Quantitative measurement of regional extravascular lung density using positron emission and transmission tomography

A technique has been developed to measure regional values of vascular and extravascular lung density using positron emission and transmission tomography. Quantitative values of lung density in a transaxial plane are obtained by recording transmission scans during the exposure of a ring source of positron emitting germanium/gallium-68, which encircles the subject in the plane of the scan. Values of blood density are obtained by scanning in the emission mode following the labeling of the subject's red blood cells with a quantity of C-carbon monoxide inhaled as a bolus. Subtraction of the normalised blood volume scan from the normalised lung density (transmission) scan provides regional values of extravascular lung density. The response of the transmission scan to changes in density was obtained by scanning different tissue equivalent materials in the density range 0.02 to 1.0 g cm-3. This resulted in a linear relationship between pixel counts and density. Density measurements made in vitro on simulated chest phantoms suggest that, at worst, random errors of 3.5% and systematic errors (due to the influence of the chest wall) of between 10 and 15% will be incurred when measurements are made in vivo on lungs of average normal density (0.3 g cm-3). The random error associated with the emission scan (arising from counting statistics alone) was found to be 1.2%. Measurements of lung density made on five normal subjects (supine) resulted in a mean density of 0.29 g cm-3 for a region in the lower (caudal) part of the lung, with a range of values between 0.26 and 0.32 g cm-3 from subject to subject. A pronounced anteroposterior gradient of both lung density and blood density was observed, while the gradient of extravascular lung density was quite small. The mean value of the ratio extravascular: vascular lung density for both caudal and cranial lung regions was 0.92 +/- 0.25.