In 99mTc-MAA lung perfusion scans, right-to-left shunt is indicated by what observation?

In 99mTc-MAA (Technetium-99m macroaggregated albumin) lung perfusion scans, the presence of a right-to-left shunt is indicated by increased uptake in systemic circulation. When a right-to-left shunt is present, blood bypasses the pulmonary circulation and directly enters the systemic circulation. This occurs because the shunt allows deoxygenated blood from the right side of the heart to flow into the left side, resulting in the ability of the 99mTc-MAA particles to enter systemic veins and arteries.

As a consequence, an unexpected distribution of the radiotracer may be observed in the systemic circulation rather than being confined to the lungs, where it typically should be present during normal perfusion. This phenomenon highlights the shunting mechanism in play, reflecting a deviation from normal physiology where blood flow should predominantly be directed toward the pulmonary vascular bed for gas exchange.

Understanding this principle is crucial in nuclear medicine diagnostics, as recognizing right-to-left shunting can aid in identifying underlying pathologies that may require intervention or further investigation.