Supine versus prone injection of Tc-99m MAA: evidence of thoracic compression or fluid shift

After a cesarean section, a woman had sharp left chest pain that radiated to the arm and was worse with inspiration. A chest radiograph revealed minimal left lung base atelectasis. A total of 5 mCi (185 mBq) Tc-99m macroaggregated albumin (MAA) was injected with the patient supine. A posterior oblique perfusion lung image revealed nearly absent perfusion of the left lung base. The patient was then injected with an equal quantity of Tc-99m MAA while prone. Images now demonstrated perfusion of the left lower lung. This might represent redistribution of fluid obscured by the atelectatic area, or the relief of pressure on pulmonary arteries to the left lower lobe by positional change.