I work at a hospital. I saw a technician doing a chest x ray the other day. He placed the x ray machine at least 5 feet about the patients chest. Wouldn’t this cause the radiation to scatter? How does it only put the lungs in focus then?
In: Engineering
Placing the x-ray source close means that the center of the target (the chest in your case) is closer to it, and therefore receives more radiation than the edges. This leads to a worse image quality and also slightly higher radiation dose.
Instead, they use a piece of metal with a hole in it to block off any radiation going the sides, producing a very narrow beam of radiation. By changing the shape and size of the hole, they can even shape the beam so it only hits what it’s supposed to hit.
X-rays are high enough energy that they don’t really scatter or refract at that scale. The upside of this is that you can see through bodies, downside is that you have very few ways to shape the beam. Taking an x-ray image is essentially casting a shadow that you want to be as close to the real object as possible, which means you want the rays to be as parallel as possible.
If you were trying to cast an accurate shadow of your hand on the wall using only a light bulb (no lenses or mirrors), the bulb would have to be pretty far away to not distort the size and shape.
A chest is very deep. If the X-ray source was very close then the front of the chest could be twice as close to the source as the back of the chest, meaning a higher dosage and also greater magnification on the final X-ray image. If you move the source back then the beam is more parallel by the time it reaches the chest so the magnification is much more constant from front to back, and so is the dose of radiation.
Imagine you wanted to evenly light a person in a dark room with a lamp. Would you put the lamp right up against them, or back a way so they were more evenly lit?
It’s essentially the same thing.
X-rays come from a tiny pont in the machine. The image is produced as a “shadow” where bones block the beam and the rest of the body only slightly blocks it allowing some rays through.
If the x-ray source was placed close, then the direction of the beam going to the left shoulder would be different to the direction of the beam going through the heart.
The different angles of the diverging beam result in the image being magnified. The closer an object is to the x-ray source the more it is magnified. This means that the anatomy closer to the x-ray source is more magnified than the anatomy close to the detector.
In a standard chest x-ray that means that the spine and back ribs are magnified compared to the sternum and front ribs (which are next to the detector). The closer the x-ray source to the anatomy, the more this magnification difference occurs. By placing the x-ray source 5 feet away, this effect is reduced and the magnification is reasonably even, which allows rough measurements to be taken from the image.
It’s not just the beam direction being different, the length is different – it’s longer to the shoulder because of the angle and x-ray beam strength is very sensitive to length (inverse square law). This would result in an uneven exposure, too many x-rays at the centre of the image, and not enough at the corners. By placing the x-ray source as far away as practical, this effect is reduced and a reasonably even exposure can be achieved.
I’ve got a question, why during an x-ray you don’t wear any protection and you ask the doctor “is this safe?” And he’s like “yeah of course” as he quickly scurried into a completely different room protected from all the radiation
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