[1] Univ Cambridge, Sch Clin Med, Box 111,Cambridge Biomed Campus, Cambridge CB2 0SP, England
[2] Cambridge Univ Hosp NHS Fdn Trust, Resp Med, Hills Rd, Cambridge, England
[3] Cambridge Univ Hosp NHS Fdn Trust, Lung Funct Unit, Hills Rd, Cambridge, England
[4] Cambridge Univ Hosp NHS Fdn Trust, Dept Radiol, Hills Rd, Cambridge, England