Computed tomography dose and variability of airway dimension measurements: how low can we go?

Quantitative CT shows promise as an outcome measure for cystic fibrosis (CF) lung disease in infancy, but must be accomplished at a dose as low as reasonably achievable. To determine the feasibility of ultra-low-dose CT for quantitative measurements of airway dimensions. Two juvenile pigs were anesthetized and their lungs scanned at 25 cm H2O face-mask pressure in apnoea using beam currents of 5, 10, 20, 40 and 100 mAs. The lumen diameters and wall thicknesses of matched airways (n=22) at each dose were measured by two observers using validated software. Measurement variability at each dose was compared to that at 100 mAs (reference dose) for large and small airways (lumen diameter <2.5 mm). Lowering CT dose (mAs) affected measurement variability for lumen diameter of small and large airways (P<0.001) and for wall thickness of small (P<0.001), but not large (P=0.63), airways. To obtain the same measurement variability at 5 mAs as at 100 mAs, four to six small airways or one to three large airways have to be measured and averaged. Quantitative airway measurements are feasible on images obtained at as low as 5 mAs, but more airways need to be measured to compensate for greater measurement variability. (orig.)