Which procedure requires the use of a contrast agent?

The correct choice involves the myelogram procedure, which specifically utilizes a contrast agent. Myelography is an imaging technique that consists of injecting a contrast dye into the spinal canal (where the spinal cord and nerve roots are located) before conducting X-rays or CT scans. This contrast agent enhances the visibility of the spinal structures and helps to identify any abnormalities such as herniated discs, tumors, or other conditions affecting the spinal cord and its protective layers.

In contrast, Magnetic Resonance Imaging (MRI) can be performed with or without contrast agents, depending on the specific diagnosis needed. While contrast can improve visualization of certain conditions, it is not universally required for all MRI procedures. Ultrasound does not utilize contrast agents; it primarily relies on sound waves to create images of the body's internal structures. Similarly, a CT scan can be performed with or without the use of a contrast agent. Contrast in CT scans enhances certain areas for better visualization of blood vessels, organs, or tumors, but again, it is not a mandatory requirement for the scan itself. Thus, in the context of procedures that inherently require a contrast agent, myelograms are uniquely positioned as the correct answer.