Dr. 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.
Then, in 1973, Dr. 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 (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. (“CT” stands for computed tomography; another moniker is CAT scan, for computer-assisted tomography.) Dr. Ledley’s prototype did not include the modern machine’s dreaded tunnel — in his original design, the patient passed through a thinner ring-like scanner — but most CT scanners today rely on his basic concept.
CT technology, originally formulated several years earlier by Nobel Prize-winning scientists
Godfrey Hounsfield and Allan M. Cormack, transformed medical imaging. Before the advent of CT scans, doctors relied on X-ray images, which showed only hard tissue such as bone.
CT scans offer near-photographic renderings of soft tissue and organs such as the brain and heart. On X-rays, tumors show up as faint shadows or not at all. With a CT scanner, doctors can use cross-sectional images to pinpoint a tumor’s location — and without subjecting the patient to potentially life-threatening surgery.
When Dr. Ledley’s machine began to become widely available in the mid 1970s, the Journal of the American Medical Association called it a “remarkable and fundamentally new technique.” Dr. Ledley was inducted into the National Inventors Hall of Fame in 1990 and in 1997 received the National Medal of Technology.
Dr. Ledley developed and built his original scanner in a basement laboratory at Georgetown University, with help from a local machinist and a final paint job by a nearby Cadillac dealership.
The machine represented a major improvement on a model built in Britain by Hounsfield, said Joseph November, a University of South Carolina history professor who is working on a biography of Dr. Ledley.
Hounsfield’s machine used a tank of water equipped with what November described as a “rubber bladder” — a watertight area within the tank where the patient positioned his head. Only the head, and sometimes extremities such as arms and legs, could be scanned because of the unwieldy water tank.
The technology’s limitation, Dr. Ledley discovered, stemmed from a mathematical weakness.