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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 = NO/(NO + ND), where NO and ND 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.