Final answer:
For a 25-year-old female certain she is not pregnant, the best action is to proceed with the CT scan of the sinuses, balancing the need for the test with any risks. Standard procedures may involve a pregnancy test for further assurance before the scan.
Step-by-step explanation:
If a 25-year-old female is scheduled for a CT scan of the sinuses and is absolutely certain she is not pregnant, as it has been three weeks since her last menstrual period, the best course of action among the provided options would likely be to perform the exam immediately while she is experiencing sinus problems. It is important in medical imaging to balance the urgency of the condition with risks. Given that she is certain she is not pregnant, there is no need to postpone, cancel indefinitely, or suggest an MRI instead if a CT scan has been deemed necessary.
It is, however, standard procedure in many healthcare facilities to perform a pregnancy test prior to a CT scan in females of childbearing age to absolutely rule out pregnancy due to the risks that radiation could pose to a developing fetus. This is important to consider even when a patient is certain they are not pregnant because there is always a possibility of an unexpected pregnancy. The radiology department or referring physician can help determine the protocol for such instances, but generally, with the assurance of non-pregnancy, proceeding with the critical diagnostic test is important for the patient's health. Confirmation of non-pregnancy through a rapid test can add an extra layer of safeguard.