Tag Archives: Chest XR

Student Corner: Air Everywhere

May 19, 2015

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This time, we have an interesting CXR to examine. There are three distinct places in the image below where air is in places it shouldn’t be. Can you identify them?

sp EGD 1

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Need a refresher on how to read a CXR? This post will help you out.

Scroll down further for the answer.

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PTX, SubQ, Pneumoperitoneum post EGD

Image Key: Blue arrows–supraclavicular subcutaneous emphysema; Purple arrows–pneumothorax; Red arrows = pneumoperitoneum

Pneumothorax: air in the pleural space

On an upright CXR, a pneumothorax is one of the more easily identifiable pathologies in the thoracic cavity. The presence of air separates the parietal pleura and visceral pleura, resulting in the lung tissue being pushed towards midline. This results in the edge of the lung tissue being easily identifiable (purple arrows). The rest of the cavity is devoid of lung markings.

It is important to note that the size of a pneumothorax can vary greatly. Therefore even if the absence of lung markings isn’t as striking as it is in this picture, the edges of the thoracic cavity should always be closely examined to see if there is any evidence of air. On the other extreme is a tension pneumothorax, which is defined as an expanding pocket of air in the thoracic cavity, which causes half of the lung to completely collapse and shift the mediastinal structures in the contralateral direction.

Pneumoperitoneum: air in the abdominal cavity

The presence of air in the abdominal cavity comes from two major sources: outside the body or the GI tract. Air from outside the body enters into the abdominal cavity through either iatrogenic (surgery, peritoneal dialysis) or traumatic (penetrating wound) routes. Air from the GI tract enters if any segment of the bowel is perforated (most commonly secondary to a duodenal ulcer). On an upright CXR, as is shown above, the air rises to the level of the diaphragm and can be identified.

Even though the subdiaphragmatic air in this picture is clearly evident, CXR’s are not the gold standard diagnostic test for pneumoperitoneum. Abdominal CT scans can pick up much smaller amounts of air that may be difficult to visualize on a plain film.

Subcutaneous Emphysema: air in subcutaneous tissue planes

The image above has distinct areas of radiolucency in the supraclavicular area as a result of air tracking in the subcutaneous tissue, which is defined as subcutaneous emphysema. The area is patchy from the infiltration of air into soft tissues.

Similarly to pneumomediastinum, the air comes from either inside the body (secondary to pneumothorax, pneumomediastinum) or outside the body (penetrating trauma, chest tube insertion site). The air travels along fascial planes between the dermal and muscular layers. Another, more serious, cause is necrotizing fasciitis. In this case, however, it is likely that the air entered into the subcutaneous tissues as a result of trauma, which also resulted in a pneumothorax.

Author: Jaymin Patel

Image Contributor: Katren Tyler, M.D.

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Student Corner: How to Read a Chest X-Ray

August 25, 2014

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In these “Student Corner” pieces, we will go over certain aspects of radiology in EM that are of interest to medical students. Topics will include: common (and interesting) case presentations with accompanying imaging, schematics for how to read different types of imaging in various anatomical locations, discussions on what types of imaging to order and when in the EM setting, and others.

In this inaugural edition of the Student Corner, we’ll take a look at how to tackle reading an anterior-posterior chest x-ray.

For starters, it is important to understand that having a “gameplan” for reading any type of image is key when you first start out trying to decipher radiological images. As a reader and interpreter, you must be systematic in your thought process as you analyze the image in front of you. For chest x-rays, there is a classic schematic: ABCDE. Any medical student will tell you that this is not the only time you will see “ABC…” used as a way to quickly memorize something, but at least it’s easy to remember.

Here’s the image we are going to use and let’s start to dissect it using the mnemonic:

Note: For the purpose of keeping this a short piece, we’ll only focus on the anterior-posterior view only.

CXR UL pna Airway

A-Airway

Legend: Red Arrows–trachea; Blue Arrows–carina; Green arrows–L and R main bronchus

The upper airway, including the trachea, carina and both main bronchi, should all be visible on an AP view. Things to look for include deviation of the trachea away from the midline (there is some deviation to the patient’s right in this image, but this is due to the aortic arch, which passes to the left of the trachea as it passes posteriorly in the mediastinum), obstruction due to aspiration of a foreign object and obscuring of the upper airway due to enlarged mediastinal lymph nodes.

Let’s explore tracheal deviation a bit further. Deviation from the midline is not associated with a defect in the trachea itself, but with a force from either the R or L side of the chest cavity that is pulling or pushing the trachea to one side or the other. For example, introduction of air into one side of the chest cavity will cause that lung to collapse due to the loss of negative intrapleural pressure. The collapsed lung will shrink to the size of a ball and “push” the trachea to the opposite side. You can think of the two lungs like bungee cords that put roughly equal force on the trachea in each direction. If one of the cords snaps or is released from where it is attached to, the cord that is still intact will pull the trachea towards one side, resulting in a deviation that will show up on a CXR.

B-Bones

CXR example Bones

Legend: Numbers–ribs; Red Arrow–clavicle; Blue Arrow–medial border of scapula

A CXR offers a good view to look for rib fractures and clavicle fractures. Clavicular fractures are usually easy to spot, as they usually reveal distinct fracture lines in the middle 3rd of the clavicle. Hairline fractures are less common. Rib fractures are sometimes hard to spot, but each rib should be followed across it’s length to look for fracture lines or step-offs (disruptions in the normal curve of the rib) that could indicate a fracture.

The number of ribs is also important to assess because it is an indirect measurement of the volume of the chest cavity. Hyperinflated lungs are usually the result of obstructive disease where the patient is unable to fully expel the air that is inhaled with every breath they take–this increase in residual volume will build up over time and overinflate the chest cavity. This overinflation will result in a greater-than-normal number of ribs being visible on an AP view. Normally, you should expect to see 8-10 ribs on an upright chest X-ray, depending on whether the patient was instructed to exhale or inhale before the picture was taken.

C-Cardiac

CXR Cardiac

 

Legend: Red Dashed Lines–heart borders

This part of the mnemonic involves the heart and surrounding structures. The silhouette of the heart should be identified and the heart borders should be clear. A general rule of thumb is that the heart base should not be wider than 1/2 the total width of the diaphragm. As with a lot of “general rule of thumb”s in medicine, it’s not quite clear whether this has any diagnostic value–for example, if the heart base is indeed 1/2 the width of the diaphragm on CXR, is that really sensitive for cardiomegaly? In any case, it’s something to keep in mind.

The aortic arch and the L pulmonary artery should be visible as two semi-circles above the left atrium. There is a space called the “AP Window” that has the following borders: ascending aortic arch (anterior), descending aortic arch (posterior), L pulmonary artery (inferior), inferior border of aortic arch (superior). The window should be “concave” in the sense that the lateral border should be caved in medially. If it is not, things like mediastinal lymphadenopathy and aorta/pulmonary artery aneurysms are possible.

D-Diaphragm

CXR Diaphragm

 

Legend: Blue Arrow–gastric air bubble; Red Arrow–costophrenic angle

The diaphragm has 3 major characteristics which you look for on CXR. One is the gastric air bubble, which allows you to identify that the stomach is on the left (as opposed to the right, as in situs inversus). Another is the contour of the diaphragm, which should be a “dome” shape. The right side should be a little higher than the left, thanks to the liver. The third is perhaps the most important: the costophrenic angle. It is the lateral point of attachment for the diaphragm and it should be a sharp, triangle-shaped region at either end. The angle should be acute. If the angle is closer to 90 degrees, then one possible explanation is that the lungs are hyperexpanded (perhaps because of COPD) and pushing the diaphragm down into the abdomen. “blunting” of the angle refers to a radio-opaque marking of the angle that usually is indicative of pleural effusion.

E-Everything Else

Everything else is…everything else. Mostly this means the lung parenchyma itself. For this, asymmetry is key. Compare left and right and see whether there is a difference. More on this particular section of the read later.

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Now you should try to read the above x-ray for yourself and type your own version of the read in the comments if you’d like. If not the entire read, then try to identify the pathology in the x-ray and post your answer in the comments. Any questions/comments would also be appreciated.

I’ll post the answer with the “correct” read a bit later on the site.

Author: Jaymin Patel

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