ASSESSMENT OF LEFT-TO-RIGHT INTRACARDIAC SHUNTING BY VELOCITY-ENCODED, PHASE-DIFFERENCE MAGNETIC-RESONANCE-IMAGING - A COMPARISON WITH OXIMETRIC AND INDICATOR DILUTION TECHNIQUES

Background Velocity-encoded, phase-difference magnetic resonance imagi ng (MRI) has been shown to provide an accurate assessment of shunt mag nitude in patients with large atrial septal defects, but its ability t o determine shunt magnitude in patients with intracardiac left-to-righ t shunts of various locations and sizes has not been evaluated in a pr ospective and blinded manner. The objective of the present study was t o determine whether velocity-encoded, phase-difference MRI can assess the magnitude of intracardiac left-to-right shunting in humans. Method s and Results Twenty-one subjects (15 women and 6 men; age range, 15 t o 72 years) underwent velocity-encoded, phase-difference MRI measureme nts of flow in the proximal aorta and pulmonary artery, followed immed iately by cardiac catheterization. The presence of left-to-right intra cardiac shunting was assessed with hydrogen inhalation, after which sh unt magnitude was measured by the oximetric and indocyanine green tech niques. Of the 21 patients, 12 had left-to-right intracardiac shunting detected by hydrogen inhalation. There was a good correlation (r=.94) between the invasive and MRI assessments of shunt magnitude. In compa rison to oximetry and indocyanine green, MRI correctly identified the 12 patients with a ratio of pulmonary to systemic flow (Qp/Qs) of <1.5 (9 without intracardiac shunting and 3 with small shunts) and the 9 p atients with a Qp/Qs of greater than or equal to 1.5 (6 with atrial se ptal defect, 1 with ventricular septal defect, 1 with patent ductus ar teriosus, and 1 with both atrial septal defect and patent ductus arter iosus). Conclusions Compared with measurements obtained during cardiac catheterization, velocity-encoded, phase-difference MRI measurements of flow in the proximal great vessels can reliably assess the magnitud e of intracardiac left-to-right shunting.