The purpose of this clinical research is to define the optimal dose of 68Ga-OPS202 as a PET imaging agent to be used to detect and localize gastro-entero-pancreatic neuroendocrine tumors (GEP-NETs). 68Ga-OPS202 is a radiolabelled imaging agent to be used in association with Positron-Emission-Tomography (PET). 68Ga-OPS202 is made of two main components: 1) OPS202, an antagonistic somatostatin analogue which binds to the somatostatin receptor (type 2) present on the surface of the tumor cells and 2) Gallium 68, a radioisotope that combined with OPS202 can be seen in the PET scanner.