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.
📐 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.