Getting into the chest x-ray (part 2)—the importance of overlap

If you want to become really good at reading chest x-rays, you need to know how borders are formed. Borders appear on the image when structures of different density (e.g., soft tissue and air) touch one another. In this video from our Chest X-ray Essentials course, you will learn how separate anatomical structures in the thorax interact to create an image.

This video is part 2 in a 4-part series on chest x-ray problems. Watch the other videos here:

• Watch video #1: The importance of grayscale
• Watch video #3: The importance of shapes and key landmarks
• Watch video #4: How pathology changes the grayscale

If you want to take your chest x-ray skills to the next level, then check out our Chest X-Ray Essentials course, which was specifically designed for non-radiologists who want to use the chest x-ray in order to make better clinical decisions.

[00:00:00] What will I gain from this video? After watching this video, you will understand how separate anatomical structures in the thorax interact to create an image. When the x-ray beam hits a tissue, we can call this a radiology absorption event, a RAE. The result in grayscale on an x-ray will depend on the density of the tissue. The greyscale concept would be simple if the human thorax was flat

[00:00:30] and one dimensional. Each anatomical structures would be assigned a single level of gray because the x-ray beam would only hit one structure. Unfortunately, the anatomy of the thorax is three dimensional and complex. So, the x-ray beam, when if courses through the body, will hit a number of different tissues of varying densities. The resulting greyscale on the x-ray will depend on the sum of all the tissue densities the x-ray beam encounters or

[00:01:00] will depend on the sum of the RAEs. The reason we can see separate structures on a chest x-ray examination is because of the density difference between them. Here, we have a rectangle, we see the rectangle because the edges of the rectangle are surrounded by white. The greyscale of the rectangle is the result of one RAE. If we have two similar rectangles, we see them as separate because of the white between them. If we overlap the two rectangles,

[00:01:30] the resulting image becomes more complex. In the real world, if we overlap two transparent structures, the overlap area is usually darker. However, in the radiological world, if we overlap the two structures, we see the overlap area as being wider. The reason the overlap area is whiter is because the x-ray beam now has to go through two structures and because of the summation of the RAEs, this area

[00:02:00] looks whiter. When we separate the two rectangles again, we can again see them as separate structures because of the whiteness between them. Each rectangle has four edges for a total of eight. If the two rectangles are positioned so that the medial edges touch on x-ray, we can no longer see the two rectangles separately but see one larger rectangle and we only see four edges. This is called the silhouette sign.

[00:02:30] Let us define the silhouette sign. The silhouette sign refers to the absence of a depiction of an anatomical soft tissue border. The silhouette sign results from the juxtaposition of structures of similar radiological density. Here, we have an example of the silhouette sign. On the right, we can identify the heart centrally. We can see the right heart border and we can see the left heart border clearly. On the left-hand

[00:03:00] image, we can see the right heart border. But we cannot identify the left heart border and this is because this patient has a pneumonia. The pneumonia is soft tissue density and lies adjacent to the heart. Because of the silhouette sign, we only see one structure in this region and in this white area, which is the sum of the pneumonia and the heart border. If one rectangle is greyer than the other

[00:03:30] and the two rectangles touch, we can still see them separately. They form an interface. Interfaces occur only if there is a significant density difference between the two adjacent tissues. Interfaces will occur when air touches a soft tissue density or when air touches bone or when soft tissue touches bone. In a non x-ray world, when two objects overlap, we can tell which one lies in front of the other. This is called interposition.

[00:04:00] In the x-ray world, this is more difficult. Which one of the two rectangles is in front? We cannot tell. On this slide, we have two images of the rectangle seen from the top. The two images are identical. However, if we look from the side, we see that on the top rectangle two lies in front of rectangle one. On the bottom, we see the rectangle one lies in front of

[00:04:30] rectangle two, even though the two images form similar rectangles, when looked at from above. This is a CT examination of the thorax. The reason I'm showing the CT examination is to outline that we can predict where interfaces or borders will be formed, on a chest x-ray, by knowledge of the anatomical structures. If we're taking a chest x-ray examination and the x-ray beam courses

[00:05:00] through the body from posterior to anterior, we can predict that there will be an interface formed by the lung and the chest wall. We can predict that there will be a border formed by the lung and the mediastinal structures, here, in this area and here. And we can also predict that there will be a interface or border created by the lung and the chest wall, here.

[00:05:30] So, let's now practice identifying the interfaces created between the lung and the other anatomical structures. Along this line, we can identify various interfaces. Can you identify what is causing this border? The answer is the superior vena cava. We can see another interface at this level. Can you identify what that is caused by?

[00:06:00] That is caused by the tissues in the right paratracheal region. We can identify this rectangular structure, in this region, which we've already learned is caused by the trachea. On this side, we can identify a clear border. And do you know what that is caused by? That is the interface between the lung and the subclavian artery. Now, if we moved our line more inferiorly, we can also identify various interfaces. Can you identify what would cause

[00:06:30] the interface between the lung and here? That is the right atrium. We can see another interface in this region and that is caused by the paraspinal tissues. There is another interface at this level. Do you know what that is caused by? That is caused by the interface between the lung and the esophagus and the azygos vein. Can you identify what is causing this interface?

[00:07:00] That is caused by the descending aorta. And at this level, that is caused by the left ventricle. So, to recap, the x-ray image will depend on the sum of the various densities, encountered by the x-ray beam, as it courses through the body. The edge of an object on a chest x-ray will be visible only if it borders a structure of significantly different radiodensity, for example, soft tissue and air.