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33 Cards in this Set
- Front
- Back
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Absorption of x-rays refers to
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the interaction of x-rays with biological tissue
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What material makes for a better absorber of x-rays?
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Heavier elements (calcium)
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In observing a photographic film, what do the darker areas indicate? the lighter areas?
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darker = film has been exposed to more x-rays signifying where more x-rays in the body have been absorbed and vice versa
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Attenuation of an x-ray beam
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the reduction in the beam intensity due to the absorption or scattering of the x-ray photons out of the beam
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Name the 3 processes of photon attenuation
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Photoelectric Effect
Compton Scattering Pair Production |
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Photoelectric Effect
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describes the effect of a high-energy photon being completely absorbed by an electron, the photoelectron is ejected having absorbed the energy; associated with ionisation energy
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Why can photoelectrons be a biological hazard?
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High energy electrons can hit other electrons producing secondary damage from radiation in tissue
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Work Function (ϕ)
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also known as ionization energy - minimum amount of energy required to free an electron from an atom
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Discuss the conservation of energy in the context of the photoelectric effect.
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The energy of the photon will be equal to the amount of energy absorbed to free the electron (work function) plus the energy of the photoelectron in the form of kinetic energy (fast moving)
[ϕ +(mv^2)/2] |
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What happens to the vacancy crerated by the ejected photoelectron?
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A higher energy electron will fall into the lower energy orbital thus producing an additional photon (ie visible light)
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Compton Scattering
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the process by which a high-energy photon transfers only some of it's energy to an electron. The original photon is scattered with lower energy
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Pair Production
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the process by which a very high-energy photon (>1.02 MeV) interacts with the electric field of the nucleus producing an electron and a positron
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What happens to an ejected positron in pair production?
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The positron will interact with an electron to produce two photons (that travel in opposite directions
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Describe the relation between photon energy and predicted form of attenuation (absorption/scattering).
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Photoelectric effects are typical of lower photon energies, compton scattering cover a broad range of energies and pair production exist only for very high photon energies.
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What are two clinical disadvantages to Compton scattering?
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Secondary tissue damage from photoelectrons & scattered x-ray photons may cause poor contrast in images
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Which form of attenuation is most common in x-ray imaging?
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Compton scattering. The original photon only loses only some of it's energy to the photoelectron
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Why do different atoms or materials absorb x-rays by different amounts?
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Efficiency of absorbance is determined by how compatible the element is with the photon. Heavier elements have energy level separations which match better with x-ray photons.
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Why do lighter elements have poor absorbance efficiency?
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The energy level separations are smaller which means the atom cannot absorb the x-ray photons directly.
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Contrast Media
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material put into an organ or tissue to increase the absorption of the target which therefore increases contrast
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What characteristic makes for a good contrast media?
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A higher atomic number (z) will have a higher absorbance of x-rays.
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Give a clinical example of contrast media. [2]
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Iodine inserted into the bloodstream revealing artery and vein structures
Barium is used to study the GI tract. |
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Digital Subtraction Angiography (DSA)
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The result of two images superimposed or masked. One is normal (or mask) x-ray, which is subtracted from a different image with the contrast media.
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How does exponential decay apply to photon absorpption?
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In a specific material, the thicker the material, the more photons will be absorbed. The decreasing intensity theough the material will follow a curve of exponential decay.
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Describe the relationship between thickness (t) of the absorbers and the intensity of the transmitted x-rays (I)
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As thickness increases the intensity decreases exponentially.
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Linear Attenuation Coefficient & the 2 factors it depends on
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How much absorption you will get for a specific material (constant)
μ depends on both the material of the absorber and the energy of the x-ray |
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Lamber-Beer Law
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The measured intensity (I) is equal to the starting intensity (Io) times e^ (exponential decay) linear attenuation coeffecient (μ) times the thickness of the material (t)
I = Ioe^-μt |
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What does μ stand for in the Lambert-Beer Law? What are the units?
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The amount of attenuation per meter of absorber (1/m)
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Which plotted variables would produce an exponential decay?
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X-axis = thickness
Y-axis = Transmitted intensity |
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Why does lead have a greater μ value than paper?
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The denser material will absorb more photons per meter.
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K-edge
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the amount of energy which corresponds to the k-shell ionisation energy and will result in the photoelectric effect raising the mass attenuation coefficient (which increases absorption and contrast)
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Half-Value Layer (HVL) or Half-Value Thichness (HVT)
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the thickness of material required to reduce the transmitted intensity by a factor of 2. [Also equals ln2/μ]
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Mass Attenuation Coefficient (μm)
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used to compare how good individual atoms of different materials are at absorbing the radiation by comparing densities. [μm = μ/density]
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A bigger μ indicates a better absorber. What is the relationship to the half-value thickness?
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Half-vvalue thickness and μ are inversely proprtional. A bigger μ would imply a smaller or thinner HVL.
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