Generation Flashcards
What are the two main ways of making x- rays?
Kilovaltage X-ray production
Megavatage X-ray production
What is elastic scatter?
Simple and low energy process that occurs when making kilovoltage X-rays
What is the conservation of energy principle?
Energy cannot be created nor destroyed but instead is changed from one form to another
X-ray production is the changing of a form of energy ( electrical energy becomes X-ray energy)
How to make X-rays
1 high energy electron
1 high atomic number target
Electrons with high kinetic energy interact with target atoms
Their KE is transferred to the target atoms
They emit this as heat and X-rays (conservation of energy)
What interactions occur when elections hit the target?
Interactions with outer electrons of the target atoms
Interactions with inner electrons of target atoms
Interactions with the nuclei of target atoms
These can be: elastic ( KE of electrons is conserved), inelastic (KE of elections is not conserved). Result in EMR production
Interactions with outer electrons
Tungsten has 74 orbiting electrons
There is a high chance of an incoming electron hitting one
Orbital electrons deflect incoming electrons via electrostatic repulsion → negative charges repel each other
Heat production
Deflections are small and so loss of kinetic energy is small
KE loss by electron emitted as photon in infra-red part of electromagnetic spectrum → i-e heat is produced
95 - 99% of energy produced is in the form of heat
What is bremsstrahlung?
Process of slowing down projectile electrons leads to ejection of X-ray and is classed as an inelastic interaction with the nuclei
Inelastic interactions with nuclei
Electrons passing near the nucleus are attracted to it → positive charge attracts the negative electron
The electrons slow down as they change course → leave in different direction with reduced KE
Extra KE is emitted as an X-ray photon
These X-rays are known as bremsstrahlung radiation
In bremsstrahlung - what does X-ray energy depend on?
Atomic number of the target material → high atomic numbers mean more protons and increased attraction
Energy of incoming electron → amount of energy available for release
Extent of deflection → large deflection will cause a larger reduction in KE of the electron
Bremsstrahlung →energy of X-ray produced
Energy of the X-ray is proportional to the atomic number times energy squared( x-ray ➰z.e squared)
Energy range of X-rays →very small if electron diverted slightly, total KE of electron if completely stopped
Direction of radiation depends on incident energy
What is characteristic radiation?
Incoming electron collides with an inner (K or L shell) electron → inelastic collision
Gives enough energy to overcome its binding energy and leave the atom → is ionised
Characteristic radiation process
The ionisation leaves a gap in an inner shell → atom cannot tolerate this
The gap is filled by an election from next outer shell → this electron loses energy by doing this. The energy is emitted as an X-ray photon with an energy equal to the difference in binding energies of the two shells
Radiation emitted is characteristic radiation
What are k and L peaks?
Characteristic radiation is only emitted at very specific energies
This corresponds to the difference in shell binding energies
This is characteristic of the target material → if you know the energy you can work out what the target material was
The highest energy is emitted when a k shell electron leaves → then the L peak
What is the X-ray spectrum?
Understanding both bremsstrahlung and characteristic radiation X-ray production process → both occur and cannot be separated
Thus a spectrum is obtained (range of energies)
The electron volt definition
EV is the amount of energy gained by an electron when it is accelerated by 1 volt
Ev = 1.602 x10 to the power of -19J
CT scanners on board imaging X-rays have Kev energy
Linear accelerators have MeV energy
What is intensity?
The amount of energy(j) per unit time crossing a unit area perpendicular to the beam
A way of indicating the quantity of radiation
What does quality mean?
A measure of the penetrating power of a X-ray beam
Increase in quality increases beam penetration
“hardening” the beam means improving it’s qualify
The X-ray spectrum
Bremsstrahlung consists of a range of energies → from zero - maximum Kev possible. A continue spectrum
Characteristic is of distinct energies →line spectra
The intensity of the different energies can be drawn on a graph → X-ray spectrum
Electron production equipment
Filament and focussing cup
What is thermionic emission? → important
Electrons are produced when metal is heated → process is called thermionic emission - heat increases the KE of orbital electrons - thermions are the electrons emitted
Works with certain metals compared to others→”work function” is the amount of work
Needed to be done to produce thermionic emission
Current used to read metal
What is space charge cloud?
When electrons leave the metal, it becomes positively charged
This attracts the electrons back
Continuous emission and re-attraction produces a space charged cloud → cloud of electrons hovering around the metal
The metal is called the cathode
The cathode (filament)
Made from tungsten
Good thermionic emitter
High atomic number - 74
Doesn’t vaporise readily
High melting point - 3387 degrees Celsius
Ductile
Spiral shape increases surface area
Cathode current heats filament
Cathode(focussing cup )
Negatively charged → squashes elections together, overcomes their mutual repulsion, makes a tight electron beam
Nickel, molybdenum or stainless steel → poor thermionic emitters with high melting points
Electron accelerators
Vacuum and anode
Opposites attract → important concept
Electrons are negatively charged
They can be attracted away from the cathode by a potential difference ( eg +80kv)
This accelerates them towards the positive end
Of the potential difference
The positive potential difference is located on the target and is known as the anode
What is the purpose of a vacuum?
Electrons have a very small mass and are easily deflected
Air molecules in their path deflect them
Their path must be through a vacuum
What is heat dissipation?
Treatment tubes are on for a long time at a high intensity →these need forced continuous cooling
Diagnostic enclosed CT tubes have a higher intensity but for a short period of time → these can rely on heat radiation (rotating anode)
Typical kv treatment tube
There is a coolant oil supply coming in from the right hand side and passes through the anode → means we can constantly pass cool liquid through hot region and remove it
What is meant by forced cooling?
In a stationary anode tube the heat produced is cooled using pumped coolant → meaning a heat exchanger and fluid pump are required
It can cool continuously → useful if the beam is going to be running at a high dose rate for a long period of time ( as in a treatment tube)
Typical kv CT tube
Cathode (on the left side ) is facing off against a truncated come shape(a disk) embedded on it is the target →rotator rotates wheel and goes around continuously but always is in line with the beam of electrons
What is the rotating anode?
Enables large quantities of heat to be radiated away quickly. Rotates a cooler part of the anode to face the electron beam while hotter part cools → cannot be continually used is ideal for short bursts as in imaging
What are the essential kv tube components? And what do they do?
Filament (and current)→generates electrons releases electrons via thermionic emission
Focussing cup→ squashing them together
Vacuum→stop collision with air molecules
Anode (and potential difference)→accelerate the electrons
Target → generates X-rays
Shielded tube housing (and window)→enables x-rays to leave the tube
Beyond the shielding window of a kv tube what other important element is present and what is its purpose/function?
Filters→ manually added to the beam to remove lower energy photons and produce a more penetrating beam
→ At <150kV, aluminium is used ( eg 1mm al,2mm al)
→ from 150 - 240kV copper is used (eg 0.4mm Cu)
→above 240kv tin is used (eg 0.25mm Sn)
Interlocks prevent inappropriate filters being used
That other essential component is present within a kv tube beyond the shielding window ? And what is its function ?
Collimation → different applicators enable treatment of different field shapes
The applicator → touches the patient - contact therapy
Different lengths for different energies
→ 15 - 30cm is for superficial
→ 30 -50cm is for orthovoltage
What is FSD? → continuation of collimation
Field size determined → approximately by the base- plate aperture and refined by the lead aperture at the end of the tube
Applicators are open or closed ended → closed ends> 200kV filter out scatter
Applications→ maintain a constant FSD and beam direction for each treatment
What is shielding in relation to treatment?
Different sizes and shapes of treatment fields can be treated using an individual lead shield
Lead thickness depends on HVL and beam energy
What is monitor unit/ time?
Dose is determined by time or monitor units
Timer → dose rate is different for different applicators
Monitor units → mainly for orthovoltage , ionisation chamber between filter and applicator, dose produced will also differ for each applicator
What is the electron gun?
(The notes following from this point relate to the LINAC)
(similar to a kv tube)→ electron guns are “pulled” towards the anode which has 50kV positive potential difference
→ it increases kinetic energy
→ not enough to produce Mev photons
→gives the electrons a running start
Electrons do not collide with a target but instead are infected into the accelerator waveguide
What is electron injection?
Electrons have some initial KE from the gun →pass through a vacuum maintained by an ion pump
Microwaves are entering the waveguide in pulses→ the same for electrons
Pulsed modulator → simultaneously delivers high voltage pulses to the klystron/magnetron and electron gun → each pulse is 2 microseconds
The concept of velocity mismatch explained
For the electrons to be carried along by the microwaves they must possess the same velocity
Microwaves move at c( 3x10 to the power of 8 m/s)
Injected elections move at 0.4c
What is the accelerator waveguide?
A hollow conducting copper tube → internal irises with apertures are spaced at varying distances
How to solve the velocity mismatch
Microwave velocity needs reducing → but can’t actually happen as all em waves have a velocity of c
Bunching section of the waveguide → constructed to physically constrain the microwaves
In turn slowing them down → allowing the electrons to “ hitch “ on to microwaves
Electron injection → after the elections “hitch” onto the microwave
Electrons are taken through and allowed to accelerate, they can reach 0.98C
The relativistic section
This is the second section of the waveguide → evenly spaced irises and does not increase actual speed of electrons they instead gain relative mass
E=mc squared
Electrons gain mass
Also gaining KE
Gain up to 0.98C
Maximum energy of elections
energy gain is dependent on microwave power and waveguide length
Typical 6 MeV
→ 2MW microwaves
→ 1.5 waveguide
Dual energy linacs work by “detuning” the timing of the electron and microwaves pulses to reduce efficiency and hence energy
Linac principles → what are the high energy X-ray production challenges
Electron production → need high initial energy electrons
Electron acceleration → high Mv potential difference is dangerous and need an alternative acceleration mechanism
X-ray production → most X-rays produced in a forward direction and need an alternative target and tube orientation
Microwaves
Are similar to radio - waves but their frequency is thousands of times higher
Linacs → use 3000 MHz and power demand is 2mw
Radio waves → microwaves → infrared →visible light → ultra violet →x-rays → gamma
How does the klystron and magnetron work
Both create electromagnetic radiation
Magnetron → power lower energy units, are cheaper than klystrons but less stable
Klystron → used for high energy units
Sited in the Stand → magnetron
or adjacent room → klystron
→ operate in 2 microseconds pulses
→ gap of 2 milliseconds between pulses
Both generate microwaves
What is microwave propagation?
Microwaves are injected into a waveguide via the rf window
Feeder waveguide conveys them to accelerator in the gantry → copper pipe filled with sulphur hexa- fluoride
This acts as an insulator and prevents “arcing”
Linac → X-ray production - what are focussing coils?
Electrons are easily diverted and repel each other → important to keep electron beam narrow - 3mm at target helps maintain small focal spot
Focus coils wrapped around beam -line constrain the beam in a magnetic field
What is the purpose of steering rails
Electrons need to hit the right spot on the target →steering coils vary their field strength continuously to push the beam line to the correct alignment
What is the purpose of bending magnets?
At MeV energies X-rays are produced in the forward direction - if the waveguide pointed directly at the target the treatment head would be too long to use
Electron beam must be bent through 90 degrees
Bending magnets →90 degrees or 270?
Elekta bend → bend beam through 90 degrees
Varian bend →do a full loop of 270 degrees
What is the target?
Thin disc of metal → typically gold or tungsten and 6mm thick
Converts many electrons as possible to photons
The orientation of the target is different do that of kilovoltage due to the different distribution of X-rays
X-ray production → megavoltage
Mostly generated in forwards direction → target is a “ transmission target” where photons appear to have been transmitted through the target as photons
Cooling system
Generation of a huge amount of heat →a complex cooling system pumps cold water around the waveguide and target
What must be performed in order to change a linear accelerator beam from photons to electrons
Remove the flattening filter
Lower the dose rate
Remove the target
Add a scattering foil
What can be found in a linear accelerator “carousel”
Flattening filter
Target
Electron scattering fail
