Incomplete rim sign | Radiology Signs

Answer:

The incomplete rim sign describes a mass whose border is sharply defined only where it is projected against aerated lung and becomes indistinct where it merges with the chest wall, mediastinum or diaphragm, so that only part of the circumference is outlined. It indicates that the lesion arises outside the lung, from the pleura, the extrapleural space, the chest wall or the mediastinum, because only the portion protruding into air has an interface capable of generating an edge. An intrapulmonary mass, being surrounded by air on all sides, is marginated around its entire circumference. The sign is read together with the other features of extrapulmonary origin: obtuse angles with the chest wall rather than acute angles, a broad base against the pleural surface, a smooth convex medial margin, displacement rather than encasement of adjacent structures, and rib or vertebral erosion or a periosteal reaction when the chest wall is involved. Together these constitute what is often taught as the incomplete border sign of extrapleural lesions. Common causes are a pleural fibrous tumour or metastasis, an empyema, extrapleural haematoma or lipoma, and a chest wall tumour or infection; a neurogenic tumour in the paravertebral gutter is the classic posterior mediastinal example. Because chest radiography can localise but not characterise, CT, with ultrasound for pleural and chest wall lesions and MRI for neurogenic and vertebral involvement, is used for the definitive assessment.

Why is it called so?

The rim of the lesion is only partly visible, being completed by aerated lung on one side and lost against soft tissue on the other.

Pathophysiology

A radiographic edge is produced only where two structures of sufficiently different attenuation are in contact along the path of the beam. A lesion arising in the chest wall or extrapleural space is continuous with soft tissues of the same attenuation over part of its surface, so no interface exists there and no border is drawn, while the part that bulges into the aerated lung is sharply outlined. The same physics explains the accompanying obtuse angle, since an extrapulmonary mass pushes the pleura ahead of it and its margins taper gradually into the chest wall, whereas a lung mass abuts the pleura at an acute angle. This is the silhouette principle applied to lesion localisation rather than to loss of a normal border.

Alternative names: Incomplete border sign; extrapleural sign

Other associated named signs: Silhouette sign, the cervicothoracic sign, the hilum overlay sign and the split pleura sign of empyema