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

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 lu ng density should reflect the changes in the PBV and extravascular lung wat er. The purpose of the study was to optimize transmission measurements of t he 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 d uring 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 Co-57 s ource were used. The coefficient of variation in density determination owin g 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 d uring exercise. Pulmonary blood volume was monitored simultaneously using T c-99m-labelled red blood cells and the registered blood pool activity was c orrected for attenuation. Highest precision in relative density determinati on was obtained with large energy window and uncollimated source. A precisi on of 1.0% was obtained with 1 min measuring time. About 10% change in tran smission corresponding to a 15% change in density was observed during exerc ise. Changes in blood pool and lung density covariated. We conclude that lu ng density changes can be monitored with a high degree of statistical preci sion in a few minutes and with a low radiation dose of radiation using a ga mma camera and a planar source.