The CT scan

Published on 23/05/2015 by admin

Filed under Internal Medicine

Last modified 22/04/2025

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CHAPTER 3 The CT scan

Although the plain chest X-ray is one of the most useful imaging techniques, it is limited by the fact that it is a two-dimensional image and small or subtle abnormalities can be overlooked. In other circumstances the chest X-ray will identify an abnormality but will give limited information as to its extent or detailed appearance. Remember also that, although particularly useful for detecting lung abnormalities, the chest X-ray is a very poor way of imaging the mediastinum.

Types of CT scan

Two main types of CT scans are performed for the chest: contiguous (spiral) and high resolution. You will need to understand the difference between these in order to know what to request and also to appreciate their limitations. If you include all the relevant history on the scan request, and clearly state the question that needs answering, the radiologist is much more likely to undertake the most appropriate scan.

Finding your way around the CT scan

It is complicated, but if you learn a few basic areas of anatomy, the rest can be built on later. As with a plain chest X-ray it is important that you have a scheme to work with. Remember, you never look down on a patient, so all CT images are taken as though you are looking up the body from the feet, and the left structures are on the right!

Now look at the lung windows. Start from the top image. You will need to know how to identify the lobes of the lung, so that you can localize any pathology.

1. See images on p. 35 and 36. You will be able to identify the trachea (20). Follow this down until it splits into the left (21) and right (22) main bronchi. Where they split is known as the carina (23).
2. See images on p. 36–38. Identify the main bronchi.

b. See images on p. 36–38. On the left, the upper lobe bronchus gives rise to the airway going to the lingula (28), and the lower lobe bronchus is left by itself (29). It may help to remember that the left lower lobe is smaller than the right because of the space taken by the heart.

Now that you have identified the normal structures, look for enlarged lymph nodes. You need to distinguish lymph nodes from blood vessels.