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:
Low limbs MRA
Given the size of the volume to explore and the limited field of view in MRI, CE-MR angiography techniques for the lower combine:
4D Dynamic MRA
It is possible to cumulate techniques for accelerating ultrafast gradient echo sequences such as:
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.
Injection-synchronization in MRA
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:
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).