There are two terms that help to explain the radiographic appearance of body structures and their components; they are:

• Radiopacity -> opaque to x-rays and appears white on radiographs
• Radiolucency -> translucent to x-rays and appears black on radiographs

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.

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.

• On the capillary side of the membrane
• Hydrostatic pressure: acts on the fluid within the capillary and forces it out of the capillary and into the interstitium
• Osmotic pressure: draws fluid from the interstitium and into the capillary
• On the interstitial side of the membrane
• Hydrostatic pressure: acts on the fluid within the interstitium and enter the capillary
• Osmotic pressure: draws fluid into the interstitium from the capillary

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.

Radiographic signs of left-sided heart failure include left atrial enlargement, pulmonary venous engorgement, peribronchial pattern, and air bronchograms. The constellation of these findings provides compelling evidence of pulmonary edema.

Left-sided heart failure occurs when the heart loses its ability to pump oxygenated blood to the rest of the body efficiently. This can result in the "backing up" of blood pressure from the left ventricle, to the left atrium, to the pulmonary veins and finally the pulmonary capillary beds. This elevated hydrostatic pressure in the pulmonary capillaries disturbs Starling Forces across the capillary membrane causing fluid to "leak" into the pulmonary interstitium resulting in edema. Radiographic signs of left sided heart failure therefore begin with evidence of pulmonary venous engorgement. As fluid "leaks" into the pulmonary interstitium it collects along the triad of artery, bronchus, and vein causing the so called "peribronchial pattern", and silhouetting of the margins of the pulmonary arteries and veins. And finally, as fluid continues to "leak" into the interstitium it fills the alveoli causing the radiographic finding of "air bronchograms". Patients often present with a history of exercise intolerance due to the reduced amount of oxygenated blood being pumped to the rest of the body by the left ventricle. Since fluid will be leaking out of the vasculature due to increased hydrostatic pressure, the patient may also have trouble breathing due to fluid in the lungs.

The increase in hydrostatic pressure results in the passage of fluid from the capillaries, through their loose junctions, into the surrounding interstitium. At this stage, peribronchial patterns would likely be detectable on radiographs.
Copyright (C) 1998, Lynne Larson .All rights reserved.

As the pressure increases, more fluid will pour out into the interstitium and the surrounding alveoli. At this stage, air bronchograms would likely be detectable on radiographs.
Copyright (C) 1998, Lynne Larson .All rights reserved.

As the left side of the heart fails, that is less blood is ejected and venous return continues, then preload (fluid volume) to the left side of the heart becomes elevated. The elevated volume in the pulmonary venous system results in dilation or pulmonary venous engorgement. As the volume continues to increase, pressures in the pulmonary venous system become elevated and also in the pulmonary capillary bed resulting in a net efflux of fluid into the pulmonary interstitium. Venous engorgement is the first sign of increased volume in the left heart, making their detection on radiographs extremely important.

Radiographic signs of pulmonary venous distension:

• On the lateral view, the cranial pulmonary veins are greater than 75% the width of the proximal 1/3 of the fourth rib
• The pulmonary vein is obviously larger than its accompanying pulmonary artery (normally they are of equal width)

See also the Pulmonary Venous Distension section of the Clinical Evaluation of the Heart.

As mentioned previously the elevated capillary hydrostatic pressure causes fluid to "leak" into the pulmonary interstitium where it collects along the triad of artery, bronchus, and vein causing the so-called "peribronchial pattern", and silhouetting of the margins of the pulmonary arteries and veins. With respect to the development of a peribronchial pattern, the fluid weeps down both the inside and outside of an airway. The walls of the tertiary bronchioles are not detectable radiographically due to the fact that they no longer have cartilaginous rings. However, once the inner and outer wall of the bronchiole is covered in fluid, it becomes more radiopaque and therefore contrasts very well against the radiolucent lumen (filled with air). As a result, on cross section the bronchiole appears as a radiopaque circle with a radiolucent center ("donut"), and if cut longitudinally it appears as two parallel radiopaque lines ("railroad track").

Peribronchial Pattern - example 1

Peribronchial Pattern - example 2

Courtesy of Dr. Jerry Owens

Peribronchial Pattern - example 3

Note: Realize that a peribronchial pattern is a term given to a radiographic sign that it caused by fluid lining either the inside or outside of an airway resulting in a "donut" (on cross-section) or a "railroad track" (if cut longitudinally). The fluid that is responsible for the peribronchial pattern can be of almost any nature: fluid of edema, hemorrhage, excessive production of mucous etc. Therefore, the presence of peribronchial patterns alone does not conclude that a patient has pulmonary edema. The suspicion does increase however when there are signs of left atrial enlargement, pulmonary venous engorgement, peribronchial pattern, and/or air bronchograms.

The final stage in the development of pulmonary edema and the most advanced stage is represented by the radiographic finding of air bronchograms. Air bronchograms are formed when continued fluid accumulation occurs into the alveoli themselves, resulting in alveolar edema. When the alveoli accumulate fluid, they appear radiopaque and only the air filled bronchioles remain visible. As you can see from the picture (side), the air bronchograms are easily detectable against such an opaque, fluid-filled lung. It is important to note that usually peribronchial patterns will be seen before air bronchograms during the stages of pulmonary edema development. Some areas of the lung however, will be at various stages and may show a mix of both peribronchial patterns and air bronchograms

Air Bronchogram - example 1

Air Bronchogram - example 2

Note: Realize that air bronchograms occur when fluid accumulates in the alveoli of the lung causing alveolar edema. Alveolar edema causes a sharp contrast between the fluid filled alveoli and the patent airway. Much like peribronchial patterns, air bronchograms will result regardless of the origin of the fluid and therefore can't be automatically credited to pulmonary edema.

Students frequently have difficulty distinguishing between peribronchial patterns and air bronchograms

The difference between these two radiographic signs is the ability to visualize the outer membrane of the airway wall. With a peribroncial pattern the outside and the inner airway walls are visualized (because fluid collects only along the airway wall and not into the adjacent alveoli and so there is radiographic contrast between the airway wall (in white) and the adjacent alveoli (in black)). With an air bronchogram only the inner airway wall, and hence lumen, is visible. The outer airway wall is obscured/silhouetted because the adjacent alveoli are flooded and thus the radiographic density of the immediate region around the airway is of the same density (radiographic color) as the airway wall itself. The absence of radiographic contrast between the airway wall and the adjacent region results in the loss of the visualization of the outer airway wall.

A lobar sign is present when a sharp line of demarcation is noted between a lung lobe that is opaque (consolidated) and adjacent to one that is either normally aerated or near normally aerated. Hence in a lobar sign, the margin of the consolidating process must extend to the margin of the lung lobe to provide the sharp radiographic contrast between the non-aerated and aerated lung lobes. Just like with fissure lines, one must ensure that this line of demarcation occurs where lung lobe fissures actually exist.

Lobar signs are seen when a patient has fluid accumulation in the alveoli and parenchyma and as a result, the affected lung lobe is radiopaque compared to its radiolucent adjacent lobes. The presence of a lobar sign indicates alveolar disease due to the presence of exudates, edema, and/or hemorrhage in that area.

Lobar Signs - example 1

Lobar Signs - example 2

As a general rule lobar signs are not observed in the setting of cardiogenic pulmonary edema. In example 2, one can see the lobar sign associated with the right cranial lung lobe. If the lobar sign was due to increased pressures within the vasculature, all lobes of the lung should be affected equally. Lobar signs suggest a focal process whereas pulmonary edema is a more diffuse process.

The cranial vena cava is responsible for draining the unoxygenated blood from the head, thorax and forelimbs into the right atrium. Meanwhile, the caudal vena cava drains unoxygenated blood from the abdomen and the hindlimbs into the right atrium. Therefore, when pressures increase in the right side of the heart, specifically the right atrium, the cranial and caudal vena cava experience an increase in hydrostatic pressure. This increase in hydrostatic pressure in the cranial vena cava or the caudal vena cava causes fluid flux out of the vessels and into the pleural cavity or abdominal cavity respectively.

During the evaluation of thoracic radiographs, fluid in the pleural cavity can be detected by observing certain radiographic features such as: fissure lines and the leafing of lung lobes.

Fissures are the normal divides between the lobes of the lungs; they are not normally visible radiographically, but become radiographically visible in the presence of fluid in the pleural space or under conditions that cause the pleural lining of the lung to become thickened. It should be noted that the most common cause of fissure lines is pleural thickening. When there is pleural effusion however, the fluid collects within the fissure, wedges the lobes apart (with an agent [fluid] that contrasts with the radiopaque lungs) making the fissure radiographically visible. The fissure will generally appear as a discrete radiopaque line. Fissure lines created by pleural fluid are wider at the lung lobe margins and become narrower centrally. If a linear structure is observed in an area that doesn't normally have a lung fissure, then other differentials must be investigated.

Normal Anatomy

A - Fissures of the lateral aspect of the left lung (looking medial to lateral), usually seen when patient is in left lateral recumbency.

B - Fissures located on the lateral aspect of the right lung (looking medial to lateral), usually seen when patient is in right lateral recumbency

C - Fissures located on the dorsal aspect of the lungs and are seen when the patient is in dorsal recumbency.

D - Fissures located on the ventral aspect of the lungs and are seen when the patient is in sternal recumbency.

Fissure Line - example 1

Fissure Line - example 2

Both fissure lines and lobar signs appear as lines along the lobes of the lung, they both occur at the same location (along a lung lobe fissure) but they are easily differentiated. Radiographically, a fissure line is observed as a single white line due to the wicking of fluid between the two lung lobes (image: left below). A lobar sign is represented by a white margin adjacent to an area of radiolucency. This white margin represents the edge of a lung lobe that is consolidated often due to the presence of exudates, edema and/or hemorrhage. These are important to differentiate because they represent different disease processes. One represents fluid collecting in the pleural space (fissure line due to fluid) and the other represents flooding of the alveoli with fluid (note that exudates and hemorrhage are essentially fluid).

The same basic principles behind lung fissure lines apply to the leafing of the lung lobes that is sometimes visible in a patient's radiograph. Instead of involving one lung fissure, leafing of the lung lobes occurs when there is enough fluid to surround multiple lung lobes, the lobes are displaced from the chest wall by fluid and fissure lines are very wide. Leafing of lung lobes and fissure lines represent two ends of a spectrum associated with fluid accumulation in the pleural space; leafing represents marked fluid accumulation and fissure line represent mild fluid accumulation. In DV or VD views, the outlines of the lung lobes and fissures can be clearly seen and was originally described as looking like staggered, layered leaves. Again, leafing of lung lobes will appear identically on radiographs regardless of the fluid type found in the pleural space around the lungs. Therefore, evidence of leafing doesn't automatically specify the lung's condition as cardiogenic and other causes and types of pleural fluid should be investigated

Leafing of Lung Lobes - example 1

The term "cotton-like density" isn't exclusive to one condition as it is a statement regarding the appearance of a disease process. It occurs when there is obliteration of the lung parenchymal architecture due to the infiltration of cells or fluid (remember that exudate and hemorrhage are essentially fluid). Unlike air bronchograms even the airways (bronchioles) in the region are obliterated or flooded. Radiographically it appears as relatively discrete areas of opacity with either discrete or non-discrete borders. Typically, a cotton-like density will be radiopaque with a "fluffy" border. Cotton-like densities may be found in individuals with: pulmonary osteomas, neoplasia, fungal infiltrates and areas of abscessation.

Cotton-like Densities - example 1

Cotton-like Densities - example 2