The Normal Thoracic Radiograph: Why You Must Understand Normal to Recognize Abnormal
David S. Biller, DVM, DACVR
Professor and Head of Radiology, Kansas State University, College of Veterinary Medicine, Department of Clinical Sciences
Manhattan, KS, USA
If you can accurately describe a radiographic lesion you are well on your way to identifying the disease process. There are 3 phases to interpretation of a radiograph. First phase is called the Recognition phase. In this phase we compare all parts of the radiographic image to normal. If any abnormalities are recognized we must consider whether they are a normal variant, an artifact, or due to improper patient positioning. The second phase is Descriptive phase. In this phase we describe how the lesion varies from normal including: changes in opacity, size, shape, contour (margination), location, position, and number. We also describe the extent of the lesion (involve entire left cranial lung or just the cranial subsegment). Lastly we describe the distribution of the lesion (focal, multifocal, or diffuse). The last phase is the Analysis phase. Take all the information regarding radiographic changes from normal, historical information (patient signalment) and abnormalities found on physical examination and develop a list of differential diagnosis. Prioritize this list by probability. At this point additional information may be necessary and require additional tests (radiographs, ultrasound, computed tomography, blood work, culture and biopsies/fine needle aspirates).
Evaluation of the chest wall or extrathoracic structures should always include the cranial abdomen and caudal cervical regions. Liver size, abdominal distension, free abdominal gas and presence of peritoneal fluid can usually be diagnosed. Position of abdominal viscera may be partially evaluated in suspected cases of diaphragmatic hernia. In cats the appearance of the falciform fat triangle, just ventral to the liver can also be used for this evaluation.
Evaluation of the chest wall radiographically includes: soft tissues, ribs, sternum, and spine. The chest wall should first be examined from a distance for symmetry. The ribs should also be examined for symmetry. Fractures, penetrating wounds and neoplasia may cause displacement of or destruction of ribs. The chest wall should be evaluated for changes in opacity (focal/diffuse opacities or lucencies). One of the biggest problems associated with interpretation of the thorax is associated with the normal chest wall, is skin folds. Skin folds can be confused with lobar borders causing erroneous diagnosis of pneumothorax. Subcutaneous masses and dirt (ticks) can also add confusion when evaluating chest wall and pulmonary parenchyma. Deformities (i.e., scoliosis, lordosis, kyphosis, pectus excavatum) are rarely of clinical significance but can cause marked changes in appearance of the internal thoracic structures. Lesions of the chest wall include: trauma (fractures, swelling, SQ emphysema), infections, degeneration, and neoplasia. Many chest wall lesions present as masses, and if they project into the thorax may demonstrate an extrapleural mass sign:
1. Well defined convex border facing the pulmonary surface
2. Tapered edges which blend into the chest wall
3. Adjacent rib lesion (destructive &/or productive reaction). These signs may be difficult to demonstrate unless the x-ray beam catches the mass tangentially.
The position of the diaphragm depends upon muscle tension and the trans-diaphragmatic pressure. The diaphragm is not actually visualized as a separate structure; the density seen (or what we call the diaphragm) is mainly the cranial aspect of the liver. If the diaphragm is seen separately as a thin band it is indicative of free gas in the abdomen (pneumoperitoneum).
The inability to visualize the diaphragm may be due to border effacement or its rupture. Border effacement may occur secondary to alveolar pattern in the caudal lung lobes or pleural effusion. Rupture of the diaphragm or a diaphragmatic hernia may occur as an acquired disease from trauma or congenital for example: hiatal, or peritoneopericardial.
Radiographic signs of a traumatic diaphragmatic hernia include: interruption of diaphragmatic outline (incomplete visualization of diaphragm), abdominal viscera displacement (presence of abdominal structures in the thorax-abdominal radiograph maybe helpful), pleural effusion &/or thoracic mass lesion which may contain loculated gas, ± rib fractures, and mediastinal shift. Pleural effusion maybe asymmetrical. The irregular herniated organs compress the lungs on one side. The spaces between the lungs and the organs fill with fluid, obscuring the organs. Additional studies which may be useful in the diagnosis of a diaphragmatic hernia: abdominal radiographs, UGI, positive contrast peritoneography, and multiple positional radiographs. Differential diagnosis for loss of diaphragmatic outline include: pleural fluid, pleural masses, mediastinal densities, and pulmonary densities.
Mediastinum is a body compartment or potential space between pleural cavities lined on each side by pleura (mediastinal pleura). This is a nebulous anatomic area which may be difficult to evaluate because of a lack of any contrasting tissue densities. Radiographic diagnosis of disease may be difficult because: 1) number of incompletely visualized organs (lack of contrasting tissue densities), 2) superimposition of spine on VD/DV film, and 3) normal fat accumulation there. It extends from dorsal to ventral and from the thoracic inlet to the diaphragm. On the lateral view a fluid opaque area can be visualized between the thoracic inlet and carina and from the ventral edge of the spine to just ventral to the trachea. This is the craniodorsal mediastinum which contains a number of important structures including: trachea, esophagus, heart, great vessels (ie., aorta, caudal and cranial vena cava), thoracic duct, and major nerves, lymph nodes, and thymus (young animals). The mediastinum is poorly seen as a distinct structure on the lateral radiograph.
On the DV radiograph the cranial and caudoventral borders are apparent. Normal thickness of the cranial mediastinum is 1.5 to 2 times the width of the vertebral bodies in the cat. Should have smooth, rather straight margins. Wide cranial mediastinum in young animals (< 1 year) usually due to thymic shadow (sail sign). Esophagus, outer tracheal walls, major subdivisions of the aorta, cranial vena cava, and azygos vein are not normally seen; their visualization would indicate the presence of gas within the mediastinum (pneumomediastinum). Esophagus may be seen without surrounding gas if it contains gas or material (fluid, food, barium).
The right and left hemithoraces are essentially equal in size when evaluated on the VD/DV radiograph. Any difference in size is referred to as a mediastinal shift. This can result from uneven inflation of lungs (right versus left) caused by a unilateral increase or decrease in lung volume or a unilateral increase or decrease in intrathoracic pressure (volume).
Mediastinum should also be evaluated for presence of diffuse or focal widening which may indicate a infiltrative process or mass lesion. Diffuse mediastinal widening, indistinctness or border effacement of contained structures, and narrowing or displacement of the trachea can be seen with fluid accumulation (mediastinitis, hemorrhage-coagulopathy, trauma).
Mediastinal masses are common in cats and appear as thoracic densities near midline and frequently cause displacement of adjacent structures. If a mass is in contact with a mediastinal structure it will be border effaced. Mediastinal masses are frequently divided up by location: cranioventral, craniodorsal, perihilar, caudoventral, and caudodorsal.
Radiographic changes associated with mediastinal masses include: increased opacity or opaque cranial thorax (mediastinum), displacement of normal mediastinal structures (displaced and or compressed trachea and/or esophagus-esophagram is helpful), widened mediastinum, and loss of distinct mediastinal borders. Differential diagnoses as well as confusing lesion that may mimic mediastinal disease includes: normal thymus young animals, obese animals, lung mass (especially of the cranial tips of L and R cranial lungs and accessory lung), and perihilar edema.
Pleura and Pleura Space
The pleural space is a potential space between the visceral (pulmonary) and parietal (costal, mediastinal, and diaphragmatic) pleura. The lung remains inflated and occupies the pleural space rather than demonstrating its normal elastic retractile property because of the surface tension of the pleural fluid and sub-atmospheric pressure present in the thorax. The normal pleural spaces (right and left) are not visualized radiographically, although a small amount of fluid is present and acts as a lubricant reducing friction between pleural surfaces. They usually communicate with each other through fenestration of the mediastinum, but not with other compartments of the body. Interlobar fissures are reflections of the visceral pleura on the lung lobes, not normally seen. Three criteria must be met for the pleura to be visualized radiographically:
1. Projection of pleura on end (tangential beam striking pleural margin or border),
2. Thickened pleura or increased pleural fluid present, and
3. Well aerated lung adjacent to diseased lung to contrast the pleura. We usually do not see pleural fissures because the x-ray beam strikes the pleural margin obliquely. When seen they will appear as very thin, sharp lines in typical locations. It is more common to see interlobar fissures in the older animal secondary to pleural thickening (aging change).
Radiology plays an extremely important role regarding pleural disease: detection, location, estimation of volume, distribution, and mobility of the air or fluid, placing needle for thoracocentesis, and evaluation for associated disease (chest wall, pulmonary, mediastinal, diaphragmatic). There are 2 broad categories of pleural disease:
1. Increased opacity to the pleural space (pleural effusion or mass lesion)
2. Decreased opacity of the pleural space (pneumothorax)
The overall opacity of the lung depends upon the ratio between air within the alveoli and bronchi and soft tissue of the vessels and interstitium. Any process that alters this ratio alters the appearance of the lungs. The components of the lung include: airways and alveoli, vessels, interstitium (lymphatics, supporting framework for vascular and bronchial structures, interalveolar septa), and bronchi. The pulmonary opacity can be increased by conditions other than pathology including: poor inspiration, severe obesity, under exposure, and advanced age. The lungs are usually divided into 3 regions for evaluation and include: central (perihilar), middle, and peripheral.
Pulmonary lesions may be described as: solitary, multiple, focal or disseminated (diffuse). Other descriptive terms that can be used in describing lesions of the lungs include: well defined, ill defined, irregular, rounded, nodular, fluffy, homogeneous, inhomogeneous, cavitary, and solid. Pulmonary parenchyma lesions may also include generalized increases and decreases in pulmonary opacity.