3 - X-ray interaction with matter Flashcards
(31 cards)
What are the different types of interactions photons can have with matter?
- transmission
- absorption
- scatter
Describe a transmission photon interaction.
Photon passes through matter unaltered
Describe an absorption photon interaction.
- photon is stopped by the matter
- energy is fulled absorbed into matter
- photon ceases to exist
Describe a scatter photon interaction.
- photon changes direction
- partial absorption of energy into matter
- can then be transmitted, absorbed or scattered again
Define attenuation.
Reduction in intensity of x-ray beam
Define x-ray beam intensity.
Quantity of photon energy passing through a cross-sectional area of the beam per unit time
How is intensity related to energy and number of photons?
- proportional
- increases in current (number of photons) and increases in potential difference (photon energy) increase the intensity of beam
How does minimal attenuation present on a radiographic image?
Black
How does partial attenuation present on a radiographic image?
Grey
How does total attenuation present on a radiographic image?
White
What are the different types of attenuation interactions?
- photoelectric effect
- Compton effect
Describe the photoelectric effect.
- complete absorption
- photon in x-ray beam interacts with inner shell electron in patient, resulting in absorption of photon
- photoelectron is created
- electrons from outer shells move down, emitting energy in the form of light and heat
When does the photoelectric effect typically happen in human tissues?
- when energy of incoming photon is equal to, or just greater than the binding energy
- with lower energy photons
- human tissues have lower binding energies so can be displaced relatively easily
How does the photoelectric effect affect the radiographic image?
Lighter area on the image, as the photons don’t reach the receptor
How is contrast produced in radiographic images using the photoelectric effect?
- photoelectric effect is proportional to the atomic number cubed (Z^3)
- small steps in atomic number result is large changes in absorption = contrast
Describe the Compton effect.
- partial absorption and scatter
- photons in x-ray interact with outer shell electron in subject
- creates a recoil electron and the photon is scattered
When does the Compton effect happen in human tissues?
- when the incoming photon has much greater energy than the binding energy of the electron
- higher energy photons
Describe the recoil electron.
- some photon energy is transferred to the electron to overcome binding energy and provide kinetic energy
- electron is ejected and can ionise other tissues
How does energy affect the direction of photon scatter?
Higher = forward scatter
Lower = back scatter
How does back scatter affect the radiographic image?
No effect
How does forward scatter affect the radiographic image?
- photons scattered obliquely forward may interact with the wrong area of the receptor
- creates fogging of image or reduced contrast
What affects the probability of the Compton effect happening?
Proportional to the density of the material being irradiated
Describe the effect of collimation on scatter.
- reduced surface area reduces the volume of irradiated tissue
- less scattered photons are produced
- less scatter interacting with receptor which results in better contrast on the image
How does the photoelectric effect affect the dose to the patient?
All energy deposited into tissue but necessary for image formation
