[Physics] Refraction of X-rays


According to Snell's Law, refraction occurs when an electromagnetic wave passes from one medium to another. My question is, how are x-ray images able to be produced if the x-rays refract as they pass from air into the body and then back into air? Sources have stated that a lead grating is used to cover the film so that only the waves which are not refracted may be used to form the image, but why would some waves be refracted when others aren't?

Best Answer

My question is, how are x-ray images able to be produced if the x-rays refract as they pass from air into the body and then back into air?

Short Answer

  • X-ray images give us information about the body by detecting the differential in the number of x-ray photons absorbed by tissues with heavier-atoms (e.g., bone - calcium) versus photons transmitted through tissues with lighter-atoms (e.g., muscle, organs, connective tissue, fat, and fluids - carbon/hydrogen/oxygen/nitrogen).
  • X-ray energies are high, so they refract very little - regardless of the material. The distortion produced by the air-tissue-air refraction differential is small.
  • Grids are used in x-ray imaging because scattered x-rays have an angle parallel to the image detector (film/semiconductor/phosphor). A grid over the image detector with a height to each cell will capture a portion of the scattered photons. The smaller the grid square, and the taller the grid height, the more scattered photons it will capture. But, a small grid size will absorb more useful photons, thus the grid square dimension and height are optimized to maximize information and minimize noise.

More Detail:

X-ray images, with their characteristic light and dark regions, result from the variation in the absorption of different tissues types. (The dark areas on the x-ray image correspond to areas of high x-ray exposure - areas of low absorbance.) Calcium in bones absorbs x-rays more strongly than the lighter atoms in other tissues.

Refraction of x-rays is a very small effect compared to the attenuation of intensity due to the various methods of x-ray photon absorption. The refractive index of x-rays for all materials is very close to 1 for all materials. That is, the phase velocity of an x-ray in tissue, air, and vacuum are almost identical. Therefore, even if the x-ray were directed at a target with a high angle of incidence, the direction of photon propagation as it passes from air to tissue to air produces little direction change. Again, refraction is not the effect utilized to produce x-ray images. While there is some refraction of x-rays as they pass between air and tissue, the difference in index of refraction between air and tissue is small, therefore, refraction has little effect on the tissue density information of the x-ray image.

X-Ray Scattering: X-ray photons interact with the tissue and scatter in several ways. Ideally, for purposes of imaging, any scattered x-ray photon is an absorbed photon. Absorbed photons do not strike the detector, which is interpreted as the presence of tissue. The x-ray image reflects the difference between absorbed photons and unscattered penetration. Scattered photons degrade the image, and a lead grid is used over the detector to absorb the scattered photons.

X-Ray Absorption Modes:

  • 1) Ionization only: An x-ray strikes an orbital electron and the x-ray completely loses its energy to an orbital electron.
  • 2) Compton Scattering: The x-ray photon strikes an orbital electron. The collision conserves energy, and both scatter at an angle from the x-ray beam. The electron is ionized and the photon loses a portion of its energy. Its frequency is reduced, as is its tissue penetrating ability. The scattered lower-energy photon may have insufficient energy to leave the tissue and will be absorbed completely by subsequent collisions with orbital electrons. But, some of the scattered, lower energy X-rays will penetrate the tissue and strike the x-ray detector. Those scattered x-rays will be randomly distributed over the film, producing a background noise, which contributes no useful information, and degrades the signal to noise ratio of the image with its bone and tissue density information.
  • Pair-Production: x-rays of sufficient energy (1.022 MeV) will produce an electron-positron pair if they pass close to the nucleus of an atom. The heavier the nucleus, the greater the distance where this effect will occur. X-rays of this energy are too energetic for imaging human tissue, which is normally done at voltages between 20-150KeV.

Sources have stated that a lead grating is used to cover the film so that only the waves which are not refracted may be used to form the image. But, why would some waves be refracted when others aren't?

  • The phenomenon utilized by x-ray imaging is absorption not refraction. The image is formed by detecting x-ray photons which are Scattered versus Not-Scattered. X-rays are scattered more strongly by heavier nuclei, thus the scattering/non-scattered ratio is a proxy for tissue with heavy nuclei. Scattering is the equivalent of absorption since we only want to record photons which have passed through the tissue without interacting. On x-ray images, the bone appears white because not as many x-rays have penetrated the bone without scattering.
  • As mentioned, some of the x-ray energy is lost to electron collision in Compton Scattering. After scattering, some (maybe most) x-rays will still have sufficient energy to penetrate the tissue. But, after scattering the angle of the x-ray will be off-axis of the beam. The intent of x-ray imaging is to capture only the photons that travel straight through the body without scattering. Bones scatter x-rays more than fleshy tissues, producing an area on the detector with fewer x-ray strikes. Thus, the variation in the shadows cast by bone and dense tissue reflect structural information.
  • Scattered photons land randomly over the image, contributing no useful information and increasing the noise. Scattered x-rays have an angle of trajectory parallel to the plane of the film. This allows for a method of differentiating scattered and non-scattered photons.
  • The photons scattered at a sufficiently large angle (as determined by the geometry of the grid - height of wall vs. area of grid), will strike the elevated walls of the lead grid. The taller the grid walls the larger the number of scattered photons the lead grid will capture before striking the x-ray film/detector.
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