X-ray Production Flashcards
What is Bremsstrahlung radiation?
“Breaking radiation” due to interaction with nuclear Coulomb field
Release of photons due to acceleration of charged particle with an amplitude proportional to acceleration
What is the acceleration equation of Bremsstrahlung photon?
a ∝ zZ / m
What is the intensity of Bremsstrahlung photons?
I ∝ Z^2 / m^2
What are core electrons?
Electrons closest to nucleus (inner shell)
What happens after collisional loss?
Heating in target material
(recoil, excitation and ionisation)
What happens after radiative loss?
X-ray production: Photons produced with set energy or various energy
How are characteristic x-rays produced?
Incoming electron interacts with core electron and leaves excited ion behind
Electron from higher orbital falls to lower hole and characteristic photon is released (seen as line in spectrum)
What is the energy of the characteristic x-rays?
hv = BEm - BEk
BEm = binding energy electron begins with
BEk = binding energy of electron in new orbital
What does each shell have?
Multiple energy states
and not all electron transitions are allowed
What are the selection rules for quantum numbers?
Δn ≠ 0
Δl = ± 1
Δj = ± 1; 0 (includes spin of electron, 1/2)
What do increasing principal quantum number mean?
Electrons are further away from nucleus as this increases (and higher ΔBE)
What is the energy of x-ray?
Difference between the energy levels of the orbits
When is characteristic line present?
When the incoming electron energy > electron binding energy
What dictates the energy loss of by e-?
The proximity of electron to nucleus
What happens to electron in near nucleus in Bremsstrahlung?
Electron feels attraction and deflects, slowing it down and energy is reduced which subsequently emits a photon
What does a greater deflection angle of e- mean?
More energy is lost by e- and higher energy Bremsstrahlung x-ray is produced
What is the assumption of a thin target?
Target is so thin that no electrons undergo more than one collision (all bremsstrahlung interactions are likely)
What is the assumption of a thick target?
That it is made of multiple thin targets so electrons pass through each layer and reduce their energy so new bremsstrahlung x-ray is produced at each layer
What increases the probability of radiative to collisional loss?
Increases directly with Z of target and energy
(at low energies: ratio is small so electron looses energy to heat)
What are the components of an x-ray tube?
glass envelope
cathode
anode
protective housing
What is the glass envelope made of?
Typically Pyrex to withstand tremendous heat and maintain vacuum
What does the tube window in the glass envelope do?
Area of envelop that is thinner
Contributes to inherent filtration
What does the cathode do?
Negatively charged electrode with two primary parts:
filament
focusing cup
What is in the filament?
Coil of metal wire, typically tungsten (current flows to heat filament through thermionic emission)
What are the properties of the coil metal wire?
High melting point
High thermionic emission (electrons boiled off)
Carry high current to produce high temperature
What is the focusing cup used for?
Metallic housing filaments which are negatively charged to keep electron cloud together and form a beam
What is the anode?
Positively charged electrode which is used for x-ray production and heat management
two types: stationary and rotating
What are the properties in anode structure?
High Z for efficient bremsstrahlung production
High melting point
High conductivity
Small ‘apparent’ source size
Materials: Tungsten, Molybdenum, Rhenium
What is protective housing made of?
Protective steel
lined with 3mm lead
oil insulation between lead and glass tube
What does protective housing prevent?
Leakage radiation
Electric shock
Dissipates heat
What happens when a high anode-cathode voltage is applied?
Electrons are accelerated and then impact anode
What is the kilovolts peak (kVp)?
Maximum voltage applied across the tube and maximum X-ray energy (kV ∝ eV of electron)
related to maximum energy carried by a thermionic electron across the tube
What does Direct AC do to tube?
damages it
What does the Half-wave (HW) rectification do and lead to?
only accepts positive voltage
Poor output -> 50%
What does the Full-wave (FW) rectification do and lead to?
All negative voltage voltage is made positive
most output is still below kVp
What is the 3 phase full wave rectification?
the peaks are overlapped at a factor of 3 and output is higher and less variability
What does DC voltage produce?
Constant potential so constant x-ray production
(used commonly in modern generators)
What is the current in the tube?
The number of thermionic electrons passing between cathode and anode (mA)
(different from filament current)
What is mAs?
miliampere seconds
a measure of radiation produced over a set amount of time in an x-ray tube
What does continuum emission produce?
Bremsstrahlung (peak)
What does a discrete emission produce?
Characteristic x-ray lines
What is the high energy cut off?
The edge of the Bremsstrahlung peak
Maximum electron energy dependent on applied kV (x-rays can’t be produced above this)
How are characteristic lines produced?
Electron can (occasionally) give up all energy to X-ray production
What is the low energy cut off?
Point at which X-rays cannot penetrate layers as low energy x-ray are easily attenuated
What does the filtration of the x-ray do?
Attenuation of x-ray beam
What effect does quantity have and which parameters are affected by it ?
Magnitude changes, shape remains unaltered
current, exposure
What effect does quality have which parameters are affected by it ?
Non-uniform change in intensities, shape changes
voltage, filtration
What is the focal spot size?
The area on anode where x-rays are emitted
size is controlled by x-ray target angle and electron beam size (effected by size of filament)
What is the anode heel effect?
When there is greater attenuation on anode side as x-ray travels through more of the target causing lower intensity on one side (beam hardening)
non-uniform intensity and energy distribution
What are the benefits and drawbacks of small focal spot size?
Better resolution
Better for smaller parts of body
Heat is concentrated in smaller area
Limit on x-ray flux
What are the benefits and drawbacks of large focal spot size?
Lower resolution images
Better for imaging large structures
Heat spreads out
Greater flux available so faster exposure (better due to patient movement)
What is the space charge effect?
Occurs when there is a cloud of electrons at filament
they generate an opposing electric field (negative) and limits the flow of electrons from the cathode to the anode at lower voltages, reducing X-ray intensity (until anode voltage is high enough)
In diagnostic x-ray what will define the design of the x-ray tube?
Cooling the electron target
What are the two types of anode design?
Stationary:
Concentrated electron impact
Localised heating
Low power applications only (dental)
Rotating:
Electron impact spread over large area
Energy delivered per unit area reduced
High power applications
What is a sign of thermal anode damage?
Anode pitting:
small regions of anode surface overheat and liquefied will flow and ‘creep’ to a new location
(effects x-ray spectrum)
What does thermal anode damage cause?
Beam hardening
X-ray must pass through more material to escape
Increased mean energy
How is the life of a tube prolonged?
By setting limits on usage (kV, mA, exposure time)
What is the effect of of increasing current and voltage?
current: more electrons move across gap and counts per second increase linearly
voltage: electrons have higher kinetic energy & increase in electrons per unit time & x-rays have higher energy
What is the effect of increasing exposure and filtration?
Exposure: more electrons when time increases
Filtration: low cut off edge shifts to right and beam hardens
