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Photon Attenuation Basics
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Photon Attenuation Basics

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@ExtraordinaryCactus

Questions and Answers

Which of the following increases attenuation? (Select all that apply)

  • Decreasing the thickness of the material
  • Increasing the energy of photons
  • Increasing the density of the material (correct)
  • Decreasing the atomic number of the material

    • Lead attenuates less than tissue.

    False

    What are the two coefficients that characterize the attenuation of a material?

    linear attenuation coefficient and mass attenuation coefficient

    Attenuation is caused by _____ and scattering.

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

    Match the interaction mechanisms with their relevance to diagnostic imaging:

    <p>Coherent scattering = Not relevant in diagnostic imaging Compton scattering = Occurs in diagnostic imaging Photoelectric absorption = Occurs in diagnostic imaging Pair production = Does not occur in diagnostic imaging</p> Signup and view all the answers

    What is the process called when a high energy photon interacts with a nucleus to create an electron and a positron?

    <p>Pair production</p> Signup and view all the answers

    Photodisintegration occurs within the diagnostic x-ray range.

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

    What is the primary characteristic of phosphor materials?

    <p>High conversion efficiency and ability to emit light.</p> Signup and view all the answers

    In pair production, the energy of the photon is converted into two _______ photons during the annihilation process.

    <p>511 keV</p> Signup and view all the answers

    Match the terms related to radiation with their descriptions:

    <p>Spatial Resolution = The ability to distinguish small details in an image Conversion Efficiency = The effectiveness of a phosphor in converting X-ray photons into visible light Luminescence = The emission of light from a substance that has absorbed energy Screen Speed = The speed at which the film reacts to the light emitted by the screen</p> Signup and view all the answers

    What occurs during coherent scattering?

    <p>The electron cloud vibrates and re-emits a photon of the same energy.</p> Signup and view all the answers

    Compton scattering involves the ejection of tightly held electrons from an atom.

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

    What is the term for the electron that is ejected during Compton scattering?

    <p>Compton or recoil electron</p> Signup and view all the answers

    During Compton scattering, energy is divided between the scattered photon, the binding energy of the electron, and the __________ energy of the recoil electron.

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

    What effect does scattering have on diagnostic imaging?

    <p>Creates film fog and decreases image contrast</p> Signup and view all the answers

    What is the significance of the incident photon's energy in scattering interactions?

    <p>It determines the energy of the scattered photon.</p> Signup and view all the answers

    Match the following types of scattering to their characteristics:

    <p>Coherent Scattering = No energy change, re-emits photon of the same energy Compton Scattering = Ejects loosely held electron, reduces energy of outgoing photon</p> Signup and view all the answers

    The formula for the energy of a photon is represented as E = hf or E = __________.

    <p>hc / lambda</p> Signup and view all the answers

    What happens to the energy of the scattered photon when the incident photon has higher energy?

    <p>The energy of the scattered photon increases.</p> Signup and view all the answers

    Backscatter radiation describes x-rays scattered in the direction opposite to the incident photon.

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

    What is the equation relating the change in wavelength between incident and scattered photons?

    <p>λ (scat) - λ (incident) = 2.40 x 10^-12 (1 - cosθ)</p> Signup and view all the answers

    The probability of photon undergoing Compton scattering is _______________ to photon energy.

    <p>inversely proportional</p> Signup and view all the answers

    Match the following terms with their descriptions:

    <p>Compton Scattering = Photon energy decreases as it transfers energy to an electron. Photoelectric Absorption = Photon disappears as it ejects an inner-shell electron. Backscatter Radiation = X-rays scattered in the direction of the incident photon. Characteristic Radiation = Produced when an outer electron fills a vacancy left by an ejected electron.</p> Signup and view all the answers

    At which angle is energy not transferred to the scattered photon?

    <p>0 degrees</p> Signup and view all the answers

    The probability of photoelectric absorption increases as the atomic number of a material increases.

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

    What must be true for photoelectric absorption to occur?

    <p>The incident photon must have energy equal to or greater than the binding energy of the electron.</p> Signup and view all the answers

    Most photons scatter in a ___________ direction as photon energy increases.

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

    What defines the maximum energy transfer during scattering?

    <p>The angle of 180 degrees</p> Signup and view all the answers

    What are the two main types of x-ray interactions that are important in diagnostic radiography?

    <p>Photoelectric absorption and Compton scatter</p> Signup and view all the answers

    Increasing kVp always results in improved image contrast.

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

    What is the primary purpose of intensifying screens in x-ray imaging?

    <p>To convert x-rays into visible light and reduce patient dose.</p> Signup and view all the answers

    The protective coating of the intensifying screens is made of ______.

    <p>cellulose acetate</p> Signup and view all the answers

    Match the type of luminescence with its description:

    <p>Fluorescence = Instantaneous light emission Phosphorescence = Delayed light emission</p> Signup and view all the answers

    What happens to image noise when higher kVp and faster screens are used?

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

    A higher atomic number in phosphors correlates with a lower probability of x-ray absorption.

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

    What is spatial resolution measured by in x-ray imaging?

    <p>The number of line pairs per millimetre (lp/mm).</p> Signup and view all the answers

    As the thickness of the body part imaged increases, the ______ radiation also increases.

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

    What determines the screen speed in x-ray imaging?

    <p>The conversion efficiency of x-rays to light</p> Signup and view all the answers

    Compton scattered x-rays provide useful information for image formation.

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

    What is the formula for calculating the Intensification Factor (IF)?

    <p>IF = exposure required without screen / exposure required with screen</p> Signup and view all the answers

    The reflective layer of the intensifying screen is made from ______.

    <p>magnesium oxide or titanium dioxide</p> Signup and view all the answers

    Match the following factors affecting scatter radiation to their descriptions:

    <p>kVp = Increases probability of scattered photons X-ray field size = Larger field size results in more scatter Tissue thickness = Thicker tissue leads to more interactions</p> Signup and view all the answers

    Study Notes

    Attenuation

    • Attenuation refers to the reduction in intensity of photons due to absorption and scattering.
    • Factors increasing attenuation include:
      • Increased material thickness.
      • Higher material density, e.g., lead attenuates more than tissue.
      • Elevated atomic number, leading to higher density.
      • Lower photon energy, as higher energy photons can pass without interaction.
    • Characterized by linear attenuation coefficient (μ) and mass attenuation coefficient (μ/ρ).

    Basic Interaction Mechanisms

    • Coherent scattering occurs for low-energy photons (<10 keV) with no energy loss; irrelevant for diagnostic imaging.
    • Compton scattering involves an x-ray photon interacting with outer-shell electrons, leading to ionization and energy loss.
    • Photoelectric absorption occurs when an x-ray photon fully transfers energy to an inner-shell electron, ejecting it and ionizing the atom.
    • Pair production creates an electron-positron pair from high-energy photons (≥1022 keV); not applicable in the diagnostic range.
    • Photodisintegration strikes nuclei with high-energy photons (>10 MeV) to emit nuclear fragments; irrelevant for x-ray diagnostics.

    Compton Scattering

    • Involves an x-ray photon ejecting a loosely held outer-shell electron, forming a Compton or recoil electron.
    • The scattered photon has lower energy and longer wavelength; energy distribution: E(in) = E(scat) + E(b) + E(KE).
    • Recoil electrons are absorbed within 1-2 mm of soft tissue and can contribute significantly to patient dose and film fog.

    Energy Transfer and Scatter Angle

    • Energy conservation principle applies during scattering; energy of incident photon equals energy of the scattered photon plus binding and kinetic energies.
    • Scattering angle affects energy change; maximum energy transfer occurs at 180 degrees, while no energy transfer occurs at 0 degrees.

    Backscatter Radiation

    • Refers to x-rays scattered in the original direction of the incident photon; most photons scatter forward, especially at higher energies.

    Probability of Scattering

    • Probability of Compton scattering is higher with loose electron density and inversely proportional to photon energy, decreasing as energy increases.

    Photoelectric Absorption

    • Occurs when an x-ray photon interacts with an inner-shell electron, where the photon transfers all its energy.
    • Photoelectric absorption requires incident photon energy to be equal to or greater than the binding energy, e.g., K-shell binding energy for oxygen is 0.53 keV.
    • High atomic number materials increase absorption probability, whereas higher photon energy decreases it.

    Differential Absorption

    • Diagnostic images rely on the difference between x-rays absorbed (photoelectric) and transmitted.
    • Compton scattering leads to film fog and reduced image contrast, more significant at lower x-ray energies.

    Intensifying Screens

    • Devices converting x-rays into visible light to reduce patient dose; consists of various layers:
      • Base: polyester for strength and flexibility.
      • Reflective layer: enhances light direction towards the film.
      • Phosphor layer: absorbs x-rays and emits visible light.
      • Protective coating: prevents damage and allows light passage.

    Luminescence

    • The phenomenon of light emission due to electron excitation; two types are fluorescence (instant) and phosphorescence (delayed).

    Phosphors Characteristics

    • Essential characteristics include high atomic number, conversion efficiency, appropriate spectral emission, and minimal afterglow.

    Screen Speed and Image Quality

    • Screen speed ranges from 100 (slow, detailed) to 1200 (very fast), influencing fluoroscopic efficiency and patient dose.
    • Higher screen speed tends to generate image noise and reduced spatial resolution.

    Beam Restricting Devices

    • Factors affecting scattered radiation intensity:
      • kVp: Higher energy decreases the likelihood of photoelectric interactions, leading to increased scatter.
      • X-ray field size: Larger fields increase scatter due to more potential interactions.
      • Tissue thickness: Thicker tissues lead to more scatter.

    Overall Concepts

    • A balance of kVp is necessary to maintain adequate contrast while minimizing patient dose.
    • Effective imaging practices require understanding relationships between absorption interactions, screen dynamics, and radiation safety.

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    Description

    This quiz covers the fundamentals of photon attenuation, focusing on how absorption and scattering contribute to the process. It explores factors such as material thickness, density, atomic number, and photon energy in relation to attenuation. Test your understanding of these concepts and their implications in different materials.

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