Photoacoustic imaging velocimetry for flow-field measurement

Ultrasonically encoded photoacoustic flowgraphy in biological tissue

Blood flow speed is an important functional parameter. Doppler ultrasound flowmetry lacks sufficient sensitivity to slow blood flow (several to tens of millimeters per second) in deep tissue. To address this challenge, we developed ultrasonically encoded photoacoustic flowgraphy combining ultrasonic thermal tagging with photoacoustic imaging. Focused ultrasound generates a confined heat source in acoustically absorptive fluid. Thermal waves propagate with the flow and are directly visualized in pseudo color using photoacoustic computed tomography. The Doppler shift is employed to calculate the flow speed. This method requires only acoustic and optical absorption, and thus is applicable to continuous fluid. A blood flow speed as low as 0.24  mm·s(-1)} was successfully measured. Deep blood flow imaging was experimentally demonstrated under 5-mm-thick chicken breast tissue.

Ultrasound-heated photoacoustic flowmetry

We report the development of photoacoustic flowmetry assisted by high-intensity focused ultrasound (HIFU). This novel method employs HIFU to generate a heating impulse in the flow medium, followed by photoacoustic monitoring of the thermal decay process. Photoacoustic flowmetry in a continuous medium remains a challenge in the optical diffusive regime. Here, both the HIFU heating and photoacoustic detection can focus at depths beyond the optical diffusion limit (~1 mm in soft tissue). This method can be applied to a continuous medium, i.e., a medium without discrete scatterers or absorbers resolvable by photoacoustic imaging. Flow speeds up to 41 mm·s-1 have been experimentally measured in a blood phantom covered by 1.5-mm-thick tissue.

Biomedical photoacoustics in China

During the last decade, along with its explosive growth globally, biomedical photoacoustics has become a rapidly growing research field in China as well. In particular, photoacoustic tomography (PAT), capable of imaging intact biological tissue in vivo at great depths, has generated intense interest among Chinese researchers. This review briefly summarizes the current status and recent progress of the research in PAT in China. The focus is on the technology development and biomedical applications of three representative embodiments of PAT: photoacoustic microscopy, photoacoustic computed tomography, and photoacoustic endoscopy. In addition, recent development and studies in other related areas are also reviewed shortly.