An X-ray image is like a photo taken in the dark: the less light, the harder it is to see details. Doctors wanted to know how much you can dim the "light" in computed tomography (CT) and still not miss a disease. To do this, they took 50 pairs of chest images: one with a full radiation dose, the other with a low dose. Then they created even weaker versions on the computer — from 1% to 75% of the original strength.
The computer program itself found organs in the images and measured their density. Liver density helps spot fatty liver, muscle density helps spot frailty, and bone density helps spot osteoporosis. It turned out that for these three tasks, a dose like a regular chest X-ray (just 3% of the full dose) is enough. But if you're looking for calcium deposits in blood vessels, you need a higher dose, like in lung cancer screening (about 25% of the full dose). Otherwise, the program can't distinguish the small white dots.
Why does this matter? If you screen thousands of people at once, the total radiation exposure is huge. The new data allow you to cut the dose by tens of times and still get reliable results. However, each measurement has its own threshold: some things are visible with almost no radiation, while others need a brighter "flash."
The dose from a regular chest X-ray is about 30 times lower than from a standard CT of the same area. And yet CT gives a 3D picture, not a flat shadow.