The lung perfusion scan is a nuclear medicine study performed to evaluate a patient for pulmonary embolus (PE) or other pulmonary disorders. Technetium (Tc-99m) is injected IV and distributed throughout the pulmonary vasculature because of the gravitational effect on perfusion. The scan, which produces a visual image of pulmonary blood flow, is useful in diagnosing or confirming pulmonary vascular obstruction. The diameter of the IV-injected macroaggregated albumin (MAA) is larger than that of the pulmonary capillaries; therefore, the MAA temporarily becomes lodged in the pulmonary vasculature. A gamma camera detects the radiation emitted from the injected radioactive material, and a representative image of the lung is obtained. This procedure is often done in conjunction with the lung ventilation scan to obtain clinical information that assists in differentiating among the many possible pathological conditions revealed by the procedure. The results are correlated with other diagnostic studies, such as pulmonary function, chest x-ray, pulmonary angiography, and arterial blood gases. A recent chest x-ray is essential for accurate interpretation of the lung perfusion scan. An area of nonperfusion seen in the same area as a pulmonary parenchymal abnormality on the chest x-ray indicates that a PE is not present; the defect may represent some other pathological condition, such as pneumonia.