Flow imaging using an integrated photoacoustic/ultrasound probe

An analysis has been made into whether an hybrid photoacoustic/ultrasound probe can be used to image flow using pulsed photoacoustic flowmetry. Until now, no 2D velocity imaging has been shown and research has focused more toward higher-frequency single-element detectors. The probe used here consists of a 7.5 MHz linear array and a 805 nm pulsed diode laser, both housed within the hand held probe itself for optimal portability of the system. The diode laser can be pulsed at up to 10 kHz, comparable to ultrasound systems used for flow estimation, and the linear array provides the 2D imaging capabilities. The probe was used here in vitro: fixed at an 55 degrees angle to polyethylene tubing of 0.58 mm inner diameter, both placed in a water basin. Black polystyrene particles with a diameter distribution of 53-63 mu m were pumped in a suspension through the tubing. The particles are sub-resolution to the linear array's transducers, and should present a challenge to distinguish while providing enough optical contrast for improved detection and signal-to-noise ratio. A full cross-correlation is used to estimate the flow, and implemented in a sliding window to extract a velocity map. 1D flow profiles are presented, from which the average velocity shows good agreement with the pump speed. There is a viscosity-related bias in the flow estimation, but the repeated measurements show less than 13% fluctuation in measured flow. Finally, a 2D flow map is shown with comparable bias and measurement precision.