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The advent of computed tomography (CT) has revolutionized diagnostic radiology. Since the inception of CT in the 1970s, its use has increased rapidly.
It is estimated that more than 62 million CT scans per year are currently obtained in the United States, including at least 4 million for children. By its nature, CT involves larger radiation doses than the more common, conventional x-ray imaging procedures ( Table 1 Typical Organ Radiation Doses from Various Radiologic Studies. We briefly review the nature of CT scanning and its main clinical applications, both in symptomatic patients and, in a more recent development, in the screening of asymptomatic patients. We focus on the increasing number of CT scans being obtained, the associated radiation doses, and the consequent cancer risks in adults and particularly in children.
Although the risks for any one person are not large, the increasing exposure to radiation in the population may be a public health issue in the future. CT and Its Use The basic principles of axial and helical (also known as spiral) CT scanning are illustrated in Figure 1 The Basics of CT. A motorized table moves the patient through the CT imaging system. At the same time, a source of x-rays rotates within the circular opening, and a set of x-ray detectors rotates in synchrony on the far side of the patient. The x-ray source produces a narrow, fan-shaped beam, with widths ranging from 1 to 20 mm.
In axial CT, which is commonly used for head scans, the table is stationary during a rotation, after which it is moved along for the next slice. In helical CT, which is commonly used for body scans, the table moves continuously as the x-ray source and detectors rotate, producing a spiral or helical scan. The illustration shows a single row of detectors, but current machines typically have multiple rows of detectors operating side by side, so that many slices (currently up to 64) can be imaged simultaneously, reducing the overall scanning time. All the data are processed by computer to produce a series of image slices representing a three-dimensional view of the target organ or body region.. CT has transformed much of medical imaging by providing three-dimensional views of the organ or body region of interest. The use of CT has increased rapidly, both in the United States and elsewhere, notably in Japan; according to a survey conducted in 1996, the number of CT scanners per 1 million population was 26 in the United States and 64 in Japan. It is estimated that more than 62 million CT scans are currently obtained each year in the United States, as compared with about 3 million in 1980 ( Figure 2 Estimated Number of CT Scans Performed Annually in the United States.
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