Measurement of lung density by photon transmission for monitoring intravascular and extravascular fluid volume changes in the lungs

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Abstract
Left ventricular (LV) heart failure increases pulmonary blood volume (PBV) and interstitial fluid volume. Continuous measurements of lung density may be a simple non‐invasive method for monitoring of the LV function as the lung density should reflect the changes in the PBV and extravascular lung water. The purpose of the study was to optimize transmission measurements of the lung with a gamma camera and a planar source. Another aim was to assess the possibility of transmission monitoring in normal subjects at rest and during exercise and to compare the results with simultaneous measurements of PBV changes. Transmission measurements were made in a water‐filled phantom containing lungs of different density. A gamma camera and a planar 57Co source were used. The coefficient of variation in density determination owing to counting statistics in a lung area was calculated for different energy windows, acquisition times and collimation. Dynamic measurements in normal subjects were carried out in a sitting and a supine position at rest and during exercise. Pulmonary blood volume was monitored simultaneously using 99mTc‐labelled red blood cells and the registered blood pool activity was corrected for attenuation. Highest precision in relative density determination was obtained with large energy window and uncollimated source. A precision of 1·0% was obtained with 1 min measuring time. About 10% change in transmission corresponding to a 15% change in density was observed during exercise. Changes in blood pool and lung density covariated. We conclude that lung density changes can be monitored with a high degree of statistical precision in a few minutes and with a low radiation dose of radiation using a gamma camera and a planar source.