What does activity in the brain or kidneys imply on a perfusion scan?
Some nuclear medicine departments will image the brain or kidneys to evaluate for a mechanical right-to-left shunt. Because the 99m Tc-MAA is too large to pass through the capillary beds of the lungs, all of the injected material should remain in the lungs unless there is a shunt that bypasses the lungs. Such a shunt is most commonly found in the heart (e.g., an atrial septal defect) or possibly as arteriovenous malformations in the lungs. Although the shunt can be detected on a V/Q scan, it is difficult to adequately assess the location or severity of the shunt, which typically requires additional studies using other structural imaging techniques.