**Core Concept**
Intravenous (I/V) contrast agents are used to enhance the visibility of vascular and soft tissue structures in imaging. They are not used in procedures that rely on intrathecal or intravenous contrast delivery, such as myelography, where contrast is injected into the spinal canal.

**Why the Correct Answer is Right**
Myelography involves injecting contrast directly into the subarachnoid space of the spinal canal, not via IV. This technique is used to visualize the spinal cord and nerve roots. I/V contrast is not used in this route because the spinal canal does not allow IV contrast to reach the target area. In contrast, HRCT, IVP, and MRI may use IV contrast (though MRI often uses IV contrast for better tissue delineation), making myelography the only option that does not use I/V contrast.

**Why Each Wrong Option is Incorrect**
Option A: HRCT (High-Resolution Computed Tomography) often uses IV contrast to improve visualization of lung parenchyma and detect pathology like nodules or interstitial disease.
Option B: IVP (Intravenous Pyelogram) uses IV contrast to visualize the kidneys, ureters, and bladder.
Option C: MRI frequently uses IV contrast (e.g., gadolinium) to enhance tumor detection and tissue characterization.

**Clinical Pearl / High-Yield Fact**
Myelography is a **spinal canal** imaging technique requiring **intrathecal** contrast injection — never IV. IV contrast is contraindicated here due to the risk of spinal cord irritation and failure to reach the target. Always remember: **IV = blood vessels; intrathecal = spinal canal**.

✓ Correct Answer: D. Myelography