CXR intepretation

• Visible structures on a chest X-ray


  • Trachea
  • Hila
  • Lungs
  • Diaphragm
  • Heart
  • Aortic knuckle
  • Ribs
  • Scapulae
  • Breasts
  • Bowel gas

  
• Important obscured/invisible structures on a chest X-ray


  • Sternum
  • Esophagus
  • Spine
  • Pleura
  • Fissures
  • Aorta

  
• Trachea and major bronchi


  • They contain air and so are of lower density (black) than the surrounding soft tissues
  • The trachea branches at the carina, into the left and right main bronchi, and these can often be followed as they branch beyond the hila and into the lungs
• Assessing the airways


  1. Start your assessment of every chest x-ray by looking at the airways

  2. The trachea should be central or slightly to the right

  3. If the trachea is deviated, it is important to establish if this is because the patient has been incorrectly positioned (rotated), or if there is pathology

  4. If the trachea is genuinely deviated you should then try to decide if it has been pushed or pulled by a disease process

  
• Hilar structures


  • The hila (lung roots) are complicated structures mainly consisting of the major bronchi and the pulmonary veins and arteries
  • These structures pass through the narrow hila on each side and then branch as they widen out into the lungs
  • The hila are not symmetrical but contain the same basic structures on each side
  • Although the hilar lymph nodes are not visible on a normal chest x-ray, they are of particular clinical importance
  • Many disease processes involve the hila
  • The hila may be at the same level, but commonly the left hilum is higher than the right
  • Both hila should be of similar size and density
  • If either hilum is bigger and more dense, this is a good indication that there is an abnormality

  
• Assessing the hila


  1. Chest x-ray assessment routinely involves checking the hilar structures for normal, size, density and position

  2. The hila are often wrongly called abnormal when normal, and vice-versa

  3. An awareness of the range of normal is important, but the best tip is to look for increase in density as well as size

  4. If the hila are out of position, ask yourself if they are pushed or pulled, just as you would when assessing the trachea

  
• Lung zones


  • It is difficult to fit the whole of both lungs into your field of view
  • When describing the lungs it is conventional to divide them into three zones - upper, middle and lower
  • Each of these zones occupies approximately one third of the height of the lungs
  • The lung zones do not equate to the lung lobes

  
• Assessing the lung zones

  1. Each zone is compared with its opposite side paying attention to any asymmetry

  2. If the lungs appear asymmetrical, it should be determined if this can be explained by asymmetry of normal structures, technical factors such as rotation, or lung pathology

  3. If there is genuine asymmetry, decide which side is abnormal

  4. Often a dense (whiter) area is abnormal, but some diseases cause reduced density (blacker)

  5. If there is an area that is different from the surrounding ipsilateral lung, then this is likely to be the abnormal area

  
• Lung lobes and fissures


  • The left lung is divided into two lobes, upper and lower
  • The right lung has an oblique fissure and a horizontal fissure, separating the lung into three lobes - upper, middle, and lower
  • Each lobe has its own visceral pleural covering

  
• Horizontal fissure


  • It separates the right upper lobe from the right middle lobe
  • It can be seen on normal chest x-rays as a thin line running roughly horizontally from the edge of the lung towards the right hilum

  
• Oblique fissures


  • They overlie each other on a lateral view and are not always seen in entirely
  • If seen at all, the lower end is usually seen most clearly
• Azygos fissure


  • It occurs in approximately 1-2% of individuals
  • The azygos vein usually runs horizontally along the right side of the mediastinum
  • It hooks forwards over the right main bronchus, draining the azygos system into the superior vena cava
  • If there is an azygos fissure, the vein appears to run within the lung, but is actually surrounded by both parietal and visceral pleura

  
• Assessing the fissures


  1. Occasionally lung disease is limited in extent by a fissure

  2. This can help locate a disease process more specifically to a lobe

  3. For most cases this degree of accuracy is not clinically important - unless further action such as biopsy or surgery is required, in which case other imaging such as CT would probably be performed

  4. In most cases it is still best to refer to the location of abnormalities in terms of lung zones

  
• Pleura and pleural spaces


  • The pleura are only visible when there is an abnormality present
  • Some diseases cause pleural thickening, and others lead to fluid or air gathering in the pleural spaces

  
• Assessing the pleura


  1. Pleural abnormalities can be subtle and it is important to check carefully around the edge of each lung where abnormalities are usually seen more easily

  2. Lung markings should be seen all the way to the edge of the chest wall

  3. If the lung edge (visceral pleura) is visible and there is black surrounding this edge then a pneumothorax should be suspected

  4. This should lead to immediate assessment of the patient's trachea and mediastinum, both on the x-ray and, more importantly, clinically

  5. If there is a deviation of these midline structures away from the side of a pneumothorax, this is a medical emergency

  6. Every chest x-ray should also be checked for pleural thickening, or collections of fluid or air in the pleural spaces

  
• Costophrenic recesses and angles


  • The costophrenic recesses are formed by the hemidiaphragms and the chest wall
  • They contain the rim of the lung bases which lie over the dome of each hemidiaphragm
  • On a PA chest x-ray the recess is seen in only one place on each side, where an angle is formed by the lateral chest wall and the dome of each hemidiaphragm
  • These angles are known as the costophrenic angles

  
• Assessing the costophrenic angles

  1. On a PA chest x-ray the costophrenic angles should form acute angles which are sharp to a point

  2. Often the term costophrenic "blunting" is used to refer to the presence of a pleural effusion

  3. This, however, is not always correct and costophrenic angle blunting can be related to other pleural disease, or to underlying lung disease

  4. Lung hyper-expansion can also lead to blunting of the costophrenic angles

  
• Diaphragm


  • The diaphragm separates the comparatively dense (white) abdominal organs below it, from the relatively less dense (blacker) lung above
  • Each hemidiaphragm should appear as a rounded, domed structure with a crisp white edge contrasted against the adjacent dark lung
  • The right hemidiaphragm is usually a little higher than the left, to accommodate the liver
  • Very often you will see air in the stomach below the left hemidiaphragm, with the appearance of a dark (less dense) bubble
  • The lowest portion of the lungs, which lie in the posterior costophrenic recesses, lie below the level of the contours of the hemidiaphragms

  
• Assessing the diaphragm


  1. The hemidiaphragms are domed structures that should be well defined and visible to the midline on a frontal view

  2. The contours of the hemidiaphragms do not form the bottom of the lungs

  3. Lung markings can be seen below the hemidiaphragms

  4. The stomach bubble, which is a normal phenomenon, and the soft tissues of the abdomen are also visible below the hemidiaphragms

  
• Hemidiaphragm


  • Normal lung is seen below
  • Lung markings are seen below on both sides
  • Medially the hemidiaphragms form an angle with the heart (cardiophrenic angles)
  • Contour of the hemidiaphragm should be seen passing medially as far as the spine
  • Left and right hemidiaphragms are almost superimposed on a lateral view
  • Anteriorly the left hemidiaphragm blends with the heart and becomes indistinct

  
• Assessing the diaphragm


  • The hemidiaphragms are domed structures that should be well defined and visible to the midline on a frontal view
  • The contours of the hemidiaphragms do not form the bottom of the lungs
  • Lung markings can be seen below the hemidiaphragms
  • The stomach bubble and soft tissues of the abdomen are also visible below the hemidiaphragms

  
• Heart size


  • Not assessed by absolute measurement, but rather in relation to the total thoracic width, and is expressed as a ratio
  • Cardiothoracic ratio (CTR) = Cardiac Width : Thoracic Width
  • A CTR of greater than 1:2 (50%) is considered abnormal
  • Assumes the projection is Posterior-Anterior (PA), and that cardiac size is not exaggerated by factors such as patient rotation or an incomplete breath in

  
• Measuring cardiothoracic ratio (CTR)

  1. Cardiac size is measured by dropping parallel lines down both sides of the heart, at the most lateral points on each side, and measuring between them

  2. Thoracic width is measured by dropping parallel lines down the inner aspect of the widest points of the rib cage, and measuring between these

  3. The cardio-thoracic ratio can then be stated

  
• CTR is approximately 15 : 33 and is therefore within the normal limit of 50%
• Cardiac contours


  • The left heart contour consists of the left lateral border of the Left Ventricle (LV)
  • The right heart contour is the right lateral border of the Right Atrium (RA)
• Assessing the heart


  • The heart size should be considered on every chest x-ray, but the cardiothoracic ratio (CTR) can only be assessed confidently if a posterior - anterior (PA) view has been acquired
  • If an anterior - posterior (AP) view has been taken, then the heart should not be called enlarged even if the CTR is >50% as an AP view exaggerates the heart size
  • If the heart contours are not clearly seen, this may be because of increase in density of the adjacent lung
  • The lingula of the upper lobe of the left lung, wraps over the left ventricle, and so loss of definition of the left heart border may be related to disease in this area of lung
  • On the right, the middle lobe is located adjacent to the right atrium, and therefore loss of definition of the right heart border may be due to increased density caused by disease in this lung lobe

  
• Mediastinal contours


  • The mediastinum itself contains the heart and great vessels (middle mediastinum) and potential spaces in front of the heart (anterior mediastinum), behind the heart (posterior mediastinum) and above the heart (superior mediastinum)
  • These potential spaces are not defined on a normal chest x-ray, but an awareness of their position can help in describing the location of disease processes
  • Structures in the superior mediastinum that should always be checked include the aortic knuckle, the aorto-pulmonary window, and the right para-tracheal stripe

  
• Aortic knuckle


  • Represents the left lateral edge of the aorta as it arches backwards over the left main bronchus, and pulmonary vessels
  • The contour of the descending thoracic aorta can be seen in continuation from the aortic knuckle
  • Displacement or loss of definition of these lines can indicate disease, such as aneurysm or adjacent lung consolidation
• Aorto-pulmonary window


  • Lies between the arch of the aorta and the pulmonary arteries
  • This is a potential space in the mediastinum where abnormal enlargement of lymph nodes can be seen on a chest x-ray

  
• Right para-tracheal stripe


  • From the level of the clavicles to the azygos vein the right edge of the trachea is seen as a thin white stripe
  • This appearance is created by air of low density (blacker) lying either side of the comparatively dense (whiter) tracheal wall
  • If this stripe is thickened (normally less than 3 mm) this may represent pathology such as a paratracheal mass or enlarged lymph node
• Assessing the mediastinal contours


  • Whenever you look at a chest x-ray it is well worth looking for abnormalities in the region of the aortic knuckle, the aorto-pulmonary window, and the right para-tracheal stripe
  • Appreciation of the range of normality of these structures will come with viewing as many chest x-rays as you can

  
• Soft tissues


  • Abnormalities of the soft tissue may give important clues to a diagnosis
  • If a patient has very thick soft tissue due to obesity, underlying structures such as the lung markings may be obscured
  • Large breasts may obscure the costophrenic angles, giving the impression of the presence of pleural effusions
• Soft tissue fat


  • Normal fat plane between layers of muscle appears blacker than muscle
  • Irregular areas of black within the soft tissues may represent air tracking in the subcutaneous layers (surgical emphysema)
• Bones


  • Bones are the densest tissue visible on a normal chest x-ray
  • The bones visible on a chest x-ray include the clavicles, the ribs, the scapulae, part of the spine, and the proximal humeri (upper arms)
  • The bones are used as useful markers of chest radiograph quality to assess patient rotation, adequacy of inspiration and x-ray penetration

  
• Ribs


  • The anterior end of approximately 5-7 ribs should be visible above the diaphragm in the mid-clavicular line
  • Less than this indicates an incomplete breath in, and more than 7 ribs or flattening of the diaphragm, suggests lung hyper-expansion
  • The subcostal grooves on the underside of the ribs contain the neurovascular bundles that accompany each rib

  
• Assessing the bones


  • Bones are useful for the initial assessment of image quality, and offer useful information regarding rotation, inspiration and penetration
  • Chest x-rays are not helpful if there is thought to be an isolated rib injury as rib fractures are often invisible
  • Occasionally you will see an important abnormality of the bones on a chest x-ray such as a metastatic deposit