Encyclopedia/CardiacChest

Cardiothoracic ratio

Synonyms: CTR, Cardiac size on chest radiograph

The maximum horizontal width of the heart divided by the maximum internal width of the thorax on a PA chest radiograph.

SystemsCardiac, Chest
Cases2
Last revisedOct 25, 2026

The cardiothoracic ratio (CTR) is a quick estimate of heart size on a PA chest radiograph: the widest transverse diameter of the heart divided by the widest internal diameter of the thoracic cage.

Normal values

  • Under 0.5 on a PA film in adults is generally considered normal
  • AP and supine films magnify the heart, so the ratio overestimates size
  • Neonates and infants normally have a larger ratio (up to about 0.6)

Causes of an increased ratio

  • Cardiomegaly from cardiomyopathy, valvular disease or heart failure
  • Pericardial effusion (a globular, water-bottle shape)
  • Technical: AP projection, poor inspiration, rotation
  • Pectus excavatum and prominent epicardial fat

Practical points

The CTR is a screening measure; echocardiography is needed to assess chamber size and function.

Cases and figures

Normal chest radiograph — imaging
X-ray
Case 1001Adult

Normal chest radiograph

Pre-employment health check. No symptoms.
Cardiogenic pulmonary edema — imaging
X-ray
2
Case 1019Older adult

Cardiogenic pulmonary edema

Worsening breathlessness and orthopnoea. Bibasal crackles and ankle swelling.

References

  1. Danzer CS. The cardiothoracic ratio: an index of cardiac enlargement. Am J Med Sci. 1919;157:513–521.

Cite this entry

Cardiothoracic ratio. Radflow Encyclopedia. Last revised 25 October 2026. https://radflow.cc/encyclopedia/cardiothoracic-ratio (accessed 15 November 2026).

Related entries

Chest radiograph: systematic reviewA structured way to read every chest radiograph, from technical quality to review areas, so that subtle findings are not missed.

Educational reference for healthcare professionals and students. It is not medical advice and does not replace clinical judgement or local guidelines.