Computed tomography (CT), originally known as computed axial tomography (CAT or CT scan) and body section roentgenography, is a medical imaging method employing tomography where digital geometry processing is used to generate a three-dimensional image of the internals of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. The word "tomography" is derived from the Greek tomos (slice) and graphein (to write). CT produces a volume of data which can be manipulated, through a process known as windowing, in order to demonstrate various structures based on their ability to block the X-ray beam. Although historically (see below) the images generated were in the axial or transverse plane (orthogonal to the long axis of the body), modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures.
History
The first commercially viable CT scanner was invented by Godfrey Newbold Hounsfield in Hayes, England at Thorn EMI Central Research Laboratories using X-rays. Hounsfield conceived his idea in 1967, and it was publicly announced in 1972. It is claimed that the CT scanner was "the greatest legacy" of the Beatles; the massive profits from their record sales enabled EMI to fund scientific research.[1] Allan McLeod Cormack of Tufts University, Massachusetts, USA independently invented a similar process and they shared a Nobel Prize in medicine in 1979.
The prototype CT scanner
The original 1971 prototype took 160 parallel readings through 180 angles, each 1° apart, with each scan taking a little over five minutes. The images from these scans took 2.5 hours to be processed by algebraic reconstruction techniques on a large computer.
The first production X-ray CT machine (called the EMI-Scanner) was limited to making tomographic sections of the brain, but acquired the image data in about 4 minutes (scanning two adjacent slices) and the computation time (using a Data General Nova minicomputer) was about 7 minutes per picture. This scanner required the use of a water-filled Perspex tank with a pre-shaped rubber "head-cap" at the front, which enclosed the patient's head. The water-tank was used to reduce the dynamic range of the radiation reaching the detectors (between scanning outside the head compared with scanning through the bone of the skull). The images were relatively low resolution, being composed of a matrix of only 80 x 80 pixels. The first EMI-Scanner was installed in Atkinson Morley's Hospital in Wimbledon, England, and the first patient brain-scan was made with it in 1972.
a historic EMI-Scanner
In the U.S., the first installation was at the Mayo Clinic. As a tribute to the impact of this system on medical imaging the Mayo Clinic has an EMI scanner on display in the Radiology Department.
The first CT system that could make images of any part of the body, and did not require the "water tank" was the ACTA scanner designed by Robert S. Ledley, DDS at Georgetown University.
