Biomedical subjects

Richard E Challis

Publications and source records attributed to Richard E Challis.

4 recordsLinked to original sources

PubMed2006

Ultrasonic imaging of biofilms utilizing echoes from the biofilm/air interface.

Ultrasonic imaging of biofilms in water is difficult due to the very low contrast in acoustic impedance between the biofilm and water. In this paper, biofilms exposed to moist air are scanned through the substrate in order to obtain echoes from the biofilm/air interface. A 50 MHz scanning system was used to scan 1 mm x 1 mm areas of biofilms in a 10 microm grid pattern. Two fast Fourier transform (FFT) based methods for enhancement of the film thickness measurement resolution are compared. Using these techniques, the surface topography of biofilms with thickness less than the acoustic wavelength can be imaged.

Air↗

PubMed2005

New architectures for feedthrough SAW recursive devices.

This paper presents an analysis of a new type of feedthrough recursive surface acoustic wave (SAW) device. The device combines a conventional SAW structure with positive feedback in a way that allows use of selective properties of the SAW structure, control of the central frequency and bandwidth, achieving significantly higher quality factors for given dimensions of the structure, and reduction of the sidelobe level. Several possible implementations are discussed from a simple one that uses external circuitry to the most advanced that includes digital supervisory control. Equations are presented that relate the central frequency, bandwidth, and sidelobe level to the parameters of the SAW structure and feedback loop. The simulation results were found to be in good agreement with experimental data. These data show the control of the central frequency within 1%, a 10-fold increase in the quality factor compared to the original SAW structure, and a reduction of the side-lobe level by 20 dB irrespective of the influence of second order effects and random manufacturing fluctuations.

Journal Article↗

PubMed2005

Errors and uncertainties in the measurement of ultrasonic wave attenuation and phase velocity.

This paper presents an analysis of the error generation mechanisms that affect the accuracy of measurements of ultrasonic wave attenuation coefficient and phase velocity as functions of frequency. In the first stage of the analysis we show that electronic system noise, expressed in the frequency domain, maps into errors in the attenuation and the phase velocity spectra in a highly nonlinear way; the condition for minimum error is when the total measured attenuation is around 1 Neper. The maximum measurable total attenuation has a practical limit of around 6 Nepers and the minimum measurable value is around 0.1 Neper. In the second part of the paper we consider electronic noise as the primary source of measurement error; errors in attenuation result from additive noise whereas errors in phase velocity result from both additive noise and system timing jitter. Quantization noise can be neglected if the amplitude of the additive noise is comparable with the quantization step, and coherent averaging is employed. Experimental results are presented which confirm the relationship between electronic noise and measurement errors. The analytical technique is applicable to the design of ultrasonic spectrometers, formal assessment of the accuracy of ultrasonic measurements, and the optimization of signal processing procedures to achieve a specified accuracy.

Journal Article↗

PubMed2002

Modeling ultrasonic compression wave absorption during the seeded crystallization of copper (II) sulphate pentahydrate from aqueous solution.

Ultrasonic compression wave absorption is investigated as a means to monitor the seeded crystallization of copper (II) sulphate pentahydrate from aqueous solution. Simple models are applied to predict crystal yield, crystal size distribution, and the changing nature of the continuous phase. The Allegra-Hawley scattering formulation is used to simulate ultrasonic absorption as crystallization proceeds. Experiments confirm that simulated attenuation is in agreement with measured results.

Absorption↗