There are two terms that help to explain the radiographic appearance of body structures and their components; they are:
Both radiopacity and radiolucency are functions of: composition, thickness and density
Many times, the only way that two adjacent structures can be differentiated from each other radiographically is by comparing radiolucencies. If the two radiolucencies are different, each structure can be easily seen. It becomes harder when two adjacent structures are of the same radiolucency.
Air is extremely radiolucent due to its low density, and as a result normal lungs tend to be appear black on a radiograph.Normal Thoracic RadiographThe black regions of this thoracic radiograph represent the lungs and appear radiolucent due to the high concentration of air within the structures. As a result, the appearance of the heart, aorta, diaphragm and liver are very distinct and allows them to be differentiated. The lung fields are generally dark, but do have grey regions that represent the pulmonary vessels.
There are many diagnostic tests that can be performed to identify cardiac conditions. An electrocardiogram (ECG) records the electrical activity of the heart, allowing one to comment on many aspects of the heart such as chamber enlargement and rhythm disturbances. An ECG can be performed for a short period of time in the clinic, or the patient can wear a Holter monitor which will monitor the heart's rhythm over 24 hours or longer. An echocardiogram is an ultrasound of the heart, which can assess morphology and function and so can make statements about cardiac chamber enlargement, valvular insufficiency or stenosis, cardiac shunts and frequently one can infer intracardiac pressures.
While heart dimensions, morphologies and functions can be determined using a plethora of diagnostic tools (e.g. electrocardiogram, echocardiogram, palpation, auscultation), the radiograph is the only diagnostic tool readily available to veterinarians to make a statement regarding the patient's lungs. It also becomes tremendously important to be able to decipher cardiogenic pulmonary edema from other forms of lung disease. Again, this is only possible by close observation of thoracic radiographs.
This differentiation becomes tremendously important when determining the treatment protocol that is suitable for the patient. Cardiogenic pulmonary edema is due to increased hydrostatic pressure in the pulmonary capillary bed due to elevated pressures in the pulmonary venous circulation (see Role of Starling Forces below). Therefore, treatment with diuretics will resolve the pulmonary edema. In the case of most forms of lung disease, treating with diuretics will have no impact on the condition of the lungs and in some cases may worsen them.
Starling forces are responsible for the movement of fluid across the permeable membranes of the capillary beds.
The net movement of fluid between the interstitium and the capillary lumen is a result of an imbalance between these pressures. An increase in hydrostatic pressure causes a net fluid movement out of the capillary and into the surrounding interstitium. Pulmonary edema occurs when there is an accumulation of fluid in the interstitium and/or the alveoli of the lungs. Ascites or pleural effusion is also due to increased hydrostatic pressures.
The lymphatic system is responsible for the retrieval of blood plasma that has been lost by the circulatory system. The lymphatic system is made up of lymph vessels that transport the lymph throughout the body and to the thoracic duct, which will drain back into the circulatory system through the subclavian veins to flow into the cranial vena cava and right atrium.
The lymphatic system acts to prevent the accumulation of excessive interstitial fluid especially in the lung. In the absence of lymphatic obstruction, lymph flow can increase 20 to 50 times to remove excess fluid accumulation. Pulmonary edema, ascites or pleural effusion occurs when the lymphatic system is overwhelmed by the amount of lost blood plasma from the circulatory system.