Contrast-enhanced MRA also depicted significantly more pedal vessel segments than did DSA in a study by Hofmann et al.⁶ The technique proved superior in predicting an appropriate distal site of graft anastomosis.

A study of 35 consecutive patients with peripheral arterial venous disease, conducted by Cronberg et al, found contrast-enhanced MRA to be 92% sensitive for the detection of significant stenosis in the crural and pedal vessels.

The specificity, however, was only 64%.7 A total of 20/129 vessel segments were classified as normal on MRA, but they were considered to harbor either a significant stenosis or occlusion on DSA. The researchers concluded that while contrast-enhanced MRA can be considered as a useful diagnostic adjunct, it should not replace DSA as a standard of reference.

A basic limitation of contrast-enhanced pedal MRA is the restriction of coverage to a single anatomic area. A complete peripheral runoff study requires the arterial tree to be imaged from the infrarenal aorta down to the pedal vessels. Techniques have been developed to achieve this coverage. Multistation acquisition of peripheral arteries is possible with high-performance gradient systems, ultrafast 3D sequences, automatic movement of the patient table, optimized 3D volume placement with flexible choice of scan parameters, and the availability of dedicated multichannel phased-array coils.

Problems could potentially arise with bolus chase techniques. The assessment of crural arteries may be limited due to early filling of adjacent veins. This is strongly accentuated when cellulitis and diabetic ulcers shorten the arteriovenous transit time of the contrast. Venous enhancement could even occur before the filling of the distal arteries. Venous compression is unlikely to be helpful in such cases, particularly if the patient has critical limb ischemia and a severely restricted arterial blood supply.⁹ The other main issue with bolus chase techniques is that coronal acquisition of crural arteries does not always include the pedal arteries.

The optimal approach for delineating the entire peripheral vasculature seems to be a hybrid dual-bolus approach. The cruropedal arteries are acquired first using sagittal slabs and a time-resolved acquisition technique.^10-12 This allows continuous access to the calf and pedal arteries and may lessen the incidence of partial- volume artifacts in the slice direction. Two- or three-station bolus chase MRA should follow next, enabling assessment of the aorto-iliac, femoropopliteal, and proximal calf vessels.

An evaluation of this dual-bolus approach found that the cruropedal arteries could be displayed with excellent or good image quality in 95% (205/216) of cases. Venous overlay was absent in 94% (203/216) of cases.10 The sensitivity of this technique ranged between 80% and 100%, compared with DSA. Specificity varied from 93% to 100%.

The lack of venous overlay when using a hybrid angiography technique has been confirmed in a study of 19 patients with critical limb ischemia.13 A further study of 53 patients, which involved parallel acquisition, showed the dual-bolus approach to be robust.¹¹