In time-of-flight MR angiography, the flow compensated gradient-echo sequences will be optimized to favor the vascular signal over that of the surrounding tissues by:
The strength of the vascular signal depends on:
The main limitations of time-of-flight MRA are:
Vascular contrast can be improved by suppressing the static tissue signal, by means of:
The direction of the flows to visualize can be selected by placing a presaturation band upstream of the volume of interest, to suppress unwanted arterial or venous flows.
In 2D acquisition, time-of-flight imaging uses a set of fine slices that are stacked up to reconstruct a pseudo-volume. The advantage of fine slices is better sensitivity to slow flows (which do not remain in the slice for long and will therefore not be saturated), with the possibility of using high flip angles (giving better stationary tissue saturation and an increased vascular signal). But the drawback with 2D acquisition is poor spatial resolution along the axis of the slice stack.
Contrary to 2D TOF, 3D TOF volumetric imaging gives good spatial resolution in the 3 spatial directions, with a better signal-to-noise ratio (figure 10.7). Each repetition excites the volume, producing a progressive saturation of the flows, even more so when they are slow. The slowest flows may even disappear entirely. Flow saturation can be reduced as it passes through the explored volume by: