Venous enhancement is virtually unknown in the aorto-iliac station, as commencement of imaging is signalled by contrast arrival in the region-of-in-terest (timing bolus/bolus detection, etc.). This is also true for the femoro-popliteal segment assuming a relatively short acquisition time for the 1st location (8-12seconds), rapid table movement (<4 seconds) and use of centric phase-encoding (i.e. sampling of central k-space data at the start of the scan) for the 2nd location. However, with MT-MRA, data collection at the 3rd location is delayed until scan data for the first two locations is completed. Venous enhancement occurs in the 3rd location when the combined imaging time for the first two locations (plus two table movements) is greater than the circulation time from aorta to calf vein. Although the transit time from aorta to calf artery varies widely, the transit time through the tissues gives additional time for arterial- phase imaging. Although superficial and mild enhancement within the deep veins is acceptable, substantial "problematic" venous enhancement is more common in patients with fast flow states (e.g. cellulitis/venous ulcers/diabetes). Because of the conflicting demands of higher resolution (lengthens scan time)
and elimination of venous enhancement (requires shorter scan time), each examination represents a challenge to trade-off one against the other. Therefore, careful optimization of MT peripheral MRA is required as follows:
1. Imaging at the first location (aorto-iliac) is commenced as soon as contrast arrives in the region-of-interest (timing bolus/bolus detection, etc.) to ensure that there is no "knock-on" delay for leg imaging. In our department we use 2D MR fluo-roscopy which allows visualization of contrast arrival in real-time (Fig. 3a) [37-39].
2. Contrast infusion rate. The infusion must be tailored to the length of the first two scans + table movement x 2 + acquisition of the central lines of k-space for the 3rd location (the first few seconds only). In our institution we use a tailored biphasic injection rate (e.g. an initial injection rate of 1cc/sec x 10 secs, followed by 0.6-1.0cc/sec x 20cc) [40-42].
3. Use image subtraction. Superimposition of subcutaneous and marrow cavity fat on the lower extremity arteries in the AP plane might not be problem with faster infusion rates. However, the lower infusion rates mean that the enhancing arteries are not be clearly differentiated from fat on MIP images. Subtraction of a pre-contrast mask essentially eliminates fat-signal from the post-contrast image and gives dramatically improved image quality .
4. Use optimized k-space filling strategies: Whilst "centric" k-space data acquisition reduces the likelihood of venous enhancement at all locations, it also facilitates higher resolution imaging of the (smaller) legs arteries by allowing acquisition of resolution-defining peripheral k-space mapping to continue regardless of venous enhancement.
5. Tailor the 3D scans to the anatomy and use individually tailored scan parameters for each ROI.
• Localizer. The localizer is more critical for MT-MRA than for any other CE-MRA application. The goal of arterial phase imaging over three consecutive locations can only be achieved if the smallest possible imaging volume per location can be prescribed. We use a low-resolution gradient-echo "time-of-flight" scan in the axial plane taking (approximately 1 minute per loca tion). Anterior and lateral MlP's generated from the axial data-set facilitate accurate 3D volume placement. Using manufacturer-specific moving-table software, localizers can now be acquired from the aorta to the level of the ankles, thus allowing accurate tailoring of the 3D volume for all three locations (Fig. 9). Resolution. 3D volumes with sufficient resolution to allow accurate grading of stenosis within the arteries of interest must be acquired at each of three successive locations. As the arteries progressively decrease in size from aorta to foot, it is advantageous to prescribe independent volumes at all three locations, with highest resolution reserved for the legs. Whilst isotropic voxels are ideal, in-plane spatial resolution and through-plane resolution must be sacrificed in favour of shorter scan times. In-plane resolution for the aorto-iliac and thigh arteries of 1.2-1.6 mm (160-200 phase-encode steps) and through-plane resolution of 3-4mm (slices reconstructed to 1.5-2mm by interpolation or zero-padding) is sufficient. For the 3rd location, higher in-plane and through-plane resolution is required. It is essential to use centric phase-encoding to minimize venous enhancement at this station.
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