CAT Scan Patent Issued November 25, 1975

X-Ray System Saves Lives By Early Detection of Illness and Injury
November 21, 2001
Press Release #01-56

Maria V. Hernandez

Robert S. Ledley, born in 1926 in New York City, received patent # 3,922,552 on November 25, 1975 for the diagnostic X-ray system, also known as the CAT (Computed Axial Tomography) scan.

Ledley's invention, the ACTA (Automatic Computerized Transverse Axial) diagnostic X-ray scanner, was the first whole-body computerized tomography (CT) machine, and provided views of the body and brain not seen with traditional X-rays. This was achieved using computers to generate three-dimensional images from flat X-ray pictures of cross-sections of the body, called slices. Ledley's technology allowed early diagnoses of illness and disease, thus saving lives.

Ledley also used computerized tomography in radiation therapy and in the diagnosis of bone disease. He was inducted into the National Inventor's Hall of Fame in 1990.

Ledley's patents, as well as the more than 6 million patents issued since the first in 1790, can be seen on the Department of Commerce's United States Patent and Trademark Office website at

Last year USPTO issued 182,223 patents and registered 127,794 trademarks.