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How do X-rays work?

  1. Feb 2, 2010 #1
    Hello everyone,

    I really don't have any idea of how the x-rays work actually. Someone told me when light goes through that place becomes dark in the x-ray or something. Anyway if anyone can give me an explanation of how an X-ray work that would be awesome.

    x ray----------man-------board

    The x-ray prints on the board right. You can't see the depth in a normal x-ray right? Does everything get printed on the same plane or can you see layers. Organs that are front of others etc.

    Also what do they mean by antero-posterior x-ray or postero-anterior x-ray? If anyone can help that would be great. Thanks a lot!! :smile:
  2. jcsd
  3. Feb 2, 2010 #2


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    That's pretty much it, the x-ray is just like light. Where it goes straight through you the film (negative) goes dark where it is blocked by your body it doesn't reach the film and the film stays white.

    No layers - the skill in reading an xray is workign out what you are seeing with everything squashed flat.

    Front to back, or back to front - depends which way you are lying when they zap you
  4. Feb 2, 2010 #3


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  5. Feb 2, 2010 #4
    Thanks mgb_phys :smile: That was a nice answer. I have few questions though. If everything is squished flat on the x-ray, are the organs that are infront still infront. I mean do they cover the organ behind them so you can only see a little bit of the organ behind. Or can you see the organ behind transparent from the organ infront depending if light is reflected or not?

    Front to back, or back to front?
    Also could you please elaborate on this. Is their difference between these two if everything is squished on a flat plane. In a front to back x-ray do I see the front of the body first and vise versa?

    Thanks a lot for the help so far :smile:

    Ok thanks berkeman I'll read it.
  6. Feb 2, 2010 #5
    Soft tissue in the body is more transparent to x rays, and bones and the like are much less. When the x ray hits the plate, it darkens it, but the bones block the rays, leaving it white. If there were something between the machine and the organ/bone, it would cover the organ because the rays were blocked before they could reach it.
  7. Feb 3, 2010 #6


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    The only thing that you know in an x-ray is the total attenuation from the source to the detector along a given ray. The information about which organ is in front and which is behind is not preserved. If you get 80% attenuation going from sternum to spine then you will get 80% attenuation going from spine to sternum.

    The way that front to back or back to front matters is in the geometry. The x-ray source is approximately a point source, and the detector is a large panel. So the rays diverge from the source to the detector. This essentially means that closer objects will cast larger fainter shadows on the film than the same object located further away.
  8. Feb 3, 2010 #7


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    For important information they often take shots at more than one angle.
    Also, if you rotate and tilt the body and present a smart computer with the resulting large set of Xray pictures, it can work out a 3D picture of what's inside (a 'scan'). See CT - Computer Tomography etc.
  9. Feb 3, 2010 #8
    Thank you everyone for their replies :smile: I found all of them useful.

    "The way that front to back or back to front matters is in the geometry. The x-ray source is approximately a point source, and the detector is a large panel. So the rays diverge from the source to the detector. This essentially means that closer objects will cast larger fainter shadows on the film than the same object located further away."

    Hey Dalespam, thanks for your response. It was just what I needed. What I have quoted is just what I wanted but unfortunately I don't understand it much, but I know it is right. Can you tell me exactly what do you mean?

    Do the x-rays diverge because it is travelling from high to low optical density or something like that. If closer objects cast a larger shadow, does that mean x-rays don't give a definite picture of a size of an organ? Do you mean one organ gets bigger or everything gets bigger maintaining the ratio. When you say objects further and far away in an posteroanterior x-ray, is the sternum the object closest. Could you please explain this a little bit I would really appreciate because your answer is what I need :smile:
  10. Feb 3, 2010 #9


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    The xrays diverge because they are from a point source - like a light bulb
    Just from geometry things closer to the bulb will cast a bigger shadow on a wall than things closer to the wall ( try it with your hands)

    On an X ray it doens't make a huge difference because the body part is fairly thin but the side of your body pressed against the film will be more accurately sized than the organs at the other side (closer to the source).
  11. Feb 3, 2010 #10


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    Objects at differing distances from the film plane cast differing-sized shadows.

    Attached Files:

  12. Feb 3, 2010 #11


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    X rays diverge because it is very hard to focus them - they tend to go through things too easily. You just have to put up with the fact that they diverge.
    It is possible to use a parabolic curved steel mirror to produce focusing but it requires the rays to be bounced off obliquely. That's how they make X ray telescopes but it's not worth the cost and bother for medical imaging. The only practical way to produce a (near) parallel beam is to have a tiny hole and a very long tube (a collimator), which would waste most of the power of the X ray beam and make more problems for health and safety.
  13. Feb 3, 2010 #12


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    It doesn't really have anything to do with the optical density or anything like that, it is a purely geometrical phenomenon, the picture DaveC426913 posted shows it well. The further from the source the smaller angle your hand represents and therefore the smaller the resulting shadow. You can do this with a regular light bulb and your hand and a wall. If you shine a light bulb on a wall and put your hand close to the wall then the shadow will be small. If you move your hand further from the wall and closer to the bulb then your shadow will be larger.

    The light bulb is analogous to the x-ray source, your hand is analogous to the bone or organ that you are trying to image, and the wall is analogous to the film, the shadow shows where the film is not exposed to the x-rays. It is a good analogy for figuring things out, and in fact it applies to your previous question. Put your left hand in front of your right hand and look at the shadow, then put your right hand in front of your left hand and look at the shadow. Can you tell, just from the shadow, which hand is in front?
  14. Feb 3, 2010 #13
    Thanks for all the wonderful answers :smile: This is one of those rare topics where all the responses were able to help me. :smile:

    Thanks for the picture Dave that was very helpful :smile:


    That was a nice experiment I tried it out. The hands were superimposed on each other and it was difficult to tell which one was which but I had this feeling in antero posterior x-ray, posterior organs are bit infront but it is hard to tell.

    I have one question remaining. Let's say two organs are directly infront of each other. Like
    x ray--------object1---object2----board

    Then the light that travels to object 2 is light defracted by object 1, does this make any difference?

    Thanks again for everyone who replied :smile:

    This a link that shows how shadows work, just to complete the topic

    http://homepage.mac.com/cbakken/obookshelf/shadows.html [Broken]
    Last edited by a moderator: May 4, 2017
  15. Feb 3, 2010 #14


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    This is demonstrated in my diagram. The two organs are superimposed. Their shadows are translucent (not opaque, like with your hands), and they overlap. So the final image simply has both images of the organs overlapped.
  16. Feb 3, 2010 #15
    lol sorry dalespam I wrote it in a rush, I edited it now :smile:

    Anyway Dave from what I see in the diagrams two shadows are superimposed on each other. How would this create a transculent shadow? When two shadows join wouldn't it get darker.

    Also with X-rays objects that let light pass through produce a dark spot in the x-ray right. Is this a completely dark spot or can you see structures in it. I don't understand what happens when light passes straight through an organ in an x-ray. Does this produce a complete dark spot or shows the structure.

    In other words what I'm asking is since x-ray is a negative image. When you turn it into colour how do dark spots look like?
    Last edited: Feb 3, 2010
  17. Feb 3, 2010 #16


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    Yes, my diagram is not representative of this; it's actually a reversal.

    Here is a typical x-ray.
    http://thumbs.dreamstime.com/thumb_39/1138133331Z731O2.jpg [Broken]

    Yes, the film goes dark when exposed to radiation. Places where the x-rays easily pass (such as through a cavity like the lungs) will expose the film a lot, thus producing a dark spot on the film. Places where the x-rays do not pass through (such as dense bone) exposes the film only a little, thus producing a light area.

    The structures you see are dependent on how thick the organ is and how dense it is. A thick organ like the liver might show up as a very light blob, because there's so much material.

    Yes, you can see structures - if there are structures to see. X-rays only pick out changes in density or in thickness. Two materials next to each other of the same density and thickness will not be distinguishable in an x-ray. This is why they have limited use. They tend to be useful where the structures of interest are high in density-contrast, such as broken bones.

    Note, BTW, that they can control the exposure as needed. If a spot is so dense that very little x-radiation reaches the film, the area appears almost totally white. To compensate for this, they can crank up the exposure, increasing the overall radiation, and producing finer grey-scale detail in the otherwise white area being observed.

    Note that the above pic is exposed with enough radiation to shine through all internal organs and is stopped only by bone. And note the subtle detail in the overlapping bones in the pelvis.
    Last edited by a moderator: May 4, 2017
  18. Feb 3, 2010 #17
    Hey thanks Dave for taking your time and posting pics to help me :smile: Thanks for the response that was just what I needed. I have one quick clarification in regard to increasing exposure by increasing radiation. Do you mean it is like increasing the amount of light sources thus getting more grey areas as well?
    Last edited by a moderator: May 4, 2017
  19. Feb 4, 2010 #18
    Yes, diffraction does occur, hence the loss in transmission. But, medical x-ray sources are not coherent, so you will not see a diffraction pattern. Secondary scattering can smear out an image, but this is a very small effect.

    Yes, if the transmission is very low through some dense region you wish to image, the image will not be very resolved on the film, due to low contrast. This is fixed by one of two things;

    Using a more brilliant source or cranking up the power. Just like a light bulb can become brighter by pushing more current through the the wire, an x-ray source can become brighter, usuallly by increasing the number of electrons involved in the process.

    Alternatively, the exposure time can be increased, if the object you're imaging is still.
  20. Feb 4, 2010 #19


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    Not increasing the number of light sources, but increasing the brightness of the one light source.
  21. Feb 4, 2010 #20


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    In practical terms diffraction does not occur in medical imaging applications. About the only application of diffraction for x-rays that I am aware of is x-ray crystallography. In medical imaging x-rays are either transmitted, scattered, or absorbed.

    I will ignore scattering for the time being, so any photon which is not transmitted is absorbed. Let's say that object 1 absorbs 50% of the incident photons and object 2 absorbs 25% of the incident photons. So, let's say that the objects are arranged as shown and 1000 photons are transmitted, then 1000 x .5 = 500 will be absorbed by object 1 and then (1000 - 500) x .25 = 125 will be absorbed by object 2, leaving 1000 - 500 - 125 = 375 that strike the board. Now, let's say that object 2 and object 1 are reversed, in this case 1000 x .25 = 250 will be absorbed by object 2 and then (1000 - 250) x .5 = 375 will be absorbed by object 1, leaving 1000 - 250 - 375 = 375 that strike the board. So the total attenuation is the same regardless of the order of the two objects.

    Note that the above analysis is for a single line of tissue, and does not consider the divergence that we were discussing above. The point is that the x-ray measurement is really a projection measurement with no information remaining about position along the projection.
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