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November 25, 1975: Patent for Full-body CAT Scan

Allan Cormack
Allan Cormack
Cat Scan
Patent drawing for Ledley's full-body machine
Godfrey Hounsfield
Godfrey Hounsfield
When German physicist Wilhelm Roentgen accidentally discovered x-rays cast by his cathode ray tube in 1895, the phenomenon quickly found practical application in medical imaging and diagnostics. It would be another 80 or so years before x-rays were harnessed in a new, improved form of diagnostic imaging: computer-assisted tomography, or CAT scanning.

In a CAT scan, the x-ray tube and detector rotate around the patient, capturing images of each cross-section of the body an organ under examination. A CAT scan uses crystal detectors that emit signals when struck by x-rays, which are stored and analyzed in a computer. The result is a 3-D image of the body part.

The two men credited with the CAT-scan's invention are Allan Cormack and Godfrey Hounsfield, who shared the 1979 Nobel Prize in Physiology or Medicine for the discovery.

A native of South Africa, Cormack became interested in astronomy as a teenager, and chose to study math and physics because they were essential to a career in astronomy.

Career prospects for astronomers weren't good, so he studied electrical engineering instead at the University of Cape Town. Within two years his interests had reverted to math and physics; he eventually earned bachelor's and master's degrees in physics.

He worked at Cambridge University's famed Cavendish Laboratory before returning to his alma mater as a faculty member. Much of his research was in nuclear physics; he became interested in what is now known as CAT-scanning in 1956.

Cormack provided the theoretical framework for CAT scans, analyzing the conditions for demonstrating a correct radiographic cross-section in a biological system, which was published in two papers in 1963 and 1964, respectively. His results didn't initially garner much attention; it wasn't until 1970 that other developments in the field caused him to devote more sub-stantial efforts to that area. Cormack understood that it was basically a matter of finding a two-dimensional mathematical function to relate the observed transmission to the varying absorbtion as the x-rays pass through a cross-section. Although others before him had deduced similar methods of calculation, Cormack was the first to state the basic principles for reconstructing a cross-section of organ tissue. He also foresaw that solving this problem would open up radiotherapy and imaging diagnostic applications in medicine.

Building a practical machine became the purview of Hounsfield. He grew up in the English countryside on a farm, the youngest of five children, and early on evinced a fascination with all things mechanical on the farm: the threshing machines, the binders, and generators.

During his teenage years he started doing his own experiments, building electrical recording machines, and investigating the principles of flight by launching himself from the tops of haystacks with a homemade glider. And he very nearly blew himself up while experimenting with water-filled tar barrels and acetylene to see how high he could propel the waterjet.

In school, he excelled primarily in math and physics. When World War II broke out, he joined the Royal Air Force, driven by his love of aeronautics. He eventually passed a City and Guilds examination in radio communications at the Royal College of Science and, later, the Cranwell Radar School. And he also built a large-screen oscilloscope and demonstration equipment as instruction aids.

After the war, he earned a bona fide degree from the Faraday House Electrical Engineering College in London. He joined the research staff of EMI in 1951, working on radar and guided weaponry, and on computers, which were then in their infancy.

After transferring to EMI's Central Research Laboratories, Hounsfield proposed a project on automatic pattern recognition. This work led, in 1967, to the idea that became known as computer- assisted tomography.

In his Nobel Prize autobiography, he recalled the many frustrations and technical hurdles he had to overcome to produce the first clinical brain-scanner—including traveling across London by public transport carrying bullock's brains for use in an experimental scanner in the lab.

Computer tomography was first used to take images of the skull, for study of diseases of the brain. A whole-body version of the CAT scan was invented by Robert Ledley, a professor of physiology and biophysics at Georgetown University.

Ledley received a patent for that device in November, 1975. Since then, CAT scans have become a mainstay of the medical profession, and not just for imaging the brain or specific organs. It has also been used to monitor effects of radioactive treatment of cancerous tumors.

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Editor: Alan Chodos
Associate Editor: Jennifer Ouellette