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 cardiothoracic ratio as a percentage, however this is incorrect, as it is a ratio. This situation is exacerbated by at least one well-known PACS vendor using percentages for their inbuilt ratio measurement. It is usual to express the ratio to two decimal places, e.g. 0.50, rather than 0.5.
The cardiothoracic ratio 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 is 0.42-0.50. A measurement <0.42 is usually deemed to be pathologic. A measurement >0.50 is usually taken to be abnormal although some radiologists feel that measurements up to 0.55 are "borderline".
The cardiothoracic ratio should not be measured on an AP chest x-ray. Objects nearer the x-ray tube appear artificially enlarged due to divergence of the x-ray beam, resulting in the heart appearing artificially large on AP radiographs.
It should be noted that this measurement is a crude marker of disease . If the patient is symptomatic then echocardiography is required but the yield of echocardiography is low if performed just for an increased cardiothoracic ratio .
In the following situations, cardiothoracic ratio is more than 0.5 on a PA chest radiograph, but heart and pericardium are normal:
A low cardiothoracic ratio is usually presumed to be abnormal and is called small heart syndrome.