X-Ray Interaction with Matter CH9

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.

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.