3.3 - X-ray Interaction with Matter Flashcards
What do we need to produce radiographic images?
we need a source of x-rays which are directed at pt and then when X-Ray reaches pt there is interaction (If none we dont get an image) - must be variable interaction with diff tissues.
Where do x-ray photons pass from and to?
tube and some go through the pt to reach the image receptor
What can happen to x-ray photons transversing tissue? 4
• Pass through unaltered – NO ENERGY LOSS
• Change direction with no energy loss – SCATTER
o The photon comes out, exposes the image receptor but does not create a useful image
• Change direction losing energy – SCATTER AND ABSORPTION
o This gives a radiation dose to the patient
• Be stopped depositing all energy within the tissue – ABSORPTION
o All energy transferred to the material and x-ray doesn’t come out other end
What is attenuation?
reduction in the number of photons (x-rays) within the beam
Why does attenuation occur?
- It occurs as a result of absorption and scatter
What affect the number of photons reaching the image receptor?
attenuation
What colour if all photons reach the film?
black as no material to interact with
What colour if there is partial attenuation?
Grey
What colour if there is complete attenuation?
white
What is atom composed of?
Nucleus - protons and neutrons (+ and 0) both with Amu of 1
orbiting electrons in shells which have charge of -1 and mass of 0
What is the atomic number?
No of protons an atom has (Z)
What is the atomic mass?
No of protons and neutrons (A)
Where are electrons kept?
In electron shells
k - 2 l - 8 m - 18 n - 32 o
2 nsquared
What holds electrons in place?
Electrostatic force (attraction of the positive nucleus)
What is binding energy?
binding energy is the smallest amount of energy required to remove a particle from a system of particles
What electrons have highest binding energy?
Those closest to the nucleus
What are he types of x-ray and electron interactions that occur inside the tube?
Continus spectrum
characteristic spectrum
What are the general aims for continuous and characteristic spectrum target interactions?
we want electron to interact with atom at target and then this will lead to production of x-ray photons
What Is the continuous spectrum target interaction?
This is where incoming electrons from the filament pass close to the target nucleus and are slowed down and deflected as the negative electrons get drawn into positive nucleus resulting in loss of kinetic energy which is transferred into photons which are released from the X-ray tube
What does continuous spectrum target interaction produce?
Lots of photons with low energy however These are useless we need high energy
What is the characteristic spectrum target interaction?
This is where incoming electrons calling with the inner shell electron and either displace it to another shell or remove it entirely. This results in an unstable atom so electrons reshuffle by filling the space and as this happens energy is released in specific values as they go from one shell to the next and there is a release of energy
What are the interactions that occur in patients tissues?
Photoelectric effect
Compton effect
What is the photoelectric effect simplified?
Absorption
What is the Compton effect?
Absorption and scatter
What is the photoelectric effect?
This involved inner shell electrons and is where photons come in and there is complete absorption of x-ray photon for this to happen energy of incoming photons must be just greater than energy of binding electron
What does incoming electron interact with in photoelectric effect?
Inner shell electron (usually k shell) and only happens if photon has energy greater than the binding energy of electron resulting in x-ray photon completely disappearing
What is most of photons energy used for in photoelectric effect?
Used to overcome the binding energy of the electron, the rest gives the electron kinetic energy and causes electron to be ejected
What is the photoelectron?
This is when the photons energy is greater than that of the electron binding energy and most of energy is used to overcome binding energy but the rest gives the electron energy to be ejected and the electron that is ejected is the photoelectron
What happens when the photoelectron leaves?
Now an empty space in electron shell and atom is pos - wants to be stable so atom regains stable state by electrons from outer shell dropping to fill the void
What is the difference in energy between the two electron shells emitted as?
head/light
What happens to the outer voids in photoelectric effect?
they are filled by free photoelectric electrons from another atom overall resulting in complete absorption of photon energy so photon doesn’t reach film
What is characteristic radiation?
This is the spare energy as electrons go between different shells and is in form of light or heat
Wha is photoelectric effect mathematically proportional to?
To the cube of atomic number - this results in large differences in photoelectric absorption
What is the Compton effect?
This involves outer shell electrons and results in scatter and partial absorption of photons
it is where energy of incoming photons is way greater than binding energy of the electron causing electron to come from outer shell and photon is left with lots of energy still. the ejected electron takes some of the photons energy as kinetic energy and is known as the recoil electron
atom now unstable so atom captures free electron
the photon following the collision has lower energy so longer wavelength and is known as the scatter photon
the recoil electron can interact with other atoms in tissue and scatter photons dependent on the energy and position of bound electron involved
What is a scatter photon?
This is where photon has way more energy than electron and ejects the electron and photon is left with less energy so longer wavelength and undergoes a change in direction
What is recoil electron?
This is the electron that id displaced and it can interact with other atoms in tissue and the scatter photon depending on energy and position of bound electron
What can scatter photons do?
Travel in any direction
What can high energy scatter photons do?
Travel forward
What can low energy scatter photons do?
Travel backwards
What is probability of Compton effect happening?
proportional to density of material - very dense material with lot of electrons packed closely together are more likely to have Compton effect occur
What is the issue with the Compton effect?
If scattered photons are produced before receptor is reached and scattered backwards then no issue but if they are produced beyond the image and scattered to the receptor then they can affect the receptor by causing darkening
Why can scatter photons cause problems?
Their path is randomly altered os if they are produced beyond image receptor and are scattered back then they can cause fogging, reduced contrast and affect image quality
How do we reduce scatter?
Via collimation
What does collimation do?
Reduces area irradiated therefore reduces volume irratated therefore reduces number of scattered photons produced as well as reducing pts dose
What happens to all x-rays?
Collimated to reduce area of beam at pt and therefore reduce dose
How do we collimate x-rays?
In film packets we have lead foil which prevents back scattered photons from oral tissues reaching the film so any back scatter photons absorbed and prevented from degrading image
Why dont digital receptors have this lead back?
They are more sensitive to x-rays at low dose
reduced exposure time
collimation
When are absorption of photons most likely?
High atomic number
object thicker
photon energy lower
What is radiographic contrast?
This is the difference in density of light and dark areas of radiograph. Allows us to see sharp junctions between diff materials
What does photoelectric absorption result in?
Deposition of all photon energy within tissue
What does photoelectric absorption do?
Increases pt dose but needed for image quality to get diagnostic image
What does Compton scatter result in?
Deposition of some photon energy within tissue - so adds to dose with no diagnostic benefit and may inc dose to operators
What happens if we lower x-ray kvp?
we would increase photoelectric interactions and contrast between tissues but dose to pt would also increase (no need)
What does a lower tube potential difference produce?
lower energy photons
What dose a high tube kVp produce?
Higher energy photons
What happens if we increase kVp on image quality and pt dose?
We have inc in Compton effect
photoelectric interactions reduced
reduced contrast
reduce dosage absorbed by pt
but can’t be reduced so much that no diagnostic image
