Clinical Indications for Blood-pool Imaging of Peripheral Arteries

Clinically, blood pool imaging is indicated: ¹) whenever the spatial resolution of first-pass imaging is insufficient to answer the clinical question; ²) when depiction of both the arterial and venous systems is desired; and ³) when the venous system is of primary interest. Furthermore, there are a number of indications – which are still considered to be experimental at present – such as perfusion imaging, plaque imaging and imaging of tumours. The latter indication is beyond the scope of this article and will not be discussed any further.

Atherosclerotic Peripheral Arterial Occlusive Disease

The most common cause of peripheral arterial occlusive disease is atherosclerosis of the infrarenal aorta and lower extremity arteries. Patients with chronic occlusive disease are generally excellent candidates for imaging with CE-MRA. The aorta and iliac arteries are also referred to as ‘inflow’ arteries in the context of peripheral arterial disease of the lower extremities. CE-MRA acquisitions are performed in the coronal plane, and a parallel imaging, capable phased-array surface coil should be used whenever possible. Truly acquired slice thickness in the first pass should not exceed 2.0–2.5mm if possible. In cases where additional coverage is needed in the anteroposterior direction – for instance in the presence of an abdominal aortic aneurysm or a femorofemoral cross-over bypass graft, or when the LeRiche syndrome is suspected – the number of slices should be increased to cover all the relevant anatomy. Preliminary experience indicates that small collateral vessels are better depicted in the first pass (see Figure 1), because of the higher relaxivity of the contrast medium, than with conventional extracellular contrast agents.⁶ In the steady state, acquisitions should be acquired during cessation of breathing. The key differentiation the radiologist must make when evaluating the upper-leg vasculature is whether there is a relatively short, focal stenosis or a complete occlusion over a long segment. This differentiation is particularly important in the setting of intermittent claudication, as patients and their vascular surgeons may be interested only in invasive treatment in case endovascular options can be considered. There can be substantial added value in acquiring equilibriumphase images when establishing whether a lesion is an occlusion or merely a high-grade stenosis (see Figure 2).

Lower Leg and Pedal Arteries

Although depiction of the infragenicular arterial system in patients with intermittent claudication is important, it is usually not the location of the lesions that causes symptoms, nor the target for invasive intervention, except in patients with diabetes mellitus.⁷ This is opposed to the group of patients with chronic critical ischemia, i.e. rest pain and/or tissue loss. The angiographic hallmark of chronic critical ischemia is bilateral, multiple stenoses and occlusions at different levels in the peripheral arterial tree. Patients with diabetes are a well-recognised subgroup with primarily distal atherosclerotic occlusive disease and preservation of normal inflow. Obtaining a full anatomical study from the infrarenal aorta down to the lower leg and pedal arteries is essential in the pre-interventional work-up of distal peripheral arterial disease. Equilibrium-phase imaging is especially well suited to characterising the distal lower-extremity vasculature. The diameter of the lower leg arteries gradually decreases from approximately 5–6mm in the distal popliteal artery to about 2–3mm in the foot. To reliably diagnose arterial occlusive disease, the spatial resolution should be in the order of 1.0 x 1.0 x 1.0mm3 or better. On modern 1.5T and 3.0T MR scanners equipped with state-of-the-art gradient systems, this resolution is certainly feasible. At this resolution, the higher signal-to-noise ratio with blood-pool agents in the first pass allows for routine high-quality imaging. In fact, Nikolaou et al. have already demonstrated the feasibility of imaging the lower legs with a 0.4 x 0.4 x 0.4 mm3 (64 microns) resolution.⁸ This represents an almost 16-fold decrease in voxel size compared with imaging at 1.0 x 1.0 x 1.0mm3. In the opinion of this author, 0.5 x 0.5 x 0.5mm3 (125 microns) represents a good compromise between spatial resolution and acquisition duration (see Figure 3).