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To quantitatively assess atrial septal defects (ASDs) with small shunts using MRI followed by transcatheter closure monitored by MR fluoroscopy.Acute ASDs were created in 14 pigs under x-ray fluoroscopy. Six animals were studied in order to select MR-compatible delivery systems and imaging strategies. ASDs in eight animals were examined with balloon sizing under MR fluoroscopy, flow measurements, and contrast media injections, after which transcatheter closure was performed under MR fluoroscopy. The delivery system was assembled from commercially available materials.The ratio of pulmonary to systemic flow (Qp/Qs) was reduced from 1.23 ± 0.15 before ASD closure to 1.07 ± 0.11 after ASD closure (P < 0.001). In two out of eight animals Qp/Qs was close to 1.0 before closure despite the presence of defects >15 mm. The ASDs were measurable with MR balloon sizing in all of the animals. Balloon sizing was identical with MR (16.9 ± 2.3 mm) and x-ray fluoroscopy (17.1 ± 1.3 mm). The in-house-assembled delivery system allowed successful placement of closure devices under MR guidance.Assessment and closure of small shunts with MR fluoroscopy is feasible. A barrier to the rapid implementation of transcatheter closure in patients is uncertainty about the MR safety of guidewires and device delivery systems. J. Magn. Reson. Imaging 2005;21:204–211. © 2005 Wiley-Liss, Inc.