Pulsed photoacoustic Doppler flowmetry using time-domain cross-correlation: accuracy, resolution and scalability.

The feasibility of making spatially resolved measurements of blood velocity using a pulsed photoacoustic Doppler technique in acoustic resolution mode has been investigated. Doppler time shifts were quantified via cross-correlation of photoacoustic waveform pairs generated within a blood-simulating phantom using pairs of light pulses. The phantom comprised micron-scale absorbers imprinted on an acetate sheet and moved at known velocities. The photoacoustic waves were detected using PZT ultrasound transducers operating at center frequencies of 20 MHz, 5 MHz and 3.5 MHz; measurements of velocity and resolution were calculated from the mean cross-correlation function of 25 waveform pairs. Velocities in the range ±0.15 to ±1.5 ms(-1) were quantified with accuracies as low as 1%. The transducer focal beam width determines a maximum measurable velocity |V(max)| beyond which correlation is lost due to absorbers moving out of the focal beam between the two laser pulses. Below |V(max)| a measurement resolution of <4% of the measured velocity was achieved. Resolution and |V(max)| can be scaled to much lower velocities such as those encountered in microvasculature (< 50 mms(-1)). The advantage of pulsed rather than continuous-wave excitation is that spatially resolved velocity measurements can be made, offering the prospect of mapping flow within the microcirculation.