Physical properties of x-rays 4.0 Flashcards
What is effect of direct interaction of x-rays with film emulsion vs light from a screen?
Direct interaction - only 5% will create latent center
Indirect interaction - much more efficient
Describe the film emulsion
Crystal lattice made up of
95-99% Silver bromide
1-5% Silver Iodide
The crystal has an overall negative charge due to Bromides being on the outside, and silver ions being centrally located
Sensitivity speck is a defect in the crystal - one positive charge sticking out
Describe formation of a latent image
Light/x-ray interact with the crystal
Br and e- produced.
Bromide absorbed in emulsion
e- moves to speck where it will attract silver ion
Silver ion is reduced to silver atom
Silver atom attracts more electrons and more silver ions
When developed which part of emulsion becomes dark? white?
What will increase probability of a region of a film to be developed?
Areas where a latent image has formed - will be dark
Areas without exposure - no latent image will form, and the crystals will be washed away.
Larger the latent image center, the more likely an area will be developed
Define fluorescence, phosphorescence and luminescenc
Luminescence - emission of light after interaction with a stimuli (intensifying screen). Umbrella term for phosphorescence and fluorescence
Fluroescence - when something emits a light immediately 10-8s
What is the inverse square law equation?
Intensity 1 / Intensity 2 = distance^2 2 / distance ^2 1
What is the colliquial way to remember inverse square?
Double your distance
Quarter the intensity of the beam
What is the direct square law equation?
mAs 1 / mAs 2 = d^2 1 / d^2 2
Describe Rayleigh/coherent?
bLow energy (10-30keV) radiation travels and interacts with an electron in an atom. The electric field causes the WHOLE atom begins to vibrate and the electron cloud will emit a scattered photon with the same energy and a slight change to its direction.
No ionization is produced (there is no transfer of energy)
What is % of rayleigh scatter that occurs?
~5%
Describe photoelectric effect
X-ray photon, with energy just greater than the orbital binding energy of the inner shell electrion, displaces an inner shell electron.
The photon loses ALL energy
electron is quickly absorbed
Void in inner shell results in a cascade of energy and release of an Auger electron or characteristic radiation
In an atom -
K-shell- 70keV
M - shell - 2.5keV
If a photoelectric effect were to occur - what would strength of photoelectron be?
characteristic radiation be (M–> K)?
Auger electron?
Characteristic -
Photoelectron 100-70 = 30keV
Characteristic radiation = 67.5keV
Auger electron = 65keV
70-2.5 - 2.5
What is the probability of the PE to occur related to?
Z^3
Inversely related to energy - 1/E^3
Why do atomic numbers have anything to do with PE?
How does this related to the energy of the beam?
As energy increases - PE is usually less likely to occur (1/E^3)
The exception to this rule is at the absorption K-edge.
example: tungsten (K-shell is 69.5) - At 65keV - the x-ray photon will be mostly Compton scatter. But at 69.5 - the interaction will predominantly be PE
What is K-edge of: Tungsten (W) Iodine (I) Barium (Ba) Lead (PB) Bismuth (Bi)
Tungsten: 69.5 Iodine: 34.5 Barium: 40.4 Lead: 93.1 Bismuth: 95.7
Why is PE desirable in imaging?
PE allows for differential absorption - why we can see shades of gray and differences between tissues
What is compton scatter?
An x-ray photon displaces an outer shell electron and also a scattered photon.
Soem of KE of x-ray photon will be transferred to electron, and some will be retained in scattered photon
What is energy of scattered photon in compton scatter?
Depends on the amount of energy transferred ot the electron. Also depends on original energy of x-ray photon
If barely interacts - minimal energy will be transferred, photon will continue moving forward and interact with emulsion –> fog
If hits at an angle - will lose more energy. Larger the angle of interaction, the more energy the photon will lose
If two photons (50keV and 100keV) hit an electron at same angle - amount transferred to electron will be higher for 100keV (this is less important in imaging)
What is energy of scattered electron in compton scatter?
Depends on the interaction with the x-ray photon
What is the maximum degree an electron can be scattered after a Compton interaction? x-ray photon?
Electron - 90 degrees
X-ray photon - 180 degrees
What will increase the probability of occurrence for a compton interaction?
Increased energy (except for at K-edge)
Increased e/g x density
Independent of atomic number
What element has the highest electrons per gram, and why?
Hydrogen - 6 x 10^23
Everything else is 3 x 10^23
This is because hydrogen has no neutrons within the nucleus contributing to it’s overall weight. More electrons can be in a gram
Describe pair production
Can only occur when energies of x-rays and gamma rays exceed 1.02MeV
High energy photon (>1.02MeV) interacts with nucleus of an atom → photon energy is transformed into electron and positron pair (via B+ decay)
The electron and positron lose their kinetic energy via excitation and ionization.
When the positron comes to rest, it interacts with a negatively charged electron – resulting in formation of two oppositely directed 0.511MeV annihilation photons
Describe photodisintegration
Part of nucleus is ejected by a high energy photon.
Ejected portion may be neutron, proton, alpha particle or cluster of particles.
Energy required has to be greater than 7-15MeV to overcome nuclear binding energy
What is the relationship of light, frequency and wavelength?
c = wavelength x frequency
wavelength and frequency are inversely related to one another.
What is the relationship of x-ray wavelength and energy?
What does this equation imply?
E = 1.24 / wavelength
Shorter wavelength (higher frequency) = higher energy (such as gamma or x-rays)
What is the wavelength of x-rays?
10^-10-10^-12
What is beam intensity? how is it calculated?
Intensity is the sum of all of the photons x their energy
What influences the beam intensity?
Filtration
mAs
kVp
Atomic number of material
How does atomic number affect beam intensity?
Effects both the quantity and the quality of the beam
Quality:
Higher the atomic number - more likely to have PE interactions, resulting in attenuation/absorption of the beam
Quantity:
Higher atomic Z = more efficient at converting x-rays
How does voltage affect the intensity of the beam?
Effects the quality
Energy of x-rays depend on energy of electron stream that bombards the anode
Higher the voltage - increase the energy of electrons
How does kVp relate to intensity mathematically?
Intensity = kvp^2
How does x-ray tube current affect the intensity beam?
Effects the quantity
Increase the current (mAs) - increase the electron cloud - more electrons available to be converted to x-rays
How does beam filtration affect the intensity of the beam?
Removes all low energy x-ray photons - strengthens quality of the beam
What is the linear attenuation coefficient?
measure of the quantity of radiation attenuated by a given thickness of its absorber
What is the non-logarithmic math equation for the linear attenuation coefficient?
What does the final answer tell you?
n = LAC No (x) No = original photons x = thickness n = # of photons that are going to be removed
How do you count for the exponential component of the LAC?
What is this equation similar to?
N = Noe ^ (LAC *x) N = # of photons that remain No = original photons x = thickness
Exact same as radioactive decay
N = No e ^ (gamma *t)
What happens to LAC when energy of photons increase?
of photons attenuated will decrease
How is density related to attenuation?
Double the density = double the attenuation
What is the half value layer?
Absorber thickness required to reduce the original x-ray beam by 50%
How do you calculate half value layer?
(1/2) ^ n
N = # of half value layers (1,2,3)
How is half value layer related to LAC?
What is similar to this equation?
HVL = 0.693 / LAC
Physical half - life for radioactive decay and decay constant
Gamma = 0.693 / T 1/2
What is the mass attenuation coefficient? how does this relate to LAC?
Used to quantitate attenuations of material independent of their physical state.
Takes density out of LAC
With LAC attenaution of for water > ice > water vapor because of density
With MAC water = ice = water vapor
How do you calculate the MAC?
MAC = LAC / density (g/cm2)
How do you substitute MAC into the attenuation equation?
N = No e ^ - ( LAC/density * x)
What effects absorption?
atomic number
energy
density
electron/gram
How does atomic number effect absorption?
Because of the PE effect -
Higher Z elements will more likely have PE effect at higher energy photon levels = more likely to be absorbed
How does energy affect absorption?
As energy increases, attenuation decreases
Less PE effect, more compton
What ist he effect of density on attenuation?
Linear - double the density, double the absorption
How does electron / gram effect attenuation?
Although hydrogen has higher electron / g, bone will have higher e / cm^3 (electron density) - will result in more interactions/absorption
Hydrogen e/g = 5.97
Bone = 3.19
Electron density (e/g / g/cm3) = e/cm3 Hydrogen = 0.0005 Bone = 5.9
How do density and atomic number relate to one another
Generally - higher atomic numbers are dense (except for Lead and gold)
Atomic # does not differ for different physical states (water will have same Z for ice or vapor)
How does density and electrons / gram relate?
No relationship
Water has same electrons/gram whether it’s spread apart or compacted into ice
However, the densities are different between different physical states
How does atomic number and electrons per gram relate to one another?
Function of # of neutrons present
Hydrogen is only element with no neutrons
electron/mass for hydrogen will all be made up of electrons = almost 2x as much as any other element
Higher Z = less electrons/gram
What factors determine scatter?
kV
Material
field size
How does kV affect scatter?
Higher energy - less PE, more Compton scatter, coherent
How does material thickness affect scatter?
thickness of patient will increase probability of interactions
How does field size affect scatter?
narrow beam only irradiates only a small volume of tissue - generating a small # of scattered photons that aer likely to miss
Wider beam - quantity of scattered photons increases rapidly
LAC example.
2 studies:
Bone at 20kev has a LAC of 4.8. Water at the same energy has LAC of 0.8
Bone at 100kev has LAC of 0.3 and water at the same energy has LAC of 0.1
Which will result in a better image?
Example 1.
More PE (lower keV)
Bone is made up of calcium/phosphorus - slightly higher then standard hydrogenous materials in the body
Differential between LAC is large = higher contrast
