Lungs in acute and chronic left heart failure: evaluation using positron emission tomography

Approximately two-thirds of the volume taken up by a normal lung is air, about one-sixth is blood and about one-sixth is extravascular water and tissue solids. The physical density of the normal lung is thus approximately 0.3 g/ml. An increase in intrapulmonary blood volume or accumulation of water in the interstitial or alveolar space causes an increase in lung density, and measurements of lung density may thus provide an index of the abnormalities in pulmonary oedema. Qualitative information about lung density is obtained in the standard chest radiograph. Quantitative measurements of lung density can be obtained with X-ray computed tomography, Compton-scatter techniques or transthoracic gamma-ray attenuation. These techniques provide information of overall lung density. It is clear, however, that any changes in overall lung density are the result of individual variations in the density of the vascular and extravascular compartments, and that a modest increase in the density of one could be masked by a corresponding decrease in the density of the other. Evaluation of water accumulation and changes in pulmonary blood volume should therefore ideally be made with a technique able to differentiate between the vascular and extravascular compartments. The paper looks briefly at one such method-positron emission tomography