Secondary CT, or computed tomography, generally has no time interval. Computed tomography belongs to the category of imaging tests and is generally used as an aid in the diagnosis of diseases. Although computed tomography produces some radiation to the human body, the amount of radiation is very small and the harm to the human body is relatively small. There is no clear limit on the interval between two computed tomography examinations. If the condition requires a review, such as a change in the condition of a patient with cerebral hemorrhage, the review can be done on the following day. In case of cancer patients, the interval between two computed tomography examinations may be shorter, mainly to observe changes in the condition, such as whether the cancer cells have spread or metastasized, and the time of the computed tomography examination should be subject to the doctor’s arrangement. If healthy people undergo medical checkups, it is sufficient to have a computed tomography scan once a year. After all, computed tomography scanning has a certain degree of radioactivity and it is not recommended to do it too often. Doctors may use their discretion to perform a second computed tomography scan within a short period of time, depending on the patient’s condition, and this should be done in accordance with the doctor’s instructions.