Contrast-enhanced MR angiography: Theory and technical optimization

Contrast-enhanced magnetic resonance angiography (CE MRA) has emerged as a technique of choice for vascular imaging [1-3]. Technical improvements in CE MRA over the past decade have significantly improved not only image quality but also its speed, reliability and ease of use. Performed using traditional extracellular gadolinium( Gd)-chelate contrast media, CE MRA yields angiographic data that are comparable to-and in some instances, superior to-those of conventional catheter angiography. CE MRA, moreover, is noninvasive and has inherent clinical benefits compared to catheter x-ray angiography and CT angiography in that there is no exposure to ionizing radiation or nephrotoxic iodinated contrast media. The latter issue of nephrotoxicity is a major consideration in patients with vascular disease as many also have diabetes mellitus and/or renal insufficiency, making the use of iodinated contrast agents undesirable. CE MRA relies on the T1 shortening effect of Gd-chelate contrast agents in blood [4-8]. This is different from the flow-based time-of-flight (TOF) and phase-contrast (PC) MRA techniques which exploit the inherent motion of blood flow to generate vascular signal. By relying on the presence of Gd within vessels, the vascular signal on CE MRA is not hampered by the numerous flow-related artifacts such as signal loss from spin saturation or slow flow that can degrade flow-based MRA techniques, often resulting in the overestimation of stenoses or the mimicking of a vascular occlusion [9]. With CE MRA, arteries will be visualized if image acquisition is performed during the arterial phase of the bolus. If, on the other hand, imaging is performed later during the venous or delayed phase of the bolus, veins will be visualized. As in conventional angiography, imaging a contrast agent during its vascular transit enables the generation of a luminogram. Since vascular enhancement is a transient and dynamic process, the critical element for CE MRA, as with catheter-based xray angiography, is timing of the imaging. Data from CE MRA can be post-processed to yield projections very similar to those of conventional catheter angiography. CE MRA, generally performed using three-dimensional (3D) MRA pulse sequences, has the added benefit of yielding volumetric data sets which can also be post-processed using multiplanar reformation and various 3D visualization techniques, notably maximum intensity projection (MIP) and volume rendered (VR) display (see Chapter 2). These tools often enable a greater appreciation of vascular segments that would otherwise be obscured by overlying structures on planar projections from conventional catheter angiography (Fig. 1). Over the past decade, CE MRA has benefited from numerous improvements in scanner hardware and from the development of specialized CE MRA software. As a result, it has progressively evolved into the technique of choice for many - if not most - common clinical vascular indications, such as the carotid arteries [10-14], the aorta [15- 19], the renal arteries [19-25], and the peripheral vasculature [25-39]. These applications will be described more extensively in the chapters to follow. In this chapter, the basic principles of CE MRA will be reviewed, and the practical issues related to patient preparation and set up, timing, imaging parameters and contrast agent administration will be discussed. In each section, potential artifacts and pitfalls, as well as strategies for their minimization or avoidance, will be highlighted.