Computed tomography (CT)

the general process of creating cross-sectional or tomographic images from projections (line integrals) of the object at multiple angles and using a computer for image reconstruction. If unmodified, the term CT generally implies images made from projections measured by transmission of X-rays (X-ray computed tomography). However, the term could also include SPECT imaging and PET imaging and even some ultrasound techniques based on projection measurements.

In all forms of CT, the input projection data are line integrals or ray sums of some property, and the tomographic image is a representation of that property. In X-ray CT, the function imaged is the distribution of linear attenuation coefficient since, for monochromatic X-rays, the logarithm of the transmitted intensity is proportional to the integral of the attenuation coefficient along the path. In emission CT, the imaged property is the distribution of radionuclide concentration. Corrections for attenuation must be applied in order for the projections and the desired property, the distribution of radionuclide concentration, to have the proper relationship. In ultrasound, the transit time for an ultrasound pulse is related to the integral of the speed of sound, and so images of the local propagation speed of sound can be formed.

The main advantage of CT as compared to projection imaging is the inherent ability of CT to separate objects according to their position in the projection direction, i.e. to avoid the confusion that arises when the shadows of multiple objects are superimposed. This, combined with high precision measurements and digital displays, gives CT the ability to resolve objects with extremely small contrast. For example, conventional X-ray CT systems are able to easily distinguish objects whose relative difference in attenuation coefficient is a fraction of one percent. This allows soft tissue structures to be imaged in a manner not possible with projection radiography.

The mathematical foundation of computed tomography was initially derived by Radon and published in 1917. However, it was not until the development of modern computers that the technique was at all viable. In addition, the scientists who developed CT were generally unaware of Radon's earlier work and used practical implementations that were not anticipated by Radon.

An X-ray CT system requires an X-ray source, one or more X-ray detectors, and means (scanning apparatus) to move the components so as to collect the needed projections. The first commercial CT scanner based on the prototype developed by Hounsfield consisted of an X-ray source and two detectors, with a third, reference, detector near the X-ray source (Fig. 1). In this early CT scanner, the tube and detector assembly is first translated to collect a set of parallel projection measurements through the object (patient). The assembly is then rotated to a second angle orientation and the translation is repeated. The translation and rotation are repeated until sufficient measurements are obtained to reconstruct an image. An iterative reconstruction algorithm, developed originally by Hounsfield, was initially used. Modern systems use hundreds or thousands of detectors to reduce the acquisition time. Different arrangements of the source and detector(s) have been used over the years, and these have been called CT generations (first generation, second generation, etc.). However, one must avoid the temptation to assume that a "generation" with a higher number is necessarily superior to one with a lower number. Both third generation and fourth generation CT systems are currently enjoying commercial success. See also helical CT scanner and multisection CT.

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