The left heart contour (red line) consists of the left lateral border of the Left Ventricle (LV). The right heart contour is the right lateral border of the Right Atrium (RA).
Assessing the heart
The heart size should be assessed on every chest X-ray.
If the CTR is <50% on either a Posterior - Anterior (PA) or an Anterior - Posterior (AP) view, then the heart size is within normal limits.
However, a PA view is required to confidently diagnose cardiac enlargement. This is because an AP view will exaggerate the heart size due to magnification.
If the heart contours are not clearly seen, this may be because of increase in density of the adjacent lung. The lingula - part of the upper lobe of the left lung - wraps over the left ventricle, and so loss of definition of the left heart border may be related to disease in this area of lung. On the right, the middle lobe is located adjacent to the right atrium, and therefore loss of definition of the right heart border may be due to increased density caused by disease in this lung lobe.