Chest x ray anatomy

Visible structures

• Trachea
• Hila
• Lungs
• Diaphragm
• Heart
• Aortic knuckle
• Ribs
• Scapulae
• Breasts
• Stomach

Invisible or obscured structures

• Sternum
• Pleura
• Oesophagus
• Fissures
• Spine
• Aorta

Trachea and major bronchi

• The large airways contain air and are therefore less dense (blacker) than surrounding tissue
• The trachea should be central, but may be displaced slightly to the right since it passes to the right of the aorta
• The trachea and bronchi are visible, and branch at the carina

Hilar structures

• Each hilum contains major bronchi and pulmonary vessels
• Lymph nodes on each side are not visible unless abnormal
• Left hilum is often higher than the right due to the presence of the heart on the left side
• Position, size, and density of hila are important
• If position is changed, has it been pushed or pulled?
• If size and density are both increased then it is indicative of pathology

Lung zones

• Lung are assessed and described by dividing them into upper, middle, and lower zones (not lobes)
• Compare left to right, and normal to abnormal
• Be careful of pathologies that give rise to bilateral abnormalities
• Note that the lower zone extends below diaphragm due to the diaphragm's dome shape

Pleura and pleural spaces

• The pleura and pleural spaces are only visible when abnormal
• Lung markings should reach the thoracic wall
• Normal invisible pleura
• Trace around the entire edge of the lung (starting and ending at the hila), paying attention to where abnormalities are most seen
• Is there pleural thickening, pneumothorax or effusion?

Lung lobes and fissues

• Left lung has two lobes and an oblique fissue (seen on lateral view)
• Right lung has three lobes and both a horizontal (seen on normal (AP/PA) view) and an oblique fissure (seen on lateral view)
• Each lobe has its own covering of pleura
• Fissures are important because some diseases are confined to a certain extent within lobes
• May have azygous fissure (azygous vein runs within it)

Costophrenic angles and recesses

• Costophrenic angles are limited views of the costophrenic recess
• They should be sharp on frontal view (although when they are blunt it isn't always pleural effusion)
• Formed by lateral chest wall and dome of each diaphragm

Diaphragm

• The hemidiaphragms are domed structures
• Each hemidiaphragm should be well defined
• The left hemidiaphragm should be visible behind the heart
• The hemidiaphragm contours do not represent the lowest part of the lungs
• Liver beneath the right
• Stomach (gastric bubble visible) beneath the left

Heart size and contours

• Cardiothoracic ratio (CTR) = Cardiac width : Thoracic width
• Heart size is assessed as CTR (>50% is abnormal but only applicable to PA)
• Cardiac contours
• Right (Right atrium)
• Left (Left ventricle)

Mediastinal contours

• Middle mediastinum contains the heart
• Aortic knuckle
• Represents the left lateral edge of the aorta as it arches backwards over the left main bronchus and pulmonary vessels
• Displacement or loss of definition may mean aneurysm or adjacent lung consolidation
• Aorto-pulmonary window
• Lies between arch of the aorta (superior) and pulmonary arteries (inferior)
• Abnormal enlargement of lymph nodes can be seen here
• Right para-tracheal stripe
• From the level of the clavicles to the azygous vein the right edge of the trachea is seen as a thin white stripe
• If this stripe is abnormally thickened (more than or equals to 3mm), it may represent a paratracheal mass or an enlarged lymph node

Soft tissues

• These need to be assessed on every chest x ray
• Thick soft tissue may obscure underlying structures
• Black within soft tissue may represent gas
• Breast asymmetry is common and should not be mistaken for pathological soft tissue
• Nipple markings may be seen but care should be taken that they are not pathological lung nodules
• Pseudo-blunting of the costophrenic angle may be due to breast, malposition, etc.
• Soft tissue fat appears less dense and therefore darker
• These are usually regular as opposed to irregular pockets of black as is in the case of surgical emphysema

Bones

• These need to be assessed on every chest x ray
• They are also helpful in assessing the quality of the x ray
• Look for abnormalities of single bones and diffuse bone disease
• Clavicles clearly seen on x ray
• Spinous processes of vertebra should lie midway between the medial ends of the clavicles (if not then the patient is rotated)
• Check of metastases in bones especially in well-defined structures such as the clavicle, scapula, and humerus
• Ribs are important in assessing the inspiratory effort made by the patient
• Anteriorly there should be 5-7 ribs
• More than 7 ribs and flattening suggests lung hyper-expansion
• Adequate penetration when spine can be seen through the heart