MRA, specifically contrast-enhanced MRA—the dominant clinical technique—offers a rapid, noninvasive evaluation of the peripheral vasculature with sensitivity and specificity for the detection and grading of stenoses near that of catheter angiography. The other major noninvasive technique, CT angiography, is limited in this application due to the lack of dynamic flow information obtainable and by its reduced accuracy in the presence of calcified plaque. Contrast-enhanced MRA sequences are typically obtained utilizing a heavily T1W 3D spoiled GRE sequence. Contrast-enhanced MRA images are also acquired with a short TE so as to reduce flow-related signal loss and susceptibility artifacts. Before the actual contrast injection the same sequence later used for the contrast-enhanced scan is acquired as a mask for subsequent subtractions.

Coordination of the arrival of the contrast bolus with image acquisition (i.e., bolus timing) is a critical consideration in contrast-enhanced MRA. Inadequate timing resulting in delays acquiring the center of k-space leading to venous contamination or scans acquired too early to achieve maximal obtainable intraarterial SI. Specifically, the center of k-space—the portion encoding data important for image contrast—should be filled when arterial enhancement is at its peak. Several methods are employed for coordinating central k-space acquisition and bolus arrival. Bolus timing consists of acquiring a series of axial T1WI at the level of interest while a small test bolus (1–2 mL) is injected at the same rate as the actual bolus. This is used to calculate a circulation time so as to estimate the timing of bolus arrival during the actual examination. In certain applications, such as whole-body MRA, more than one test bolus is needed. In distinction, with MR fluoroscopy, the full contrast dose is given, and serial coronal 2D images obtained until the bolus has arrived within the desired region, as determined through region of interest analysis by the system or manually by the technologist. Upon arrival, the full scan is then acquired. Using rapid, time-resolved MRA techniques such as TRICKS (time-resolved imaging of contrast kinetics), TWIST (time-resolved angiography with interleaved stochastic trajectories), and TREAT (time-resolved echo-shared angiographic technique), the need for bolus timing is eliminated. These techniques undersample the periphery of k-space, while repeatedly sampling the center of k-space, allowing acquisition of image-contrast information at a high rate. Due to the subsequent data acquisition at a high frame rate both arterial and venous phase images are acquired. This is especially useful in eliminating problems stemming from venous overlay in patients with altered hemodynamics due to inflammation and collateral flow.