Chest X-ray Anatomy
Heart size and contours

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

Heart size is not assessed by an absolute measurement, but rather is measured in relation to the total thoracic width - the Cardio-Thoracic Ratio (CTR).

CTR = Cardiac Width : Thoracic Width

The CTR is frequently expressed as a percentage. A CTR of greater than 1:2 (50%) is considered abnormal.

Accurate assessment of heart size assumes the projection is Posterior-Anterior (PA), and that cardiac size is not exaggerated by factors such as patient rotation.

Cardiothoracic ratio (CTR)

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Cardiothoracic ratio (CTR)

  • Cardiac size is measured by drawing vertical parallel lines down the most lateral points on each side of the heart, and measuring between them.
  • Thoracic width is measured by drawing vertical parallel lines down the inner aspect of the widest points of the rib cage, and measuring between them.
  • The cardio-thoracic ratio can then be calculated.
  • Here the CTR is approximately 15 : 33 (cm) and is therefore within the normal limit of 50%.

Normal cardiac contours

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Normal 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).

Assessing the heart

The heart size should be assessed on every chest X-ray.

If the CTR is <50% on either a Posterior - Anterior (PA) or an Anterior - Posterior (AP) view, then the heart size is within normal limits.

However, a PA view is required to confidently diagnose cardiac enlargement. This is because an AP view will exaggerate the heart size due to magnification.

If the heart contours are not clearly seen, this may be because of increase in density of the adjacent lung. The lingula - part 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.