"Computed tomography (CT), also known as Computed Axial Tomography (CAT), is a painless, sophisticated x-ray procedure. Multiple images are taken during a CT or CAT scan, and a computer compiles them into complete, cross-sectional pictures ("slices") of soft tissue, bone, and blood vessels."  CT scans can be performed on every region of the body for a variety of reasons (e.g., diagnostic, treatment planning, interventional, or screening). The “tome” in tomography is the Greek word for “slice.” At the core of the scanner is a computer that not only controls the radiation source, the rotation of the x-ray tube and detector, and the movement of the table, but also generates anatomical slices, or tomograms, from the measured projections. The mathematical technique that allows an image of the head to be recovered from its projections is referred to as the back projection algorithm.  Because the patient is positioned horizontally on the table, the back projection algorithm yields slices that are trans-axial, which means the slices are oriented at right angles to the long axis of the body. Lastly, a contrast media, or “dyes” are used both in brain CT and MRI to provide another mechanism for modulating image intensity beyond what is possible using intrinsic tissue contrast. The device operates on 480VAC, 3-phase delta-wye, 100Amp power circuits.
Tomography had been one of the pillars of radiological diagnostics until the late 1970s, when the availability of minicomputers and of the transverse axial scanning method, this last due to the work of Godfrey Hounsfield and South African-born Allan McLeod Cormack, gradually supplanted it as the modality of CT. Mathematically, the method is based upon the use of the Radon Transform invented by Johann Radon in 1917. But as Cormack remembered later, he had to find the solution himself since it was only in 1972, that he learned of the work of Radon, by chance.
The first commercially viable CT scanner was invented by Sir Godfrey Hounsfield in Hayes, United Kingdom at EMI Central Research Laboratories using X-rays. Hounsfield conceived his idea in 1967. The first EMI-Scanner was installed in Atkinson Morley Hospital in Wimbledon, England, and the first patient brain-scan was done on 1 October 1971. It was publicly announced in 1972.
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. Allan McLeod Cormack of Tufts University in Massachusetts independently invented a similar process, and both Hounsfield and Cormack shared the 1979 Nobel Prize in Medicine.
The first CT system that could make images of any part of the body and did not require the "water tank" was the ACTA (Automatic Computerized Transverse Axial) scanner designed by Robert S. Ledley, DDS, at Georgetown University. This machine had 30 photomultiplier tubes as detectors and completed a scan in only 9 translate/rotate cycles, much faster than the EMI-scanner. It used a DEC PDP11/34 minicomputer both to operate the servo-mechanisms and to acquire and process the images. The Pfizer drug company acquired the prototype from the university, along with rights to manufacture it. Pfizer then began making copies of the prototype, calling it the "200FS" (FS meaning Fast Scan), which were selling as fast as they could make them. This unit produced images in a 256×256 matrix, with much better definition than the EMI-Scanner's 80×80.
The original 1971 prototype took 160 parallel readings through 180 angles, each 1° apart, with each scan taking a little over 5 minutes. The images from these scans took 2.5 hours to be processed by algebraic reconstruction techniques on a large computer. The scanner had a single photomultiplier detector, and operated on the Translate/Rotate principle. 
CT versus MRI
Basically, a CT Scan is a medical imaging obtained using X-rays. The radiation is passed through the body and received by a detector and then integrated by a computer to obtain a cross sectional image that is displayed on the screen. On the other hand, Magnetic resonance imaging (MRI) is a medical imaging technique most commonly used in radiology using a very powerful magnet and pulsing radio waves to detect medical anomalies.
The basic mathematics of the 2D-Fourier transform in CT reconstruction is very similar to the 2D-FT NMRI, but the computer data processing in CT does differ in detail, as for example in the case of the volume rendering and artifact elimination algorithms that are specific to CT. Usually the time taken to complete a a CT scan is within 5 minutes while an MRI takes 30 about mintues.
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- ↑ http://www.ehealthmd.com/library/ctscan/CTS_whatis.html
- ↑ FDA. Medical Imaging: CT. Page Last Updated: 06/20/2013. http://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/MedicalImaging/MedicalX-Rays/ucm115317.htm
- ↑ Cormack AM. Representation of a function by its line integrals, with some radiological applications. J Appl Physics 1963; 34: 2722-2727.
- ↑ http://en.wikipedia.org/wiki/X-ray_computed_tomography
- ↑ http://www.diffen.com/difference/CT_Scan_vs_MRI%7C CT Scan versus MRI