1. We have developed a method for non-invasive measurement of lung tissue mass, thoracic blood and interstitial volumes by a combination of transmission and emission scanning with technetium isotope (99mTc). 2. In a lung model we demonstrated that emission counts could be successfully corrected for attenuation with data obtained by transmission scanning, despite an uneven distribution of radioactivity and attenuation in the model. 3. In dogs we compared regional transthoracic tissue thickness, measured by transmission scanning, and regional 'thickness' of blood measured by transmission/emission scanning with direct gravimetric measurements made post mortem. Scanning and direct measurements correlated significantly. 4. In man we used a [99mTc]pertechnetate (99mTcO4) flood source to obtain antero-posterior transmission scans with a gamma-camera. The thickness of attenuating tissue was estimated in each pixel. Scans were obtained of thoracic blood (by labelling erythrocytes with 99mTcO4) and of interstitium (with 99mTc-labelled diethylenetriaminepenta-acetic acid and subtraction of the blood image). We used a computer program to correct the emission scans for attenuation using the transmission scan derived tissue thickness, pixel by pixel. Finally we took a lateral chest radiograph to estimate chest wall thickness. 5. In normal man lung tissue thickness at hilar level was 3.1 +/- 0.5 cm (n = 8). Thoracic blood thickness increased from the apex downwards in the upright lung, being 1.2 +/- 0.1 cm at the hilar level and 2.0 +/- 0.3 cm at the lung base. Interstitial thickness was 0.8 +/- 0.3 cm at the hilum and 0.85 +/- 0.2 at the base. These values compare well with data in the literature.(ABSTRACT TRUNCATED AT 250 WORDS)

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