Time-of-flight MR angiography
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:
- saturating the stationary tissue signal with very short TR: thus the longitudinal magnetization of these tissues does not have time to regrow and their signal weakens
- favoring the inflow effect (figure 10.4): because the blood flowing into the explored zone has not been saturated, its longitudinal magnetization is maximal. The signal from the blood flow is thus stronger than that of the saturated tissues.
The strength of the vascular signal depends on:
- flow velocity and type
- the length and orientation of the explored vessel (the vascular signal will be better if the slice is perpendicular to the axis of the vessel)
- the sequence parameters: TR, flip angle, TE, slice thickness
The main limitations of time-of-flight MRA are:
- Signal loss linked to spin dephasing when flows are complex or turbulent (stenosis), when flows are too slow or oriented parallel to the slice plane
- Poor signal suppression of the stationary tissues with short T1 relaxation time (fat, atheroma, hematoma, thrombus)
Vascular contrast can be improved by suppressing the static tissue signal, by means of:
- a magnetization transfer preparation pulse
- selective excitation of the water or fat saturation
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.
2D TOF imaging
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.
3D TOF imaging
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:
- dividing 3D acquisition into « slabs » MOTSA: Multiple Overlapping Thin Slab Acquisition), SHUNKS (figure 10.6)
- using a variable excitation angle that is weaker as the flow enters the volume and stronger as it leaves the volume (TONE: Tilted Optimized Nonsaturating Excitation), thus compensating relaxation of short T1 tissues.