMI), and low sensor cost, suggest that this type of hydrophone may provide a practical alternative to piezoelectric based hydrophones.

Characterization of a polymer film optical fiber hydrophone for use in the range 1 to 20 MHz: A comparison with PVDF needle and membrane hydrophones

A small aperture wideband ultrasonic optical fiber hydrophone is described. The transduction mechanism is based on the detection of acoustically induced changes in the optical thickness of a 25-/spl mu/m thick parylene polymer film acting as a low finesse Fabry Perot (FP) interferometer that is deposited directly onto the end of a single mode optical fiber. The acoustic performance compares favorably with that of PVDF needle and membrane hydrophones with a peak noise-equivalent-pressure (without signal averaging) of 10 kPa over a 25-MHz measurement bandwidth, a wideband response to 20 MHz, and a near omnidirectional performance at 10 MHz. The dynamic range was 60 dB with an upper limit of linear detection of 11 MPa and a temporal stability of <5% over a period of 20 h. The hydrophone can also measure temperature changes with a resolution of 0.065/spl deg/C, offering the prospect of making simultaneous acoustic pressure and temperature measurements. The transduction parameters of the FP sensing element were measured, yielding an ultrasonic acoustic phase sensitivity of 0.075 rad/MPa and a temperature phase sensitivity of 0.077 rad//spl deg/C. The ability to achieve high acoustic sensitivity with small element sizes and to repeatably fabricate rugged sensor downleads using polymer deposition techniques suggests that this type of hydrophone can provide a practical alternative to piezoelectric hydrophone technology.

Comparison of a miniature, ultrasonic, optical fibre hydrophone with PVDF hydrophone technology

A miniature optical fibre hydrophone has been developed for the measurement of ultrasound in the range 1-30 MHz. The acoustically sensitive element comprises a 23 /spl mu/m thick polymer film mounted at the end of an optical fibre. When illuminated by laser light launched into the fibre, the polymer film acts as a Fabry Perot interferometer. An incident acoustic wave modulates the optical thickness of the interferometer thereby producing a corresponding intensity modulation in the light reflected from the film. The system was characterised in terms of sensitivity, frequency response and directivity using a broadband (1-30 MHz) ultrasonic field produced by nonlinear propagation obtained by driving a 1 MHz PZT source with a high amplitude 1 MHz toneburst. PVDF needle and membrane reference hydrophones were used as comparisons. The minimum detectable acoustic pressure of the optical fibre hydrophone was found to be 10 kPa in a 25 MHz measurement bandwidth with a wideband response to 30 MHz. The -3dB beamwidth at 10 MHz was 60/spl deg/. Such performance is comparable to that achieved with PVDF hydrophone technology, with additional advantages of immunity to EMI, small physical size, a flexible probe-type configuration, robustness and potentially low cost. Among the applications that might benefit from these advantages are single-use applications such as the measurement of industrial CW fields in hostile environments and in vivo measurements of medical ultrasound exposure.

The Practicalities of Obtaining and Using Hydrophone Calibration Data to Derive Pressure Waveforms

This paper considers the means by which calibration data are used to convert hydrophone output voltage into pressure. Hydrophone frequency responses are complex-valued quantities, and only by correcting for the magnitude and phase variations, is it possible to accurately recover the original pressure waveform. The limitations of current hydrophone calibration techniques are discussed, and a new method of obtaining hydrophone phase data is presented. Magnitude and phase information is measured via both coarse increment (1 MHz) and fine increment (50 kHz) calibration techniques for three exemplar hydrophones (0.5 mm needle, 0.2 mm needle, and 0.4 mm membrane). Frequently hydrophone calibration data are available at frequency increments that do not match that required by the deconvolution process. Therefore, a variety of methods to interpolate the calibrated system response to obtain correctly spaced data are considered, and two spline interpolation methods are found to offer best performance. Data preconditioning and filtering to address artifacts above and below the 1 to 40 MHz bandwidth of the coarse frequency increment calibration are also investigated, and a simple procedure for selecting an appropriate low-pass filter is presented. The revised calibration data are used to deconvolve the hydrophone frequency response for experimentally derived waveforms. Standard ultrasonic output parameters (such as peak compressional and peak rarefactional pressures, pulse intensity integral, and temporal peak and pulse average acoustic intensities) are calculated from these waveforms. Although the three hydrophones used in this paper are of different types and have a range of active element sizes, all output parameters derived from the deconvolved waveforms have <;5% variation from their respective population means (with the majority being within <;2%).

Development of a 50mhz optical fibre hydrophone for the characterisation of medical ultrasound fields

A wideband fibre optic hydrophone system based on a polymer film Fabry-Perot sensing interferometer has been developed for the measurement of medical ultrasound fields. The sensor transduction mechanism is based upon the interfer- ometric detection of acoustically-induced changes in the optical thickness of the polymer film. The advantage of this concept is that it offers the prospect of providing sufficiently small element sizes to avoid spatial averaging at frequencies in the tens of MHz range. The sensor is interrogated using a fibre coupled tunable laser operating in the range 1520-1600nm. The acoustic performance of the sensor was characterised in terms of its sensitivity and frequency response by comparison with a calibrated PVDF membrane hydrophone. The noise-equivalent- pressure was approximately 3kPa over a 25MHz measurement bandwidth with a frequency response that extended to 50MHz. The concept lends itself to a wide range of applications, from the laboratory characterisation of medical ultrasound sources, to in- vivo measurement of therapeutic and diagnostic ultrasound. I. INTRODUCTION Modern medical ultrasound equipment increasingly uses higher frequencies to improve spatial resolution. Characteri- sation of the output of such equipment requires a hydrophone with broadband frequency response and an acoustically small element size to minimize spatial averaging. The hydrophone must also achieve these two characteristics without sacrific- ing sensitivity. The devices presently used for this type of characterisation are piezoelectric PVDF needle and membrane hydrophones. Whilst developments in fabrication techniques have allowed the production of needle hydrophones with small active areas (∼40µm), the reduction in sensitivity as active element size decreases is still a limitation with this type of sensor. Several alternatives to the needle hydrophone based on fibre-optic sensing methods have been suggested in the past. These techniques employ reflectometry (1), interferometry (2), diffractivity (3), polarimetry (4) or the inclusion of Bragg reflectors (5) within the fibres to detect the ultrasound. In this paper we present an extrinsic optical fibre sensor based on a polymer film Fabry-Perot Interferometer (FPI). This is a development of previous work in which it was shown that this type of sensor can provide a bandwidth of 20MHz and a wideband noise equivalent pressure of 10kPa (6). The concept has been advanced by making two key developments. Firstly a new range of sensors with bandwidths in excess of 50MHz has been designed. Secondly, in order to develop a relatively low cost system with the necessary robustness for practical field use, the operating wavelength region of the sensors has been shifted to the 1540-1610nm range. This enables the rapidly tuneable, stable and inexpensive fibre-coupled C-L band lasers developed for optical telecommunications applications to be used as the interrogating source.

2F-4 A Fabry-Perot Fibre-Optic Hydrophone for the Measurement of Ultrasound Induced Temperature Change

A wideband fibre optic hydrophone system based on a polymer film Fabry-Perot interferometer has been developed for the measurement of ultrasound fields. The sensor transduction mechanism is based upon the interferometric detection of acoustically-induced changes in the optical thickness of the polymer film. The system is also sensitive to temperature change due to thermal expansion of the polymer film. This permits the sensor to be used to measure temperature changes caused by ultrasound exposure. The advantage of this concept is that it offers the prospect of providing simultaneous measurement of ultrasound fields and induced temperature changes at the same spatial location. Characterisation of the thermal performance of the sensor shows its response to be linear up to 65 degC and the resolution nominally 0.25 degC. Ultrasonically induced temperature rises of 50 degC above ambient were measured when insonating with a HIFU transducer. The response time of the sensor is currently limited to approximately 120 ms due to the tuning speed of the laser

A finite difference analysis of the field present behind an acoustically impenetrable two-layer barrier

The interaction of an incident sound wave with an acoustically impenetrable two-layer barrier is considered. Of particular interest is the presence of several acoustic wave components in the shadow region of this barrier. A finite difference model capable of simulating this geometry is validated by comparison to the analytical solution for an idealized, hard-soft barrier. A panel comprising a high air-content closed cell foam backed with an elastic (metal) back plate is then examined. The insertion loss of this panel was found to exceed the dynamic range of the measurement system and was thus acoustically impenetrable. Experimental results from such a panel are shown to contain artifacts not present in the diffraction solution, when acoustic waves are incident upon the soft surface. A finite difference analysis of this experimental configuration replicates the presence of the additional field components. Furthermore, the simulated results allow the additional components to be identified as arising from the S(0) and A(0) Lamb modes traveling in the elastic plate. These Lamb mode artifacts are not found to be present in the shadow region when the acoustic waves are incident upon the elastic surface.

Moving towards an ideal frequency response with fibre-optic hydrophones

The frequency response of a polymer film Fabry-Perot fibre-optic hydrophone developed for the characterisation of ultrasound fields, has been investigated. The transduction mechanism of the hydrophone is based upon the detection of acoustically and thermally induced thickness changes in a polymer film Fabry-Perot interferometer deposited at the tip of a single mode optical fibre. The frequency response of the original sensor has been found to be significantly non-uniform over the 50 MHz operating bandwidth. A finite difference simulation of acoustic interactions with the sensor has successfully been used to predict the response and investigate the origin of the non-uniformities. Additionally, the model has been used to predict the response of a sensor with modified tip geometry in order to find a design capable of providing an improved response. Sensors with a hemispherical tip have now been fabricated, characterised experimentally and found to provide a significantly improved response. Measurements were made on a shockwave toneburst (fc = 1 MHz), using both types of fibre-optic hydrophone and a 0.4 mm PVdF membrane hydrophone. Deconvolution was used (following IEC62127 - 1) in order to produce accurate pressure waveforms from the measured data. It was found that, in addition to providing an improved frequency response, the modification to the hydrophone geometry improved the efficacy of the deconvolution process.

In vitro validation of three‐dimensional cavitation‐based pressure mapping for quality assessment of clinical high intensity focused ultrasound devices.

Sufficiently robust and reliable quality assessment (QA) procedures are vital in assuring the widespread adoption of high intensity focused ultrasound (HIFU) for use in both thermal ablation and enhanced drug delivery. Mapping of broadband cavitation emissions in a tissue‐mimicking material with a repeatable cavitation threshold offers the potential for rapid, 3‐D, cavitation‐based pressure mapping of the field produced by a given HIFU transducer. Previous work has demonstrated the viability of this concept, including the design and optimization of a 50‐element, cylindrical array capable of mapping a collection of broadband sources distributed throughout a region comparable to the size of a typical HIFU focal volume. The work presented here relates to in vitro experimentation using the array to map cavitating fields produced by a number of HIFU transducers at a range of insonation amplitudes. Results are compared to the predicted size of the cavitation region determined via hydrophone‐based characterizati...

Three‐dimensional passive localization of cavitation activity for quality assessment of clinical high‐intensity focused ultrasound devices.

The widespread adoption of high‐intensity focused ultrasound (HIFU) as a modality for non‐invasive treatment of malignant tissue will rely heavily on the implementation of robust and reliable quality assessment (QA) procedures. Previous work has demonstrated the viability of using cavitation emissions as a QA tool for rapid mapping of the acoustic fields generated by HIFU transducers. The present work details the design, implementation, and experimental validation of a cylindrical array surrounding the HIFU focus for 3‐D passive mapping of cavitating fields produced by clinical HIFU transducers. The configuration of array elements was first optimized for accurate mapping of cavitation activity both on‐axis and off‐axis by modeling the received signal from a collection of broadband sources. A novel 50‐element array was subsequently manufactured from PVDF using a novel printed circuit board technique. Testing of the array has been conducted by mapping the extent and evolution of the cavitation field produce...