CT scanners, from first principles
An interactive guide to what a CT scanner does while you lie in the ring, from X-rays and shadows to rebuilt slices.
Finished
I wanted to understand what a CT scanner actually does in the few seconds it asks you to breathe in and hold, so I had Claude build a first-principles explainer.
It starts with what an X-ray is and how the tube makes one, then works through the idea the whole machine depends on: one detector reading is only a total along a line, and totals from hundreds of angles are enough to rebuild a slice. A small scanner model runs in the page on a simple chest. You can change the number of angles, switch the sharpening filter on and off, move the patient halfway through a turn, add contrast dye or a metal pendant, and change the display window.
Later chapters cover why you hold your breath, why some scans use iodine dye and others don’t, why metal comes off (it spoils the picture rather than harming you), and how radiation dose and risk are estimated. The dose figures are UKHSA’s typical adult values, and the risk estimates use the cautious linear no-threshold model. These are population averages, not medical advice.
The chapters, diagrams, glossary and check questions read without JavaScript. The scanner models, calculators and step-by-step playback need it.