The cardiothoracic (CT) ratio of the heart is defined as the ratio of the heart's maximal transverse diameter to the thorax's maximal transverse diameter, measured on a posteroanterior (PA) chest radiograph [1].
Here's a more detailed explanation:
-
Definition: The CT ratio assesses the relative size of the heart compared to the chest cavity. A high CT ratio can indicate cardiomegaly (enlarged heart).
-
Measurement: The CT ratio is calculated as follows [1]:
- Heart Diameter (A): Measure the widest horizontal distance of the heart from the right and left borders.
- Thorax Diameter (B): Measure the widest internal diameter of the rib cage at the level of the diaphragm.
- CT Ratio = A / B
-
Normal Value: The normal CT ratio is generally considered to be less than 0.5 in adults. A CT ratio greater than 0.5 may suggest cardiomegaly and warrants further investigation.
-
Significance: The CT ratio helps to screen for heart enlargement. While it's a simple and readily available measurement, it has limitations and isn't always accurate, especially in certain conditions (e.g., chest deformities, pleural effusions).
-
Example: If the heart's transverse diameter (A) is 12 cm and the chest's transverse diameter (B) is 25 cm, the CT ratio is 12/25 = 0.48. This would be considered within the normal range.
In summary, the CT ratio is a simple measurement used to assess heart size relative to chest size on a chest X-ray, calculated by dividing the heart's widest diameter by the chest's widest diameter [1].
