We read with interest the paper of Marshall et al entitled ‘Direct visualisation of collateral ventilation in COPD with hyperpolarised gas MRI’.1 Collateral ventilation (CV) is defined as ‘the ventilation of alveolar structures through passages that bypass the normal airway’.2 This happens through the intra-alveolar pores of Kohn, the bronchioloalveolar communications of Lambert and the intrabronchiolar pathways of Martin. These pathways exist in normal lungs, but CV is ineffective since flow resistance is 50 times greater than through the normal airway. In emphysematous lungs, the alveolar wall destruction enlarges the accessory pathways and the presence of airway obstruction increases expiratory resistance; this allows gas to move freely from one …