Time-of-Flight-MRA
Time of flight angiography (TOF) is an MRI technique to visualize flow within vessels, without the need to administer contrast. It is based on the phenomenon of flow-related enhancement of spins entering into an imaging slice. As a result of being unsaturated, these spins give more signal than surrounding stationary spins.
With 2-D TOF, multiple thin imaging slices are acquired with a flow-compensated gradient-echo sequence. These images can be combined by using a technique of reconstruction such as maximum intensity projection (MIP), to obtain a 3-D image of the vessels analogous to conventional angiography.
With 3-D TOF, a volume of images is obtained simultaneously by phase-encoding in the slice-select direction. An angiographic appearance can be generated using MIP, as is done with 2-D TOF.
Key points
- short TR
- image-plane kept perpendicular to flow direction
Potential pitfalls
- slow flow or flow from a vessel parallel to the scan plane may become desaturated just like stationary tissue, resulting in a signal loss from the vessel
- turbulent flow may undergo spin-dephasing and unexpectedly short T2 relaxation: again resulting in a signal loss from the vessel
- acquisition times are relatively long.
- retrograde arterial flow may be obscured if venous saturation bands have been applied.
- artifacts: ghosting, susceptibility artifact
- very T1 bright signal will be visible, e.g. hemorrhage
Siehe auch:
und weiter:
Assoziationen und Differentialdiagnosen zu Time-of-Flight-MRA:
