Despite some shortcomings  , DSA remains the present day gold standard for evaluating the peripheral arteries. It must also be stated that selective angiography (unilateral injection into iliac, femoral, or popliteal artery) is vastly superior to a non-selective exam where the contrast is injected into the distal aorta. [10,46]. Most peripheral vascular studies are for one of two generalized indications. The first is claudication, in which case most emphasis is on the aortoiliac and femoral arteries, with the infrapopliteal arteries much less important to patient management. The second is for peripheral ischemic disease - non-healing ulcers, gangrene, rest pain, chronic osteomyelitis etc. In these circumstances, the below-knee arteries, including the pedal arteries, are extremely important to determining optimum patient therapy. Peripheral MRA has become quite reliable for work up of the claudication patient, but is in general somewhat lacking for the peripheral ischemic patient, as below-knee resolution limitations and problems with obscuring venous enhancement hinder peripheral MRA [28,47-50]. Until recently, many investigators have been satisfied with MRA examinations that extend from the aorta to the ankle, yet exclude the pedal arteries. While this is striking as an advance for MRA, in the case of an ischemic patient, a vascular surgeon or angiographer will not consider the study complete until the pedal arch is depicted.
Thus pedal MRA, while presently not typically performed as a stand alone exam, is increasingly recognized as an important component of the complete peripheral MRA exam, and MR angiog-raphers are attempting to include high quality pedal imaging as part of the peripheral MRA examination [24, 26]. In addition, with the recogni tion that even selective DSA does not visualize all lower extremity and pedal arteries, including pedal arteries that have been proven capable of receiving grafts , there is heightened interest in MRA being the examination of choice for lower extremity ischemic disease. In fact,Velazquez et al. believe that for patients in whom conventional contrast angiography fails to show suitable runoff vessels for use in a limb-salvage procedure, MRA should always be performed . With regard to pedal vascular imaging, we can perhaps extend this logic to state that when considering a patient for possible limb salvage, if a pedal bypass is felt to be viable surgical option but DSA or a non pedal-focused peripheral MRA fails to demonstrate a suitable pedal bypass artery, dedicated pedal MRA as described in this chapter should be performed before proceeding with amputation.
Finally, this section would not be complete without a brief mention of Doppler Ultrasound (DUS) and Multi-Detector CT Angiography (MD-CTA). A recent large series (n=485) of patients with peripheral vascular disease evaluated with DUS showed that ultrasound alone was adequate for surgical planning in all but 36 patients (7%) . The author goes on to state that such ultrasound examinations demand the sonographer have a high level of technical proficiency, anatomic understanding, and advanced training. This is another way of saying that the quality of DUS is highly operator dependant. Another study comparing pedal DUS, CE-MRA, and selective DSA in 37 patients concluded that DSA is questionable as a gold standard, showing both DUS and CE-MRA superior to DSA in predicting the ultimate distal anastomotic site . Furthermore, the probability that a vessel was either faintly or not visualized was greater for DSA than for DUS or MRA, with no significant difference between DUS and MRA. Thus with a well-trained technician, DUS shares many advantages of MRA in terms of surgical planning and finding DSA occult arteries for bypass. One advantage of DUS is the relative availability of the modality compared to MRI. On the downside, however, DUS is only as good as the technologist, and results are provided in a tabular or chart form rather than a true anatomic roadmap image.
Multi-Detector CTA is appealing, primarily because of its relative simplicity, with preliminary studies suggesting peripheral MD-CTA performs quite well (although no dedicated pedal CTA studies have been published to date) [53-55]. Spatial resolution is quite similar to pedal MRA, at ~0.7 x 0.7 x 1.25 mm , and agreement between MD-CTA (peripheral studies that include the dorsalis pedis) and DSA is quite good, particularly for the
Fig. 10. Lateral MIP view of the pedal arch with MultiHance
Fig. 10. Lateral MIP view of the pedal arch with MultiHance above-knee arteries (sensitivity and specificity 91% and 92% in one study , 87% agreement with DSA above the knee vs. 80% agreement below in another ). CTA, however, suffers several drawbacks . First, data is acquired in a transverse plane, meaning very large datasets are collected. Second, venous enhancement can be a problem, as it can with MRA. Third, bony segmentation is mandatory, and this is difficult, particularly in the lower extremities when there is venous enhancement and opacified arteries in near proximity to bones. And finally, calcified vascular plaques can confuse image interpretation, being a main reason for misinterpretation . Of course, the nephrotoxic effects of iodinated contrast must be considered as well. All said, MRA remains the preferred modality over CTA at this time, although more work remains to be done .
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