Thoracic radiographs (or chest x-rays) allow the evaluation of heart size and the lungs.  Since echocardiograms can not see into the lungs, thoracic radiographs compliment an echocardiogram in determining the severity of cardiac disease, particularly when an animal is coughing. 

Coughing is often due to an enlarged heart pressing on a pet’s airway rather than congestive heart failure and thoracic radiographs are needed to determine which is the source of the cough.  Thoracic radiographs also allow diagnosis of other reasons for cough or difficulty breathing including some types of cancer and airway disease such as asthma and bronchitis.