In MR angiography, gadolinium-based contrast is used to improve vessel visualization. Which technique is used when gadolinium is contraindicated?

• A. Intra-arterial digital subtraction angiography
• B. Time-of-Flight non-contrast MRA
• C. CT angiography with iodine contrast
• D. Ultrasound Doppler

Answer: (B)

When gadolinium cannot be used, imaging the vessels with MR without contrast relies on flow-related signal to visualize arteries. Time-of-Flight non-contrast MR angiography uses the natural inflow of fresh, unsaturated blood into the imaging volume. This blood appears bright against the saturated background tissues, so arteries stand out without any contrast agent. It’s especially helpful for patients with renal impairment or gadolinium allergy.

Other options are not MR-based alternatives that avoid contrast: intra-arterial digital subtraction angiography is an invasive procedure using iodinated contrast; CT angiography uses iodinated contrast and radiation; and ultrasound Doppler depends on sound waves rather than MR signal.