Magnetic Resonance Angiography - Artifacts

Artifacts

MRA techniques in general are sensitive to turbulent flow, which causes a variety of different magnetized proton spins to mix inside a single voxel thus causing a local loss of signal (intra voxel dephasing). This can cause mis-diagnosis of stenosis. Other types of MRA related artifacts include:

• Phase-contrast:
  • Phase wrapping: caused by the under estimation of maximum blood velocity in the image. The fast moving blood about maximum set velocity for phase-contrast MRA gets aliased and the signal wraps from pi to -pi instead making flow information unreliable. This can be avoided by using velocity encoding (VENC) values above the maximum measured velocity. It can also be corrected with the so called phase-unwrapping.
  • Maxwell terms: caused by the switching of the gradients field in the main field B0. This cause the over magnetic field to be distort and give inaccurate phase information for the flow.
• Time-of-flight:
  • Laminar flow: In certain vessels, the blood flow is slower on the vessel walls than near the center. This causes some of the blood near the walls to become saturated further upstream in the vessel.
  • Venetian blinds: Since the technique acquires images in slabs, non-uniform flip angles due to uneven distribution of the sinc pulses resulting in non-uniform signal intensity