The x-rays that bounce off are not measured. Simplistically X-ray machines emit a certain amount of rays, and do not pass through bone or metal. Essentially for each part of the image they measure the *proportion* of x-rays that make it through (conventionally this is adjusted such that the areas with more transmission are darker and the areas with less are brighter). The reader is on the opposite side of the machine that emits the rays, with the subject between them.
Here’s the ELI5:
As an analogy, imagine you have a line of people and 100 people trying to run through them under a certain time limit. With just a single line of people you can imagine that most people would get through. But what happens if you add a second line? Fewer will make it through in time because there’s more in the way. If you have a super thick line or crowd even fewer people, perhaps none, will make it through.
Now if those people running are also in a line and have to take a straight path, and the line is uneven (maybe 1 person thick in some places, maybe 10 in others, and in-between elsewhere). You record each spot where a person got through and put a transparent piece of red plastic over that square of wall. Now repeat that many times–in the thin areas of the blocking line the multiple layers of plastic will make the wall really dark red, the medium thickness areas some people will get through sometimes, but not always, so you’ll get medium shades of red proportional to the thickness of the wall, and where there are lots of blockers the wall will remain white.
Based on the pattern of white and darkness of the red you can make statements about which areas were thicker and thinner–and this is essentially how X-rays work, too. Through accumulation of scientific evidence we know what proportion of rays should get through certain tissues normally and can make assessments about areas that do not fit their “normal” appearance.
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