Figure 1: There is mild diffuse atherosclerotic disease bilaterally, with multifocal mild stenosis. The left
anterior tibial artery is visualized laterally in both lower legs. Incidentally noted is a high insertion of the right posterior tibial artery.

Figure 2: Sagittal MIP image of left lower leg demonstrates multifocal moderate to high grade stenosis in
the proximal anterior tibial artery. There is apparent patency of the distal anterior tibial artery.

Movie 1: Again noted is mild diffuse atherosclerotic disease bilaterally, with multifocal mild stenosis. Three vessel runoff is present to the right foot. On the left side, there is only two vessel runoff to the foot. The anterior tibial artery demonstrates diffuse moderate to severe stenosis proximally. There is retrograde flow into the anterior tibial artery, probably supplied by the posterior tibial artery. Notice that the retrograde flow cannot be diagnosed by MRA without time resolution (Figure 1).

Movie 2: Sagittal time-resolved MRA of the left leg again demonstrates only two vessel runoff to the foot.
The anterior tibial artery demonstrates diffuse moderate to severe stenosis proximally. Again, there is retrograde flow probably supplied by the posterior tibial artery. The high resolution spatial of each 3D time resolved acquisition permits reconstruction of images in all projections without loss of image quality.

Bilateral lower extremity arterial occlusive disease with retrograde filling of the left anterior tibial artery.

Clinical manifestations of peripheral arterial occlusive disease are either intermittent claudication (pain upon exercise that is relieved by rest) or rest pain. These symptoms depend on whether the distribution of disease is predominantly proximal or distal and on the severity of the disease. Patients with peripheral arterial disease are also at risk of myocardial infarction and ischemic stroke. Diagnostic evaluation of the presence and extent of peripheral arterial obstruction generally has been performed with conventional diagnostic intra-arterial angiography; that is, digital subtraction angiography (DSA). However, this conventional method is invasive, has an increased frequency of adverse events with age, and can fail to demonstrate distal vessels. MR angiography is a safer technique, especially in the elderly, and has great potential as a screening and diagnostic tool to guide interventional procedures. With current MR angiography techniques, the location, severity, and extent of atherosclerotic disease can be demonstrated.

3D contrast-enhanced MR angiography (3D CE-MRA) has become a clinically accepted technique for vascular imaging. In combination with a T1 shortening contrast agent, T1-weighted acquisition provides 3D angiograms of high quality and excellent contrast. Limitations to CE-MRA include a limited spatial resolution, a limited image acquisition window and scan duration, and contrast bolus timing. One solution to the contrast bolus timing problem has been time-resolved CE-MRA. In this technique, multiple 3D data sets are acquired consecutively during the passage of the contrast bolus, ensuring that at least one acquisition interval coincides with the desired arterial bolus phase. This technique provides information regarding the anatomy and flow dynamics in vessels without the need for post-acquisition extraction of arteries from superimposed veins and contrast bolus timing. Time resolved CE-MRA is limited by its requirement for fast data collection which makes high spatial resolution more difficult to accomplish and by time-demanding image reconstruction algorithms because of variability in k-space sampling. Until recently, most of our strategies have used either 2D sequences or 3D imaging with view-sharing or key-holing methods to shorten acquisition times.

Recently a more efficient way of encoding resonance signal, called parallel acquisition technique (PAT), has become available. Parallel acquisition techniques include SENSE (sensitivity encoding) and SMASH (simultaneous acquisition of spatial harmonics). PAT utilizes multicoil arrays that can be used for spatial encoding which would normally be performed using gradients, thereby reducing imaging time. In a typical PAT acquisition, only a fraction (typically ½ or 1/3) of the phase encoding lines are acquired compared to the conventional acquisition. A specialized reconstruction is then applied to the data to reconstruct the missing information, resulting in the full FOV image in a fraction of the time. Using PAT the scan time is typically reduced by a factor equal to the number of parallel receiver coils.. There is a SNR penalty proportional to the square root of the time savings. However, this may be at least partially compensated by the higher SNR generated from the multicoil arrays compared to volume coils or large surface coils. For any PAT factor, all of the savings can be exploited in the interests of more rapid imaging, the resolution can be doubled for any scan time, or a combination of the two can be used. Thus PAT provides time-resolved, contrast enhanced, 3D MR angiography with sub-millimeter in-plane resolution and adequate coverage that does not require view sharing, or temporal interpolation. PAT is particularly attractive for the peripheral vasculature, where most of the time savings can be invested in more rapid imaging for the first two locations with use of much higher resolution for the legs and combined with a time resolved approach in the calves and feet.

References:

1. Weiger M, Pruessmann KP, Kassner A, et al. Contrast Enhanced 3D MRA Using SENSE. Journal Of Magnetic Resonance Imaging. 2000; 12: 671-677.
2. Golay X, Brown SJ, Itoh R, Melhem ER. Time-Resolved Contrast Enhanced Carotid MR Angiography Using Sensitivity Encoding (SENSE). American Journal of Neuroradiology. 2001 September; 22: 1615-1619.
3. Meaney JFM. Magnetic Resonance Angiography of the Peripheral Arteries: Current Status. European Radiology. 2003; 13: 836-852.