The cardiothoracic ratio can be used as a measure of cardiomegaly.
cardiothoracic ratio =
= (transverse diameter of the heart on AP view) / (transverse diameter of the bony thoracic cage)
where:
• The transverse diameter of the heart is the sum of the maximum projections of the heart to the left and to the right of the midline
• Variation can be introduced by variation in posture, by inspiration and by the degree of adipose tissue
• The comparison of heart size should only be done on similarly performed x-rays; standard and portable x-rays should not be intermixed for measurement
Interpretation:
• Cardiac enlargement is felt to be present when the cardiothoracic ratio (C/T ratio) exceeds 50% (some use 60%).
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