Biomedical subjects

D R Billson

Publications and source records attributed to D R Billson.

11 recordsLinked to original sources

PubMed2005

The radiated fields of focussing air-coupled ultrasonic phased arrays.

This paper presents an investigation into the fields radiated into air by ultrasonic phased arrays under transient excitation. In particular, it includes a theoretical prediction of spatial variations in amplitude throughout the both the near-field and far-field of such arrays. The approach has been used to predict the result of phasing to produce a focus in air, which can be seen to be particularly effective in the near-field of the array. Interesting features are observed, which are then described in terms of the performance of both individual elements and the resulting array. It is shown how some elements of design can be used to improve performance in focussing. The predictions are compared to the results of experiments in air using electrostatic arrays, where good focussing could be achieved provided the appropriate design principles were followed. The approach has been developed specifically for use in air, but the results would also hold for modelling in certain medical arrays where a focussing requirement might be needed close to the array itself.

Journal Article↗

PubMed2004

Modelling of the radiated field from multi-element capacitive micromachined ultrasonic transducers.

This paper presents results from a theoretical model of the ultrasonic fields radiated by a 3x3 assembly of capacitive micromachined ultrasonic transducer (cMUT) sources on the same silicon substrate. These predictions have, for the first time, been compared directly to the fields measured experimentally using a scanned miniature detector. This work indicates that there is minimal cross-coupling between source elements, and demonstrates that it is possible to predict successfully the field characteristics of various geometries of such cMUT elements, with a view to the development of future imaging systems.

Journal Article↗

PubMed2004

Imaging using air-coupled polymer-membrane capacitive ultrasonic arrays.

Polymer-membrane capacitive ultrasonic linear and 2-D arrays have been fabricated for use in air-coupled imaging. By using arrays as receivers, there is a possibility of much faster imaging as the need for physically moving the receiver to scan a sample can be replaced by electronic multiplexing. In order to utilise this, a through-thickness air-coupled image of a composite plate has been made using a 2-D array as a receiver and a comparatively large planar source in air. This was made possible by the use of a chirp drive signal and cross-correlation on the measured waveform. Larger 2-D arrays with an increased number of elements have been simulated using a small scanned single receiver, and excellent imaging potential demonstrated. In addition two array receivers have been used in conjunction with two methods of post-processing, SAFT and ellipse crossing, to locate objects accurately.

Journal Article↗

PubMed2003

A conical air-coupled capacitance transducer for surface imaging.

This paper describes the construction and operation of an air-coupled capacitance transducer with a conical backplate. This was designed to produce a focal region over an extended distance along the transducer axis. Measurements were performed to examine both the frequency response of the transducer in pulse-echo mode, and the lateral resolution for imaging purposes. The radiated field was measured and compared to theory, and the extent of the focal region determined. Images of surface topography are presented, to illustrate the range of application of the transducer.

Journal Article↗

PubMed2003

Gas jets as ultrasonic waveguides.

The study of the propagation of ultrasound within a gas jet is extended to obtain waveguide effects, where the jet collimates the ultrasonic beam from a transducer within the flow. Two methods have been investigated to achieve this--cooling the gas within the air jet, and using a different gas whose acoustic velocity is lower than air. Cooling an air jet to a temperature less than that of the surrounding air produced a noticeable waveguide effect. In addition, studies have been carried out using other selected gases, such as carbon dioxide (CO2) with a lower acoustic velocity than air, and helium (He) with a higher value. The former gas enhanced confinement of the ultrasonic beam, whereas the latter caused divergence. An ideal solution was found to be a CO2/air mixture, which produced a well-collimated sound field along the axis, while limiting the excess attenuation of pure CO2 gas jets. The effectiveness of the waveguide using mixed gas jets in producing images in air-coupled testing of solids is demonstrated.

Journal Article↗

PubMed2002

Gas jet as a waveguide for air-coupled ultrasound.

The directional characteristics of an ultrasonic signal have been studied during propagation within an axial gas jet. The effects of nozzle shape, nozzle diameter, and variations in jet velocity, temperature and gas composition have been investigated. At high flow velocities of an air jet, divergence of the ultrasonic beam was observed. This was attributed to the effects of refraction, caused by increased acoustic velocities in the direction of the flow. An effective waveguide was also demonstrated by cooling the air jet to below ambient temperatures, so that the acoustic velocity in the air jet was lower than that in the surrounding atmosphere. This could also be achieved by using carbon dioxide mixed with air, whereas the use of helium led to increased divergence. The result is likely to be of use in air-coupled ultrasonic materials inspection.

Journal Article↗

PubMed2002

The characterization of capacitive micromachined ultrasonic transducers in air.

Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. They exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz, when driven from a standard ultrasonic voltage source. Experiments are described using 1 mm square devices in air, operating in both pitch-catch and pulse-echo modes. The dependence on d.c. bias voltage is examined, together with calibration measurements using 1/8 in. microphones. The radiated beam profile, and the farfield directivity pattern, have been measured for both broad bandwidth and one-burst excitation, using a scanned miniature receiver. A 16 element square array is also presented, which has been used to measure the beam cross-sections from a focussed source.

Journal Article↗

PubMed2001

The use of broadband acoustic transducers and pulse-compression techniques for air-coupled ultrasonic imaging.

A pulse-compression technique has been applied to air-coupled testing of solid materials. Capacitance transducers were used to generate wide bandwidth swept-frequency (chirp) signals in air, which were then used to measure and image solid samples in through transmission. The results demonstrate that such signal processing techniques lead to an improvement in the signal to noise ratio and timing accuracy for air-coupled testing. Measurements of thickness and spectroscopic experiments are presented. Images of defects in a wide range of materials, including metals and carbon-fibre composites have also been obtained. This combination of capacitive transducers with pulse-compression techniques is shown to be a powerful tool for non-contact air-coupled ultrasonic measurements.

Image Processing, Computer-Assisted↗

PubMed2001

The use of air-coupled ultrasound to test paper.

Capacitance transducers containing a thin polymer membrane have been used to transmit ultrasonic signals with frequencies in excess of 1 MHz through various paper products such as paper and cardboard. At normal incidence, a resonance was visible in thicker samples, the frequency of which could be correlated to parameters such as the thickness of the paper sample and the moisture content. It has also been demonstrated that images can be obtained of changes in structure across paper and card samples.

Journal Article↗

PubMed2001

Novel, wide bandwidth, micromachined ultrasonic transducers.

Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. This process allows inherent control of parameters such as membrane size and thickness, cavity size and the intrinsic stress in the membrane to be achieved. Devices fabricated using this process exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz when driven from a standard ultrasonic voltage source. Experiments have been performed with devices containing silicon nitride membranes of variable thicknesses over a 2 microm thick air cavity and with device dimensions of up to 5 mm square. This is much larger than has been reported for a device with a single membrane. Calibration measurements using 1/8 inch microphones in air, and miniature PVDF hyrdophones in water, have been performed. The dependence on d.c. bias voltage is examined, involving static membrane deflection measurements and received peak voltages. Pulse-echo and pitch-catch mode operation have been achieved. Interferometric measurements of membrane displacement have been performed in air to illustrate the membrane deflection characteristics. Operation in liquids is also discussed.

Equipment Design↗

PubMed1999

Neural network correction of ultrasonic C-scan images.

A neural network-based approach to the correction of C-scan images is presented. This allows the effects of a finite ultrasonic beam diameter in an immersion experiment to be considered, by training the network on a series of defects of known characteristics. The result is an image which is a better representation of the actual defect.

Humans↗