Contrast-enhanced MR angiography.—Gadolinium-based contrast agents (GBCAs) strongly increase signal from the blood in T1-weighted spoiled gradient-echo sequences. Single-phase contrast-enhanced MR angiography (CE-MRA) is an excellent method for visualizing Fontan pathways, great vessels, and venovenous and aortopulmonary collaterals (Fig 6). Ideally, the sequence should be performed when the contrast agent arrives in the vessel of interest at its maximum concentration (first pass). Scanning too early or too late may result in missing the passage of contrast material bolus, or inadequate vascular visualization owing to contrast material dilution or overlapping of venous and arterial phases. Consistently good results depend on sufficient expertise and ideally optimized planning that is individualized to the patient. Two data sets are routinely acquired for all patients undergoing cardiac MRI at our institution: The early data set is acquired to assess the venovenous collaterals and pulmonary arteries before the contrast material enters the heart, and the second data set is acquired 15 seconds after the first. This allows visualization of the aorta, aortopulmonary collaterals, and Fontan pathway. The acquired non–electrocardiographically (ECG) gated three-dimensional (3D) isotropic data sets can be displayed as maximum intensity projection, multiplanar reconstruction, or volume rendering (Fig 7). Time-resolved CE-MRA has lower spatial resolution but provides dynamic information that helps clarify flow patterns, thereby highlighting potential stenoses or blockage of the Fontan pathway and collateral circulations (11). The advantage of time-resolved CE-MRA is the ability to follow blood flow in real time, including direction of flow through collaterals, which may be more difficult in a single data set. For example, time-resolved CE-MRA may detect complete obstruction of brachiocephalic vein and filling of the pulmonary veins via collateral vessels instead of the left lung, as shown in Figure 8. Although we routinely use conventional single-phase CE-MRA at our institution, time-resolved CE-MRA has a role in certain circumstances, such as when using a 3-T scanner or if particular collaterals are suspected and patient size allows sufficient spatial resolution when using a 1.5-T scanner.