The CT scan

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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.

CT scanning

A CT scanner takes multiple cross-sectional images of the body. A completed CT scan of the chest usually consists of two complete sets of images, one showing mediastinal structures (in which the lung fields look relatively black) and one showing the lung fields in which the mediastinum looks relatively white. These are called the mediastinal and lung windows. These two sets are necessary because there are not enough shades of grey to display adequately both the high-density mediastinal structures and the low-density lungs using one linear scale. In the lung windows the grey scale is expanded across the range of lower density structures, whereas in the mediastinal windows the scale is expanded over the higher-density structures.

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.

Interpreting the images

Interpretation of the CT scan requires significant expertise and should only be done by a radiologist. As well as having the experience to interpret the image slices, radiologists have access to software that will allow them to display the images in other forms, for example in the sagittal and coronal planes as well as cross-sectional and three-dimensional images. The CT scans we have included in this book have been chosen because they illustrate situations in which, working on the wards, emergency department or outpatients, you are likely to order scans and a basic knowledge of how they are interpreted will be of use. The next section will give you a basic knowledge of the anatomy of the scan that should help you visualize abnormalities reported on by the radiologist.

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!

1. Start with the mediastinal windows. Identify the image that shows the arch of the aorta (1) and on this slice:

a. Look for the trachea, the black circle medial to the arch (2).

b. Look for the superior vena cava (SVC) (3) which lies to the right of the aorta, and tends to be oval rather than round because the blood is under low pressure.

2. You now need to look at the images above (or cranial) to the one you have just looked at. Make your way up the chest looking at each slice. The lungs will get smaller as you move upwards and the aorta will disappear.

a. The vessels now visible are the vessels that have come off from the top of the aortic arch. These are the brachio-cephalic artery (4) (this will soon split to form the right subclavian and carotid artery), the left subclavian (5), and the left carotid (6).

b. There are two further vessels that can be seen, both slightly more anterior than the arteries. These are the left (7) and right (8) brachio-cephalic veins, which will join together to form the SVC that you could see on the first slice you looked at.

3. Now look even higher. You cannot see any lungs because you are now looking above the level of the lung apices. The black hole of the trachea can be seen. Either side of this is a high density (white) structure (9), this is not a blood vessel, but is the thyroid. This is white because it contains iodine, as well as being a very vascular structure. It will appear white even on an unenhanced scan.

4. Now go back to the original slice, and follow the scans downwards. The lungs will get bigger, and the heart will gradually appear.

a. Now that you are below the level of the arch, the aorta appears twice – the ascending aorta (10), which appears in the middle, and the descending aorta (11), which lies posteriorly.

b. The other white structure is shaped like an inverted Y. This is the pulmonary trunk (12), splitting into the left (13) and right (14) pulmonary arteries. There is no longer a black circle for the trachea, as this has split into the main bronchi and will be better seen on the lung windows later.

c. There is a very small black circle in front of the descending aorta. This is the oesophagus (15), which is sometimes collapsed, or may contain air, as in this case. It is often difficult to see.

5. Move further down. Now you can see the heart.

a. Embryologically the heart was a midline structure that then rotated to the left; this means the left chambers will lie behind the right chambers. The contrast used to enhance the scan lies mainly in the blood pool, so this helps to define the chambers versus the walls of the heart. The thick-walled left ventricle can be seen and is oval in shape (16). The right heart (17) has a much thinner wall and fits around the left ventricle, so it has a more complex outline. Remember to look at the cardiothoracic ratio, since, as with the plain chest X-ray, this should be less than 50% of the bony diameter of the thorax.

b. A few smaller bright areas are seen in the lungs. These are the smaller pulmonary arteries and veins.

6. The lowest image we have given you shows the diaphragm and liver. It is an area that causes a lot of confusion, as the top of the diaphragm can easily be mistaken for a mass.

a. The rounded descending aorta is seen posteriorly (18).

b. There is an area of poorly defined whiteness seen adjacent to the liver/diaphragm. This is the inferior vena cava coming from below to drain blood upwards into the right side of the heart (19).

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.

a. On the right, the right main bronchus splits to give off an oval black tube, the upper lobe bronchus (24). The remaining airway is now called the bronchus intermedius (25). This gives off an anterior branch which is the middle lobe bronchus (26), and the rest continues downwards and backwards as the lower lobe bronchus (27).

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.

3. You can follow the bronchi to identify the main lobes of the lung. The lobes of the lung are surrounded by fissures and you should try to identify these in order to work out the borders of each lobe. The appearance of a fissure will differ depending on the slice thickness. On an HRCT the fissure will appear as a thin white line. On a spiral CT with thicker slices the fine white line is often lost. The blood supply to each lung comes from the hilum, and the vessel branches get smaller as you go to the periphery of the lung, therefore on a thicker slice CT instead of a white line, you see a darker linear strip. No blood vessels are seen to cross the fissure.

See images on p. 35–38. Identify the major fissures on the left (30) and right (31) and the minor fissure (32) on the right. On the left the major fissure separates the upper and lower lobes. On the right the major fissure separates the lower lobe from the rest of the lung and the minor fissure separates the upper and middle lobes. The minor fissure can be difficult to spot.

4. Now look at the edge of the lungs. You should not be able to see normal pleura, as it is so thin; if there is density here, it must be abnormal.

5. Being able to identify the bronchi is important as it confirms which lobe any pathology lies within, but also remember to look inside the bronchi themselves, as there may be a tumour or an aspirated foreign body.