Stationary Arterial Waves in Magnetic Resonance Angiography

Abstract

Editor: Stationary arterial waves refer to the rare and inconsistent angiographic finding of sinusoidal changes in arterial diameter involving a focal segment of vessel. This finding has been reported to occur on approximately 3% of peripheral angiograms, most often involving the femoral and renal arteries, and to a much lesser extent on carotid angiograms (1). Arterial waves have been attributed to several possible etiologies, including arterial spasm and artifact resulting from flow-related disruption of contrast medium layering in vessels and from cardiac pulse and flow waves, which create standing waves when interacting with local interfaces (1–3). To date, this phenomenon has been appreciated predominantly on angiography, with only one report suggesting a sonographic correlate (4). We offer a case report of stationary arterial waves observed on magnetic resonance (MR) angiography and discuss the implication of this finding in regard to proposed etiologies. In 2001, a 34-year-old woman with a history of complex arteriovenous malformation (AVM) of the right lower extremity was evaluated at our department. A multiinjection, multistation, gadolinium-enhanced three-dimensional MR angiogram was obtained on a standard 1.5-T MR system (Siemens Vision; Siemens Medical Systems, Malvern, PA). MR angiography demonstrated a complex AVM involving the right lower extremity with massive enlargement of inflow, outflow, and runoff vessels. Stationary arterial waves were present from the left external iliac artery to the distal superficial femoral artery (Figure, parts a,b). The patient returned for follow-up 2 years later and MR angiography was repeated this time on a high-performance cardiovascular 1.5-T MR system (Sonata; Siemens) with use of a gadolinium-enhanced stepping table MR runoff technique. Again, stationary arterial waves were present in the left lower extremity arterial system in a similar distribution as seen previously (Figure, parts c,d). Stationary arterial waves demonstrate a markedly regular periodicity and smoothness that differentiate them from fibromuscular hyperplasia, which is irregular and nonperiodic. Lehrer (1) theorized that arterial stationary waves are standing waves created by oscillations resulting from retrograde flow within an artery during the pulse cycle. This was in contradistinction from earlier theories that suggested that the pressure pulse wave was responsible for this phenomenon. He noted that the scale of distance of retrograde flow in medium-sized arteries (as measured in animal models) was of the proper magnitude to produce angiographic stationary arterial waves, whereas pressure pulse waves would result in a wavelength on the order of meters. Animal studies had also shown that the magnitude of backflow within an artery is increased with upstream runoff and downstream impedance (1). Conversely, retrograde flow is decreased with distal hyperemia and increased upstream impedance. We speculate that, in our case, the significant flow to the AVM of the right lower extremity caused an increase in magnitude of oscillatory retrograde flow within the left lower-extremity arteries. This condition permits the formation of stationary arterial waves. It should be mentioned that the appearance of stationary arterial waves is rare even with the precondition of oscillatory retrograde flow. Mathematic conditions require that the magnitude of the velocity components of the forward and retrograde flow be equal for standing waves to form. The presence of standing waves implies increased magnitude of retrograde flow, not absence of net forward flow, as there is a nonzero steady-state forward flow (1). The presence of an AVM in the right lower extremity in our patient may stabilize this condition, allowing for imaging of arterial waves in two consecutive MR angiography studies 2 years apart. This case, in addition to the cases presented by Kroger and Massalha (4), suggest that stationary arterial waves are not a modality-specific phenomenon related to intraarterial injection of contrast media. Instead, these cases support Lehrer’s assertions (1) that stationary arterial waves are a physical phenomenon mediated by the physiologic flow conditions in the arteries in which they occur.