In non-contrast MR angiography, which technique uses flow-related enhancement to visualize vessels?

• A. Time-of-Flight MRA
• B. Phase-contrast MRA
• C. Spin-echo MRA
• D. Diffusion-weighted MRA

Answer: (A)

Flow-related enhancement in non-contrast MRA comes from fresh blood entering the imaging volume that hasn’t been saturated by RF pulses. In Time-of-Flight imaging, stationary tissue within the slab is repeatedly saturated, so its signal drops, while blood flowing into the slab carries unsaturated spins. When these unsaturated spins reach the imaging region, they recover signal more strongly, making vessels appear bright against a darker background. This inflow-based bright signal lets us visualize arteries without any contrast agent.

Phase-contrast MRA, in contrast, relies on velocity-encoded phase shifts to depict flow, which isn’t about inflow enhancement of spins. Spin-echo MRA isn’t typically used for this flow-contrast mechanism in non-contrast angiography, and diffusion-weighted approaches aren’t used to visualize vessels in the same way.