Phase contrast angiography relies on dephasing the moving spins submitted to a bipolar gradient. For a bipolar gradient of a given intensity and time, the moving spins will dephase in proportion to their velocity.
Similar to spatial encoding in the phase direction, the possible phase values range from – π to + π. Beyond this range of values, aliasing occurs, causing poor velocity encoding.
The encoding gradient characteristics are thus defined in order to encode flows within a certain velocity range from – venc to + venc to be determined by the user. Any velocity outside this range will be poorly encoded (similar to what happens in pulsed and color Doppler with PRF).
In a bipolar gradient, the dephasing of the spins moving along the gradient axis is proportionate to their velocity, gradient intensity and the square of the application time of a gradient lobe.
Phase contrast imaging from a thick 2D slice can be compared to projection angiography. Sequence adjustments are needed to suppress the stationary tissue signal and to calculate the phase difference.
The advantage of 2D single-slice acquisition is that it is fast, which is useful for testing different encoding speeds. This technique can also be employed in vascular flow cine imaging, using pulse or ECG synchronization (figure 10.10).
For the quantitative study of flows, the slice plane must be perpendicular to flow direction. Thus a flow velocity curve can be obtained as a function of time, which, when coupled with morphologic data (vascular section) will calculate the flow rate.
In phase contrast volumetric acquisition, each partition is encoded in 3 directions. To reduce acquisition time, the number of phase encoding steps must be reduced. The images are then reconstructed in maximum intensity projection (MIP).
The finesse of the acquisition volume partitions gives better quality images than in 2D single-slice acquisition. Moreover, this technique is more sensitive than time-of-flight for slow flows (which are saturated in time-of-flight).
Its applications namely concern 3D cerebral venous imaging (thrombosis of cerebral veins, fistula) and may also be used after injection of a contrast agent (particularly in cases of defective bolus and contrast-enhanced MRA acquisition). The scan time of these sequences can be greatly reduced by parallel imaging.