A computed tomography (CT) scan (formerly known as computed axial tomography or CAT scan) is a medical imaging technique used in radiology to obtain detailed internal images of the body  for diagnostic purposes.

The personnel that perform CT scans are called radiographers or radiology technologists.

Early Experiments

The invention of this incredible technology has been widely attributed to the success of the scientists in the 1900s. This extraordinary invention was made possible through the work of several scientists, most notably Godfrey Newbold Hounsfield and Allan MacLeod Cormack, a physics professor at Tufts University in Medford, Massachusetts who share Nobel prize in Medicine and Physiology in the year 1979, for their outstanding contributions to the development of Computed tomography.

The history of computed tomography goes back to 1917 when Johann Radon developed the basic mathematical equation with the theory of the Radon transform. Back then to the early 1920s, when many scientists were developing methods to image a specific layer or section of the body, at that time, ‘‘body section radiography’’ and ‘‘stratigraphy’’ (from stratum, meaning ‘layer’) were used to describe the technique and in 1935, Grossman refined the technique and named it as “Tomography”.

With this tomographic concept, Watson developed another Tomography technique, which was based on transverse/axial scans and was referred to as “Transverse Axial Tomography” in 1936.

Another mathematical advancement was the “Algebraic Reconstruction Technique,” which was formulated by Polish mathematician Stefan Kaczmarz in 1937. Both Reconstruction Technique and Radon transform theory were adopted by Hounsfield to create one of the greatest inventions in the medical history.

The basic principle of tomographic scan was published by Frank & Takashi in 1940 which was a revolutionary step towards its creation on which the tomographic scanning was based on with the theory of Image Reconstruction given by Allan MacLeod Cormack in 1956.

In 1967, Hounsfield was investigating pattern recognition and reconstruction techniques by using the computer. From this work, he deduced that, if an x-ray beam were passed through an object from all directions and measurements were made of all the x-ray transmission, information about the internal structures of that body could be obtained. This information would be presented to the radiologist in the form of pictures that would show 3D representations.

With encouragement from the British Department of Health and Social Security, an experimental apparatus was constructed to investigate the clinical feasibility of the technique.

The radiation used was from an americium gamma source coupled with a crystal detector. Because of the low radiation output, the apparatus took about 9 days to scan the object. The computer needed 2.5 hours to process the 28,000 measurements collected by the detector. This procedure was too long, due to which various modifications were made and the gamma radiation source was replaced by a powerful x-ray tube. The results of these experiments were more accurate, and it took 1 day to produce a picture.

In 1971, the first clinical prototype CT brain scanner (EMI Mark 1) was installed at AtkinsonMorley’s Hospital and clinical studies were conducted under the direction of Dr. James Ambrose. The processing time for the picture was reduced to about 20 minutes. Later, with the introduction of minicomputers, the processing time was reduced further to 4.5 minutes.

GODFREY NEWBOLD HOUNSFIELD

Godfrey Newbold Hounsfield was born in 1919 in Nottinghamshire, England. He studied electronics and electrical and mechanical engineering. In 1951, Hounsfield joined the staff at EMI Limited (Electric and Musical Industries) where he began work on radar systems and later on computer technology. Hounsfield died on August 12, 2004, at the age of 84 years.

In 1972, First clinical ‘Brain’ Scanner was been developed and the first patient scanned by this machine was a woman with a suspected brain lesion, and the picture showed clearly in detail a dark circular cyst in the brain. By developing the first practical CT scanner, Hounsfield opened up a new domain for technologists, radiologists, medical physicists, engineers, and other related scientists.

Hounsfield’s invention revolutionized medicine and diagnostic radiology. It was him, who called the technique ‘computerized transverse axial scanning’ (tomography) in his description of the system, which was first published in the British Journal of Radiology in 1973.

Terms such as ‘‘computerized transverse axial tomography,’’ ‘‘computer-assisted tomography or computerized axial tomography,’’ ‘‘computerized transaxial transmission reconstructive tomography,’’ ‘‘computerized tomography,’’ and ‘‘reconstructive tomography’’ were also used.

The term computed tomography and its acronym, CT, was established by the Radiological Society of North America in its major journal ‘Radiology’. In addition, the American Journal of Roentgenology accepted this term, which is now used worldwide within radiology.

Between 1973 and 1983, a number of CT units were installed globally. The first significant technical development came in 1974 when Dr. Robert Ledley , a professor of radiology, physiology, and biophysics at Georgetown University, developed the first ‘whole-body CT scanner’.

In 1975, Dynamic special reconstructor (DSR) was installed at Myo clinic. The goal of the DSR was to carry out dynamic volume scanning for imaging of the dynamics of organ systems and the functional aspects of the cardiovascular and pulmonary systems with high temporal resolution as well as imaging anatomic details.

In the mid-1980s, high speed CT Scanner was introduced that used electron beam technology, a result of work by Dr. Douglas Boyd and colleagues, at the University of California at San Francisco. The scanner was invented to image the cardiovascular system to overcome motion artifacts.

At that time this scanner was called as the cardiovascular CT scanner but later, this scanner was acquired and marketed by Siemens Medical Systems under the name Evolution and was subsequently referred to as the “electron beam CT” (EBCT) scanner.

The U.S. Food and Drug Administration (FDA) cleared the EBCT scanner in 1983. As of 2007, the EBCT scanner is marketed by General Electric (GE) Healthcare under the name e-Speed and it now features proprietary technologies that play a significant role in imaging the heart.

During the 1990’s “Portable/Mobile CT” scanners grew in popularity. The portable CT scanners allow the clinicians to maximize the availability of stationary CT equipment in a hospital as improving the workflow of standard scanners creates faster imaging for ICU and non-ICU patients. In addition, minimizing the need for extra transport contributes to economic benefits, as well as improved the use of other equipment and enhance the overall quality of patients care.

The spiral/helical CT scanners were developed after 1989 and were referred to as “single-slice” spiral/helical (SSCT) or volume CT scanners. In 1992, “Dual-slice” spiral/helical (volume scanner) CT Scanner was introduced to scan two slices per 360-degree rotation, thus increasing the volume coverage speed compared with single-slice volume CT scanners.

In 1998, “multi-slice” CT (MSCT) was introduced at RSNA meeting in Chicago, which was based on the use of multi-detector technology to scan four or more slices per revolution of the x-ray tube and detectors, thus increasing the volume coverage speed of single-slice and dual-slice volume CT scanners.

• 2000 = 8-16 and 32-40 slice CT Scanner was introduced
• 2004 = Introduction of 64 slice CT Scanner
• 2006-2007 = Introduction of 256 slice CT Scanner(prototype) & 320 slice CT scanner

By 2005, 90% of PET scanners were actually “PET-CT” fusion imaging scanners and in 2006, “Dual source” CT (DSCT-CT) scanner with two x-ray tubes coupled to two detector arrays was introduced. In 2009, at the International Symposium on Multi-detector-Row CT (MDCT), Dr. Mathias Prokop discussed the clinical implications of the 16 cm wide detector CT. The wider coverage per gantry rotation enabled more dynamic scanning and the ability to do multiple acquisitions in less time.

The 640-slice CT scanner is the ultimate testament to the application of modern technology that is equipped with a gantry rotation of 0.275 seconds, a 100 kw generator and 320 detector rows (640 unique slices) covering 16 cm in a single rotation, with the industry’s thinnest slices at 500 microns (0.5 mm).

It has emerged as the most powerful tool to diagnose cardiovascular disease, diseases like cancer at an early stage and is used in Cardiology, Neurology, Oncology, Gastroenterology and Paediatrics. Also, abdominal, neck, brain and lower extremity angiograms can be done rapidly with 4D DSA with more precision, contrast and less radiation.

Toshiba has unveiled a 640-slice CT scanner at the 2012 annual meeting of the RSNA. The system was cleared by the U.S. Food and Drug Administration (FDA) in September 2012 and currently has one install in the United States at the National Institutes of Health (NIH). Presently, Apollo Hospitals, Chennai, has introduced India’s First State-Of-The-Art, The Advanced Aquilion One Prism 640-Slice CT scanner for non-invasive assessments.

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