**Core Concept**
Myelography is a radiographic imaging technique used to visualize the spinal cord and its surrounding structures by injecting a contrast agent into the spinal canal. The contrast agent used in myelography should be an iodinated compound that is radiopaque and has a high viscosity to maintain its position within the spinal canal.

**Why the Correct Answer is Right**
Myodil, also known as iophendylate, is an oily contrast agent that has been used for myelography due to its high viscosity and ability to remain within the spinal canal. Its oily nature allows it to coat the nerve roots and spinal cord, providing clear visualization of these structures on radiographs. Myodil is metabolized slowly, which helps maintain its position within the spinal canal for an extended period.

**Why Each Wrong Option is Incorrect**
**Option A:** Conray 320 is an iodinated contrast agent used in various radiographic imaging modalities, including intravenous urography and computed tomography (CT) scans. However, its low viscosity makes it unsuitable for myelography.
**Option C:** Dianosil is not a commonly used contrast agent in myelography. Its properties are not well-suited for this specific application, and it is not widely recognized as a myelographic contrast agent.
**Option D:** Iopanoic acid is an iodinated contrast agent used in cholecystography and other imaging modalities. However, it is not used in myelography due to its low viscosity and rapid metabolism.

**Clinical Pearl / High-Yield Fact**
Myodil was one of the first contrast agents used in myelography, and its use has largely been replaced by non-ionic, water-soluble contrast agents. However, it is still worth remembering as a historical example of a contrast agent used in this specific application.

**✓ Correct Answer: B. Myodil**