Week 5- Interactions Of X-Rays Flashcards
How does EMR behave
Like streams or packets or energy called photons that have no mass or electric charge
How does EMR energy travel
Straight lines
What is E(photon energy) proportional to
Wave frequency (F)
How many fates are there for X-Ray photons when they pass through matter
1) penetration
2) absorption
3) scattered with energy loss
4) scattered without energy loss
Describe penetration
The photon penetrates the matter withoutinteracting with it
Describe absorption
The photon interacts with the matter and iscompletely absorbed by depositing ALL of its energy into thematter
Describe scattered
The photon interacts with the matter and isscattered (deflected) from its original direction, depositing eitherPART or [4] NONE of its energy into the matter
What is attenuation
the attempt to numerically describe the collectivefates of a large number of photons as they interact with
Represents deduction in beam intensity , reduction in energy content, reduction in number of photons
Name the 2 main components of attenuation
Scatter
Absorption
How many processes are involved in attenuation
5
List 1,2,3 attenuation processes
-elastic or unmodified scatter (no energy loss)
2) Compton or modified scatter (energy loss)
3) photoelectric absorption
Describe elastic scatter
occurs at very low X-Ray energies and involvesscattering (deflection) of photons without loss of energy.§
This type of scatter is of little practical relevance to diagnosticimaging; but is discussed for the sake of completenes
Energy of photon is less than energy gap. Doesn’t excite to shell so electron doesn’t absorb energy.
Does elastic scatter have energy loss
No
What is in elastic scatter
involvesscattering (deflection) of photons with an associated loss of photonenergy.§
main scattering process important from a diagnosticimaging perspective is Compton scatter
Energy loss
Energy from photon is transferred to atomic electron, raising to next shell. Electron falls back to original shell, emitting radiation
The greater the angle of scatter, the…
Greater the energy and range of the recoil electron
Does a side or forward scatter photon have more energy
Forward
Greater loss of energy of scattered photon
What does scattered radiation make up
50-90% of total number of photons emerging from patient
What is photoelectric absorption
an inelastic collision of a photonwith an orbiting electron in an atom of an absorbing material, withall the energy of the photon given up to the electron.
photon disappears (absorbed). There is recoil of theelectron, emission of characteristic radiation, and the productionof Auger electrons
Energy of photon is greater than energy gap and binding energy of inner shell electron. All energy is transferred to electron, allowing ionisation of atom. Electron vacancy is filled by an electron from another shell, EMITTIJNG RADIATIOMN
How is they’re a signal difference (contrast) in soft tissue and bone
Due to different atomic numbers
Tissue- 7
Bone- 14
Magnitude is 8x greater
Explain cascade effect
INCIDENT PHOTON ISCOMPLETELY ABSORBEDLEAVING A VACANCY INTHE K SHELl
2) VACANCY IS FILLED BY AN ELECTRON FROM HIGHER ENERGY LEVEL
3) RADIATION IS EMITTED AS HIGHER ENERGY ELECTRON DROPS TO LOWER LEBEL
What is the Auger effect
the emission of an electronfrom an atom that accompanies the filling of an innerelectron shell vacancy
Inner shell electron is removed. Ionised and left in excited energy state. Relaxation to ground occurs rapidly. Vacancy filled by outer shell electron, releasing energy. If energy released is small it is absorbed immediately with ejection of very low energy AUGER ELECTRON
What is the linear attenuation coefficient
is the fraction of photons removedfrom the beam per unit thickness of the attenuating material
What is photon fluence
Photons per unit area
What is energy fluence
Energy per unit area
When dopes intesnity of the beam decrease
As thickness of absorbing material increases
Linear attention coefficient is affected by pressure or temperature
What is linear attenuation coefficient affected by
Pressure and temperature
As pressure increases and vol decreases what happens to density
Increases
As temp increases and vol increases what happens to density
Decreases
When is photoelectric coefficient high
When photon energy is greater than binding energy of electrons within a shell
The more tightly the electron is bound to the atom, and the nearerthe photon energy is to its binding energy
When does probability of photoelectric absorption decrease
asphoton energy increases. It increases markedly as the atomicnumber [ Z ] increases
When does scatter increase
When kv, thick body parts, field size increases
Why is scatter a problem
It can cause foggy image
Affects radiology workers
Does lead have high or low attenuation
HIGH
Give symbol for tube voltage
KV
What does tube voltage determine
thephoton energies within the beam.
The greater the tube voltage, the greater the range of photon energieswithin the beam, and the greater the penetrating power of the beam.
rise in penetration through all tissues – a decreasein attenuation differences – resulting in a lower contrast radiograph and alarger range of shades of grey
What is increased if photon energy and kV Is reduced
Photoelectric absorption
Scatter photons reduced
When is scatter generally lower
At lower kV values
Name 2 factors which reduce amount of scatters
1] thin body tissue is easier for photons topenetrate; [2] the presence of bone [high Z] means thatmost interactions are photoelectric
What does a higher tube voltage mean
Decrease in attenuation differences
Lower contrast
When is a high kV technique good
produces a more uniformly dense image to betterdemonstrate lung markings. There is a reduction in dose, too
As decrease in attention differences so lower contrast
what are anti-scatter grids
placed between the patient and the image receptor to reduce the scattered photons reaching the receptor produced by Compton scatter; thus, improving image quality. GRIDS are made of parallel strips of high attenuating material [ Lead ]with an interspace filled with low attenuating material
scatter increases withincreased tissue thickness andlarger field sizes; thus, GRIDS arerequired to improve image contrast.
what is scatter air gap
an additional distance between a patient and an image detector. The gap decreases the likelihood for scattered x-ray radiation to reach the detector, as radiation is partially absorbed and scattered in the air.
what is the amount of scattered radiatipn related to
tube voltage [controlledby kV], body part thickness [controlled using grids], and field size
