A system and method for imaging a subject includes a clock that generates a clock signal and a radiation source that directs photons through the subject in response to the clock signal. A detector system is included that detects the photons and a memory module records a time of detection of the photons by the detector system with respect to the clock signal. The system includes a processor that calculates a time of flight (TOF) of the photons from the radiation source to the detector system and compares the TOF to a reference TOF to determine a delay in the TOF attributable to the photons passing trough the subject.