A Monte Carlo study on the effects of erythrocyte oxygenation on photoacoustic signals

A theoretical model to study the effects of erythrocyte oxygenation on photoacoustic (PA) signals is described. An erythrocyte was considered as a fluid sphere and for such a sphere the PA field was computed by using a frequency domain approach. The linear superposition principle was used to obtain the resultant PA field generated by a collection of red blood cells (RBCs). A Monte Carlo algorithm was used to simulate 2D tissue realizations consisting of oxygenated RBCs (RBCOs) and deoxygenated RBCs (RBCDs). The oxygen saturation level of RBCOs was assumed to be 100% and 0% for RBCDs. The proportion of RBCOs and RBCDs fixed the oxygen saturation (SO₂) of a blood sample as, SO₂ = N_O/(N_O + N_D), where N_O and N_D represent the numbers of RBCOs and RBCDs. The simulation results showed that the mean PA signal amplitude decreased monotonically as the SO₂ level increased for the 700 nm laser radiation. The same quantity exhibited a monotonic rise as the SO₂ level increased for the 1000 nm optical source. The PA amplitude demonstrated nearly 6 fold decrease and 5 fold increase, respectively at those wavelengths when SO₂ level varied from 0 to 100%. Spectral intensity in the low frequency range (<; 10 MHz) also decreased for the first laser and increased for the second laser with increasing SO₂. However, these trends were not distinctly observed between 10-100 MHz. The simulated trends were in accordance with other experimental works. This suggests the suitability of this formulation to model the PA signal behaviors at different SO₂ levels.