Monte Carlo simulations of time-of-flight resolved blood flow index: times-of-flight beyond 1 ns permit selective measurement of pulsatile cerebral blood flow

Monte Carlo simulations of time-of-flight (ToF) resolved blood flow index (BFI) are presented. These simulations are validated against continuous-wave (CW) and ToF-resolved BFI measurements in phantoms and in-vivo. We use these simulations to directly compare the benefits of ToF gating and increasing source-detector separation (SDS) for a range of acquisition rates and demonstrate that ToF gating at 920 ps or later provides superior brain selectivity to CW methods at 30 mm SDS. To recover a BFI signal that better represents the brain pulse than the scalp pulse requires a ToF ⪆1 ns. Brain selectivity for both CW and TD devices is highest when the minimum resolvable autocorrelation lag is below 1 μ𝑠 and rapidly diminishes if the minimum lag exceeds 10 μ𝑠.