Lung shunt fraction is a term used in nuclear medicine to describe the proportion of blood flow that bypasses the lungs and goes directly from the right side of the heart to the left side. This fraction is typically measured using a radioactive tracer injected into a vein, which allows for the calculation of how much blood is not being oxygenated in the lungs. Lung shunt fraction is important in assessing the risk of pulmonary complications during certain procedures, such as radioembolization therapy for liver cancer, as well as in evaluating lung function and overall heart health.