Normal Chest Xray
WHICH TISSUE DENSITIES CAN WE SEE ON THE RADIOGRAPH?
In order of increasing density, they are:
air – lungs
fat – in breast & subcutaneous tissues, & around muscle
water (soft tissue) – heart, muscle, blood
metal or Ca++ - bone, metallic hardware, contrast agents
On the radiograph, anatomic structures are recognized by their different densities and by their different abilities to absorb or attenuate the X-ray beam
FILM EXPOSURE:
- Traditional radiography has been performed with film-screen cassettes.
- Direct radiography (DR) is a group of technologies that can indirectly or directly convert X-ray intensity into electric signals.
- Simplistically, when the radiographic film is exposed to radiation, the silver ion in its emulsion precipitates.
- What color is silver ion?? It is BLACK.
- An area of film exposed to more radiation will be blacker than an area that receives less.
- In the chest
* Air density is low-->allow the silver ions to go through-->more ions will precipitate on film-->appear black
* Bone density is high--> block(absorb) the silver ions--> fewer ions will precipitate on film-->appear white
With an optimally exposed image, the disc spaces of the spine are barely perceptible through the heart.
- When we accidentally over-expose (over-penetrate) an image, it receives excess radiation, and all structures will appear too black.
- In contrast, when we use insufficient exposure or when a patient is obese, the image is under-exposed (under-penetrated). The image then receives too little radiation, very little silver ion is deposited, and all structures will be too white.
over-exposed = too black
under-exposed = too white
- Both over- and under-exposure limit the detection of pathology.
THE SILHOUETTE SIGN:
- The silhouette sign helps us identify and localize pathology, especially in the thorax.
- The silhouette sign is the normal interface between two structures of different densities
air in the lungs and soft tissue of the heart
- It is the loss of a normal silhouette that indicates pathology.
- Normally, when two substances of water density are in anatomic contact, they cannot be differentiated from each other, and their interfaces cannot be perceived.
- Blood within the lumen of the heart cannot be distinguished from contiguous heart muscle.
- Both look “white” on radiography.
- However, when the lung (air density) is filled in with pneumonia (water density)-->the normal air-water interface between the lung and the adjacent heart border will be obscured.
- Thus, the heart border is obliterated or “silhouetted” by the pneumonia.
HOW TO DISPLAY THE CHEST RADIOGRAPH?
- We display the PA or AP as if we are facing the patient.
- The patient’s right is on our left, and his left is on our right. The films will be labeled with side.
- We place the lateral CXR as if the patient is facing towards our left.
STANDARD RADIOGRAPHIC VIEWS:
A FUNDAMENTAL rule of radiology is to put the area of interest as close as possible to the image cassette.
1. The Postero-Anterior (PA) radiograph:
- Most CXRs are taken in a PA position
- The term PA refers to the direction of the X-ray beam, which horizontally traverses the patient from posterior to anterior.
- The patient stands in front of the x-ray film cassette with their chest against the cassette and their back to the radiographer.
- The X-ray beam passes through the patient from back to front (i.e. PA) onto the film.
- The patient’s anterior chest wall is placed next to the film cassette. (Why? Because the heart is an anterior structure, placing the heart next to the cassette reduces MAGNIFICATION and increases image SHARPNESS.)
- Heart and mediastinum are thus closest to the film
- This view is taken during deep inspiration with the patient upright and 6 feet from the X-ray tube.
- PA is preferred when possible because:
- It’s quicker
- Inspiration is improved
- Heart is less magnified
- Special considerations in PA film:
- It is important to examine all the areas where the lung borders the diaphragm, the heart, and other mediastinal structures.
- At these borders, lung-soft tissue interfaces are seen resulting in either:
- Line or stripe:
- right para tracheal stripe
- paraspinal line
- para-aortic
- azygoesophageal line or recess
- Vena azygos lobe
2. Silhouette:
- for instance the normal silhouette of the aortic knob or left ventricle
- The anterior and posterior junction lines are formed where the upper lobes join anteriorly and posteriorly. These are usually not well seen
2. The Antero-Posterior (AP) radiograph
- Substituted for the PA when the patient is unable to stand or sit.
- In this instance, the patient’s back is placed against the film cassette, and the X-ray beam traverses the patient from anterior => to posterior.
- When a patient is unable to be moved, an AP radiograph may be performed with portable equipment.
- Special considerations in AP film:
- Because of low ceilings and portable equipment, the AP is usually taken at a distance of 3 feet. Compared to the PA, this increases magnification and decreases image sharpness.
- heart and mediastinum are distant from the cassette and are therefore subject to x-ray magnification.
- As a result it is very difficult to make an accurate assessment of the cardio mediastinal contour on an AP film.
- Neonatal xrays are always AP so, remember to look for the thymus:
3. The other routine view is the Left Lateral CXR (LL CXR).
- The left side of the chest is placed against the film cassette, and the X-ray beam traverses the patient from right to left.
- We perform a LEFT LATERAL CXR rather than a right lateral because
- the heart is on the left. This places the heart as close as possible to the film cassette, thus decreasing cardiac magnification and increasing sharpness.
- The LL CXR also gives a better look at the lung (left lower lobe) behind the heart.
CXR APPROACH PATTERN:
- Identity:
- Name, date, gender, date of study.
- Errors do occur and those relating to labelling of the radiograph are the most common.
2. Technical: (C-A-F-E)
C = centralized/rotated.
- First, assess patient's position
- The spinous processes of the thoracic vertebrae should be midway between the medial ends of the clavicles.
- Length of anterior ribs showed to be equal on both sides.
- The anterior ribs should be equidistant from the lateral margins of the spine and posterior spinous processes of the vertebra.
- the lateral view, there should be a very small distance between the posterior right and left rib margins.
* Slight rotation to the left--> enlargement of the left superior mediastinum and left cardiac structures.
* Slight rotation to the right--> enlargement of the mediastinal and cardiac structures
- A lordotic image can exaggerate the size of the cardiac apex
- If the patient is rotated, it can be either to the left or to the right.
The rotation can be assessed by:
- Measuring the distance between the spinous process and the medial ends of one of the ribs
* If unequal (one side shorter than the other) --> the patient is rotated
2. Comparing the densities of the two hemi-thoraces
* The side with the darker(more black) hemithorax--> is the side to which the patient is rotated
(irrespective of whether the CXR has been taken PA or AP)
A = AP or PA view:
PA
- You can see the stomach bubble and the apices of the lungs above the clavicles.
- The mediastinum appears normal.
AP
- You cannot see the stomach bubble and the apices of the lungs above the clavicles.
- An anterior-posterior film will always be labeled as AP, so if nothing is written on the film it is safe to assume it is PA.
F = Film inspiratory/expiratory
In the Inspiratory film:
- Count: 7 complete anterior ribs or 9 posterior ribs are visible. If more than six anterior ribs imply hyper-expanded lungs.
- A good inspiratory image is one in which the anterior sixth or posterior eighth rib is visualized above the apex of the left hemidiaphragm.
- The lower third of the heart is visible above the diaphragm.
- Diaphragms: should lie at the level of the sixth ribs anteriorly. The right hemidiaphragm is usually higher than the left because the liver pushes it up.
In the expiratory film:
- less than 7 complete anterior ribs or 9 posterior ribs are visible.
- the lower third of the heart is visible below the diaphragm.
Several potentially false findings can result from assessing films with poor expansion:
- Apparent cardiomegaly
- Apparent hilar abnormalities
- Apparent mediastinal contour abnormalities
- Lung parenchyma tends to appear of increased density, i.e. ‘white lung'.
E = exposure under/over:
- On a high-quality radiograph, the vertebral bodies should just be visible through the heart.
- If the vertebral bodies are not visible, then an insufficient number of X-ray photons have passed through the patient to reach the x-ray film (underexposure)-->as a result, the film will look ‘whiter’ leading to potential ‘overcalling’ of pathology.
- Similarly, if the film appears too ‘black’, then too many photons have resulted in (overexposure) of the x-ray film-->this ‘blackness’ results in pathology being less conspicuous and may lead to ‘undercalling’.
Underexposure --> ‘whiter’ --> ‘overcalling’ of pathology
Overexposure --> ‘blacker’ --> ‘undercalling’ of pathology
3. Anatomical:
The trachea
- The trachea lies in the mediastinum and is displaced slightly to the right by the left-sided aortic arch (mainly the lower third of the trachea) but can be pathologically pushed or pulled to either side, providing indirect support for an underlying abnormality
- It divides at the carina (T4-6) into the L & R main bronchi, which continue into the lung.
The left main bronchus
- It is longer (5 cm), inferior, and more horizontal than the right.
- Divides into LUL and LLL bronchi.
The right main bronchus
- It is shorter (2.5 cm) and more superior and vertical.
- Divides into the RUL and bronchus intermedius, which then divides into RML & RLL bronchi.
- Smaller bronchi are not normally detected in the lung unless diseased.
Para-tracheal stripe
- The right wall of the trachea should be clearly seen as the so-called right para-tracheal stripe.
- The para-tracheal stripe is visible by quality of the silhouette sign: air within the tracheal lumen and adjacent right lung apex outline the soft-tissue-density tracheal wall.
- Loss or thickening of the para-tracheal stripe intimates adjacent pathology.
- Widening of the paratracheal line (> 2-3mm) may be due to
- Lymphadenopathy
- Pleural thickening
- Hemorrhage
- Fluid overload and heart failure
The aortic arch
- Is the first structure on the left, followed by the left pulmonary artery; then you can trace the pulmonary artery branches fanning out through the lung.
- Dilation of the trachea is obvious on the plain radiograph, with the tracheal shadow often being as wide as the vertebral bodies.
The hilar regions
- The hila are where the pulmonary arteries and veins and main bronchi enter the lung.
- The normal hilar shadow is 99% composed of vessels - pulmonary arteries and to a lesser extent veins
- Both hilar should be of similar density and concave.
- The vessel margins are smooth and the vessels have branches.
- The left hilum is superior to the right in 98% of normal CXRs.
- The right and left hila have the same height in 2% of normal CXRs.
- The right hilum is NEVER normally higher than the left.
- The left hilum is usually superior to the right by up to 1 cm.
- This results from the superior pulmonary vein crossing the lower lobe pulmonary artery.
- The point of intersection is known as the hilar point (HP).
- The pulmonary arteries (PA’s) parallel the bronchi. They are vertically oriented and divide into upper and lower branches.
- The right pulmonary artery (RPA) passes in front of the bronchi.
- The left pulmonary artery (LPA) arches over the L main bronchus and is more superiorly oriented than the RPA.
- The pulmonary veins are more horizontal than the PA’s and do not parallel the bronchi. They form a single trunk from each lobe of the lung and drain into the left atrium. The inferior pulmonary veins form the lowest part of the hila.
- In this illustration the lower lobe arteries are coloured blue because they contain oxygen-poor blood.
- They have a more vertical orientation, while the pulmonary veins run more horizontally towards the left atrium, which is located below the level of the main pulmonary arteries.
- Both pulmonary arteries and veins can be identified on a lateral view and should not be mistaken for lymphadenopathy.
- Sometimes the pulmonary veins can be very prominent.
- The left main pulmonary artery passes over the left main bronchus and is higher than the right pulmonary artery which passes in front of the right main bronchus.
- These images are thick slab sagittal reconstructions of a chest-ct to get a better view of the hilar structures.
- The lower lobe pulmonary arteries extend inferiorly from the hilum.
They are described as little fingers because each is the size of a little finger. - On the right side, the little finger will be visible in 94% of normal CXRs and on the left side in 62% of normals.
Normal hili are:
- Normal in position - left higher than right
- Equal density
- Normal branching vessels
Enlarged hilar area (bulky):
- Enlargement of the hili is usually due to lymphadenopathy or enlarged vessels.
- In this case there is an enlarged hilar shadow on both sides. This could be the result of enlarged vessels or enlarged lymph nodes. A very helpful finding in this case is the mass on the right of the trachea.
- This is known as the 1-2-3 sign in sarcoidosis, i.e. enlargement of left hilum, right hilum and paratracheal.
some more examples of sarcoidosis.
- Lymphadenopathy and ground glass appearance of the lungs
- Lymphadenopathy, 1-2-3 sign
- Bulky lymphadenopathy
- 1-2-3 sign
- Nodular lung pattern, no lymphadenopathy
- Hilar and paratracheal lymphadenopathy
Heart and mediastinum
Assessment of heart size
- Two thirds of the heart lies on the left side of the chest, with one third on the right
- The heart size is normally less than half the widest diameter of the thorax
- The cardiothoracic ratio should be less than 0.5. i.e. A/B < 0.
- A cardiothoracic ratio of greater than 0.5 (in a good quality film) suggests cardiomegaly.
- in neonates < 0.6
Assessment of cardiomediastinal contour
Special consedirations:
- Mediastinal lines or stripes are interfaces between the soft tissue of mediastinal structures and the lung
- Displacement of these lines is helpful in finding mediastinal pathology
1) The paraspinal line: may be displaced by a
- Paravertebral abscess
- Hemorrhage due to a fracture
- Extravertebral extension of a neoplasm
2) Para-aortic line: Displacement of the para-aortic line can be due to
- Elongation of the aorta
- Aneurysm
- Dissection
- Rupture
3) The azygoesophageal recess
- An important mediastinal-lung interface to look for is the azygoesophageal line or recess, the most important mediastinal line to look for
- The azygoesophageal recess is the region inferior to the level of the azygos vein arch in which the right lung forms an interface with the mediastinum between the heart anteriorly and vertebral column posteriorly, it is bordered on the left by the esophagus.
- Deviation of the azygoesophageal line is caused by (SBAHE):
- Subcarinal lymphadenopathy
- Bronchogenic cyst
- Atrial (Left) enlargement, mitral stenosis
- Hiatal hernia
- Esophageal disease (dilatation/achalasia, carcinoma)
- Notice the deviation of the azygoesophageal line on the PA-film
- Caused by a hiatal hernia
- A hiatal hernia is the most common cause of displacement of the azygoesophageal line
- Notice the air within the hernia on the lateral view
- Another common cause of displacement of the azygoesophageal line is subcarinal lymphadenopathy
- Notice the displacement of the upper part of the azygoesophageal line on the chest x-ray in the area below the carina
This is the result of massive lymphadenopathy in the subcarinal region - There are also nodes on the right of the trachea displacing the right paratracheal line
- There is displacement of the azygoesophageal line both superiorly an inferiorly
- There is an air-fluid level (arrow)
Combined with the above this must be a dilated esophagus with residual fluid. The final diagnosis was achalasia - The density on the left in the region of the lingula is the result from prior aspiration pneumonia
- The AP-film shows a right paratracheal mass
The azygoesophageal recess is not identified, because it is displaced and parallels the border of the right atrium
The large round density in the left lung is the result of aspiration - Notice the massive dilatation of the esophagus on the CT
4) Aortopulmonary window
- The aortopulmonary window is the interface below the aorta and above the pulmonary trunk and is concave or straight laterally.
- AP-window can be convex laterally due to a mass that fills the retrosternal space
Assessment of mediastinal compartments
It is useful to consider the contents of the mediastinum as belonging to three compartments:
- Anterior mediastinum: anterior to the pericardium and trachea
- Middle mediastinum: between the anterior and posterior mediastinum
- Posterior mediastinum: posterior to the pericardial surface
Each compartment, has it's own pathology
* The thymus shrinks with
- Sepsis
- Sickness
- Steroids
Anatomy of Lungs and pleura:
Lobar anatomy
- Three lobes in the right lung (RUL) (RML) (RLL)
- Two in the left (LUL) (LLL)
- The left lobe also contains the lingula:
- Functionally a separate ‘lobe'
- Anatomically part of the upper lobe
Pleura:
- The pleura is normally 1mm thin
- There are two layers of pleura:
- The parietal pleura lines the diaphragms, chest wall, and mediastinum
- The visceral pleura covers the lobes of the lung
- Both of these layers come together to form reflections that separate the individual lobes
- These pleural reflections are known as fissures
- The fissures are seen as a pencil thin line when tangent to the X-ray beam
- On the right there is an oblique and horizontal fissure; the right upper lobe sits above the horizontal
fissure (HF), the right lower lobe behind the oblique fissure (OF) and the middle lobe between the
two
- On the left, an oblique fissure separates the upper and lower lobes
- Lung zones:
Upper zone at the level of rib 1 +2
Middle zone at the level of rib 3+4
Lower zone at the level of rib 5+6
- It takes about 200-300 ml of fluid before it comes visible on an CXR About 5 liters of pleural fluid are present when there is total opacification of the hemithorax.
Special consideration:
- Vena azygos lobe: A common normal variant is the azygos lobe
- The azygos lobe is created when a laterally displaced azygos vein makes a deep fissure in the upper part of the lung
- On a chest film it is seen as a fine line that crosses the apex of the right lung
- Here another patient with an azygos lobe.
The azygos vein is seen as a thick structure within the azygos fissure
Diaphragms
