RESUMO
We have observed that the companion shadow of the upper rib may be misinterpreted as a small pnemothorax or pleural plaque associated with asbestosis. To observe the radiographic characteristics of the normal companion shadow, we analyzed, on the posteroanterior(PA) chest radiographs, the companion shadow of 50 normal cases. Factors such as occurrence on each rib, the sharpness of the margin, the relative position to the rib, the shape and the thickness were observed. Also, we analyzed the displaced pleura of 4 pneumothorax cases to differentiate their frndings from the findings of normal companion shadows. On 50 normal chest radiographs, 192 compaion shadows were observed on the first to fourth ribs. In 173 of those shadows, the visceral margin of the companion shadow on the second rib simulated pneumothorax more closely than those on any othe rivs due to its apical location and thinness. In six of 50 normal cases, the companion shadow on the first or second rib showed an inw rdly convex lower margin, mimicking pleural plaque. The compaion shadow was suggested on the plain chest radiograph by the following characteristics imultiplicity(47/50), thicker than normal pleura(3/4), persistent on serial filma with the same shape and specific location(4/4).