Brunker, J; (2014) Photoacoustic Doppler velocity measurements using time-domain cross-correlation. Doctoral thesis , UCL (University College London).

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Abstract

The feasibility of making spatially resolved measurements of blood velocity using a pulsed photoacoustic Doppler technique has been investigated. Doppler time shifts were quantified via cross-correlation of photoacoustic waveform pairs. The waveforms were generated within a blood-simulating phantom using pairs of light pulses and detected using an ultrasound transducer. Two types of blood-simulating phantom were investigated. The first was a rotating wheel phantom consisting of micron-scale absorbers imprinted on an acetate sheet and moved at known velocities; this simulated plug flow. A time-correlation data processing scheme was used to quantify velocities in the range 0.15 to 1.5 m/s with accuracies as low as 1% and a measurement resolution <4%. The transducer beam width determines a maximum measurable velocity |Vmax| beyond which correlation is lost due to absorbers moving out of the focal beam between the two laser pulses. Resolution and |Vmax| can be scaled to much lower velocities such as those encountered in microvasculature (< 50 mm/s). Velocities in this range were investigated for the second type of phantom comprising absorbers, such as red blood cells or microspheres, flowing in a suspension within a transparent tube; this demonstrated non-plug flow. The absorber-filled tube could also be manually shifted for direct comparison between the plug and non-plug flow cases. Laminar flow gave rise to under-reading of the known velocities, which was exacerbated by increasing absorber concentrations and tube diameters, presumably due to inadequate light penetration into the tube. A novel signal processing scheme (“waveform segmentation”) was developed to surmount this difficulty, and also adds the potential for mapping out the flow velocity profile across the tube. The results show that the absorber spatial heterogeneity can be resolved even using a relatively low frequency detector, and thus pave the way for applying the cross-correlation technique to make blood velocity measurements in vivo.

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