1. Apparatus for interacting with a number of particles which can be introduced into the blood of a subject and which can provide signals from within a vessel of interest that can be detected outside the body, the apparatus comprising: detector means positionable near the body of the subject for detecting the signals from the number of particles at successive locations of each of the number of particles at successive times during the travel of each particle through a vessel of interest; circuit means coupled to said detector means for providing output manifestations from successive positions of each of the number of particles at successive times during the travel thereof through the vessel of interest; and output means responsive to the output manifestations from said circuit means for providing an indication of the relative internal cross-sectional areas of the vessel of interest at successive positions along the cource of the vessel of interest.