Time-of-flight (TOF) angiography 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 that 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. Several 3-D TOF volumes can be combined to visualize longer segments of vessels. 3-D TOF angiography will allow greater spatial resolution in the slice-select direction than 2-D TOF; however, with thick volumes and slow flowing blood, loss of signal is seen with the 3-D TOF method.