Week 6- X-Ray Photon Production Flashcards
List components of a diagnostic X-ray
Tube insert
Tube housing (shield)
Light beam diaphragm
Describe tube housing
Provides protection from RADIATION and ELECTRICAL HAZARDS resulting from tube insert
Describe light beam diagrpham
Provides protection from radiation hazards from tube insert
(Filters - remove lobe energy
Reduces irradiated areas from scattered radiation)
Controls field size of Beam/ reduces area irradiated
Protects body parts not required on radiograph
Is anode or cathode positive
ANODE
Describe X-ray photon production
X-Ray photons are produced when fast-moving electrons [ accelerated from thecathode ] are suddenly stopped by theimpact with a metal target [ the anode ]
The Kinetic Energy (KE) of the electrons isconverted into X-Ray photons (≈ 1%) andinto heat (≈ 99%
Describe thermionic emission
E- emitted from cathode by tHERMONIC EMMISION
Material is heated, KE atoms^, ^ collisions
-outer e- loosely bound
-collisions of atoms near material has sufficient KE to cause some e- to be LIBERATED
-each e- LIBERATED will leave material with a positive charge
Describe thermionic emmission- space charge effect
A state of equilibrium is created where:▪ the rate of electrons that are liberated from the material [ W ]EQUALS the rate of electrons that are attracted back into thematerial [ W ]▪
Thus, an electron cloud [ or space charge ] is created
In terms of tHERMONIC emissions, the higher the temperature, the…
Greater KE of atoms
More e- liberated
Define thermonic work functiom
Min energy needed to liberate an e- from material
The lower the thermionic work function of a material,the less energy is required to liberate the electronsfrom the surface of a material
What does scattered radiation do to image quality
Degrade quality of image
What else needs to increase in order to increase tube current
Increase number of e- in the charge space BY INCREASING TEMP OF FILAMENT
What does increasing temp of filament do to electrons
Increase temp of filament
What causes filament to heat
Passage of the current through the filament
E- are liberated by thermionic emmission
Describe the process of the tube current/ filament current
The passage of the current through the filament causesthe filament to heat, with the temperature rise causingelectrons to be liberated by thermionic emission▪ The liberated electrons are drawn towards the positiveanode and form the TUBE CURRENT [ mA ]▪ The liberated electrons are then accelerated towards themetal target to create x-ray photons▪ To be able to increase the tube current, it is necessaryto increase the number of electrons in the chargespace by increasing the temperature of the filament,causing more electrons to be liberated▪ Thus, the tube current [ mA ] selection determines thefilament heating current required to produce therequired mA
How do we increase tube current
Increase number of e- own charge space by increasing temp of filament so more e- can be liberated
What does kV control
peak potential differenceacross the X-Ray tube [TUBE VOLTAGE]
▪ The higher the kV, the greater the peak potentialdifference, and the greater the force of attractionbetween the cathode filament and the anode▪
Thus, the liberated electrons will strike the anode withgreater KE, producing more energetic X-ray photons
▪ The maximum x-ray photon energy is determined by thetube voltage
Describe the process occurring within the target
▪ Each liberated electron arrives at the surface of thetarget material with a kinetic energy determined by thetube voltage [ kV ]
▪ The liberated electrons penetrate several millimetres intothe target material and lose their energy by acombination of processes:
▪ [1] As many very small energy losses, by interactionwith the shell electrons of the atoms in the targetmaterial, constituting unwanted heat and causing a risein temperature]
▪ [2] As large-energy losses producing X-Ray photons,caused by the interaction with either the inner shells ofthe atoms in the target material OR [3] the positiveattractive field of the atomic nuclei within the targetmaterial
What happens when filament e- interact with shell e-
they are briefly excited (releasing some KE) before returning to their original shape
Energy emitted is infrared- HEAT
99% energy is heat
Describe Bremsstrahlung (braking) radiation
Inelastic collisions with atomic nuclei:
▪ filament electrons are suddenly decelerated ordeflected in the vicinity of the positively chargedatomic nuclei
▪ the deceleration causes the filament electronsto lose some KE, which may be converted intoX-ray photons known as Bremsstrahlung (orbraking) radiation▪
the X-ray photons produced have a range ofenergies [ in keV ] lower than maximum kVsetting
Describe max photon energy
Inelastic collisions with atomic nuclei:
▪ a filament electron is suddenly halted byimpacting directly with the positively chargednucleus
▪ the sudden halting causes the filamentelectron to lose all of its KE, which is thenconverted into an X-ray photon
▪ the X-ray photon produced has the maximumpossible energy [ in keV ] equal to themaximum kV setting
Describe characteristic radiation
Inelastic collisions with inner shells:
▪ filament electrons can liberate shell electronswhen they have sufficient KE to overcome thebinding energies of the shell electrons
▪ the vacancy left by an ejected inner shellelectron leaves the inner shell unstable
▪ the vacancy is immediately filled by anotherelectron dropping down from a higher shell [ anelectron transition ] with the release of electronPE in the form of an X-ray photon
▪ the energy of the X-ray photon is dependent onthe difference between the binding energies ofthe shells involved in the electron transition
Define electron transitions
the energy of the X-ray photon is dependent onthe difference between the binding energiesof the shells involved in the electron transitions
What do we use to manipulate size, shape, start and end point of X-Ray spectrum
KV (tube voltage)
MA (tube current)
Z (atomic number)
Filtration
Waveform
Symbol for tube voltage
KV
Symbol for tube current
MA
Symbol for atomic number
Z
What happens to the x-ray spectrum when tube voltage is increase
Up and right
^ number of X-ray photons
but increase change in AverageE, Effective E, Maximum E, or Minimum E
Equation for voltage
V= i x r
What occurs when we increase tube current
X-ray spectrum shifts ^
^ in number of X-ray photons
but no change in AverageE, Effective E, Maximum E, or Minimum E
What happens to figrak when target material to lower Z
Reducesnumber of x ray photons
And radiation produced
Describe inherent filtration
Low e xray photons are filtered by tube snort and cooling oil surround tube insert
What does the quality of Bremsstrahlung radiation represent
The penetrating power of X-RAY Beam
