A PET/CT scanner combines two imaging technologies, Positron Emission Tomography (PET) and Computed Tomography (CT), in one machine to create detailed pictures showing both body structure (CT) and function/metabolic activity (PET). It's crucial in oncology for detecting, staging, and monitoring cancer, but also helps diagnose conditions like Alzheimer's and heart disease, allowing doctors to pinpoint problems, plan treatments, and assess therapy effectiveness with high precision.