PET/CT scans can detect cancer as small as 5mm. Each scan is different, used for different purposes, and and one may be more specific, sensitive than the other depending on what they're looking for, location like bone, soft tissue, nerves, veins, whole body, lymph nodes, and have had some used in conjunction with one another like CT then PET or PET then MRI, etc. CT is the most often used scan since it's the lowest cost, and doesn't require special training to interpret, doesn't requires dieting, have other restrictions like a PET scan does.