X-ray interaction with matter Flashcards
basic production of radiographic image
source of X-rays - X-ray machine
- production of X-rays
object - teeth and jaws
- interaction of X-rays with matter
X-ray photons pass from tube, and some through patient to reach image receptor (IR)
Interaction with different tissues alters number of photons exiting patient
Variation in numbers of photons reaching IR produces radiographic appearance of different tissues
4 options for X-ray photons traversing image
pass through unaltered
change direction with no energy loss (scatter)
change direction losing energy (scatter and absorption)
be stopped, depositing all energy within tissue (absorption)
scatter
X-ray photons change direction with no energy loss
scatter and absorption
X-ray photons change direction and losing energy
absorption
x-ray photons be stopped, deposition all energy in tissue
effect on photon absorption on image
All photons reach film = black (e.g. air)
Partial attenuation = grey
Complete attenuation = white (e.g. amalgam)
structure of atoms
Central nucleus
- Protons (positive charge)
- Neutrons (no charge)
Orbiting electrons (negative charge)
atomic number Z
number of protons
Equivalent to no. electrons in neutral atom
atomic mass A
number of protons + number of neutrons
orbiting electrons
electrons orbit nucleus in “shells” K,L,M,….
maximum number of electrons in orbit greater in outer orbits (2 x n^2)
K shell electrons have highest binding energy (requires more energy to eject electron from shell)
outer shells have lower binding energies
2 principles of interactions of diagnostic X-rays in tissue
Photoelectric effect - Absorption
Compton effect – Scatter, and absorption
photoelectric effect
energy from X-ray photon is transferred to object
X-ray photon interacts with inner shell electron (usually K shell)
photon has energy just higher than the binding energy of electron
X-ray photon disappears
most of photon energy used to overcome binding energy of electron, remainder gives electron kinetic energy
electron is ejected (photoelectron)
Atom has “hole” in electron shell: +ve charge
- Not balanced
Ionised atom is unstable
Electron drops from outer shell, filling void
Difference in energy between 2 levels is emitted as light/heat (characteristic radiation)
Outer voids filled by “free” electrons
Results in complete absorption of photon energy: photon does not reach film
effect of photoelectric absorption
Results in complete absorption of photon, preventing any interaction with active component of image receptor:
image appears white if all photons involved,
grey if some photons not involved
photoelectric occurence proportional to
Atomic Number3 (Z^3)
1/photon energy3 (1/kV^3)
Density of material
- Noticeable differences between cubes
so Easy to differentiate materials
compton effect of X-ray interaction
X-ray photon interacts with loosely bound outer shell electron (not passing through, lower binding energy so lots of energy left in photon)
Photon energy considerably greater than electron binding energy
Electron is ejected taking some of photon energy as kinetic energy: recoil electron
Atom is then positively charged
what happens to excess energy in original photon in Compton effect?
Following collision, photon has lower energy (longer wavelength)
Called a scatter photon
Undergoes a change of direction
- Related to how much energy it has lost
Different to incoming energy direction (at least slightly different)
following scatter effect (in compton effect)
Atomic stability is regained by capture of free electron as described before (see diagram in Photoelectric effect 2)
Recoil electron can interact with other atoms in tissue
Scatter photon, dependent on energy and position of bound electron involved, can be involved in more Compton or photoelectric interactions
what happens to scattered photons
Scattered photons can travel in any direction
Direction of scatter is affected by energy of scatter photon:
- high energy -> forward direction
- backward direction
probability of compton effect occurring
Proportional to density of material (electron density)
- Dense packed electrons – more likely to happen
Independent of atomic number
Not related to photon energy, although forward scatter more likely with high energy photons
effect of compoton scattered photons
Scattered photons produced before the image receptor is reached, and scattered backwards, do not reach image receptor and do not contribute to the image
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
reduction of scatter - methods
Reduction of area irradiated (collimation), and therefore volume irradiated, will reduce the number of scattered photons produced as well as reducing patient dose
Lead foil within film packet prevents back scattered photons from oral tissues reaching film (in addition to absorbing some of the energy in the primary beam)
- Not used with digital receptors
absorptions of photons more likely if: (3)
Object traversed has high atomic number
Object traversed is thicker
Photon energy is lower
radiographic contrast
Difference in density of light and dark areas of radiograph
Image showing both light and dark areas with clear borders has high contrast
Contrast is greatest when difference in absorption by adjacent tissues is greatest
- E.g. amalgam and tooth (completely different material) clearer than caries and healthy tooth (same material but poorer state)
effect of interaction on dose
Photoelectric absorption results in deposition of all photon energy within tissue:
- Increased patient dose but necessary for image quality
Compton scatter results in deposition of some photon energy within tissue:
- Adds to patient dose but does not give useful information
- May increase dose to operators/ people in the vicinity (stand far away should be find)
photoelectric interaction effect on dose
results in deposition of all photon energy within tissue:
- Increased patient dose but necessary for image quality
compton interaction effect on dose
deposition of some photon energy within tissue:
- Adds to patient dose but does not give useful information
- May increase dose to operators/ people in the vicinity (stand far away should be find)
effect of low kVp on image quality and pt dose
Low tube potential difference (kVp) produces lower energy photons
Photoelectric interactions are increased
Contrast between different tissues increases BUT Dose absorbed by patient is increased
effect of high kVp on image quality and pt dose
High tube kVp produces higher energy photons
Photoelectric interactions are reduced
Contrast is reduced but Dose absorbed by patient is reduced
No point reducing dose so much that image is of no diagnostic use
- Wasted dose
how to choose kVp
compromise between diagnostic quality of the image and dose
Currently recommended that dental units have kVp 60-70kV
At GDH&S we currently use 70kV for all intra-orals.
