Radiographic interpretation of the normal thorax (Proceedings)

Exposure techniques for the thorax

Thoracic radiographs should initially be evaluated for positioning, technique (exposure factors), and the presence of any artifacts that might affect interpretation. Radiographic quality may be the single most overlooked factor in accurate interpretation of thoracic radiographs.

The thorax presents a unique anatomy necessitating specific technical consideration. Because of the inherent high contrast present in the thorax, a low contrast, long grey scale technique is needed to help minimize inherent contrast and allow visualization of a wide range of opacities. This can be accomplished by using high kVp, low mAs exposure settings. The low mAs setting will decrease the potential for motion artifact. Less than 1/60th of a second time exposure is preferred. The high kVp setting decreases contrast, and provides added latitude and room for technical error (Digital radiology systems accomplish this same thing). A good technique will allow faint visualization of the spine through the cardiac shadow on ventrodorsal (VD) or dorsoventral (DV) views. As for any other body part, a grid is necessary if the thorax is thicker than 10 cm. Small dogs and non-obese cats measuring less than 10 cm can be imaged using table top (non-grid) technique and a detail (slow film-screen combination) cassette. The exposure factors for regular cassette and grid versus non-grid and detail cassette are very similar, and there is an added advantage of enhanced detail available with this technique.

Respiratory cycle

The respiratory cycle can have a significant effect on the appearance of thoracic radiographs. Expiratory films, combined with obesity and/or underexposure, can create a significant increase in pulmonary opacity that mimics pulmonary disease. While full inspiratory radiographs should always be attempted, it is not always possible. It is essential then, to be able to differentiate expiratory artifacts from true lung pathology. Radiographic signs of full inspiration include (lateral view) diaphragmatic crura caudal to T-12, (T-13 in the feline), extension of the left cranial lung lobe to first rib, elongated, thinner, horizontal caudal vena cava, expanded radiolucent lung fields, flatter, more caudally located diaphragm, increased ventral angulation of trachea, and increased separation of heart and diaphragm. On ventrodorsal (VD) or dorsoventral (DV) views, signs of true inspiration include smaller appearing cardiac shadow (also on lateral view), increased thoracic cavity length and width, dome of diaphragm caudal to T8, increased separation of heart and diaphragm, and caudal lung lobes caudal to T10.

There are times when an expiratory thoracic film is preferred. Subtle pneumothorax is better visualized on an expiratory film (expiration enhances the opacity of the collapsed lung versus the radiolucent pleural air). Intra-thoracic collapsing trachea is also visualized best on expiration.

Thoracic radiographic views

For routine thoracic radiology, a lateral and either VD or DV view are required. Some differences are visible between right and left lateral views. On right lateral views, the left and right crura lie parallel, with the right crus cranial to the left. The heart is more upright and oval, and there is more overlap of right and left cranial lobar vessels, making them harder to distinguish individually. The caudal vena cava inserts into the right atrium at the level of the cardiac silhouette. On left lateral views, the left crus of the diaphragm extends more cranially (fundus of stomach associated with the left crus), and the left and right crus intersect at the level of the caudal vena cava. The heart appears more rounded, with slight dorsal displacement of the apex (a gap between the cardiac apex and sternum is often seen). The cranial lobar vessels are more distinct and parallel to each other.

These differences are not typically seen in the feline thorax. The diaphragmatic crura are usually superimposed on both right and left lateral views. When taking lateral thoracic views on either dogs or cats, it is important to center the x-ray beam on the caudal aspect of the scapula, and pull the front legs forward, off the cranial thorax. It is important not to stretch the patient, as this results in distortion of the thorax. The head and neck should be extended slightly to avoid a "kink" in the thoracic trachea. A true lateral radiograph will have superimposition of the paired dorsal rib arches and ventral costochondral junctions.

Either a VD or DV view can be chosen as the preferred second view. There are visible differences between the two positions, but not significant enough to choose one over the other. It is best to choose one view, and use it consistently. One exception to this rule occurs with patients in respiratory distress. A DV view is less stressful, and should be used over the VD view.

On DV views, the diaphragm appears as a single, smooth, "hump", and the caudal lung lobes are narrow and taper. The caudal pulmonary arteries and veins are better visualized due to better caudal lung lobe inflation and magnification. The cardiac silhouette is shorter in length. On VD views, three "humps" of the diaphragm are visible (left and right crus, and central cupula). The caudal lung lobes appear wider and less tapered, and the caudal pulmonary vessels are less distinct. The cardiac silhouette appears longer and narrower. The differences between DV and VD are not well visualized on small dog and cat thoracic radiographs. On both views, the sternum and spine should be superimposed. Obliquity on VD or DV views can mimic cardiac enlargement or mediastinal shift.

Three view thorax

When nodular or unilateral disease is suspected, 3 views of the thorax (right AND left lateral and either VD or DV) are required for complete evaluation. Pulmonary lesions are best seen in the "up" or non-dependent lung. A mild, physiologic atelectasis occurs in the dependent lobes, decreasing the amount of air present to contrast soft tissue lesions. Therefore, for example, left lung lesions such as pneumonia or nodular disease will be best visualized on a right lateral view. Very large lesions can be missed by not taking the correct lateral view.

Horizontal beam views are useful for a variety of thoracic disorders. The patient may be placed in lateral recumbency, and a VD view using a horizontal X-ray beam taken. This view allows redistribution of fluid and air, aiding in the diagnosis of subtle pneumothorax or pleural effusion. Evaluation of suspected chest wall or mediastinal masses may be enhanced when pleural fluid falls away from the area of interest. An alternative position with the patient held upright, or "sitting", can be done using a VD view and horizontal X-ray beam. This is especially good for cranial mediastinal masses which are obscured by pleural fluid. In this alternative position, pleural fluid falls ventrally, leaving the cranial mediastinum better evaluated.

Normal radiographic anatomy

The entire thoracic radiograph should be evaluated, and it is important to develop a system so that every film is read in a consistent manner. Evaluation of the thorax also includes the extra-thoracic structures, such as chest wall, ribs, vertebra, sternum, diaphragm, and cranial abdomen (if visible).

Chest wall

Diseases of the chest wall are rare, but artifacts are very common. The opacity of the soft tissue chest wall is due to skin, fat, muscle and parietal pleura. Chest wall thickness varies with body condition. Usually homogeneous, the chest wall may appear stippled when subcutaneous emphysema is present. This stippling can mimic pulmonary infiltrate when superimposed over the lungs. Subcutaneous chest wall masses such as skin tumors, enlarged ticks, and nipples, create smoothly marginated well defined opacities. When superimposed over the lungs, these opacities can mimic pulmonary nodules. Skin folds are another chest wall phenomenon that often create confusing artifacts. Skin folds create tissue opacity lines with distinct dense margins. On VD/DV views they are visible in the caudal lung fields, and can usually be traced beyond thoracic boundaries. At times, however they can mimic pneumothorax, creating a distinct margin between the more radiolucent lung periphery and more opaque central lung field. Pulmonary vessels should cross skin fold lines if no pneumothorax is present, although a hot light may be necessary to visualize these structures. Skin folds are also present on lateral views, extending from cranioventral to caudodorsal, across the ventral thorax.

The ribs, sternebrae, and spine should be evaluated for signs of trauma, or lysis or proliferation that might indicate an active disease process such as osteomyelitis or neoplasia. The ribs of the Bassett Hound and Dachshund breeds have very prominent costochondral junctions that result in a shadow over the periphery of the lung (the chest wall is superimposed). This can sometimes be confused with pleural effusion. Marked proliferative changes are common at the costochondral junction in older dogs, and should not be mistaken for active disease.

Mediastinum

The mediastinum is a real space which separates the two hemithoraces. It is formed by the reflection of the parietal pleura around the heart and other midline structures, and although located within the thoracic cavity, is considered part of the extrapleural space. While the mediastinum does not communicate with the pleural space, it is not a completely closed space, communicating with the fascial planes of the neck cranially, and the retroperitoneal space (via the aortic and esophageal hiatus) caudally. Contained within the mediastinum are the heart and great vessels, trachea, esophagus, thymus, lymph nodes (sternal, cranial mediastinal, and tracheobronchial), and various nerves and blood vessels. With the exception of the heart, descending aorta, and caudal vena cava, which are visible due to surrounding air-filled lung, the remaining mediastinal structures are not seen, either because they are too small to absorb a sufficient number of X-rays, or because they are in contact with each other leading to obliteration of their margin (silhouette sign or border effacement). The lumen of the trachea is visible as it contains air.

The mediastinum is best evaluated on the lateral view, where the portion of the cranial mediastinum can be identified immediately ventral to the trachea, as a homogenous soft tissue opacity. On the VD or DV view, the cranial mediastinum is superimposed on the midline, and in normal animals, should be no wider than twice the width of the spine. In obese animals, fat is deposited in the cranial mediastinum causing widening with smooth, straight margins (up to 3.5 times the width of the spine). This should not be mistaken for a mediastinal mass. Likewise, in young dogs and cats, the presence of the thymus may cause increased width and opacity of the cranial mediastinum on the lateral view, and a prominent triangular soft tissue density just to the left of midline in the VD view (often referred to as the sail sign). These opacities should not be mistaken for fluid or mass in the cranial mediastinum. Remnants of the thymus can be seen normally in some adult dogs and cats. Mediastinal reflections are frequently seen on thoracic radiographs, and should not be mistaken for pathologic opacities. The first is a reflection of the cranioventral mediastinum to the left, accommodating the extension of the right cranial lung lobe across the midline to the left, just below the tip of the left cranial lung lobe (lingual portion). This is visualized on the VD or DV view, but the same reflection is seen on the lateral view as a line of soft tissue opacity running obliquely in a caudoventral direction from the region of the distal aspect of the first rib to the sternum. It represents the mediastinum between the left cranial and right cranial lung lobes, and can occasionally give the appearance of a hyperlucent "bullous" type lesion. Another mediastinal reflection is noted on the VD/DV view, accommodating the extension of the accessory lung lobe to the left of midline. This soft tissue opacity has sometimes been called the cardiophrenic ligament.

The mediastinum is elastic, and can shift with uneven lung inflation, shifting towards the underinflated side (or away from the overexpanded side). The diaphragm frequently will appear elevated on that same side. Artifactual shift can occur with an oblique VD or DV radiograph, but can be recognized as artifactual by noting that the heart has shifted to the same side as the rotated sternum.

Pleural space

Unlike the mediastinum, the normal pleural space is not visible on radiographs. It is actually a potential space formed between the visceral and parietal pleural layers, which is represented by a single line at the periphery of the lung lobes. Although the pleural space does not communicate with other body parts, the right and left pleural space can communicate via mediastinal fenestrations (pleural fluid and air are typically bilateral). Occasionally fissure lines (reflections of visceral pleura) are visualized in normal animals if the x-ray beam strikes these lines tangentially. The fissure line between the right middle and right caudal lung lobe is commonly seen. The potential pleural space becomes visible with the addition of air (pneumothorax), or fluid (hydrothorax, pleural effusion)

Normal anatomy of the lungs

In all species except the horse, the lungs are divided into individual lobes, with cranial, middle, caudal, and accessory on the right side, and cranial (divided into cranial and caudal segments) and caudal on the left side. While it is not possible to distinguish individual lung lobes radiographically in normal animals, it is important to know general locations as certain lobes are more susceptible to disease than others. Parenchymal components within the lung include alveoli, interstitial tissue, bronchial walls, and blood vessels. Vessels make up the majority of background opacity in normal thoracic radiographs. It is important to identify individual pulmonary arteries and veins in all patients, as a vessel abnormality is an important indication of disease. On the left lateral view, peripheral arteries and veins extending into the cranial lung lobes are well visualized. The larger of the two sets of vessels is the magnified artery and vein supplying the right cranial lung lobe. The arteries lie dorsal to the veins, with the radiolucent bronchus lying between the two. Arteries and veins should be about the same size, and the magnified pair should be slightly smaller than the proximal portion of the 4th rib. Peripheral arteries and veins are best visualized on the DV view in the caudal lung lobes. The artery lies lateral to the vein, with a bronchus in between. The arteries and veins should be approximately equal in size, and no bigger than the diameter of the 9th rib where they intersect.