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Cardiothoracic
Demonstration data

Cardiothoracic Ratio (CTR)

Ratio of maximal cardiac width to maximal internal thoracic width on a frontal chest radiograph, screening for cardiomegaly.

The cardiothoracic ratio is the classic radiographic index of heart size: the widest transverse diameter of the cardiac silhouette divided by the widest internal diameter of the bony thorax measured at the level of the right hemidiaphragm.

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📐 Formula

CTR = maximal cardiac width / maximal internal thoracic width

When to Use

  • Screening for cardiomegaly on a frontal chest radiograph.
  • Serial follow-up of known cardiac enlargement or pericardial effusion.
  • Triage before dedicated echocardiography or CT.

How It Works

  • Cardiac width is the sum of the maximal rightward and leftward projections of the cardiac silhouette from the midline (measured directly as the widest transverse cardiac diameter).
  • Thoracic width is the maximal internal diameter of the rib cage, measured from the inner margins of the ribs at the level of the right hemidiaphragm.
  • The ratio is the cardiac width divided by the thoracic width.

Interpreting the Result

  • On a well-inspired erect PA film, CTR ≤ 0.50 is normal; > 0.50 indicates cardiomegaly.
  • AP and supine projections magnify the cardiac silhouette and routinely overestimate CTR — a threshold of ~0.55–0.57 is often applied, and the ratio should not be used to diagnose cardiomegaly on portable films.
  • A rising CTR on serial films is more meaningful than a single borderline value.

Limitations & Pitfalls

  • Poor inspiration, rotation, AP/supine technique and pectus deformity all distort the ratio.
  • A large silhouette may reflect pericardial effusion or mediastinal fat rather than true chamber enlargement.
  • Normal CTR does not exclude significant cardiac disease.

References

  • Danzer CS. The cardiothoracic ratio: an index of cardiac enlargement. Am J Med Sci. 1919;157:513–521.
  • Simkus P, et al. Limitations of cardiothoracic ratio derived from chest radiographs. Semin Ultrasound CT MR / Acta Radiol reviews, 2021.

⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.