Time of flight angiography

Time of flight angiography (TOF) is an MRI technique to visualise flow within vessels, without the need to administer contrast. It is based on the phenomenon of flow-related signal hyperintensity 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 saturated just like stationary tissue, resulting in a signal loss from the vessel ⁴

• 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 (more than 5 minutes) ³.

• 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. haemorrhage