Srinath Rajagopal

A random phased array device for delivery of high intensity focused ultrasound

Randomized phased arrays can offer electronic steering of a single focus and simultaneous multiple foci concomitant with low levels of secondary maxima and are potentially useful as sources of high intensity focused ultrasound (HIFU). This work describes laboratory testing of a 1 MHz random phased array consisting of 254 elements on a spherical shell of radius of curvature 130 mm and diameter 170 mm. Acoustic output power and efficiency are measured for a range of input electrical powers, and field distributions for various single- and multiple-focus conditions are evaluated by a novel technique using an infrared camera to provide rapid imaging of temperature changes on the surface of an absorbing target. Experimental results show that the array can steer a single focus laterally to at least +/-15 mm off axis and axially to more than +/-15 mm from the centre of curvature of the array and patterns of four and five simultaneous foci +/-10 mm laterally and axially whilst maintaining low intensity levels in secondary maxima away from the targeted area in good agreement with linear theoretical predictions. Experiments in which pork meat was thermally ablated indicate that contiguous lesions several cm(3) in volume can be produced using the patterns of multiple foci.

REFERENCE CHARACTERISATION OF SOUND SPEED AND ATTENUATION OF THE IEC AGAR-BASED TISSUE-MIMICKING MATERIAL UP TO A FREQUENCY OF 60 MHz

To support the development of clinical applications of high-frequency ultrasound, appropriate tissue-mimicking materials (TMMs) are required whose acoustic properties have been measured using validated techniques. This paper describes the characterisation of the sound speed (phase velocity) and attenuation coefficient of the International Electrotechnical Commission (IEC) agar-based TMM over the frequency range 1 to 60 MHz. Measurements implemented a broadband through-transmission substitution immersion technique over two overlapping frequency ranges, with co-axially aligned 50 MHz centre-frequency transducers employed for characterisation above 15 MHz. In keeping with usual practice employed within the technical literature, thin acoustic windows (membranes) made of 12-μm-thick Mylar protected the TMM from water damage. Various important sources of uncertainty that could compromise measurement accuracy have been identified and evaluated through a combination of experimental studies and modelling. These include TMM sample thickness, measured both manually and acoustically, and the influence of interfacial losses that, even for thin protective membranes, are significant at the frequencies of interest. In agreement with previous reports, the attenuation coefficient of the IEC TMM exhibited non-linear frequency dependence, particularly above 20 MHz, yielding a value of 0.93 ± 0.04 dB cm(-1) MHz(-1) at 60 MHz, derived at 21 ± 0.5°C. For the first time, phase velocity, measured with an estimated uncertainty of ±3.1 m s(-1), has been found to be dispersive over this extended frequency range, increasing from 1541 m s(-1) at 1 MHz to 1547 m s(-1) at 60 MHz. This work will help standardise acoustic property measurements, and establishes a reference measurement capability for TMMs underpinning clinical applications at elevated frequencies.

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%).

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

Primary ultrasonic interferometer photodiode characterization using frequency-modulated laser wavefront radiation

Photodiodes play an important role in many optical systems and, as such, their stability and performance are of great importance. Manufacturers of such detectors typically only provide information regarding their sensitivity as a function of the optical wavelength and this is often sufficient for applications such as telecommunications. However, in certain applications such as primary ultrasonic standards based on interferometry, the frequency response of the photodiodes is of critical importance because, for accurate calibration, a correction factor must be applied, which contributes a major source of measurement uncertainty. Most optical calibration systems reported in the literature operate either in the GHz range or a very limited MHz range. This work reports on the development of a system based on rotational optical components that produces light patterns scanned through a slit at varying speeds using different deflection mechanisms. This results in the generation of spatially dependent interference fringes in the range 1 Hz up to 115 MHz with an expanded uncertainty of less than 5% for the majority of its frequency range of operation.

A comparison between heterodyne and homodyne interferometry to realise the SI unit of acoustic pressure in water

Optical approaches for hydrophone calibrations offer significant advantages over existing methods based on reciprocity. In particular, heterodyne and homodyne interferometry can accurately measure particle velocity and displacements at a specific point in space thus enabling the acoustical pressure to be measured in an absolute, direct, assumption-free manner, with traceability through the SI definition of the metre. The calibration of a hydrophone can then be performed by placing the active element of the sensor at the point where the acoustic pressure field was measured and monitoring its electrical output. However, it is crucial to validate the performance and accuracy of such optical methods by direct comparison rather than through device calibration. Here we report on the direct comparison of two such optical interferometers used in underwater acoustics and ultrasonics in terms of acoustic pressure estimation and their associated uncertainties in the frequency range 200 kHz–3.5 MHz, with results showing agreement better than 1% in terms of pressure and typical expanded uncertainties better than 3% for both reported methods.

Inter-laboratory comparison of HITU power measurement methods and capabilities

High Intensity Therapeutic Ultrasound (HITU) is gaining in importance among the spectrum of therapeutic options to combat cancer. HITU has already been approved and is in clinical use for the treatment of organs like the prostate, the liver and the uterus. Nevertheless, the metrology of the applied high power ultrasound fields, and in consequence, reliable treatment planning and monitoring, is still a challenge. As part of a European Metrology Research Programme project, the four National Metrology Institutes from the UK, Germany, Italy and Turkey conducted an inter-laboratory comparison of their power measurement capabilities at power levels of 5, 25, 75 and 150 W each at frequencies of 1.1, 1.5 and 3.3 MHz. The task was to measure the total, time-averaged ultrasonic output power, emitted by the circulated transducers under specified electrical excitation conditions into an anechoic water load, and the actual rms transducer input voltage. The output value to be reported was the electro-acoustic radiation conductance including the associated standard and expanded uncertainties. Several different measurement techniques were applied to gain further insight into HITU power measurement. The deviations from the calculated comparison reference value found for the different techniques are discussed and conclusions for the further improvement of measuring procedures are drawn.

Laser generated ultrasound sources using carbon-polymer nanocomposites for high frequency metrologya)

The characterization of ultrasound fields generated by diagnostic and therapeutic equipment is an essential requirement for performance validation and to demonstrate compliance against established safety limits. This requires hydrophones calibrated to a traceable standard. Currently, the upper calibration frequency range available to the user community is limited to 60 MHz. However, high frequencies are increasingly being used for both imaging and therapy necessitating calibration frequencies up to 100 MHz. The precise calibration of hydrophones requires a source of high amplitude, broadband, quasi-planar, and stable ultrasound fields. There are challenges to using conventional piezoelectric sources, and laser generated ultrasound sources offer a promising solution. In this study, various nanocomposites consisting of a bulk polymer matrix and multi-walled carbon nanotubes were fabricated and tested using pulsed laser of a few nanoseconds for their suitability as a source for high frequency calibration of hydrophones. The pressure amplitude and bandwidths were measured using a broadband hydrophone from 27 different nanocomposite sources. The effect of nonlinear propagation of high amplitude laser generated ultrasound on bandwidth and the effect of bandlimited sensitivity response on the deconvolved pressure waveform were numerically investigated. The stability of the nanocomposite sources under sustained laser pulse excitation was also examined.

Laser generated ultrasound sources using polymer nanocomposites for high frequency metrology

Accurate characterization of ultrasound fields generated by diagnostic and therapeutic transducers is critical for patient safety. This requires hydrophones calibrated to a traceable standard and currently the upper calibration frequency range available to the user community is limited to a frequency of 40 MHz. However, the increasing use of high frequencies for both imaging and therapy necessitates calibrations to frequencies well beyond this range. For this to be possible, a source of high amplitude, broadband, quasi-planar and stable ultrasound fields is required. This is difficult to achieve using conventional piezoelectric sources, but laser generated ultrasound is a promising technique in this regard. In this study, various polymer-carbon nanotube nanocomposites (PNC) were fabricated and tested for their suitability for such an application by varying the polymer type, carbon nanotubes weight content in the polymer, and PNC thickness. A broadband hydrophone was used to measure the peak pressure and bandwidth of the laser generated ultrasound pulse. Peak-positive pressures of up to 8 MPa and −6dB bandwidths of up to 40 MHz were recorded. There is a nonlinear dependence of the peak pressure on the laser fluence and the bandwidth scales inversely proportionally to the peak pressure. The high-pressure plane waves generated from this preliminary investigation has demonstrated that laser generated ultrasound sources are a promising technique for high frequency calibration of hydrophones.

Calibration of miniature medical ultrasonic hydrophones for frequencies in the range 100 to 500 kHz using an ultrasonically absorbing waveguide

Enhancements to the existing primary standard optical interferometer and narrowband tone-burst comparison calibration methods for miniature medical ultrasonic hydrophones of the membrane type over the frequency range 100 to 500 kHz are described. Improvements were realized through application of an ultrasonically absorbing waveguide made of a low-frequency-absorbing tile used in sonar applications which narrows the spatial extent of the broad acoustic field. The waveguide was employed in conjunction with a sonar multilayered polyvinylidene difluoride (PVDF) hydrophone used as a transmitting transducer covering a frequency range of 100 kHz to 1 MHz. The acoustic field emanating from the ultrasonically absorbing waveguide reduced the significance of diffracted acoustic waves from the membrane hydrophone ring and the consequent interference of this wave with the direct acoustic wave received by the active element of the hydrophone during calibration. Four membrane hydrophone make/ models with ring sizes (defined as the inner diameter of the annular mounting ring of the hydrophone) in the range 50 to 100 mm were employed along with a needle hydrophone. A reference membrane hydrophone, calibrated using the NPL primary standard optical interferometer in combination with the ultrasonically absorbing waveguide, was subsequently used to calibrate the other four hydrophones by comparison, again using the ultrasonically absorbing waveguide. In comparison to existing methods, the use of the ultrasonically absorbing waveguide enabled the low-frequency calibration limit of a membrane hydrophone with a ring diameter of 50 mm to be reduced from 400 kHz to 200 kHz.