Fig. 4 Noncontrast magnetic resonance angiography A) before and B) after localized shimming of the ascending aorta and arch in a patient with prior coronary artery bypass grafting. Before local shimming, artifact from sternal wires obscures the ascending aorta (arrowheads), and consequently the bypass graft origin (arrow). Local shimming also improves visualization of the great-vessel origins (curved arrows).