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Unit 2 Attenuation HVL and Beam quality - Med Rad 2
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Unit 2 Attenuation HVL and Beam quality - Med Rad 2

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Questions and Answers

What is the result of plotting the dosimeter reading against the attenuator thickness?

  • A linear curve
  • A logarithmic curve
  • An exponential growth curve
  • An exponential decay curve (correct)

    • What is the definition of Half Value Layer (HVL)?

  • The thickness of a material that reduces the beam intensity by one half (correct)
  • The thickness of a material that blocks the beam completely
  • The thickness of a material that reduces the beam intensity by one quarter
  • The thickness of a material that increases the beam intensity by one half

    • At energy levels below 120kV, HVLs are measured in:

  • mm of Al (correct)
  • mm of Cu
  • gm of lead
  • cm of water

    • What is the purpose of the Half Value Layer (HVL)?

    <p>To describe the penetrating ability of an x-ray beam</p> Signup and view all the answers

    What is the thickness of aluminum that would halve the original intensity?

    <p>1.4mm</p> Signup and view all the answers

    At energy levels between 120-400 kV, HVLs are expressed in:

    <p>mm of Cu</p> Signup and view all the answers

    What is the percentage of the beam transmitted through a tin sheet 9mm thick if the HVL of the beam is 3mm of tin?

    <p>12%</p> Signup and view all the answers

    What type of plot is obtained when the logarithm of the number of photons transmitted is plotted against the thickness of the attenuating material?

    <p>Semi-logarithmic plot</p> Signup and view all the answers

    What is the value of e in the natural logarithmic function?

    <p>2.718</p> Signup and view all the answers

    What is the intensity of the beam transmitted through an absorber of thickness x, I(x), in terms of the initial intensity I0?

    <p>I(x) = I0e^{-μx}</p> Signup and view all the answers

    What is the attenuation coefficient μ in terms of the beam intensity I(x) and the initial intensity I0?

    <p>μ = - ln(I(x)/I0)</p> Signup and view all the answers

    What is the value of μ if the relative intensity at a given distance x is 50%?

    <p>0.1386 cm^-1</p> Signup and view all the answers

    What is the thickness of a material that will reduce the incident intensity by a factor of 10?

    <p>TVL</p> Signup and view all the answers

    What happens to the quality of a heterogeneous beam as it penetrates through matter?

    <p>It increases</p> Signup and view all the answers

    What is the relationship between μ and HVL?

    <p>μ = - ln (0.5) / HVL</p> Signup and view all the answers

    What is the purpose of TVL in shielding calculations?

    <p>To determine the thickness of material required to reduce the intensity by 90%</p> Signup and view all the answers

    What happens to the effective photon energy of a polychromatic beam as it penetrates further through an attenuator?

    <p>It increases</p> Signup and view all the answers

    What is the primary purpose of added filtration in diagnostic radiography?

    <p>To remove low energy photons that increase patient radiation dose</p> Signup and view all the answers

    What happens to the HVL of an x-ray beam when added filtration is increased?

    <p>It increases</p> Signup and view all the answers

    What is the effect of beam hardening on the transmission of x-rays through an absorber?

    <p>It decreases the transmission</p> Signup and view all the answers

    What happens to the dose when the HVL of an x-ray beam increases?

    <p>It decreases</p> Signup and view all the answers

    Why is molybdenum (Mo) almost transparent to radiation at photon energies just below 20.002keV?

    <p>Because of its K absorption edge</p> Signup and view all the answers

    What is the effect of the absorption edge in a filter on its transmission?

    <p>It creates a transparent window of transmission at specific energies</p> Signup and view all the answers

    What is the shape of the logarithmic graph of relative dose plotted against attenuator thickness for a polychromatic beam?

    <p>A non-linear curve</p> Signup and view all the answers

    What is the purpose of the Safety Code 35 requirement for x-ray beam filtration?

    <p>To ensure the HVL of the radiation beam meets minimum requirements</p> Signup and view all the answers

    What happens to the x-ray beam when it is filtered by an absorber?

    <p>It becomes harder</p> Signup and view all the answers

    What is the purpose of using a Mo filter in conjunction with a Mo target in mammography?

    <p>To allow most useful x-ray energies to be transmitted with minimal loss of intensity</p> Signup and view all the answers

    What is the energy range of the K-series characteristic X-ray emissions from a Mo target?

    <p>17.5-19.6 keV</p> Signup and view all the answers

    What is the effect of using a Mo target in mammography?

    <p>Improved image quality</p> Signup and view all the answers

    What is the property of Mo that makes it transparent to its own characteristic radiation?

    <p>Emissions always occur at energies just below the energy of its K-absorption edge</p> Signup and view all the answers

    If 6 x 10^7 x-ray photons enter a 2 cm thick layer, what will be the number of x-ray photons transmitted through the layer if μ = 1.42 cm^-1?

    <p>3.5 x 10^6 photons</p> Signup and view all the answers

    What is the attenuation coefficient of a material that absorbs 80% of gamma rays in 5 cm?

    <p>0.322 cm^-1</p> Signup and view all the answers

    What is the effect of absorption edge in a filter?

    <p>It removes lower-energy photons</p> Signup and view all the answers

    What is the purpose of using a Mo target in mammography?

    <p>To produce X-ray beams with a specific energy range</p> Signup and view all the answers

    What is the relationship between the attenuation coefficient and the half-value layer (HVL)?

    <p>The attenuation coefficient is inversely proportional to the HVL</p> Signup and view all the answers

    What is the formula to calculate the intensity of the radiation after passing through a material?

    <p>I(x) = I0e^(-μx)</p> Signup and view all the answers

    Study Notes

    Attenuation, HVL, and Beam Quality

    • Exponential reduction in the number of photons occurs when an x-ray beam penetrates through an attenuator, resulting in an exponential decay curve.
    • The Half Value Layer (HVL) is the thickness of a material that reduces the beam intensity by one half, and it is used to describe the penetrating ability (quality) of an x-ray beam.

    HVL Measurement

    • HVL is measured in mm of Al (at energy levels below 120 kV) and mm of Cu (at energy levels of 120-400 kV).
    • The HVL of a beam of radiation is the thickness of a material that reduces the beam intensity by one half.

    Attenuation Coefficient (μ)

    • μ is the attenuation coefficient, which is a measure of how much a material absorbs or scatters radiation.
    • I(x) = I0e^(-μx), where I(x) is the intensity of the beam after passing through a material of thickness x, I0 is the initial intensity, and μ is the attenuation coefficient.
    • μ is related to HVL by the formula: μ = 0.6931 / HVL.

    Logarithmic Functions

    • Logarithmic functions are used to describe the relationship between the intensity of the beam and the thickness of the attenuator.
    • The natural logarithm (ln) is used to linearize the exponential decay curve.

    Homogenous vs. Heterogeneous Beams

    • Homogenous beams have a single energy, and their intensity reduces exponentially as they pass through a material, but their quality remains the same.
    • Heterogeneous beams have a range of energies, and their intensity and quality change as they pass through a material, resulting in beam hardening.

    Filtration

    • Filtration is used to remove low-energy photons from the beam, which reduces patient dose and increases the effective energy of the beam.
    • Added filtration increases the HVL of an x-ray beam, making it more penetrating.

    Beam Hardening

    • Beam hardening occurs when a polychromatic beam passes through a material, and the low-energy photons are absorbed more rapidly than the high-energy photons, resulting in an increase in the effective energy of the beam.

    TVL (Tenth Value Layer)

    • TVL is the thickness of a material that reduces the incident intensity by a factor of 10 (90% attenuation, 10% transmission).
    • TVL is used for shielding calculations in which barriers can be specified in the number of TVLs.

    Polychromatic X-ray Spectrum Attenuation

    • The attenuation of a polychromatic beam through an absorber does not follow a precise exponential curve, but instead, the intensity of the beam decreases more slowly due to the range of energies present.

    Effect of Absorption Edge

    • The absorption edge of a filter material can cause unusual behavior in the beam, resulting in a "transparent window" of transmission at certain energies.
    • Mo (molybdenum) is a common filter material used in mammography, and its absorption edge is at 20.002 keV.

    Attenuation Coefficient and HVL Calculation

    • The attenuation coefficient (μ) can be calculated from the HVL using the formula: μ = 0.6931 / HVL.
    • The HVL can be calculated from the attenuation coefficient (μ) using the formula: HVL = 0.6931 / μ.

    Practice Questions

    • Questions 1-2: Calculate the number of x-ray photons transmitted through a layer of material given the initial intensity and attenuation coefficient.
    • Question 3: Calculate the attenuation coefficient of a material given the transmission and absorption of gamma rays.

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    Description

    Test your understanding of radiation attenuation, HVL, and beam quality in medical imaging. This quiz covers exponential reduction of photons, x-ray beam penetration, and dosimeter readings.

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