Single photon emission computed tomography ventilation images of a female asthmatic at a, b) baseline and c, d) post-methacholine challenge, at approximately corresponding anatomical locations. Single photon emission computed tomography ventilation images of a female asthmatic at a, b) baseline and c, d) post-methacholine challenge, at approximately corresponding anatomical locations. The centre colour bar indicates relative activity of Technegas® (highest relative activity being red). The white arrows indicate ventilation defects at baseline (a and b) and new defects post-methacholine challenge (c and d), after a ∼20% fall in forced expiratory volume in 1 s. As well as the development of new ventilation defects, the ventilation patterns have also changed. The challenge currently is to quantify the changes in ventilation patterns, which will require complex statistical methods, and to relate these to clinically relevant parameters. Gregory G. King et al. Eur Respir Rev 2019;28:180111 ©2019 by European Respiratory Society