Gadolinium-enhanced three-dimensional MR angiography of the thoracoabdominal aorta

Abstract

MR angiography (MRA) of the thoracoabdominal aorta is a noninvasive technique that can rapidly acquire a volume of data with the capability of multiplanar reformations (MPR) and “bright blood” maximum intensity projection (MIP) angiographic images. These MIP images can display long tortuous vessels in a single three-dimensional (3D) volume, with excellent delineation of branch vessel disease and without the flow artifacts or long examination times of conventional spin-echo (SE) imaging. Two-dimensional (2D) time-of-flight (TOF) imaging is used most widely because of familiarity and ease of implementation, but this method has limitations in evaluating thoracoabdominal aortic disease. Sequential 2D axial imaging (which maximizes flow-related enhancement) is time consuming, subject to slice-to-slice misregistration, and can be degraded by pulsatile and turbulent flow. Coronal or parasagittal imaging is more time efficient, but image degradation secondary to inplane saturation and stagnant or turbulent flow from aneurysmal disease may lead to nondiagnostic studies. Three-dimensional TOF techniques offer the advantages of higher signal-to-noise ratio, better spatial resolution with near isotropic voxels, and shorter echo times (TEs), which lessen signal loss because of intravoxel phase dispersion. Although these techniques provide excellent image quality in the carotid arteries, they are of limited use in the aorta because of saturation effects. However, the addition of gadolinium chelates shortens the T1 relaxation rate of blood, which obviates the need for flow-related enhancement, allowing for inplane imaging without saturation effects, even in regions of virtually stagnant flow. The enormous signal enhancement of gadolinium chelates enables imaging with high-resolution matrices, providing diagnostic angiograms in as little as 2 minutes. With improved hardware, faster and stronger gradients, and phased-array coils that increase the signal-to-noise ratio, breath-hold gadolinium-enhanced 3D schemes with ultrashort TEs will become the optimal method for imaging the aorta and its branch vessels. Using this technique, the aorta can be imaged in less than 1 minute.