How to Read a Chest X Ray
Today we will be discussing the topic of chest radiographs. We will be going over why you might get this image, how to understand the different views, how things appear on the radiograph. We will go over how to read the chest x ray and chest x ray interpretation, normal x-ray findings, and then lastly, abnormal chest radiographs; including how does a chest x ray detect pneumonia. So, let’s get started!
Introduction to the Chest X-Ray
The chest radiograph is a diagnostic image that is used to see the lungs, heart, blood vessels, airways, ribs, and spine.
Why order a Chest X-Ray?
There are several reasons why a clinician might order a chest x-ray including trauma, cough, chest pain, or shortness of breath.
Chest X-Ray Views (PA and Lateral)
There are two main views that are used when a chest radiograph is completed. These are PA and lateral.
In the PA view, the patient stands with the shoulders rotated anteriorly and depressed (hands on the hips). The patient will place their chest against a plate which digitally records the image. The X-ray tube is position six feet away from the patient.
With the lateral view, the patient stands with the left side pressed again the plate.
PA vs. AP view
Other less common views might include an AP view, which can be done in a hospital with a mobile X-ray unit. The heart will be magnified with a widened mediastinum in this view. This is due to the heart being farther away from the plate and the X-ray closer to the patient.
A supine view can be selected for trauma patients, and a lateral decubitus where there is concern for pneumothorax, foreign body, or pleural effusions.
How different tissues display on X-ray (absorbing radiation)
As the radiograph beam travels through the body, it is absorbed differently by different parts of the body.
Bone will absorb radiation the most, which is why it will appear white on the image, as opposed to the lungs absorbing very little, thus why it appears darker.
Initial Approach to Reading the CXR
First and foremost, we should double check the patient’s name, date of birth, and medical record number on the image. Next we should assess if it is an AP or PA image and if the patient is standing or supine.
Then we should look to see the quality of the film, such as the positioning, rotation, penetration, and if the radiograph was taken during inspiration, and then lastly, examine the structures on the chest X-ray.
Inspiration – Anterior vs Posterior Ribs
Full inspiration will be noted when the diaphragm is located between the 5th and 6th anterior rib; or the 9th and 10th posterior rib. The ribs that are angulating downward are the anterior ribs, and the ribs that are horizontal lie are the posterior ribs.
Hyperventilation - pathology
If the diaphragm is located past the 7th anterior rib, this is considered hyperventilation and may be consistent with pathology such as COPD with air trapping. A flattened hemidiaphragm will also signify hyperinflation.
Position, Rotation, and Penetration
The position during the chest x-ray should be that with the scapula retracted laterally out of the lung views. The spinous processes of the thoracic vertebrate should be in the middle of both clavicles, signifying no rotation.
A well penetrated chest x-ray will be one where you can see the vertebrate faintly behind the heart, as well as the intersection of the left hemidiaphragm and spine being well demarcated.
Normal Chest X Ray Findings
Next we will dive into the normal chest x ray findings and normal anatomy revealed with this imaging technique.
A quick overview and way to remember the big structures revealed on the chest radiography is:
- A: Apices/airway
- B: Bones/soft tissue
- C: Cardiac
- D: Diaphragm
- E: Edges/effusion
- F: Fields (Lung Fields)
Next we will dive into more detail when scanning the anatomy revealed on the chest radiograph. We will dive into the:
- and bones
The trachea will either be centered or slight shifted to the left or right, which is normal. The trachea will branch into the right and left main bronchi.
The first thing you should do is make sure that the patient is not rotated. Again, we do this by making sure that the spinous processes of the thoracic vertebrate are in the middle between the two clavicles.
If the trachea is shifted, the next step is to figure out of it is being pushed or pulled in the respective direction. Things that may “push” the trachea include those that increase the pressure, such as a mass, pneumothorax, or effusion. Things that pull are those that decrease the volume, such as atelectasis, pulmonary fibrosis, and a past lobectomy.
The structure of the hila are the left and right main bronchi, pulmonary arteries and veins, and hilar lymph nodes. Both the left and right hilum are often the same size and density in comparison. The left and right hilum may be at the same level, but it is also normal for the left hilum to be slightly higher than the right.
Bilateral hilar adenopathy
Bilateral hilar adenopathy might be seen is disease pathologies such as sarcoidosis, lymphoma, or pulmonary arterial hypertension.
Other causes of hilar enlargement include:
- lung cancer
- metastatic disease
- viral infections
- and histoplasmosis
The lungs can be divided into four zones.
- Apical: Above the clavicles
- Upper: Below the clavicles and above the cardiac silhouette
- Middle: The level of the hilar structures
- Lower: The base
The left lung is divided into two lobes, whereas the right lung has three lobes.
Consolidation of the lungs may be seen as a white density when the alveoli are filled with pus, fluid, or cells. This can be secondary to edema, infection, or malignancy.
The borders of the lungs overlap with certain other anatomic structures on the chest radiograph, which helps to give a rough estimate as to where the pathology is occurring. These include:
- Left upper lobe: Hits the aortic notch/knob
- Lingula: Hits the left heart border
- Right upper lobe: Hits the right mediastinal border/ascending aorta
- Lower lobes: Hit the diaphragm
Silhouette Sign/Air Bronchograms
The last couple of signs to assess for when looking at the lungs are the silhouette sign and air bronchograms.
Silhouette sign is present when a mass or fluid accumulation leads to loss of the normal silhouette. Whenever there is a mass or fluid collection is present; anything it comes in contact with, will become obscured on imaging.
Lastly, air bronchograms are tubular outlines that is seen secondary to fluid filled alveoli. Air bronchograms are commonly seen in S. pneumo, pneumonia.
The pleura is not normally seen unless there is a disease process.
A pneumothorax is diagnosed when a white visceral pleura is seen separating the parietal pleura. The lateral decubitus positioning can help to see a small pneumothorax more clearly.
Most pleural gas accumulates in the non-dependent lateral location and as little as 5 mL of pleural gas may be visible. Overall, the edges of the lungs must be reviewed to ensure that the lung markings go to the end, and if they do not, then a pneumothorax should be considered.
The first question to ask yourself is if the mediastinum is widened over 8 cm? Just like with all chest radiographs, we need to make sure we have a quality image, assessing for rotation, AP vs. PA view, and inspiration. Remember if the image is an AP view that the patient might have a falsely widened mediastinum.
If we have a good film, we should assess for any vascular disease such as an aneurysm or dissection, or other abnormalities such as a mass.
When looking at the heart, we should first assess the size. A normal heart size is less than 50 percent of the thoracic diameter. Anything that is over this ratio is cardiomegaly.
Cephalization might also be seen in cardiomegaly associated with congestive heart failure. Dilation occurs when these vessels are equal to or greater in size when compares to the lower zone.
Pulmonary edema is also seen as curly B lines (thickening of the interlobular septa) and alveolar edema.
Typically, when looking at the diaphragm, the right side will be slightly higher than the left side due to the liver located inferior to the right hemidiaphragm. There might also be gas bubbles in the stomach below the left hemidiaphragm which is normal.
An elevated hemidiaphragm can be seen in:
- Phrenic nerve paralysis
- Subphrenic abscess formation
- or Hepatosplenomegaly
Free Air under the Diaphragm
However, there should not be free air located underneath the diaphragm, which can be a sign of bowel perforation.
The costophrenic angles should be well defined, and if they are not a pleural effusion should be considered. Fluid, such as a pleural effusion, is often gravity dependent, where a lateral decubitus view can be helpful.
We should assess the proximal humerus, clavicles, vertebrate, ribs, and scapula for lytic and sclerotic lesions as well as for fracture.
Lytic vs Sclerotic Lesions
Lytic lesions are present with decreased bone density whereas sclerotic bone lesions are present with increased bone density.
How Does a Chest X Ray Detect Pneumonia?
Chest radiograph is often our go to when we suspect that a patient has pneumonia. Various findings on chest x-ray can point us towards a diagnosis of pneumonia especially where there is a patient in the clinical setting with cough, fever, and leukocytosis.
On imaging lobar consolidation with air bronchograms that are patchy, bilateral, or multifocal may be present and indicative of bacterial pneumonia.
Bronchopneumonia may present on chest radiograph with reticular and or nodular opacities, pleural effusion, or cavitation which is again concerning for bacterial pneumonia.
Whereas, with viral pneumonia we might anticipate seeing focal consolidation in the peripheral, mid, and lower lung zones, or unilateral or patchy bilateral areas of consolidation
I hope this article today helped to review the clinical importance of the chest radiograph including how to evaluate a chest radiograph in your own clinical practice and what to look for as well as when something is pathologic. For more information and an in-depth review see our Medgeeks Master the Chest X-Ray Course at Medgeeks. Until next time!
- Master the Chest X-Ray Course. Accessed: January 31st, 2020.
- Pneumonia, Bacterial. Accessed: January 31, 2020.
- Statdx, Pneumonia, Viral. Accessed: January 31, 2020.
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