In vivo photoacoustic time-of-flight velocity measurement of single cells and nanoparticles

Optical techniques for in vivo measurement of blood flow velocity are not quite applicable for determination of velocity of individual cells or nanoparticles. Here, we describe a photoacoustic time-of-flight method to measure the velocity of individual absorbing objects by using single and multiple laser beams. Its capability was demonstrated in vitro on blood vessel phantom and in vivo on an animal (mouse) model for estimating velocity of gold nanorods, melanin nanoparticles, erythrocytes, leukocytes, and circulating tumor cells in the broad range of flow velocity from 0.1 mm/s to 14 cm/s. Object velocity can be used to identify single cells circulating at different velocities or cell aggregates and to determine a cell's location in a vessel cross-section.