Imaging description

Images with inadequate arterial opacification from suboptimal bolus timing demonstrate gradually decreasing opacification in the lower extremity arteries as they course distally. Often, the distal calf vessels are most severely affected, sometimes demonstrating a complete lack of opacification, which can simulate occlusion due to atherosclerotic or thrombotic disease. (Figures 87.1 and 87.2). However, immediate delayed images through the lower extremities will show complete opacification of the distal vessels, allowing for accurate characterization of vascular disease in the calf (Figure 87.3).

Importance

The use of multidetector CT for the evaluation of the abdominal aorta and lower extremity vasculature has become a routine clinical examination in multiple patient populations. It is important to design protocols for these studies to avoid the common pitfall of suboptimal bolus timing. Disease in the aortoiliac or the femoropopliteal bed that results in sluggish downstream flow can be misinterpreted as occlusion in the calves if three-vessel runoff is not observed. Similarly, patients with very low cardiac output may require modifications to a standard protocol to account for the slow transit of contrast from the aorta to the feet. For this reason, it may be helpful to obtain immediate delayed images through the calves and feet, as the scanner may progress faster than the contrast bolus during the arterial phase of imaging. For dedicated lower extremity angiography, placement of the bolus tracker on the popliteal artery will also improve timing.

Typical clinical scenario

CT angiography of the abdominal aorta and lower extremities is performed for a variety of reasons. In the emergent setting, it is often required to assess for vascular injury in the setting of trauma or for acute vascular occlusion in a patient with a cold extremity. When ordered non-emergently, the goal is often to evaluate patients with peripheral vascular disease to determine if there is a need for surgical intervention. Regardless of the acuity, bolus timing is an important consideration. If there is stenosis proximally, distal vessels will demonstrate delayed opacification due to slower flow. However, if a study is obtained without proper timing, stenoses may erroneously be interpreted as occlusions.