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  • In 1895, a physicist named Wilhelm Roentgen

  • was doing experiments with a cathode tube,

  • a glass container in which a beam of electrons lights up a fluorescent window.

  • He had wrapped cardboard around the tube

  • to keep the fluorescent light from escaping,

  • when something peculiar happened.

  • Another screen outside the tube was glowing.

  • In other words, invisible rays had passed through the cardboard.

  • Wilhelm had no idea what those rays were, so he called them X-rays,

  • and his discovery eventually won him a Nobel Prize.

  • Here's what we now know was happening.

  • When high energy electrons in the cathode tube hit a metal component,

  • they either got slowed down and released extra energy,

  • or kicked off electrons from the atoms they hit,

  • which triggered a reshuffling that again released energy.

  • In both cases, the energy was emitted in the form of X-rays,

  • which is a type of electromagnetic radiation

  • with higher energy than visible light, and lower energy than Gamma rays.

  • X-rays are powerful enough to fly through many kinds of matter

  • as if they are semi-transparent,

  • and they're particularly useful for medical applications

  • because they can make images of organs, like bones, without harming them,

  • although they do have a small chance

  • of causing mutations in reproductive organs, and tissues like the thyroid,

  • which is why lead aprons are often used to block them.

  • When X-rays interact with matter, they collide with electrons.

  • Sometimes, the X-ray transfers all of its energy to the matter and gets absorbed.

  • Other times, it only transfers some of its energy,

  • and the rest is scattered.

  • The frequency of these outcomes

  • depends on how many electrons the X-rays are likely to hit.

  • Collisions are more likely if a material is dense,

  • or if it's made of elements with higher atomic numbers,

  • which means more electrons.

  • Bones are dense and full of calcium, which has a relatively high atomic number,

  • so they absorb X-rays pretty well.

  • Soft tissue, on the other hand, isn't as dense,

  • and contains mostly lower atomic number elements,

  • like carbon, hydrogen, and oxygen,

  • so more of the X-rays penetrate tissues like lungs and muscles,

  • darkening the film.

  • These 2-D pictures are only useful up to a point, though.

  • When X-rays travel through the body,

  • they can interact with many atoms along the path.

  • What is recorded on the film reflects the sum of all those interactions.

  • It's like trying to print 100 pages of a novel on a single sheet of paper.

  • To see what's really going on,

  • you would have to take X-ray views from many angles around the body

  • and use them to construct an internal image.

  • And that's something doctors do all the time in a procedure

  • called a CT, Computed Tomography scan,

  • another Nobel Prize winning invention.

  • Think of CT like this.

  • With just one X-ray,

  • you might be able to see the density change due to a solid tumor in a patient,

  • but you wouldn't know how deep it is beneath the surface.

  • However, if you take X-rays from multiple angles,

  • you should be able to find the tumor's position and shape.

  • A CT scanner works by sending a fan or cone of X-rays through a patient

  • to an array of detectors.

  • The X-ray beam is rotated around the patient,

  • and often also moved down the patient's body,

  • with the X-ray source tracing a spiral trajectory.

  • Spiral CT scans produce data that can be processed into cross sections

  • detailed enough to spot anatomical features, tumors,

  • blood clots, and infections.

  • CT scans can even detect

  • heart disease and cavities in mummies buried thousands of years ago.

  • So what began as Roentgen's happy accident has become a medical marvel.

  • Hospitals and clinics now conduct over 100 million scans each year worldwide

  • to treat diseases and save lives.

In 1895, a physicist named Wilhelm Roentgen


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B1 中級

TED-ED】レントゲンは肌を透視する方法 - 葛王 (【TED-Ed】How X-rays see through your skin - Ge Wang)

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    Ann に公開 2021 年 01 月 14 日