Spread spectrum time-resolved photon migration imaging system: the principle and simulation results

The image quality of optical tomography depends heavily on the amount of independent information and the signal to noise ratio. Time-resolved photon migration imaging (TPMI) measures a temporal point spread function (TPSF) of light diffusing through a turbid medium. The temporal profile can generally provide more useful information for accurate image reconstruction than continuous wave (CW) and frequency domain (FD) measurements, providing that adequate signal to noise ratio has been achieved. Currently, measurement of TPSF is most commonly realized with the time-correlated single photon counting (TCSPC) technique. Nonetheless, the principle of single photon counting requires that no more than one photon is detected in each cycle. This usually results in a too long data acquisition time, or a rather low signal to noise ratio in a short time. We propose a more efficient coded excitation and correlation detection mechanism that borrows ideas from spread spectrum communications. A train of excitation pulses modulated by a pseudo-random bit sequence is used as the light source, while a cross-correlation scheme is used to retrieve the impulse response. We have simulated a TPMI system and evaluated its performance. It has been demonstrated that the data acquisition time can be reduced by several orders, in comparison with a TCSPC system.