The “BIG RIB SIGN” and “VERTICAL DISPLACEMENT SIGN” In Lateral Chest X Ray.

Pulmonary Medicine Blog By Dr Deepu

The “BIG RIB SIGN” and “VERTICAL DISPLACEMENT SIGN”- To Differentiate The Right and Left Ribs on Lateral Chest Radiographs

Today I would like to share with you two important signs in Chest X Ray- The Big Rib Sign and Vertical Displacement Sign.

Diseases of the ribs and diaphragm are often identified only at lateral chest radiography. Therefore, it is important to use a reliable technique for differentiating the right and left ribs. Two techniques—the “big rib” sign and the “vertical displacement “sign—can be used to localize the right and left ribs at lateral chest radiography. Conventially lateral radiographs are taken with left side near the film. But lateral view can be obtained as Right or left  as per clinicians request, then the principle remains the same but only the magnified sides change .

The big rib sign is a technique that exploits the difference in magnification between the right and left sides on lateral chest radiographs.

The side of the rib cage farther from the film is magnified more than the side closer to the film. On a well-positioned left lateral chest radiograph, the right ribs appear larger than the left ribs . This difference in rib size is more easily detected posteriorly where the x-ray beam is tangential to the ribs but can be appreciated at all corresponding points along the curvature of the two ribs. In addition, rotating the patient may enhance or reduce the magnification effect because, with such rotation, the x-ray beam is transmitted through different portions of the ribs, which are thin medially and thick laterally. In a left lateral projection, when the posterior portions of the right and left ribs appear comparable in size, the hemidiaphragm traceable to the most anterior ribs is the right hemidiaphragm. Otherwise, the significantly larger ribs are the right ribs, which are farther from the film.

The big rib sign is very useful but is not perfect because the magnification difference between the right and left ribs is only 10%. For example, if the width of a rib is 5 mm, the observed difference between sides is only 0.5 mm, which is not always sufficient to enable differentiation of the two sides. Furthermore, the big rib sign is not applicable when the posterior ribs are superimposed.

Another Sign is the vertical displacement sign, which is an easy, reliable, and precise method for differentiating the right and left ribs on lateral radiographs .

 The vertical displacement sign is based primarily on the vertical divergence of the x-ray beam rather than on the magnification of the ribs. Because the right rib cage is farther from the film, the projection of the right ribs on a lateral radiograph will fan out and diverge in a vertical direction to a greater degree than that of the left ribs. Thus, the right side can be distinguished from the left by the vertical displacement of the paired ribs.

The vertical displacement sign is usually applicable even when the posterior ribs are partially superimposed. It is usually possible to identify which rib is higher or lower even when they overlap. The vertical displacement sign can be used as an alternative when the big rib sign is not applicable.

Also Read

Air Bronchogram Sign.

The ‘Dark Bronchus’ Sign

Easy Way To Interpret Chest X Ray

reference:
1. http://pubs.rsna.org/doi/pdf/10.1148/radiographics.19.1.g99ja02105