Change in regional pulmonary perfusion as a result of posture and lung volume assessed using technetium-99m macroaggregated albumin SPET.

The purpose of this study was to evaluate the effects of gravity and lung volume on regional pulmonary perfusion using technetium-99m macroaggregated albumin (99mTc MAA) single-photon emission tomography (SPET). Twenty-five subjects were classified into three groups according to their position during the injection of the tracer [11 subjects sitting, six supine and eight both supine and prone (S+P) positions]. All of these subjects were injected with the tracer during normal tidal breathing. In the S+P group, half of the tracer was injected while the subject was in each position. The remaining 11 subjects were classified into two groups according to their lung volume during the injection. Supine patients were instructed to hold their breath at residual volume (RV) (five subjects) or total lung capacity (TLC) (six subjects) while receiving the tracer injection. A region of interest with a ventrodorsal axis was defined in the centre of each lung. Profile curves were produced by plotting and normalizing the perfusion values as a percentage of the maximum value. The perfusion distributions for the sitting and S+P positions and at RV were relatively uniform. However, the distributions for the supine position and at TLC showed a gravitational influence [sitting vs. TLC: 87.8%+/-10.4% vs. 67.3%+/-8.7% for % maximum perfusion at +5 pixels from the midpoint of the upper lobe (P<0.00002)]. The gravity-related perfusion inhomogeneity was more prominent in the lower lobe than in the upper lobe. It is concluded that the physiological vertical gravitational gradient should be taken into consideration during the interpretation of lung SPET images. Preferably, patients should be injected with the tracer twice, once in the supine position and once in the prone position, while breathing normally. Alternatively, they may be injected with the tracer once while in the supine position and holding their breath at RV. Either of these protocols should ensure a uniform distribution of tracer.