Abstract : 3γ imaging is a new nuclear medical imaging technique which has been suggested by Subatech laboratory. This technique involves locating three-dimensional position of the decay of an innovative radioisotope (β+, γ) emitter the 44Sc. The principle consist in the detection of two photons of 511 keV gamma rays from the decay of the positron, provided by a PET ring detector, associated to the detection of the third photon by a Liquid xenon Compton telescope. The energy deposited in the interaction between the photon and xenon and its position are identified by measuring the ionization signal with a MICROMEGAS chamber (MicroMesh Gaseous Structure), while the trigger and time measurement of the interaction are provided by the detection of thescintillation signal. The principle of the TPC is thus usedto Compton imaging.In order to demonstrate experimentally the feasibility of imaging 3γ, a small prototype, XEMIS (Xenon MedicalImaging System) was developed. This thesis is an important step towards the proof of feasibility. In this work are exposed the characterization of the detector response for a beam of 511 keV gamma rays and the analysis of data derived from it. The measurement of energy and time resolutions will be presented, as well as the purity of the liquid xenon.