- AP and transverse diameter equal, giving a cylindrical configuration
- rotation assessed by comparing length of anterior ribs
- normal cardiothoracic ratio up to 0.6
- variable thymic size
- thymic involution: prenatal or postnatal stress, hyaline membrane disease, corticosteroid treatment
- known artefacts: hole in incubator top looks like pneumatocele. skin fold mimic pneumothorax