interactions of xray with matter and exposure factors Flashcards
types of interaction
scattering (x-ray photon goes off in a different direction)
absorption (x-ray photon lost)
no interaction (passes thru without any interaction)
attenuation coefficient
value to show how good an element attenuates the beam
attenuation processes
coherent - S
compton - S
photoelectric - A
pair production - A
photodisintegration - A
coherent scattering
If the energy of a photon is considerably less than binding energies of orbiting electrons of an atom the photon may be deflected from its path with NO loss in energy
- photon interacts with e- in atom of medium = causes raise in energy
- energy rise not sufficient for e- to become ionised
- e- then returns to og energy level and emits a photon with same energy as incident photon
- emitted photon has different direction to incident therefore scattered
- no energy has been permanently transferred to material
- coherent scattering only significant at energies lower than those normally encountered
photoelectric absorption
- x-ray photon involved with interacting with an orbiting e-
- photon gives up all energy and therefore disappears (absorbed)
- e- ejected from atom
- can only take place of photon energy is =/> the e- binding energy
- vacancy created in inner most shell
- filled by e- in next shell
- quantum jumps produces characteristic radiation
- energy of characteristic photon is = to energy difference between shells
photoelectric effect
causes both attenuation and absorption
- individual photons are removed from beam is attenuation
- energy is imparted to the absorbing medium is absorption
ENERGY ABSORBED:
- kinetic energy of ejected e-
- energy of recoil of absorbing atom
- energy of characteristic radiation
compton scattering
INTERACTION BETWEEN FREE E- AN PHOTON
- when a photon collides with an e-, if the photon energy is > than the e- binding energy, the e- may be considered a free e-
- photon may be scattered in any direction
- partial absorption of photon energy (outgoing photon has lower energy than incoming)
- e- can only travel forward relative to incident photon
- radiation scattered in all directions although with higher energies scattered more forwards
pair production
formation of 2 charged particles- an e- + positron pair- from a single high- energy photon
- process converts energy into mass, and can only occur for photon energies greater than 1.02 Mev
- only significant for high energies and materials with high atomic number
- Photon interacts with electric field of nucleus
- Photon energy is converted into mass
- Any remaining photon energy is passed to particles as kinetic energy
- Absorption, not scattering
photodisintegration
Very high-energy photons (>10MeV) can escape interaction with electrons and nuclear electric field, and reach the nucleus.
- The stability of the nucleus may be disrupted by absorption of the photon energy
- Absorption
exposure factors
mAs (milliampere seconds)
kVp (kilovoltage peak)
mAs
product of current (flowing from cathode filament to anode target) and time (current allowed to flow to anode)
- Correlates with the current applied across the cathode filament circuitry
- Indicative of the quantity of photons in the x-ray beam
Effects Intensity (amount)
“Total energy per second flowing through a unit area”
- Does not effect x-ray quality (penetration)
exposure time (s)
- length of time current is allowed to flo to anode target (facilitate x-ray production
- ideally kept as short as possible to minimise blue from patient motion
- REDUCED TIME= INCREASED mA
mAs proportion
mAs is directly proportional to the number of photons produced
short time and high mA may increase tube loading
kVp
- refers to max voltage applied between the cathode ad anode at exposure
- effects the speed and energy with which e-s cross from cathode to anode
- directly related to the max energy of x-ray photons produced
changing kVp
- affects the quality of photons and quantity of photons produced
the MORE ENERGETIC e- will produce HIGHER ENERGYphotons (QUALITY)
a higher kVp will apply a greater potential difference across cathode and anode
therefore e- will be ACCELERATED FASTER towards anode and can be involved in multiple interactions at the anode= producing MORE PHOTONS
kVp quality
changing kVp will alter max energy of resultant x-ray photons
photon energy is measured in keV
this will influence:
- ability of photons to penetrate matter
- image contrast
- patient dose
ability to penetrate matter
- At higher kVp LESS ATTENUATION will occur overall
- At higher kVp, the relative likelihood of the PHOTOELECTRIC EFFECT IS DECREASED
- At higher kVp, the likelihood of COMPTON SCATTERING IS DECREASED, but its proportion relative to PHOTOELECTRIC ABSORPTION INCREASES as it is LESS DEPENDENT on the photon energy
- no interaction probability increases
effect of kVp on contrast
- A more penetrating beam results in a lower contrast radiograph than one made with an x-ray beam having less penetrating power.
- This is because of the reduced comparative differential attenuation between adjacent tissue
- The more energetic the beam, the less effect different levels of tissue density and atomic number will have in attenuating that beam.
HIGH kVp LOWERS IMAGE CONTRAST
effect of kVp on contrast (low)
- photoelectric effect is more prevalent, this increases contrast
- as kVp increases the proportion of compton scattering interaction increases leading to more image noise and lower contrast
- increased photon energy also means more days can reach the IR contributing to an image
HIGH kVp CAN NCREASE IMAGE DETAIL AND PENETRATION
effects of kVp on patient dose
- at higher kVp there is less chance of absorption and / or scattering within the patient
- therefore higher kVp will actually reduce dose where the mAs has been adjusted accordingly
