|
The feasibility of making spatially resolved measurements of blood flow using pulsed photoacoustic Doppler techniques
has been explored. Doppler time shifts were quantified via cross-correlation of pairs of photoacoustic waveforms
generated within a blood-simulating phantom using pairs of laser light pulses. The photoacoustic waves were detected
using a focussed or planar PZT ultrasound transducer. For each flow measurement, a series of 100 waveform pairs was
collected. Previous data processing methods involved rejection of poorly correlated waveform pairs; the modal velocity
value and standard deviation were then extracted from the selected distribution of velocity measurements. However, the
data selection criteria used in this approach is to some extent arbitrary. A new data analysis protocol, which involves
averaging the 100 cross-correlation functions and thus uses all of the measured data, has been designed in order to
prevent exclusion of outliers. This more rigorous approach has proved effective for quantifying the linear motion of
micron-scale absorbers imprinted on an acetate sheet moving with velocities in the range 0.14 to 1.25 ms-1. Experimental
parameters, such as the time separation between the laser pulses and the transducer frequency response, were evaluated
in terms of their effect on the accuracy, resolution and range of measurable velocities. The technique was subsequently
applied to fluid phantoms flowing at rates less than 5 mms-1 along an optically transparent tube. Preliminary results are
described for three different suspensions of phenolic resin microspheres, and also for whole blood. Velocity information
was obtained even under non-optimal conditions using a low frequency transducer and a low pulse repetition frequency.
The distinguishing advantage of pulsed rather than continuous-wave excitation is that spatially resolved velocity
measurements can be made. This offers the prospect of mapping flow within the microcirculation and thus providing
insights into the perfusion of tumours and other pathologies characterised by abnormalities in flow status.
|
