FEV_{1} = forced expiratory volume in 1 second; FVC = forced vital capacity; TLC = total lung capacity; Tlco = carbon monoxide transfer factor; Kco= carbon monoxide transfer coefficient; Pao _{2} = arterial oxygen tension; Paco _{2} = arterial carbon dioxide tension; Sao _{2} = oxygen saturation; HRCT = high resolution CT;t _{0.5} F= half time for the fast component of the bi-exponential DTPA clearance curve; t _{0.5} = half time for mono-exponential approximation to the early part of the DTPA clearance curve;t _{0.5} S= half time for the fast component of the bi-exponential DTPA clearance curve; f F = fraction of tracer cleared by the fast component.