1. A system for measuring a physiological parameter of blood in a patient, the system comprising: a transmission module configured to emit a plurality of photon density waves into tissue of the patient from a plurality of modulated light sources; a receiver module configured to detect characteristics of the plurality of photon density waves; and a processing module configured to: identify time based characteristics of a pulsatile perturbation of the tissue based on the characteristics of the plurality of photon density waves; and identify a value of the physiological parameter based on at least the characteristics of the pulsatile perturbation of the tissue and the characteristics of the plurality of photon density waves.