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Blood flow measurements have been demonstrated using the acoustic resolution mode of photoacoustic sensing. This is
unlike previous flowmetry methods using the optical resolution mode, which limits the maximum penetration depth to
approximately 1mm. Here we describe a pulsed time correlation photoacoustic Doppler technique that is inherently
flexible, lending itself to both resolution modes. Doppler time shifts are quantified via cross-correlation of pairs of
photoacoustic waveforms generated in moving absorbers using pairs of laser light pulses, and the photoacoustic waves
detected using an ultrasound transducer. The acoustic resolution mode is employed by using the transducer focal width,
rather than the large illuminated volume, to define the lateral spatial resolution. The use of short laser pulses allows
depth-resolved measurements to be obtained with high spatial resolution, offering the prospect of mapping flow within
microcirculation. Whilst our previous work has been limited to a non-fluid phantom, we now demonstrate measurements
in more realistic blood-mimicking phantoms incorporating fluid suspensions of microspheres flowing along an optically
transparent tube. Velocities up to 110 mm/s were measured with accuracies approaching 1% of the known velocities, and
resolutions of a few mm/s. The velocity range and resolution are scalable with excitation pulse separation, but the
maximum measurable velocity was considerably smaller than the value expected from the detector focal beam width.
Measurements were also made for blood flowing at velocities up to 13.5 mm/s. This was for a sample reduced to 5% of
the normal haematocrit; increasing the red blood cell concentration limited the maximum measurable velocity so that no
results were obtained for concentrations greater than 20% of a physiologically realistic haematocrit. There are several
possible causes for this limitation; these include the detector bandwidth and irregularities in the flow pattern. Better
results are obtained using a detector with a higher centre frequency and larger bandwidth and tubes with a narrower
diameter.
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