Hey everyone, have you ever tried reading an x-ray? Most of you might have. But did you understand it? when you look at it for the very first time, it looks like a puzzle. You need to find the right pieces to understand it clearly. Well, in this article we will be discussing the basics of reading a chest x-ray. But before that, we must have a idea of how x-ray images are produced.
Image: Normal Chest X-ray (Credits, Wikimedia, Reuse and modification license)
Well, to read an x-ray, we must be aware of how images are produced, and why images are black and white. Xrays are produced by the source of x-ray, in this case, the source can be an x-ray machine in a lab. These x-rays are a form of electromagnetic radiation and have a higher frequency and higher energy. So, they can penetrate tissues in the body. but unfortunately, this can also lead to DNA damage and cancer. Therefore x-ray radiation must be limited. Now to capture the image, a detector is used on the other side. For a very long time, a photographic film was used as a detector, but now mostly digital detectors are used, which helps us to see the x-ray in real-time.
When a patient stands in between a source of x-ray and a detector, the high energy radiation is sent. When the radiation is passed through the patient, some of it will completely pass through, some of it will be absorbed by the tissues, and some will scatter. We are mainly interested in the radiation that is absorbed by the tissues because that is what creates the shadows of the organs. Therefore, an x-ray image tells us about these three factors.
The density of the Tissue: As we know, the air has a very low density. Xray passes through it easily and produces a black background. Other tissues have a higher density, so they produce grey to white background according to their density. Bones have very high density, Xrays cant pass through it, making a white shadow on the image. Those structures, which let the x-ray pass through are called radiolucent and those which block x-rays are called radio-opaque. We have air which is the most radiolucent and bone is highly radioopaque.
The thickness of the tissue: Same as density, less the thickness of the tissue, more Xray will pass, making it black and more the thickness, less Xray will pass, making it white.
Duration of Exposure: If there is short exposure of x-ray, less Xray hits the detector, so the image is very bright. when the exposure is too long, more Xray hits the detector making it too dark. [NOTE: LESS Xray makes it bright and MORE Xray makes it dark.]. This is what we mean by underexposed or overexposed Xray.
Now, we have a basic idea of how x-ray images are produced and what are the factors that affect the images, we can now move a step ahead to interpret a chest Xray. When we look at a chest Xray, there is always a systematic approach. It makes things easier. The system which is mostly followed is the "ABCDE" system. Here's what it means:
A- Airways
B- Bones
C- Cardiac silhouette(Mediastenum)
D- Diaphragm
E- Effusions
Now, We must know where to look for these in an Xray.
Mainly, Three airways structures are seen in an Xray. Trachea, Right bronchus, and left bronchus. Always note that the patient's right side will be on the left side of the screen. The left bronchus is slightly more angled than the right. The right bronchus is more vertical. So, any foreign body that goes into the airways, is more likely to enter the right lung. The trachea is always centrally located in a healthy individual. If the trachea has deviated, then we must look for the cause.
Mostly, There are 4 sets of bones, that are easily interpreted in a chest X-ray. The ribs can be easily seen with two parts, posterior and anterior. The posterior part is more horizontal and easily visualized but anterior is more vertical and hard to visualize. Normally, 8-10 ribs are expected to be visualized. Next Bone is a clavicle, as you can see, it's pretty easy to visualize. of course on both sides. it must be assessed for any kind of fracture. Another one is vertebra and finally, sternum, which is hard to see from this view but can be clearly seen in a lateral view.
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There are a lot of structures present here and defines the normal anatomy of the heart. The heart is outlined in the picture and labeled. Now, keep in mind that this is a normal chest X-ray. The heart must be accessed for any type of enlargement. Normal cardiac size is 50% of the thoracic width. If the size is more than 50% then it suggests there is cardiac enlargement. Cardiac enlargement can occur for various reasons like pulmonary hypertension, cardiomyopathy, etc. The heart border as shown in the image must be clearly visible in a healthy individual. If it's not visible, then there must be some pathology that must be studied.
As labeled you can clearly see the right and left diaphragms. Right is slightly above the left because of the liver being directly under it. You must keep in mind that they are not flat structures, they look flat on this view, but truly they are 3-d curved structure. Also, Pleura is outlined with blue color, but normally its invisible in a normal X-ray. Also, a pocket of air can be seen right beneath the left diaphragm, which usually represents air bubbles in the stomach also known as the gastric bubbles. The diaphragm must be indistinguishable from the liver beneath it. However, if there is air present under the diaphragm, then it becomes separate from the liver.
Pulmonary effusion basically means the collection of fluid in the pleural cavity surrounding the outline of the lungs. Obviously, this is not seen in a normal X-ray. but, it's a very common pathology. Therefore it must be looked for and excluded.
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You can see, the pleural effusion in the left lower side of the lung. As explained above, fluid has a high density than air, therefore it renders a white shadow. Also, notice how the trachea a pushed towards the right side due to pressure from the left lung.
These were the basics of reading a normal chest X-ray. In Part-II, we will be discussing the abnormal findings in a chest X-ray and how to understand them.
I hope you enjoyed this one.
*All images used are copyright free and provided with appropriate credits*
*The modified image is under reuse and modification license*
References:
[1]https://geekymedics.com/chest-x-ray-interpretation-a-methodical-approach/
[2] https://radiologyassistant.nl/chest/chest-x-ray-basic-interpretation
