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Questions and Answers
What are the two interactions important for making an x-ray image?
What are the two interactions important for making an x-ray image?
What factor significantly influences the likelihood of a photoelectric interaction occurring with an x-ray?
What factor significantly influences the likelihood of a photoelectric interaction occurring with an x-ray?
Which interaction involves no energy transfer and does not cause ionization?
Which interaction involves no energy transfer and does not cause ionization?
What is produced during the pair production process when an incident x-ray has sufficient energy?
What is produced during the pair production process when an incident x-ray has sufficient energy?
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In Compton scattering, what happens to the incident x-ray?
In Compton scattering, what happens to the incident x-ray?
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What is the minimum energy required for pair production to occur?
What is the minimum energy required for pair production to occur?
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Which statement about coherent scattering is true?
Which statement about coherent scattering is true?
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What is the result of a Compton scattering interaction?
What is the result of a Compton scattering interaction?
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In which imaging technique is pair production particularly important?
In which imaging technique is pair production particularly important?
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Which process involves an x-ray being absorbed directly by the nucleus?
Which process involves an x-ray being absorbed directly by the nucleus?
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Which process is primarily associated with low-energy x-rays and does not contribute significantly to medical imaging?
Which process is primarily associated with low-energy x-rays and does not contribute significantly to medical imaging?
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What happens to the nucleus when it absorbs an x-ray during photodisintegration?
What happens to the nucleus when it absorbs an x-ray during photodisintegration?
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During Compton scattering, how is the energy of the scattered x-ray determined?
During Compton scattering, how is the energy of the scattered x-ray determined?
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What is the primary outcome of Compton scattering for the scattered x-ray?
What is the primary outcome of Compton scattering for the scattered x-ray?
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What happens to the excess energy from an incident x-ray after pair production occurs?
What happens to the excess energy from an incident x-ray after pair production occurs?
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What is the consequence of the positron uniting with a free electron during pair production?
What is the consequence of the positron uniting with a free electron during pair production?
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What happens to the probability of Compton scattering as x-ray energy increases?
What happens to the probability of Compton scattering as x-ray energy increases?
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Which statement is true regarding Compton scattering and atomic number?
Which statement is true regarding Compton scattering and atomic number?
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What type of radiation exposure hazard can occur during fluoroscopy?
What type of radiation exposure hazard can occur during fluoroscopy?
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What is a photoelectron?
What is a photoelectron?
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What characterizes the x-rays produced after a photoelectric interaction?
What characterizes the x-rays produced after a photoelectric interaction?
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The probability of a given x-ray undergoing a photoelectric effect is dependent on which factors?
The probability of a given x-ray undergoing a photoelectric effect is dependent on which factors?
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What is required for a photoelectric interaction to occur?
What is required for a photoelectric interaction to occur?
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How does the probability of undergoing a photoelectric effect change with photon energy?
How does the probability of undergoing a photoelectric effect change with photon energy?
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Study Notes
X-Ray Interaction with Matter
- X-rays interact with matter in five ways: coherent scattering, Compton scattering, photoelectric effect, pair production, and photodisintegration.
- Only Compton scattering and photoelectric effect are crucial for creating x-ray images.
- The conditions governing these two interactions control differential absorption, which affects image contrast.
Coherent Scattering
- X-rays with energies below approximately 10 keV interact with matter through coherent scattering (also called classical or Thompson scattering).
- J.J. Thompson first described coherent scattering.
- Coherent scattering involves an interaction between low-energy x-rays and atoms.
- The x-ray changes direction slightly but loses no energy; the wavelength of the scattered x-ray equals that of the incident x-ray.
- In coherent scattering, the incident x-ray excites a target atom.
- The target atom quickly releases the excess energy as a scattered x-ray with a wavelength equal to the incident x-ray.
- The direction of the scattered x-ray differs from that of the incident x-ray.
- Coherent scattering only changes the direction of the x-ray, not its energy.
- There's no energy transfer and no ionization.
- Most coherently scattered x-rays travel in the forward direction.
- Coherent scattering contributes minimally to the medical image because it involves low-energy x-rays.
Compton Scattering
- X-rays in the diagnostic range interact with outer-shell electrons, causing scattering and energy reduction. This ionization of the atom is called Compton scattering.
- The scattered x-ray changes direction and loses energy.
- The energy of the Compton scattered x-ray equals the difference between the energy of the incident x-ray and the energy of the ejected Compton electron.
- Most of the energy is split between the scattered x-ray and the Compton electron.
- The scattered x-rays do not contribute useful information on the radiograph.
- In general, the probability of Compton scattering decreases as x-ray energy increases.
- The Compton scattering probability does not depend on the atomic number of the atom involved.
Photoelectric Effect
- X-rays in the diagnostic range can undergo ionizing interactions with inner-shell electrons, resulting in complete x-ray absorption.
- The ejected electron is called a photoelectron.
- The photoelectron has kinetic energy equal to the difference between the incident x-ray's energy and the electron's binding energy.
- Photoelectric effect is total x-ray absorption.
- Characteristic x-rays result from the photoelectric interaction, similar to those in Chapter 7.
- Ejection of a K-shell photoelectron creates a K-shell vacancy.
- An outer-shell electron typically from the L shell fills the vacancy, releasing characteristic x-rays.
- These characteristic x-rays contribute nothing to the diagnostic image and don't penetrate the image receptor.
- The probability of photoelectric interaction depends on both x-ray energy and the target atom's atomic number.
- A photoelectric interaction can only take place if the incident x-ray has energy equal to or higher than the electron binding energy.
- The probability that a given x-ray will undergo photoelectric interaction decreases with the third power of the x-ray energy.
- The probability of photoelectric interaction is directly proportional to the third power of the atomic number of the absorber.
- Photoelectric interactions are more likely to occur with high-Z atoms compared to low-Z atoms.
Pair Production
- If an incident x-ray has sufficient energy, it can escape electron interaction and come close to the nucleus.
- The interaction between the x-ray and the nuclear field causes the x-ray to disappear.
- Two electrons appear: a positively charged positron and a negatively charged electron.
- This process is known as pair production.
- Pair production is unimportant in x-ray imaging but is crucial for positron emission tomography (PET).
- Pair production requires x-rays with energies exceeding 1.02 MeV.
- Any excess energy above 1.02 MeV is distributed equally as kinetic energy to the positron and electron.
- The resulting electron loses energy through excitation and ionization before filling a vacancy in an atomic orbital.
- The positron will combine with a free electron. This annihilation process converts their mass into energy.
Photodisintegration
- X-rays with energy greater than approximately 10 MeV can escape interaction with electrons and be absorbed directly by the nucleus.
- When this occurs, the nucleus gets excited and emits a nucleon or other nuclear fragments.
- This process is known as photodisintegration.
- Photodisintegration has no significance in diagnostic imaging.
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Description
This quiz covers the various ways X-rays interact with matter, including coherent scattering, Compton scattering, the photoelectric effect, pair production, and photodisintegration. It highlights the importance of Compton scattering and the photoelectric effect for X-ray imaging and how they influence image contrast through differential absorption.
