Imaging description

Time-of-flight (TOF) magnetic resonance angiography (MRA) images are prone to several artifacts that may suggest stenosis or occlusion of vascular segments. TOF MRA is based on the acquisition of 2D or 3D gradient echo images which are optimized to saturate protons in stationary tissues while maximizing flow-related enhancement from inflowing blood protons that enter the slice during the acquisition. Saturation bands are added to selectively null signal from venous or arterial blood that is flowing in the opposite direction of the vessel of interest. For example, if an image of the abdominal aorta is desired, a saturation band below the plane of interest is used to null inflowing venous blood from the inferior vena cava. Several well-known artifacts occur with TOF imaging. In-plane flow can result in pseudostenosis or artifactual occlusion because blood protons flowing within a vessel parallel to the imaging plane will become saturated during the acquisition (Figure 52.1). Slow-flowing blood may also become saturated before it reaches the end of the TOF slab, such that the distal vessel appears attenuated in luminal diameter and signal. Reversal of flow, which can occur due to retrograde filling with collateral arteries, will be undetectable with TOF techniques due to the use of saturation bands to suppress venous contamination (Figure 52.2). Susceptibility artifacts from surgical clips or adjacent hardware may attenuate the MR signal, and the gradient echo (GRE) sequences used for TOF are especially sensitive for this type of artifact. Finally, dephasing of protons that occurs due to turbulent flow at vessel bifurcations may mimic stenoses, while accelerated and turbulent flow at existing stenoses may lead to overestimation of the degree of stenosis.

Importance

TOF MRA is a widely used unenhanced MRA method. Knowledge of artifacts that may mimic vascular stenosis or occlusion is essential for accurate interpretation. Misinterpretation of the absence or presence of vascular stenosis or occlusion can lead to unnecessary intervention or surgery, or fail to diagnose a treatable cause for the patient's symptoms.