193.The cardiothoracic ratio
To estimate the cardiothoracic ratio, the widest diameter of the heart (upper double arrow) is compared to the widest internal diameter of the thoracic cage from the inside of rib to the inside of rib (lower double arrow). The widest internal diameter of the thorax is usually at the level of the diaphragm. The cardiothoracic ratio should be less than 50% in most normal adults on a standard PA frontal radiograph taken with an adequate inspiration (about nine posterior ribs showing).
Recognizing an Enlarged Cardiac Silhouette
The cardiac silhouette can appear enlarged for three main reasons:
• The heart is enlarged (cardiomegaly).
• A pericardial effusion mimics the appearance of cardiomegaly on conventional radiographs.
• An extracardiac factor produces apparent cardiac enlargement.
Effect of Projection on Perception of Heart Size
Because the heart resides anteriorly in the chest, on a posteroanterior (PA) chest radiograph (the standard frontal chest study in which the x-ray beam enters posteriorly and exits anteriorly where the imaging cassette is positioned), the heart appears truer to its actual size because it is nearer the imaging surface.
On an anteroposterior (AP) chest radiograph (the usual bedside, portable chest radiograph in which the x-ray beam enters anteriorly and exits posteriorly where the cassette is positioned), the heart is slightly magnified because it is farther from the imaging surface.
Therefore, the heart will appear slightly larger on an AP chest radiograph like a portable chest radiograph than will the same heart on a PA chest radiograph (see Fig. 2-17).