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We introduce a simple physics-based model of RA-950 emphysema scoring. Our model assumes that the lung is strictly composed of healthy tissue and emphysematous tissue, each described by a single attenuation value and contaminated with Gaussian noise. We show that when combined with curve-fitting, the model can accurately capture change in RA-950 score with respect to image noise and subject breath hold, and accurately compute “true” RA-950 scores (relative to a clinical reference scan) in a cohort of 16 patients. To validate the model, noise realizations of 10 lung screening subjects and 6 COPD patients were created using various combinations of reconstruction parameters and simulated reduced dose acquisitions. Least-squares curve fitting software was utilized to determine the amount of emphysema and the attenuation value of healthy lung tissue for each subject using the model. The derived model provided accurate emphysema scores (difference between model value and clinical reference of < 0.02) in all cases except one. Upon radiologist review of this case, the score derived from our model was deemed more appropriate than RA-950 from the clinical reference scan. The R-squared values were < 0.9 in all cases except one, and < 0.95 in 12 of 16 cases. The case with low R2 value was also reviewed by a radiologist and found to have substantial other disease that violated key model assumptions. The model appears to be robust to breath hold, image noise, and amount of emphysema present, factors that have been found to confound other approaches (such as denoising) to improving emphysema scoring.
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