Robert S. Ledley, a Georgetown University physicist who was credited with inventing the first full-body CT scanner, a machine that revolutionized medical diagnostics by allowing doctors to gaze inside their patients' tissues without ever touching a scalpel, died Tuesday at the Arden Courts nursing facility in Kensington, Md. He was 86.

His death, of Alzheimer's disease, was confirmed by his son, Fred Ledley. Robert Ledley was a Laurel, Md., resident.

Ledley was trained as a dentist -- in case his career in physics didn't pan out -- and became a scientific polymath. In the late 1950s, when most doctors still worked with pen and paper, he became a prominent advocate for using data processors to help diagnose disease.

In 1973, Ledley introduced one of the most powerful diagnostic aides since the discovery of X-rays in 1895. He called his invention the automatic computerized transverse axial scanner, or ACTA. It was, in effect, the first machine capable of producing cross-sectional images of any part of the body.

Today, the CT scan is a commonplace medical procedure. Ledley's prototype did not include the modern machine's dreaded tunnel -- in his original design, the patient passed through a thinner ringlike scanner -- but most CT scanners today rely on his basic concept.

Ledley was inducted into the National Inventors Hall of Fame in 1990, and in 1997 received the National Medal of Technology.

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