Introduction

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 gadolini-um(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 non-invasive and has inherent clinical benefits compared to catheter x-ray angiography and CT an-giography 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 x-ray 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 [1519], 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,

The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or reflecting the views of the Department of Defense or the Uniformed Services University of the Health Sciences.

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Fig. 1a-j. CE MRA versus conventional x-ray catheter angiography. Two saccular pseudo-aneurysms of the thoracic aorta in this patient with a remote history of a severe motor vehicle accident are well visualized on 3D CE MRA. The MR exam included pre-contrast 3D MRA (a, sagittal MIP) to assure that artifacts (e.g. phase wrap, etc) are minimal and that adequate anatomic coverage is achieved. The pre-contrast MRA also serves as a useful practice scan for both the patient and the operator prior to contrast administration. The visualization of the two saccular aneurysms (*) on dual phase 3D CE MRA, consisting of arterial-phase (b, sagittal MIP) and delayed-phase (c, sagittal MIP) 3D MRA, compares favorably with that of conventional x-ray angiography (d and e, left anterior oblique projections; f, frontal projections). While traditional MIP projections (b and c) give a good large view of the thoracic aorta, 3D CE MRA benefits from its ability to provide improved illustration of the actual neck of the saccular aneurysms as well as their relationship with adjacent arch vessels using multi-planar reformation (MPR) of the volumetric data sets. Oblique sagittal MPR images (g, arterial phase; h, delayed phase) show the neck of the smaller saccular aneurysm (*) and aberrant origin of the left vertebral artery (arrow) from the aortic arch. The left vertebral artery is better visualized on the delayed phase image (h) because of its small size and relatively slower flow that hampered adequate Gd concentration during the arterial phase. This highlights the benefit of dual phase imaging when performing CE MRA (especially when using ultra fast 3D MRA acquisitions) as slow filling structures may not be seen as they have yet to accumulate sufficient Gd concentration. 3D CE MRA has the advantages of MPR for improved visualization of individual vascular segments and their relationships. In this case, the aberrant left vertebral artery origin is better seen on MPR of the 3D CE MRA than on corresponding planar projections of conventional x-ray angiography (d and e), in which filling of the large superior saccular aneurysm quickly obscures the left vertebral and left subclavian artery origins. On coronal MPR (i, arterial phase; j, delayed phase), the neck of the larger superior saccular aneurysm (*) and its relationship to the origin of the left subclavian artery (large arrow) can be evaluated en face (compare to conventional x-ray angiogram image, f). On delayed phase images (j), peri-aneurysmal enhancement (small arrows) aids in the identification of thrombus (T) within the larger superior saccular aneurysm (Reprinted and adapted with permission from [17])

b a g 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.

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