X-ray Interactions with Matter Stacy Kopso, M.E.d., RT(R)(M) X-rays travel in straight line

Photon Interactions with Matter Purpose: How X-ray photons interact w/ matter(human tissue) Reason: To minimize harm to the patient and produce a quality radiographic image X-rays travel in straight line

Compton Interaction Scattering events that ionize the atom Incident photon(primary beam) interacts w/ an outer- shell electron of a atom and removes the electron The energy from the x-ray photon must exceed the energy required to remove the electron from its orbital shell The remaining energy of the photon undergoes a change in direction and produces a scatter photon There is partial absorption of the incident photon Incident photon interacts w/ an outer- shell electron, producing a scatter photon and recoil electron

Incident photon interacts w/ an outer-shell electron, producing a scatter photon and recoil electron Ionize the atom

Compton Interaction Affects on image quality Decreases image contrast Results in Image fog Affects on patient dose Backscatter- increase pt dose Sidescatter from patient- occupational exposure Influenced by Energy level of the beam (kVp) Thickness of the tissue irradiated Incident photon interacts w/ an outer- shell electron, producing a scatter photon and recoil electron

Photoelectric Interaction Incident photon interacts with an inner (K) shell electron of atom and removes the electron The energy of the incident photon must be equal to or greater than the binding energy of the k shell Total absorption of the incident photon An ejected photoelectron exits the atom The vacancy is filled by an outer shell electron resulting in the emission of characteristic (secondary radiation) photon within the body An increase in contrast= increase in pt dose

Total absorption of the incident photon The atom is ionized

Photoelectric Interaction Affects on image quality Directly impacts contrast(white) Affects on patient dose Primary source of patient radiation exposure Influenced by energy level of the xray beam Density of the tissue Atomic number of the tissue An increase in contrast= increase in pt dose

Classical Interactions AKA: Coherent Scattering or Thomson Scattering Occurs at very low x-ray photon energy levels At 70kVp only 3% of the xrays undergo this Incident x-ray photon interacts with the atom and excites the atom No electron is removed (no ionizaton) The incident photon undergoes a change in direction with no energy loss

Classical Interactions Affects on image quality Degrades image quality Affects on patient dose Contribute only to patient skin exposure

Photon Interactions with Matter Pair Production Does not occur in radiography because the energy levels required exceed the range used in diagnostic x-ray production Radiation therapy/Nuclear medicine X-ray photon interacts with the nucleus of the atom Results in a positron and negatron Photodisintegration High energy levels/Does not occur in radiography Xray photon is absorbed in the nucleus Nucleus is excited and emits a nuclear fragments

Incident photon energy Compton Photoelectric Coherent Interaction Middle & outer shells Inner-shell electrons Excites the atom Incident photon energy Loses 1/3 Loses all energy No energy loss Ionization Must exceed the energy of the orbital electron Must be equal to or greater than the energy of the orbital electron No ionization occurs Pt dose Backscatter-pt Side scatter-Occupational Significant Skin dose Image Quality Fog Decrease contrast White/increase in brightness Minimal effect Incident photon Partial absorption Total absorption No absorption Energy Moderate energy Higher kVp Low energy Lower kVp Much lower energy Interaction produces Scatter photon Secondary electron Photoelectron

Classwork

Differential Absorption Difference in x-ray interaction by various body tissue X-ray image is formed by those x-rays that undergo photoelectric absorption (white) and x-rays that are transmitted (black) bone vs lung Affected by; Thickness of body part (mass density) Type of tissue (atomic number) Photon energy (kVp)

Differential Absorption Transmission xray photons that pass through the body and reach the image receptor Create dark shades of the image Absorption photons that are attenuated (weakened of the beam) by the body and do not reach the image receptor Creates light shade (brightness) Need both for an image Radiopaque absorb xrays Radiolucent lower probability of absorption

As Tissue thickness increases; Attenuation Increases/Decreases Photoelectric Interaction Compton Interaction As atomic # of tissue increases; X-ray transmission As photon energy increases;

Classwork