Objectives. The purpose of this study was to evaluate the ability of velocity-encoded nuclear magnetic resonance (NMR) imaging to quantify left to right intracardiac shunts in patients with an atrlal septal defect. Background. Quantification of intracardiac shunts is clinically important in planning therapy. Methods. Velocity-encoded NMR imaging was used to quantify stroke flow in the aorta and in the main pulmonary artery in a group of patients who were known to have an increased pulmonary to systemic flow ratio ( Qp Qs). The velocity.encoded NMR flow data were used to calculate Qp Qs, and these values were compared with measurements of Qp Qs obtained with oximetric data derived from cardiac catheterization and from stroke volume measurements of the two ventricles by using volumetric data from biphasic spin echo and cine NMR images obtained at end-diastole and end-systole,. Results. Two independent observers measured Qp Qs by using velocity-encoded NMR imaging in 11 patients and found Qp Qs ranging from 1.4:1 to 3.9:1. These measurements correlated well with both oximetric data (r = 0.91, SEE = 0.35) and ventricular volumetric data (r = 0.94, SEE = 0.30). Interobserver reproducibility for Qp Qs by velocity-encoded NMR imaging was good (r = 0.97, SEE = 0.20). Conclusions. Velocity-encoded NMR Imaging is an accurate and reproducible method for measuring Qp Qs, In left to right shunts. Because it is completely noninvasive, it can be used to monitor shunt volume over time.
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