Computed Tomography


Computed tomography (CT) comprises an X‐ray tube and a bank of detectors that rotate around a patient, taking multiple exposures from different angles. The data from the detectors, essentially a density map of the body, is reconstructed to form an image which may be manipulated to any plane. The development of CT during the 1970s heralded the start of sophisticated cross‐sectional imaging. Such cross‐sectional images have, in turn, revolutionized the diagnostic investigation for many clinical conditions and demand for imaging grows inexorably; in 2006, an estimated 62 million CT studies were performed in the United States alone, almost five times the number in 1990. Increasingly, the radiation dose involved in CT has become a major issue and, alongside high‐speed imaging, is the focus of current developments.

Key Concepts:

  • The first CT machine was built in the early 1970s by Sir Godfrey Hounsfield.

  • CT comprises an X‐ray tube and a bank of detectors rotating around a patient to produce a dataset from which an image can be reconstructed.

  • Current technology enables rapid acquisition (<10 s) of submillimetre resolution images.

  • The contrast adjacent structures may be enhanced by the administration of oral, intravascular or rectal contrast medium.

  • CT images may be displayed in any plane and manipulated in a number of ways depending on the clinical indication.

  • There are innumerable indications for CT across many clinical specialties, with many other investigations becoming obsolete.

  • CT exposes patients to a relatively high dose of radiation; efforts should be made to ensure that the investigation is necessary and that the technique employed uses the lowest dose possible while answering the clinical problem.

  • Dual‐energy CT enables one to determine the chemical composition of structures in addition to their morphology.

  • There is currently extensive research in the use of CT perfusion imaging, adding functional information to the anatomical detail.

  • CT machines are often combined with nuclear medicine machines to give hybrid functional and anatomical information.

Keywords: computed tomography; CT; clinical applications; development; radiation; X‐ray

Figure 1.

A modern spiral computed tomography system. Note the aperture within the gantry, through which the patient moves as the examination progresses.

Figure 2.

Schematic diagram of spiral technology. The tube rotates continuously around the patient. The patient moves through the gantry. In this way a helical volume of data is obtained which can then be analysed in many different ways. After Kalendar et al..

Figure 3.

Schematic diagram showing the tube travelling around the patient. The X‐ray photons from the anode pass to the detectors, which convert the incident photons to an electrical signal.

Figure 4.

Diagram showing the Hounsfield scale. The approximate values of some of the common tissues are indicated.

Figure 5.

Images illustrating several displays of the same electronic data obtained through a normal thorax (1‐mm thick slice). (a) Viewed at lung windows: window width (WW) 1200 HU and window level (WL) −600 HU. (b) Viewed at soft tissue windows: WW 400 HU and WL 20 HU. (c) Viewed at bone windows: WW 3000 HU and WL 570 HU.

Figure 6.

(a) Unenhanced CT of the head in a patient with right sided weakness shows a small area of low attenuation on the left. Perfusion imaging (b) shows increased time to peak enhancement on the left, indicating a large area of infarction. Images courtesy of Dr J Cross.

Figure 7.

Patient with a pulmonary embolus. Thrombus in the pulmonary arteries is shown as a negative filling defect (arrow) surrounded by contrast medium.

Figure 8.

Volume rendered image of the heart depicting the coronary vessels. Image courtesy of Dr D Gopalan.

Figure 9.

(a) Coronal image from a CT colonography study demonstrating a 7 mm polyp. The virtual colonoscopy view (b) shows the same lesion.

Figure 10.

Volume rendered image of the heart, aorta and arterial supply to the legs.



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Further Reading

Prokop M, Galanski M, Schaefer‐Prokop C and van der Molen A (2002) Spiral and multislice computed tomography of the body. Thieme.

Sahani DV and Yaghmai V (2009) Advances in MDCT. Radiologic Clinics of North America 47(1) .

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How to Cite close
Shaw, Ashley S, and Dixon, Adrian K(Feb 2010) Computed Tomography. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0002338.pub2]