Evaluation of Peripheral Arterial Bypass Grafts

Considering the chronic nature of the atherosclerotic disease process, many patients will ultimately present with renewed complaints after having been treated successfully for intermittent claudication or chronic critical ischemia. Equilibrium-phase imaging may also confer substantial added value over first-pass imaging in patients with bypass grafts. Ultrahigh spatial resolution imaging may actually evolve into an important adjunct to first-pass imaging in these patients for two main reasons: first, steady-state imaging is better suited to characterising the exact degree of stenosis at the sites of proximal and distal anastomosis; and second, because of the ability to perform multiple subsequent ultra-high spatial resolution acquisitions, the field of view becomes virtually unlimited, thus in effect greatly extending anatomic coverage compared with what can be imaged during first arterial passage of contrast medium.

Acquired Conditions Presenting with Symptoms of Peripheral Arterial Disease

Fibromuscular Dysplasia

Arterial fibrodysplasia encompassess a heterogeneous group of vascular occlusive and aneurysmal disorders that can affect virtually any large artery in the body. Most often, the renal, extracranial and intracranial cerebral, and proximal upper extremity arteries are involved. Four principal forms of fibrodysplasia exist: intimal fibroplasia; medial hyperplasia; medial fibroplasia; and perimedial fibroplasia.9 In 85% of cases, the renal arteries are affected by medial fibrodysplasia, producing the characteristic ‘string-of-beads’ appearance due to a series of stenoses interspersed with aneurysmal outpouchings.¹⁰ It is notoriously difficult to make this diagnosis with high confidence on first-pass images due to the limited spatial resolution. Equilibrium-phase ultra-high-resolution imaging, however, can readily demonstrate the characteristic ‘string-ofbeads’ narrowing at the site of involvement and even the fibrous septae lining the arterial lumen (see Figure 4).

Popliteal Artery Entrapment

Popliteal artery entrapment results from an anatomic variant in which the popliteal artery passes medial to and underneath the medial head of the gastrocnemius muscle or a slip of that muscle, with consequent compression of the artery.¹¹ There are five slightly different anatomical variants, and a sixth, ‘functional’ type in which the popliteal artery becomes occluded with plantar flexion but no anatomic abnormality exists.¹² Popliteal entrapment is a disease of young men (male to female ratio is 9:1), and presents with calf or foot claudication. In up to 25% of cases, the abnormality is bilateral.¹² Angiographically, the diagnosis is suggested when there is medial deviation of the proximal popliteal artery (P1 segment) in combination with segmental occlusion or post-stenotic dilatation.¹¹ If no abnormality is seen, additional ‘stress’ views should be obtained during dorsiflexion (with contracted gastrocnemius muscles).¹³ Because of the prolonged intravascular retention of Vasovist, this contrast medium is ideally suited to this indication. The relationship of the popliteal artery to the surrounding soft tissues can be easily demonstrated by reviewing the source images.

Thoracic Outlet Syndrome

The thoracic outlet includes three compartments – the interscalene triangle, the costoclavicular space and the retropectoralis minor space – which extend from the cervical spine and mediastinum to the lower border of the pectoralis minor muscle. Dynamically induced compression of the neural, arterial or venous structures crossing these compartments leads to thoracic outlet syndrome (TOS). The diagnosis of TOS is based on the results of clinical evaluation, particularly if symptoms can be reproduced with various dynamic manoeuvres, including elevation of the arm. Imaging is required to demonstrate neurovascular compression and to determine the nature and location of the structure undergoing compression and the structure producing the compression.¹⁴ As in patients suspected of popliteal entrapment, the added value of equilibrium imaging lies in the possibility of visualising both arteries and veins in various positions, allowing the diagnosis to be made with a high degree of certainty in a non-invasive fashion (see Figure 5).

Venous Imaging

Vasovist-enhanced MR imaging of the venous system is ideally suited to the detection of venous thromboembolic disease and its long-term sequelae. In fact, it is the personal opinion of this author that blood-poolenhanced imaging is the current gold standard for imaging peripheral veins. Deep venous thrombi, even below the knee, are readily detected (see Figure 6).¹⁵ Furthermore, in patients with massive thromboembolic occlusion of central veins, the collateral pathways can readily be visualised in a fashion that is superior to conventional X-ray-based venography (see Figure 7).

Conclusions

CE-MRA of the peripheral vasculature has evolved over the past few years from an experimental imaging modality to a technique that is now widely applied in clinical practice. The recent introduction of the higher relaxivity blood-pool agent Vasovist expands the diagnostic armamentarium of the radiologist by opening up new opportunities in the field of peripheral MRA. The higher relaxivity and prolonged intravascular residence time of Vasovist yield better first-pass image quality, as well as the possibility of obtaining additional steady-state MRA data. The latter property will lead to a fundamental paradigm shift in MR imaging of the vasculature, enabling the migration to equilibrium-phase ultra-high spatial resolution imaging sequences. In combination with current hard- and software, equilibrium-phase ultra-high spatial resolution images of the abdominal aorta and lower extremity vasculature, including the venous circulation, can be obtained that yield the necessary diagnostic and preinterventional information in the vast majority of patients.