This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nuclear Medicine, Subject name: Radiology
Radioisotope used in PET scan is? 
Select and explain. 
A. Tachnetium 99m, B. Iodine 123, C. Iodine 131, D. Fluoride-18
D: Fluoride-18
Answer: D: Fluoride-18 
Explanation: Ans. (d) Fluoride-18Ref. Sutton's radiology, 7th ed. H826