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.

High-resolution CT scanning (HRCT)

This is a sampling test, which means that not all the lung is examined. With this technique 1–2 mm slices are taken about 10 mm apart. This technique is very good for detecting interstitial lung disease since the thin slices allow for a very detailed assessment of lung architecture (rather like a thin tissue slice enables a pathologist to see more detail). It is not a good technique for finding mass lesions (for example, lung tumours) because the lesion could occur in one of the spaces between the slices and so be missed. The HRCT scan is usually acquired over several breath holds.

Intravenous contrast would not be used with HRCT, since contrast would need to be given before each individual image was taken – something that is clearly impossible.

Spiral CT

This is also called helical CT or volumetric CT. In this technique the anode of the scanner rotates continuously while the table on which the patient lies is moved at a predetermined speed. This technique has the advantage that it is extremely quick – 7 to 10 seconds. The whole chest is scanned, so that small lesions are not likely to be missed, unlike in high-resolution CT where there is a gap between the slices. Sometimes patients are so breathless that they breathe during the scan, causing blurring of the images and making interpretation more difficult.

Spiral CTs are used when intravenous contrast needs to be administered, for example when staging a lung cancer or detecting a pulmonary embolus. The exact timing of the administration of contrast depends on the abnormality being sought. For example, contrast will be injected at a different time for the detection of a pulmonary embolus than it would be if it was being used to enhance the structures of the mediastinum. This is another reason why it is important to be specific about the clinical question being asked when requesting a scan. In addition, this needs to be noted when interpreting scans. For example, a scan performed to stage a lung cancer cannot be used to exclude a pulmonary embolus.

Combined imaging

Many CT scanners are able to acquire spiral images, and with the same data set can process the image to produce a high-resolution study. If a patient only needs a high-resolution CT, this should be requested, as combined imaging exposes the patient to a higher radiation dose.

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

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