A2487 - Estimation of Tissue Volume of Lung Tissue Using Computed Tomography Imaging and Local Reference Values

RATIONALE Chronic obstructive pulmonary disease (COPD) is typically characterized by the volume of lung tissue with a CT density below a specified Hounsfield Unit (HU) threshold. Our purpose was to determine the effects on estimation of tissue volume of using local measurements of air and tissue radio-density within each scan as opposed to the standard practice of assuming air and lung tissue radio-density of -1000 and 0 HU respectively. METHODS Excised calf tissue containing the heart and lung, with the lung inflated to 25 cmH2O, served as a phantom. Multiple CT scans of the phantom were performed with various reconstruction algorithms, with and without containers of water to simulate tissue surrounding the lung, and at various times after an initial scan. Air and tissue volumes were estimated using standard reference values (air = -1000 HU, lung tissue = 0 HU) and compared to air and tissue volumes estimated from local reference values using cardiac tissue and tracheal or extracorporeal air within a specific scan. Finally, the tissue volume calculated from the measured weight of the calf lung-heart was compared to the estimated tissue volume assuming a tissue mass density of 1.04g/mL. RESULTS The Figure shows total tissue volume when using different references for each of 9 scans, which are shown in different colors. Repeat scans using three different local reference values for air (ET tube, tracheal superior, and peripheral; middle three set of points in the Figure) exhibited significant variation. Standard reference values and the local reference values of inferior tracheal air and cardiac tissue and their estimated tissue volumes (extreme left and right points in the Figure) exhibited little variance among scans. The most accurate tissue volume estimates (red line) were given by local reference values from inferior tracheal air and heart tissue (4% deviation from true tissue volume), as compared to standard reference values (12% deviation from true tissue volume). CONCLUSIONS Estimates of total tissue volume were most accurate using local reference values of inferior tracheal air and cardiac tissue, yielding estimates within 4% of actual tissue volume. We conclude that the estimation of lung tissue volume from non-contrast CT is subject to systematic and significant errors when using standard calibration protocols and reference values for tissue and intrapulmonary air radio-density. By contrast, using local CT radio-density air and tissue (inferior tracheal air and cardiac tissue) improved accuracy.