Chest x-ray anatomy
Heart size and contours
Heart size is not assessed by absolute measurement, but rather in relation to the total thoracic width, and is expressed as a ratio.
Cardiothoracic ratio (CTR) = Cardiac Width : Thoracic Width
A CTR of greater than 1:2 (50%) is considered abnormal. This however, assumes the projection is Posterior-Anterior (PA), and that cardiac size is not exaggerated by factors such as patient rotation or an incomplete breath in.
Key points
- The heart size is assessed as the cardiothoracic ratio (CTR)
- A CTR of >50% is abnormal - PA view only
- The left hemidiaphragm should be visible behind the heart
- The hemidiaphragm contours do not represent the lowest part of the lungs
Hover over image to show findings
Cardiothoracic ratio (CTR)
Cardiac size is measured by dropping parallel lines down both sides of the heart, at the most lateral points on each side, and measuring between them. Thoracic width is measured by dropping parallel lines down the inner aspect of the widest points of the rib cage, and measuring between these. The cardio-thoracic ratio can then be stated.
Here the CTR is approximately 15 : 33 (arbitrary units) and is therefore within the normal limit (expressed as a percentage) of 50% .
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Hover over image to show findings
Cardiac contours
The left heart contour (red line) consists of the left lateral border of the Left Ventricle (LV). The right heart contour is the right lateral border of the Right Atrium (RA).
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Assessing the heart
The heart size should be considered on every chest x-ray, but the cardiothoracic ratio (CTR) can only be assessed confidently if a posterior - anterior (PA) view has been acquired. The rule is, if an anterior - posterior (AP) view has been taken, then the heart should not be called enlarged even if the CTR is >50%. This is because an AP view exaggerates the heart size. If the CTR is <50% on an AP view, then clearly the heart size is within normal.
If the heart contours are not clearly seen, this may be because of increase in density of the adjacent lung. The lingula of the upper lobe of the left lung, wraps over the left ventricle, and so loss of definition of the left heart border may be related to disease in this area of lung. On the right, the middle lobe is located adjacent to the right atrium, and therefore loss of definition of the right heart border may be due to increased density caused by disease in this lung lobe.