Cardiac Shadow

Image 1: X-ray showing cardiomegaly (click on x-ray to enlarge)

Fig 2: Demonstrating the correct technique for measuring the width of the heart. CTR = a+b/c

Size

The maximum width of the heart on a PA film should be less than half the maximum internal diameter of the chest. If this cardiothoracic ratio (CTR) is more than 50%, cardiomegaly may be diagnosed (Fig 1).

The technique to measure the width of the heart correctly is shown in Fig 2. Echocardiography is the most accurate method of detecting the enlargement of individual chambers within the heart.

Position

Approximately one-third of the heart should lie to the right of the midline and two-thirds to the left.