Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why do X-rays only show bone?

  1. Aug 28, 2010 #1
    Does it have to do with the density of the bone, like are x rays only absorbed by dense matter?
     
  2. jcsd
  3. Aug 28, 2010 #2

    Danger

    User Avatar
    Gold Member

    The intensity of the image is directly proportional to the density of the medium. X-rays show a lot of things, such as tumours, cysts, tendons, blood vessels, etc.. to various degrees. In a picture of a surgically repaired break, for instance, the screws and plates show up brighter than the bone, and the bone in turn brighter than the surrounding tissues.
     
  4. Aug 29, 2010 #3
    If you select the x-ray energy spectrum to be just above the calcium K x-ray edge, the contrast for calcium is maximized relative to soft tissue. See plot and table, and sharp increase in x-ray attenuation between 4 and 5 KeV in

    http://physics.nist.gov/PhysRefData/XrayMassCoef/ElemTab/z20.html

    Very roughly the K-shell binding energy in calcium (Z=20) is

    E = 13.6Z·2 eV =202· 13.6 eV = ~5440 eV
    Thus the L-shell to K-shell transition energy is ~5440(1-1/22) = ~4080 eV.

    Using differential x-ray energy subtraction measurements will enhance bone (calcium) contrast even more.

    Bob S
     
  5. Aug 29, 2010 #4

    Danger

    User Avatar
    Gold Member

    Cool, Bob. I didn't know that you can "tune" them.
     
  6. Aug 29, 2010 #5
    Oh yeah, just adjust the voltage on the x-ray tube anode (actually I think the cathode voltage is adjusted). The x-ray spectrum includes both a continuous spectrum (bremsstrahlung) and characteristic x-rays from the anode material.

    Bob S
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook