X-ray interaction with matter Flashcards
When interacting with matter, what is the X-rays 4 possibilities
- Pass through unaltered
- Scatter
- Scatter and absorption
- Absorption
What is attenuation
• Attenuation is a reduction in the number of photons (X-rays) within the beam reaching the image receptor
What is the effect of photon absorption on image
- All photons reach the film results in a black shade - this may be in front of the patient or soft tissues
- Partial attenuation will result in grey shades - these give us detail and allow us to see anatomical features and pick up pathology
- Complete attenuation results in white - this is seen in metal restorations that completely absorb photon energy
What is the structure of the atom
• Central nucleus
○ Protons (positive)
○ Neutrons (no charge)
• Orbiting electrons (negative charge)
How do electrons orbit
- Electrons orbit nucleus in ‘shells’ K, L, M
- The maximum number of electrons in the orbit is greater than the outer orbits (2 x n2)
- The K shell electrons have the highest binding energy (it requires more energy to eject the electron from the shell)
- Outer shells have lower binding energies
What is the principle interactions of diagnostic x-rays in tissue
- Photoelectric effect (absorption)
- Compton effect (scatter and absorption)
Describe the photoelectric effect
- X-ray photon interacts with inner shell electron and it deposits all its energy and disappears (process of pure absorption)
- The inner shell electron is ejected with energy into the tissues and will undergo further interactions - it is called a photo electron
- The atom now has a hole in its electron shell giving it a positive charge and it is unstable
- The vacancy is filled by an outer shell electron
- The outer voids are filled by ‘free’ electrons and this results in complete absorption of photon energy: the photon does not reach the film and so there will be no image
What is the effect of the photoelectric absorption
- Results in complete absorption of photon, preventing any interaction with active components of the image receptor
- The image appears white if all photons are involved
- The image appears grey if some photons are not involved
It provides good contrast but results in a higher absorption dose
What is the compton effect
- X-ray photon interacts with loosely bound outer shell electron
- Photon energy is considerably greater than electron binding energy as outer shell electron has a low binding energy
- Electron is ejected taking some of photon energy as kinetic energy: recoil electron
- Remainder of incoming photon energy is deflected or scattered from its original path as a scattered photon
What happens to the excess energy in the original photon in the Compton effect
○ Following collision, the photon has a lower energy (longer wavelength)
○ It is called a scatter photon
○ It undergoes a change of direction and the change in direction is proportional to how much energy is lost
What happens following the scatter events
- The atomic stability is regained by capture of free electron as described before
- The recoil electron can interact with other atoms in tissue
- The scatter photon, dependent on energy and position of bound electron involved, can be involved in more compton or photoelectric interactions
- We are interested in what happens following scatter events as it has implications for radiation dose not just for the patient but other people too
What happens to scattered photons
- The scattered photons can travel in any direction
- The direction of scatter is affected by the energy of scatter photons
- If they have high energy they will go in a forward direction (but not the same as the continuing path of the original photon) and if they have lower energy then they will go relatively backwards
- We can get a whole range of directions between the 2 extremes dependent on energy
What is the probability of the compton effect occurring
- It is proportional to the density of material (electron density)
- It is independent of atomic number
- It is not related to photon energy, although forward scatter is more likely with high energy photons
What is the effect of compton scattered photons
• Scattered photons produced before the image receptor is reached and scattered backwards, do not reach image the image receptor and do not contribute to the image so the image is not degraded
• Scattered photons produced beyond image receptor and scattered back towards it, may reach image receptor producing darkening
○ As their path is randomly altered, they do not contribute useful information to the image
○ Results in fogging of image, reducing contrast and image quality
What is the method of reduction of scatter
• Reduction of area irradiated (collimation) and therefore volume irradiated will reduce the number of scattered photons produced as well as reducing patient dose
○ Less volume of tissue to interact with reduces the amount of scatter radiation
• Lead foil within film packets prevents back scattered photons from oral tissues reaching the film (in addition to absorbing some of the energy in the primary beam)
