Why is Qp Qs important, and how is it measured on MRI?
In patients with intracardiac shunting, the evaluation of the ratio of pulmonic to systemic blood flow (Qp/Qs) is important because it often determines subsequent therapy. A Qp/Qs of <1.5 is usually managed conservatively in asymptomatic patients. A Qp/Qs of >1.5 in a patient whose pulmonary vascular resistance is not markedly elevated is usually referred for surgical correction. Velocity-encoded images are used to measure left-to-right shunting by comparing the flow in the proximal main pulmonary artery with the flow in the proximal aorta.