One grainy, gray-scale image of a brain changed science and medicine forever.

Half a century ago, the first CT image of a patient lifted the veil of invisibility that cloaks the interior of the human body, providing scientists a window on our innards unlike any before.

Today, doctors in the United States alone order more than 80 million scans per year. X-ray computed tomography, or CT, is frequently the quickest way of getting a handle on what’s causing a mysterious woe. CT scans can ferret out heart disease, tumors, blood clots, fractures, internal bleeding and more. The technique can give surgeons a heads-up about what they will encounter inside a patient, and guide treatment for cancer and other diseases.

“It answers so many questions quickly. That’s why it’s used,” says medical physicist Cynthia McCollough of the Mayo Clinic in Rochester, Minn.

A CT scan involves thousands of X-ray measurements taken from multiple angles. Here’s how it works: A source of X-rays rotates around the body, sending a beam of radiation through bone, blood and tissue, while rotating detectors measure the beam that makes it through.

Different materials in the body absorb X-rays differently. The calcium in bone vigorously sops up X-rays, for example, while soft tissues absorb less. So when the data collected by the detectors are stitched together by a computer, it can form a cross-sectional view of what’s inside based on where X-rays are absorbed more or less. Moving the table holding the patient so that the X-ray beam and detectors slide along the body enables 3-D reconstructions of organs and other parts.

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Over the years, scientists have continually improved the technology, making it faster and higher resolution, and cutting the amount of radiation that patients receive. These improved CT scans have painted ever more detailed landscapes of the human body. It’s hard not to marvel at the beauty of the inner world that the scans bring to the surface.

Here’s a look back at some of the major advances in CT scans over the last 50 years.

The CT scan is born

In 1971, radiologist James Ambrose at Atkinson Morley Hospital in London performed the first CT scan of a patient in collaboration with engineer Godfrey Hounsfield, the technology’s inventor. Hounsfield worked for the British electronics company EMI, best known for its role as the record company for the Beatles.

first CT image of a patient showing the brain area

CT scanner manufacturers kept pushing up the number of detectors, making scanners that could capture more slices at once. When detectors hit 64 slices in the early 2000s, McCollough says, “the real ‘Wow’ happened.” Scans could be fast, high resolution and cover a significant length of the body, all at once. Today, scanners are even more sophisticated, using up to 320 slices.

Finally, the fragile complexity hidden inside the human body — from the intricate webs of blood vessels to the gracefully branching air passages of the lungs to the delicate yet sturdy structure of bone — was there for all to see.

Doubling up

When McCollough’s father-in-law landed in the emergency room with a painful wrist, he languished for hours while doctors tried to figure out what was wrong. Eventually, McCollough remembers, she asked, “Can’t we send him over to CT?”

Thanks to a technique called dual-energy CT, doctors were able to pinpoint the problem. Dual-energy CT, introduced in 2006, uses two beams of X-rays at different energies, instead of just one beam. Taking images this way allows the scanners to work out exactly what materials are within. Different materials absorb X-rays in different amounts, but in order to zero in on a specific type of material, you want to know how that absorption changes as the X-rays’ energy changes.

For example, dual-energy CT can distinguish between different types of crystals that can form in the joints, causing arthritis. Urate crystals indicate gout, and crystals containing calcium point to pseudogout. For McCollough’s father-in-law, the scan quickly uncovered the cause of his pain: pseudogout. In this way, CT scans can reveal the human body at its most basic level, the materials that make it up.

CT scan of a hand showing urate crystals around the wrist and finger joints in green

X-rays are a type of high-energy light, and like all light, they are made up of particles called photons. Photon-counting CT scanners measure individual X-ray photons. The technology also allows for crisper, more detailed images and provides a measure of the energies of the photons, which, like dual-energy CT, identifies different materials inside the body.

images of a broken wrist in a regular CT scan and a photo-counting CT scan
colorized CT scan showing areas of the lung damaged by covid in green and non-damaged areas in blue