3D MRA visualization and artery-vein separation using blood-pool contrast agent MS-325.

Magnetic resonance angiography (MRA) is established as an important complementary technique to conventional angiography, and contrast–enhanced MRA (CE-MRA) offers even higher contrast between the vascular lumen and surrounding structures. MS-325 is a gadolinium-based MR contrast agent designed specifically for blood-pool imaging, or MRA, and is the only gadolinium-based intravascular contrast agent undergoing trials in humans. MS325 provides excellent vascular and selective arterial enhancement during dynamic MRA. The long blood residence time also allows acquisition of steady-state images of the arteries and veins with excellent spatial resolution (1,2). With the increasing use of CE-MRA, venous contamination of arterial images becomes a common concern (ie, venous enhancement may confound the visualization of arteries). Currently available viewing techniques, such as targeted maximum intensity projection, multiplanar reformation, and “fly through” used in virtual endoscopy, can be used only to minimize this problem but not to solve it (3,4). These techniques are also time intensive and require more operators. Although the ability to acquire dynamic images may facilitate artery-vein separation by providing an artery mask that can be applied to steady-state images, this approach requires motion correction and image registration between dynamic and steady-state images. The separation of artery and vein is of significant importance to correctly diagnose and treat peripheral vascular diseases. The strategies for artery-vein separation include both acquisition methods and postprocessing techniques. Among the current developments, acquisition methods include phase-contrast and time-resolved acquisition approaches (5–7), and postprocessing techniques cover correlation analysis and graph searching methods (8–10). The shortcomings of these approaches are the limitations of their applications and the costs. For instance, phase-contrast acquisition approaches (5) are limited to cases in which the blood flow directions in artery and vein are opposite to each other. In the time-resolved acquisition approaches (6,7), the image must be acquired during the first pass of a contrast agent or accomplished with cardiac gating. Correlation analysis (8) requires seven or eight MRA data sets in a single breath hold for a three-dimensional (3D) angiogram of the lung. In graph searching approaches (9), the node costs require 3D edge strength and a model of preferred branch direction. The enhanced artery visualization method (10) is limited to the segmentation of a small number of the main overlapping veins in the peripheral vasculature. Clearly, a more general approach for artery-vein separation is desirable.