Final answer:
The authorized user must complete the written directive for a patient who has undergone radioactive material administration within 48 hours. Employing radiopharmaceuticals with short half-lives, such as I-131 for thyroid imaging, limits the radiation dose to the patient due to faster decay.
Step-by-step explanation:
If a patient undergoes a therapeutic or diagnostic administration of radioactive material, the authorized user must complete the written directive within 48 hours. Utilizing radiopharmaceuticals with different half-lives is critical for varying diagnostic and treatment purposes. For medical imaging, employing isotopes with shorter half-lives helps to limit the radiation dose to the patient, as these isotopes decay more quickly and therefore reduce the time during which radiation is emitted.
For example, small doses of I-131 which has a half-life of approximately 8 days, are used for imaging the thyroid. This relatively short half-life helps to ensure that after a few weeks, almost all radioactive iodine exits the patient's system, reducing prolonged radiation exposure.