Simulation of Photoacoustic Imaging of Red Blood Cell Aggregation Using a Numerical Model of Pulsatile Blood Flow

Photoacoustic (PA) imaging of blood flow can provide label free and non-invasive assessment of red blood cell (RBC) aggregation and the oxygen saturation (sO ₂ ). Our group has previously demonstrated that the interrelationship between RBC aggregation and the sO ₂ during a pulsatile blood flow could be potentially assessed using PA imaging. The pulsatile blood flow yields spatiotemporal changes in RBC aggregation, affecting PA imaging. A simple particle motion model was developed based on the blood flow velocity measured from the human radial artery (RA). The positions of randomly distributed, identical circular particles (2.7 um radius) in the lateral-axial plane (20 mm by 2 mm) were traced at each time step of an experimentally measured velocity profile. At each step, the time dependent PA power (PPA) from each single cell (or particles interacting to form aggregates) was computed by modeling and accounting for the directivity of a 21 MHz (9.2 to 32.8 MHz bandwidth) linear array. In-vivo PA images of the RA of healthy volunteers were acquired using the VevoLAZR equipped with a 21 MHz linear-array probe. The measured PA images were compared to the simulated PA images. The aggregates formed a parabolic front along the axial direction and were driven to the right-hand side along the lateral direction as the simulation propagated in time. The PPA was also large at the parabolic front, and was also driven to the right-hand side for every time step. The spatiotemporal distribution of the computed PPA was comparable to the experimental PPA. Specifically, the PPA increased by 12 dB along the lateral direction. These results can be used to study the label-free, non-invasive assessment of the spatiotemporal distribution of sO ₂ in vivo. Furthermore, the improved particle model can provide insights into the mechanism of PA wave generation from RBC aggregation during in vivo blood flow.