Noninvasive imaging of {alpha}vß3 expression in living mice by positron emission tomography (PET). The images show transaxial sections of the thorax of one nude mouse which was imaged three times with a small animal PET system using the radiolabeled {alpha}vß3 ligand [¹⁸F]Galacto-RGD. Ninety minutes after tracer injection the xenotransplanted, {alpha}vß3 positive tumor (human melanoma M21) shows intense tracer uptake (top). Treatment with increasing amounts of an unlabeled {alpha}vß3 antagonist leads to a dose dependent decrease in tracer uptake (center, bottom). This indicates that serial PET studies with [¹⁸F]Galacto-RGD may be used to monitor noninvasively the blockade of the receptor during therapy with specific antagonists.