In the future, pedal MRA will likely become recognized as an integral part of the complete peripheral MRA exam, and techniques, coils, and contrast agents that allow for optimal inclusion of the pedal vasculature will be refined. As pedal MRA is not in general performed or interpreted without the remainder of the peripheral exam (abdominal aorta down), new techniques will focus on ways of incorporating high-resolution pedal arterial images into complete peripheral mRa studies. Whether these exams will be single injection moving table vs. multiple injection multi-station examinations vs. time-resolved acquisitions remains to be seen.
A significant part of future advances will likely come from new coil technology [27,56]. Improved coils not only increase SNR and field of view, but allow for higher parallel imaging factors with techniques such as SENSE [26, 28, 57]. This can speed up acquisition time and improve spatial resolution at each station of a moving table peripheral MRA, thereby decreasing chances of venous enhancement and providing improved pedal coverage and spatial resolution.
Another recent advance in peripheral MRA designed to decrease venous enhancement and prolong the arteriovenous window (which can in turn allow for better pedal imaging) uses venous compression. By inflating thigh or calf blood pressure cuffs to sub-systolic pressures, several preliminary studies demonstrate a substantial decrease in venous enhancement (58, 59) While this technique is in its infancy, it may prove extremely useful for pedal imaging in certain cases, although caution must be used, as many patients with critical ischemia cannot tolerate venous compression.
Contrast agents will likely also play a role in the future of pedal MRA. Several new gadolinium chelates have been developed, including conventional extracellular agents at higher concentrations (1.0 Molar vs. 0.5 Molar - gadobutrol - Schering AG, Berlin, Germany), agents with higher relaxivi-ties due to weak protein interaction (Gd-BOPTA -MultiHance - Bracco Imaging, Milan, Italy), and blood pool agents such as MS-325 (EPIX Medical, Cambridge, MA) and B-22956 (Bracco Imaging, Milan, Italy) . None of these agents has, at the time of this writing, been approved for clinical use in the United States, but near future approval is likely in several cases. Early trials (non-pedal) with both the higher concentration and weak protein interaction contrast agents suggest they may be advantageous compared to conventional contrast agents, particularly for imaging of smaller arteries, as would be the case in the foot (Fig. 10) [61-64]. Blood pool agents can be used for first pass MRA in a fashion identical to conventional contrast agents, but then open up the possibility of steady state vascular imaging at very high resolution (Figs. 7, 11) [65,66]. How well AV segmentation algorithms [67, 68] can eventually strip away the veins in a study such as Figure 11 remains to be seen.
One significant advantage of MRI has always been that it does more than purely anatomic imaging. A recent study by Zhang et al. evaluated soft tissue enhancement in the feet of diabetic patients undergoing time-resolved pedal CE-MRA .They found enhancing lesions in 92% of diabetic patients (vs. 51% in non-diabetics, diabetics had greater intensity of enhancement). These regions of soft tissue enhancement occurred in regions where diabetics tend to get ulceration, and the authors suggest these represent precursor lesions to diabetic ulceration. If true, this could add an entirely new dimension to pedal MRA - not only the ability to evaluate the pre-operative diabetic foot, but also help screen for and predict potential sites of ulceration, thereby allowing for pre-emptive preventative treatment.
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