Contrast-enhanced MR Angiography

These techniques are faster and offer a better signal-to-noise ratio than methods without contrast agent injection (figures 10.11 to 10.13).
The benefits linked to shortening T1 induced by the Gadolinium chelates are numerous:

• Increase in the vascular signal which becomes predominant in relation to the inflow effect
• Avoidance of blood signal saturation, enabling large volumes to be explored
• Better turbulent flow imaging

3D CE-MRA sequences

The sequences best suited to this imaging are fast 3D gradient echo sequences, with short TR (<= 5 msec) and short TE (1-2 msec). They are T1 weighted with destruction of residual transverse magnetization. Given that echo time is very short, the gradients are not flow compensated.
3D acquisition is used to obtain high spatial resolution and a good signal-to-noise ratio.

To deliver the best contrast, k-space center filling must correspond to the moment of peak intravascular contrast. K-space filling can be optimized (spiral or elliptical centrifugal trajectory, partial refilling, shared data on the k-space periphery) to rapidly acquire its center (image contrast) and meet the constraints of the transient passage of the contrast agent.
With the gain in speed that it entails, parallel imaging is particularly well suited to contrast-enhanced MRA.
A fat signal suppression method is added to weaken the background signal.

Given the size of the volume to explore and the limited field of view in MRI, CE-MR angiography techniques for the lower combine:

• specific coils to fully cover the limbs
• an examination table with fast, automated movement (MobiTrack)
• possible compression of the root of the thigh to reduce venous flow

It is possible to cumulate techniques for accelerating ultrafast gradient echo sequences such as:

• parallel acquisition (SENSE, GRAPPA)
• segmented readout of k-space with variable refreshment at each phase (TRICKS: Time Resolved Imaging of Contrast KineticS, CENTRA: Contrast-ENhanced Timing AngiogRAphy, TREAT: Time-Resolved Echo-shared Angiographic Technique, KeyHole),
• Partial time-correlated k-space acquisitions (k-t BLAST: k-t Broad-use Linear Acquisition Speed-up Technique, k-t SENSE, TWIST: Time-resolved Angiography With Interleaved Stochastic Trajectories)

Combining these techniques will produce sequences with reduced acquisition, improved temporal resolution and even dynamic imaging: 4D angiography (4D – TRAK: 4D Time-Resolved Angiography using Keyhole, TRICKS, TWIST). The first images, before enhancement, serve as a subtraction mask to extract the vascular tree in the succeeding images.
The major drawback with these techniques is the length of processing time.

These sequences demand accurate timing when injecting the bolus, to ensure that acquisition coincides with the intravascular passage of the contrast agent (risk of contamination by the venous signal if too late). The optimal interval between injection and imaging can be determined in several ways:

• Fixed interval (empirical)
• Bolus test
• Real-time tracking of the bolus arrival (« MRI Fluoroscopy ») with manual or automatic triggering: BolusTrack (figure 10.14)
• Dynamic angiography

The duration of the injection may be less than the duration of the sequence as the bolus disperses over time, due to circulation in the venous system then in pulmonary circulation before reaching the systemic circulation. Furthermore, while it is vital to acquire the k-space center during the contrast peak, it is less important for the contrast agent to be present during filling of the k-space peripheral region.
These MRA techniques obviously present the disadvantages inherent to injecting a contrast medium (venipuncture, allergy, renal risk, NSF, high cost).