Evolution of CT Scan

The History

In 1960's, after so many research, the world's first CT scanner was invented. The inventor was Godfrey Newbold Hounsfield, who was born in England 1919. He and Alan Cormack, a medical physicist, together developed and placed the first brain scanner into operation in 1971 for a company called EMI Ltd. Hounsfield conceived his idea in 1967 In 1979, they were awarded the Nobel Prize in medicine and physiology. 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.

 

Left; Godfrey Newbold Hounsfield

Right; Alan McLeod Cormack

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.

The first production X-ray CT machine (in fact 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 × 80 pixels.

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 nine 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.

Since the first CT scanner, CT technology has vastly improved. Improvements in speed, slice count, and image quality have been the major focus primarily for cardiac imaging. Scanners now produce images much faster and with higher resolution enabling doctors to diagnose patients more accurately and perform medical procedures with greater precision. In the late 1990s CT scanners broke into two major groups, "Fixed CT" and "Portable CT". "Fixed CT Scanners" are large, require a dedicated power supply, electrical closet, HVAC system, a separate workstation room, and a large lead lined room. "Fixed CT Scanners" can also be mounted inside large tractor trailers and driven from site to site and are known as "Mobile CT Scanners". "Portable CT Scanners" are lightweight, small, and mounted on wheels. These scanners often have built-in lead shielding and run off of batteries or standard wall power.

In 2008 Siemens introduced a new generation of scanner that was able to take an image in less than 1 second, fast enough to produce clear images of beating hearts and coronary arteries.

Generations of CT Scanner

There are four generation of CT Scanner. In the first and second generation designs, the X-ray beam was not wide enough to cover the entire width of the 'slice' of interest. A mechanical arrangement was required to move the X-ray source and detector horizontally across the field of view. After a sweep, the source/detector assembly would be rotated a few degrees, and another sweep performed. This process would be repeated until 360 degrees (or 180 degrees) had been covered. The complex motion placed a limit on the minimum scan time at approximately 20 seconds per image. In the 3rd and 4th generation designs, the X-ray beam is able to cover the entire field of view of the scanner. This avoids the need for any horizontal motion; an entire 'line' can be captured in an instant. This allowed simplification of the motion to rotation of the X-ray source.

The a, b, c and d images are the x-ray beam distribution of 1st, 2nd, 3rd and 4th generation of CT scanner respectively. First generation  detectors: one type of beam: pencil-like X-ray beam tube-detector movements: translate-rotate duration of scan (average): 25-30 mins Second generation  detectors: multiple (up to 30) type of beam: fan shaped x-ray beam tube-detector movements: translate-rotate duration of scan (average): less than 90 s Third generation  detectors: multiple, originally 288; newer ones used over 700 arranged in an arc type of beam: fan shaped x-ray beam tube-detector movements: rotate-rotate duration of scan (average): approximately 5s Fourth generation detectors: multiple (more than 2000) arranged in an outer ring which is fixed type of beam: fan shaped x-ray beam tube-detector movements: rotate-fixed  duration of scan (average): few seconds