  Significance: Diffuse in-vivo Flow Cytometry (DiFC) is an emerging
fluorescence sensing method to non-invasively detect labeled circulating cells
in-vivo. However, due to Signal-to-Noise Ratio (SNR) constraints largely
attributed to background tissue autofluorescence, DiFC's measurement depth is
limited. multiplies Aim: The Dual-Ratio (DR) / dual-slope is a new optical
measurement method that aims to suppress noise and enhance SNR to deep tissue
regions. We aim to investigate the combination of DR and Near-InfraRed (NIR)
DiFC to improve circulating cells' maximum detectable depth and SNR.
  Approach: Phantom experiments were used to estimate the key parameters in a
diffuse fluorescence excitation and emission model. This model and parameters
were implemented in Monte-Carlo to simulate DR DiFC while varying noise and
autofluorescence parameters to identify the advantages and limitations of the
proposed technique.
  Results: Two key factors must be true to give DR DiFC an advantage over
traditional DiFC; first, the fraction of noise that DR methods cannot cancel
cannot be above the order of 10% for acceptable SNR. Second, DR DiFC has an
advantage, in terms of SNR, if the distribution of tissue autofluorescence
contributors is surface-weighted.
  Conclusions: DR cancelable noise may be designed for (e.g. through the use of
source multiplexing), and indications point to the autofluorescence
contributors' distribution being truly surface-weighted in-vivo. Successful and
worthwhile implementation of DR DiFC depends on these considerations, but
results point to DR DiFC having possible advantages over traditional DiFC.
