Unilateral hypertransradiancy is the correct term for the chest radiograph appearance of decreased attenuation affecting one lung. Hyperlucency is a commonly used alternative but is inaccurate; the chest is not penetrated by light!
An outside-in approach is helpful to identify the cause:
technical factors such as grid cutoff: the ipsilateral chest wall will appear transradiant
patient rotation (check the clavicles): transradiant on the side to which the patient is turned
chest wall abnormalities e.g. mastectomy, contralateral chest wall tumor, Poland syndrome (absent pectoralis major muscle), surgical removal of pectoralis major muscle for flap surgery, unilateral fatty atrophy to chest wall muscles
ipsilateral pneumothorax
contralateral pleural opacity, e.g. supine pleural effusion, pleural thickening or tumor, pneumonectomy
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contralateral increased lung opacity:
unilateral lung fibrosis (single lung transplant)
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primary decrease in lung perfusion:
asymmetrical large pulmonary embolism
unilateral pulmonary artery obstruction due to leiomyosarcoma or extrinsic compression from hilar mass
congenital heart disease and contralateral shunt
unilateral absent pulmonary artery
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secondary decrease in lung perfusion due to air-trapping and hyperinflation:
Swyer-James syndrome
bronchial narrowing with check-valve, e.g. foreign body, endobronchial tumor
asymmetrical constrictive bronchiolitis
compensatory emphysema due to profound ipsilateral lobar collapse
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