The cardiothoracic ratio (CTR) aids in the detection of enlargement of the cardiac silhouette, which is most commonly from cardiomegaly but can be due to other processes such as a pericardial effusion.
Some report CTR as a percentage, however this is incorrect, as it is a ratio.
The CTR is measured on a PA chest x-ray, and is the ratio of maximal horizontal cardiac diameter to maximal horizontal thoracic diameter (inner edge of ribs/edge of pleura). A normal measurement should be ≤0.5.
It should be noted that this measurement is a crude marker of disease 4. If the patient is symptomatic then echocardiography is required but the yield of echocardiography is low if performed just for an increased CTR 3.
In the following situations, cardiothoracic ratio is more than 0.5 on a PA chest radiograph, but heart is normal: