Abstract: An apparatus for determining the depth position of venous thrombi in the lower extremities utilizes a unique property in .sup.125 I. This resides in the fact that in approximately half of all disintegrations, two photons are emitted simultaneously. These two coincident photons have, like separately emitted photons, an energy of approximately 28 keV. A scintillation detector perceives the coincidently emitted photons as one photon of an energy of 56 keV. The quotient between the number of separately emitted photons and the number of coincident photons may thus be determined. This quotient constitutes a parameter which is greatly dependent on the distance between the detector and the source of radiation, whereby there is obtained a possibility of determining the depth of the thrombus.