5 & 6 X-Ray Contrast, Radiation Dose, & Image Artifacts

[[Intro[[

  • All photons impart energy to the object
    • All photons deliver a dose of energy
  • As photons interact with matter, they are absorbed, scattered, or transmitted
  • Exposure: The amount if radiation in air measured by radiation monitors
    • Exposure is measured in roentgen (R) and coulomb/kg
  • Absorbed dose: The dose of radiation (energy) you are delivering to the tissue, measured by the energy absorbed
  • The absorbed dose is measured in units of gray (Gy)
    • 1 Gy = 1 J absorbed by 1kg of tissue
    • Used to be rad (radiation absorbed dose) which is 0.01 Gy
  • Math
    • 1 J = 6.2 x 10^15 keV
    • All can be done with just units

Biological Effect of Dose

  • Risk to different tissue is dependant on tissue type and the type of radiation
  • Dose equivalent: A measure of the biological damage to living tissue as a result of the absorbed dose; the biological dose that delivers the same degree of risk to a tissue regardless of the radiation type
    • Dose equivalent is measured in Severt (Sv) which is Gy times quality factor (f)
    • Usually in mSV (small numbers)
  • Effective dose: An estimate ofo the stochastic effect that a non-uniform radiation dose has on the whole body; weighted sum of dose equivalent by all organs
    • Sum of each organ: weighting factor times (absorbed dose of the organ times f)

Image Contrast

  • Contrast: The difference between foreground and background on an image
  • In x-rays, contrast represents the difference in attenuation properties (μ) of materials along a path
  • When μ increases, image brightness increases
  • When the change in μ increases, image contrast increases
  • Generally, μ decreases with energy, but it depends on the material
  • Iodine injections have a higher attenuation coefficient and are used to increase contrast in imaging
  • Increasing the energy decreases the change in attenuation coefficient, which can also be used to improve contrast (so that more particles hit your receptor)

Beam Intensity

  • Beam intensity: The rate of change of the number of photons per unit area, represented by I
  • I = (number photons/area)/change in time
  • I(x) = I0 * e^-μx
  • We will use “I” (intensity) interchangeably with “N” (number of photons), especially as a relative measure, however, keep in mind that they are representing different things
  • Beams are polychromatic or polyenergetic (can be used interchangably)

Spectrum Effective Energy

  • Effective energy is the weighted average of the spectrum energies (needed because x-rays are polychromatic)
  • Because effective energy is different for different materials, we pick the μ that has the biggest difference between materials
    • Attenuation vs energy. We want the biggest difference between the blue and green lines, so we pick the lower attenuation (around 45 keV)
  • Filters: Something used to reshape the radiation spectrum to eliminate energies that don’t contribute to the image, but do deliver dose
  • Removing photons from the spectrum depends on their energy, the filter material, and the filter’s path length
    • For math, remember you cannot use the same μ for different energies

[[System & Beam Geometry[[

  • Source-object distance (SOD): How far is the sample from the source?
  • Source-image distance (SID): How far is the source from the detector image?
  • Object size (h)
  • Image size (l)
  • Magnification factor: How much larger is the image from the actual sample?
  • Parallel beam geometry is the ideal geometry, stating that all the beams move parallel to one another
    • Parallel beam geometry has some limitations, because the source size must be as large as the largest object imaged, it’s inconsisent with particle physics, and complex
    • Parallel beam geometry suggests simple math and no need to calculate the true size of the object from the image
    • No magnification effect from a parallel beam
  • Fan (divergent) beam geometry (non-ideal) suggests that the beams come out from the source in a cone-like shape
    • Fan beam geometry have a more compact, practical design, and is more consistent with physics
    • Fan beam geometry requires magnification factor to be taken into account, as well as depth dependent magnification (objects must be the same distance from tube)
  • To reduce the radiation sent to parts of the body that we don’t want to image, we use a tube-side collimator, which focuses beam on the center of the object, making the beams closer to parallel
  • To reduce non-parallel beams and scattered photons, we use a detector-side collimator, or more commonly a radiographic grid, to absorb any scattered photons that are trying to reach the detector that would otherwise create noise

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