I want to have a brain CT. The reason is to have an exam, that's all. But if there are no symptoms to write, will the doctor agree?

Yes, you need it, and it doesn't violate hospital regulations. The doctor will definitely give you this examination. As for the diagnosis, you can write a physical examination or something. It's normal for a person to have a headache and brain fever. However, I suggest a magnetic vibration of the skull, which has no radiation, but the cost is slightly higher.