• The heart is located anteriorly in the chest and it is bordered by the lingula of the left lung. The difference in density between the heart and the air in the lung enables us to see the silhouette of the left ventricle. When there is something in the lingula with the same 'water density' as the heart, the normal silhouette will be lost (blue arrow).
• When there is a pneumonia in the left lower lobe, which is located more posteriorly in the chest, the left ventricle will still be bordered by air in the lingula, and we will still see the silhouette of the heart (red arrow).

• The PA-film shows a silhouette sign of the left heart border.
  Even without looking at the lateral film, we know, that the pathology must be located anteriorly in the left lung.
  This was a consolidation due to a pneumonia.

• Here we see a consolidation which is located in the left lower lobe.
  There is a normal silhouette of the left heart border.

• On this lateral film there is too much density over the lower part of the spine.
• By only looking at the interfaces of the left and right diaphragm on the lateral film, it is possible to tell on which side the pathology is located.
• First study the lateral film.

• On a normal lateral chest film the silhouette of the left diaphragm 2- can be seen from posterior up to where it is bordered by the heart, which has the same density (blue arrow).
• One should be able to follow the contour of the right diaphragm -1- from posterior all the way to anterior, because it is only bordered by the lung.
• Here we cannot follow the contour of the right diaphragm all the way to posterior, which indicates that there is something of water-density in the right lower lobe (red arrow).

• On the PA-film there is a normal silhouette of the heart border, so the pathology is not in the anterior part of the chest, which we already suspected by studying the lateral view.
• Why do we still see the silhouette of the right diaphragm on the PA-film?
• What we see is actually the highest point of the right diaphragm, which is anterior to the pneumonia in the right lower lobe.
  The pneumonia does not border the highest point of the diaphragm.

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.

    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 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.

Assessment of cardiomediastinal contour

• Deviation of the azygoesophageal line is caused by (SBAHE):  

1. Subcarinal lymphadenopathy
2. Bronchogenic cyst
3. Atrial (Left) enlargement, mitral stenosis
4. Hiatal hernia
5. 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:

1. Anterior mediastinum: anterior to the pericardium and trachea
2. Middle mediastinum: between the anterior and posterior mediastinum
3. Posterior mediastinum: posterior to the pericardial surface

Special consideration: 

1. 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

• The highest point of the right diaphragm is usually 1–1.5 cm higher than that of the left.
• In 3% of cases, the left is higher but only by 1 cm. A scalloped contour is common.
• Each costophrenic angle should be sharply outlined.
• The outlines of both hemidiaphragms should be sharp and clearly visible along their entire length (except the medial most aspect of the left hemidiaphragm).
• The ‘curvature' of both hemidiaphragms should be assessed to identify diaphragmatic flattening.
• The highest point of a hemidiaphragm should be at least 1.5 cm above a line drawn from the cardiophrenic to the costophrenic angle.

• Remember to follow every rib, (from the anterior to posterior), the clavicles vetebrae and the shoulders.
• Special consideration:

1. In some patients, an extra joint is seen in the anterior part of the first rib at the point where the bone meets the calcified cartilaginous part (arrow).
2. This may simulate a lung mass.

look for pathology in the ‘hidden areas:

• Some areas need special attention, because pathology in these areas can easily be overlooked:
• apical zones
• hilar zones
• retrocardial zone
• zone below the dome of the diaphragm (free air, dilated bowel, abnormal calcification, and masses)

These areas are also known as the hidden areas.

• Notice that there is quite some lung volume below the dome of the diaphragm, which will need your attention (arrow).

• Here an example of a large lesion in the right lower lobe, which is difficult to detect on the PA-film, unless when you give special attention to the hidden areas.
• Click on the image for an enlarged view.

• Here a pneumonia which was hidden in the right lower lobe mainly below the level of the dome of the diaphragm (red arrow).
• Notice the increase in density on the lateral film in the lower vertebral region.
• You may have to enlarge the image to get a better view.

• Notice the subtle increased density in the area behind the heart that needs special attention (blue arrow).
  This was a lower lobe pneumonia.
• There is a subtle consolidation in the left lower lobe in the hidden area behind the heart.
  Again there is increased density over the lower vertrebral region.

• There should be a decrease in density from superior to inferior in the posterior mediastinum as the airspaces are more in lower lobes so they should be more blackish.
• The retrosternal airspace should be of the same density as the retrocardiac airspace.

Diaphragm

• The right hemidiaphragm is usually ‘higher’ than the left.
• The outline of the right can be seen extending from the posterior to anterior chest wall.
• The outline of the left hemidiaphragm stops at the posterior heart border. Air in the gastric fundus is seen below the left hemidiaphragm

• There are only two spaces to look at on the lateral film.
• The heart lies antero-inferiorly. Look at the area anterior and superior to the heart. This should be black, because it contains aerated lung.
• Similarly the area posterior to the heart should be black right down to the hemidiaphragms.
• The blackness in these two areas should be equivalent; therefore you can compare one with the other. If the area anterior and superior to the heart is opacified, suspect disease in the anterior mediastinum or upper lobes.
• If the area posterior to the heart is opacified suspect collapse or consolidation in the lower lobes

• The left main pulmonary artery (in purple) passes over the left main bronchus and is higher than the right pulmonary artery (in blue) which passes in front of the right main bronchus.

• Once you know how the normal hilar structures look like on a lateral view, it is easier to detect abnormalities.
• In this case on the PA-view there is hilar enlargement.
  On the PA-view it is not clear whether this is due to dilated vessels or enlarged lymph nodes.
  On the lateral view there are round structures in areas where you don't expect any vessels. So we can conclude that we are dealing with enlarged lymph nodes.
• This patient has sarcoidosis.
  Notice also the widening of the paratracheal line (or stripe) as a result of enlarged lymph nodes.

• On the lateral view spondylosis may mimick a lung mass.
• Any density in the area of the vertebral bodies should lead you to the PA-film to look for spondylosis, which is usually located on the right side (arrows).
  On the left side the formation of osteophytes is hampered by the pulsations of the aorta.

• On the PA-view the superior mediastinum is widened.
  The lateral view is helpful in this case because it demonstrates a density in the retrosternal space.
  Now the differential diagnosis is limited to a mass in the anterior mediastinum (4 T's).
• This was a Hodgkin's lymphoma

• A common incidental finding in adults is a Bochdalek hernia, which is due to a congenital defect in the posterior diaphragm (arrows).
  In most cases, it only contains retroperitoneal fat and is asymptomatic, but occasionally it may contain abdominal organs.
• Large hernias are sometimes seen in neonates and can be complicated by pulmonary hypoplasia.
• A hernia of Morgagni is also a congenital diaphragmatic hernia, but is less common.
  It is located anteriorly.

• Old films: It is extremely important to always compare with old films, as we will demonstrate in this case.
  Actually, someone said that the most important radiograph is the old film, since it gives you so much information.
  For instance a lung mass, which hasn't changed in many years, is not a lung cancer.

• Congestive heart failure. compared to the old film, things become more obvious and you will be much more confident in your diagnosis:

1. The size of the heart is slightly increased compared to the old film.
2. The pulmonary vessels are slightly increased in diameter indicating increased pulmonary pressure.
3. There are subtle interstitial markings as a result of interstitial edema.
4. There is pleural fluid bilaterally. Notice that the inferior border of the lower lobes has changed in position.

• All these findings indicate the presence of heart failure.