What should fill the needle before making an injection of a contrast agent?

Filling the needle with the contrast agent before making an injection is essential to ensure that the agent flows properly and is delivered effectively into the patient's body. By using the contrast agent itself, it also helps to eliminate any air bubbles that may be present in the syringe, which could lead to complications or errors in dosage during the injection procedure.

Using air would introduce potential emboli, which can be dangerous if injected into the bloodstream. Water is not suitable as it may dilute the contrast agent or interfere with its effectiveness. Blood is also not appropriate because it could lead to contamination and adversely affect the patient's health during the imaging process. Therefore, filling the needle with the contrast agent is the standard practice to ensure safety and effectiveness in imaging procedures.