Normal Cardiothoracic Index Size on Radiography
The cardiothoracic index (CTI) is the ratio of the maximum transverse cardiac diameter to the maximum transverse thoracic diameter, measured on a posteroanterior (PA) chest radiograph. It provides a rapid, reproducible estimate of cardiac size and is one of the most widely used measurements in chest imaging. Accurate interpretation requires age-appropriate reference values, as normal thresholds differ substantially between neonates, infants, children, and adults.
Normal Reference Values
| Orientation | Age | Measurement |
|---|---|---|
| Posteroanterior | Adult | <0.5 |
| — | 1-2 y/o | 0.6 |
| — | 2-3 y/o | 0.5 |
| — | 3-5 y/o | 0.52 |
| — | >5 y/o | 0.5 |
| — | 0-3 weeks | 0.65 |
| — | 4-7 weeks | 0.7 |
| — | 8-23 weeks | 0.67 |
| — | 24-47 weeks | 0.66 |
| — | 48 weeks | 0.61 |
Clinical Significance
In adults, a CTI of ≥0.5 on a PA radiograph is the conventional threshold for cardiomegaly. Exceeding this value warrants further evaluation with echocardiography to assess chamber size, wall motion, and pericardial effusion. It is important to note that the CTI is less reliable on anteroposterior (AP) projections — commonly obtained in supine or portable studies — where cardiac magnification artificially elevates the ratio.
In infants and young children, the heart occupies a proportionally larger thoracic volume, and higher thresholds apply. Neonates (0–3 weeks) may normally reach a CTI of 0.65, while values up to 0.70 are accepted at 4–7 weeks of age. Failure to apply pediatric norms leads to both false-positive and false-negative diagnoses. By age 5 years, the adult threshold of approximately 0.5 is approached.
- Cardiomegaly (adults CTI ≥0.5): dilated cardiomyopathy, severe valvular disease, hypertensive heart disease
- Pericardial effusion: produces a globular cardiac silhouette with rapid CTI enlargement
- High-output states: anemia, thyrotoxicosis, arteriovenous fistula
- Apparent cardiomegaly (technical): AP projection, poor inspiration, low lung volumes
- Pediatric congenital heart disease: left-to-right shunts (e.g., VSD, ASD) causing volume overload
Reference: Maresh MM, Washburn AH. Size of the heart in healthy children. Am J Dis Child 56:33, 1936. Weissman NJ, Adelmann GA. Cardiac Imaging Secrets. Elsevier Health Sciences. (2004).
Imaging Notes
The CTI is measured on a standard PA chest radiograph obtained at full inspiration, with the patient erect. Draw a vertical midline through the spine; the cardiac diameter is the sum of the maximum rightward and leftward cardiac projections from this midline. The thoracic diameter is the maximum inner width of the rib cage at the level of the right hemidiaphragm. Divide cardiac by thoracic diameter to obtain the ratio. Reproducibility is highest on a true PA projection; AP or rotated films should be interpreted with caution and the projection explicitly noted in reports.
On pediatric radiographs, the thymus can overlap the cardiac silhouette and spuriously widen the apparent cardiac border, particularly in infants under 2 years. When thymic tissue is prominent, clinical correlation and, if necessary, echocardiography are preferred over relying solely on the CTI.