2.5. Image reconstruction and analysis

All acquired data were first reconstructed in Labview (NI, USA) in real-time by using back-projection algorithm [33]. The reconstruction parameters can be adjusted in the program. The raw data were stored for offline processing by Matlab (R2016b, Mathworks, Inc., MA, USA). In the offline processing, 0.2-10 MHz filtering was implemented to remove high frequency noise and images were reconstructed in back-projection schemes [33].

Relative photoacoustic signal intensity (PSI_re) were calculated to reflect the ICG concentration change as follows:

In order to quantify the kinetics of PSI_re change after it reached the peak after ICG injection, an exponential decay model [34] was used as follows:

A schematic diagram of this model is shown in Fig. 3 in red line. PSI_re(t) is the relative PSI, which reflected the ICG concentration at time t, and PSI_re(0) is the peak PSI_re after ICG injection. The elimination time (t₀) indicates the elapse time from PSI_re(0) to the base line. Rate constant (k) gives the decay rate.

An exponential decay model was used to determine the PSI_re change after it reached the peak after ICG injection (red line).