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

RATIONALE AND OBJECTIVESMagnetic resonance angiography (MRA) is establishedas an important complementary technique to conventionalangiography, and contrast–enhanced MRA (CE-MRA)offers even higher contrast between the vascular lumenand surrounding structures. MS-325 is a gadolinium-basedMR contrast agent designed specifically for blood-poolimaging, or MRA, and is the only gadolinium-based intra-vascular contrast agent undergoing trials in humans. MS-325 provides excellent vascular and selective arterial en-hancement during dynamic MRA. The long blood resi-dence time also allows acquisition of steady-state imagesof the arteries and veins with excellent spatial resolution(1,2).With the increasing use of CE-MRA, venous contami-nation of arterial images becomes a common concern (ie,venous enhancement may confound the visualization ofarteries). Currently available viewing techniques, such astargeted maximum intensity projection, multiplanar refor-mation, and “fly through” used in virtual endoscopy, canbe used only to minimize this problem but not to solve it(3,4). These techniques are also time intensive and requiremore operators. Although the ability to acquire dynamicimages may facilitate artery-vein separation by providingan artery mask that can be applied to steady-state images,this approach requires motion correction and image regis-tration between dynamic and steady-state images.The separation of artery and vein is of significant im-portance to correctly diagnose and treat peripheral vascu-lar diseases. The strategies for artery-vein separation in-clude both acquisition methods and postprocessing tech-niques. Among the current developments, acquisitionmethods include phase-contrast and time-resolved acquisi-tion approaches (5–7), and postprocessing techniquescover correlation analysis and graph searching methods(8–10). The shortcomings of these approaches are thelimitations of their applications and the costs. For in-stance, phase-contrast acquisition approaches (5) are lim-ited to cases in which the blood flow directions in arteryand vein are opposite to each other. In the time-resolvedacquisition approaches (6,7), the image must be acquiredduring the first pass of a contrast agent or accomplishedwith cardiac gating. Correlation analysis (8) requiresseven or eight MRA data sets in a single breath hold fora three-dimensional (3D) angiogram of the lung. In graphsearching approaches (9), the node costs require 3D edgestrength and a model of preferred branch direction. Theenhanced artery visualization method (10) is limited tothe segmentation of a small number of the main overlap-ping veins in the peripheral vasculature. Clearly, a moregeneral approach for artery-vein separation is desirable.MATERIALS AND METHODS